Composition of riparian litter input regulates organic matter decomposition: Implications for headwater stream functioning in a managed forest landscape

Abstract

Although the importance of stream condition for leaf litter decomposition has been extensively studied, little is known about how processing rates change in response to altered riparian vegetation community composition. We investigated patterns of plant litter input and decomposition across 20 boreal headwater streams that varied in proportions of riparian deciduous and coniferous trees. We measured a suite of in-stream physical and chemical characteristics, as well as the amount and type of litter inputs from riparian vegetation, and related these to decomposition rates of native (alder, birch, and spruce) and introduced (lodgepole pine) litter species incubated in coarse- and fine-mesh bags. Total litter inputs ranged more than fivefold among sites and increased with the proportion of deciduous vegetation in the riparian zone. In line with differences in initial litter quality, mean decomposition rate was highest for alder, followed by birch, spruce, and lodgepole pine (12, 55, and 68% lower rates, respectively). Further, these rates were greater in coarse-mesh bags that allow colonization by macroinvertebrates. Variance in decomposition rate among sites for different species was best explained by different sets of environmental conditions, but litter-input composition (i.e., quality) was overall highly important. On average, native litter decomposed faster in sites with higher-quality litter input and (with the exception of spruce) higher concentrations of dissolved nutrients and open canopies. By contrast, lodgepole pine decomposed more rapidly in sites receiving lower-quality litter inputs. Birch litter decomposition rate in coarse-mesh bags was best predicted by the same environmental variables as in fine-mesh bags, with additional positive influences of macroinvertebrate species richness. Hence, to facilitate energy turnover in boreal headwaters, forest management with focus on conifer production should aim at increasing the presence of native deciduous trees along streams, as they promote conditions that favor higher decomposition rates of terrestrial plant litter.

Keywords: boreal; introduced species; land use; litter quality; priming effect.

Figure 1

Locations of study sites in northern Sweden, including map coordinates. The inset shows the location of the study region in Sweden

Figure 2

Results from principal component analyses (PCAs) showing associations among physical and water chemistry variables (gray arrows), and different types of riparian litter input (black arrows) in (a) summer and (b) autumn. Variance explained by PC1 and PC2, respectively, was 30.2% and 22.0% in summer and 26.7% and 18.1% in autumn

Figure 3

Litter mass loss rates in fine‐mesh litterbags (i.e., microbial decomposition) for alder, birch, spruce, and lodgepole pine (n = 20). Different small letters indicate significant differences at p = .05. Error bars represent ±1 SE

Figure 4

Results from PLS regression on litter mass loss in fine‐mesh litterbags for (a) alder, (b) birch, (c) spruce, and (d) lodgepole pine. Variance explained was 54.8%, 79.0%, 70.5%, and 60.5% (two components) for alder, birch, spruce, and lodgepole pine, respectively. Predictor variables with a VIP >0.7 are presented, and gray color indicates a VIP > 1.0

Figure 5

Results from PLS on birch litter mass loss in coarse‐mesh litterbags. Variance explained was 60.7% (two components). Predictor variables with a VIP > 0.7 are presented, and gray color indicates a VIP > 1.0