The ability of a plant to occupy an ecological niche depends on its functional traits, the features like shape, biochemistry or behaviour that affect its performance. How functional traits vary with environmental conditions is of fundamental importance in trait-based community ecology. However, how intraspecific variability, the variability within a species, in functional traits connects to species distribution is not well understood.
Dong He and colleagues investigated inter- and intraspecific variation of specific leaf area, the leaf area per unit of dry mass (SLA). The authors tested SLA against soil factors and tested if trait variation is more closely associated with specific environmental regimes for low-variability species than for high-variability species.
They did this by visiting a subtropical evergreen forest in southern China. They collect 106 700 leaves from 5335 individuals of 207 woody species were intensively collected 106,700 leaves from 5335 individuals of 207 woody species. That’s a lot of leaves. The reason they got so many is that they wanted 30 individuals of each species in their sample. This way they could build an idea of what a typical tree was producing instead of taking the gamble of sampling one tree and basing their conclusions on a freak result.
They found that interspecific variation was more important that intraspecific variation. The key factors causing variation in specific leaf area were down to total soil nitrogen and total organic carbon. Not surprisingly the less nitrogen there was in the soil, the smaller the specific leaf area became. Species with higher intraspecific variation had a broader habitat range, again as expected, because they had more flexibility to build leaves.
The authors concluded that low specific leaf area is a phenotypic and probably adaptive response to nitrogen stress, which drives the predominance of species with ever-decreasing SLA towards less fertile habitats. This result, they say, highlights the importance of quantifying intraspecific trait variation to improve our understanding of species distributions across a vegetated landscape.
One of the authors of this paper, Hans Cornelissen, has written elsewhere on the wider challenges facing trait-based ecology. In that earlier article, he and his co-authors concluded: “Given the incredible diversity of plant species and habitats, a proper evaluation of the foundational assumptions of trait-based ecology cannot be obtained from only a few studies. On the contrary, this requires many replicated studies of these questions.”
The new study should go some way to plugging the data gap. He et al. say their study represents the most extensive mapping of specific leaf area variation and its environmental determinants in forests, based on over 100,000 leaves across plants in a large forest plot. With such a large sample it offers useful data for creating general rules linking trait patterns to niche occupancy, and explaining why you find certain species here and not there.