Throughfall exclusion and fertilization effects on tropical dry forest tree plantations, a large-scale experiment

Across tropical ecosystems, global environmental change is causing drier
climatic conditions and increased nutrient deposition. Such changes
represent large uncertainties due to unknown interactions between drought
and nutrient availability in controlling ecosystem net primary
productivity (NPP). Using a large-scale manipulative experiment, we
studied for 4 years whether nutrient availability affects the individual
and integrated responses of aboveground and belowground ecosystem
processes to throughfall exclusion in 30-year-old mixed plantations of
tropical dry forest tree species in Guanacaste, Costa Rica. We used a
factorial design with four treatments: control, fertilization (F), drought
(D), and drought + fertilization (D + F). While we found that a 13 %–15 %
reduction in soil moisture only led to weak effects in the studied
ecosystem processes, NPP increased as a function of F and D + F. The
relative contribution of each biomass flux to NPP varied depending on the
treatment, with woody biomass being more important for F and root biomass
for D + F and D. Moreover, the F treatment showed modest increases in
maximum canopy cover. Plant functional type (i.e., N fixation or
deciduousness) and not the experimental manipulations was the main source
of variation in tree growth. Belowground processes also responded to
experimental treatments, as we found a decrease in nodulation for F plots
and an increase in microbial carbon use efficiency 25 for F and D plots.
Our results emphasize that nutrient availability, more so than modest
reductions in soil moisture, limits ecosystem processes in tropical dry
forests and that soil fertility interactions with other aspects of drought
intensity (e.g., vapor pressure deficit) are yet to be explored.