Effects of changes in leaf quality and diversity of the riparian vegetation on headwater streams ecosystem functioning

Abstract

In forested regions, stream primary production is low due to light limitation. In consequence, most streams’ food webs appear to be mainly fueled by allochthonous organic matter inputs from their riparian vegetation. Under a global change scenario, the quality and quantity of such inputs are expected to change with potential implications for stream ecosystems. However, the effects of such modifications on stream ecosystem functioning are difficult to understand due to the existence of complex trade-offs within and among communities making use of leaf litter assemblages. This thesis aims to shed some light on how different changes in the properties of organic matter inputs to headwater streams may alter their ecosystem functioning. To address that objective, I developed different field and laboratory experiments and analyzed the leaf litter decomposition process as an integrative indicator of stream ecosystem status. In the first chapter, I assess how leaf traits may be affected by climate change focusing on persistent after-life traits that have been usually reported to affect leaf litter decomposition, and thus have the potential to impair pivotal effects on the functioning of stream ecosystems. The results from this chapter suggest a decrease of intraspecific leaf quality in riparian deciduous species with global warming in a relatively short term and point to significant implications for Mediterranean mountain streams currently under deciduous gallery forests. In chapters 2 and 3, I investigate how the loss of riparian plant functional diversity or the establishment of dense pine plantations on the slopes of the basins may alter key stream processes (litter decomposition, nutrient cycling, secondary production, fungal biomass), and the likely role of key plant species to alleviate such effects. Our results support a consistent slowing down of the decomposition process, and hence effects on stream ecosystem functioning, derived from plant biodiversity loss but not from pine plantations, as long as riparian vegetation strips along streams are present. Moreover, outcomes from both chapters, underscore the importance of key (N-fixing) species at different scales (instream and catchment) as drivers of plant diversity effects or as buffer of plantation-derived effects in the studied ecosystem processes. In chapter 4, I analyse how detritivores can cope with the invasion of riparian areas of the streams by alien plants. Results from this chapter suggest that big detritivores, with outstanding digestive capacity to process low-quality leaf litter from native or invasive species, may play a key role facilitating the access to nutrients of recalcitrant leaf litter to sympatric small detritivore species via coprophagy. Overall, the results presented in this thesis may help managers and policymakers in the design of ecologically sound conservation programs.