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Flow interruption in intermittent rivers (IRs) generates a mosaic of terrestrial and aquatic habitats across the river network affecting ecosystem processes, as organic matter (OM) decomposition. Water use for farming in arid and semi-arid climates intensifies the dry conditions and affects local river characteristics. In that way, flow intermittence and the distribution of land uses may affect the OM processing along the river. To understand the role of IRs in global OM dynamics and how global change affecting water flow regimes determines these dynamics, it is important to estimate OM-processing rates at a basin scale. The aim of this study was to evaluate the effect of the intensity of flow intermittence on OM processing, and how this effect was modulated by local environmental factors related to land uses across a Mediterranean river basin. To do this, wood decomposition (mass loss and fungal biomass) was selected as a functional indicator. Drying duration and frequency were measured to characterize flow intermittence in different reaches along the river, as well as local environmental factors. Linear models stablished the role of factors on decomposition. The results showed that differences in decomposition rates across the river network were negatively related to the duration of flow interruption. Dissolved inorganic nitrogen associated with agriculture counteracted the negative effect of intermittence on mass loss (increasing by up to three times); but with a higher duration of dry conditions, its effect was insignificant. An increase of 20% of canopy (higher in natural areas) resulted in increases of up to 5% of mass loss. Overall, our study is relevant to understanding the interaction between flow intermittence and land uses on OM processing, especially considering the intensification of flow intermittence and its increased distribution to other regions, which is expected to be a consequence of climate warming and human activities.

Drought limits tree water use and growth of Mediterranean trees. However, growth and water use strategies are rarely addressed simultaneously across species and drought conditions. Here, we investigate the link between stem diameter variations and sap flow in four co-existing Mediterranean trees ( Pinus halepensis Mill., Quercus pubescens Willd., Quercus ilex L. and Arbutus unedo L.), under relatively wet (2011) and dry (2012) conditions. Continuous stem diameter variations were converted to basal area increment (BAI) and de-trended to estimate tree water deficit (Δ W ), an indicator of stem hydration. P. halepensis and Q. pubescens showed the most and the least conservative sap flow density ( J S ) regulation under drought, respectively, with Q. ilex and A.unedo showing intermediate drought responses. All species, except A. unedo , showed some between-year variability in the environmental control of J S . Seasonal stem shrinkage in response to drought (i.e., increasing Δ W ) and subsequent trunk rehydration after rainfall (i.e., decreasing Δ W ) occurred in all species. Vapor pressure deficit (VPD) and soil moisture ( θ ) interacted to determine seasonal variation in Δ W . Interestingly, in the dry year, 2012, more species-specific differences were found in the responses of Δ W to θ and VPD. Across species, J S and Δ W began to decline at similar soil moisture thresholds, underpinning the tight link between J S and Δ W under varying drought conditions. Annual BAI decreased proportionally more than tree-level transpiration ( J T ) between the wet (2011) and the dry (2012) year, hence growth-based WUE (WUE BAI = BAI/ J T ) decreased for all species, albeit less acutely for P. halepensis . Overall, despite their contrasting leaf habit and wood type, the studied Mediterranean tree species show coordinated responses of transpiration, water storage dynamics and growth-based WUE which allow them to cope with seasonal and interannual drought.