Factors contributing to leaf decomposition vary with temperature in two montane rivers of the Intermountain West, Utah

Terrestrial organic matter (OM) is an essential energy source that fuels many food webs. The factors contributing to OM decomposition and the rate at which OM decomposes can influence carbon fluxes through ecosystems. Previous research demonstrates that the factors driving OM decomposition can vary with environmental condition, prompting more research that characterizes the relative importance of each factor driving OM decomposition under differing environmental conditions. This is especially important for ecosystems that may be particularly vulnerable to climate change, like temperate, montane ecosystems. We used a 126-day leaf-pack study to compare and identify the most important factors (i.e., physical abrasion, microbial activity, and shredding macroinvertebrates) regulating OM decomposition rates ( k ) in two montane rivers. We used a structural equation model (SEM) to evaluate the relative importance of each factor contributing to OM decomposition. We found that k were significantly faster in the Blacksmith Fork. But when temperature differences were accounted for, k were approximately 1.5 times faster in the Logan River. Macroinvertebrate abundance and biomass, physical abrasion, nutrients, and temperature were significantly greater in the Blacksmith Fork, while microbial activity was the only factor significantly greater in the Logan River. We estimated that by day 100, microbes contributed 2.1 times more to decomposition in the Logan River (0.88 g; 14.6%) compared to the Blacksmith Fork (0.41 g; 6.9%). Relative to shredders (0.39 g; 6.5%), microbial contributions were approximately 2.2 times greater in the Logan River by day 100. Our SEM also revealed that microbes were more important to decomposition in this system relative to shredding macroinvertebrates. The reversal of k when day was replaced with degree-day and the significant direct effect of degree-days in our SEM suggests that temperature is a key factor regulating OM decomposition in these montane rivers. These findings contrast with many other studies conducted in montane systems, showing that microbes are less important contributors to OM decomposition at higher elevations, and further demonstrate that the relative importance of the factors driving OM decomposition is highly context dependent, even across small geographic scales.