The U-shaped pattern of size-dependent mortality and its driving factors in a subtropical monsoon evergreen forest

1. Tree mortality is an important ecological process influencing multiple
functions of forest ecosystems. Previous studies have shown two basic
size-mortality patterns, including a competition-driven declining and a
disturbance-driven increasing mortality rate with tree size. Subtropical
forests, which have a high species diversity and subject to frequent
monsoon disturbances, are widely distributed in eastern Asia. However, the
tree size-mortality pattern in the mature subtropical forests remains
unclear. 2. Here we analyzed patterns of size-dependent mortality from
tree species to forest community using a 5-year inventory data from 117
species and 163,612 individuals in a 20-ha forest dynamic plot in a mature
subtropical monsoon evergreen forest in eastern China. To explain the
spatial variability in mortality patterns, two major biotic drivers
(competition and tree size) and multiple local-scale environmental factors
were further analyzed. 3. Our results showed that tree size was the best
predictor of tree mortality at the scales of both species and community. A
species-level analysis identified four size-mortality patterns that are
shaped by species-specific attributes such as maximum size and life form.
For 27 out of 92 species that comprised 59% of tree individuals, the
relationship between size and mortality exhibited a U-shaped pattern of a
first decline followed by an increase. An overall community-scale
size-dependent mortality also showed a U-shaped pattern. 4. Tree mortality
was also influenced by the competition and environmental conditions, but
the relative importance varied widely across tree sizes and species. The
competition showed significant correlations with the mortality of small
trees, while the effect of environmental conditions on mortality was
strongest for large trees. A principal component analysis showed that a
combination of biotic and abiotic factors explained 42.3% of the spatial
variation in mortality at large sizes. Synthesis. Our results reveal four
identifiable size-dependent mortality patterns that differ across diverse
species, jointly leading to a U-shaped mortality size pattern at the
community level. This finding calls for the need to establish the details
of every potential size-mortality pattern with consideration of the
different effects of biotic and abiotic factors on tree mortality of
specific size.