- Top of page
- Materials and methods
1. The drying of stream channels resulting from flow interruption is expected to increase as a consequence of climate change. Implications for aquatic organisms and processes are profound. We assessed whether riparian diversity can partially buffer against consequences of drying on fungal decomposers and leaf litter decomposition, an important ecosystem process.
2. Our central hypothesis was that during dry periods recalcitrant leaf litter with high water-holding capacity would extend the window of opportunity for microbial activity in less recalcitrant litter when both litter types are mixed, and that this would lead to a positive litter diversity effect on decomposition. To test for such interactive effects, we conducted a diversity experiment in a Mediterranean stream, in which alder and oak litter, and a mixture of both, was subjected to various drying regimes differing in intensity and timing.
3. Drying regime affected both fungal decomposers and the decomposition rate of alder litter. Effects were observed both immediately and 3 weeks after stream flow resumed. Small differences in the timing of the dry period influenced both decomposition rate and measures of fungal performance (i.e. biomass and sporulation activity). Litter mixing, in contrast, had no effect on either decomposition or fungal decomposers, although mixing increased moisture retention in alder litter as required for the mechanism hypothesized to lead to a diversity effect.
4. Given the contrasting traits of the litter types used in the experiment, our results imply that riparian tree diversity is unlikely to buffer against increased frequencies of stream flow disruption expected in the face of climate change. It appears, however, that the precise timing of dry periods and high-flow events will strongly influence the extent to which stream food webs can exploit the resources supplied by riparian vegetation in the form of leaf litter.
- Top of page
- Materials and methods
An important fraction of the world’s streams and rivers are subjected to dry periods that cause occasional disruption of flow (Larned et al. 2010). The resulting flow intermittency involves contraction and fragmentation of running waters and temporary loss of aquatic habitat (Lake 2003). Climate change is an important factor likely to alter stream flow in the future, exacerbating both the spatial and temporal extent of intermittency in different climatic zones around the world (Milly, Dunne & Vecchia 2005). Particularly susceptible are Mediterranean streams, where climate change is projected not only to increase the extent of intermittency (Milly, Dunne & Vecchia 2005; IPCC 2007), but also to induce shifts from currently perennial to intermittent flow.
Changes in flow regimes have direct impacts on aquatic communities and processes (e.g. Boulton 2003; Lake 2003) and can also affect the density, composition and dominance patterns of riparian vegetation (Poff & Zimmerman 2010). Such changes in the structure of riparian communities affect both the amount and quality of leaf litter delivered to stream channels. The potential repercussions for stream communities and ecosystem processes are large, as allochthonous litter is a main source of carbon and nutrients for stream food webs and metabolism (Wallace et al. 1997; Webster 2007; Tank et al. 2010). Key in this respect is the utilization of leaf litter by decomposers and the resultant litter decomposition.
Diversity effects are particularly likely to arise when the species within a community contrast in their functional traits (Hillebrand & Matthiessen 2009; Gessner et al. 2010). In the case of litter decomposition, relevant litter traits include nutritional quality and properties that influence microenvironmental conditions afforded to decomposers. The significance of litter traits for microenvironmental conditions is unclear, but it could be tested and be particularly relevant in intermittent streams. As stream channels are drying, microenvironmental conditions in litter packs retained in the channels fundamentally change, leading to strong declines in decomposition rates (Herbst & Reice 1982; Maamri et al. 1998; Langhans & Tockner 2006; Leberfinger, Bohman & Herrmann 2010). However, different litter species differ greatly in their water-holding capacity (Dirks et al. 2010). Therefore, in litter mixtures subject to desiccation, the presence of species with high water-holding capacity might slow the loss of moisture in adjacent species with low water-holding capacity (Wardle et al. 2003). This would extend the window of decomposer activity during dry periods and enhance survival of microbes until flow resumes (Langhans & Tockner 2006), notwithstanding the fact that some aquatic decomposers can occur in terrestrial environments as well (Sanders & Webster 1978; Sridhar & Bärlocher 1993). Litter-mixing effects on decomposition of individual species can thus arise.
Particularly important drivers of litter decomposition in streams are a group of microfungi known as aquatic hyphomycetes (Gessner et al. 2007; Krauss et al. 2011). Their role is twofold: They directly degrade leaf litter, and they stimulate litter consumption by detritivores. This stimulation occurs because fungi produce biomass rich in nutrients and because they enzymatically change the physical and chemical properties of decomposing litter, thereby enhancing litter quality and palatability for consumers (Gessner et al. 2007). Unlike invertebrates, which also can be important for litter decomposition in streams (Graça 2001; Hieber & Gessner 2002), fungi are intimately associated with their substrate. As a consequence, they cannot evade desiccation when stream flow recedes and water levels drop. This makes fungal decomposers particularly vulnerable to desiccation stress accompanying stream intermittency.
This study aimed at assessing litter mixture effects on decomposition under simulated stream intermittency. By determining litter mass loss, fungal biomass and sporulation activity in mixed- and single-species litter under different drying regimes in a field experiment, we tested whether (i) aquatic fungi and litter decomposition are affected by drying resulting from stream intermittency; (ii) litter mixing alleviates effects on fungal biomass and activity induced by desiccation stress; and (iii) any effect of litter mixing, desiccation and the interaction of both factors on fungi translates into effects on litter decomposition. Effects on decomposers and decomposition during dry periods might extend well beyond the time when flow resumes because microbial decomposers and litter-consuming invertebrates might have to recolonize the litter, resume growth or both. Therefore, assessments of drying effects need to consider both immediate and propagated effects.