Abstract
The worldwide plant economic spectrum hypothesis predicts that leaf, stem, and root traits are correlated across vascular plant species because carbon gain depends on leaves being adequately supplied with water and nutrients, and because construction of each organ involves a trade‐off between performance and persistence. Despite its logical and intuitive appeal, this hypothesis has received mixed empirical support. If traits within species diverge in their responses to an environmental gradient, then interspecific trait correlations could be weakened when measured in natural ecosystems. To test this prediction, we measured relative growth rates (RGR) and seven functional traits that have been shown to be related to fluxes of water, nutrients, and carbon across 56 functionally diverse tree species on (1) juveniles in a controlled environment, (2) juveniles in forest understories, and (3) mature trees in forests. Leaf, stem, and fine root traits of juveniles grown in a controlled environment were closely correlated with each other, and with RGR. Remarkably, the seven leaf, stem, and fine root tissue traits spanned a single dimension of variation when measured in the controlled environment. Forest‐grown juveniles expressed lower leaf mass per area, but higher wood and fine root tissue density, than greenhouse‐grown juveniles. Traits and growth rates were decoupled in forest‐grown juveniles and mature trees. Our results indicate that constraints exist on the covariation, not just the variation, among vegetative plant organs; however, divergent responses of traits within species to environmental gradients can mask interspecific trait correlations in natural environments. Correlations among organs and relationships between traits and RGR were strong when plants were compared in a standardized environment. Our results may reconcile the discrepancies seen among studies, by showing that if traits and growth rates of species are compared across varied environments, then the interorgan trait correlations observed in controlled conditions can weaken or disappear.
Highlights
Identifying general principles and trade-offs that underlie the diversity of organism form and function is a central goal of functional ecology because trade-offs constrain demographic rates and their linkages to ecosystem processes (Díaz et al, 2016; Shipley et al, 2016)
Because carbon gain depends on leaves being adequately supplied with water and nutrients, and because construction of other plant organs involves trade-offs between performance and persistence, it has recently been hypothesized that leaf, stem, and fine root traits will all be closely correlated across species (Freschet, Aerts, & Cornelissen, 2012; Freschet, Cornelissen, Van Logtestijn, & Aerts, 2010; Pérez-Ramos et al, 2012; Reich, 2014; de la Riva et al, 2016)
Given that some of the strongest evidence against whole-plant integration has been found in mature trees sampled in the field (Baraloto et al, 2010), we predicted that mature trees would exhibit weaker interspecific trait correlations than juvenile trees
Summary
Identifying general principles and trade-offs that underlie the diversity of organism form and function is a central goal of functional ecology because trade-offs constrain demographic rates and their linkages to ecosystem processes (Díaz et al, 2016; Shipley et al, 2016). Leaf mass per area (LMA) and leaf lifespan are positively correlated among species, reflecting evolutionary adaptation to shade, but intraspecific correlations across light gradients are negative, as ecological acclimation to shade decreases LMA but increases leaf lifespan (Lusk, Reich, Montgomery, Ackerly, & Cavender-Bares, 2008; Russo & Kitajima, 2016) This could apply to traits from different organs: If the two traits show divergent responses to an environmental gradient, trait correlations among species measured across varied environments could be weakened, or disappear (Figure 1a). If the intraspecific response to environmental gradients aligns with the interspecific relationship, the correlation of traits measured on species from multiple environments will remain strong This conceptual hypothesis does not depend on the length of the reaction norms within a species (note that the length of all the dotted lines in Figure 1a,b are approximately equal). Given that some of the strongest evidence against whole-plant integration has been found in mature trees sampled in the field (Baraloto et al, 2010), we predicted that mature trees would exhibit weaker interspecific trait correlations than juvenile trees
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