Fire-plume exposed leaf litter alters decomposition and macroinvertebrate assemblages in headwater streams

Abstract Wildfires are increasing in frequency and spatial extent due to climatic warming and land use changes, with diverse effects on terrestrial ecosystems. However, despite the growing literature about wildfire impacts on streams, the specific mechanisms by which wildfire alters terrestrially derived leaf litter are not completely known, especially in ecosystems where riparian cover is not totally lost. Here, we examined the potential effects of heated smoke plumes on leaves that remain in the canopy after a wildfire in riparian areas that eventually get deposited in stream ecosystems. The heat from the smoke harms leaf cuticle and tree xylem, inducing leaf senescence and altering their chemical composition. With that aim, we exposed leaf litter of two native trees in northern Spain, alder ( Alnus glutinosa ) and oak ( Quercus robur ), and the most abundant exotic tree in the area, eucalypt ( Eucalyptus globulus ), to high temperatures that simulated fire plume exposure (150 °C for 3 h). Afterwards, we conducted a field experiment where treated and control (non-exposed) leaf litter was deployed in three streams, and decomposition rates and aquatic hyphomycete and macroinvertebrate assemblages were examined after 21 days. Fire-plume exposure reduced total and microbial decomposition rates, with the highest reduction occurring for alder. Sporulation and taxon richness of aquatic hyphomycetes varied across species, being highest in alder, but they were not affected by fire-plume exposure. In contrast, the abundance of macroinvertebrates within leaf packs responded to fire-plume exposure, although differently across species, being reduced in alder and increased in oak and eucalypt leaf litter. Our results demonstrate that wildfires can alter stream ecosystem functioning and structure, even when exposure to fire is brief and leaves remain on trees. The effects, however, can vary across tree species, which highlights the relevance of preserving riparian diversity to maintain the performance of stream ecosystems after a fire.