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The effects of tropical forest fragmentation and land use on leaf litter decomposition and nutrient cycling

Noble, Virginia Rose. 2013. University of British Columbia

Abstract

Forest conversion and fragmentation are major causes of diminished ecosystem function and biodiversity loss at a global scale. Understanding landscape effects on leaf litter decomposition and recycling of available nutrients is critical for conservation, especially in diverse tropical rainforests. Here, we studied the effects of land conversion and forest fragmentation on these two processes in a tropical ecosystem. We examined decomposition and nutrient cycling during the wet season in 22 sites across four habitat types: continuous forests, large fragmented forest patches, small fragmented forest patches, and orange plantations, in the tropical moist forest zone of NW Guanacaste, Costa Rica. The study employed a two-by-two factorial design to explore the effects of leaf litter type (site-specific litter vs. control litter) and mesh size (9 mm vs. 1 mm) on decomposition rate across land cover types. Litter bags with different mesh sizes either included (9 mm) or excluded (1 mm) macroinvertebrates. We removed litter bags from the field after 3 and 8 weeks, and cleaned and weighed the contents. Additionally, we utilized Plant Root Simulator (PRS™) Probes to examine nutrient cycling of 10 cations and anions including N, P, K, Ca, and Mg in all site-specific litter types and both mesh treatments. We found that the rate of decomposition differed depending on habitat type, leaf litter type and mesh size. Decomposition was faster in orange plantations compared to any other forest type, and decomposition was faster in forest fragments compared to continuous forests. We determined that these differences in decomposition rates were due predominantly to differences in site-specific litter quality. Only in forest fragments did the distance to forest edge impact macroinvertebrate feeding behaviour (seen in control litter), presumably due to spatial variation in leaf litter quality. Finally, mineralization of organic nutrients differed across the four habitat types. For example, nitrogen, which is an important compound for plant cell structure and function, mineralized more quickly in orange plantations than in forest habitats. This study highlights the importance of litter quality differences due to fragmentation and land use in altering leaf litter decomposition and nutrient cycling.