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., 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)for upper- and
lower-layer tree communities within a 20-ha plot in a
subtropical, evergreen broad-leaved forest in the Dinghushan 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 trait 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 dimeter, 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.