Pieter De Frenne
email: pieter.defrenne@ugent.be@tel

Jonathan Roger Michel

Henri Lenoir

Miska Luoto

Brett R Scheffers

Florian Zellweger

Juha Aalto

Michael B Ashcroft

Ditte M Christiansen

Guillaume Decocq

Karen De Pauw

Jonathan Lenoir

Sanne Govaert

Caroline Greiser

Eva Gril

Arndt Hampe

Tommaso Jucker

David Klinges

Irena A Koelemeijer

Jonas J Lembrechts

Ronan Marrec

Camille Meeussen

Jérôme Ogee

Vilna Tyystjärvi

Pieter Vangansbeke

Kristoffer Hylander

Forest microclimates and climate change: Importance, drivers and future research agenda

Keywords: biodiversity, buffering, climate change, ecosystem function, forest, future research, microclimate

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Introduction: the importance of forest microclimates

Forest organisms living below or within tree canopies experience distinct climatic conditions that deviate considerably from the climate outside forests [START_REF] Chen | Microclimate in forest ecosystem and landscape ecology -Variations in local climate can be used to monitor and compare the effects of different management regimes[END_REF][START_REF] Geiger | The climate near the ground[END_REF][START_REF] De Frenne | Global buffering of temperatures under forest canopies[END_REF]). Below forest canopies, direct sunlight and wind speed are strongly reduced, leading to a dampening of temperature and humidity variations. Temperature extremes are often strongly buffered in forests compared to open habitats, with cooler below-canopy maximum temperatures, warmer minimum temperatures, and lower seasonal and inter-annual variability ( The physiological and ecological importance of forest microclimates has long been recognized [START_REF] Grubb | The maintenance of species-richness in plant communities: the importance of the regeneration niche[END_REF][START_REF] Geiger | The climate near the ground[END_REF], a book with a first edition already published in 1927). Due to the increasing biodiversity loss in response to current macroclimatic warming, studies on forest microclimates are receiving much attention in global change biology (Fig. 2). However, most studies on forest biodiversity rely on gridded macroclimate data that are based on free-air temperature data from weather stations in open areas outside forests, thus neglecting forest microclimate variation in space and over time [START_REF] Potter | Microclimatic challenges in global change biology[END_REF]; [START_REF] Barry | Microclimate and local climate[END_REF]De Frenne & Verheyen, 2015). This introduces significant uncertainty into predictions of climate change impacts on forest biodiversity and functioning [START_REF] Zellweger | Forest microclimate dynamics drive plant responses to warming[END_REF]). Forests and their understories harbour the majority of terrestrial biodiversity, and are essential for the provision of many ecosystem services. Thus, addressing the uncertainties is a fundamental task for global change biologists, land managers, and policy makers alike (MEA, 2005; Landuyt et al. 2019; IPBES, 2020). Viewing forest ecology through a microclimate lens can help tease out mechanistic relationships of organisms with their environment. Buffered forest microclimates and the microhabitats within forests (e.g., root caverns, tree holes, fallen trunks) enable organisms to avoid extreme heat and drought [START_REF] Kearney | The potential for behavioral thermoregulation to buffer "coldblooded" animals against climate warming[END_REF]; Scheffers et al. 2013a[START_REF] Scheffers | Asplenium bird's nest ferns in rainforest canopies are climatecontingent refuges for frogs[END_REF]. The microclimate buffering capacity of forests may provide climatic microrefugia during macroclimate warming (Ewers and Banks-Leite, 2013; von Arx et al. 2013, [START_REF] Lenoir | Climatic microrefugia under anthropogenic climate change: implications for species redistribution[END_REF], De Frenne et al. 2019). Therefore, the pressure on individuals, populations, species and communities to respond to rapid anthropogenic climate change may be reduced, at least in the short term, by the presence of climatic microrefugia for cold-adapted organisms (Keppel et [START_REF] Greiser | Hiding from the climate: Characterizing microrefugia for boreal forest understory species[END_REF]. Through these mechanisms, forest microclimates can determine the distribution of individuals, populations, and species. Thus, incorporating microclimates into species distribution models is expected to significantly improve the accuracy of predictions [START_REF] Slavich | Topoclimate versus macroclimate: how does climate mapping methodology affect species distribution models and climate change projections[END_REF]; [START_REF] Lembrechts | Incorporating microclimate into species distribution models[END_REF]Zellweger et al. 2019b). The forest microclimate is also a driver of species interactions. Low light availability and heterogeneous moisture can enhance plant competition [START_REF] Connell | On the prevalence and relative importance of interspecific competition: evidence from field experiments[END_REF][START_REF]Effects of root competition and canopy openness on survival and growth of tree seedlings in a tropical seasonal dry forest[END_REF], although microclimates can also facilitate co-existence, such as when shade offers refuge to mixed-species seedling assemblages [START_REF] Holmgren | The interplay of facilitation and competition in plant communities[END_REF], or when centipedes share epiphytic ferns as cool and moist nest sites [START_REF] Phillips | Bird's nest ferns promote resource sharing by centipedes[END_REF]). In some cases, species interactions can result in a re-engineering of the microclimate environment itself, for example canopy gaps produced by leaf-cutter ant herbivory [START_REF] Swanson | Welcome to the Atta world: A framework for understanding the effects of leaf-cutter ants on ecosystem functions[END_REF]). Microclimate therefore shapes -and in turn, is shaped by -the composition of forest communities [START_REF] Parker | Structure and microclimate of forest canopies[END_REF][START_REF] Woods | Microhabitat associations of vascular epiphytes in a wet tropical forest canopy[END_REF]Frey et al. 2016a[START_REF] Jucker | Canopy structure and topography jointly constrain the microclimate of human-modified tropical landscapes[END_REF]. At the ecosystem level, microclimate is of paramount importance as a key regulator of many ecosystem functions. Rates of litter decomposition, carbon sequestration and microbial activity tend to be greater in forests than in neighbouring open habitats (Riutta et [START_REF] Köchy | Litter decomposition and nitrogen dynamics in aspen forest and mixed-grass prairie[END_REF], and also vary spatially within forests due to, among other things, gap dynamics [START_REF] Zhang | Effects of gap size on litter decomposition and microbial activity in a subtropical forest[END_REF]. Tree recruitment, via seedling growth and sapling survival, is heavily contingent upon microclimatic conditions [START_REF] Aussenac | Interactions between forest stands and microclimate: Ecophysiological aspects and consequences for silviculture[END_REF][START_REF] Campanello | Tree regeneration and microclimate in a liana and bamboo-dominated semideciduous Atlantic Forest[END_REF][START_REF] Harper | The demography of plants[END_REF]. Every forest tree starts as a small seedling on the floor and thus understorey conditions shaped by trees in the overstorey eventually feedback to tree recruitment and future forest structure. Therefore, threats to forest biodiversity and functioning from deforestation, forest degradation, and fragmentation are inherently linked to the loss and modification of forest microclimates by these activities [START_REF] Chen | Microclimate in forest ecosystem and landscape ecology -Variations in local climate can be used to monitor and compare the effects of different management regimes[END_REF][START_REF] Jucker | A research agenda for microclimate ecology in human-modified tropical forests[END_REF][START_REF] Laurance | The fate of Amazonian forest fragments: A 32-year investigation[END_REF]). Despite the potentially broad impact of microclimates on the response of forest ecosystems to global change, our understanding of how forest microclimates modulate biotic responses to climate warming and land use change at the species, community and ecosystem level is still limited. However, ecologists are increasingly making progress in filling this major research gap. This development is expected to benefit substantially from recent advances in modelling, remote sensing and mapping of forest microclimates [START_REF] Greiser | Monthly microclimate models in a managed boreal forest landscape[END_REF][START_REF] Jucker | Canopy structure and topography jointly constrain the microclimate of human-modified tropical landscapes[END_REF]Zellweger et al. 2019b). Here, considering the growing interest and recent advances in microclimatology, we provide a summary of where the field currently is, and where it is heading. To do so, we review the known drivers, processes and ecological importance of forest microclimates in current and future macroclimates, and lay out future research directions for this emerging field of research. Our structure for this review is premised on drawing contrasts between forests vs. open habitats in tropical, temperate and boreal biomes. We discuss the physical mechanisms driving forest microclimates, present an organism's perspective on microclimates, review the effects of microclimate on biodiversity and ecosystem functioning, and discuss how and when microclimates feed back to macroclimate warming. We end with a future research agenda for forest microclimates, focused on: (1) forest microclimate feedbacks; (2) forest microclimate mapping; and (3) microclimate impacts on forest biodiversity and ecosystem functioning.

Drivers of vertical and horizontal microclimate variation

Horizontal distribution of microclimates: forest vs open habitats

The horizontal distribution of microclimates within forests and open habitats is driven by vegetation, topography, soil, water, prevalent meteorological conditions, and their interactions [START_REF] Geiger | The climate near the ground[END_REF], Lembrechts et al. 2020a). Perhaps the most evident characteristic of forest microclimates is that the understorey is buffered against macroclimate temperature extremes (Fig 1). During clear and warm days, much of the incoming shortwave solar radiation is absorbed and reflected by the canopy, which, together with increased evapotranspirative cooling, leads to a cooling of the understorey maximum temperature by a global mean of 4.1°C compared to open-field conditions (De Frenne et al. 2019). On the other hand, minimum temperatures of forest understories are on average 1°C warmer, mainly as a result of understorey heat retention through shielding of the outgoing longwave radiation by the canopy [START_REF] Geiger | The climate near the ground[END_REF][START_REF] De Frenne | Global buffering of temperatures under forest canopies[END_REF]). Evaporative cooling and emitted longwave radiation both act to reduce canopy and soil surface temperatures whereas net shortwave radiation acts to warm the soil and canopy surfaces [START_REF] Geiger | The climate near the ground[END_REF][START_REF] De Frenne | Microclimate moderates plant responses to macroclimate warming[END_REF]. Heat exchange of surfaces with the air may contribute to warming or cooling depending on the temperature difference between the air and the surfaces [START_REF] Huang | Wind-induced leaf transpiration[END_REF]. The efficiency of these sensible heat fluxes and of evaporative cooling (latent heat fluxes) varies with wind and turbulence conditions. Vegetation structure and composition affect these processes of heat exchange and cause a horizontal variation in buffering of ambient temperatures (Fig. 3). In particular, vegetation density (e.g., in terms of canopy cover, basal area, plant area index) via effects on albedo, evapotranspiration and radiation absorption and reflection have strong influences on understorey microclimate, especially in the warm season [START_REF] Greiser | Monthly microclimate models in a managed boreal forest landscape[END_REF], Zellweger et al. 2019a). The vertical and horizontal composition and distribution of forest canopies (e.g., gaps, tree age distribution, leaf clumping, distance to forest edge) directly affect the amount and variability of sunlight [START_REF] Sprugel | Spatially explicit modeling of overstory manipulations in young forests: effects on stand structure and light[END_REF]Valladares & Guzman, 2006). At the stand level, small-scale variations in sun-flecks cause strong gradients in near-ground temperatures and there are often strong microclimatic gradients towards forest edges, due to increased solar radiation and wind [START_REF] Matlack | Microenvironment variation within and among forest edge sites in the eastern United-States[END_REF]. Microclimate gradients from forest core to edge are very large but differ strongly among microclimatic variables (e.g., light, wind, temperature), and are impacted by the edge orientation [START_REF] Hylander | Aspect modifies the magnitude of edge effects on bryophyte growth in boreal forests[END_REF], the cloudiness (e.g., [START_REF] Chen | Contrasting microclimates among clear-cut, edge, and interior of oldgrowth douglas-fir forest[END_REF]), the slope of the terrain or the wind direction (Davies- [START_REF] Davies-Colley | Microclimate gradients across a forest edge[END_REF]. This horizontal distribution in microclimate buffering varies not only at the stand scale, but also at landscape, continental and global scales. The effects of landscape topography on near-ground temperatures can be attributed to variations in incoming solar radiation driven by slope and aspect, pooling of cold air in depressions and exposure to winds (), variations in soil moisture, and the adiabatic lapse rate due to elevational gradients, all of which have been well documented (Ashcroft et [START_REF] Davis | Microclimatic buffering in forests of the future: the role of local water balance[END_REF]. At the continental scale, airmixing and lateral heat transfer by wind decrease when moving further away from the coast and mountain chains, which, together with fewer cloudy days, commonly leads to larger magnitudes of the temperature offsets in continental lowland forests (Zellweger et al. 2019a). Moreover, regional precipitation patterns and the size and adjacency to water bodies influence latent and sensible heat fluxes [START_REF] Meleason | Influence of riparian buffer width on air temperature at Whangapoua Forest, Coromandel Peninsula, New Zealand[END_REF]Zellweger et al. 2019a). At the global scale, the largest buffering of maximum temperatures is found in tropical forests, whereas buffering of cold extremes is largest in boreal forests (De Frenne et al. 2019), due to differences in forest structure, sun angle, seasonality and snow cover., Therefore, drivers of forest microclimates differ across latitudes (Fig. 3).

Vertical distribution of microclimates: from the ground to the top of the canopy

In open areas, air temperature at 1-2 m above ground is mostly controlled by local topography, radiation balance and turbulent mixing of air. Inside forests, however, canopy elements interfere with these processes by influencing radiation fluxes into and out of the forest as well as decreasing turbulent mixing of air through decreased wind speeds [START_REF] Chen | Contrasting microclimates among clear-cut, edge, and interior of oldgrowth douglas-fir forest[END_REF][START_REF] Chen | Microclimate in forest ecosystem and landscape ecology -Variations in local climate can be used to monitor and compare the effects of different management regimes[END_REF]). Vertical temperature gradients inside forests are the result of a complexity of microclimatic layers, formed and controlled in large part by the vegetation itself (Fig. 4; [START_REF] Vanwalleghem | Predicting forest microclimate in heterogeneous landscapes[END_REF][START_REF] Davies-Colley | Microclimate gradients across a forest edge[END_REF]. Forest management often causes a simplification of the vertical structure of the vegetation with drastic implications on the vertical microclimate profile [START_REF] Onaindia | Vegetation diversity and vertical structure as indicators of forest disturbance[END_REF]. Air temperature differences between ground and canopy range from 0.15 to 0.25 °C m -1 in temperate coniferous and mixed hardwood-conifer and tropical forests [START_REF] Harley | Effects of light, temperature and canopy position on net photosynthesis and isoprene emission from sweetgum (Liquidambar styraciflua) leaves[END_REF][START_REF] Zweifel | Midday stomatal closure in Norway spruce-reactions in the upper and lower crown[END_REF][START_REF] Hardwick | The relationship between leaf area index and microclimate in tropical forest and oil palm plantation: Forest disturbance drives changes in microclimate[END_REF][START_REF] Bauerle | Exploring the importance of within-canopy spatial temperature variation on transpiration predictions[END_REF]). During the day, air temperature peaks can occur near the ground, but are most often located within the top canopy, where most of the incoming energy is absorbed (Chen et al. 1999; Didham & Ewers, 2014) (Fig. 4). The exact vertical location of air temperature maxima will depend on the density of the canopy (leaf and plant area index as a function of height) and on the intensity of turbulent air mixing (Fig. 4). However, even when understorey air is cooler than above-canopy air, leaf and litter temperatures can rise well above the local air temperature in the understorey of open forests, due to decreased wind speeds and absorption of shortwave radiation [START_REF] Martin | Boundary layer conductance, leaf temperature and transpiration of Abies amabilis branches[END_REF][START_REF] Scheffers | Extreme thermal heterogeneity in structurally complex tropical rain forests[END_REF]). [START_REF] Leuzinger | Tree species diversity affects canopy leaf temperatures in a mature temperate forest[END_REF] showed that leaf temperature regimes in canopies vary enormously over short vertical distances in several coniferous and deciduous broad-leaved tree species. Finally, snow cover in the winter will effectively decouple the near ground temperature from the temperature above the snow (Fig. 1).

Consequences of microclimates for forest biodiversity

Microclimates influence an organism's physiology, activity patterns, behaviour, and fitness. In general, by virtue of the differences in their size, mobility and lifespan, organisms respond differently to microclimate conditions with respect to their life cycle processes. In other words, the "power of resolution" of organisms is inversely proportional to their living space [START_REF] Carlile | Determination of ecological scale[END_REF][START_REF] Decocq | The 'masking effect' of silviculture on substrate-induced plant diversity in oak-hornbeam forests from northern France[END_REF], so that the abundance and diversity of smaller, short-lived, and less mobile organisms often more readily reflect the small-scale variations in micro-environmental conditions. As such, the consequences of microclimates on biodiversity are scale-dependent, with the scale of operation of an organism, population, or community matching the scale of climate exposure. Although microclimate research aims to match the scale of climate and organisms, the concept of microclimate describes a spectrum of spatiotemporal scales (from centimetres to several hundred metres, from hours to years); i.e., perception of "micro" by woodlice is different from an elephant's perception of "micro" (Weins, 1989, Lembrechts et al. 2020b). However, an interesting aspect in forests is that the trees that modify the understorey microclimate have been small in the beginning of their life cycle. This illustrates that the same individual might respond to climate at different scales across its life stages, but also how forest microclimates can be created by reinforcing feedback mechanisms. Bearing this in mind, we here describe the influence of microclimate on biodiversity across space and time.

Spatial impacts of forest microclimate on biodiversity

At the meso-to macroscale, niche partitioning occurs horizontally and vertically in ecotones, whereby plant, animal, fungal, and bacterial communities turnover from one ecosystem to another (e.g., wet rainforests to dry woodlands) or across elevational gradients (e.g., [START_REF] Yuan | Ecophysiological variation across a forest-ecotone gradient produces divergent climate change vulnerability within species[END_REF]). At the microscale, organisms are also distributed horizontally (e.g., from a tree fall gap to closed canopy) and vertically (e.g., from the ground up to the canopy), following their environmental preferences, or niches. Vertical stratification of animal and plant communities is a prime example of how habitat and climate interact to derive localized partitioning of niches [START_REF] Nakamura | Forests and their canopies: achievements and horizons in canopy science[END_REF]), which includes a broad suite of organisms such as epiphytes, wasps, beetles, moths, amphibians, birds and mammals (for a vertical gradient of moths in forests, see De Smedt et al. 2019). Species have also been shown to shift their locations in response to changes in the spatial gradients of microclimates. For example, frogs of the Philippines shift their vertical niche upwards towards the canopy at higher elevations as microclimates become more favourable [START_REF] Scheffers | Increasing arboreality with altitude: a novel biogeographic dimension[END_REF]) and canopy epiphytes grow much further down when trees grow sparse [START_REF] Hylander | Complementary roles of home gardens and exotic tree plantations as alternative habitats for Ethiopian montane rainforest plant biodiversity[END_REF]. Birds in western North America and moose in Finland respond to changes in microclimate by shifting their horizontal distribution [START_REF] Melin | Moose (Alces alces) reacts to high summer temperatures by utilizing thermal shelters in boreal forests -an analysis based on airborne laser scanning of the canopy structure at moose locations[END_REF][START_REF] Frey | Microclimate predicts within-season distribution dynamics of montane forest birds[END_REF]). Warm-edge populations of boreal understorey plants inhabit sites with more stable microclimates, cooler maximum temperatures and later snowmelt (Greiser et

Temporal impacts of forest microclimate on biodiversity

Organisms also partition their niches according to microclimates in time [START_REF] Jonason | Surveying moths using light traps: effects of weather and time of year[END_REF]. Daily cycles of organism activity are apparent in Lepidopterans with butterflies primarily active during the day and moths active at night. However, activity can also vary within diel periods with activity peaks relative to a daytime thermal threshold [START_REF] Wikström | Monitoring of butterflies in semi-natural grasslands: diurnal variation and weather effects[END_REF]), a threshold that differs spatially from open habitats to closed forests [START_REF] Xing | Cool habitats support darker and bigger butterflies in Australian tropical forests[END_REF]). Similarly, leaf litter lizards will exploit sunspots or rare microclimates for thermoregulation, but only during cold morning hours [START_REF] Nordberg | Heat seekers: A tropical nocturnal lizard uses behavioral thermoregulation to exploit rare microclimates at night[END_REF]. Here, lizard activity varies with thermal heterogeneity driven both in time and by topographic roughness and aspect [START_REF] Sears | Configuration of the thermal landscape determines thermoregulatory performance of ectotherms[END_REF]). The dispersal mechanism of a moss is suggested to be most effective in morning hours when the moisture decreases along with increasing temperatures and wind [START_REF] Johansson | Air humidity thresholds trigger active moss spore release to extend dispersal in space and time[END_REF]). At a weekly or monthly scale, weather patterns strongly influence small mammal habitat use and activity [START_REF] Vickery | The influence of weather on habitat use by small mammals[END_REF]. Seasonal shifts in activity are apparent with regional and local climates. For example, arboreal frog communities shift from being highly vertically stratified in the tree canopies during the cooler, wet season to dramatically accumulating in the understorey during the hotter, dry season [START_REF] Basham | Distance-decay differs among vertical strata in a tropical rainforest[END_REF].

Consequences of microclimates on forest functioning

Microclimates strongly influence soil decomposition, primary productivity, plant communities and forest density, which further influences groundwater and carbon sequestration -via its influence on soil dynamics. For example, forest edge to interior climatic gradients are primary drivers of carbon storage and cycling [START_REF] Laurance | Forest-climate interactions in fragmented tropical landscapes[END_REF]; Uriarte et al. 2016; [START_REF] Meeussen | Drivers of carbon stocks in forest edges across Europe[END_REF]). In temperate forests, carbon sequestration is usually higher at the edge than in forest interiors [START_REF] Meeussen | Drivers of carbon stocks in forest edges across Europe[END_REF]. By contrast, in the tropics forest fragmentation generally leads to a loss of aboveground carbon stocks due to drier and warmer conditions at forest edges (Silva Junior et al. 2020). One might argue that microclimates, which dictate localized processes such as decomposition, scale up to ecosystem functioning indirectly via species interactions [START_REF] Petraglia | Litter decomposition: effects of temperature driven by soil moisture and vegetation type[END_REF] or bottom-up processes to which species respond. For example, changes in understorey microclimate due to changed overstorey composition affect the herb layer composition as well as soil conditions [START_REF] Decocq | Silviculture-driven vegetation change in a European temperate deciduous forest[END_REF]). Sometimes the ecosystem functions are maintained, despite changed microclimates. A Bornean tropical rainforest was shown to exhibit functional resilience after heavy logging, with different taxa taking over ecosystem processes such as litter decomposition and seed predation [START_REF] Ewers | Logging cuts the functional importance of invertebrates in tropical rainforest[END_REF]. Research on the mechanisms of how changes of microscale processes scale up to ecosystems remains largely theoretical. It can be expected that the collective contribution of temperature offsets provided by forest structure simultaneously impacts many aspects of ecosystem functioning. Yet, no studies exist to our knowledge that collectively assess the key processes determining ecosystem multifunctionality, which is likely due to the enormous empirical information required for such inference to be made (see also our research agenda below). 2020). Assuming a space-for-time substitution, this suggests that the magnitude of the offset on maximum temperature could potentially increase under macroclimate warming (Fig. 1 and Fig. 5). This assumption only holds if: (i) the relationship between offsets and macroclimate continues to be linear; (ii) the forest canopy layer is not disturbed; (iii) we assume that the equilibrium point at which temperatures inside and outside forests are the same (cf. Fig. 1), does not shift; and (iv) other variables such as microhabitat moisture levels remain comparable [START_REF] Scheffers | Asplenium bird's nest ferns in rainforest canopies are climatecontingent refuges for frogs[END_REF][START_REF] Zellweger | Forest microclimate dynamics drive plant responses to warming[END_REF]. Slow, interannual climate change can, however, directly change the equilibrium point, while changes in canopy cover, moisture, etc. could directly act on the buffering and hence slope (Fig. 1). Nevertheless, if macroclimatic increases in daily maximum temperatures can be buffered, it might provide forest organisms with more time for adaptation and migration [START_REF] Zellweger | Forest microclimate dynamics drive plant responses to warming[END_REF]), a phenomenon that is reminiscent of the concept of microrefugia (that is, spatially-restricted habitats that sustain a favourable microclimate, which enables species to persist in an otherwise inhospitable matrix; [START_REF] Gavin | Climate refugia: joint inference from fossil records, species distribution models and phylogeography[END_REF]). The pattern is opposite for minimum temperatures: higher ambient air temperatures decrease minimum temperature offsets (De Frenne et al. 2019). Hence, still under the assumptions of a space-for-time substitution, the magnitude of the offset in minimum temperature could potentially decrease under macroclimate warming, contributing to reduce the buffering effect on minimum temperature and thus alter the microrefugial capacities of boreal forests for cold-adapted species surviving at the warmer range margin (Fig. 5). In the following subsections, we first discuss changes in forest microclimate dynamics due to macroclimate warming in different forest biomes, and then the potential impacts of macroclimate warming on future offsets. Biome-specific effects on temperature offsets (Fig. 5) In temperate forests, temperature buffering may happen for both maximum and minimum temperatures (De Frenne et al. 2019). Yet, during the cold season, deciduous trees shed their leaves, the primary drivers of buffering, making buffering in temperate forests likely to be more important and relevant during the growing season. Additionally, Zellweger et al. (2019a) showed that the magnitude of the thermal offset during the summer season in European temperate forests was more pronounced for daily maximum temperatures than for daily minimum temperatures. As a consequence, canopy cover density directly affects buffering capacity, with likely implications on organismal responses to climate change. For example, the thermophilisation rate -the rate of community shift towards more warm-adapted species -in understorey plant communities of temperate forests is better related to the rate at which the daily maximum temperature changes in forest interiors (i.e. the rate of microclimate warming) during the growing season than the rate of macroclimate warming [START_REF] Zellweger | Forest microclimate dynamics drive plant responses to warming[END_REF]). In boreal forests, buffering of minimum temperatures is most pronounced, while tropical forests have more pronounced offsets of maximum temperatures (De Frenne et al. 2019). Albeit the velocity of macroclimate warming is highest at high latitudes, tropical species will likely be more severely impacted due to their narrow thermal niches and safety margins [START_REF] Tewksbury | Putting the heat on tropical animals[END_REF][START_REF] Antão | Temperature-related biodiversity change across temperate marine and terrestrial systems[END_REF][START_REF] Lenoir | Species better track climate warming in the oceans than on land[END_REF]). Worryingly, daily maximum temperatures in the next decades will likely be more extreme than what tropical species have ever experienced in their recent evolutionary history [START_REF] Deutsch | Impacts of climate warming on terrestrial ectotherms across latitude[END_REF][START_REF] Kingsolver | The Well-Temperatured Biologist[END_REF].

Macroclimate warming effects on temperature offsets

In their review covering the second half of the 20th century, [START_REF] Boisvenue | Impacts of climate change on natural forest productivity -evidence since the middle of the 20th century[END_REF] reported that both satellite and ground-based data support an increase in forest productivity across many temperate parts of the globe owing to climate warming. Hence, at temperate latitudes, forests with ample water and soil nutrients may become denser, thereby increasing temperate forest offsets [START_REF] Zellweger | Forest microclimate dynamics drive plant responses to warming[END_REF]). On the other hand, recent reports show cross-European canopy opening due to an increase in natural and anthropogenic disturbances [START_REF] Senf | Mapping the forest disturbance regimes of Europe[END_REF] and thus a potential reduction in temperature offset. And finally, as macroclimate warms, earlier timing of bud burst and leaf flush will impact the seasonal course of forest microclimates, potentially leading to phenological mismatches between trees and understorey species [START_REF] Heberling | Phenological mismatch with trees reduces wildflower carbon budgets[END_REF]). Earlier leaf flush might effectively shorten the growing season for understorey plants, if shade levels are enhanced earlier in the season and the temperature sensitivity of phenological advances of wildflowers is lower than trees [START_REF] Heberling | Phenological mismatch with trees reduces wildflower carbon budgets[END_REF]). In the tropics, satellite-driven measures of vegetation greenness (NDVI), a surrogate for photosynthetic activity and productivity, show reduced productivity in warmer years [START_REF] Braswell | The response of global terrestrial ecosystems to interannual temperature variability[END_REF][START_REF] Asner | Satellite observation of El Niño effects on Amazon forest phenology and productivity[END_REF], suggesting a reduced future buffering capacity. Conversely, in boreal forests, the impact of changes in primary productivity on the buffering capacity of forests is less clear. On the one hand, old growth boreal forests in North America showed no net increase in stem growth (Giguère-Croteau et al. 2018). On the other hand, [START_REF] Beck | Changes in forest productivity across Alaska consistent with biome shift[END_REF] have reported changes in forest productivity across Alaska that are consistent with a complete biome shift: decreased productivity at the warmer (southern) versus enhanced productivity at the colder (northern) edge of the boreal biome. Thus, the buffering capacity of boreal forests may mirror the climatically-induced changes in primary productivity with the magnitude of the offsets decreasing and increasing towards the warmer and colder edge of the boreal zone, respectively.

Extreme event effects on temperature offsets

The current and future increase in daily maximum temperatures during the warm season will in many areas lead to more intense, more frequent and persistent heat waves [START_REF] Meehl | More intense, more frequent, and longer lasting heat waves in the 21st Century[END_REF][START_REF] Russo | Top ten European heatwaves since 1950 and their occurrence in the coming decades[END_REF]. Therefore, some temperate forests are becoming increasingly water-limited during the summer season, generating drought stress and inducing physiological constraints in trees that make them more susceptible to pests [START_REF] Trumbore | Forest health and global change[END_REF]. This combination of stressors may ultimately lead to widespread crown defoliation, tree mortality and higher risks of forest wildfires due to forest fuel accumulation ( 2019) have predicted that some forests of the northwestern United States will lose their capacity to buffer extremes of maximum temperature and vapor pressure deficit due to changes in water balance combined with accelerating heatinduced canopy losses. A threshold in canopy cover of c. 75 % exists below which buffering properties in temperate forests largely decrease (Zellweger et al. 2019a). Tree die-off causing canopy cover to drop below this threshold will thus severely reduce the degree to which forest microclimates and biodiversity will be buffered from climatic extremes. Additionally, wildfires and other disturbances such as forest management can accelerate these processes as well [START_REF] Davis | Microclimatic buffering in forests of the future: the role of local water balance[END_REF][START_REF] Senf | Mapping the forest disturbance regimes of Europe[END_REF].

Interactions between human land-use and macroclimate warming

Forest microclimates can be heavily influenced by management practices and policies that change the canopy composition and structure at the stand level and the spatial arrangement of stands across landscapes (Frey et al. 2016a,b;[START_REF] Greiser | Monthly microclimate models in a managed boreal forest landscape[END_REF][START_REF] Jucker | Canopy structure and topography jointly constrain the microclimate of human-modified tropical landscapes[END_REF]). Forest management activities that have the potential to affect microclimate include the management system (such as shelterwood, single-tree selection, clear-cutting, thinning and tending), choice of tree species (and making a deliberate choice on their shade casting ability, for instance), regeneration type (natural vs artificial such as tree planting or sowing), fertilization, rotation length, presence of a shrub layer, control of large herbivores, as well as the size and distribution of management units [START_REF] Vanwalleghem | Predicting forest microclimate in heterogeneous landscapes[END_REF][START_REF] Brang | Suitability of close-to-nature silviculture for adapting temperate European forests to climate change[END_REF][START_REF] Latimer | Forest fragmentation alters winter microclimates and microrefugia in humanmodified landscapes[END_REF]. Thus, depending on the type of management, forest managers can influence many aspects of the below-canopy microclimate, with important consequences for biodiversity and ecosystem processes [START_REF] Selva | Misguided forest action in EU Biodiversity Strategy[END_REF]). In boreal forests, and to a lower extent in temperate and tropical forests, intensive forest management for timber and other woody biomass harvest has led to a biotic, genetic, structural and functional homogenization of forest stands across large spatial extents [START_REF] Rousseau | Woody biomass removal in harvested boreal forest leads to a partial functional homogenization of soil mesofaunal communities relative to unharvested forest[END_REF]). The even aged single species stands typical of managed boreal forests have reduced the resilience of the whole boreal system to increasing frequency and severity of climate-induced pest outbreaks and wildfires (Cudmore et 5). In the tropics, the combined effects of logging, droughts and fires on canopy loss (i.e., deforestation and degradation) can locally increase air dryness [START_REF] Staal | Feedback between drought and deforestation in the Amazon[END_REF]) and daily maximum temperatures more than the warming associated with high emission scenarios [START_REF] Senior | A pantropical analysis of the impacts of forest degradation and conversion on local temperature[END_REF]). Hence, by letting in direct sunlight and warm and dry air, large canopy gaps following deforestation strongly alter understorey microclimate (Fig. 3 and5), reducing the capacity to buffer macroclimatic fluctuations and thus causing many species to decline in abundance, e.g. termites that are especially sensitive to desiccation [START_REF] Cornelius | Effect of soil type and moisture availability on the foraging behavior of the Formosan subterranean termite (Isoptera: Rhinotermitidae)[END_REF]; see De Smedt et al. 2018 for a study from temperate forests). However, small canopy gaps (< 400 m²) in tropical forests, which occur under natural forest dynamics, can quickly regain their thermal environment in a few years [START_REF] Mollinari | Rapid recovery of thermal environment after selective logging in the Amazon[END_REF]), while secondary forests can regain their thermal environments within 20 -30 years after logging (del Pliego et al 2016). These drastic changes in microclimatic conditions are not only due to tree removal, but at a finer resolution also to epiphyte loss. Indeed, epiphytes represent a significant functional group for microclimate dynamics in tropical forests, reducing water loss through evaporative drying [START_REF] Scheffers | Asplenium bird's nest ferns in rainforest canopies are climatecontingent refuges for frogs[END_REF]) and providing buffered microhabitats for canopy-dwelling organisms [START_REF] Seidl | Bird's nest fern epiphytes facilitate herpetofaunal arboreality and climate refuge in two paleotropic canopies[END_REF]) (Fig. 3,arrow J).

Forest microclimate feedbacks on macroclimate warming

Although we now have a better understanding of the impact of macroclimate warming on forest microclimate dynamics, the potential feedback of forest microclimates on macroclimate warming itself remains understudied [START_REF] Barry | Microclimate and local climate[END_REF]). Yet, the implications are important for solutions to mitigate and adapt to climate change. Changes in microclimates may feed back to the macroclimate by affecting localized water and carbon balances and microgradients of CO2 within forests. The release of water vapor into the atmosphere by trees through transpiration affects local as well as regional precipitation patterns [START_REF] Bonan | Forests and climate change: forcings, feedbacks, and the climate benefits of forests[END_REF][START_REF] Spracklen | Observations of increased tropical rainfall preceded by air passage over forests[END_REF]). For instance, in the tropics, air that passed over extensive areas of forests produced at least twice as much rain as air that has passed over short or no vegetation [START_REF] Spracklen | Observations of increased tropical rainfall preceded by air passage over forests[END_REF]. Regional tropical rainfall usually decreases (in quantity and frequency) after a threshold of 30 -50 % deforestation, especially when large forest patches are cleared, while small clearings may actually enhance rainfall via convective processes leading to cloud formation [START_REF] Lawrence | Effects of tropical deforestation on climate and agriculture[END_REF]. The importance of vegetation in land-atmosphere-ocean feedback processes is remarkably illustrated by the last Sahara desertification episode (c. 5000 yrs ago), when precipitationvegetation feedbacks due to deforestation by humans are considered to have played a crucial role [START_REF] Pausata | The greening of the Sahara: past changes and future implications[END_REF]). Studies on afforestation projects in the Saharan and Sahelian zones are limited to their role in mitigating the effects of warming by carbon drawdown, while their impacts on microclimates currently remain understudied [START_REF] Pausata | The greening of the Sahara: past changes and future implications[END_REF]). Another example with feedbacks between forest cover and climate is the poleward expansion of boreal forests, which decreases the albedo and thus the ratio of incoming and outgoing radiation [START_REF] Bonan | Forests and climate change: forcings, feedbacks, and the climate benefits of forests[END_REF][START_REF] Pearson | Shifts in Arctic vegetation and associated feedbacks under climate change[END_REF], and increases snow depths, as a consequence of more shrubs, thus isolating the ground from deep frost during the winter leading to permafrost thaw [START_REF] Lantz | Recent shrub proliferation in the Mackenzie delta uplands and microclimatic implications[END_REF][START_REF] Connon | The influence of shallow taliks on permafrost thaw and active layer dynamics in subarctic Canada[END_REF]). The positive feedback on macroclimate warming is derived from permafrost thaw releasing stored carbon dioxide under aerobic conditions and methane under wet, anaerobic conditions (Fig. 5). This example links to the role of snow cover in decoupling the near-ground temperature from ambient temperatures and how forest structure moderates this (Fig. 1). However, in this example, shrubs act as accumulators of snow because strong winds in the tundra remove snow from open areas, while in many other situations the snow cover and thus the buffering of near-ground temperatures is higher in open than in forested sites (Fig. 1 and Fig. 5).

A research agenda and identification of research gaps

To identify current knowledge gaps and formulate a research agenda on forest microclimates, we followed an approach adapted from [START_REF] Sutherland | Identification of 100 fundamental ecological questions[END_REF]. First, the authors of the underlying paper submitted questions via online forms, which were summarized and grouped. These updated questions were then presented and discussed with co-authors followed by live voting at a joint physical meeting (Ekenäs, Sweden in Feb. 2020). From these voting results, we identified three key directions for future forest microclimate research as discussed below (Supporting Information Table S1).

1) Drivers of forest microclimate buffering and future changes

Major unknowns in the quantification of the relative importance of the drivers of below-canopy microclimates are related to (1) abiotic, (2) biotic environmental effects, and (3), how the contribution of both might change in the future as a result of anthropogenic global change. Concerning the latter, forest microclimates will indeed be affected by changes in the abiotic as well as biotic part of the environment (climate change, urbanization, changes in hydrology and alteration of soil characteristics, etc.), and we need to address the key uncertainties, especially with regard to interactions of climate change with other globalchange drivers such as land-use changes, changes in forest management or enhanced atmospheric inputs of nitrogen. Given the complexity of the effects of anthropogenic global change on biotic factors, they must be a key part of the future research agenda. These factors include forest age and structure (multistorey vs. monostorey), tree species composition and forest fragmentation, all of which are linked to forest management and global environmental change (mortality due to pests and pathogens, invasive species). Future research should therefore focus on how changes in the climate system and land use interactively affect forest structures and thus the microclimate buffering, magnitude of offsets and potential level of decoupling. Besides modelling studies, there is a place for empirical work such as manipulative experiments or comparative studies on how the magnitude of forest offsets change as a means of drought, Nfertilization, changed tree species composition, introduction of exotic species, etc. Land managers and policy makers could use this information to identify management regimes that maximize temperature buffering, to aim at optimal forest functioning and guide biodiversity conservation [START_REF] Greiser | Hiding from the climate: Characterizing microrefugia for boreal forest understory species[END_REF]).

2) Mapping and predictions of forest microclimates

While the mechanisms driving the buffering between forest microclimate and macroclimate, and other global-change drivers get disentangled, focus should also go towards the creation of (1) open-access, freeto-use, global gridded products of forest microclimate and (2) automated protocols for past and future microscale geospatial data (Zellweger et al. 2019b;Lembrechts et al. 2020a). This can, for example, be achieved by applying correction factors based on the offset between micro-and macroclimate to existing macroclimate maps (e.g., WorldClim and CHELSA) (Fig. 6). Further increases in the spatial resolution of such microclimate maps is possible thanks to the recent emergence of both large-scale global databases of in-situ measured (forest) microclimate (De Frenne et al. 2019; Lembrechts et al. 2020a) as well as everhigher resolution remotely-sensed global forest cover products (down to 30 m resolution, and better). More methodological development is, however, needed to incorporate the vertical and temporal components of forest microclimate in these mapping efforts, as reliable and repeated info about 3D forest structure (e.g., using laser scanning) is only now becoming available, for instance via GEDI LiDAR data. Obtaining accurate microclimate time series for forest understories (for the past, present and future) are further complicated by the interactions between climate change and land use changes, as discussed in the previous paragraph [START_REF] Zellweger | Forest microclimate dynamics drive plant responses to warming[END_REF]Lembrecht & Nijs, 2020). Other important challenges are the dynamic nature of managed forest landscapes, how to incorporate wind effects in models of complex fragmented landscapes and, for global applications, the current computer power. Obtaining high-resolution long-term microclimate time series for the whole world requires effective assimilation of in-situ measurements, and mechanistic and statistical models. While existing mechanistic models of microclimate currently largely focus on open terrain (e.g., [START_REF] Maclean | Predicting future climate at high spatial and temporal resolution[END_REF], this is a rapidly expanding field where workable solutions for forest microclimates can be expected in the near future. Complementing these models with in-situ measurements for calibration, and statistical models for global extrapolations, should be able to deliver the gridded projects we need (Lembrechts & Lenoir, 2020).

3) Impacts on biodiversity and ecosystem functioning in forests

In addition to characterizing the physiographic and biophysical processes that drive forest microclimates (Fig. 345) as well as developing approaches for mapping microclimate at appropriate scales (Fig. 6), careful thought is needed on how to best integrate these new data streams into biodiversity research [START_REF] Jucker | A research agenda for microclimate ecology in human-modified tropical forests[END_REF]). Access to climate data that better reflect local conditions experienced by living organisms should improve our ability to model species distributions and predict how they will respond to rapid global change [START_REF] Mod | What we use is not what we know: environmental predictors in plant distribution models[END_REF][START_REF] Lenoir | Climatic microrefugia under anthropogenic climate change: implications for species redistribution[END_REF][START_REF] Lembrechts | Incorporating microclimate into species distribution models[END_REF]. However, few studies have actually tested this assumption [START_REF] Lembrechts | Incorporating microclimate into species distribution models[END_REF][START_REF] Ohler | Microclimatic effects on alpine plant communities and flowervisitor interactions[END_REF], particularly in the context of forests (Frey et al. 2016a). A key question that remains to be addressed is at what spatial scale (horizontal and vertical) microclimate should be measured and modelled, and how this varies for different groups of species (e.g., in relation to body size, dispersal and thermoregulation, [START_REF] Potter | Microclimatic challenges in global change biology[END_REF]Scheffers et al. 2014a). Similarly, we also need to determine which aspects of microclimate best predict species distributions in forests (e.g., air temperature, humidity, soil moisture, solar radiation) and how to effectively summarize these metrics (e.g., means, extremes, fluctuations, thresholds, growing degree hours/days; [START_REF] Hylander | Microrefugia: not for everyone[END_REF][START_REF] Bramer | Advances in monitoring and modelling climate at ecologically relevant scales[END_REF]). Empirical and modelling approaches that allow different facets of microclimate to be manipulated independently are crucial to addressing these questions (for an example to separate light and temperature effects, see De Frenne et al. 2015). Beyond the immediate need to better characterize how microclimate shapes current-day ecological processes in forests, a major challenge is to determine how long different types of forests can continue to act as microrefugia (also referred to as hold-outs in this context) for species in a warming world [START_REF] Hannah | Fine-grain modeling of species' response to climate change: holdouts, stepping-stones, and microrefugia[END_REF]. As global mean temperatures continue to rise, so too will those in forest understoreys (albeit slower if buffering is at play). But perhaps more importantly, long-term climate change in interaction with forest management will eventually lead to changes in the species composition and structure of forests (e.g., the number and size of trees, as well as canopy height and density) [START_REF] Coomes | Wood production response to climate change will depend critically on forest composition and structure[END_REF][START_REF] Albrich | Climate change causes critical transitions and irreversible alterations of mountain forests[END_REF]) -with clear cascading effects for understorey microclimate [START_REF] Jucker | Canopy structure and topography jointly constrain the microclimate of human-modified tropical landscapes[END_REF]. Very few studies have effectively evaluated ecosystem multifunctionality, and translated this to services, let alone relate it to microclimates (e.g., of the type suggested by [START_REF] Byrnes | Investigating the relationship between biodiversity and ecosystem multifunctionality: challenges and solutions[END_REF]. Although policy documents abound with statements about climate change mitigation and adaptation, there is a lack of understanding about forest (micro)climate and biodiversity, which might lead to misguided actions [START_REF] Selva | Misguided forest action in EU Biodiversity Strategy[END_REF]). There are thus large knowledge gaps in biodiversity -ecosystem functioning -microclimate research. While these longer-term effects of climate change on forest microrefugia have been largely overlooked, a promising avenue for exploring them would be to integrate microclimate projections into forest dynamics models used to simulate forests under future conditions [START_REF] Albrich | Climate change causes critical transitions and irreversible alterations of mountain forests[END_REF].

Concluding remarks

In sum, we have outlined the contemporary research interests and gaps linking microclimatic variation to biodiversity and the functioning of forest ecosystems worldwide. The urgency is clear; compelling evidence is accumulating to suggest that distinct below-canopy microclimatic conditions in forests arising from vertical and horizontal processes can mediate the amount of macroclimate warming in forests all around the globe. However, even though the microclimatic changes in forests due to macroclimate warming may be smaller than those in other ecosystems, the ecological impact may be just as large if forest species have narrower niches and thus are more sensitive. Moreover, other global changes such as forest disturbance and widespread canopy opening (Senf & Seidl, 2020) might accelerate the effects of climate change in forests through their impact on microclimates. Our priority voting of important questions suggested that future forest microclimate research should focus on three overarching themes (drivers & global change, mapping & predictions, and biodiversity & ecosystem functioning). These themes reflect the wealth of fundamental research gaps that still exist in forest microclimate research. Recent studies highlighting the role of microclimate in helping to sustain local biodiversity and ecosystem functions have paved a way towards "microclimate forest restoration", or in other words, restoring forest ecosystems with the explicit purpose to increase their capacity to buffer the local microclimates from macroclimatic change. Such arguments are to date hardly considered in the pros and cons of the global tree restoration debate (e.g., [START_REF] Bastin | The global tree restoration potential[END_REF]). In tandem with the steadily increasing number of microclimate monitoring sites (Lembrechts et al. 2020a), novel microclimate modelling approaches have been developed. These crucial methodological advances are likely to encourage the use of microclimate data instead of settling for coarsescale climate data of long-term average conditions. Once the global variation in forest microclimates is properly documented and analysed, more efforts should be placed in order to implement this information into further analyses of ecosystem functioning. Doing so is expected to greatly increase our understanding of the impacts of climate change on forest ecosystems. Although the importance of microclimate in regulating many biophysical processes has been acknowledged by ecologists and biologists for nearly a century, we are finally stepping into an era where we have a solid conceptual and methodological foundation for testing many fundamental research questions related to forest functioning. This is important as a better understanding of the magnitude, drivers and implications of forest microclimate on biodiversity is urgently required in order to better manage forests, support their sustainable use and secure viable ecosystem services for future generations in a warmer climate.

Figure captions

Fig. 1. Definitions of the main processes underlying microclimate dynamics: offsets, buffering, coupling and decoupling. To be read in conjunction with Box 1. Fig. 2. Number of publications on the topics "microclimates & forests" (dark red) and "microclimates & biodiversity" (blue) according to a Web of Science search on 23 Oct. 2020 (results included till 2019). Fig. 3. Multiple vegetation drivers of microclimate might be of different importance in forest at boreal (top), temperate (middle), and tropical (bottom) latitudes, respectively, even if most processes are general. Increasing tree density from open non-forest habitats (A), to plantations with a simple canopy structure (B), to (semi-)natural forest with complex structure (C) reduces below-canopy wind speeds above ground. Forest canopies can reduce ground snow cover and thus decrease the insulating effect of snow cover on cool soil temperatures during the cold season (D). Vertical layering of vegetation (E) influences the amount and quality of incoming shortwave radiation, outgoing longwave radiation and moisture exchange. Disturbances can create canopy gaps (F), providing a local shift in microclimate. Seasonal reductions in canopy cover (tree phenology, G) during the cool and/or dry season increases the exposure of the internal forest to ambient conditions. Forests also buffer the temporal (i.e. diurnal, seasonal and interannual) variability in temperature conditions relative to adjacent non-forest systems (bottom panel). This buffering effect varies with vegetation height and structure, with reduced buffering in secondary, post-agricultural forests (H) relative to primary or ancient, (semi-)natural forests (I). Microhabitats within a forest, such as those created by epiphytic plants (J) can offer an even more buffered microclimate, critical for the ecology and physiology of many forest species. Finally, the temperature offset in forests can change throughout the diel cycle, with cooler forest interiors vs. open areas during the day (K) and warmer at night (L). For the sake of simplicity, we chose to depict wind, shortwave radiation, and temperature in the boreal, temperate, and tropical panel, respectively. However, of course all of these microclimate variables can be relevant to systems across latitudes. Macroclimate change effects on microclimates. Climate warming and climatic extremes affect microclimates and microrefugia by influencing forest composition and structure in boreal (top panel), temperate (middle) and tropical forests (lower panel). We here highlight the processes influencing microclimatic offsets that are most relevant for each biome, although most processes are important in all the biomes. Complex, indirect effects of climate change on microrefugia involve feedback with natural and anthropogenic factors. Fig. 6. The four dimensions of improving gridded microclimate products for forests. a) one can turn coarse-grained free-air temperature grids (products such as CHELSA and WorldClim) into coarse-grained forest temperature maps using the offset between weather station and forest temperatures. Next, to increase the temporal (b) and spatial (c) resolution of forest microclimate maps, and to create the full vertical temperature profile (d), one should aim for the integration of in-situ measurements, and mechanistic and statistical models.

Box 1 Definitions of offsets, buffering, coupling and decoupling

Many terms related to microclimate dynamics have been used in the scientific literature, such as 'buffering', 'coupling', 'decoupling' and 'offset' to imply divergence from macroclimatic fluctuations over time. However, no uniform definition of these terms exists yet. For this reason, we here suggest a uniform terminology including all terms by illustrating the processes behind each of them (Fig. 1). First of all, we define the temperature offset as the instantaneous difference between a reference temperature at a given time t0 and the focal temperature under study at the same time t0. For instance, the horizontal temperature offset due to the presence of a forest canopy is the instantaneous difference between the free-air temperature in open conditions (i.e., macroclimate) and the sub-canopy temperature at the same height (i.e., microclimate), with positive and negative offset values meaning colder and warmer conditions in the forest understorey, respectively (Fig. 1, left panel). Similarly, the vertical temperature offset due to snow cover is the instantaneous temperature difference between the air above the snow and inside the snow layer, with positive and negative offset values meaning colder and warmer conditions inside the snow layer, respectively (Fig. 1, right panel). Then, depending on the magnitude and distribution of the temperature offsets over time, it is possible to distinguish three contrasting situations (Fig. 1): (1) perfect coupling; (2) buffering; and (3) perfect decoupling:

(1) Perfect coupling occurs when microclimatic temperatures (Tmicro) equal macroclimatic temperatures (Tmacro). In other words, the slope (β1) of the linear relationship between Tmacro and Tmicro (Tmicro = β0 + β1 × Tmacro) is equal to one (identity) and the offset is zero and constant over time. [START_REF] Aalto | Biogeophysical controls on soil-atmosphere thermal differences: implications on warming Arctic ecosystems[END_REF] Buffering means a dampening of Tmacro fluctuations over time such that temporal fluctuations in Tmicro still exist but are much less pronounced than for Tmacro. This generates a cycle of positive and negative offset values which tend to diminish the positive correlation between Tmacro and Tmicro, such that β1 is lower than 1 but greater than 0. The closer β1 is to zero, the more pronounced the magnitude of buffering. (3) Perfect decoupling occurs when Tmicro behaves independently from Tmacro, i.e. when the slope (β1) is zero and the buffering is so strong that the positive correlation between Tmicro and Tmacro is totally lost. For instance, temperatures inside the snow layer during winter are completely decoupled from temperatures above the snow layer (Fig. 1, right panel).

  Ewers & Banks-Leite, 2013; von Arx et al. 2013, De Frenne et al. 2019) (see Fig. 1 and Box 1 for the definitions of technical terms used in this paper). The magnitude of such positive and negative temperature differences or offsets between open lands and forest interiors can vary due to both the structure of the forest and the ambient temperatures (De Frenne et al. 2019).

  al. 2019). The performance and distribution of forest lichens and bryophytes often show clear patterns along local temperature and moisture gradients (Hylander, 2005; Stewart & Mallik, 2006; Åström et al. 2007; Gauslaa, 2014; Löbel et al. 2018). Noteworthy, the influence of microclimates on local species diversity can be so strong that entire amphibian communities can abruptly change across a microclimate gradient spanning just a few metres (Basham & Scheffers, 2019; Basham et al. 2020).

  al. 2010; Gauthier et al. 2015). Although fires are part of the natural disturbance dynamics in many boreal systems, large stand-replacing wildfires have resulted in shrub proliferation and enhanced snow accumulation, with possible implications for longer decoupled ground temperatures (Lantz et al. 2013; Aalto et al. 2018) (Fig. 1 and Fig.

Fig. 4 .

 4 Figure captionsFig.1. Definitions of the main processes underlying microclimate dynamics: offsets, buffering, coupling and decoupling. To be read in conjunction with Box 1. Fig.2. Number of publications on the topics "microclimates & forests" (dark red) and "microclimates & biodiversity" (blue) according to a Web of Science search on 23 Oct. 2020 (results included till 2019). Fig.3. Multiple vegetation drivers of microclimate might be of different importance in forest at boreal (top), temperate (middle), and tropical (bottom) latitudes, respectively, even if most processes are general. Increasing tree density from open non-forest habitats (A), to plantations with a simple canopy structure (B), to (semi-)natural forest with complex structure (C) reduces below-canopy wind speeds above ground. Forest canopies can reduce ground snow cover and thus decrease the insulating effect of snow cover on cool soil temperatures during the cold season (D). Vertical layering of vegetation (E) influences the amount and quality of incoming shortwave radiation, outgoing longwave radiation and moisture exchange. Disturbances can create canopy gaps (F), providing a local shift in microclimate. Seasonal reductions in canopy cover (tree phenology, G) during the cool and/or dry season increases the exposure of the internal forest to ambient conditions. Forests also buffer the temporal (i.e. diurnal, seasonal and interannual) variability in temperature conditions relative to adjacent non-forest systems (bottom panel). This buffering effect varies with vegetation height and structure, with reduced buffering in secondary, post-agricultural forests (H) relative to primary or ancient, (semi-)natural forests (I). Microhabitats within a forest, such as those created by epiphytic plants (J) can offer an even more buffered microclimate, critical for the ecology and physiology of many forest species. Finally, the temperature offset in forests can change throughout the diel cycle, with cooler forest interiors vs. open areas during the day (K) and warmer at night (L). For the sake of simplicity, we chose to depict wind, shortwave radiation, and temperature in the boreal, temperate, and tropical panel, respectively. However, of course all of these microclimate variables can be relevant to systems across latitudes. Fig. 4. Typical vertical air temperature profiles inside forests of various canopy depth and density, during nighttime and daytime and for cloudy or clear sky conditions. These typical examples are mainly based on e.g. Roberts et al. (1990), Chen et al. (1999), Ogée et al. (2003), Geiger et al. (2009), and Brower et al. (2011). Fig. 5.Macroclimate change effects on microclimates. Climate warming and climatic extremes affect microclimates and microrefugia by influencing forest composition and structure in boreal (top panel), temperate (middle) and tropical forests (lower panel). We here highlight the processes influencing microclimatic offsets that are most relevant for each biome, although most processes are important in all the biomes. Complex, indirect effects of climate change on microrefugia involve feedback with natural and anthropogenic factors. Fig.6. The four dimensions of improving gridded microclimate products for forests. a) one can turn coarse-grained free-air temperature grids (products such as CHELSA and WorldClim) into coarse-grained forest temperature maps using the offset between weather station and forest temperatures. Next, to increase the temporal (b) and spatial (c) resolution of forest microclimate maps, and to create the full vertical temperature profile (d), one should aim for the integration of in-situ measurements, and mechanistic and statistical models.

Fig. 5 .

 5 Figure captionsFig.1. Definitions of the main processes underlying microclimate dynamics: offsets, buffering, coupling and decoupling. To be read in conjunction with Box 1. Fig.2. Number of publications on the topics "microclimates & forests" (dark red) and "microclimates & biodiversity" (blue) according to a Web of Science search on 23 Oct. 2020 (results included till 2019). Fig.3. Multiple vegetation drivers of microclimate might be of different importance in forest at boreal (top), temperate (middle), and tropical (bottom) latitudes, respectively, even if most processes are general. Increasing tree density from open non-forest habitats (A), to plantations with a simple canopy structure (B), to (semi-)natural forest with complex structure (C) reduces below-canopy wind speeds above ground. Forest canopies can reduce ground snow cover and thus decrease the insulating effect of snow cover on cool soil temperatures during the cold season (D). Vertical layering of vegetation (E) influences the amount and quality of incoming shortwave radiation, outgoing longwave radiation and moisture exchange. Disturbances can create canopy gaps (F), providing a local shift in microclimate. Seasonal reductions in canopy cover (tree phenology, G) during the cool and/or dry season increases the exposure of the internal forest to ambient conditions. Forests also buffer the temporal (i.e. diurnal, seasonal and interannual) variability in temperature conditions relative to adjacent non-forest systems (bottom panel). This buffering effect varies with vegetation height and structure, with reduced buffering in secondary, post-agricultural forests (H) relative to primary or ancient, (semi-)natural forests (I). Microhabitats within a forest, such as those created by epiphytic plants (J) can offer an even more buffered microclimate, critical for the ecology and physiology of many forest species. Finally, the temperature offset in forests can change throughout the diel cycle, with cooler forest interiors vs. open areas during the day (K) and warmer at night (L). For the sake of simplicity, we chose to depict wind, shortwave radiation, and temperature in the boreal, temperate, and tropical panel, respectively. However, of course all of these microclimate variables can be relevant to systems across latitudes. Fig. 4. Typical vertical air temperature profiles inside forests of various canopy depth and density, during nighttime and daytime and for cloudy or clear sky conditions. These typical examples are mainly based on e.g. Roberts et al. (1990), Chen et al. (1999), Ogée et al. (2003), Geiger et al. (2009), and Brower et al. (2011). Fig. 5.Macroclimate change effects on microclimates. Climate warming and climatic extremes affect microclimates and microrefugia by influencing forest composition and structure in boreal (top panel), temperate (middle) and tropical forests (lower panel). We here highlight the processes influencing microclimatic offsets that are most relevant for each biome, although most processes are important in all the biomes. Complex, indirect effects of climate change on microrefugia involve feedback with natural and anthropogenic factors. Fig.6. The four dimensions of improving gridded microclimate products for forests. a) one can turn coarse-grained free-air temperature grids (products such as CHELSA and WorldClim) into coarse-grained forest temperature maps using the offset between weather station and forest temperatures. Next, to increase the temporal (b) and spatial (c) resolution of forest microclimate maps, and to create the full vertical temperature profile (d), one should aim for the integration of in-situ measurements, and mechanistic and statistical models.

  al. 2012; Ashcroft et al. 2012; Hampe & Jump, 2011; Lenoir et al. 2017;

How will macroclimate warming affect forest microclimates?

  How macroclimate warming affects forest microclimate dynamics, and vice versa, remains an open question in global change ecology (Lenoir et al. 2017; De Frenne et al. 2019). For instance, it is unclear whether the magnitude of temperature offset between macroclimate and forest microclimates (De Frenne et al. 2019) will remain stable, increase or decrease over time as macroclimate warms. As discussed previously, the magnitude of the temperature offset between forests and open habitats depends on ambient, macroclimatic conditions: forest offsets of maximum temperatures increase with ambient temperatures (De Frenne et al. 2019; Su et al. 2020; Zhang et al.

  Abatzoglou & Williams, 2016; Allen et al. 2010, Trumbore et al. 2015). Davis et al. (

Acknowledgements

This review resulted from extensive preparations and discussions at a scientific workshop at Ekenäs Herrgård, Sweden, in February 2020, funded by "The Oscar and Lili Lamm Memorial Foundation" obtained by K.H. K.H. also received funding from the Swedish Research Council Formas (grant 2014-530 and 2018-2829) and the Bolin Centre for Climate Research, Stockholm University. P.D.F., P.V. and C.M. received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ERC Starting Grant FORMICA 757833), K.D.P. and S.G. from the Research Foundation Flanders (FWO, grant number ASP 035-19 and project G0H1517N, respectively), J.L. and E.G. from the French National Research Agency (ANR) within the framework of the IMPRINT project "IMpacts des PRocessus mIcroclimatiques sur la redistributioN de la biodiversiTé forestière en contexte de réchauffement du macroclimat" (grant number: ANR-19-CE32-0005-01), D.H.K. from the US National Science Foundation Graduate Research Fellowship Program, J.J.L. from the Research Foundation Flanders (FWO, grants OZ7828 and OZ7792). T.J. was supported by a NERC Independent Research Fellowship (grant number: NE/S01537X/1), J.O by the French National Research Agency (ANR) in the frame of the Cluster of Excellence COTE (project HydroBeech, ANR-10-LABX-45), and A.H. by the EU ERA-NET BiodivERsA (project SPONFOREST, BiodivERsA3-2015-58). MBA received a travel grant from the University of Wollongong.

Supplementary Information Table

 S1. Ranked submitted research questions on microclimates, grouped separately for 'forests' and 'soil temperatures' (more generally, also including other ecosystems), from high to low voting results (scores). Each participant (18 participants in total) of the workshop had to score their top 3 questions, both in the forests and in the soil category. Each participant's number 1 question received 3 points, the second ranked question received 2 points, and the third ranked question received one point. After this voting procedure, all points across all participants were summed per question. With 18 participants, and assuming each participant would vote for the same question as their top question, the maximum score was 54.