Shifts in trait means and variances in North American tree assemblages: species richness patterns are loosely related to the functional space

Abstract

One of the key hypothesized drivers of gradients in species richness is environmental filtering, where environmental stress limits which species from a larger species pool gain membership in a local community owing to their traits. Whereas most studies focus on small‐scale variation in functional traits along environmental gradient, the effect of large‐scale environmental filtering is less well understood. Furthermore, it has been rarely tested whether the factors that constrain the niche space limit the total number of coexisting species. We assessed the role of environmental filtering in shaping tree assemblages across North America north of Mexico by testing the hypothesis that colder, drier, or seasonal environments (stressful conditions for most plants) constrain tree trait diversity and thereby limit species richness. We assessed geographic patterns in trait filtering and their relationships to species richness pattern using a comprehensive set of tree range maps. We focused on four key plant functional traits reflecting major life history axes (maximum height, specific leaf area, seed mass, and wood density) and four climatic variables (annual mean and seasonality of temperature and precipitation). We tested for significant spatial shifts in trait means and variances using a null model approach. While we found significant shifts in mean species’ trait values at most grid cells, trait variances at most grid cells did not deviate from the null expectation. Measures of environmental harshness (cold, dry, seasonal climates) and lower species richness were weakly associated with a reduction in variance of seed mass and specific leaf area. The pattern in variance of height and wood density was, however, opposite. These findings do not support the hypothesis that more stressful conditions universally limit species and trait diversity in North America. Environmental filtering does, however, structure assemblage composition, by selecting for certain optimum trait values under a given set of conditions.

Mapping local and global variability in plant trait distributions

“…It has been shown that using trait distributions leads to different estimates of carbon dynamics (32) and that higher-order moments of trait distributions contribute to sustaining multiple ecosystem functions (33). While species-level mapping (21,23,24) does capture trait distributions, it has been limited geographically and restricted to subsets of functional groups.…”

“…Previous work has extrapolated trait measurements across continental or larger regions through three methodologies: (i) grouping measurements of individuals into larger categories that share a set of properties [a working definition of plant functional types (PFTs)] (4, 15), (ii) exploiting trait-environment relationships (e.g., leaf Nm and mean annual temperature) (1,(16)(17)(18)(19)(20), or (iii) restricting the analysis to species whose presence has been widely estimated on the ground (21)(22)(23)(24). Each of these methods has limitations-for example, trait-environment relationships do not well explain observed trait spatial patterns (1,25), while species-based approaches limit the scope of extrapolation to only areas with well-measured species abundance.…”

Morphological structure of ant assemblages in tropical and temperate forests

“…Annual precipitation (bioclimatic variable 12) is defined as the sum of all the monthly precipitation estimates; minimum temperature of coldest month (bioclimatic variable 6) is defined as the lowest temperature of any weekly minimum temperature. and non-linear responses (Símová et al 2015). We examined these complex relationships using multiple regression in which the eight trait values entered as response variables, while all the environmental variables (linear and quadratic terms) and their interaction terms entered the model as driver variables.…”

Regional rainfall and local topography jointly drive tree community assembly in lowland tropical forests of New Caledonia

“…For instance, Šímová et al. () in North America and Muscarella et al. () in Puerto Rico found a positive relationship between MAP and specific leaf area (SLA), while Santiago, Kitajima, Wright, and Mulkey () found a negative relationship in Panama and Ordoñez et al.…”