Abstract
Understanding how environmental change alters the composition of plant assemblages, and how this in turn affects ecosystem functioning is a major challenge in the face of global climate change. Assuming that values of plant traits express species adaptations to the environment, the trait‐based approach is a promising way to achieve this goal. Nevertheless, how functional traits are related to species’ environmental tolerances and how trait spectra respond to broad‐scale environmental gradients remains largely unexplored. Here, we identify the main trait spectra for US angiosperm trees by testing hypotheses for the relationships between functional traits and species’ environmental tolerances to environmental stresses, as well as quantifying the environmental drivers of assemblage means and variances of these traits. We analyzed >74,000 community assemblages from the US Forest Inventory and Analysis using 12 functional traits, five traits expressing species’ environmental tolerances and 10 environmental variables. Results indicated that leaf traits, dispersal traits, and traits related to stem hydraulics were related to cold or drought tolerance, and their assemblage means were best explained by minimum temperatures. Assemblage means of traits related to shade tolerance (tree growth rate, leaf phosphorus content, and bark thickness) were best explained by aridity index. Surprisingly, aridity index, rather than minimum temperature, was the best predictors of assemblage variances of most traits, although these relationships were variable and weak overall. We conclude that temperature is likely to be the most important driver of functional community structure of North American angiosperm trees by selecting for optimum strategies along the cold and drought stress trade‐off. In turn, water availability primarily affects traits related to shade tolerance through its effect on forest canopy structure and vegetation openness.
Highlights
Documenting large-scale patterns of plant form and function is necessary to understand how functional traits drive the response of species to the environment as well as how they mediate key ecosystem functions, such as carbon, nutrient, or water cycling (Lavorel & Garnier, 2002)
We extend the approach of Stahl et al (2013) by including cold tolerance, likely the most important evolutionary adaptation for North American angiosperm trees (Hawkins et al, 2014; Latham & Ricklefs, 1993)
Aridity index best explained the means of those functional traits related to shade tolerance, where assemblage means increased with increasing aridity (i.e., decreased with increasing aridity index defined as the ratio of annual precipitation to potential evapotranspiration (Table 3, Figs 1 and S3–S4 in Appendix S4)
Summary
Documenting large-scale patterns of plant form and function is necessary to understand how functional traits drive the response of species to the environment as well as how they mediate key ecosystem functions, such as carbon, nutrient, or water cycling (Lavorel & Garnier, 2002). Leaf nutrient traits, whole plant growth, and mortality rate (or lifespan) are among the fundamental components of the slow-fast trait spectra, and presumably they should be related to gradients of drought and nutrients as adaptations to these stresses involve resource maintenance (Coley et al, 1985; Reich et al, 2003; Wright et al, 2004) These traits could be further related to shade tolerance as shade-tolerant species invest in resistant tissues in order to tolerate periods of low light, reflecting a “slow” strategy (Valladares & Niinemets, 2008). Trait variances could decrease with strong rates of competition (Enquist et al, 2015; Mayfield & Levine, 2010) that should be more intense in warm and wet environments
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