Abstract: 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.

Key words: alpine forest river, foliar litter decomposition, river water characteristics, temperature