高山森林林窗对凋落叶分解的影响

林窗对降水和光照等环境条件的再分配以及分解者群落的影响可能深刻作用于森林凋落物分解过程,但有关高山森林林窗大小对凋落物分解的影响尚无研究报道。采用凋落物分解袋法,研究了川西高山森林不同大小林窗对非生长季节和生长季节红桦(Betula albo-sinensis)和岷江冷杉(Abies faxoniana)凋落叶质量损失的影响。结果显示,经过1a的分解,不同生境下红桦和岷江冷杉凋落叶分别分解了27.25%—30.12%和27.04%—27.96%,其中非生长季节占53.83%—60.18%和50.23%—59.09%。林窗对红桦和岷江冷杉凋落叶质量损失的影响因物种不同而呈现季节差异。总体上,林窗加快了岷江冷杉凋落叶的分解而延缓了红桦凋落叶的分解。与郁闭林下相比,林窗显著增加了2种凋落叶非生长季节的质量损失速率,显著降低了生长季节2种凋落叶的质量损失速率;2种凋落叶质量损失速率在非生长季节随林窗面积增大而加快,在生长季节随林窗面积增大而减慢。林窗显著影响了初冻期、深冻期和融化期岷江冷杉凋落叶的质量损失率,但对红桦凋落叶质量损失率影响不显著。可见,高山森林凋落物分解过程受到林窗的显著影响,并且阔叶和针叶凋落叶在非生长季节和生长季节对林窗的响应具有明显差异。

Effects of gap sizes on foliar litter decomposition in alpine forests

Forest gaps are one of the most normal interferences in primary forest ecosystems, and they play an important role in not only aboveground processes but also belowground processes. However, many studies have paid more attention to the effects of forest gaps on aboveground processes. Increasing evidence has demonstrated that forest gaps have a strong effect on mass loss and carbon and nutrient release in litter by redistributing precipitation, light, and other environmental factors. At high latitudes and altitudes, litter decomposition due to forest gaps may be more complex because of seasonal snow cover and freeze-thaw cycles. Theoretically, interception of rainfall and snowfall in the winter by the canopy and canopy shading may cause the hydrothermal dynamics on the forest floor to vary with respect to gap sizes and critical periods. Currently, little information is available on the effects of gap sizes on litter decomposition at different critical periods. In order to understand the effects of forest gaps on litter decomposition in forest ecosystems at high altitudes, litterbags with red birch (Betula albosinensis) and Minjiang fir (Abies faxoniana) foliar litter were incubated on the forest floor in small, medium, and large gaps and the adjacent closed canopy from November 21, 2011 to October 29, 2012. Then, the litterbags were sampled at the periods of onset of soil freezing (OF), soil deep-freezing (DF), soil thawing (TP), early growth (EP), mid-growth (MP), and late growth (LP), and the mass loss was calculated. After a year of litter decomposition, red birch and Minjiang foliar litter lost 27.25-30.12% and 27.04-27.96% mass, respectively. The mass loss of red birch and Minjiang fir foliar litter in the non-growing season accounted for 53.8-60.18% and 50.23-59.09% of the total mass loss, respectively. Gap sizes have strong effects on the mass loss of Minjiang fir and red birch foliar litter, depending on tree species and sampling periods. Forest gaps accelerated the litter decomposition of Minjiang fir, but slowed down the litter decomposition of red birch. Regardless of tree species, forest gaps significantly accelerated the rates of mass loss of foliar litter in the non-growing season (P < 0.05), but significantly lowered them in the growing season (P < 0.05). The rates of mass loss of foliar litter increased with gap sizes in the non-growing season, but decreased with gap sizes in the growing season. Moreover, the rates of mass loss of Minjiang fir foliar litter in the periods of OF, DF, and TP increased significantly with gap sizes (P < 0.05), but the effects on red birch foliar litter were not significant (P > 0.05). In addition, the contribution of litter mass loss at different critical periods to a year of mass loss was in the order of DF > MP > OF > EP > LP > TP. Mass loss of red birch foliar litter in the non-growing season accounted for 60.18%, 54.46%, 55.34%, and 53.83% of the one-year mass loss in large gaps, middle gaps, small gaps, and closed canopy, respectively; similarly, the contributions of mass loss of Minjiang fir foliar litter to the one-year mass loss in the non-growing season were 59.09%, 54.37%, 52.22%, and 50.23%, respectively. These results suggest that forest gaps accelerated the mass loss of foliar litter in the snow-cover season, but slowed down the mass loss in the growing season. The responses of litter decomposition to gap sizes in the snow-cover season were different from those in the growing season, implying that gap sizes could have strong effects on litter decomposition at different critical periods in the coniferous forest ecosystem of alpine regions.