高山林线交错带高山杜鹃的凋落物分解

凋落物分解是维持生态系统生产力、养分循环、土壤有机质形成的关键生态过程。高山林线交错带是陆地生态系统中对气候变化响应的敏感区域。季节变化和海拔梯度上的植被类型差异可能会影响该区域凋落物的分解,进而对高山生态系统的碳氮循环产生重要影响。采用凋落物分解袋的方法,研究了川西高山林线交错带优势种高山杜鹃(Rhododendron lapponicum)凋落叶在雪被期和生长季的分解特征。结果显示:(1)季节变化和植被类型对高山杜鹃凋落物的分解均具有显著影响(P0.05),凋落叶的质量损失主要发生在生长季且在高山林线最大,暗针叶林中雪被期的质量损失略高于生长季,但差异不显著;(2)林线交错带上高山杜鹃凋落叶分解缓慢,一年干物质失重率为9.62%,拟合分解系数k为0.145;(3)高山杜鹃凋落叶的质量变化主要体现在纤维素降解显著且集中在雪被期,木质素无明显降解,在高山林线上C/N、C/P、木质素/N变化幅度较小且C、N、P的释放表现得稳定而持续。结果表明,季节性雪被对林线交错带内高山杜鹃分解的影响不仅局限在雪被期内,雪被融化期间频繁的冻融作用和雪融水淋洗作用可能会促进高山杜鹃凋落物在生长季初期的分解。总的来看,在气候变暖的情景下,雪被的缩减、生长季的延长和高山杜鹃群落的扩张可能加速高山林线交错带高山杜鹃凋落物的分解。

Litter decomposition of Rhododendron lapponicum in alpine timberline ecotone

Litter decomposition is one of the key ecological processes in forest ecosystem, which plays an important role in ecosystem productivity, nutrient cycling and the formation of soil organic matter. In alpine ecosystem, freeze-thaw cycle and snow cover play an important role of litter decomposition processes. Climate change has changed the snow cover and circulation pattern of soil freezing and thawing in the alpine area, however, it could cause timberline upward shift and a trend for a shrub expansion, which will change community structure and species composition in the alpine region. Alpine timberline ecotone indicates the limit distribution of the forest and is a sensitive area of global climate change. From alpine coniferous forest to alpine shrub meadows, different vegetation types on snowfall distribution may cause dramatic variations in snow depth and snowmelt timing, soil temperatures and freezing depth in ecosystem, which in turn control litter decomposition process, and subsequently influence carbon and nitrogen cycle in alpine ecosystem. Rhododendron lapponicum is one of constructive species or dominant species in alpine/subalpine of western China, which was common in the fir forest and alpine scrub meadow. We present here one year study of R. lapponicum leaf litter decomposition in snowing season and growing seasonin alpine timberline ecotone located in the eastern Tibetan Plateau, China, where the ground is completely covered with snow for 5-6 months each year. A field experiment using litterbag method was carried out in different vegetations along an elevation gradient in an alpine timberline ecotone, mass loss, nutrient release and changes of leaf litter were studied. The results showed that: 1) both seasonal variation and vegetation type had significantly effects on leaf litter decomposition of R. lapponicum. Mass loss mainly occured in the growing season and showed the highest value in timberline compared with other vegetations. This may be relatively higher average temperatures in the growing season than snowing season, and there were the most dramatic temperature fluctuations and most frequent freeze-thaw cycles in the timberline. However, slightly higher mass loss in coniferous forest was observed in snowing season compared with that in growing season. 2)Litter mass loss (9.62%) was relatively slow with a coefficient k (0.145) in the first year decomposition in alpine timberline ecotone. 3)Change in quality of leaf litter was mainly reflected in a significant cellulose degradation that concentrated in snowing season, but not significantly degradation of lignin of R. Lapponicum. The C/N, C/P, lignin/N ratios changed little and C, N, P release performed stable and sustainable as the first year decomposition process in the timberline. The results suggested that seasonal snow cover do not only affect litter decomposition in the period of snowing season, but may further accelerate leaf litter decomposition of R. lapponicum in the beginning of the growing season according to frequent freeze-thaw cycles and snow melting in the alpine timberline ectone. In conclusion, with the reduction of snow cover, extension of growing season and the expansion of shrub community as R. lapponicum, would accelerate the litter decomposition of R. lapponicum in alpine timberline ecotone in the scenario of climate warming.