Legacy of Summer Drought on Autumnal Leaf Litter Processing in a Temporary Mediterranean Stream

Abstract

Temporary Mediterranean streams dry out in summer due to low precipitation and high temperatures, thus affecting biota and ecosystem functioning. The consequences of exposing leaf litter to an extreme summer drought, that is, high solar radiation, low humidity, and high temperatures, were evaluated. It was hypothesised that litter exposure to summer drought conditions in a dry streambed would change litter chemical composition, affecting decomposition and associated benthic communities after autumnal flow resumption. Recently fallen black poplar leaves were enclosed in litterbags and separated into two sets: (1) control leaves, kept air-dry in the dark, and (2) drought leaves, exposed to harsh summer conditions in a simulated dry streambed. Both sets were then immersed in a temporary Mediterranean stream in autumn, and leaf litter decomposition monitored. As expected, summer drought exposure increased nitrogen, total fibre, lignin, and cellulose content in drought leaves. After litterbag immersion, higher cellulose-degrading enzyme activity was observed in drought leaves while lignin-degrading enzyme activity was greater in the control leaves. Fungal and bacterial assemblages clearly differed between treatments. Inter-replicate variability was lower in drought leaves for most of the measured variables, suggesting that drought homogenised decomposer and detritivore communities. Despite these drought-related effects, decomposition rates did not vary, apparently due to counterbalancing effects (that is, drought treatment enhanced recalcitrant compounds and nutrient content in leaf litter) probably related to terrestrial microbial colonisation and photodegradation. Our study demonstrates that summer drought has bottom-up critical legacy effects in temporary Mediterranean streams, affecting the structure and function of decomposer communities.