Ontogenetic trait variation influences tree community assembly across environmental gradients

Abstract

Abstract. Intraspecific trait variation is hypothesized to influence the relative importance of community assembly mechanisms. However, few studies have explicitly considered how intraspecific trait variation among ontogenetic stages influences community assembly across environmental gradients. Because the relative importance of abiotic and biotic assembly mechanisms can differ among ontogenetic stages within and across environments, ontogenetic trait variation may have an important influence on patterns of functional diversity and inferred assembly mechanisms. We tested the hypothesis that variation in functional diversity across a topo-edaphic gradient differs among ontogenetic stages and that these patterns reflect a shift in the relative importance of different assembly mechanisms. In a temperate forest in the Missouri Ozarks, USA, we compared functional diversity of leaf size and specific leaf area (SLA) of 34 woody plant species at two ontogenetic stages (adults and saplings) to test predictions about how the relative importance of abiotic and biotic filtering changes among adult and sapling communities. Local communities of adults had lower mean SLA and lower functional dispersion of SLA than expected by chance, particularly at the resource-limited end of the topo-edaphic gradient, suggesting an important role for abiotic filtering among co-occurring adults. In contrast, local communities of saplings often had higher functional dispersion of leaf size and SLA than expected by chance regardless of their location along the topo-edaphic gradient, suggesting an important role for biotic filtering among co-occurring saplings. Moreover, the overall strength of trait-environment relationships varied between saplings and adults for both leaf traits, generally resulting in stronger environmental shifts in mean trait values and trait dispersion for adults relative to saplings. Our results illustrate how community assembly mechanisms may shift in their relative importance during ontogeny, leading to variable patterns of functional diversity across environmental gradients. Moreover, our results highlight the importance of integrating ontogeny, an important axis of intraspecific trait variability, into approaches that use plant functional traits to understand community assembly and species coexistence.

Ontogenetic shifts in trait‐mediated mechanisms of plant community assembly

“…Four of these traits (SLA, leaf N, wood density, and seed mass) possibly vary across size classes within species, potentially limiting our ability to learn about community assembly via adult traits. Nevertheless, interspecific variation in traits may be largely consistent across ontogeny (Iida et al 2014, Spasojevic et al 2014, and traits measured on adults often correspond to major axes of niche, demographic history, and life history variation among species (Kraft et al 2010, Uriarte et al 2010, Wright et al 2010, Ru¨ger et al 2012, Lasky et al 2013, 2014a). Here we focus on adult traits as predictors of demography across different life stages.…”

“…In order to make inferences about the importance of ontogeny, we made ontogenetic comparisons, i.e., smaller individuals of a species were compared with larger individuals of that same species (Spasojevic et al 2014). By contrast, dividing individuals into size classes that are invariant across species would result in comparisons of individuals of small-statured species with individuals of large-statured species, which is not an ontogenetic comparison.…”

“…This intraspecific variation is often associated with ontogeny (Werner andGilliam 1984, Parish andBazzaz 1985), especially for forest trees (Niinemets 2010). Similarly, interspecific differences in traits that affect community assembly may vary in relevance across ontogenetic stages (Werner and Gilliam 1984, Parish and Bazzaz 1985, Poorter 2007, Muscarella et al 2013, Spasojevic et al 2014. Such ontogenetic differences could promote species coexistence if there are trade-offs in performance across life stages, (i.e., if species with high performance at early life stages have poor performance in later stages) or if species partition niches across life stages (Loreau andEbenhoh 1994, Moll andBrown 2008).…”

Using codispersion analysis to quantify and understand spatial patterns in species–environment relationships

“…The second dataset is from the Tyson Research Center Plot (TRCP), a 25-ha forest dynamics plot located at Washington University in the St Louis Tyson Research Center, MO, USA (Spasojevic et al, 2014). We analyzed species-environment relationships for five woody species in the central 20-ha of the plot: Frangula caroliniana (Walter) A.…”

“…Together, these five species comprised 78% of the total basal area of TRCP in the 2013 census (Table 2b). Principal components (PC) analysis (see Notes S3) was used to summarize, in two composite principal axes, the variation in 17 physicochemical soil properties that were measured at points across TRCP in 2013 and kriged to 20 9 20-m 2 raster cells (Spasojevic et al, 2014). Maps of individual environmental variables are available on the TRCP website (http:// www.ctfs.si.edu/site/Tyson+Research+Center%2C+Missouri) and the data used in this paper are provided in Tables S1, S2.…”