Effects Of Forest Gap Locations In An Alpine Forest On Element Release During Foliar Litter Decomposition

Plant litter decomposition is a crucial contributor to element cycling in terrestrial ecosystems, there are many factors influencing the decomposition of litter and canopy gaps are recognized as important determinants of forest regeneration. Forest gap formation by disturbances can alter the snow thickness, light intensity, and other environmental conditions that may influence the temporal and spatial dynamics of element at different scales. Although many studies have addressed element release from litter, knowledge of the effects of forest gaps on element dynamics during litter decomposition in alpine regions is limited. In addition, under a warming climate, the seasonal freezing and thawing pattern will be changed, the process of element dynamics in response to global warming need to be further researched urgently in the future. A field experiment was conducted to investigate foliar litter decomposition in an alpine forest of western Sichuan by using litterbag method. Air-dried foliar litter of willow (Salix paraplesia), larch (Larix mastersiana), fir (Abies faxoniana), azalea (Rhododendron lapponicum), birch (Betula albosinensis), and cypress (Sabinct saltuarid) were placed in nylon litterbags, and placed on the forest floor from forest gap to under the canopy cover. The samples were placed and retrieved from November 2012 to October 2014 with intermediate sampling dates. The litterbags were sampled at the snow formation stage, snow coverage stage, snow melt stage and during the growing season. According to the specific characteristics of alpine forest gaps, the objective of this study was to elucidate the effects of gaps on element dynamics as litter decomposed during snow cover season and growing season, intending to give a theoretical base for the researching of the litter decomposition and understanding the effects of gaps on element cycling in the alpine forest during potentially warmer winters.The research result shows that N accumulation and release from the foliar litter in the first year and second year, respectively. Compared with the extended gap edge and the closed canopy, the gap center and the canopy gap edge showed higher N accumulation rates and release rates in willow, larch and cypress litter. Over one year of decomposition, P release from the foliar litter occurred, regardless of the foliar litter species. The P release rates were greater in the gap center and the canopy gap edge for larch, azalea and cypress litter. P accumulated in willow, fir, azalea and cypress litter in the second year and the accumulation rates for willow and azalea litter were greater in the gap center.K concentrations showed decline as litter decomposed during two years, the K release rates were greater in the gap center and the canopy gap edge for all species. Na accumulation from litter except willow and larch litter in each year, and the highest Na accumulation rates in litter were recorded in the gap center and the canopy gap edge in the first year. Ca was released from all species of litter not only in the first year but in the next year, the Ca release rates were greater in the gap center and the canopy gap edge for willow, fir and birch litter. The Mg released for all litter except larch during the first and second year of decomposition, the release rates of Mg were greater at the extended gap edge and under the closed canopy.Cu accumulated in the fir, azalea and cypress litter in the first year and was released from all types of litter in the second year. The accumulation rates of Cu from the azalea and cypress litter and the release rates from willow, larch and azalea litter were greater in the gap center and the canopy gap edge. Pb accumulated in litter, regardless of the foliar litter species, and the accumulation rates of Pb were greater at the extended gap edge and under the closed canopy. Cd was released from litter in the first year, the highest Cd release rates in willow, birch and cypress litter were recorded at the gap center and the canopy gap edge. The Cd accumulation rates from larch and fir litter were higher at the extended gap edge and under the closed canopy.In conclusion, our results indicate that the alpine forest gaps may influence the element dynamics during litter decomposition by changing either the snow cover or the hydrothermal environment. The gap center and the canopy gap edge could promote the release of N for willow and larch litter and the release of K for all types of litter during winter and the growing season. The frequent freeze-thaw cycles in winter and the suitable hydrothermal environment during the growing season at the extended gap edge and under the closed canopy could promote the release of Na、Mg、Pb and Cd during foliar litter decomposition. Results showed that the decreased snow depth in the winter warming scenario would inhibit the release of N and K, while promoting the release of Na、Mg、Pb and Cd from decomposing litter, with the degree of release being related to litter quality. Overall, alpine forest gap formation may promote the release of N and K, and inhibit biogeochemical cycling of Na、Mg、Pb and Cd during litter decomposition in the alpine forest.