Impacts of Catastrophic Hurricanes on Stream Chemistry in Tropical Montane Forests are Long-Lasting, Context Dependent, and Vary by Critical Zone Architecture

Abstract:

Long-term studies and experiments in the Luquillo Mountains of Puerto Rico are designed to capture the effects of infrequent extreme events on stream chemistry. Past work has shown that in most watersheds, hurricanes that have sufficient wind speed to damage or kill canopy trees result in a flush of nitrate in stream water that lasts for a few years, paralleling the regrowth of vegetation in the forest. This conclusion is based on following the response of stream chemistry to two major hurricanes, Hugo in 1989 and Georges in 1998. Experimental manipulations of the forest canopy result in similar changes in soil solution as those observed following the hurricanes. In both the actual hurricanes and the experimental manipulations (canopy trimming), damage to the canopy resulted in elevated nitrate for 1.5 y. The response to the second hurricane or second hurricane simulation was less than the first, suggesting that effects of extreme events in the forest are context-dependent and not additive. Differences are also evident across bedrock types, with a much longer period of elevated concentrations before return to background on quartz diorite than on volcaniclastic bedrock. An “overshooting” of background concentrations, with nitrate in some streams dipping well below pre-disturbance levels 10-15 years after the last hurricanes, is now observed in some watersheds. Taken together, these results suggest that extreme events can shape the nitrogen dynamics of these tropical catchments for decades.