Evolutionary diversity in tropical tree communities peaks at intermediate precipitation

Abstract

Global patterns of species and evolutionary diversity in plants are primarily determined by a temperature gradient, but precipitation gradients may be more important within the tropics, where plant species richness is positively associated with the amount of rainfall. The impact of precipitation on the distribution of evolutionary diversity, however, is largely unexplored. Here we detail how evolutionary diversity varies along precipitation gradients by bringing together a comprehensive database on the composition of angiosperm tree communities across lowland tropical South America (2,025 inventories from wet to arid biomes), and a new, large-scale phylogenetic hypothesis for the genera that occur in these ecosystems. We find a marked reduction in the evolutionary diversity of communities at low precipitation. However, unlike species richness, evolutionary diversity does not continually increase with rainfall. Rather, our results show that the greatest evolutionary diversity is found in intermediate precipitation regimes, and that there is a decline in evolutionary diversity above 1,490 mm of mean annual rainfall. If conservation is to prioritise evolutionary diversity, areas of intermediate precipitation that are found in the South American 'arc of deforestation', but which have been neglected in the design of protected area networks in the tropics, merit increased conservation attention.

Figure 1

Time-calibrated molecular phylogeny of 852 angiosperm genera found in lowland tree communities of tropical South America. Phylogenetic reconstruction based on sequences of rbcL and matK plastid regions from plants collected during fieldwork or available in GenBank. Tree topology and divergence times of taxa were estimated using a Bayesian Markov Chain Monte Carlo approach. Branch lengths were time-scaled using a relaxed molecular clock with fossil-based age constraints implemented on nodes (Appendix 1). Colours represent mean annual precipitation (MAP), with warmer colours indicating drier conditions. The minimum and maximum MAP are given. Scale (length) is in myrs and is equivalent to branch lengths in the phylogeny (80 myrs). Dotted lines indicate 1,200 mm and 1,800 mm of MAP. Black circles indicate the nodes comprising lineages from the major angiosperm clades: Magnoliids, Monocots, Superrosids, Superasterids. Black squares indicate nodes comprising some of the dry-adapted lineages that are absent or have a much lower frequency of occurrence in wet environments, at least as trees (e.g., Cactaceae, Zygophyllaceae, Asteraceae; see Discussion).

Figure 2

Relationship between mean annual precipitation (MAP) and lineage diversity (standardised effect size of phylogenetic diversity, a measure of the evolutionary diversity of communities) across 2,025 lowland tree communities of tropical South America. (a) Effect of MAP on lineage diversity (LD). Break point (1,490 mm) was determined by piecewise regression. r² = coefficient of determination from generalized least squares (GLS) models that account for spatial autocorrelation. GLS was calculated for before (y = 0.002326x − 2.634307) and after (y = −0.0006x + 0.711) the break point. (b) Geographical variation of lineage diversity and MAP. Colours of the symbols illustrate lineage diversity and are identical to colours in (a) (warmer colours indicate higher values). Circles indicate communities below and triangles above the precipitation break point (1,490 mm). Grey areas around the curves in (a) are 99% confidence intervals. These represent, for a given value of MAP, the interval estimate for the mean of LD, thus reflecting the uncertainty around this mean. Dashed lines represent national borders and contours represent mean annual precipitation in (b).

Figure 3

Conservation assessment of lineage diversity across lowland tree communities in tropical South America. Distribution of the top 5% unprotected tree communities with highest lineage diversity (80 sites; red circles). Unprotected status was determined by overlaying the distribution of our sites on to the coverage of protected areas across South America.