Large areas of tropical forests are vulnerable to both anthropogenic and natural disturbances, and important issues determining their sensitivity to disruption must be addressed to gain improved management practices. One of these is how changes in land-use or tropical storm patterns affect the resiliency of phosphorus-limited Neotropical forests where vegetation is sustained in the long term by atmospheric phosphorus inputs through rainfall, dust, or fog. Past researchers have found that dust and fog deposition depend on canopy density (leaf area). Canopy traps phosphorus enabling a positive feedback between vegetation and phosphorus deposition. This study is investigating the ecohydrological mechanism controlling the occurrence and strength of these feedbacks and their susceptibility to changes in climate and land cover. The principle objectives are: 1. To quantify the total phosphorus input from deposition to a dry neotropical forest and to investigate its fluctuations between dry and wet seasons, associated with fires, and proximity to ?slash and burn? management. 2. To quantify the dependence of phosphorus deposition on canopy properties and thus the strength of the canopy-phosphorus deposition feedback. 3. To determine the relative magnitudes and roles of rainfall, dust, and canopy condensation in total phosphorus input. These three processes are differently affected by changes in climate and land cover. Students will be trained and teaching materials developed for use in protecting these sensitive ecosystems.
Background The growth of many tropical forests is limited by the availability of phosphorus, particularly on highly weathered soils. Unlike nitrogen, phosphorus is a nutrient that cannot be replenished biologically and thus, losses of phosphorus from the system need to be balanced by inputs, which are typically provided by atmospheric deposition in rain water, fog, and dry atmospheric particulate coming from forest fires in the surrounding areas or dust emitted by desert landscapes or dry lake sediments. Some of these sources may be far from the deposition areas. Intellectual Merit The project investigated how the presence of forest canopies can enhance phosphorous deposition by trapping with its leaves fog droplets and airborne particulate. Subsequent rainfall events then wash the leaf surfaces and deliver these nutrients to the forest floor. With a number of different field measurements the PIs have demonstrated that such an enhancement consistently occurs. This project concentrated on the case of dry seasonal forests in the Yucatan peninsula of Mexico. This region is affected by shifting cultivation, whereby land parcels are cleared of forest vegetation, cultivated for a few years and then abandoned when the soils become relatively unproductive. At that point a secondary forest grows back until the land is cleared again after a few years. This land use pattern has a cultivated period followed by a fallow period in which the forest grows back and the soil resources are replenished. During that period the enhancement of phosphorus deposition is crucial to the recovery of forest vegetation and the replenishment of soil phosphorus pools. Measurements included the collection of rainwater in open areas and under forest canopies, sampling of fog water, and measurements of fog deposition. Water samples were carried to the University of Virginia and analyzed for organic and inorganic phosphorus content. The results showed that the rates of phosphorus deposition under the forest canopies were more than three times bigger than those in open areas. Some additional experiments were performed to verify that this phenomenon was not a trivial effect of rainwater removal of phosphorus containing compounds from the leaf tissues. The results conclusively demonstrated that there is a canopy-induced enhancement of phosphorus deposition. The significance of this result stems from the fact that, by enhancing the supply of this limiting nutrient, forests build their own habitat. Thus the deforestation can shift tropical forests to a state in which no forest vegetation can grow after clearcut because it would miss the additional supply of phosphorus afforded by the "canopy trapping" effect investigated in this project. Through process-based analyses of the phosphorus budgets the investigators have shown that in the case of landscapes affected by shifting cultivations the effect of canopy trapping can explain the downward spiral of land degradation caused by the reduced deposition rates during the cultivation periods. Broader impacts Two graduate students and one postdoctoral research associate from the University of Virginia were funded by this project. One of the graduate students served as a guest lecturer in university seminars for first year (undergraduate) students. Her lectures - on hydrological feedbacks in tropical deforestation - were based on the results obtained from this research project. Moreover the results of this research were presented at the local high school in Charlottesville (VA) to show high school students the methods used in the environmental sciences to address questions related to land use and land cover change. Preliminary results from this research were also presented by the PI at a workshop for K12 teachers held at the University of Virginia Experimental Farm in August 2009. The PI was one of the instructors at this workshop and his lectures provided the school teachers with a general background of new advances in forest hydrology, including the hydrologic controls on phosphorus deposition investigated as part of this project. One of the graduate students involved in the project was invited by faculty at El Colegio de la Frontera Sur (ECOSUR) in Chetumal, Mexico to make a presentation about his dissertation research for a graduate student seminar. Two field assistants and a lab technician were hired in Mexico to provide assistance wth field sampling, maintaining data loggers and rain gauges.
