An extensive suite of functional traits distinguishes Hawaiian wet and dry forests and enables prediction of species vital rates

Abstract

The application of functional traits to predict and explain plant species’ distributions and vital rates has been a major direction in functional ecology for decades, yet numerous physiological traits have not yet been incorporated into the approach. Using commonly measured traits such as leaf mass per area (LMA) and wood density (WD), and additional traits related to water transport, gas exchange and resource economics, including leaf vein, stomatal and wilting traits, we tested hypotheses for Hawaiian wet montane and lowland dry forests (MWF and LDF, respectively): (1) Forests would differ in a wide range of traits as expected from contrasting adaptation; (2) trait values would be more convergent among dry than wet forest species due to the stronger environmental filtering; (3) traits would be intercorrelated within “modules” supporting given functions; (4) relative growth rate (RGR) and mortality rate (m) would correlate with a number of specific traits; with (5) stronger relationships when stratifying by tree size; and (6) RGR and m can be strongly explained from trait‐based models. The MWF species’ traits were associated with adaptation to high soil moisture and nutrient supply and greater shade tolerance, whereas the LDF species’ traits were associated with drought tolerance. Thus, on average, MWF species achieved higher maximum heights than LDF species and had leaves with larger epidermal cells, higher maximum stomatal conductance and CO2 assimilation rate, lower vein lengths per area, higher saturated water content and greater shrinkage when dry, lower dry matter content, higher phosphorus concentration, lower nitrogen to phosphorus ratio, high chlorophyll to nitrogen ratio, high carbon isotope discrimination, high stomatal conductance to nitrogen ratio, less negative turgor loss point and lower WD. Functional traits were more variable in the MWF than LDF, were correlated within modules, and predicted species’ RGR and m across forests, with stronger relationships when stratifying by tree size. Models based on multiple traits predicted vital rates across forests (R2 = 0.70–0.72; p < 0.01). Our findings are consistent with a powerful role of broad suites of functional traits in contributing to forest species’ distributions, integrated plant design and vital rates. A plain language summary is available for this article.

Leaf drought tolerance cannot be inferred from classic leaf traits in a tropical rainforest

“…In contrast, leaf area and π tlp were found to be unrelated across species within a wet montane forest (Medeiros et al, ) and across species ranging strongly in drought tolerance and native habitat (Scoffoni et al, ), suggesting the coordination between leaf area and π tlp is context‐dependant. Similarly, and more generally, correlations between hydraulic traits, such as π tlp , and economic traits, such as N mass , varied across studies, with some finding significant relationships (Medeiros et al, ; Rosas et al, ; Zhu et al, ), while others finding no relationship (Bartlett, Zhang, et al, ; De Guzman, Santiago, Schnitzer, & Álvarez‐Cansino, ; Li et al, ; Medeiros et al, ; Rosas et al, ).…”

“…For each sampled leaf, we estimated the time‐integrated CO 2 assimilation rate per area (, in μmol m −2 s −1 ) and the time‐integrated stomatal conductance to water vapour (, in mol m −2 s −1 ) using the following approach (Medeiros et al, ). A time‐integrated estimate of the leaf intercellular CO 2 mole fraction, (in μmol CO 2 per mole air; μmol/mol), was estimated from leaf δ 13 C using the following relationship: (Cernusak et al, ; Farquhar, O'Leary, & Berry, ), with c a the atmospheric CO 2 concentration taken as 390 ppm.…”

“…Intraspecific variation can thus blur interspecific trait relationships, especially when trait values are drawn from independent studies led under various conditions (Clark et al, ; Laughlin et al, ). As an illustration, accounting for variation in tree size can substantially strengthen trait relationships across species (Medeiros et al, ). Overall, higher trait dimensionality than typically assumed under global trait spectra may operate within plant communities, with important implications for species coexistence and responses to changing environmental conditions (Clark, ; Laughlin, ; Medeiros et al, ; Rosas et al, ).…”

“…(+) ‘fast‐slow’ hypothesis (Medeiros et al, ), with less negative π tlp being related to higher maximal g w (Henry et al, ) and higher g w overall in a wet environment…”

Plant Trait Networks: Improved Resolution of the Dimensionality of Adaptation

“…As many traits are intercorrelated, the relationships among multiple traits has become a strong focus in studies of plant adaptation and environmental responses for the past several decades [6][7][8]. Recent studies have considered extensive suites of functional traits [9,10], including leaf, stem, root, reproductive, and whole-plant integrative traits. Positive or negative trait correlations are generally considered to represent trade-offs, co-optimization, and/or allometric relationships based on biomechanical and/or physiological requirements [11,12].…”