Foggy futures: How drought affects soil microbes in a tropical montane cloud forest

Tropical montane cloud forests (TMCFs) are undergoing rapid climate change. As recent studies report that lowland tropical forests have become net carbon (C) sources, TMCF now store more C aboveground and belowground than lowland forests. It is imperative to determine the fate of this C under future change. In particular, climate change forecasts predict hotter, drier conditions in TMCFs. We set up drought shelters in Armando Bermúdez National Park in the Cordillera Central, Dominican Republic, to study the effects of drying on carbon cycling. We imposed "chronic" and "intense" drought treatments along a temperature gradient, in a montane forest at 1350 m.a.s.l. and in a cloud forest at 2180 m.a.s.l. The "chronic" treatments had shelter roofs with 50% open area and were left up year round. The "intense" treatments deepened and extended the dry season for 6 months with 100% PPT-reduction shelters. Carbon cycling and microbial processes were evaluated in the treatments and paired control plots. We collected a range of soil data, analyzed microbial communities, and investigated the effects of drought on decomposition (via extracellular enzyme activity (EEA) and litter mass loss).

Drought shelters decreased soil moisture compared to control plots in both treatments, and set off a cascade of changes in C cycling and microbial function. In particular, decomposition (as a function of litter mass loss) decreased significantly in intense treatments in comparison to control in both the lower elevation (P < 0.001) and in the cloud forest (P = 0.022). Chronic treatments did not differ significantly from controls; however, there was higher percent mass loss in the chronic vs. intense treaments in the lower elevation site (P = 0.024). In addition, theabundance and diversity of microbes shifted markedly under the various treatments, and other soil properties and nutrients (e.g. mineralization rates) differed across treatments. The frequently highly organic and waterlogged soils in TMCFs, which store significant amounts of C, are especially vulnerable to climate change. Drier conditions in these forests will have long lasting effects on the composition and diversity of the microbes present and how these soils function, with important direct and indirect effects on rates of decomposition and C cycling in general.