Factors controlling leaf processing in stream ecosystems and a hierarchical model of their interactions.

摘要

Deciduous leaf litter represents a critical energy source for stream organisms. Leaves that enter a stream undergo a series of steps during the course of decomposition, referred to as processing. Processing involves leaching of soluble compounds, enzymatic breakdown by microbes, physical breakage, and consumption by invertebrates. I conducted studies to examine three topics: the processing of leaves in a hardwater, mountain stream; the role of spatial scales in leaf processing; and the application of hierarchy theory to leaf processing in stream ecosystems.;The South Fork of Mink Creek (Bannock County, ID) contains high concentrations of dissolved inorganic nitrogen and phosphorus (approximately 50 mug/L, each) and hardwater (total hardness > 300 mg CaCO3/L). Nutrient enrichment did not accelerate the processing of aspen (Populus tremuloides ), birch (Betuld occidentalis), or dogwood ( Cornus stolonifera) in SF Mink Creek; leaf processing was not nutrient limited. Traditional measures of leaf quality (% N, C/N ratio) did not correspond to relative rates of processing among the three leaf types examined. Rather, the % of the carbon in the form of lignin corresponded to relative processing rates and further supports the conclusion that leaf processing in SF Mink Creek was not nutrient limited. Terrestrial preconditioning for up to 14 days did not affect rates of in-stream processing of aspen and dogwood leaves. Leaf litter decayed faster in SF Mink than in streams elsewhere in North America. Similarly, rates of microbial respiration on the leaves in SF Mink were higher than those reported for similar leaves in other systems.;When examined across nine streams in central/southern Idaho, leaf processing was determined to be scale-dependent; indicating that conclusions drawn about leaf processing depend on the spatial scale of observation. Factors controlling processing among the nine sites included temperature, nitrogen concentration, and the abundance of shredding macroinvertebrates---factors that were expected to be constant within a stream. The factors explaining variation in leaf processing at the regional scale did not operate equally across leaf types. The processing of dogwood leaves appeared to be primarily microbial whereas the processing of aspen leaves was strongly related to the relative abundance of shredders.;Lastly, a conceptual framework for examining leaf processing across spatial scales is presented in the context of hierarchy theory. The conceptual framework for leaf processing in stream ecosystems consists of a dual hierarchy: one of constraints and one of structural units on which those various constraints act. The implication of this dual hierarchy is that as one examines leaf processing at increasing spatial scales there is a concomitant increase in the spatial scale at which the constraints act.