亚高山森林林窗大小对凋落叶木质素降解的影响

木质素降解是认识高寒森林凋落物分解过程的关键环节,可能受到林窗大小及其在不同季节水热环境的影响。采用分解袋法,研究了川西亚高山森林不同面积大小林窗下红桦(Betula albo-sinensis)和岷江冷杉(Abies faxoniana)凋落叶在初冻期、深冻期、融化期、生长季节初期、生长季节中期和生长季节后期的木质素分解动态特征。研究结果表明,采样时间和林窗面积大小对两种凋落叶的木质素降解均有显著影响。经历1a分解,红桦凋落叶的木质素降解了21.53%—27.65%,而岷江冷杉凋落叶的木质素富集了7.95%—19.40%。较大林窗促进了冬季岷江冷杉凋落叶和生长季节红桦凋落叶木质素的降解,抑制了冬季红桦凋落叶木质素的降解;而生长季节岷江冷杉凋落叶木质素富集速率则为林下大林窗中林窗小林窗。逐步回归分析表明,凋落叶木质素的降解过程在冬季主要受到负积温和土壤冻融循环次数的影响(木质素结构的物理破碎),而在生长季节则主要受到平均温度和正积温的影响(木质素的生物降解)。可见,川西亚高山森林木质素降解受林窗格局变化的显著影响,且林窗大小对凋落叶木质素降解的影响与物种和分解时期有关。

Effects of gap sizes on the lignin degradation of foliar litter in a subalpine forest

Lignin, a major component of foliar litter, plays a crucial role in litter decomposition process in forest ecosystems. However, forest gap may affect lignin degradation of foliar litter in subalpine forests, since gaps with different sizes not only regulate snow cover and its associated freeze-thaw events in winter, but also affect temperature and moisture conditions during the following growing season; however, little information about the gap size-effect is currently available. To test the seasonal effects of forest gap sizes on litter lignin degradation, a field experiment using litterbags was conducted in a subalpine forest in western Sichuan, which is located along the upper reaches of the Yangtze River and in the eastern part of the Tibetan Plateau. Litterbags containing red birch (Betula albo-sinensis) and Minjiang fir (Abies faxoniana) foliar litter were placed on the forest floor under four gap sizes conditions:a closed canopy, a large gap, an intermediate gap and a small gap. The litterbags were sampled at the onset of the soil freezing, during soil deep freezing and soil thawing, and during the early, middle and late growing season. The experiment was conducted from November 2011 to October 2012. The lignin concentrations of the litter were measured, and the rates of lignin degradation were calculated. The results indicated that season and gap size had significant effects on lignin degradation of both red birch and Minjiang fir litter. Over the course of the one-year study period, lignin losses from the foliar litter were 21.53-27.65% for red birch and-7.95-19.40% for Minjiang fir, across the four size gaps. The large gap promoted the degradation of lignin in the Minjiang fir litter during winter, and that of lignin in the red birch litter during the growing season. Conversely, the large gap inhibited the degradation of lignin in the red birch litter during winter, while the lignin degradation rate of the Minjiang fir litter during the growing season was as follows:closed canopy > large gap > intermediate gap > small gap. In addition, a step-wise regression analysis result revealed that lignin degradation of foliar litter is significantly correlated with negative accumulated temperature and frequency of soil freeze-thaw cycle in winter, suggesting that the degradation of lignin that occurs during winter is mainly driven by physical damage to its molecular structure. Furthermore, close correlations were detected between litter lignin degradation rate and average temperature, and positive accumulated temperature, suggesting that biological processes may contribute to lignin degradation during the growing season. These results imply that differences in forest gap size significantly affect the lignin degradation process of foliar litter in subalpine forests; however, the effects may depend on plant species and season.