Wood trait–decay relationships vary with topography and rainfall seasonality in a subtropical forest in China

Plant traits affect wood decomposition. Far less acknowledged, wood
trait-decay relationships may vary with environmental conditions that
alter the spatiotemporal distribution of decomposer activities.
Unfavorable environments can suppress decomposer activities and weaken the
wood trait-decay relationships. We hypothesize that waterlogged soils in
the valley during the rainy season may decrease termite activities and
decomposition rates, especially of palatable deadwood. By comparison, wood
trait-decay relationships could be less affected at the ridge and hilltop
during the rainy season. We conducted wood decay experiments of
137 tree species in three sites (valley/ridge/hilltop) differing in
microclimate in a 50-ha plot in a subtropical forest. Eight physical and
chemical traits were measured to determine the most influential wood
properties for termite feeding activities and wood decomposition rates.
Samples were harvested after the rainy or after a whole year
(rainy-plus-dry season). Mass loss and volume loss (i.e. termite feeding
activities) of the first harvest were attributed to the rainy season,
while the difference between the two harvests was attributed to the dry
season. We found that termite feeding activities and wood mass
loss during the rainy season were decreased by soil waterlogging in the
valley but not on the ridge or hilltop. Wood density predominantly
controlled termite feeding and wood mass loss, but its negative effects
appeared at the ridge and hilltop only during the rainy season, and in the
valley only during the dry season. Notably, warmer soil temperatures in
the valley accelerated termite activities during the dry season, causing
similar final wood mass loss as higher-elevation sites. Synthesis. Our
study reveals that environmental effects on wood decomposition are not
only mediated by trait changes directly but also via the spatiotemporal
variations of wood trait-decay relationships indirectly. Moreover, wood
decomposition does not strictly follow the elevational temperature
gradients due to the negative effects of rainfall at lower elevations.
Notably, future droughts may reduce soil waterlogging and accelerate
termite activities and the decomposition of lower-density wood,
particularly in valleys. We are concerned that deadwood carbon stocks once
waterlogged by rainfall could become a C source under future droughts,
especially in regions where termites are dominant decomposers.