Abstract:
Across tropical ecosystems, global environmen-
tal change is causing drier climatic conditions and increased
nutrient deposition. Such changes represent large uncertain-
ties due to unknown interactions between drought and nutri-
ent availability in controlling ecosystem net primary produc-
tivity (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 ef-
fects 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 experimen-
tal manipulations was the main source of variation in tree
growth. Belowground processes also responded to experi-
mental treatments, as we found a decrease in nodulation for
F plots and an increase in microbial carbon use efficiency
for F and D plots. Our results emphasize that nutrient avail-
ability, more so than modest reductions in soil moisture, lim-
its 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.
