Wood decomposition in European rivers increases with temperature but decreases with human population density

Plant litter decomposition in rivers is shaped by multiple environmental conditions, which are modified by riparian zone char-acteristics and human activities, thereby impacting in situ plant litter decomposition rates. However, disentangling the relativeimportance of these conditions for plant litter decomposition rates is challenging without large-scale studies encompassing wideenvironmental and anthropogenic gradients. We carried out a continental-scale study on plant litter decomposition in 72 riverlocations across 7 catchments in Europe (Germany, Italy, Portugal, Spain, Sweden), representing wide gradients in climatic con-ditions, riparian zone characteristics, land-use intensity, and human population density. We used remote sensing data and fieldsurveys to quantify catchment, riparian, and river habitat characteristics. To assess decomposition rates, we used standardizedwood sticks as a model substrate, representing a globally important source of organic matter in river systems. Wood decompo-sition rate (percent mass loss per day) increased by 4.3% per 1°C rise in mean annual air temperature but decreased by 3.4% per10 people increase in population density (per 3.14 km2), 2.3% per 100 mm increase in mean annual precipitation, and 0.5% per1 m increase in channel width. Land-use intensity and riparian zone characteristics showed no significant effects on wood de-composition rates across the studied gradients. Our results show that wood decomposition rates in the studied rivers are likely toincrease linearly with ongoing global warming, reducing the longevity of wood substrates and their reliability as carbon sinks.However, this warming effect might be offset in rivers experiencing concurrent increases in precipitation and human populationdensity. Consequently, the net effect of global change on wood decomposition rates in rivers may be difficult to predict