Organic matter decomposition along a temperature gradient in a forested headwater stream

Background/Question/Methods

By the end of the century, stream water temperature is predicted to increase by 1-9°C, potentially altering the rates of temperature-dependent ecosystem processes, such as organic matter decomposition. Under warmer temperatures, decomposition rates may increase due to increased microbial activity, but may decrease due to decreased shredder densities. Further, the response of decomposition to warming may also vary by plant species due to the intrinsic characteristics (e.g., lignin, tannins, C:N) of litter that enters streams. In this study, we examined the effect of temperature and leaf litter quality on decomposition along the naturally-occurring temperature gradient in Walker Branch, a forested headwater stream in eastern Tennessee, USA. Water temperature varies up to 5°C between upstream and downstream sites, as upstream springs result in fairly stable water temperatures year-round, while downstream water temperatures are lower in autumn and winter. We used standard litterbag techniques to quantify breakdown rates (k; d^-1) of leaves from three plant species (tulip poplar [Liriodendron tulipifera], red maple [Acer rubrum], and white oak [Quercus alba]) that vary in litter quality at five locations along the downstream temperature gradient. We also measured microbial respiration and invertebrate density through time to examine the biological factors influencing decomposition. 

Results/Conclusions

In general, litter breakdown rates were higher in the warmer, upstream sites, and slower in the cooler, downstream sites. Mass loss data collection is ongoing, but there was greater variation in breakdown rates along the temperature gradient for higher-quality organic matter (red maple: upstream k = 0.0363 d^-1, downstream k = 0.0242 d^-1; tulip poplar: upstream k = 0.0280 d^-1, downstream k = 0.0197 d^-1) than lower-quality organic matter (white oak: upstream k = 0.0100 d^-1, downstream k = 0.0072 d^-1). Microbial respiration rates were also higher on red maple and tulip poplar than white oak; however, microbial respiration rates for a given species were similar along the temperature gradient. Further work will examine whether other extrinsic factors (invertebrate density, nutrient concentrations) influence breakdown rates along the temperature gradient. Overall, examining whether temperature and litter quality interact to affect decomposition will help inform how organic matter processing may respond to warmer stream water temperature.