Plants: Diversity and Evolution

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Figure 3. Patterns of functional distinctiveness across biomes. Distinctiveness represents how species are functionally distant from each other within a biome i. The larger the value, the more distant a species is to the centroid of the biome's functional space. Widespreadness measures how geographically common a species is within a biome.

A value of 0 indicates that a species is present in a single biome cell.

Bin values are the number of species on a log e scale. Rectangles represent the species that are considered functionally similar and widespread in each biome using cut-off values of 0.

We found considerable variation in the proportional distribution of growth forms within and among biomes. Woody species represent the dominant growth forms in tropical biomes in terms of species numbers, whereas herbaceous species dominate in temperate environments Figure 4. When we considered only those species that are widespread and functionally common in each biome, the distribution of growth forms across biomes changed, especially in the proportion of trees, herbs and grasses. For instance, in tropical biomes, the proportion of trees decreased in each biome except for moist forest.

In temperate biomes, the proportion of grasses increased, especially toward the polar regions. Interestingly, the distribution of growth forms in xeric woodlands more closely resembles the distribution in temperate biomes when only widespread and functionally common species are considered. Figure 4. Distribution of growth forms in each biome left using the total number of species, and right using only those species that are functionally similar and widespread within the biome see Figure 3.

Pairwise comparisons of species composition among biomes reveal three main clusters representing the tropical, temperate and polar climatic zones Figure 5A , reflecting the high taxonomic overlap within, but not between, these regions Figure 1. Pairwise comparisons of trait hypervolumes among biomes show a less clear clustering of climatic zones Figure 5B. In this case, the functional space of xeric woodlands overlaps significantly with temperate biomes. This analysis also reveals the overlap in functional space of taigas with temperate grasslands and mixed forests.

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The pairwise comparison of trait hypervolumes among biomes using only those species considered as functionally common and widespread shows less overlap in trait spaces among and within climatic zones Figure 5C. However, even though xeric woodlands are now clustered with the rest of tropical biomes, these habitats along with tropical grassland exhibit great overlap in functional space with temperate biomes such as Mediterranean woodlands and temperate grasslands.

Figure 5. A Pairwise dissimilarity in species composition among biomes.

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The lighter the cell the greater the dissimilarity. Our analyses yield three important insights for understanding terrestrial biomes through a functional lens. First, we show that despite progress in the compilation and synthesis of primary biodiversity data, significant knowledge shortfalls persist that may limit our ability to quantify the functional biodiversity of biomes on continental to global scales. Second, our analyses of the available data nevertheless show that all of the biomes in North and South America share a remarkable common pattern in which the most geographically widespread species in any biome tend to share common functional traits while the most functionally distinctive species are invariably restricted in their distribution.

Third, when only the widespread and functionally common species are considered, biomes can be more readily distinguished functionally, and patterns of dissimilarity between biomes appear to reflect a correspondence between climate and plant functional niche space.

Taken together, our results suggest that while the study of the functional diversity of biomes is still in its formative stages, further development of the field will likely yield insights linking evolution, biogeography, community assembly, and ecosystem function. The BIEN database is specifically designed to close gaps in our knowledge of plant biodiversity, yet as Hortal et al.

Thus, the Raunkiaerian shortfall in functional trait data sensu Hortal et al. And while we can use phylogenetic knowledge to reasonably impute those missing values Swenson et al. For example, the existing growth form data suggest that woody species dominate in all tropical biomes, whereas the proportional diversity of herbaceous growth forms is much higher in temperate and polar biomes see also Engemann et al. Does this latitudinal increase in relative diversity of herbaceous plants reflect sampling biases and a lack of taxonomic knowledge about tropical herbaceous plants, or does it reflect the differing evolutionary and biogeographic histories in the Nearctic and Neotropical realms?

Dominant growth forms are one of the key distinguishing features of biomes, so systematic changes in the most diverse growth forms whether dominant or not will necessarily influence our predictions about how functional diversity might change across biomes, and how those changes will affect ecosystem function and services. We used species range maps as input data mainly to overcome the undeniable issue of sampling bias that is characteristic of datasets compiled from multiple sources see Supplementary Figure 1.

However, range maps drawn from species distribution models could represent an additional element of uncertainty in our results. For example, given the geographic resolution of this study and the spatial complexity of certain biomes, these models could have overestimated the geographical extent of some species from cells of a single biome to cells of nearby and interdigitated ones.

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Using occurrence data did not change the general results of this study see Supplementary Figures 2 — 6 ; however, a slight decrease in the functional overlap among biomes could indeed reveal an overestimation of some species ranges see Supplementary Figures 2 , 5 , and 6. Using occurrence information also changed the general relationships between functional distinctiveness and widespreadness in our results, due to the relative oversampling of temperate vs. In this case, our measure of widespreadness is limited by the extremely limited sampling of most species in tropical biomes.

As datasets and sampling improve, there is likely to be scope to reduce these uncertainties in the future. The other data gap that needs to be addressed to understand the functional diversity of biomes is the Prestonian shortfall in abundance information.

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Abundance is particularly important in studies of functional diversity because the traits that matter most for community assembly, ecosystem functioning, and biogeochemical cycling are those of the most dominant species. Because we based our assemblages on range maps derived from species distribution models, we could only address the geographic component of commonness, but the positive relationship between local abundance and geographic range, the so-called occupancy-abundance relationship, suggests that the most widespread species in each biome may frequently be among the more abundant as well Borregaard and Rahbek, ; Novosolov et al.

Because biomes are generally characterized by the dominant growth forms in the region, integrating abundance information might result in biomes showing less functional overlap. However, even though BIEN has compiled voluminous plot data that allow estimates of local abundance, making reasonable comparisons across different regions and growth forms is hampered by uneven sampling, regional differences in gamma diversity, and incommensurate methods of quantifying abundance.

Despite the persistent gaps in the available data, our analyses uncovered a previously undocumented relationship common to all the biomes of the Western Hemisphere: in any biome, the most widespread species also tend to exhibit low functional distinctiveness, whereas the most functionally distinctive species are almost invariably restricted in their distribution Figure 3. Moreover, together with the high degree of both taxonomic and functional overlap among biomes Figures 1 , 5 , the fact that common, widespread species are functionally similar reinforces the notion that land plants across a wide range of environmental conditions share common characteristics near the core of the functional trait spectrum Wright et al.

Thus, the question remains open whether more functionally distinctive species are specialists in particular environments or whether their trait combinations result in demographic or physiological trade-offs that limit their geographic distribution.

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Our comparison of trait hypervolume distributions across biomes Figure 2B is consistent with the observation that more species-rich environments are also more functionally diverse Swenson et al. Our results are more equivocal concerning the hypothesis that seasonal and extreme climatic environments consistently limit the functional diversity of species de la Riva et al. All tropical biomes display higher average functional diversity than all temperate biomes, and the polar biomes do display the smallest hypervolumes. However, within each group, drier or more seasonally variable biomes do not always display smaller hypervolumes e.

From these results alone we cannot determine whether temperate and polar biomes are less taxonomically diverse because of limits on functional niche space, or whether their functional hypervolumes are small because they are not taxonomically diverse. Null-modeling approaches could potentially help to disentangle taxonomic and functional diversity Swenson et al. More importantly, our results reinforce the importance of understanding how evolutionary and biogeographic history shape the functional diversity of biomes Woodward et al. The extensive sharing of species and higher lineages across biomes within, but largely not between, different biogeographic realms could serve both to homogenize functional diversity within realms and to provide clues about the characteristic traits that are selected for, or against, by different environments Douma et al.

Despite substantial hypervolume overlap among all the biomes Supplementary Figure 2 , tropical, temperate, and cold biomes all appear to occupy distinguishable regions of functional space Figures 5B,C. The main traits differentiating biomes appear to be traits related to overall plant size, including both mature height and seed mass Supplementary Figures 7 — 10 , rather than leaf economics traits, as observed in more local, plot-based analyses e.

However, Leaf P does exhibit a significant phylogenetic signal Kerkhoff et al. Leaf P is also significantly higher in herbaceous growth forms Kerkhoff et al. When we limited hypervolume analyses to only the most widespread and functionally common species in each biome, the individual biomes within each biogeographic realm overlapped less in functional trait space compare Figures 5B,C , suggesting that these species may reflect phenotypes better adapted to particular environments. In this context, the xeric woodland biome is particularly interesting.

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In part due to the inclusion of the Chaco ecoregion, xeric woodlands cluster with the tropical biomes taxonomically Figure 5A. But when the functional hypervolumes of all of the species are considered, they show much stronger similarity to the temperate biomes Figure 5B. Finally, when only the most widespread and functionally common species are analyzed, xeric woodlands again show increased similarity to tropical biomes, but also they maintain high similarity with temperate grassland and Mediterranean biomes Figure 5C.

Transitions from warm, mesic environments to colder, drier, and more seasonal environments are facilitated by similar traits, e. Furthermore, the position of xeric zones at the boundary of the inter-tropical convergence zone makes them a geographical transition between the tropical and temperate realms.

The fact that xeric woodlands are intermediate between the tropical and temperate realms both functionally and biogeographically further reinforces the idea that to better understand the functional diversity of biomes we must take into account their biogeographic and phylogenetic histories Pennington et al. Any classification of terrestrial biomes imposes a small number of discrete categories on continuous gradients in climate and species distributions, and thus represents a gross simplification of the complex ecological landscape Moncrieff et al.

Yet despite their potential for oversimplification, biomes are useful constructs for organizing and understanding the biodiversity and functioning of Earth's major terrestrial ecosystems, and trait-based approaches have high potential to help dynamically model global vegetation distributions. In this study we have shown that much of the taxonomic diversity of all biomes represents species that are both narrowly distributed and functionally similar. Further, within biomes the most functionally distinctive species in each biome also tend to be geographically rare, while widespread species uniformly display low functional distinctiveness.