Linking soils and streams: Response of soil solution chemistry to simulated hurricane disturbance mirrors stream chemistry following a severe hurricane

Abstract

Understanding the drivers of forest ecosystem response to major disturbance events is an important topic in forest ecology and ecosystem management. Because of the multiple elements included in most major disturbances such as hurricanes, fires, or landslides, it is often difficult to ascribe a specific driver to the observed response. This is particularly true for the long-term effects of hurricanes on forest ecosystem nutrient cycling. Hurricane disturbance opens the forest canopy by removing leaves and branches or snapping boles, and in so doing turns living biomass into debris that is deposited on the forest floor. At the watershed scale, past work in the Luquillo Mountains of Puerto Rico shows that these changes in forest structure and detrital dynamics result in large increases in stream water nitrate and potassium concentrations in streams draining volcaniclastic terrain. The Canopy Trimming Experiment (CTE) was designed to simulate the major effects of hurricane disturbance, and disentangle the effects of canopy opening and debris addition on forest biology and biogeochemistry following a hurricane. Using the chemistry of soil solution as an integrated indicator of biogeochemical response to hurricane simulation, the experimental manipulations show that the synergistic effects of both canopy opening and debris addition are needed to elicit one of the whole-watershed responses to hurricanes, a large pulse of nitrate (NO3) concentration in stream water that lasted approximately 18months. Manipulation of either canopy openness or debris addition alone had little effect on soil solution chemistry for NO3, or for any other solute measured (dissolved organic matter, phosphate, ammonium, major cations and anions, and silica). None of the treatments resulted in the increased potassium (K) seen in stream water following hurricane disturbance. For NO3, the time course of response and recovery following combined treatments of canopy opening and debris addition was similar to that observed in stream water after actual hurricanes. The CTE provided further evidence that tree regrowth following hurricane disturbance controls the return of NO3 concentrations to pre-hurricane levels in this tropical forest. The lack of response in K to hurricane simulation suggests that leaching of the added debris, which was not captured in the experimental manipulations, is a major driver of K concentrations following disturbance. Hurricane disturbance, which is significant in many humid tropical forests, results in pulsed outputs of nitrogen in stream water that can be clearly ascribed to interactions between damage and recovery of canopy vegetation, and decomposition of detrital inputs on the forest floor.