岷江上游高山森林凋落叶在冬季河流中的质量损失特征

凋落叶在高山森林河流中的分解不仅是生态系统物质循环的重要内容,而且与森林养分流失以及下游水体环境密切相关,并可能受到冬季雪被变化和土壤季节性冻融的影响,但一直缺乏必要的关注。因此,以岷江上游高山森林4种代表性植物康定柳(Salix paraplesia)、高山杜鹃(Rhododendron lapponicum)、方枝柏(Sabina saltuaria)和四川红杉(Larix mastersiana)凋落叶为研究对象,采用凋落叶分解袋法,研究了一个冬季不同冻融时期(冻结初期、冻结期和融化期)的质量损失特征。经过一个冬季(162d)的分解,康定柳、高山杜鹃、方枝柏和四川红杉凋落叶分别完成了初始干重45.5%、18.9%、26.4%和23.8%的分解;除康定柳凋落叶质量损失在融化期最大外,其余3种凋落叶均表现为冻结初期最大;康定柳、方枝柏和四川红杉凋落叶质量损失与河流水环境的平均温度和正积温均表现出显著或极显著的正相关关系,且与河流流速和硝态氮显著正相关,而与河流pH显著负相关;高山杜鹃凋落叶质量损失除与HCO3-含量显著相关外,与河流水温及其它水质特征均无显著相关关系。这些结果表明高山森林河流流速及水环境特征显著影响了凋落叶分解及其相关的物质循环过程,但影响程度受到凋落叶特性的调控。

FOLIAR LITTER MASS LOSS IN WINTER IN AN ALPINE FOREST RIVER IN THE UPPER REACHES OF THE MINJIANG RIVER

Litter decomposition in forested rivers is an important component of material cycle and energy flow in forest ecosystems, and it is a key process of forest nutrient output and information exchange across ecosystems as well. However, little is known on the process of litter decomposition in alpine forest rivers during different freeze-thaw periods in winter. The present study was conducted to investigate litter mass loss during different freeze-thaw periods of winter in an alpine forest river. Using the litterbag method, a field experiment was conducted to investigate foliar litter mass losses of four regional typical plants willow (Salix paraplesia), azalea (Rhododendron lapponicum), cypress (Sabina saltuaria), and larch (Larix mastersiana) in pre-freezing period, freezing period, and thawing period during temperature fluctuation in winter in an alpine forest river in the upper reaches of the Minjiang River. The results suggested that foliar litter of willow, azalea, cypress, and larch lost 45.5%, 18.9%, 26.4% and 23.8% of the initial dry mass after the incubation of a whole winter (162 days), respectively. Compared with the other freeze-thaw periods, foliar litter mass losses were the highest in pre-freezing period except for that of willow, which was the highest in thawing period. Foliar litter mass losses of willow, cypress, and larch were significantly and positively correlated to river average temperature, positive accumulated temperature, flow velocity, and nitrate concentration regardless of sampling periods. The mass losses of these three foliar litter types were significantly and negatively correlated to water pH in river. Except for a significant correlation between azalea foliar litter mass loss and river water HCO₃^- concentration, no significant relation was observed between azalea foliar litter mass loss and water temperature or other water characteristics. These results indicated that water characteristics of the alpine forest river significantly influenced foliar litter decomposition and its related processes of material cycles, but the magnitude of this influence was controlled by foliar litter features. Under a scenario of global warming, the freezing and thawing patterns in alpine forests could be influenced significantly, subsequently having influences on the water physicochemical characteristics and litter decomposition in forested rivers. A warmer temperature in winter may generate a higher river flow velocity, which would promote litter decomposition. Furthermore, warming-induced microbial activities could also accelerate litter decomposition because of constant moisture in aquatic ecosystems. As a result, litter decomposition in alpine forest rivers would be meaningful for nutrient cycling and energy flow relative to that in forest floors under a scenario of global changes.