Functional traits restrict plant community assembly at high elevations

Community structure is most likely regulated by both elevation and soil nitrogen availability.

Predicting the effects of climate change on plant communities relies on a good understanding of plant community structure, including species richness and species composition. Yet, the process of plant community assembly remains unclear due to the complex interactions between biotic and abiotic factors. Recently, the importance of environmental filters and interspecific competition on plant community assembly has been examined by exploring plant functional traits. The theory being that a species cannot tolerate an environment unless it has the traits necessary to tolerate that environment. For instance, soil nitrogen content can act as an environmental filter, restricting species number when availability is low but allowing for intense interspecific competition when it is more freely available. This means that the trait range of a plant community is narrower in more severe environmental conditions, such as high elevation, soil nutrient and water limitation, and will likely narrow further under climate change.

A subalpine conifer-hardwood mixed forest (2000 m above sea level) on Mt. Norikura in central Japan. Image credit: K. Takahashi.

In their new study published in AoBP, Ohdo and Takahashi examined the community assembly of vascular plants along gradients of elevation (45‒2500 m) and soil nitrogen availability in central Japan. They specifically investigated trait distribution patterns of four functional traits (plant height, leaf area, specific leaf area and leaf nitrogen content). Their results suggest that severe climatic conditions reduce the number of tree species and canopy height at high elevations, leading to increases in the number of herb and fern species due to increased light transmission to the forest floor. They further suggest that lower nitrogen availability decreases the number of tree, herb, and fern species by excluding those species with leaf traits unsuited to lower nitrogen availability. The authors acknowledge that using single trait analyses such as these assumes that environmental filtering and interspecific competition can be detected by the distribution pattern of each functional trait. However, they hope that future works will build upon this foundation to reveal how the multi-dimensional nature of functional traits affects community assembly along gradients of elevation and soil nitrogen availability.