Quantifying the effects of ecological constraints on trait expression using novel trait-gradient analysis parameters

Shifting Importance of Abiotic versus Biotic Filtering from Intact Mature Forests to Post-Clearcut Secondary Forests

Although ecologists often emphasize the roles of environmental- versus biotic-filtering in structuring forest communities, the relative importance of these processes could vary among undisturbed versus disturbed forests. To test this assumption, we gathered leaf traits and site conditions data from intact mature forests (control), moderately disturbed shrublands, and severely disturbed plantations from subtropical China. We found that plantations had higher leaf area, specific leaf area, leaf nitrogen and phosphorus concentrations but lower leaf thickness, dry matter content, and C:N than the shrubland or mature forest, suggesting the dominance of resource acquisition strategy in plantations versus conservation strategy in the mature forests. Plantations also had significantly lower trait ranges than mature forest or shrubland, suggesting the play of stringent environmental filtering in the plantation. However, intraspecific trait variations in leaf dry matter content and C:N were substantial in plantation, while interspecific variation in leaf thickness was high in mature forests, suggesting the importance of intra- versus inter-specific competition in plantation versus mature forests. Results from our species-level analysis were consistent with the community-level results mentioned above. Overall, our study demonstrates the shifting importance of environmental and biotic filtering from disturbed to undisturbed forests.

Precipitation seasonality promotes acquisitive and variable leaf water-economics traits in southwest Australian granite outcrop species

Southwestern Australia (SWA) is a global biodiversity hotspot characterized by a mediterranean climate and is among the world’s oldest and resource-poorest landscapes. Within this region, granite outcrops provide habitat complexity, and contribute to high levels of species diversity and endemism. Granite outcrop plant species are characterized by distinct anatomical, morphological and eco-physiological traits. So far, functional studies of SWA granite outcrop plants examined trait patterns along the stark aridity gradient that occurs in the region. Despite precipitation seasonality being an important climatic driver of plant species richness and distribution in mediterranean-type ecosystems, relationships between plant traits and precipitation seasonality remain understudied. Here, we investigated granite outcrop species’ responses to changes in precipitation seasonality in terms of average and variability values of leaf traits related to water use efficiency. We found that plants displayed more acquisitive and variable trait values with increasing precipitation seasonality. These patterns are likely associated with the long-term effect of the stable and predictable precipitation regime in the old SWA that may have fine-tuned plant water use strategies to maximize water acquisition during the most favourable season.

Quantifying the effects of ecological constraints on trait expression using novel trait-gradient analysis parameters

Complex processes related to biotic and abiotic forces can impose limitations to assembly and composition of plant communities. Quantifying the effects of these constraints on plant functional traits across environmental gradients, and among communities, remains challenging. We define ecological constraint (C_i) as the combined, limiting effect of biotic interactions and environmental filtering on trait expression (i.e., the mean value and range of functional traits). Here, we propose a set of novel parameters to quantify this constraint by extending the trait‐gradient analysis (TGA) methodology. The key parameter is ecological constraint, which is dimensionless and can be measured at various scales, for example, on population and community levels. It facilitates comparing the effects of ecological constraints on trait expressions across environmental gradients, as well as within and among communities. We illustrate the implementation of the proposed parameters using the bark thickness of 14 woody species along an aridity gradient on granite outcrops in southwestern Australia. We found a positive correlation between increasing environmental stress and strength of ecological constraint on bark thickness expression. Also, plants from more stressful habitats (shrublands on shallow soils and in sun‐exposed locations) displayed higher ecological constraint for bark thickness than plants in more benign habitats (woodlands on deep soils and in sheltered locations). The relative ease of calculation and dimensionless nature of C_i allow it to be readily implemented at various scales and make it widely applicable. It therefore has the potential to advance the mechanistic understanding of the ecological processes shaping trait expression. Some future applications of the new parameters could be investigating the patterns of ecological constraints (1) among communities from different regions, (2) on different traits across similar environmental gradients, and (3) for the same trait across different gradient types.