Aboveground net primary productivity in regenerating seasonally dry tropical forest: contributions of rainfall, forest age, and soil

Identifying factors controlling forest productivity is critical to
understanding forest-climate change feedbacks, modeling vegetation
dynamics, and carbon finance schemes. However, little research has focused
on productivity in regenerating tropical forest which are expanding in
their fraction of global area have an order of magnitude larger carbon
uptake rates relative to older forest. We examined aboveground net primary
productivity (ANPP) and its components (wood production and litterfall)
over ten years in forest plots that vary in successional age, soil
characteristics, and species composition using band dendrometers and
litterfall traps in regenerating seasonally dry tropical forests in
northwestern Costa Rica. We show that the components of ANPP are
differentially driven by age and annual rainfall and that local soil
variation is important. Total ANPP was explained by a combination of age,
annual rainfall, and soil variation. Wood production comprised 35% of ANPP
on average across sites and years, and was explained by annual rainfall
but not forest age. Conversely, litterfall increased with forest age and
soil fertility yet was not affected by annual rainfall. In this region,
edaphic variability is highly correlated with plant community composition.
Thus, variation in ecosystem processes explained by soil may also be
partially explained by species composition. These results suggest that
future changes in annual rainfall can alter the secondary forest carbon
sink, but that this effect will be buffered by the litterfall flux which
varies little among years. In determining the long-term strength of the
secondary forest carbon sink, both rainfall and forest age will be
critical variables to track. We also conclude that a detailed
understanding of local site variation in soils and plant communities may
be required to accurately predict the impact of changing rainfall on
forest carbon uptake. Synthesis We show that in seasonally dry tropical
forests, annual rainfall has a positive relationship with the growth of
aboveground woody tissues of trees and that droughts lead to significant
reductions in aboveground productivity. These results provide evidence for
climate change – carbon cycle feedbacks in the seasonal tropics and
highlight the value of longitudinal data on forest regeneration.