Charuta Kulkarni
email: ckulkarni@gradcenter.cuny.edu

Walter Finsinger

Pallavi Anand

Sandra Nogué

Shonil A Bhagwat

Synergistic impacts of anthropogenic fires and aridity on plant diversity in the Western Ghats: Implications for management of ancient social-ecological systems

Keywords: Agroforestry, Biodiversity conservation, Evidence-based policymaking, Fire management, Social-ecological systems, Wet tropics

Identifying the impacts of anthropogenic fires on biodiversity is imperative for human-influenced tropical rainforests because: i) these ecosystems have been transformed by human-induced fires for millennia; and ii) their effective management is essential for protecting the world's terrestrial biodiversity in the face of global environmental change. While several short-term studies elucidate the impacts of fires on local plant diversity, how plant diversity responds to fire regimes over long timescales (>100 years) is a significant knowledge gap, posing substantial impediment to evidence-based management of tropical social-ecological systems. Using wet evergreen forests of the Western Ghats of India as a model system, we discuss the synergistic effects of anthropogenic fires and enhanced aridity on tropical plant diversity over the past 4000 years by examining fossil pollen-based diversity indices (e.g., pollen richness and evenness, and temporal β-diversity), past fire management, the intervals of enhanced aridity due to reduced monsoon rainfall and land use history. By developing a historical perspective, our aim is to provide region-specific management information for biodiversity conservation in the Western Ghats. We observe that the agroforestry landscape switches between periods of no fires (4000-1800 yr BP, and 1400-400 yr BP) and fires (1800-1400 yr BP, and 400-0 yr BP), with both fire periods concomitant with intervals of enhanced aridity. We find synergistic impacts of anthropogenic fires and aridity on plant diversity uneven across time, pointing towards varied land management strategies implemented by the contemporary societies. For example, during 1800-1400 yr BP, diversity reduced in conjunction with a significant decrease in the canopy cover related to sustained use of fires, possibly linked to large-scale intensification of agriculture. On the contrary, the substantially reduced fires during 400-0 yr BP may be associated with the emergence of sacred forest groves, a cultural practice supporting the maintenance of plant diversity. Overall, notwithstanding apparent changes in fires, aridity, and land use over the past 4000 years, present-day plant diversity in the Western Ghats agroforestry landscape falls within the range of historical variability. Importantly, we find a strong correlation between plant diversity and canopy cover, emphasising the crucial role of maintenance of trees in the landscape for biodiversity conservation. Systematic tree management in tropical social-ecological systems is vital for livelihoods of

Introduction

One important objective for conservation managers is to mitigate past environmental degradation and simultaneously strive to design resilient future ecosystems (Society for Ecological Restoration International Science & Policy Working Group, 2004). The value of the palaeoecological approach in the field of conservation has been widely recognised [START_REF] Jackson | Looking forward from the past: history, ecology, and conservation[END_REF][START_REF] Froyd | Emerging issues in biodiversity & conservation management: The need for a palaeoecological perspective[END_REF][START_REF] Gillson | Biodiversity conservation and environmental change: using palaeoecology to manage dynamic landscapes in the Anthropocene[END_REF][START_REF] Whitlock | Land-use history as a guide for forest conservation and management[END_REF] because of the insights it lends into past ecological information relevant to management frameworks (e.g., baseline conditions, historic land management). For instance, fires have often been viewed as a damaging disturbance in several forest ecosystems and, therefore, fire prevention is routinely implemented as a mechanism for landscape restoration and biodiversity conservation [START_REF] Wright | The ecological role of fire in natural conifer forests of western and Northern North America-Introduction[END_REF][START_REF] Gadgil | This Fissured Land: An Ecological History of India[END_REF][START_REF] Tacconni | Fires in tropical forestswhat is really the problem? Lessons from Indonesia[END_REF]. However, diverse lines of palaeoecological research (e.g., [START_REF] Swain | A history of fire and vegetation in northeastern Minnesota as recorded in lake sediments[END_REF][START_REF] Clark | Post-glacial fire, vegetation, and human history on the northern alpine forelands, south-western Germany[END_REF][START_REF] Dunwiddie | Using Historical Data in Ecological Restoration: A Case Study from Nantucket[END_REF][START_REF] Burrows | Linking fire ecology and fire management in south-west Australian forest landscapes[END_REF][START_REF] Leys | Historical range of fire frequency is not the Achilles' heel of Published[END_REF][START_REF] Anderson | Native Peoples' Relationship to the California Chaparral[END_REF] suggest that fires are a fundamental ecological process in most ecosystems and strict fire prevention could be detrimental to biodiversity [START_REF] Colombaroli | Diverse knowledge informing fire policy and biodiversity conservation[END_REF][START_REF] Mclauchlan | Fire as a fundamental ecological process : Research advances and frontiers[END_REF]. While such scientific understanding has led to comprehensive fire management and conservation plans in some parts of the world (e.g., [START_REF] Brown | Prescribed burning as a conservation management practice: A case history from the Cederberg mountains, Cape Province, South Africa[END_REF][START_REF] Keeley | Fire management impacts on invasive plants in the western United States[END_REF], there currently is a shortfall in evidence-based policymaking for effective biodiversity conservation in human-influenced tropical landscapes [START_REF] Karanth | Conservation and management in human-dominated landscapes: Case Published in[END_REF][START_REF] Juárez-Orozco | Causes and effects of forest fires in Tropical Rainforests: a bibliometric approach[END_REF][START_REF] Nogueira | Impacts of Fire on Forest Biomass Dynamics at the Southern Amazon Edge[END_REF]. Considering the close association between tropical landscapes and fires since prehistoric times [START_REF] Bowman | Fire in the earth system[END_REF][START_REF] Roberts | The deep human prehistory of global tropical forests and its relevance for modern conservation[END_REF], it is important to identify the impacts of fires on biodiversity in these ancient social-ecological systems. How tropical plant diversity responds to fire regimes over long timescales is a significant knowledge gap that needs to be addressed for sustainable management of these human-influenced landscapes (Driscoll et al., 2010;[START_REF] Seddon | Looking forward through the past: Identification of 50 priority research questions in palaeoecology[END_REF].

Among human-influenced landscapes, agroforestrya practice of planting crops under or alongside native shade treesholds special promise for biodiversity conservation [START_REF] Mcneely | Agroforestry and biodiversity conservation -Traditional practices, present dynamics, and lessons for the future[END_REF][START_REF] Bhagwat | Agroforestry: a refuge for tropical biodiversity?[END_REF]. Agroforestry landscapes promote intentional management of trees in productive agriculture and support the livelihoods of people including those living in world's biodiversity hotspots Published in Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 5 [START_REF] Ashley | The policy terrain in protected area landscapes: Challenges for agroforestry in integrated landscape conservation[END_REF][START_REF] Miller | Indigenous agroforestry systems in Amazonia: From prehistory to today[END_REF][START_REF] Fisher | Poverty and biodiversity: Measuring the overlap of human poverty and the biodiversity hotspots[END_REF][START_REF] Chavan | National Agroforestry Policy in India: A low hanging fruit[END_REF]. The rainforests of the Western Ghats of India (Fig. 1) is one such biodiversity hotspot with ancient agroforestry systems and well-established fire management practices [START_REF] Asouti | Trees and Woodlands of South India: Archaeological Perspectives[END_REF][START_REF] Krishna | History of South Indian Agriculture and Agroecosystems[END_REF]. Notably, fires in these forests rarely occur naturally because, first, there is a very low coincidence of lightning with climate conditions suitable to carry fire, and secondly, even if a fire is initiated, the high fuel moisture in these rainforest ecosystems generally prevents propagation [START_REF] Kodandapani | A comparative analysis of spatial, temporal, and ecological characteristics of forest fires in seasonally dry tropical ecosystems in the Western Ghats, India[END_REF][START_REF] Bowman | The human dimension of fire regimes on Earth[END_REF].

However, anthropogenic fires i.e. those ignited by small-scale farmers who typically burn fallow land very locally, can spread over wide areas during exceptionally hot and dry years [START_REF] Kodandapani | Conservation threat of increasing fire frequencies in the Western Ghats, India[END_REF][START_REF] Cochrane | The past, present, and future importance of fire in tropical rainforests[END_REF][START_REF] Silvério | Fire, fragmentation, and windstorms: A recipe for tropical forest degradation[END_REF]. The intensification of fires is particularly alarming in light of the projected weakening of the Indian Summer Monsoon (henceforth, monsoon), which is expected to excarbate rainfall extremeties in the Indian Subcontinent, extending the length of hot-dry months [START_REF] Sinha | The leading mode of Indian Summer Monsoon precipitation variability during the last millennium[END_REF][START_REF] Roxy | Drying of Indian subcontinent by rapid Indian ocean warming and a weakening land-sea thermal gradient[END_REF][START_REF] Mishra | Future exacerbation of hot and dry summer monsoon extremes in India[END_REF] but see [START_REF] Bhowmick | Projected precipitation changes over the south Asian region for every 0.5 °C increase in global warming[END_REF]. Such enhanced regional aridity could mean drier fuel loads, resulting in more frequent spread of fires beyond agricultural lands. In this context, we examine the combined effects of anthropogenic fires and aridity on plant diversity in the Western Ghats agroforestry landscapes over the past 4000 years, a period known for incipient weakening of monsoon and enhanced aridity in the Subcontinent [START_REF] Dixit | Abrupt weakening of the summer monsoon in northwest India ~4100 yr ago[END_REF][START_REF] Kathayat | The Indian monsoon variability and civilization changes in the Indian subcontinent[END_REF]. By developing a historical perspective, our aim is to provide region-specific management information for biodiversity conservation in the Western Ghats and wet tropical agroforestry landscapes at large. Thus, we address the following questions:

1) How has the plant diversity changed over the past 4000 years?

2) What are the synergistic effects of anthropogenic fires and the weakening of monsoon (enhanced aridity) on plant diversity and vegetation composition?

3) What are the potential implications of historical fire regimes in sustaining plant diversity in a tropical socialecological system? These questions are relevant for understanding transformation of tropical forests into intricate socialecological systems, assessing the role of fire management and its effects on plant diversity under current and future environmental change. Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 Figure 1: (a) The Western Ghats of India (after [START_REF] Mirza | Preliminary review of indian eumenophorinae (Araneae: Theraphosidae) with description of a new genus and five new species from the Western Ghats[END_REF]. The circle represents the location of the study site in the Central part of the Western Ghats, located in Kodagu District, Karnataka. (b) Google image of the Kodagu region; the sedimentary sequence under study (12°9'14"N, 75°42'47"E; shown with a circle) is derived from a coffee agroforestry setting from the middle reaches of the Western Ghats (Data source: https://earth.google.com/). The site is less than 10 km away from a forest reserve currently under government protection. (c) An example of a coffee plantation grown under the shade of native trees in Kodagu, Western Ghats, India. A layer of coffee bushes is seen in the understorey and a canopy of native trees is seen above this layer. Such plantations of shade-grown coffee in many tropical regions have canopy structure similar to that of secondary forests. The shade of native trees provides habitat for many forest-dwelling species in otherwise highly human-dominated landscapes (Image credit: Shonil A. Bhagwat). Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 7

Published in

Published in

Materials and methods

The study site in the Western Ghats

The Western Ghats is a mountain range running along the west coast of peninsular India (Fig. 1a). The midelevation (500-1500 m asl) terraces constitute over 90% of the Western Ghats, presenting a mosaic of wet evergreen rainforest and grasslands shaped by human activities, particularly fire [START_REF] Premathilake | Relationship of environmental changes in central Sri Lanka to possible prehistoric land-use and climate changes[END_REF][START_REF] Kodandapani | Forest fire regimes and their ecological effects in seasonally dry tropical ecosystems in the Western Ghats, India[END_REF]. Here we utilised one such mid-elevational sedimentary sequence (hereafter, Agroforest-1), derived from the Kodagu district of Karnataka i.e. the central part of the Western Ghats (Figs. 1a-b). The Kodagu district boasts the extensive network of wet evergreen rainforest reserves primarily of Mesua-Palaquium type [START_REF] Ramesh | Atlas of endemics of the Western Ghats (India): distribution of tree species in the evergreen and semi-evergreen forests[END_REF], occupying about 30% of the landscape (Fig. 1b; Bhagwat et al., 2005a). In addition to these formally protected forests, 10% of the landscape is under rice cultivation while c. 60% of the landscape is managed as agroforestry with high tree-canopy shading coffee plantations (Bhagwat et al., 2005b). Coffee (Coffea arabica var. robusta) is often planted in the understorey of shade trees (Fig. 1c), many of which are representative of native vegetation mixed with betel nut palm (Areca catechu; Bhagwat et al., 2012). Extracted from a small swamp situated in one of the coffee estates (12°09'14" N, 75°42'47" E; 910 m asl; Fig. 1), the Agroforest-1 sedimentary sequence is among the two overlapping cores, which was sub-sampled and processed in the laboratory for pollen and charcoal analyses (Bhagwat et al., 2012;Nogué et al., 2018). The robust age-depth model was established using several 14 C dates and 210 Pb dates (Bhagwat et al., 2012;Nogué et al., 2018). The further details on the sediment core extraction, processing and chronology can be found in Supplementary Material.

Extent and components of canopy cover

We classified Agroforest-1 fossil pollen taxa into three groups: canopy-forming woody taxa (i.e. arboreal pollen types including trees, shrubs, lianas, climbers), grassland taxa (Poaceae and Cyperaceae), and herbaceous understorey taxa (all other non-arboreal pollen types). While only pollen-based landcover modelling applications can translate pollen taxa percentages into forested and non-forested fractions (e.g. [START_REF] Sugita | Theory of quantitative reconstruction of vegetation I: Pollen from large sites REVEALS regional vegetation composition[END_REF], the lack of key parameters (e.g., pollen productivity estimates, pollen fall speeds) in tropical regions, including India, does not yet allow landcover modelling [START_REF] Anupama | Holocene land cover and land use in South and Southeast Asia for climate modeling[END_REF].

REVEALS model by

Published in Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 8 However, recent modern pollen dispersal studies in India [START_REF] Quamar | Modern pollen dispersal studies in India: a detailed synthesis and review[END_REF] and references therein) attest that there is a good correlation between the abundance of pollen and extant vegetation in tropical wet evergreen forests with certain 'indicator' taxa (e.g., Mesua, Elaeocarpus, Ilex, Garcinia) well-represented in the pollen assemblages. Following this lead, we estimated the extent of canopy cover, grassland, and herbaceous understorey in the Western Ghats agroforestry landscape based on the collective abundance of canopy-forming woody taxa, grassland taxa, and herbaceous understorey taxa respectively. Additionally, we calculated the relative abundance of 20 long-lived habitat-specialist evergreen trees that are indicative of low disturbance and closed-canopy forest in the Western Ghats [START_REF] Laurance | Habitat fragmentation, variable edge effects, and the landscape-divergence hypothesis[END_REF]Nogué et al., 2018). We also juxtaposed the relative abundance of commonly cultivated plant taxa (e.g. Coffea) to differentiate the anthropogenic component of the canopy cover.

Plant diversity

Pollen richness can accurately reflect plant richness and landscape structure within one vegetation or climatic region (Birks et al., 2016 and references therein). To eliminate the bias due to variable pollen-count sums, we rarified the terrestrial pollen dataset by repeated random re-sampling 1000 times without replacement to a constant base-sum (E(Sn)=243; [START_REF] Felde | Modern pollen-plant richness and diversity relationships exist along a vegetational gradient in southern Norway[END_REF], using a custom-made computer code (https://github.com/wfinsinger/rrarefy.record; [START_REF] Finsinger | Holocene vegetation and fire dynamics at Crveni Potok, a small mire in the Dinaric Alps (Tara National Park, Serbia)[END_REF] and the 'vegan' package in the R environment [START_REF] Oksanen | Vegan: Community Ecology Package[END_REF]R Core Team, 2014). We quantified plant richness i.e. plant diversity using Hill numbers [START_REF] Hill | Diversity and Evenness : A unifying notation and its Consequences[END_REF] i.e. N0: the effective total number of taxa present, N1: the effective number of common taxa, N2: the effective number of dominant taxa. These measures provide easily interpretable data based on how rare and abundant taxa are weighted. We also calculated the pollen evenness as the ratio of dominant taxa relative to all taxa present in an assemblage (N2/N0; [START_REF] Hill | Diversity and Evenness : A unifying notation and its Consequences[END_REF][START_REF] Odgaard | Species richness of the past is elusiveevenness may not be[END_REF][START_REF] Tuomisto | An updated consumer's guide to evenness and related indices[END_REF]. Considering the size of the swamp under study (<1 ha) and corresponding small-intermediate pollen source area (~10-20 km; [START_REF] Prentice | Pollen representation, source area, and basin size: Toward a unified theory of pollen analysis[END_REF][START_REF] Sugita | A Model of Pollen Source Area for an Entire Lake Surface[END_REF], our record would more likely show a high correlation between pollen richness and evenness (e.g. [START_REF] Colombaroli | Changes in biodiversity and vegetation composition in the central Swiss Alps during the transition from pristine forest to first farming[END_REF]. To estimate diversity of the canopyforming woody taxa, we applied the same re-sampling approach exclusively to the arboreal pollen sum (E(Sn)=39).

Change in vegetation composition over time

In palaeoecological time series, temporal β-diversity is a measure of change in vegetation composition between adjacent samples that can be quantified using rate-of-change [START_REF] Birks | Estimating the amount of compositional change in late-Quaternary pollenstratigraphical data[END_REF]. To calculate rate-ofchange, we linearly interpolated the pollen percentages on equal time intervals and subsequently estimated dissimilarity between adjacent samples using the chord distance of square-root transformed percentage values [START_REF] Birks | Biological responses to rapid climate change at the Younger Dryas-Holocene transition at Krakanes, western Norway[END_REF]. Chord distance values range between zero (no change in vegetation composition) and two (complete change in vegetation composition, i.e. the two assemblages have no species in common). Due to the varying sample resolution across the 4000-yr long sequence (range = 22-670 years; mean=121 years; 3rd quartile=120 years; see Fig. S1), we calculated two sets of temporal β-diversity records selecting an interpolation-window width greater than the third quartile of the sampling-resolution distribution. Thus, the β-diversity record for the entire 4000-yr period is at 260-yr resolution while another is for the last 2200 yr at 150-yr resolution (see details in Supplementary Material). Employing a shorter interpolation-window width for the past 2200 years allows us to visualise transformations in vegetation composition associated with intensified burning and land use change. The trends are illustrated by a LOESS-smoothed curve [START_REF] Cleveland | Locally Weighted Regression: An Approach to Regression Analysis by Local Fitting[END_REF].

Past fire management

We pooled our new macroscopic charcoal record (particles >150 µm) with the existing dataset by Nogué et al., (2018) that covered only the past 1000 years, extending the fire history up to 4000 yr BP. In addition, we utilised microscopic charcoal dataset previously quantified in tandem with pollen analysis of the Agroforest-1 core (Bhagwat et al., 2012). We discussed the associated chemical and analytical procedures in detail in Supplementary Material. We transformed charcoal concentrations to charcoal accumulation rates (CHAR; pieces cm -2 yr -1 ) to account for variations in sediment accumulation rates. Overall, macroscopic charcoal (<600 µm) and microscopic charcoal pieces can be dispersed over large distances (up to 30-50 km;[START_REF] Tinner | Pollen and charcoal in lake sediments compared with historically documented forest fires in southern Switzerland since AD 1920[END_REF][START_REF] Oris | Charcoal dispersion and deposition in boreal lakes from 3 years of monitoring: Differences between local and regional fires[END_REF][START_REF] Adolf | The sedimentary and remote-sensing reflection of biomass burning in Europe[END_REF]. Thus, the low-frequency background component of macroscopic charcoal (CHARback) and the microscopic charcoal trends are considered as proxies for the fire activity at the landscape-scale [START_REF] Marlon | Climate and human influences on global biomass burning over the past two millennia[END_REF][START_REF] Adolf | The sedimentary and remote-sensing reflection of biomass burning in Europe[END_REF]. We estimated the background component by Published in Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 11 interpolating the macro-charcoal record using a constant temporal resolution of 13 years (i.e. the median sediment accumulation rate), followed by LOWESS smoothing.

Monsoon variability and intervals of enhanced aridity

The annual monsoon precipitation cycle in the Indian Subcontinent is a manifestation of the seasonal migration of the intertropical convergence zone with ~80% of yearly rainfall between June and September (JJAS; [START_REF] Webster | Monsoons: Processes, predictability, and the prospects for prediction[END_REF][START_REF] Sinha | Monsoon rainfall over the last two millennia[END_REF]. The study area, the Central Western Ghats, in particular, receives c. 2000 mm of average rainfall during JJAS [START_REF] Ambinakudige | Comparing tree diversity and composition in coffee farms and sacred forests in the Western Ghats of India[END_REF]. While February-March can be the driest months, the pre-monsoon months (April-May) bring a small amount of rainfall (c. 200 mm/month; [START_REF] Sukumar | Mudumalai forest dynamics plot, India[END_REF]. We used two independent lines of evidence to detect the intervals of enhanced aridity in response to reduced monsoon rainfall. First, using a Community Climate System Model-based PaleoView [START_REF] Fordham | PaleoView: A tool for generating continuous climate projections spanning the last 21 000 years at regional and global scales[END_REF], we obtained mean JJAS rainfall for the Kodagu region at 20-yr resolution. Although the absolute JJAS rainfall values are lower than the instrumental average (1300 vs. 2000 mm), it is the relative changes in rainfall that we focus on for identifying past intervals of enhanced aridity on the regional scale. In addition, we employed a fine-resolution (c. 1.8 years on average) speleothem record from North India to identify the weakening of the monsoon in the Indian Subcontinent [START_REF] Kathayat | The Indian monsoon variability and civilization changes in the Indian subcontinent[END_REF]. Despite possible regional heterogeneities within the subcontinent, we compared both records to identify common arid intervals across the past 4000 years.

Land use history

The Western Ghats has a long history of human occupation and modification as early as 800000 yr BP [START_REF] Pappu | Man-land relationships during Palaeolithic times in the Kaladgi Basin[END_REF][START_REF] Gaillard | Lower and Early Middle Pleistocene Acheulian in the Indian sub-continent[END_REF]. While the roots of agriculture-pastoralism in the region can be traced back to c. 5000 yr BP [START_REF] Fuller | Early plant domestications in southern India: Some preliminary archaeobotanical results[END_REF], the agroforestry systems in particular is thought to be in existence for the past 2000 years [START_REF] Krishna | History of South Indian Agriculture and Agroecosystems[END_REF]. Based on the review of archaeological and environmental history literature (e.g., [START_REF] Rice | Mysore and Coorg: gazetteer compiled for the Government of India[END_REF][START_REF] Chandran | On the ecological history of the Western Ghats[END_REF][START_REF] Asouti | Trees and Woodlands of South India: Archaeological Perspectives[END_REF][START_REF] Krishna | History of South Indian Agriculture and Agroecosystems[END_REF], we compiled a summary of land use history of the Kodagu region through notable ecological, social and political changes. The key societal transitions and changes in land management through Published in Journal of Environmental Management; https://doi.org/10. 1016/j.jenvman.2021.111957 12 intensification of existing strategies as well as through introduction of new means help us explore the historic roots of fires in the Western Ghats.

Statistical analyses

To examine the inter-relationships among fires, plant diversity, and canopy cover, we conducted simple linear regressions and time-series (cross-correlation) analyses for the entire 4000 yr timeline. We calculated the coefficients of determination (R 2 ), identifying the strength of association between the independent and the dependent variables for each scatterplot. To explore lag effects of one variable on another [START_REF] Green | Time-series and post-glacial forest ecology[END_REF][START_REF] Tinner | Long-term forest fire ecology and dynamics in southern Switzerland[END_REF][START_REF] Rey | Causes and mechanisms of synchronous succession trajectories in primeval Central European mixed Fagus sylvatica forests[END_REF], we computed cross correlations between key variables including fire (macro-CHAR values), plant diversity (total number of terrestrial taxa present (N0)), diversity of canopyforming woody taxa (total number of woody taxa present (N0)), and canopy cover (arboreal pollen %) in R (R Core Team, 2014). As cross-correlation analysis requires evenly spaced data points, we resampled the records at regular intervals using LOESS smoothing (span=0.2). To avoid inflating the sampling resolution for the entire 4000-yr dataset (see Fig. S1), we estimated the correlation coefficients at 260 yr-intervals and presented with 95% confidence intervals (p < 0.05). (d-e) Anthropogenic fires based on macroscopic (CHAR; pieces cm -2 yr -1 ) and microscopic (CHAR; x10 3 pieces cm -2 yr -1 ) charcoal analysis; the thick brown line in macro-charcoal record is a 13-yr wide LOWESSsmoothing depicting charcoal background;

(f) Regional monsoonal months (JJAS) rainfall (mm) at 20-yr interval;

(h) δ 18 O speleothem record from North India demonstrating Indian Summer Monsoon (ISM) intensity (after [START_REF] Kathayat | The Indian monsoon variability and civilization changes in the Indian subcontinent[END_REF]. [START_REF] Asouti | Trees and Woodlands of South India: Archaeological Perspectives[END_REF][START_REF] Krishna | History of South Indian Agriculture and Agroecosystems[END_REF]. Within the Historic Period, S = Satavahana Dynasty (2050-1800 yr BP), K = Kadamba Dynasty (1600-1350 yr BP), H = Hoyasala Kingdom (850-650 yr BP), V = Vijayanagara Empire (600-350 yr BP), and BE = British Era (150-0 yr BP). SFG stands for sacred forest groves while CAF stands for coffee agroforestry. Grey areas highlight periods of societal transitions i.e. changes in land management either through intensification of existing strategies and/or through introduction of new means. The years 1780±60 yr BP, 1436±60 yr BP, and 359±30 yr BP mark the switches between the no-fire and fire periods (white and red arrows respectively) and are considered as points of major transformations in the system.

Results

Trends in canopy cover and plant diversity

The canopy cover exhibits considerable changes over the past 4000 years (Figs. 2-3a). It substantially declines from initially stable (23-29%) to its lowest value (14%) c. 2250 yr BP. Subsequently, a notable increase in canopy cover is visible until 2000 yr BP (~28%), followed by a series of fluctuations between 2000 and 1450 yr BP. Over the next 850 years, the canopy cover substantially increases, reaching its peak (48-51%) c. 1100-750 yr BP. From 600 yr BP onwards, it gradually shrinks to constitute c. 30% of the present-day landscape (Figs. 2-3a).

The pollen richness (N0 = 24-58) and evenness (N2/N0 = 0.07-0.13) show largely analogous trends, collectively identifying plant diversity changes over time (Fig. 4 Published in Journal of Environmental Management; https://doi.org/10.1016/j.jenvman.2021.111957 15 higher than its oldest, no-fire counterpart (Figs. 3 and5). Importantly, except a visible setback c. 1550 yr BP, the number of canopy-forming woody taxa hardly varies over the past four millennia (Fig. 4).

In case of vegetation composition, there is an overall increasing trend in temporal β-diversity (Fig. 3c).

During 4000-2350 yr BP, it increases and decreases between 0.4 to 1.22, followed by an upward trend until 1050 yr BP (visible in both curves; more peculiar the 2200-yr curve in Fig. 3c). There is a modest drop in the temporal β-diversity during 1000-700 yr BP, however, it successively stabilises to constitute the present-day value of 1.5.

Anthropogenic fires and intervals of enhanced aridity

We find no evidence of fire occurrence between 4000 and 1800 yr BP (Figs. 3d-e). Fires first appear in the Western Ghats landscape c. 1800 yr BP and continue until 1400 yr BP. Negligible fires are noted over the period of next ~1000 years. Fires reappear in the landscape c. 400 yr BP, albeit at a much lower degree (Figs.

3d-e). Comparing regional JJAS rainfall and the overall monsoon intensity (Figs. 3e-f), three pronounced aridity intervals are observed: regional aridity during 3550-2950 yr BP, subcontinental aridity during 2550-1450 yr BP and the last 800 yr BP (except a short, wet regional spell c. 500 yr BP). The warm interval of the Medieval Warm Period is observed to be overall wet while the cooler Little Ice Age conditions are coterminous with substantial aridity and rainfall variability.

Relationships among plant diversity, canopy cover and fire activity

Canopy cover is significantly and positively correlated with the overall plant diversity (Figs. 6d andS2), its correlation with the diversity of woody taxa is rather weak (Fig. 6e). Canopy cover and plant diversity show positive responses to fire at lags > +2, corresponding to c. 500-750 years (Figs. 6a and 6c respectively). There is also a positive correlation between fire and the diversity of woody taxa but with a shorter lag (+1; ~260 years; Fig. 6b). Published in Journal of Environmental Management; https://doi.org/10. 1016/j.jenvman.2021.111957 Figure 5: A relationship between plant diversity and anthropogenic fires in the Western Ghats agroforestry landscape. The pollen richness (N0) values for all terrestrial taxa and canopy-forming woody taxa are plotted against macro-and micro-CHAR values (pieces cm -2 yr -1 ) and the points are connected in a chronological sequence. The solid circle represents the initial (oldest) plant diversity value in the sequence while the triangle shows the present-day value. Two fire periods, 1800-1400 yr BP and 400-0 yr BP are visible in Figs. 234: the years 1780±60 yr BP, 1436±60 yr BP, and 359±30 yr BP mark the switches among the no-fire and fire periods.

The years associated with highest and lowest points in plant diversity and significant biomass burning events are also shown. Insets represent scatterplots of variables and the coefficients of determination (R 2 ) show the strength of association. Note: The sample interval for pollen-based plant diversity indices and microscopic charcoal is 4-cm while that of for macroscopic charcoal is 1-cm. Thus, the corresponding macroscopic charcoal dataset in this comparative analysis resulted in a different CHAR ranges (0-4 cm -2 yr -1 ) as compared to the complete data range (0-8 cm -2 yr -1 ) seen in Figs. 23. Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957

Published in

Discussion

In the Western Ghats agroforestry landscapes, we find two distinct periods of fire i.e. 1800-1400 yr BP and 400-0 yr BP that are concomitant with enhanced aridity (Fig. 3). As fire and aridity together could exacerbate drying of these wet tropical landscapes, we interpret the switches between the fire and no-fire periods in this Published in Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 record as points of major landscape transformations (Figs. 2345) and explore their relationships with plant diversity and canopy cover. We subsequently interpret these results in the context of past landscape management and future conservation strategies for the Western Ghats and human-influenced social-ecological systems in the wet tropics.

Linkages between plant diversity and the canopy cover in the Western Ghats

We find a strong correlation between plant diversity and canopy cover: higher the canopy cover, higher the number of taxa in the landscape (Figs. 6d andS2). The high canopy cover could also favor the diversity of woody taxa (Fig. 6e). In human-influenced tropical landscapes, high canopy cover could mean less fragmented landscape [START_REF] Chazdon | Tropical forest recovery: Legacies of human impact and natural disturbances[END_REF]. Tree canopies often reduce the severity of microclimatic changes including higher temperatures, increased wind speed, lower humidity, and lower soil moisture [START_REF] Kapos | Edge-related changes in environment and plant responses due to forest fragmentation in Central Amazonia[END_REF][START_REF] Freidenburg | Physical Effects of Habitat Fragmentation[END_REF]. As a result, there may be less edge-related disturbance and more habitat availability for forest-dwelling species and less for non-forest-dwelling species (Bhagwat et al., 2005b;[START_REF] Muthuramkumar | Plant community structure in tropical rain forest fragments of the Western Ghats, India[END_REF]. The congruence between the extent of canopy cover and plant diversity (Fig. 6d) is suggestive of the contribution of forest-dwelling herbaceous understorey in the plant diversity, especially with a stable number of woody taxa in this record over the past 4000 years. Overall, the maintenance of plant diversity in this tropical socialecological system is closely linked with the extent of canopy cover i.e. landscape fragmentation.

Synergistic effects of anthropogenic fires and enhanced aridity on plant diversity and vegetation transformation

Over the course of 4000 years, plant diversity changes lag behind fires by as many as 250-500 years (Fig. 6ab). Amid the first fire period (1800-1400 yr BP), the plant diversity substantially decreased around 1550 yr BP inclusive of the only significant decline in the woody taxa (Fig. 4). Considering the strong positive correlation between the canopy cover and the plant diversity (Fig. 6d), we argue that the sustained biomass burning coupled with enhanced aridity during 1800-1400 yr BP could have resulted in landscape fragmentation and a consequent reduction of both woody and herbaceous taxa. By opening and increasing canopy gaps and creating snags and deadwood patches, the prolonged use of fire can increase the number of potential ecological niches [START_REF] Pausas | Wildfires as an ecosystem service[END_REF]. In keeping with this, we find a greater temporal β-Published in Journal of Environmental Management; https://doi.org/10. 1016/j.jenvman.2021.111957 20 diversity during this fire period, indicating a higher degree of ecological transformation in response to fires, increased seasonality, and the extensive establishment of rice cultivation in the Western Ghats (Fig. 3).

Overall, the loss of plant diversity, especially that of woody taxa, c. 1550 yr BP could have had serious implications for contemporary societies. In this context, the reduction in fires c. 1400 yr BP onwards, perhaps associated with the emergence of arboriculture, and subsequent substantial upsurge in the diversity of woody taxa and in overall plant diversity over the next 850 years is interesting (Figs. 3c-e). Indeed, during this nofire period (1400-400 yr BP), several habitat-specialist trees also found their way back after long hiatuses (e.g., Dipterocarpus, Myristica) while some made their first appearance (e.g., Dichapetalum, Poeciloneuron) in this Western Ghats agroforestry sequence, indicating a post-disturbance transition towards a closed canopy evergreen forest (Figs. 2-3a). Interestingly, temporal β-diversity remained high until 1000 yr BP, c. 400 years after the end of the first fire period (Fig. 3c), suggestive of a long vegetation recovery time or of other factors (e.g. land use) that had been critical in forest transformation. Overall, the increased plant diversity and canopy cover during the no-fire period seem to be coterminous with organised slash-and-burn agriculture, intensification of arboriculture, and later with the onset of agroforestry during the wet Medieval Warm Period (Fig. 3 top panel; [START_REF] Asouti | Trees and Woodlands of South India: Archaeological Perspectives[END_REF]. A collective strategy for curtailing the use of fire (e.g., infrequent, very localised burns) and planting trees and shrubs on productive agricultural land could have played an important role in invigorating plant diversity in the region (Figs. 3 and5). Such large-scale changes could be achievable largely due to paradigm shifts in land use by the local communities (e.g., [START_REF] Luoga | Land cover and use changes in relation to the institutional framework and tenure of land and resources in eastern Tanzania Miombo woodlands[END_REF][START_REF] Dalle | Integrating analyses of local land-use regulations , cultural perceptions and land-use/land cover data for assessing the success of community-based conservation[END_REF]. This could have included changes in their attitude towards forests in recognising their value as a provider of key resources essential for their subsistence including medicinal herbs, culturally important plants, and water [START_REF] Bhagwat | Cultural drivers of reforestation in tropical forest groves of the Western Ghats of India[END_REF]. Appreciating the importance of forests, native trees, and their ability to foster biodiversity in connection with certain 'ecosystem services' could be attributed to a timely response to changing climate (i.e. the Medieval Warm Period) from predominantly agro-pastoral societies. The subsequent reduction of canopy cover and loss of plant diversity over the course of Vijayanagara Empire (Figs. 3a-b and top panel), however, could indicate deleterious impacts of intensified urbanism and consequent resource pressures on the Western Ghats rainforests [START_REF] Morrison | The Human Face of the Land: Why the past matters for India's environmental future[END_REF]. Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 21 Lastly, the return of fires c. 400 yr BP is in concurrence with the enhanced aridity the emergence of sacred forest groves in the Kodagu region (Fig. 3; [START_REF] Bhagwat | Cultural drivers of reforestation in tropical forest groves of the Western Ghats of India[END_REF]. Although created as dedicated places of worship, the formation of sacred forest patches could have led to the protection and conservation of native trees (see the highest extent (16%) of habitat-specialist trees in Fig. 2; [START_REF] Bhagwat | Sacred groves : potential for biodiversity management[END_REF]. In maintaining sacred forest groves, a strict demarcation of forested and agricultural land parcels was implemented by the local dynasty and indigenous communities living under its rule [START_REF] Belliappa | Nuggets from Coorg History[END_REF], which could have effectively resulted into increased canopy cover in the region (Fig. 2). Thus, more coherent socialecological systems in the Western Ghats can be attributed to the communities actively managing the landscape through a moderated use of fires and through maintenance as well as restoration of trees (Nogué et al., 2018).

Published in

The British occupation of India over the past 150 years established commercial agroforestry as well as state forestry, setting the new for forestry operations and overall landscape management [START_REF] Chandran | On the ecological history of the Western Ghats[END_REF]. The completely novel forest management plans, essentially modelled after European forestry systems, were extensively adopted [START_REF] Rangarajan | Fencing the Forest: Conservation and Ecological Change in India's Central Provinces 1860-1914[END_REF][START_REF] Ribbentrop | Forestry in British India[END_REF], streamlining the production of large timber volumes and generation of revenue from forest resources [START_REF] Guha | Forestry in British and Post-British India: an historical analysis[END_REF][START_REF] Rajan | Imperial environmentalism or environmental imperialism? European forestry, colonial foresters and the agendas of forest management in British India 1800-1900[END_REF]. The colonial view of forests as a commodity also resulted in banning of the slash-and-burn practice, which came in direct conflict with traditional forest management practices [START_REF] Gadgil | This Fissured Land: An Ecological History of India[END_REF]. Under such completely altered land management strategy, the Kodagu region experienced the establishment of commercial coffee estates c. 100 yr BP (1854 CE; [START_REF] Ambinakudige | Global coffee market influence on land-use and landcover change in the Western Ghats of India[END_REF]. Forests were first substantially cleared to plant coffee but later shade grown coffee became the cultivation practice as coffee plants could not survive heavy monsoon rains [START_REF] Ambinakudige | Comparing tree diversity and composition in coffee farms and sacred forests in the Western Ghats of India[END_REF]. Both the phases of reduction and revival of the canopy cover and that of plant diversity are visible in this Western Ghats agroforestry sequence, so is the gradual rise in vegetation transformation (Figs. 3a-c). Despite the banning of slash-and-burn practice, fires continued to occur in the Western Ghats landscape, perhaps as a societal response to the reduced rainfall (Fig. 3) and/or resistance towards colonial forest management [START_REF] Thekaekara | Notes from the Other Side of a Forest Fire[END_REF]. The overall degree of biomass burning during this fire period (400-0 yr BP), however, has been much lower than that of during the previous fire period (1800-1400 yr BP; Figs. 3d-e). From a historic perspective, the present-day plant diversity in the Western Published in Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 22 Ghats falls within the range of historical variability: while the present-day plant diversity values are slightly lower than those in the previous fire period, they are higher in comparison with their oldest (no-fire) counterparts (Fig. 5). In other words, extensive coffee cultivation in the Western Ghats may have reduced the plant diversity to an extent (Fig. 3). However, due to the practice of maintaining native canopy cover, this cash-crop intensification may also have paved the way to reviving plant diversity when the conditions allowed.

Today's landscape under shade-grown coffee cultivation (c. 60% of the landscape) where native shade trees constitute a major share of the evergreen forest canopies (Fig. 1c).

Implications for ecological management in the Western Ghats and tropical landscapes

Importance of conservation beyond forest reserves

Alongside other long-term, pollen-based diversity studies (e.g., [START_REF] Weng | Challenges in estimating past plant diversity from fossil pollen data: statistical assessment, problems, and possible solutions[END_REF][START_REF] Figueroa-Rangel | Late-Holocene successional dynamics in a transitional forest of west-central Mexico[END_REF][START_REF] Palazzesi | Fossil pollen records indicate that Patagonian desertification was not solely a consequence of Andean uplift[END_REF]Birks et al., 2016b;[START_REF] Rodríguez-Zorro | Forest stability during the early and late Holocene in the igapó floodplains of the Rio Negro, northwestern Brazil[END_REF], the relationship between canopy cover and plant diversity in the Agroforest-1 record echoes the unique ecological character of the tropics: the tree-covered matrix is an important determinant of sustaining biodiversity in the tropics. From a conservation point of view, our results further underline the need for maintaining trees in human-influenced tropical landscapes. While forest reserves in the tropics are strictly protected for biodiversity conservation, such areas might fall short of fulfilling their purpose without conservation management in the landscapes surrounding them [START_REF] Defries | Interactions between protected areas and their surroundings in human-dominated tropical landscapes[END_REF][START_REF] Karanth | Conservation and management in human-dominated landscapes: Case Published in[END_REF]. Tree management in tropical landscapes under mounting anthropogenic pressures is challenging, calling for pragmatic solutions that support both biodiversity and people. As envisaged in India's National Agroforestry Policy [START_REF] Chavan | National Agroforestry Policy in India: A low hanging fruit[END_REF], increasing area under agroforestry would be one of the most practical ways to do soits potential in providing habitats outside formally protected land, connecting nature reserves, and alleviating resource-use pressure on conservation areas is already substantiated [START_REF] Bhagwat | Agroforestry: a refuge for tropical biodiversity?[END_REF][START_REF] Schroth | Agroforestry and biodiversity conservation in Tropical Landscapes[END_REF]. Interestingly, positive canopy cover-biodiversity correlation is already visible in the modern-day coffee-dominated agroforestry landscapes in the Western Ghats [START_REF] Kushalappa | Can coffee based agroforestry conserve biodiversity? Evidence of exotics replacing native trees[END_REF] and in other kinds of tropical agroforestry systems Published in Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 23 elsewhere [START_REF] Tscharntke | Multifunctional shade-tree management in tropical agroforestry landscapes -A review[END_REF][START_REF] Acabado | The Ifugao agrocultural system: bridging culture and nature to enhance tropical biodiversity[END_REF][START_REF] Ticktin | Linkages between measures of biodiversity and community resilience in Pacific Island agroforests[END_REF], providing region-specific analogues for wider implementation.

Furthermore, the Agroforest-1 record (Fig. 2) shows that habitat-specialist trees have persisted in the landscape over the past 4000 years (e.g., Eleocarpus, Syzygium), reappeared at <1000-yr intervals (e.g., Holigarna, Myristica, Reinwardtiodendron), or even "arrived" during the first fire period (e.g., Poeciloneuron, Litsea). Throughout the record, these habitat-specialist trees continue to constitute c. 25-33% of the Western Ghats wet evergreen forest cover (Fig. 2), suggesting the resilience of old-growth trees towards varying degrees of fire management and land use. While a few of them (e.g., Dipterocarpus, Hopea, and Palaquium) are already part of historic baselines that conservationists value in planning and restoration of the Western Ghats [START_REF] Muthuramkumar | Plant community structure in tropical rain forest fragments of the Western Ghats, India[END_REF], our long-term pollen record provides further, empirical support to this assumption. Patches of native trees on agroforestry landscapes would, thus, be natural places to focus organised conservation and restoration efforts beyond forest reserves. However, to sustain the conservation potential of agroforestry systems, a balance between production and protection of natural features in the landscape is essential [START_REF] Bhagwat | Agroforestry: a refuge for tropical biodiversity?[END_REF]. Thus, keeping remnants of such forests within agroforestry plantations and redesigning annual croplands to include features of old-growth trees (e.g. hedgerows) can contribute to landscape-level connectivity, thereby making biodiversity conservation more effective [START_REF] Bobo | From forest to farmland: Species richness patterns of trees and understorey plants along a gradient of forest conversion in Southwestern Cameroon[END_REF][START_REF] Morel | Combining Contemporary and Paleoecological Perspectives for Estimating Published in[END_REF].

Towards effective fire management

We find a complex relationship between anthropogenic fires and plant diversity in the wet evergreen forests of the Western Ghats (Figs. 56): a significant impact on the diversity of woody and herbaceous taxa seems to be associated with intensified canopy openings through continued, extensive use of fires. Importantly, the relationship between fires and the diversity of woody taxa (Fig. 5) provides a new perspective to look at slashand-burn agriculture, which is a localised, short-term burning practice. In the Indian Subcontinent, it is practiced as a fertilising process by burning of crop residues and returning some nutrients back to soil before the onset of the pre-monsoon rainfall during the months of April and May [START_REF] Sukumar | Mudumalai forest dynamics plot, India[END_REF]. While fire is restricted to agricultural lands, the probability of spread of fire from agricultural lands into forest lands Published in Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 24 increases with the fragmentation of the landscape and dryness of fuels [START_REF] Kodandapani | Conservation threat of increasing fire frequencies in the Western Ghats, India[END_REF]. This often leads to conflicts between rural, indigenous communities and the Indian Forestry Departments (IFDs), resulting into incendiarism, rapid fragmentation of landscape and loss of forests that IFDs hope to avoid [START_REF] Singh | Exploring biodiversity and climate change benefits of community-based forest management[END_REF][START_REF] Thekaekara | Notes from the Other Side of a Forest Fire[END_REF]. Bringing past analogues of indigenous people's practices to curb fires and subsequent recovery of the Western Ghats rainforest and plant diversity, we demonstrate that the limited, planned use of fires on agricultural land on localised scales may not leave a negative impact on plant diversity including canopy-forming woody taxa (see the discussion on sacred forest groves in Section 4.2). Since agroforestry landscapes hold promise of connecting nature reserves by increasing tree cover in the landscape [START_REF] Bhagwat | Agroforestry: a refuge for tropical biodiversity?[END_REF], we propose that low intensity biomass burning on agricultural lands would be a more practical solution, accommodating both traditional practice and avoidance of fires into forest reserves in the Western Ghats. These could be considered as "prescribed burns" on agroforestry landscapes, an evidencebased land management for collective engagement of two key actors in executing National Agroforestry Policy. We argue that such measures will sincerely help facilitate the effective implementation of this economically and ecologically important policy and its desired positive social-ecological outcomes.

Conclusions

The synergistic impacts of anthropogenic fires and enhanced aridity on plant diversity in one of world's ancient social-ecological systems point out the complex relationship between plant diversity and fire activity.

While there are examples of reduced plant diversity because of the combine effects of fire and enhanced aridity, our work also brings positive narratives where the limited, localised use of fires could have promoted the revival of plant diversity in the Western Ghats. Despite apparent changes in fires, aridity, and land use, the present-day plant diversity in the Western Ghats agroforestry landscape falls within the range of historical variability. Interestingly, the diversity of the canopy-forming woody taxa remained almost constant over the course of past 4000 years, indicating the contribution of forest-dwelling herbaceous understorey in plant diversity in this tropical landscape. Furthermore, in accordance with other tropical records, our data demonstrate that the canopy cover is the crucial determinant of sustaining plant diversity in a tropical Published in Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 25 landscape. Thus, the tree-covered matrix, even if fragmented, is one key landscape feature that needs to be conserved because of its role in providing refugia for important elements of tropical biodiversity (Bhagwat et al., 2012). Therefore, biodiversity conservation in the tropics needs to go beyond reserves; strictly protected areas might be inadequate to fulfil their purpose without conservation management in the landscapes surrounding them [START_REF] Karanth | Conservation and management in human-dominated landscapes: Case Published in[END_REF]. Under mounting anthropogenic pressures, tree management in the tropical social-ecological systems calls for pragmatic solutions that support both ecological and social components of the landscape. Through the use of palaeoecological data from the Western Ghats, we show that people can play an active role in forest conservation and in sustaining plant diversity through reduced biomass burning and intentional woodland management. Thus, we argue that for the success of ecological management in human-influenced tropical regions, it is important to recognise that people are part of the landscape.

Conservation-restoration efforts for sustaining biodiversity in the face of future monsoon variability can succeed only if they are planned in tandem with careful, evidence-based incorporation of traditional land management practices. Our work demonstrates that ancient tropical agroforestry systems form a good practice example of such collaborative efforts and have the potential to benefit both biodiversity and people.

Figure

  Figure 2: The abundances (%) of canopy-forming woody taxa, grassland, and herbaceous understorey are juxtaposed with those of

  of Environmental Management; https://doi.org/10.1016/j.jenvman.2021.111957

Figure 3 :

 3 Figure 3: Temporal transformation of the Western Ghats agroforestry landscape over the past 4000 years:

  Published in Journal of Environmental Management; https://doi.org/10.1016/j.jenvman.2021.111957 13 Yellow bars highlight the intervals of increased aridity. Pink and blue bars indicate the Medieval Warm Period (MWP) and the Little Ice Age (LIA). The top panel shows the summary of land use history of the Kodagu region through notable ecological, social and political changes (based on Rice, 1878; Chandran, 1997; Asouti

  ). Starting from intermediate values (N0 = 35-37), the pollen richness consistently declines reaching its minima (N0 = 24) c. 2250 yr BP and the evenness shows modest fluctuations between 0.08 and 0.1. During 2250-1450 yr BP, richness recovers to a stable state (N0 = 35-42 except 25 c. 1550 yr BP) whereas the evenness decreases until 1550 yr BP before increasing to 0.1. A perpetual rise is visible in both richness and evenness during 1450 and 1050 yr BP; both reach the highest values (N0 = 58 and N2/N0 = 0.13) around 700 yr BP. While richness remains high until 450 yr BP, the evenness starts to drop. Subsequently, both indices show an overall declining trend. The present-day pollen richness value (N0= 41) is slightly lower than its immediate no-fire counterpart in the Historic Period but

Figure 4 :

 4 Figure 4: The pollen richness (N0 = Total number of taxa present; N1 = Total number of common taxa present;

Figure 6 :

 6 Figure6: Cross-correlation analysis for the Western Ghats agroforestry sequence calculated at 260-yr intervals: (a) fire activity (macro-charcoal influx; cm -2 yr -1 ) and plant diversity (total number of terrestrial taxa

  Published inJournal of Environmental Management; https://doi.org/10.1016/j.jenvman.2021.111957

	14

Acknowledgments

Our work is part of 'EARNEST: Examining the Agroforestry Landscape Resilience in India to inform Social-Ecological Sustainability in the Tropics', a project funded by the European Union Horizon 2020 research and innovation programme through a Marie Skłodowska-Curie Actions Individual Fellowship awarded to Charuta Kulkarni, under the grant agreement no. 795557. We thank T. Brncic for help with coring, C.G. Kushalappa, and the College of Forestry for field assistance. We are grateful to R. Premathilake, K. Anupama, S. Prasad, and the Palynology Laboratory at the French Institute, Pondicherry, for their help with pollen identification.

We thank D. Sinclair, Oxford Long Term Ecology Laboratory for her help in sub-sampling of the core, and S. Subitani and I. Figueiral from ISEM, Montpellier for their help in macroscopic charcoal processing and identification. Special thanks to P. Rodríguez-Zorro, R. Cassino, S. Yoshi Maezumi for interesting outlooks Published in Journal of Environmental Management; https://doi.org/10.1016/j.jenvman.2021.111957

funded through Marie Skłodowska-Curie Actions Individual Fellowship under the European Commission's Horizon 2020 Programme (Grant number 795557).

Supplementary Material

Western Ghats Agroforest-1 sedimentary sequence

Core retrieval and lithological description

Using a GeoCore sediment coring system, two 172-cm long overlapping sediment sequences were retrieved from a small swamp (12° 9' 14"N, 75° 42' 47"E; 910 m asl), situated at the confluence of two perennial streams in a coffee estate (Bhagwat et al., 2012). Both sedimentary sequences are composed of alternating sequences of clay and silty clay varying between yellowish brown and dark brown hues with a sporadic appearance of sand lenses and granules.

Chronology

The chronology of the Agroforest-1 core is based on five radiocarbon dates (Bhagwat et al., 2012) and four 210 Pb dates (Nogué et al., 2018a). Radiocarbon dates were measured at the Oxford Radiocarbon Accelerator Unit and 14 CHRONO Centre at Queens University Belfast and were calibrated using the IntCal13 data set [START_REF] Reimer | IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP[END_REF]. 210 Pb dating was carried out at School of Geography and Environment, University of Oxford. The age-depth model was established using Clam [START_REF] Blaauw | Methods and code for "classical" age-modelling of radiocarbon sequences[END_REF] and can be found as Figure S1(a) in Nogué et al., (2018b). The current study utilises the top 150 cm of Agroforest-1 sequence, which is equivalent to 4000 yr BP.

Pollen and charcoal extraction and analysis

Sediment cores were sub-sampled at 4-cm intervals. Pollen and microscopic charcoal particles were extracted using standard palynological methods [START_REF] Bennett | Pollen[END_REF]. Known quantities of an exotic marker, Lycopodium spores were added to each sample prior to chemical treatment to calculate pollen-charcoal concentration and influxes [START_REF] Stockmarr | Tablets with spores used in absolute pollen analysis[END_REF]. A minimum of 500 terrestrial pollen grains was counted for each depth, except for 76 cm and 140 cm due to slightly poor pollen preservation. As many as 225 pollen taxa were identified and classified into arboreal pollen types (e.g., trees, shrubs, lianas, climbers), grassland taxa Published in Journal of Environmental Management; https://doi.org/10.1016/j.jenvman.2021.111957 40 (Poaceae and Cyperaceae), and herbaceous taxa inclusive of all other non-arboreal pollen types. The percentage abundances of the above groups were interpreted as the extents of the canopy cover, grassland, and herbaceous understorey in this study. In Figure 2, the pollen and other datasets are plotted using TILIA Graph [START_REF] Grimm | Tilia and Tilia-Graph Software[END_REF].

Microscopic charcoal pieces were counted on the Agroforest-1 pollen slides (Bhagwat unpublished data) using a point-counting method [START_REF] Clark | Point count estimation of charcoal in pollen preparations and thin sections of sediments[END_REF]. For macroscopic charcoal (<150 µm), this study extends the existing Agroforest-1 macroscopic-charcoal record (Nogué et al., 2018b) that covered only the past 1000 years, to 4000 years. Thus, the core was sampled at 1-cm intervals (~28 years sample -1 on average) for the top 150 cm. Following [START_REF] Finsinger | A guide to screening charcoal peaks in macrocharcoal-area records for fire-episode reconstructions[END_REF], 1 cm 3 of sediments were deflocculated and bleached overnight with a 10% sodium hexametaphosphate + 5% sodium hypochloride mixture followed by wet sieving through a 150 µm mesh. The samples were then suspended in 15% hydrogen peroxide for 15-20 minutes followed by wet sieving. Macroscopic charcoal pieces in each sample were manually counted under a binocular microscope. To ensure the consistency in data collection, we re-analysed the macro-charcoal data from 0-56 cm (0-1197 yr BP) utilised in Nogué et al., (2018b).

Supplementary Figure 1: Variation in the sample resolution across the Agroforest-1 sequence. For the entire timescale under study (last 4000 years; 4500-15 cal yr BP), the sample resolution ranges between 22 and 671 years with mean=121 years, median=52 years, and 3rd quartile=120 years whereas for the higher resolution section (2200-15 cal yr BP) where the sampling resolution is less variable (range=22-198 years; mean=65 years; median=49 years; 3rd quartile=120 years). For calculating temporal β-diversity, we purposely select two interpolation-window widths of 260 years (for the entire 4000-yr period) and 150 years (for the last 2200 yr) that are greater than the third quartile of the sampling-resolution distributions, thereby avoiding sampling resolution inflection. We also use the 260-yr window for performing cross-correlation analysis for the entire timeline of the study. Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957 Supplementary Figure 2: Regression plot shows the high strength of association (R² = 0.78) between the canopy cover (Arboreal Pollen %) and the overall plant diversity (total number of terrestrial taxa present (N0)) in the Western Ghats agroforestry landscape.

Published in

Published in Journal of Environmental Management;https://doi.org/10.1016/j.jenvman.2021.111957


Marc Devillard

Carlos Antonio

Albuquerque Pinheiro

Elsa Caytan

Claire Roiland

Iker Chiara Dinoi

Iker Del

Gilles Alcaraz
email: gilles.alcaraz@univ-rennes1.fr

Carlos Antonio De

Iker Del Rosal

Del Rosal

Uncatalyzed Formation of Polyaminoboranes from Diisopropylaminoborane and Primary Amines: a Kinetically Controlled Polymerization Reaction

Keywords: aminoboranes, BN compounds, boron, polymerization, Inorganic polymers Scheme 1. Strategies toward polyaminoboranes synthesis. 10

come   L'archive ouverte pluridisciplinaire

Introduction

Inorganic polymers and particularly polymers incorporating boron [1] represent a fascinating and active research area that has shown promise for a wide range of applications. Among them, polyaminoboranes (RHN-BH2)n (PABs) have attracted considerable attention over the past decade, strongly benefiting from the numerous studies on metal-catalysed dehydropolymerization of amineboranes with concomitant production of dihydrogen. [2] With a Bsp3Nsp3 backbone, polyaminoboranes are often presented as the inorganic analogues of the well-known polyolefins and show great potential as piezoelectric materials [3] or as preceramic polymers for advanced BN materials. [4] To date, this family of BN polymers remains relatively underdeveloped. While catalytic routes should offer interesting prospects for the efficient and tailored synthesis of polyaminoboranes (Scheme 1A), the polymerization process predominantly depends on the catalyst system. [2a-l] In terms of functionality, limitations are imposed by the compatibility between the catalyst, mostly based on late transition metals, and the nitrogen functional group of the amine-borane substrate. The scope is generally restricted to N-alkyl substituents although improvements have been very recently achieved by Manners and co-workers with early transition metals complexes based on titanocene catalysts. [5] Recently, metal-free routes have appeared as a promising alternative (Scheme 1B), [6] providing a simple and straight-forward access to processable polyaminoboranes with high molecular masses.

2

In this framework, we showed [6b] for the first time that a) polyaminoboranes could be efficiently produced with neither dihydrogen evolution nor salt formation by simple reaction of diisopropylaminoborane i Pr2N-BH2 (DIAB) with a primary amine RNH2, b) , the reaction tolerates olefinic functional groups enabling the design of polyaminoboranes containing N-unsaturated sidechains, as shown by the isolation of polyallylaminoborane, and c) the reaction conditions and particularly the temperature plays a crucial role on the reaction outcome. This last statement is intimately linked to the reaction mechanism and, as in metal-catalysed routes, the generation and selfassociation of monomeric aminoborane units [RNH-BH2], which is at the heart of the polymerization process, deserves a specific attention.

Following our recent results on this metal-free polymerization reaction, we have undertaken a more detailed study on the polymer characterization and on the reaction mechanism mostly based on the products previously observed and characterized.

Results and Discussion

We have recently reported a general procedure for the synthesis of polyaminoboranes (PABs) consisting in the addition of a primary amine to diisopropylaminoborane (DIAB) neat, at -40°C. Performing the same reaction at room temperature led to a different reaction outcome with the formation of a mixture of boron containing species II-RVI-R that have been unambiguously identified by multinuclear NMR spectroscopy when involving the n-butylamine species (R = Bu) (Scheme 2).

Scheme 2. Reaction of DIAB with primary amines at -40°C and at room temperature.

It is important to note that the reaction can be also conducted in solution, at low temperature, affording the corresponding PABs in similar yields to those obtained by the same reactions performed under neat conditions. [START_REF]solvent such as dichloromethane does not alter the course of the reaction but helps to maintain a correct stirring when gelation of the solution occurs under neat conditions[END_REF] Because of the extremely poor solubility in common organic solvents of prototypical PABs I-H and I-Me with which our study had begun, we erroneously considered that all PABs prepared thereafter would display a low solubility. This led us to spectroscopically characterize them by solid state [START_REF] Stubbs | [END_REF] C{ 1 H} and in-solution 11 B{ 1 H} NMR [6b] at first, and later on by 11 B{ 1 H} solid state NMR (SI-Figure S0). Further analyses were conducted and except for I-H and I-Me, in-solution 1 H, [START_REF] Stubbs | [END_REF] C{ 1 H} and 1 H- [START_REF] Stubbs | [END_REF] C HSQC spectra could be recorded for the prepared PABs (SI-Figure S1-S21). Well-resolved spectra were obtained and PABs I-R (R = Et, Pr, Bu, Et/Bu (1/1), Et/Pr (1/2), Allyl) were fully characterized. In the 1 H NMR, they all exhibit a broad singlet at  2.5-3.0 ppm for the NH group (SI-Figure S1, S6, S10, S14, S17 and S18) in agreement with the NMR data reported so far for this family of polymer. [2k] In the case of I-Allyl, the integrity of the allylaminoborane repeating unit was ascertained by selective 1D 1 H-TOCSY experiments showing the correlation between vicinal hydrogens and even beyond for the CH protons (SI-Figure S22). More surprising is the signature of the alpha-and betamethylene groups of the N-substituent for the I-Et, I-Pr and I-Bu polymers (Figure 1 -See also SI-Figure S1, S6 and S10). In the 1 H NMR spectra, each of these CH2 groups resonates as a set of two broad signals separated by 45 to 75 Hz, presumably due to the non-equivalence of the two methylenic protons [START_REF] Manners | presence of the alkyl chains used that render the 1 H spectra interest area less easily analysable[END_REF] (Table 1). Their connectivity to the same carbon atom has been unambiguously ascertained by the 2D 1 H- [START_REF] Stubbs | [END_REF] C HSQC and 1 H-1 H COSY NMR experiments (SI-Figure S4, S5, S8, S9, S12 and S13). The magnetic nonequivalence of these geminal protons can only be detected in the close vicinity (-and -methylenes) of the nitrogen and vanishes for the -methylene (I-Bu). This feature could be linked to the rigidity of the Nalkyl substituent that would get more important close to the B-N backbone thus hindering the rotation around the -CH2-CH2bonds. Related to this, the 1 H-1 H COSY NMR spectrum displays distinct correlations between pairs of vicinal methylenic protons within the alkyl chain due to the well-known dependence of the 3 JHH coupling constant on the HCCH dihedral angle. [START_REF] Fraser | [END_REF] The case of the BH2 group that is easily observable in the case of I-Et and I-Allyl is somewhat different. The BH2 gives a broad resonance signal integrating for 2H in the region  1. For I-Allyl, they are separated by 37 Hz at  1.83 and 1.73 ppm, respectively. In the case of I-Et, the peak separation is more pronounced with two overlapping broad signals separated by 88 Hz at  1.84 and 1.62 ppm, respectively. On this basis, the splitting pattern observed upon boron decoupling can't be rationalized by involving a scalar coupling of the BH2 hydrogens with the neighbouring NH protons. Indeed, the peak separation vastly exceeds the expected value for vicinal couplings, as measured for the MeNH2.BH3 and Me2NH.BH3 compounds that display similar Nsp3H-Bsp3H linkage ( 3 JBH-NH = 4.2 and 3.3 Hz), respectively (see SI-Figure S23-S25). Concerning the possible involvement of geminal couplings within the H-B-H fragment, expected at ca. 10 Hz, [10] the broadness of the observed signals precludes any accurate interpretation to validate this possibility. We envisioned that the tacticity of the PABs could be responsible for this peculiar BH2 NMR signature by analogy with the well resolved signals of methylene groups in previously described carbon-based polymers such as polymethylmethacrylate (PMMA) or polybutylmethacrylate (PBMA). [11] For an ideal isotactic structure, the geminal CH2 protons are chemically inequivalent resulting in a AB system whereas they are isochronous in the welldefined syndiotactic structure, due to the local symmetry of the -CH2unit. For the atactic polymer where the two types of linkage are present, the 1 H NMR spectrum displays both patterns (Figure 3). [11a] Practically, the extensive overlap of many stereosequences [START_REF] Mirau | NMR characterisation of polymers[END_REF] results in broadened signals for the methylenes moieties of many polymers. In order to get more insight in the structure of these PAB polymers and in the splitting patterns observed for the BH2 groups both in I-Allyl and I-Et, the synthesis of two new PABs that should display easy to observe BH2 groups was undertaken. The I-Benz and I-TMSM polymers were analogously prepared in good yields from benzylamine and (trimethylsilyl)-methylamine, respectively. They display a N-CH2-Ph and a N-CH2-SiMe3 chain that we thought NMR-silent in the region  1.5 to 2.0 ppm allowing a better characterization of the BH2 resonance signal in the corresponding PAB polymer.

Accepted Manuscript

Although perfectly characterised by multinuclear solid state NMR (SI-Figure S26-S27), their poor solubility prevented the recording of any significant spectrum precluding their characterization in solution. Further experimental evidence is still needed to definitively support the relationship between NMR signature and tacticity in the case of PABs. The results reported here emphasize the importance of the presence of alkyl chains for a good solubility in organic solvents. This particular point is illustrated with the additional syntheses of I-Myrt and I-Hex (scheme 3) performed in solution from (-)-cismyrtanylamine and hexylamine, respectively. I-Myrt and I-Hex have been characterized by multinuclear in-solution NMR (SI-Figures S28-S37). To go one step further, studies concerning the formation of PABs by amine-transfer reaction were undertaken. In order to validate the hypothesis of the transient formation of the amine-DIAB adduct followed by the elimination of diisopropylamine and the formation of the corresponding alkylaminoborane (Scheme 4), the stoichiometric reaction of DIAB with n-butylamine was performed in d8-THF at 205K and monitored by multinuclear NMR spectroscopy.

The 11 B NMR spectrum shows the immediate and complete disappearance of DIAB at 37 ppm and the presence of a broad signal at  -7 ppm in agreement with the presence of I-Bu. Traces of a compound at  -19.6 ppm consistent with traces of an amine-borane adduct are also observed (SI-Figure S38). In the 1 H spectrum, I-Bu, recognisable by its peculiar spectroscopic signature, and diisopropylamine were both clearly identified as the major components of the reaction medium (SI-Figure S39). In the 13 C{ 1 H} and 1 H- [START_REF] Stubbs | [END_REF] C HSQC NMR spectra, traces of unreacted nbutylamine and IV could be identified by comparison of the data with authentic samples (SI-Figure S40-S41), thus confirming the presence of an amineborane species previously detected by 11 B NMR spectroscopy. The multinuclear NMR experiments performed at 205K clearly showed that the formation of I-Bu takes place at very low temperature and contrarily to NHC-stabilized aminoboranes, [START_REF] Stubbs | [END_REF] neither the n-butylamine-DIAB adduct nor the monomeric butylaminoborane could be detected at this temperature.

Scheme 4. Suggested mechanism involving DIAB-amine adduct and alkylaminoborane intermediates.

In order to better understand the role of temperature and unveil the mechanism involved in the reaction, a DFT study was carried out at the DFT-B3PW91 level of theory. In accordance with the experimental results at RT, the computed reaction of n-butylamine with DIAB, chosen as a model reaction, results mainly in a mixture of Bsp2H-, Bsp3H2-and Bsp3H3-containing species (III-BuVI-Bu) with the cyclotriborazane compound (II-Bu) being the major product (scheme 2). By changing the temperature, on the other hand, the outcome of the reaction is completely modified since a high mass polybutylaminoborane I-Bu is formed as experimentally observed at 40°C and below. [6b] The computed enthalpy profiles are shown in figures 4-6. [14] Starting from the n-butylamine (BuNH2) and DIAB ( i Pr2NBH2), the first step involves the formation of the Lewis acid-base adduct B (BuNH2BH2-N i Pr2) which was considered in analogy with the case previously reported for the NHCs. [START_REF] Stubbs | [END_REF] Formation of adduct B takes place with almost no energy barrier in a slightly exothermic process (-2.2 kcal.mol -1 , Figure 4). From B, the formation of the corresponding Lewis adduct C (BuNH-BH2HN i Pr2) occurs through transition state TS-BC, displaying an activation barrier of 13.7 kcal.mol -1 . It involves an intramolecular N-to-N hydrogen shift from the nbutylamine moiety to the diisopropylamino group with C located at -1.4 kcal.mol -1 . The dissociation of adduct C finalizing the amine-transfer reaction by the release of the diisopropylamine and the BuNH-BH2 intermediate may then take place with almost no energy barrier via a weakly exothermic process (D, -4.8 kcal.mol -1 , Figure 4). As shown in Figure 4, an alternative hydrogen transfer route involving a bimolecular process from two molecules of B (Bu-NH2- i Pr2) in C1 may also occur. After the release of the n-butylamine, the proton/hydride transfer operates through a sixmembered transition state TS-C1D1 with an energy barrier of 16.1 kcal.mol -1 with respect to C1. Interestingly, such hydrogen transfer between amineboranes and aminoboranes has been previously reported by Manners and co-workers. [15] This reaction leading to D1 is exothermic by -13.5 kcal.mol -1 and results in the formation of the corresponding diaminoborane BuNH-B(H)-N i Pr2 (III-Bu) and i Pr2NHBH3 (IV) compounds. When considering these two routes, it is important to note that the intermolecular process via TS-C1D1 is higher by 2.4 kcal.mol -1 than that of the intramolecular process through TS-BC. While both processes are accessible at room temperature and can operate simultaneously, the intramolecular process is less energy demanding and should be preferential at low temperature. Starting from D, three different scenarios that could rationalise the experimental results can be envisaged (Figure 5). The first route is the trimerization of nbutylaminoborane into the cyclotriborazane species II-Bu through TS-E1F1. This route is highly exothermic (F1, -44.7 kcal.mol -1 ) and displays a high activation barrier (21.4 kcal.mol -1 with respect to E1). The second route involves a hydrogen transfer reaction between adduct B and the nbutylaminoborane (BuNH-BH2) compound in D. This intermolecular process passes through TS-E2F2 leading to the formation of III-Bu and VI-Bu in F2 (-22.0 kcal.mol -1 ). The reaction is therefore strongly exothermic (-15 kcal.mol -1 with respect to E2) only requiring a moderate activation barrier of 5.8 kcal.mol -1 . The third pathway consists in the formation of the Lewis acid-base adduct F (Bu-NH2-u) from the n-butylaminoborane intermediate (BuNH-BH2) from D and the nbutylamine. The process passes through TS-EF and results in a moderately exothermic reaction (-9.9 kcal.mol -1 ). It exhibits a low activation barrier (4.4 kcal.mol -1 with respect to E) which is even lower by 1.4 kcal.mol -1 than that associated to TS-E2F2. It is important to note that adduct F is thermodynamically and kinetically more stable than its analogue C (Figure 4). This can probably result from both the greater steric hindrance of diisopropylamine in C and the greater basicity of n-butylamine [16] in F, respectively weakening and strengthening the B-N bond in the corresponding adduct. Among these three routes, two of them are more particularly temperature dependent. Although the trimerisation barrier can be easily overcome at room temperature, to afford the cyclotriborazane compound II-Bu as the thermodynamic product of the reaction, the formation of the Lewis adduct F from n-butylamine and nbutylaminoborane (BuNH-BH2) remains kinetically favoured at lower temperatures. In addition, the slower formation of the n-butylaminoborane (D), at low temperature, probably makes the n-butylamine more widely available to react with the nbutylaminoborane intermediate (BuNH-BH2), as soon as it is formed, to give adduct F. The follow up of the reaction from species F is shown in Figure 6. Once formed, adduct F can evolve along two distinct reactions, providing the products experimentally observed in the reaction medium. One possible pathway involves a proton/hydride transfer between 10.1002/adsc.202001458 F and B through an energy demanding six-membered transition state (TS-G1H1, at 12.1 kcal.mol -1 with respect to G1), leading to the bis(butylamino)borane V-Bu and diisopropylamine-borane IV compounds at -18.0 kcal.mol -1 (H1). In the second possible pathway, on the other hand, the reaction between F and the nbutylaminoborane intermediate (BuNH-BH2) evolves to two thermodynamically favourable issues involving either i) the formation of bis(butylamino)borane V-R along with n-butylamineborane IV (H2 at -27.8 kcal.mol -1 ) via a sixmembered proton/hydride transfer transition state TS-GH2 or ii) the formation of the linear dimer (I-Bu)dimer (H at -23.8 kcal.mol -1 ) through transition state TS-GH. Interestingly, the activation barrier corresponding to TS-GH2 (5.9 kcal.mol-1) is higher by 2.5 kcal.mol -1 than that of TS-GH (3.4 kcal.mol -1 ) indicating that at low temperature, under kinetic control, the polymerization reaction is the predominant process. The dimer (I-Bu)dimer (H) may in turn react with one equivalent of nbutylaminoborane (BuNH-BH2) to provide the corresponding trimer (I-Bu)trimer (L, -34.8 kcal.mol -1 ) through an accessible activation barrier of 4.3 kcal.mol -1 (TS-IL). Once initiated, the polymerization process can carry on, the lack of solubility of the growing polymer eventually acting as an additional driving force. From a global point of view, the profiles in Figures 456indicate that the cyclotriborazane II-Bu and polybutylaminoborane I-Bu compounds represent the thermodynamic and kinetic products, respectively. While the formation of II-Bu is a highly exothermic process (-44.7 kcal.mol -1 ), passing through a high energy-demanding transition state (TS-E1F1, 21.5 kcal.mol -1 ), the formation of the corresponding linear trimer (I-Bu)trimer in L displays a smaller exothermicity (-34.8 kcal.mol -1 ) involving much lower barrier steps. The highest activation barrier for the whole polymerization process from the starting DIAB and n-butylamine (A) compounds to the product L is indeed of 13.7 kcal.mol -1 and corresponds to the intramolecular N-to-N hydrogen shift in TS-BC.

Accepted Manuscript

In accordance with the experimental results, while at room temperature the cycloborazane compound is the major isolated product together with smaller amounts of III-Bu, IV, V-Bu and VI-Bu, at low temperature the polybutylaminoborane I-Bu polymer is selectively formed.

Conclusion

We recently disclosed a solventless uncatalyzed synthesis of PABs (I) from diisopropylaminoborane ( i Pr2N-BH2) and primary amines (RNH2) and evidenced that temperature plays a determining role on the outcome of the reaction: while PABs I are produced in good yield at low temperature, at room temperature the same experiment leads to cyclotriborazane II as the main compound with diaminoboranes (III and V) and amine-boranes (IV and VI) to a lesser extent. Interestingly, we described here a complete solution NMR study of the previously synthesized PABs. Despite initial solubility issues, we realized that the PABs polymers and copolymers, produced from primary nalkylamines with hydrocarbon chains ranging beyond Accepted Manuscript solution NMR studies. Multinuclear NMR studies revealed that the protons of the alpha-and beta-methylene groups of the alkyl moiety in N-substituted PABs are magnetically nonequivalent. This can be rationalized by the hindered rotation around the -C α H2-C β H2-bonds of the alkyl chains, thus resulting in preferential conformations of these alkyl chains within the polymer. Additionally, the BH2 groups present a peculiar spectroscopic signature that clearly raises the question of the tacticity in PABs. In order to tackle this point, still unknown for the best of our knowledge, we prepared model-PABs from benzylamine and (trimethylsilyl)methylamine with the hope to obtain a more readable 1 H NMR spectrum.

Here again we faced with solubility issues, preventing the proper characterization of these PABs polymers in solution. The question of tacticity in PABs remains therefore open and investigations in this context are currently underway in our laboratory.

Theoretical calculations were carried out at the DFT level of theory, enabling the comprehension of the role of the temperature in the PABs synthetic process and the rationalization of the formation of the species experimentally observed at room temperature. The mechanism we propose from this in-depth theoretical study reveals that the formation of PABS is a kinetically controlled process starting, as previously postulated, by the initial formation of an amine-DIAB Lewis acid-base adduct which affords the n-alkylaminoborane (RNH-BH2) key intermediate via an intramolecular N-to-N proton exchange. The resulting polymerization process involves the successive formation of Lewis acid-base adducts. Each incoming key n-alkylaminoborane monomer generates a new dative B-N bond between its BH2 group and the amino moiety of the chain-end nalkylaminoborane group of the growing polymer, ensuring the propagation of the polymeric chain. At a more elevated temperature, the trimerizaton of the nalkylaminoborane into cyclotriborazane is the thermodynamically favored process. According to the computational study, the experimentally observed formation of amine-boranes and diaminoboranes mainly results from the bimolecular NH proton/BH hydride transfer through 6-membered transitions states.

Experimental Section

PABs I-H, I-Me, I-Et, I-Pr, I-Bu, I-Allyl, I-Et/Bu (1/1), and I-Et/Pr (1/2) were prepared as previously reported. [6b] Synthesis of I-Benz: To a -40 °C solution of N,Ndiisopropylaminoborane (2.17 mL, 1.52 g, 13.4 mmol) in dichloromethane (10 mL) was added a -40 °C cooled solution of benzylamine (1.50 mL, 1.44 g, 13.4 mmol, 1 eq.) in dichloromethane (10 mL) dropwise over 15 minutes and the resulting solution was allowed to warm up slowly to 0 °C in the cold bath. At this stage the cold bath was removed and the reaction mixture allowed warming up to room temperature. Volatiles were removed under vacuum giving a transparent residue that was then washed with acetonitrile (4 times 30 mL). The resulting white powder was then dried under vacuum (1.2 g, 75 %). I-Benz was characterized by solid-state 11 B{ 1 H} MAS NMR spectroscopy (see SI-Figure S26). iso ( 11 B) = -7.7 ppm  CQ = 1.3 Hz -Q = 0.9.

Synthesis of I-TMSM:

To a -40 °C solution of N,Ndiisopropylaminoborane (1.67 mL, 1.17 g, 10.4 mmol) in dichloromethane (10 mL) was added a -40 °C cooled solution of (aminomethyl)trimethylsilane (1.50 mL, 1.07 g, 10.4 mmol, 1 eq.) in dichloromethane (10 mL) dropwise over 15 minutes and the resulting solution was allowed to warm up slowly to 0 °C in the cold bath. At this stage the cold bath was removed and the reaction mixture allowed warming up to room temperature. Volatiles were removed under vacuum giving a transparent residue that was suspended in acetonitrile (30 mL) and transferred on a filter frit. It was then washed 3 more times with acetonitrile giving a white powder that was dried under vacuum (0.55 g, 46 %). I-TMSM was characterized by solid-state 11 B{ 1 H} and 29 Si MAS NMR spectroscopy (see SI-Figure S26-27).iso ( 11 B) = -6.25 ppm  CQ = 0.6 Hz -Q = 0.95.  ( 29 Si) = 45.2 ppm.

Synthesis of I-Myrt:

A -40 °C cooled solution of (-)-cismyrtanylamine (3.12 mL, 2.85 g, 18.6 mmol, 1 eq.) in dichloromethane (10 mL) was added dropwise over 15 minutes to a -40 °C cooled solution of N,Ndiisopropylaminoborane (3.0 mL, 2.1 g, 18.6 mmol) in dichloromethane (10 mL) and the resulting solution was allowed to slowly warm up to room temperature in the cold bath over 10 hours giving a white precipitate and a colourless solution. Volatiles were removed under vacuum giving a transparent residue that was then washed with acetonitrile (3 times 20 mL) and methanol (3 times 20 mL). The resulting white powder was then dried under vacuum (70 %). I-Myrt was characterized by solution NMR spectroscopy (see SI Figures S28-33). 1 H NMR (CDCl3, 500 MHz):  = 0.86-0.99 (m, 1H, CHH), 1.02 (m, 3H, CH3), 1.22 (m, 3H, CH3), 1.31-1.52 (m, 1H, CHH), 1.66-2.19 (m, 7H, 3CH + CH2+BH2), 2.34-2.47 (br., 1H, CHH), 2.47-2.70 (br., 3H, N-CH2+CHH), 2.75 (br., 1H, NH); 11 B{ 1 H} (CDCl3, 128 MHz):  = -5.2 (br.); 13 C{ 1 H} NMR (CDCl3, 126 MHz)  = 19. CH2),CH3),CH2),CH3),CH2),37.6 (s,CH),C IV ),CH),CH),.

Synthesis of I-Hex: A -40 °C cooled solution of nhexylamine (2.77 mL, 2.122 g, 20.97 mmol, 1 eq.) in dichloromethane (10 mL) was added dropwise over 15 minutes to a -40 °C cooled solution of N,Ndiisopropylaminoborane (3 mL, 2.37 g, 20.97 mmol) in dichloromethane (10 mL) and the resulting transparent colourless solution was allowed to slowly warm up to 0°C. The cold bath was removed and once the solution at room temperature, the volatiles were removed under vacuum leading to a white gummy solid that was then washed with acetonitrile (3x20 mL) and methanol (3x20 mL). The resulting white gummy powder was then dried under vacuum and I-hex was isolated in 74 % yield (1.76 g). (see SI Figures S34-S37). 1 H NMR (CDCl3, 400.16 MHz):  = 0.88 (br s, 3H, CH3), 1.29 (br. s, 6H, CH3-(CH2)3-), 1.60 and 1.77 (br., 2H, NCH2-CH2), 2.36 and 2.50 (br., 2H, NCH2-CH2), 2.68 (br, 1H, NH); 11 B{ 1 H} (CDCl3, 128.38 MHz):  = -6.8 (br.); 13 C{ 1 H} NMR (CDCl3, 120.62 MHz)  = 14.14 (s, CH3), 22.74 (s, CH3-CH2), 27.33 (s, CH3-CH2-CH2-CH2), 28.05 (s, NCH2-CH2), 31.81 (s, CH3-CH2-CH2), 51.14 (s, NCH2).

Figure 1 .

 1 Figure 1. Enlarged 1 H NMR spectra of I-Et, I-Pr and I-Bu.

  5 to 2.0 ppm. Upon 11 B decoupling, this signal sharpens into two overlapping broad signals (Figure 2-See also SI-Figure S2 and S19).

Figure 2 .

 2 Figure 2. 1 H{ 11 B} NMR spectra of I-Et (bottom) and I-Allyl (top).

Figure 3 .

 3 Figure 3. Illustration of the tacticity in PMMA, PBMA and PAB.

Scheme 3 .

 3 Scheme 3. Synthesis of I-Benz, I-TMSM, I-Myrt and I-Hex in dichloromethane at low temperature and influence of the N-substituent on the solubility of the PABs.

Figure 4 .

 4 Figure 4. Enthalpy profiles of the reaction of n-butylamine and DIAB. Formation of the BuNH-BH2 and i Pr2NH compounds (D) (black) together with III-Bu and IV (D1) (red).

Figure 5 .

 5 Figure 5. Enthalpy profile for the evolution of the BuNH-BH2 species in D in the course of the reaction. Formation of adduct F (black) together with II-Bu (green), and III-Bu and VI-Bu (red).

Figure 6 .

 6 Figure 6. Enthalpy-profile for the evolution of F in the course of the reaction. Formation of the linear I-Budimer (H) and I-Butrimer (L) compounds (black) together with compounds IV, V-Bu and VI-Bu (green and red).

CH 3 CH 2 CH 2 CH 2 NH CH 2 CH 2 NH CH 2 + NH BH 2 CH 3

				10.1002/adsc.202001458
	(1)	(1) (1)	+ BH 2 BH 2 (2) (2)	(3)	CH 3	Accepted Manuscript

Table 1 .

 1 Chemical shifts of the protons of the methylene groups in I

-Et, I-Pr and I-Bu.

		 (ppm) N-CH2	(ppm) N-CH2-CH2	(ppm) N-CH2-CH2-CH2
	I-Et	2.50 and 2.63	-	-
	I-Pr	2.36 and 2.48	1.62 and 1.78	-
	I-Bu	2.37 and 2.52	1.57 and 1.76	1.31

carbon atoms, are indeed soluble enough for in-

Acknowledgements

We thank the C.N.R.S. and the University of Rennes 1. C.A.D.A.P. thanks the Brazilian Ministry of Education and the CAPES for his fellowship. G.A. and M.D. thank the Region Bretagne for its financial support (SAD-2 n°17048). This work was performed using HPC resources from CALMIP (Grant 2017-[p17010]) 10.1002/adsc.202001458

Accepted Manuscript

Advanced Synthesis & Catalysis