Abstract:

Aims: Functional differences between species play a vital role in enabling plant species to coexist within an ecosystem. An example of this phenomenon is the vertical stratification of tree communities within a forest, whereby the upper-layer of trees (i.e., canopy) exhibit different ecological structures and functions than the lower-layer of trees (i.e., understory). To identify specific structural and function differences between these communities, we analyzed how upper-layer trees in a subtropical forest impacted lower-layer trees. We further analyzed how these traits changed after a typhoon impacted these communities.

Methods: To perform these analyses, we measured 20 unique plant functional traits (e.g., maximum area-based leaf carbon assimilation rate and instantaneous water use efficiency), structural characteristics (e.g., species abundance and evenness) and functional characteristics (e.g., relative growth rate and mortality rate) for upper- and lower-layer tree communities within a 20-ha forest dynamics plot in a subtropical, evergreen broad-leaved forest in the Dinghushan National Nature Reserve of Guangdong Province, China. To control for spatial autocorrelation (dependence) between the upper- and lower-layers within each plot, Lee’s L statistic was used to characterize how similar the spatial clustering patterns were for functional traits across these layers, as well as for functional traits of the upper-layer communities and structural or functional characteristic of the lower-layer communities. To quantify how the functional traits of the upper-layer communities impacted the structural and functional characteristic in the lower-layer communities, multivariate spatial autoregressive models were utilized, revealing the relative importance of each trait. Finally, we tested how a typhoon affected these relationships by incorporating data into the multivariate regression model prior to and after the typhoon occurred.

Results: Variations in functional traits of the upper-layer trees explained most variations in the structural and functional characteristics of the lower-layer. The spatial structure and the efficiency of photosynthesis and water use in upper-layer trees significantly affected the structure and function of the lower-layer. In particular, as tree species in the upper-layer captured more light, those in the lower-layer were less abundant, rich, and diverse, and their growth rate and recruitment decreased. Conversely, when species in the upper-layer were utilized more water, those in the lower-layer more abundant, rich, and diverse, and their growth rate and recruitment increased. Additionally, the maximum area-based leaf carbon assimilation rate, leaf area, fresh leaf weight, petiole diameter, and xylem specific conductivity of leaves for species in the upper-layer strongly influenced the structure and function of species in the lower-layer. Finally, the typhoon altered the vertical structure of these communities, leading to a corresponding change in how the upper-layer of community impacted the structure and function of the lower-layer.

Conclusion: Our research demonstrated that the vertical structure of subtropical tree communities significantly impact the structure and function of these communities.

Key words: forest canopy, functional traits, vertical stratification, community structure and functions, canopy structure