Functional diversity supports the physiological tolerance hypothesis for plant species richness along climatic gradients

Journal of Ecology 2014, 102, 447–455 doi: 10.1111/1365-2745.12204 Functional diversity supports the physiological tolerance hypothesis for plant species richness along climatic gradients Marko J. Spasojevic 1 *, James B. Grace 2 , Susan Harrison 1 and Ellen I. Damschen 3 Department of Environmental Science and Policy, University of California Davis, Davis, CA 95616, USA; 2 U.S. Geological Survey, National Wetlands Research Center, 700 Cajundome Boulevard, Lafayette, LA 70506, USA; and Department of Zoology, University of Wisconsin-Madison, 451 Birge Hall, Madison, WI 53706, USA Summary 1. The physiological tolerance hypothesis proposes that plant species richness is highest in warm and/or wet climates because a wider range of functional strategies can persist under such conditions. Functional diversity metrics, combined with statistical modelling, offer new ways to test whether diversity–environment relationships are consistent with this hypothesis. 2. In a classic study by R.H. Whittaker (Ecological Monographs, 1960), herb species richness declined from mesic (cool, moist, northerly) slopes to xeric (hot, dry, southerly) slopes. Building on this data set, we measured four plant functional traits (plant height, speciﬁc leaf area, leaf water con- tent and foliar C:N) and used them to calculate three functional diversity metrics (functional rich- ness, evenness and dispersion). We then used a structural equation model to ask whether ‘functional diversity’ (modelled as the joint responses of richness, evenness and dispersion) could explain the observed relationship of topographic climate gradients to species richness. We then repeated our model examining the functional diversity of each of the four traits individually. 3. Consistent with the physiological tolerance hypothesis, we found that functional diversity was higher in more favourable climatic conditions (mesic slopes) and that multivariate functional diver- sity mediated the relationship of the topographic climate gradient to plant species richness. We found similar patterns for models focusing on individual trait functional diversity of leaf water content and foliar C:N. 4. Synthesis. Our results provide trait-based support for the physiological tolerance hypothesis, sug- gesting that benign climates support more species because they allow for a wider range of functional strategies. Key-words: determinants of plant community diversity and structure, plant functional traits, R.H. Whittaker, Siskiyou-Klamath, species richness, structural equation modelling, topographic microclimate Introduction Species richness is higher not only in the tropics, but also more generally where the climate is warm and moist rather than cold or arid (e.g. Hawkins, Porter & Diniz 2003; Kissling, Field & Bohning-Gaese 2008). One of the leading hypotheses to explain this pattern (along with the ‘faster spe- ciation’ (Fisher 1960) and ‘more individuals’ hypotheses (Rohde 1992)) is the ‘physiological tolerance hypothesis’ (Currie et al. 2004). According to this hypothesis, environ- ments with abundant water and solar energy permit a wider *Correspondence author. E-mail: mspasoje@ucdavis.edu range of functional strategies; in turn, higher numbers of spe- cies are able to tolerate such benign environments, whereas only smaller subsets of species can tolerate the more demand- ing conditions of cold and/or aridity. Many correlative studies of species richness appear to support this hypothesis (Haw- kins, Porter & Diniz 2003; Normand et al. 2009; Kessler et al. 2011). Kleidon & Mooney (2000) linked this hypothesis explicitly to plant functional traits by simulating plant ‘species’ as random sets of functional traits related to water uptake and growth. They found that more ‘species’ could per- sist in warm and wet than in cold or dry environments and that this model could broadly reproduce observed global climate–richness relationships. © 2014 The Authors. Journal of Ecology © 2014 British Ecological Society