Effects of elevated atmospheric CO2 concentration and temperature on litter decomposition in streams: A meta-analysis

The metabolism of forest streams depends on the decomposition of plant litter of terrestrial origin. In turn, the rate at which litter decomposes depends on litter characteristics, decomposer activity, environmental characteristics, and their interactions. Atmospheric changes, such as increases in atmospheric carbon dioxide concentration ([CO 2 ]) and in temperature, may affect all these variables. Here, we report the results of a meta ‐ analysis of 41 studies conducted worldwide between 1993 and 2017 on the effects of elevated atmospheric [CO 2 ], elevated temperature, or both (temperature + [CO 2 ]) on litter decomposition in streams. Elevated temperature significantly increased litter decomposition rates, whereas elevated [CO 2 ] and temperature + [CO 2 ] did not significantly affect litter decomposition rates. The effect of elevated temperature did not depend on the type of study (i.e., laboratory or field study, correlative field or manipulative field study) but in correlative field studies, the temperature effect was stronger over latitudinal than altitudinal gradients. Effects of elevated temperature also did not depend on the type of decomposer community (microbial or microbial and macroinvertebrates) but effects were always significant for total litter decomposition (both microbes and macroinvertebrates involved), whereas microbial ‐ driven litter decomposition was significantly affected only in manipulative studies. Effects of elevated temperature did not depend on the litter identity, although significant effects were found for some litter genera but not others. In terrestrial ecosystems, the elevated temperature was found to increase litter decomposition rates, whereas elevated [CO 2 ] decreased litter decomposition rates. Study type (laboratory or field) and litter identity were important moderators of the response of litter decomposition to elevated temperature and [CO 2 ] in terrestrial ecosystems. These differences between soil and stream ecosystems may be partially due to intrinsic differences (such as moisture that is not limiting in streams) between these ecosystems. In addition, our meta ‐ analysis is geographically biased with most studies being conducted in Europe. More studies in other parts of the world could allow for a better understanding of the effects of climate warming and [CO 2 ] increases on litter decomposition, the global carbon cycle, and biochemistry in streams.