Fernando T Maestre
email: ft.maestre@ua.es

Blas M Benito

Miguel Berdugo

Laura Concostrina-Zubiri

Manuel Delgado-Baquerizo

David J Eldridge

Emilio Guirado

Nicolas Gross

Sonia Kéfi

Yoann Le Bagousse-Pinguet

Raúl Ochoa-Hueso

Santiago Soliveres

Sonia Kefi

Biogeography of global drylands

Keywords: macroecology, diversity, spatial pattern, biological soil crusts, woody encroachment, functional traits, plant-soil interactions, plant-plant interactions

II. Geographical patterns of plant diversity are linked to the long history of dryland biomes and their plants III. The functional paradox of drylands IV. Productivity of dryland vegetation: drivers, trends and patterns V. A single size does not fit all: biogeography of vegetation spatial patterns VI. Biogeography of biocrusts, the "living skin" of drylands VII. Environmental conditions and functional traits drive variations in plant-plant and plant-soil interactions VIII. Tradeoffs between traits of encroaching woody plants have a biogeographical basis IX.

I. Introduction

Drylands, areas characterized by Aridity Index (mean annual precipitation/mean annual potential evapotranspiration) values below 0.65, cover ~41% of the terrestrial surface [START_REF] Cherlet | World Atlas of Desertification: Rethinking Land Degradation and Sustainable Land Management[END_REF] and include 35% and 20% of the global diversity and plant diversity hotspots, respectively [START_REF] White | Drylands, people, and ecosystem goods and services: a web-based geospatial analysis[END_REF]; [START_REF] Davies | Conserving Dryland Biodiversity. Nairobi: Global Drylands Initiative[END_REF]. They play key roles regulating the global carbon [START_REF] Ahlström | The dominant role of semi-arid ecosystems in the trend and variability of the land CO2 sink[END_REF], nitrogen [START_REF] Tian | A comprehensive quantification of global nitrous oxide sources and sinks[END_REF] and water [START_REF] Wang | Dryland ecohydrology and climate change: critical issues and technical advances[END_REF] cycles, and are thus fundamental for sustaining life on Earth. Drylands are also crucial to achieve the sustainability of our planet because they host ~38% of the global human population, including most of the fastestgrowing population areas in the world, ~44% of global cropland areas and ~50% of global livestock [START_REF] Davies | Water in drylands: Adapting to scarcity through integrated management[END_REF][START_REF] Cherlet | World Atlas of Desertification: Rethinking Land Degradation and Sustainable Land Management[END_REF]. Drylands are typically divided into hyperarid (AI < 0.05), arid (0.05 < AI < 0.20), semi-arid (0.20 < AI < 0.50) and dry sub-humid (0.50 < AI < 0.65) areas, which occupy 6.6%, 10.6%, 15.2% and 8.7%, respectively, of global land area (Fig. S1).

The study of drylands and their vegetation has a long history. Classical authors such as the Roman naturalist Gaius Plinius Secundus (AD 23/24 -79) or the Greek geographer Strabo (BC 63/64 -AD 24) compiled the natural history and uses of many dryland plants in the Mediterranean Basin (Serrano Luque, 2018). During the XX th Century, detailed studies of the distribution of vegetation were conducted in drylands from multiple continents (e.g., [START_REF] Shreve | The desert vegetation of North America[END_REF][START_REF] Soriano | La vegetación de la República Argentina. IV. Los distritos florísticos de la Provincia Patagónica[END_REF]Keast et al., 1959), and studies of the ecology of dryland vegetation and their interactions with humans, soils, microorganisms and abiotic factors have grown exponentially over the past two decades [START_REF] Greenville | 75 years of dryland science: Trends and gaps in arid ecology literature[END_REF].

Despite the growing interest in drylands, a comprehensive biogeographical synthesis of key organisms, ecosystem attributes and processes characterizing these ecosystems is still lacking. Such a synthesis could identify those factors that shape their current distribution patterns. This is important for accurately forecasting what drylands will look like in the future and for designing more efficient restoration and conservation actions. Here, we combined a literature review with the analyses of global standardized databases and remote sensing products to synthesize our current understanding of the biogeography of dryland vegetation, its spatial and productivity patterns, and the functional traits that shape them at the global scale. Crucial for understanding these patterns are those of plant-plant and plant-soil interactions, which shape community structure and functioning at the local scale but that have scarcely been explored across large geographical scales in drylands (Soliveres et al., 2014;[START_REF] Ochoa-Hueso | Soil fungal abundance and plant functional traits drive fertile island formation in global drylands[END_REF]. We also address the biogeography of biocrusts, another fundamental biotic component of drylands whose biogeography has been little studied (García-Pichel et al., 2013;[START_REF] Maestre | Structure and functioning of dryland ecosystems in a changing world[END_REF], and that of the response and effect traits of woody species that are encroaching in herbaceous communities. This major vegetation change occurring in drylands has important implications for their structure and functioning worldwide [START_REF] Eldridge | Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis[END_REF]. Finally, we briefly discuss important knowledge gaps that need to be addressed to better understand the biogeography of global drylands. We do not, however, provide an in-depth coverage of key topics such as the importance of climatic attributes as drivers of the structure and functioning of dryland ecosystems or their responses to global environmental change drivers because they have been reviewed elsewhere (e.g. [START_REF] Austin | Water pulses and biogeochemical cycles in arid and semiarid ecosystems[END_REF][START_REF] Maestre | Structure and functioning of dryland ecosystems in a changing world[END_REF][START_REF] Collins | A multiscale, hierarchical model of pulse dynamics in arid-land ecosystems[END_REF]. Our review addresses major gaps and key questions, and provides novel syntheses and analyses that both summarize the state-of-the-art in our knowledge and serve as hypotheses to guide future work in dryland biogeography (Fig. 1).

II. Geographical patterns of plant diversity are linked to the long history of dryland biomes and their plants

To understand current plant diversity patterns and the distribution of different plant lineages in drylands, we need to start with their origin. The earliest establishment of arid conditions was asynchronous in different continents. In Africa and South America, dryland ecosystems appeared in the Paleocene (66 -56 Ma) [START_REF] Partridge | The evidence for Cainozoic aridification in southern Africa[END_REF][START_REF] Graham | Late Cretaceous and Cenozoic History of Latin American Vegetation and Terrestrial Environments[END_REF], in central Asia by the end of the Eocene (34 Ma) [START_REF] Sun | Onset of aridification by 34 Ma across the Eocene-Oligocene transition in Central Asia[END_REF], and in Australia in the Middle Miocene (16 to 11.6 Ma) [START_REF] Byrne | Birth of a biome: insights into the assembly and maintenance of the Australian arid zone biota[END_REF]. The Namib, arguably the oldest desert in the world, has experienced continuous arid conditions since at least the beginning of the Late Cenozoic (33.9 Ma, [START_REF] Lancaster | Late Cenozoic fluvial deposits of the Tsondab Valley, central Namib Desert[END_REF], whereas the southwestern deserts of the USA, or the Atacama Desert and the Caatinga in South America, are more recent (De [START_REF] Oliveira | Late Pleistocene/Holocene climatic and vegetational history of the Brazilian caatinga: the fossil dunes of the middle São Francisco River[END_REF][START_REF] Thompson | Biomes of western North America at 18,000, 6000 and 0 14C yr bp reconstructed from pollen and packrat midden data[END_REF]. In Central Asia, the semi-arid Loess Plateau began to appear around 8 Ma likely due to global precipitation changes triggered by the second phase of the uplift of the Tibetan Plateau, which had a major role in the expansion of C4 grasses [START_REF] Pagani | Late Miocene atmospheric CO2 concentrations and the expansion of C4 grasses[END_REF]. During the Last Glacial Period, Central Asia went through a cold arid stage that allowed the spread of steppes dominated by species of the Asteraceae (Artemisia spp.) and Poaceae families [START_REF] Lioubimtseva | Climate change in arid environments: revisiting the past to understand the future[END_REF]. The semi-arid climate became widespread in Australia during the Pliocene (5.3-1.8 Ma), featuring open woodlands, arid shrublands, and grasslands [START_REF] Martin | Cenozoic climatic change and the development of the arid vegetation in Australia[END_REF]. Later, during the glacial-interglacial cycles of the Quaternary, glacial periods featured a cool-arid climate, while interglacials were warm and slightly wetter. The Last Glacial Period brought an extreme arid climate featuring large areas of mobile dunes, now stabilized by woodlands, in western Australia between 25 and 12 ka BP [START_REF] Kershaw | Palynological evidence for Quaternary vegetation and environments of mainland southeastern Australia[END_REF].

Molecular clocks have confirmed that the long history of global drylands is coupled with the history of its major plant lineages, and that major dryland clades diversified more or less in synchrony during the interval between the Late Miocene (11.63 -5.33 Ma) and the Early Pliocene (5.3 to 3.6 Ma). This is the case of the Aizoaceae family inhabiting the Succulent Karoo in South Africa and Namibia, the Agavaceae and Cactaceae now living in North American deserts, and members of the Camphorosmeae family in Australia, among many others [START_REF] Arakaki | Contemporaneous and recent radiations of the world's major succulent plant lineages[END_REF][START_REF] Wu | Insights into the historical assembly of global dryland floras: the diversification of Zygophyllaceae[END_REF]. However, a striking exception to this pattern is the long-lived phreatophyte Welwitschia mirabilis. This monotypic taxon differentiated from other genera of the division Gnetopsida (Gentum and Ephedra) before the opening of the Equatorial Atlantic Gateway between Africa and South America during the Early Cretaceous (145 -100 Ma). Today, the remainder of a past larger distribution is restricted to the Kaokoveld Desert between Namibia and Angola [START_REF] Jacobson | A first assessment of genetic variation in Welwitschia mirabilis Hook[END_REF].

The long history of dryland ecosystems across all continents, and their role as the origin of many unique plant lineages, makes them an important host to a diverse flora featuring important diversity hotspots in Southern Africa, the Mediterranean basin, Western and Central Asia, North and South America, and Oceania (Fig. 2, Table S1).

The tropical dry forests of southern Africa (Miombo and Mopane woodlands) host a remarkable plant diversity [START_REF] Frost | The Miombo in transition: woodlands and welfare in Africa[END_REF][START_REF] Maquia | Diversification of African tree legumes in Miombo-Mopane woodlands[END_REF]. Another important center of plant diversification in southern Africa is the Cape Floristic Region, formed by sclerophyll shrublands and heathlands (also named fynbos) hosting ~6,000 endemic species [START_REF] Goldblatt | Cape Plants. A Conspectus of the Cape Flora of South Africa[END_REF].

Finally, among the most idiosyncratic plant diversity hotspots in drylands worldwide is the Succulent Karoo, a coastal band in Namibia and South Africa with ~5,000 plant species, of which 40% are endemic (Table S1). About 1,750 of these species are dwarf succulents belonging to the Aizoaceae family, Crassulaceae, and annual plants of the Asteraceae family (Hilton-Taylor, 1996).

Hyperarid areas of northern Africa are less diverse, though areas such as the Algerian Sahara are inhabited by at least 1200 plant species [START_REF] Ozenda | Flore et Vegetation du Sahara[END_REF].

The Mediterranean drylands of southern Spain and northern Morocco and Algeria are also among the richest drylands of the world [START_REF] Médail | Biodiversity hotspots in the Mediterranean Basin: Setting global conservation priorities[END_REF], and share many sclerophyllous trees (e.g., Quercus suber, Q. Ilex, Olea europaea, and Pinus halepensis) accompanied by understory shrubs dominated by species like Cistus spp., Rosmarinus officinalis and Genista spp.

The Irano-Anatolian biogeographic region, featuring steppes dominated by the perennial Prosopis farcta (FAO, 2019), is the center of taxonomic diversification of annual legumes, and particularly of the genus Astragalus spp., with around 1,500 species [START_REF] Ehrman | Reproductive patterns in annual legume species on an aridity gradient[END_REF]. This region also had an important role in the diversification of the families of halophytic succulents such as Chenopodioidea and Zygophyllaceae [START_REF] Wu | Insights into the historical assembly of global dryland floras: the diversification of Zygophyllaceae[END_REF].

The dryland belt of Northern Eurasia, the largest continuous set of drylands in the world, encompasses from the Great Hungarian Plain (Hungary, Serbia, Croatia, and Romania) to the Manchurian mixed forests in northeastern China [START_REF] Groisman | Dryland belt of Northern Eurasia: contemporary environmental changes and their consequences[END_REF]. Its hyperarid areas are the contiguous Taklimakan Desert, Qaidam Basin semi-desert, and Alashan Plateau semi-desert in northwestern China. Their shifting sand dunes are devoid of vegetation, but more stable areas are colonized by the small halophytic tree Haloxylon ammodendron and the perennial shrub Reaumuria songarica [START_REF] Gong | High Air Humidity Causes Atmospheric Water Absorption via Assimilating Branches in the Deep-Rooted Tree Haloxylon ammodendron in an Arid Desert Region of Northwest China[END_REF]. The permanent Tarim River crosses the Taklimakan, creating the conditions for well-developed riparian forests of Populus euphratica and P. pruinosa [START_REF] Thomas | Growth and water relations of riparian poplar forests under pressure in Central Asia's Tarim River Basin[END_REF]. The dryland belt of Northern Eurasia also includes important arid and semi-arid areas. For example, the Kazakh semi-desert is a large Artemisia spp. shrubland that limits in the north with the Kazakh steppes, rich in Stipa spp. and Festuca spp. The Central Asian and Eastern Gobi deserts are, respectively, xeric shrublands dominated by Haloxylon persicum and H. ammodendron, and extensive steppes and shrublands dominated by the endemics Caragana bungei and C. leucocephala, Potaninia mongolica and Nitraria sibirica [START_REF] Thomas | Ecological basis for a sustainable management of the indigenous vegetation in a Central-Asian desert: presentation and first results[END_REF] The Qinghai-Tibetan Plateau (4000 m.a.s.l) has been identified as a center of diversification of genera such as Pedicularis spp., Rhododendron spp., and Primula spp., among many others (see [START_REF] Wen | Evolutionary diversifications of plants on the Qinghai-Tibetan Plateau[END_REF] for further details).

North America holds a vast array of dryland ecosystems, from the Sonoran Desert to the northernmost drylands of the world, the conifer taiga forests of Canada. The family Cactaceae, with Carnegiea gigantea as its most conspicuous representative, reaches its maximum levels of diversity in the southern United States and Mexico [START_REF] Shreve | The desert vegetation of North America[END_REF]. The Colorado Plateau and the Canyonlands region is dominated by Pinus ponderosa and P. edulis forests, and by Juniperus spp.

In open areas between the trees, shrubs like Artemisia tridentata and Cercocarpus montanus, an important number of Astragalus spp. and cacti such as Echinocereus spp. find their place to thrive [START_REF] Shreve | The desert vegetation of North America[END_REF].

South America has a large surface of important dry forests mainly located in the Gran Chaco, the Maranhão Babaçu, and the Caatinga, the driest forest of South America that features a xeric shrubland with succulents and thorny trees with a high level of endemism [START_REF] Fernandes | An updated plant checklist of the Brazilian Caatinga seasonally dry forests and woodlands reveals high species richness and endemism[END_REF]. The Caatinga is also an important center of diversification of the Cactaceae family, along

III. The functional paradox of drylands

The morphological, physiological and phenological characteristics of species -functional traitsrelate to how they acquire, conserve and release resources [START_REF] Díaz | The global spectrum of plant form and function[END_REF]. They are increasingly used to explore how species assemble within communities and respond to their environment, and how changes in communities feedback on ecosystem functioning [START_REF] Suding | Scaling environmental change through the community-level: a trait-based response-and-effect framework for plants[END_REF]. Strong environmental constraints such as high aridity conditions, scarce and unpredictable rainfall, and low soil nutrient contents should reduce plant functional diversity, as predicted by the environmental filtering theory [START_REF] Keddy | Assembly and response rules: two goals for predictive community ecology[END_REF]. However, drylands contradict these theoretical predictions and exhibit a strikingly high diversity of plant forms and functions (Notes S1, Fig. S2), perhaps precisely because of plants´ response to such unpredictable conditions. We used data on leaf morphology and physiology [START_REF] Maire | Global effects of soil and climate on leaf photosynthetic traits and rates[END_REF][START_REF] Wright | Global climatic drivers of leaf size[END_REF] to evaluate the functional diversity of drylands, and to quantify their overlap with that of remaining terrestrial ecosystems (Fig. 3). The dataset used includes trait data for 1,502 species distributed worldwide, and offers a relatively well-balanced representation of dryland species compared with other trait databases (e.g., [START_REF] Kattge | TRY plant trait database-enhanced coverage and open access[END_REF]. We found that leaf functional diversity from drylands largely overlaps with that observed across the rest of terrestrial ecosystems. Moreover, the variance in dryland trait distributions is as large, and sometimes larger, than that observed across other terrestrial ecosystems. These results illustrate what we define as the functional paradox of drylands, i.e. the higher than expected functional diversity in dryland plants compared to those from less environmentally-constrained environments. They contrast with what has been recently observed in other harsh biomes such as the cold tundra, wherein species occupy a constrained subset of the global functional trait space [START_REF] Thomas | Global plant trait relationships extend to the climatic extremes of the tundra biome[END_REF]. The high variance observed in leaf size and leaf economic traits across drylands reflects the remarkable phenotypic diversity of their plants (Figs. 1 andS2), which allows them to cope with the environmental constraints of these areas. For instance, prostrate shrub species characterized by small leaves often co-occur with longleaved tussock grass species and large trees (e.g. [START_REF] Frost | The Miombo in transition: woodlands and welfare in Africa[END_REF]. Also, stress-tolerant species often coexist with species with succulent leaves, and with stress-avoidant species with thin and summerdeciduous leaves, which may explain the wide variety of leaf forms and functions observed in drylands (Noy-Meir, 1973; [START_REF] Gross | Uncovering multiscale effects of aridity and biotic interactions on the functional structure of Mediterranean shrublands[END_REF]. Furthermore, species characterized by small leaves, with low specific leaf area and high photosynthetic capacity per unit of leaf surface are overrepresented in drylands [START_REF] Noy-Meir | Desert ecosystems: environment and producers[END_REF]. This likely helps them to cope with water shortage (Notes S1). It is also remarkable that drylands exhibit leaf-trait distributions characterized by lower kurtosis than communities from the rest of the world (Fig. 3). In other words, drylands host a high plant functional diversity of plant species that are more evenly represented than in other biomes.

The high functional diversity of drylands observed at the global scale is also evident at the local scale. A maximization of local plant functional diversity in drylands has been recently documented [START_REF] Gross | Functional trait diversity maximizes ecosystem multifunctionality[END_REF], even under prevailing environmental filtering (Le Bagousse-Pinguet et al., 2017). Such a pattern likely results from co-occurring species exhibiting distinct strategies to cope with the environmental conditions found in these areas (Notes S1), from spatiotemporal storage effects (Noy-Meir, 1973) and from positive and intransitive interactions (e.g., [START_REF] Butterfield | Regeneration niche differentiates functional strategies of desert woody plant species[END_REF][START_REF] Saiz | Intransitivity increases plant functional diversity by limiting dominance in drylands worldwide[END_REF], discussed in section VII below.

IV. Productivity of dryland vegetation: drivers, trends and patterns

The high taxonomic and functional plant diversity observed in drylands plays a major role in maintaining the functioning of these ecosystems and the stability of their productivity (García- Vegetation productivity in drylands not only responds to biotic attributes, but also to abiotic ones. Indeed, productivity patterns closely match the aridity gradients found naturally across global drylands (Figs. 3a andS1). The mean (standard deviation) NDVI of dryland vegetation during the period 2001-2019 was 0.06 (0.03), 0.09 (0.06), 0.18 (0.1) and 0.26 (0.11) in hyper-arid, arid, semiarid and dry-sub humid environments, respectively (Fig. 4a). However, there is substantial variation within aridity classes driven by both the biotic attributes mentioned above (plant richness The most abundant land cover types in drylands are grasslands, followed by areas with less than 10% vegetation cover and shrublands (Fig. 4b). Savannas and forests, including deciduous, evergreen and mixed forests, occupy ~11% and <5% of global dryland area, respectively. It must be noted, however, that the remote sensing products typically used to quantify land cover, such as MODIS [START_REF] Friedl | MCD12Q1 MODIS/Terra+Aqua Land Cover Type Yearly L3 Global 500m SIN Grid V006[END_REF], have insufficient resolution to adequately quantify discontinuous forest stands such as those found in many drylands. Recent global estimates using high resolution imagery indicate that 1327 million hectares of drylands had more than 10% treecover, and 1079 million hectares comprised forest in 2015 [START_REF] Bastin | The extent of forest in dryland biomes[END_REF]. A major feature of land cover in drylands, the sparse, discontinuous vegetation cover with isolated trees and shrubs (Fig. S2), is also not captured properly by most remote sensing data currently available. However, this is beginning to change as high-resolution remote sensing products become more widely available. For example, Brandt et al. (2020) found ~1.8 billion individual trees (crown size > 3 m 2 ) over 1.3 million km 2 in drylands of West Africa, with canopy cover ranging from 0.1% (0.7 trees per hectare) in hyper-arid areas to 13.3% (47 trees•hectare -1 ) in dry sub-humid areas. Although previously ignored, isolated trees play a key role in drylands by capturing and re-distributing resources, providing habitat and refugia for fauna and flora, and producing goods and services crucial for local human populations, including timber, food and forage (FAO, 2019).

From 1982 to 2009, the global increase in vegetation productivity observed [START_REF] Zhu | Greening of the Earth and its drivers[END_REF], is also apparent in many drylands. An updated analysis (Fig. S3; Notes S2) indicates that 26 million km 2 show positive trends in vegetation productivity (greening) during the 2001-2019 period. Greening increased with reductions in aridity across global drylands (e.g., 66% of hyperarid areas experienced greening vs. 84% of dry sub-humid areas; Fig. S3). A recent analysis of greening trends in global drylands [START_REF] Burrell | Anthropogenic climate change has driven over 5 million km 2 of drylands towards desertification[END_REF] indicates that their major drivers were increases in soil moisture and water use efficiency associated with a CO2 fertilization effect, followed by land use and climate change. Climate variability and land use were, however, major greening drivers in the Sahel, India, China and Australia [START_REF] Burrell | Anthropogenic climate change has driven over 5 million km 2 of drylands towards desertification[END_REF]. Despite the overall greening trend observed, a total of 6 million km 2 of drylands showed significant negative trends in vegetation productivity (browning) between 2001 and 2019 (Fig. S3). Browning varied also with the degree of aridity, and ranged from 34% in hyper-arid areas to 16% in dry sub-humid areas. A recent analysis of browning trends in global drylands [START_REF] Burrell | Anthropogenic climate change has driven over 5 million km 2 of drylands towards desertification[END_REF] indicates that land use was the most important browning driver, followed by climate change and climate variability.

V. A single size does not fit all: biogeography of vegetation spatial patterns

The relatively low productivity of dryland vegetation prevents it from covering all the soil surface.

Instead, drylands are spatially heterogeneous environments, wherein vegetation tends to form islands, or "patches", surrounded by bare soil [START_REF] Aguiar | Patch structure, dynamics and implications for the functioning of arid ecosystems[END_REF][START_REF] Tongway | Banded Vegetation Patterning in Arid and Semiarid Environments: Ecological Processes and Consequences for Management[END_REF]. This The spatial patterns of dryland vegetation can be broadly classified into two major types (regular and irregular), which are not evenly distributed across global drylands (Fig. 5). Regular patterns occur when a certain spatial configuration of plants and bare soil is periodically repeated through the landscape (Fig. S4). They tend to resemble patterns observed on animal coats, such as tiger stripes or "brousse tigree" (see Tongway et In the same way as for plant productivity, aridity is the most important predictor of the occurrence of regular vegetation patterns, followed by mean temperature of the wettest quarter [START_REF] Deblauwe | The global biogeography of semi-arid periodic vegetation patterns[END_REF]. High (> 24ºC) or low to medium (2-6ºC) temperature seasonality also favored the formation of regular spatial patterns. Other studies have shown that the shape of regular patterns (bands, stripes, gaps, spots) is driven by the combination of rainfall and the slope of the terrain [START_REF] Deblauwe | Determinants and dynamics of banded vegetation pattern migration in arid climates[END_REF]. Gaps are more likely to occur in drylands where annual rainfall is higher (~500 mm per year), followed by labyrinths (400-450 mm) and spots (<400 mm). Bands become increasingly more frequent as slope increases [START_REF] Tongway | Banded Vegetation Patterning in Arid and Semiarid Environments: Ecological Processes and Consequences for Management[END_REF].

A biogeographical analysis of dryland vegetation patterns (Berdugo et al. 2019b) indicates that they tend to shift from irregular to regular as aridity increases, coinciding with the collapse of positive plant-plant interactions under the most arid conditions (Aridity Index < 0.

VI. Biogeography of biocrusts, the "living skin" of drylands

In addition to vascular plants, the functioning of dryland ecosystems worldwide is largely determined by the presence, cover and composition of biological soil crusts (biocrusts), diverse communities composed of lichens, bryophytes and other soil microorganisms (such as cyanobacteria, algae, and fungi) coexisting in the uppermost soil layers [START_REF] Weber | Biological Soil Crusts: An Organizing Principle in Drylands[END_REF]. They are typically found in plant interspaces and under plant canopies that are not covered by litter (Fig.

VII. Environmental conditions and functional traits drive variations in plant-plant and plantsoil interactions

The interactions between different plant species, and between plants and the soils beneath them, are not only fundamental drivers of vegetation patterns (section V) but can also shape biogeographical patterns (reviewed in [START_REF] Godsoe | Integrating Biogeography with Contemporary Niche Theory[END_REF]. Plant-plant and plant-soil interactions are involved in macro-ecological processes, including range expansions (e.g., [START_REF] Zhang | Soil-microorganism-mediated invasional meltdown in plants[END_REF], or plant evolution (e.g., [START_REF] Thorpe | Interactions among plants and evolution[END_REF] in many biomes worldwide. However, no previous study has specifically evaluated how plant-plant or plant-soil interactions (the latter including soil microbes and soil physico-chemical attributes) shape the biogeography of dryland ecosystems.

Plant-plant and plant-soil interactions are sensitive to climate, soil type and land use (e.g., Mazía Latitudinal gradients in biodiversity are less apparent in drylands than in other ecosystems (e.g., [START_REF] Ulrich | Functional diversity enhances the resistance of ecosystem multifunctionality to aridity in M editerranean drylands[END_REF]. Similarly, plant-plant interactions do not show clear relationships with latitude in drylands (Fig. 7). For example, although the positive effects of trees on grass biomass peak near the tropics, this pattern is overridden by prevailing conditions of aridity or tree functional traits [START_REF] Mazía | The sign and magnitude of tree-grass interaction along a global environmental gradient[END_REF]. Indeed, positive plant-plant interactions are stronger and more prevalent in arid and semi-arid environments than in lower latitude tropical biomes (Gómez-Aparicio, 2009).

Latitudinal patterns are not evident in plant-soil interactions either (Ochoa-Hueso et al., 2018; but see [START_REF] Delavaux | Mycorrhizal fungi influence global plant biogeography[END_REF][START_REF] Soriano | La vegetación de la República Argentina. IV. Los distritos florísticos de la Provincia Patagónica[END_REF]. Instead of following latitudinal gradients, macroecological patterns in plant-plant and plant-soil interactions are largely driven by variation in environmental conditions and their interaction with the functional traits of the interacting plant species. However, the interactions between vegetation and environment as drivers of plant-plant interactions may themselves exhibit biogeographical patterns, as shown by the large shared variance explained by vegetation, environment and geography, and the large importance of latitude and longitude as predictors of these interactions across global drylands (Fig. 7).

At the core of plant-plant and plant-soil interactions in drylands is the "fertility island" phenomenon, which refers to the higher contents in organic matter and available nutrients, coupled For example, [START_REF] Berdugo | Global ecosystem thresholds driven by aridity[END_REF] identified an important aridity threshold associated with a transition from semiarid to arid ecosystems (Aridity Index = 0.2), wherein small increases in aridity dramatically increased the proportion of fungal pathogens and reduced that of plant fungal symbionts. This could partly explain why the fertility island effect, tightly linked to these fungal communities, is less pronounced under arid than under semi-arid conditions (Ochoa-Hueso et al., 2018). These findings also suggest that climate change could shift the balance between positive and negative plant-soil interactions, negatively impacting the fitness of plant communities in drylands. Even without further aridification, drylands may have generally weaker or more negative plant-soil interactions than more mesic environments. This is due to a greater proportion of plant antagonists, compared with decomposers or symbionts, in drylands than in other terrestrial ecosystems (Fig. S8, Notes S3), or to the lower abundance of soil microorganisms observed as aridity increases [START_REF] Maestre | Increasing aridity reduces soil microbial diversity and abundance in global drylands[END_REF]. Aridity also accounts for a substantial proportion of the variation in the effects of plant-plant interactions on the structure and composition of drylands (~50% for biomass [START_REF] Mazía | The sign and magnitude of tree-grass interaction along a global environmental gradient[END_REF] ~29% for biodiversity [Soliveres & Maestre, 2014]).

Considered collectively, existing research suggests that the effects of plant-plant interactions tend to become more positive for biomass and for biodiversity in tree-or annual-dominated ecosystems when aridity increases [START_REF] Mazía | The sign and magnitude of tree-grass interaction along a global environmental gradient[END_REF][START_REF] Rey | Facilitation contributes to Mediterranean woody plant diversity but does not shape the diversity-productivity relationship along aridity gradients[END_REF]Berdugo et al., 2019a). Therefore, in these cases, and contrary to expectations for plant-soil interactions, plant-plant interactions should become more positive, and perhaps more important in shaping dryland biodiversity and productivity patterns, under future climatic scenarios.

The effects of plant-plant interactions on biodiversity across aridity gradients are far less consistent in grass-or shrub-dominated ecosystems than in savannas or annual-dominated communities (Soliveres & Maestre, 2014;[START_REF] Rey | Facilitation contributes to Mediterranean woody plant diversity but does not shape the diversity-productivity relationship along aridity gradients[END_REF]. In these cases, it is more likely that the traits of the interacting species play a greater role in modulating the outcome of plant-plant interactions than environmental conditions per se (Soliveres et al., 2014). Nurse and beneficiary traits are a crucial driver of the outcome of plant-plant interactions in drylands (Gómez-Aparicio, 2009; [START_REF] Butterfield | Regeneration niche differentiates functional strategies of desert woody plant species[END_REF][START_REF] Hayek | Differential effects of contrasting phenotypes of a foundation legume shrub drive plant-plant interactions in a Mediterranean mountain[END_REF][START_REF] Mazía | The sign and magnitude of tree-grass interaction along a global environmental gradient[END_REF]. Existing evidence suggests that woody species are generally better nurses than grasses (Gómez-Aparicio, 2009; Soliveres et al., 2014), particularly if they are N-fixers (e.g., [START_REF] Mazía | The sign and magnitude of tree-grass interaction along a global environmental gradient[END_REF] or have open and large canopies (Al [START_REF] Hayek | Differential effects of contrasting phenotypes of a foundation legume shrub drive plant-plant interactions in a Mediterranean mountain[END_REF]. These traits are also those behind more pronounced fertility island effects and can alter the abundance of fungi and bacteria beneath plant canopies (Ochoa-Hueso et al., 2018). Tall woody species are more efficient at capturing airborne particles [START_REF] Gonzales | Ecohydrological implications of aeolian sediment trapping by sparse vegetation in drylands[END_REF] and redistribute nutrients and water via their highly developed and deep root systems [START_REF] Prieto | Water release through plant roots: new insights into its consequences at the plant and ecosystem level[END_REF]. Such features of root systems are also important determinants of the association of plants with microbial symbionts such as mycorrhizae [START_REF] Schenk | The global biogeography of roots[END_REF]. This could explain why woody plants are better facilitators than grasses. In addition, population growth rates in soil microbes increase more strongly after rainfall pulses in tree-than in grassdominated ecosystems [START_REF] Fierer | Influence of drying-rewetting frequency on soil bacterial community structure[END_REF], which may cause a higher sensitivity of plant-microbe interactions to changes in rainfall amount and frequency expected under future climate scenarios in grasslands than woodlands. Whether or how plant functional traits drive plant-microbe interactions in drylands, and how they interact with aridity, is still poorly understood, mainly because of the short duration and highly species-specific responses often reported in the few existing studies (Van der [START_REF] Putten Wh Van Der | Where, when and how plant-soil feedback matters in a changing world[END_REF]. Ding and Eldridge, 2019) share common traits, so a trait-based assessment of their biogeography can help us to understand their global distribution and impacts on dryland ecosystems. We did so by combining global databases of woody encroachment [START_REF] Eldridge | Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis[END_REF], woody plant removal following encroachment [START_REF] Ding | Multiple trade-offs regulate the effects of woody plant removal on biodiversity and ecosystem functions in global rangelands[END_REF] and woody plant functional traits [START_REF] Ding | Multiple trade-offs regulate the effects of woody plant removal on biodiversity and ecosystem functions in global rangelands[END_REF]. These combined datasets (315 independent studies of 100 species) included traits that are related to the effects of woody plants on ecosystem functioning (i.e. how they affect functional outcomes such as nutrient cycling, hydrological function or habitat quality). For the purposes of our analyses, we separated them into traits linked to their morphology (structural traits) and to their physiology and phenology (functional traits). Our structural traits related to size (plant height), canopy shape (v-shaped to round), root type (mixed to surface roots) and foliage contact with the soil surface. The five functional traits related to whether plants were deciduous, allelopathic, resprouting, palatable, or nitrogen fixers. These traits have previously been ranked according to whether they increase or reduce ecosystem functions [START_REF] Ding | Multiple trade-offs regulate the effects of woody plant removal on biodiversity and ecosystem functions in global rangelands[END_REF]). After assigning a numerical value to each of these traits, these data were standardized such that a higher value corresponded to a greater function (see [START_REF] Ding | Multiple trade-offs regulate the effects of woody plant removal on biodiversity and ecosystem functions in global rangelands[END_REF]

VIII. Tradeoffs between traits of encroaching woody plants have a biogeographical basis

for details).

Encroaching woody plants from North American and African drylands were significantly taller (7.8 -9.9 m) than those from South American, Asian or Australian drylands (1.3 -1.5 m; Fig. S9). Encroaching woody plants from Africa were more likely to have tap roots, foliage that touches the ground, and fix nitrogen. Woody plants encroaching in Australia were more likely to be palatable, evergreen, tap-rooted, resprouting species, whereas encroaching species from North America were less likely to resprout or fix nitrogen. Encroaching species from Asia were more likely to have tap roots, and those from Africa more likely to be v-shaped than expected by chance.

Finally, species from Europe were more likely to have fibrous roots but less likely to be allelopathic.

Average values of structural and functional traits at the continental scale reveal that sites encroached by woody plants with high value of functional traits tend to have low values of structural traits, and vice versa (Fig. 8). For example, African woody plants had high values of function but relatively low structure, whereas North America exhibited the opposite, with generally higher structural values but low values of functional traits. Europe and to a lesser extent Australia and Asia, had average values of structural and functional plant traits.

Our synthesis shows the tradeoffs between structural and functional trait values of woody plants that encroach in drylands. It also demonstrates that the idiosyncratic portfolio of traits that confer functional outcomes have a biogeographical basis. For example, the larger than expected number of nitrogen-fixing shrubs from Australia may reflect a competitive advantage of these species in Australia's soils, which have low nitrogen contents compared to other drylands [START_REF] Eldridge | Australian dryland soils are acidic and nutrient-depleted, and have unique microbial communities compared with other drylands[END_REF]. Similarly, taller shrubs in Africa may be an evolutionary advantage under higher levels of vertebrate browsing, compared with continents such as South America or Australia, which have long been dominated by vertebrate herbivores such as camelids or macropods, respectively (Dantas & Pausas, 2013).

IX. Concluding remarks and future research directions

Drylands host a diversity of plants that capture a surprisingly large portion of the variation in foliar traits observed globally. This extraordinary functional diversity opens up relevant questions for future research, including: i) Could the high-functional diversity of drylands confer them a greater resistance or resilience to climate change compared with other biomes?, ii) How does plant functional diversity correlate with soil microbial diversity and soil-borne pathogens?, and iii) Does the phenotypic variability expressed at the individual level (intraspecific trait variability) play an important role for the functioning of drylands at the global scale? To address these questions, however, we need to better characterize the functional traits of dryland plants, which are largely underrepresented in global databases [START_REF] Kattge | TRY plant trait database-enhanced coverage and open access[END_REF][START_REF] Thomas | Global plant trait relationships extend to the climatic extremes of the tundra biome[END_REF] Such technological developments offer great promise to better characterize vegetation patterns in drylands, and to further advance our understanding of their functioning and productivity. Our knowledge of the biogeography of vegetation patterns in drylands, occurring mostly from studies in Africa, North America and Australia, is more advanced for regular than irregular patterns. The latter, however, comprise the vast majority of vegetation spatial patterns across global drylands (Fig. 5), and are the next frontier for studying their biogeography. There is also a paucity of experiments about mechanisms of vegetation pattern formation in drylands, a gap that should be addressed by future studies. Understanding the uncertainty about whether vegetation greening observed in recent decades will be maintained under future climates is a priority for future research. This uncertainty is due to contrasting effects of greater water efficiency, through elevated CO2 (Walker et al., 2020) on vegetation productivity, which will likely be offset by negative effects due to increased evapotranspiration and reduced soil moisture [START_REF] Huang | Dryland climate change: Recent progress and challenges[END_REF][START_REF] Soong | CMIP5 Models Predict Rapid and deep soil warming over the 21st century[END_REF].

There is also considerable uncertainty in our projections of future aridity, depending on whether the effects of elevated CO2 on vegetation are considered or not (see [START_REF] Huang | Dryland climate change: Recent progress and challenges[END_REF][START_REF] Lian | Multifaceted characteristics of dryland aridity changes in a warming world[END_REF]. Understanding the impacts of future aridity conditions on vegetation productivity is essential, as productivity has been found to decline abruptly in drylands worldwide when aridity index exceeds values of 0.46, leading to multiple cascading, non-linear effects on key structural and functional ecosystem attributes (see Berdugo et al., 2020 for details). Furthermore, it has been suggested that total dryland gross primary production will increase by 123% relative to the 2000-2014 baseline, largely due to the expansion of drylands into formerly more productive ecosystems by 2100 [START_REF] Yao | Accelerated dryland expansion regulates future variability in dryland gross primary production[END_REF]. However, forecasted changes in primary production also show large regional variations and important declines across drylands worldwide [START_REF] Yao | Accelerated dryland expansion regulates future variability in dryland gross primary production[END_REF]. How elevated CO2 and other factors may modulate future aridity conditions and their impacts on ecosystem productivity in drylands is thus a key, yet unsolved, question with major implications for the global carbon cycle and climate change mitigation actions. The use of ecosystem models parameterized across a wide variety of drylands, and the inclusion of biocrust and soil microbial components into them, could provide important insights into these important questions.

Despite impressive advances in biocrust research over the past few decades, our knowledge of biocrust biogeography is still limited, particularly in regions such as Central Eurasia, North Africa, Mexico and South America. Similarly, despite the increasingly available information on ecological and trait information for mosses and liverworts at regional scales (e.g. Bottom-up community approaches may also be successful for dryland restoration. For example, inoculating the soil with fungal species that create densely connected networks of hyphae may help plants to tolerate water stress and capture scarcely available soil nutrients [START_REF] Collins | Pulse dynamics and microbial processes in aridland ecosystems[END_REF]. Thus, studying plant-plant and plant-soil interactions in drylands will provide us with information that is relevant to restoration goals using nature-based solutions.

Despite our fascination with drylands and the renewed research efforts over the past few decades, we still have a relatively poor understanding of their biogeography at the global scale compared with other ecosystems such as tropical forests (e.g. [START_REF] Primack | Tropical Rain Forests: An Ecological and Biogeographical Comparison[END_REF]). However, there is a growing interest in drylands, as evidenced by a burgeoning dryland research community, with its increasing network of coordinated dryland research studies across the globe (Table S2).

Given the extent of drylands, and their contrasting evolutionary histories, environmental conditions and habitat types, their responses to environmental changes or biotic factors can only be properly understood through systematic and coordinated research efforts conducted worldwide. Such global collaborative efforts have proven fruitful, and have provided key insights into the biogeography and functioning of dryland vegetation and associated ecosystem processes, and how they respond to major climate change drivers (e.g., [START_REF] Maestre | Plant species richness and ecosystem multifunctionality in global drylands[END_REF][START_REF] Ulrich | Functional diversity enhances the resistance of ecosystem multifunctionality to aridity in M editerranean drylands[END_REF][START_REF] Gross | Functional trait diversity maximizes ecosystem multifunctionality[END_REF]Berdugo et al., 2019b). Networks of scientists working together are now in a position to test experimentally some of the major paradigms related to the biogeography and functioning of drylands under different global environmental change scenarios, to collect much-needed field data (e.g. plant functional traits and biocrusts) and to set up in situ temporal monitoring programs of vegetation and ecosystem processes across global drylands. These are major challenges for such networks and a priority theme for future research. We hope that this review will serve to stimulate future research on, and discussion of, dryland biogeography, so that we all have a better understanding of the fate of drylands, one of Earth's most important biomes, as we move to a warmer and more unpredictable world.

  with the southwestern Andes (Ortega-Baes & Godínez-Alvarez, 2006). Australia features 28 arid ecoregions inhabited by 23,436 plant species, ranging from the 8,625 species of the temperate forests of Southeast Australia, to the 650 of the Hampton mallee and woodlands, located in the coast of Southern Australia (GBIF.org 2020; Dinerstein et al., 2017). The broadleaved forests of Oceania include 803 species of the Eucalypteae tribe (genera Angophora, Corymbia, and Eucalyptus) in wetter areas, and 994 species of Acacia in drier areas (GBIF.org 2020). The quintessential Hummock Grasslands are located in the arid and hyperarid regions of the Australian outback and are typified by Triodia spp., which occupy a vast proportion of the continent (Keast et al., 1959). The Tussock grasslands of Northern Australia are rich in endemic tufted grasses, such as Dichanthium sericeum and Astrebla spp. (Keast et al., 1959).

Palacios

  et al., 2018; Le Bagousse-Pinguet et al., 2019). The productivity of vegetation, which provides essential ecosystem services, including food production, soil fertility and climate regulation (Ahlström et al., 2015; Maestre et al., 2016; Cherlet et al., 2018), is typically measured across large geographical scales using satellite measurements such as the normalized difference vegetation index (NDVI; Smith et al., 2019). While in areas with low vegetation canopy cover, such as drylands, the soil background can significantly influence NDVI estimates (Smith et al., 2019), this index shows good correlations with vegetation productivity measured in situ across drylands (e.g., Paruelo et al., 1997; Tian et al., 2017).

  and functional traits) and by other factors (e.g., topography, climatic variability, herbivory, soil type or land use; Collins et al., 2014; Maestre et al., 2016; Venter et al., 2018; Burrell et al., 2020).

  Multiple drivers often act together to amplify browning trends, as found in areas of Central Asia and the semi-arid Caatinga of Brazil (climate change and land use) or in South America (climate change and variability)[START_REF] Burrell | Anthropogenic climate change has driven over 5 million km 2 of drylands towards desertification[END_REF].

  discontinuous vegetation is characterized by multiple spatial configurations, including fairy circles and irregular, regular, spotted, stripped or labyrinth patterns (Fig. S4; Deblauwe et al., 2008; Berdugo et al., 2017, 2019b; Getzin et al., 2019). These spatial patterns have fascinated ecologists, geographers, mathematicians and physicists alike since their discovery after the second world war (see Tongway et al., 2001 and references therein). They have also been associated with ecosystem functioning (Pringle et al., 2010; Berdugo et al., 2017), and have been proposed as potential early warning signals for the onset of land degradation and desertification[START_REF] Rietkerk | Self-organized patchiness and catastrophic shifts in ecosystems[END_REF][START_REF] Kéfi | Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems[END_REF] in drylands. Thus, their study is not only relevant to our understanding of the structure and functioning of dryland ecosystems, but also for the monitoring of degradation processes affecting them.

  al., 2001 and references therein), and are characterized by a typical patch size[START_REF] Kéfi | Bistability and regular spatial patterns in arid ecosystems[END_REF]. Fairy circles, which manifest as an arrangement of bare soil circles surrounded by vegetation, and are therefore a special case of regular patterns, have been reported from the Namib and Australia[START_REF] Getzin | Contrasting Global Patterns of Spatially Periodic Fairy Circles and Regular Insect Nests in Drylands[END_REF]. Irregular patterns occur when patches of a broad range of sizes occur across the landscape (Fig.S4;[START_REF] Kéfi | Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems[END_REF]. Although external factors such as soil or resource spatial heterogeneity and vegetation growth form affect vegetation spatial patterns (e.g., Couteron et al., 2014), they have been shown to result largely from plant-plant and plant-soil interactions (Lefever & Lejeune, 1997; Kéfi et al., 2010). Mechanisms of vegetation pattern formation have been identified using theoretical models (e.g., Lefever & Lejeune, 1997; von Hardenberg et al., 2010) and are supported by field observations from different environments (e.g., Barbier et al., 2008; Berdugo et al., 2017; Getzin et al., 2021). Irregular patterns emerge when plant facilitation processes occur at a much smaller spatial scale than competitive processes (e.g., von Hardenberg et al., 2010). In turn, regular patterns result from a dominance of competitive mechanisms, whose spatial scale determines the regular distancing between patches (von Hardenberg et al., 2010). The formation of fairy circles is controversial, as they can be explained by either plant allelopathic interactions, an interaction with mound-forming termites and plant competition, or by the role of grasses as ecosystem engineers of soil water diffusion and infiltration (see Tarnita et al., 2017; Getzin et al., 2019, 2021 and references therein).

  3; Berdugo et al., 2019a). Aridity and plant-plant interactions are not, however, the sole drivers of changes in plant spatial patterns. Indeed, vegetation type strongly modulates the importance of abiotic drivers of vegetation patterns (e.g. precipitation seasonality and soil texture are important drivers in grasslands and shrublands, respectively), and contrasting mechanisms of facilitation (soil amelioration in shrublands vs. percentage of facilitated species in grasslands) operate to form irregular patterns (Berdugo et al., 2019b).Different plant growth forms (trees, shrubs or grasses) often display different spatial patterns in drylands, even at small spatial scales (Fig.S4). For example, trees might be regularly patterned whereas grasses are often irregular. Moreover, the drivers of the overall vegetation pattern formation can involve multi-scale patterning (patterns within the patterns) due to multiple mechanisms of ecological self-organization at different scales, as it occurs with fairy circles[START_REF] Sun | Onset of aridification by 34 Ma across the Eocene-Oligocene transition in Central Asia[END_REF]. Addressing these mechanisms in the field has remained an elusive task so far due to the difficulty of measuring plant-plant interactions within and across these hierarchical spatial scales.

  Fig.S6) and are particularly sensitive to climate change, which can seriously reduce their distribution and functional roles in drylands[START_REF] Ferrenberg | Biological soil crusts: diminutive communities of potential global importance[END_REF]. Algae and liverworts are important biocrust constituents in Chinese deserts, calcareous drylands in Australia and siliceous and sandy drylands in South Africa, also contributing to carbon fixation and soil stabilization in these regions[START_REF] Seppelt | Bryophytes Within Biological Soil Crusts[END_REF][START_REF] Büdel | Cyanobacteria and Algae of Biological Soil Crusts[END_REF].

  et al., 2016; Van der Putten et al., 2016), and, therefore, are expected to shape dryland´s diversity patterns. Plant-plant interactions are also influenced by the biogeographic patterns of herbivores and the co-evolution between them (Stebbins, 1981), a topic beyond the scope of this review. A quarter of dryland plant species seem to depend on positive plant-plant interactions (facilitation; Soliveres & Maestre, 2014; Vega-Alvarez et al., 2019). These patterns hold particularly true for those species less adapted to dry conditions (Valiente-Banuet et al., 2006; Berdugo et al., 2019a), which also greatly benefit from associations with symbiotic microbes like mycorrhiza. This influence has allowed, for example, the continuation of Mediterranean plant lineages that evolved during the wetter conditions of the Tertiary to today´s harsher conditions (Valiente-Banuet et al., 2006), and could be a potential explanation of the high functional diversity observed in drylands (Section III). Plant-associated microbes are a fundamental driver of the colonization of plants into new habitats (e.g., Delavaux et al., 2019). Conversely, if plant species manage to disperse far enough as to escape their soil antagonists, they can outcompete their neighbors and successfully invade new habitats (Zhang et al., 2020). Thus, existing empirical evidence leaves little doubt about the importance of plant-plant and plant-soil interactions in shaping species´ niches, and therefore influence dryland biodiversity and biogeographical patterns.

  with cooler and moister environments, typically found beneath plant patches compared with adjacent open areas without vegetation[START_REF] Schlesinger | Plant-soil Interactions in Deserts[END_REF][START_REF] Aguiar | Patch structure, dynamics and implications for the functioning of arid ecosystems[END_REF].Vegetated patches in drylands capture air-borne particles, contributing to nutrient input and conservation beneath them[START_REF] Schlesinger | Plant-soil Interactions in Deserts[END_REF][START_REF] Gonzales | Ecohydrological implications of aeolian sediment trapping by sparse vegetation in drylands[END_REF]. They also intercept water and nutrients from surface run-off after rainfall events, thus altering the soil and microclimatic conditions underneath them. Macro-ecological patterns in the fertility island effect across global drylands are determined by: (i) environmental conditions, including aridity and grazing pressure, (ii) soil properties, including soil parent material and age, which determine soil texture and pH, and (iii) the structure and composition of plant communities, including their functional traits (Allington & Valone, 2014; Ochoa-Hueso et al., 2018; Fig. S7; section VIII). Plant patches are comparatively more fertile than adjacent bare soils when soils are more alkaline, have greater sand content, under semiarid climates or when grazed (Allington & Valone, 2014; Ochoa-Hueso et al., 2018). Aridity is a major driver of the structure and functioning of drylands (e.g. Maestre et al., 2016; Berdugo et al., 2020; sections IV and V), and thus of plant-plant and plant-soil interactions there (e.g., Maestre & Soliveres 2014; Ochoa-Hueso et al., 2018). Increases in aridity such as those forecasted by the end of XXI th century (Huang et al., 2017) drastically alter the structure and function of the soil microbiome in drylands (Berdugo et al., 2020; Delgado-Baquerizo et al., 2020).

  Woody encroachment, perhaps the most dramatic form of dryland vegetation cover change, continues to increase over large dryland areas of the United States (Archer et al., 2017), Africa (Venter et al., 2018), Australia (Fensham et al., 2005), South America (Rosan et al., 2019) and Europe (Maestre et al., 2009). The causes of encroachment are many and complex, but generally relate to altered intensities of land-use (e.g., overgrazing and changes in fire regimes) and increases in atmospheric carbon dioxide, all of which give woody plants a competitive advantage over herbaceous vegetation (see Archer et al., 2017 and references therein). This global phenomenon summarizes well the importance of plant-plant and plant-soil interactions to shape the structure and functioning of drylands. Although the ecosystem consequences of encroachment have been extensively studied (e.g., Eldridge et al., 2011; Maestre et al., 2016; Archer et al., 2017), we still have relatively poor appreciation of the biogeography of the main encroaching species.Many of the more than 100 woody species that are known to encroach[START_REF] Eldridge | Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis[END_REF]

  . A significant challenge is therefore the development of large-scale trait databases comprising in situ individuallevel measurements directly coupled with environmental and soil data. The development of such databases would provide key insights into how plant functional diversity regulates ecosystem functioning and help to develop sound conservation and restoration strategies aimed at enhancing their capacity to provide essential ecosystem services. New remote sensing techniques, such as solar-induced fluorescence, near infrared reflectivity, thermography, hyperspectral imaging and lidar (reviewed in Smith et al., 2019), coupled with the use of high-resolution satellite images allowing the characterization identification of individual shrubs and trees characteristics across large regions (Brandt et al., 2020) are substantially improving our ability to monitor vegetation across multiple spatio-temporal scales.

  , Bernhardt-Römermann et al., 2018), we still lack comprehensive databases of a wide range of biocrust species and associated functional traits at the global scale. Increases in aridity linked to climate change are expected to result in considerable shifts in the abundance and distribution of dryland biocrusts (Rodríguez-Caballero et al., 2018). Thus, renewed efforts to examine the biogeography of biocrusts would allow us to better understand current patterns and predict future changes in the structure and functioning of dryland ecosystems, and to develop sound management, conservation and restoration strategies that account for these important communities. The collection of standardized spatio-temporal data on the abundance of multiple biocrust components and associated traits (e.g., tissue nutrient content, albedo, hydrophobicity) and ecosystem functions across a wide range of drylands remains as one of the next major challenges in dryland research. Nurse plants enhance both phylogenetic and functional diversity in drylands (e.g., Valiente-Banuet et al., 2006; Butterfield & Briggs, 2011). Our understanding of the extent to which these nurse plant effects are consistent across environments or among different components of biodiversity (e.g., taxonomic, functional or phylogenetic; but see Vega-Alvarez et al., 2019) is still in its infancy. Both plant-plant and plant-soil interactions are crucial determinants of spatial and biodiversity patterns in drylands, yet we ignore their relative importance, in comparison toenvironmental factors such as climate, in shaping these patterns. Addressing these issues can help us to better link biotic interactions with ecosystem structure and functioning in drylands, and to establish a mechanistic understanding of the biogeographical patterns of their vegetation. Although not free of limitations, which are discussed in Notes S4, the map and the analyses shown in Fig.7also serve as a working hypothesis to further explore the biogeography of plant-plant interactions in drylands and elsewhere. A better knowledge of plant-plant and plant-soil interactions can also help, for example, to aid in the restoration of degraded drylands by helping us to select species with traits that enhance ecosystem functioning[START_REF] Gross | Functional trait diversity maximizes ecosystem multifunctionality[END_REF][START_REF] Bagousse-Pinguet | Phylogenetic, functional, and taxonomic richness have both positive and negative effects on ecosystem multifunctionality[END_REF].

Figure S7 .

 S7 Figure S7. Fertile island effect for soil functions associated with the carbon, nitrogen, and phosphorus biogeochemical cycles by aridity class (a) and conceptual representation of the main ecological drivers of fertile island formation in drylands (b).

Figure S8 .

 S8 Figure S8. Global distribution of essential soil fungal groups for plant communities (a, plant pathogens; b, decomposers and c, mycorrhizal fungi) across global drylands.

Figure S9 .

 S9 Figure S9. Mean (± SE) values for average structural and functional traits for woody plant species that are encroaching across drylands worldwide.

Figure S10 .

 S10 Figure S10. Relation between predicted and observed values for the percentage of positive plantplant interactions (A). Relative importance of the geographical, climatic and vegetation predictors used to perform the random models (B).

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Plant species richness of the world's dryland ecoregions and examples of plant species and vegetation types that can be found in drylands worldwide. Plant richness was computed as the number of species in the GBIF Plantae dataset located on ecoregions with a mean aridity index lower than 0.65 (GBIF.org, 2020). Please note that the boundaries of the ecoregions presented in the map do not fully match those of drylands presented in the rest of maps within this review. Aridity values and ecoregions were obtained from Trabucco and Zomer (2019) and Dinerstein et al. (2017), respectively. Picture credits are available in Notes S5. See Fig. S2 for additional examples of major dryland vegetation types.

Figure 3 .

 3 Figure 3. The diversity of leaf forms and functions in global drylands (areas with an aridity index < 0.65, orange) and in the rest of the terrestrial ecosystems (grey). We show the biome-scale distributions (mean [M], variance [V], skewness [S] and kurtosis [K]) of six leaf morphological

Figure 4 .

 4 Figure 4. Normalized difference vegetation index (NDVI, a) and land cover types (b) across global drylands. The data shown in panel a represent average NDVI data for the period 2001-2019 obtained from the MODIS MOD13Q1 Version 6 product (https://lpdaac.usgs.gov/products/mod13q1v006/). The data shown in panel b represent the main land cover types in 2019 obtained from the MODIS MCD12Q1 Version 6 product (https://lpdaac.usgs.gov/products/mcd12q1v006/). The Others class in panel b includes urban areas, those covered by snow/ice and water bodies.

Figure 5 .

 5 Figure 5. Distribution of major vegetation spatial patterns across global drylands. Dark brown areas are those in which vegetation cover is too low to create patterns (<5% of cover); green areas are fully covered by vegetation (>95% of cover); blue areas are those showing regular patterns as identified by Deblauwe et al. (2008); dark orange areas contain fairy circles (according to Juergens, 2013; Ravi et al., 2017; Getzin et al., 2019); light orange areas represent those where their spatial patterns remain underexplored (probably holding irregular or mixed patterns). Cover data (averaged for the period 2000-2019) were estimated using the MODIS MOD44B Version 6 product (https://lpdaac.usgs.gov/products/mod44bv006/). See Fig. S4 for examples of these spatial patterns.

Figure 6 .

 6 Figure 6. Distribution of biocrust communities across global drylands. Different colors indicate the dominant biocrust components (i.e., cyanobacteria, hypolithic, lichens, mosses) at each study site. The data plotted come from the syntheses conducted by Rodríguez-Caballero et al. (2018,

Figure 7 .

 7 Figure 7. Distribution of positive plant-plant interactions (facilitation) across global drylands and variation partitioning analysis showing the relative proportion of variation explained from major predictors of these interactions. Geographical predictors include latitude and longitude; vegetation predictors include the cover and dominance of grasses, shrubs and trees; and environmental predictors include 19 climatic variables, elevation, soil carbon, pH and sand content. The scale represents the percentage of positive interactions (in %). See Notes S4 for an explanation of the methodology used to obtain the map and of the variation partitioning analyses and Fig. S10 for additional details on the performance of the model used and on the relative importance of predictors used to obtain this map.

Figure 8 .

 8 Figure 8. Biogeography of structural (a) and functional (b) traits of woody plants that have encroached into former grasslands across global drylands. Structural traits are plant size (average height), shape (v-shaped to round), root type (mixed to surface roots) and foliage contact with the soil surface (contact vs. no contact). The functional traits are whether or not plants are deciduous, allelopathic, resprouters, palatable, or nitrogen fixers. Values represent the average (standardized) values assigned to different traits (see Ding & Eldridge, 2019) according to whether they increase or reduce structure or function. A larger value equates with greater structure or function.

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7

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Figure 8

 8 Figure 8

Acknowledgements

We thank Emilio Rodríguez Caballero and Ning Chen for kindly providing the data needed to make Figs. 6 and S6, Dolores Ruiz-Lupión of MUSHER Scientific Illustration Studio (www.musherstudio.com) for drawing Figure 1 and three anonymous reviewers for their comments on our manuscript. This review was done thanks to the support of the European Research Council (ERC Grant agreement 647038 [BIODESERT]) and Generalitat Valenciana (CIDEGENT/2018/041). MB was supported by a Juan de la Cierva Fellowship from the Spanish Ministry of Science and Innovation (FJCI-2018-036520-I). LCZ was supported by the Marie Sklodowska-Curie Actions (Grant Agreement 795380 [INDECRUST]) under the European Community's H2020-EU.1.3.2 Programme. YLB-P was supported by a Marie Sklodowska-Curie Actions Individual Fellowship (MSCA-IF) within the European Program Horizon 2020 (Grant Agreement 656035 [DRYFUN]). SS was supported by the Ramón y Cajal program from the Ministry of Science and Innovation (RYC-2016-20604). MD-B was supported by the Ramón y Cajal program from the Spanish Ministry of Science and Innovation (RYC2018-025483-I), and by the British Ecological Society grant agreement No LRB17\1019 (MUSGONET). RO-H was supported by the Ramón y Cajal program from the Spanish Ministry of Science and Innovation (RYC-2017 22032).

Data Accessibility

The data used to make Figure 1 are available at Zenodo (https://doi.org/10.5281/zenodo.4252661). The data used to run the variance partitioning analyses shown in Figure 7 are available at Figshare (https://doi.org/10.6084/m9.figshare.14237702). The rest of data used in our analyses come from either public datasets or other published studies, and can be accessed from the links and references provided.

Author contribution

FTM planned the review. All authors contributed to data synthesis, analysis, and mapping. All authors contributed to the writing of the review.

Supporting Information

Additional Supporting Information may be found online in the Supporting Information section at the end of the article.