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Throughfall kinetic energy in young subtropical forests: Investigation on tree species richness effects and spatial variability
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Agricultural and Forest Meteorology
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Soil erosion threatens ecosystem functioning by reducing soil organic carbon stocks or relocating nutrients. A common measure to protect soil against erosion is afforestation. There is growing evidence that mixed-species forest stands have beneficial effects on ecosystem functions (growth rates, nutrient cycling). In addition, species-rich forests tend to have higher and denser crown cover and thus might affect soil erosion. This study investigated the role of tree species richness on throughfall kinetic energy (TKE) as an important part of the soil erosion process and examined the spatial variability of TKE in mixed-species forest stands. The research was conducted within BEF-China, a large-scale forest biodiversity experiment in subtropical China. In summer 2013, 1800 TKE measurements were carried out during five rainfall events. TKE was measured using splash cups and related to tree height, crown base height, number of branches, leaf area index, stem ground diameter and crown area. Our experiment showed that TKE was not influenced by tree species richness at the plot level. This is likely due to the young age of the experimental forest where a dense and high tree canopy has not yet been developed. However, TKE was influenced by neighborhood tree species richness indicating that tree species richness only affected TKE on a small spatial scale within the direct neighborhood in young forests. TKE showed distinct spatial variability. Directly below the first branch of the tree individuals TKE was lowest (430 J/m2) while it was highest in the middle of four tree individuals (556 J/m2). Mean freefall kinetic energy (FKE) was 480 J/m2. Lower TKE below the first branch than FKE can be attributed to low rain drop velocities due to short falling heights. Higher TKE in the middle of four tree individuals than FKE can be ascribed to a larger crown area on which drops can confluence resulting in an increase of drop mass. Furthermore, TKE was positively affected by the number of influencing tree individuals with a 13% increase of TKE from one to four influencing tree individuals. However, further investigation on TKE below mature trees and dense canopies is needed to confirm our findings for later successional stages.
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