Effects of burn status and conditioning on colonization of wood by stream macroinvertebrates

Abstract

The combination of changing climate and anthropogenic activities is increasing the probability of fire around the world. When fires occur in riparian zones, associated tree mortality can add wood to streams directly, or wood may fall onto the forest floor and remain there for some time before moving into stream channels. Because wood provides critical structure for aquatic macroinvertebrates, our objectives were to assess the effects of wood burn status, conditioning, and their interaction on (1) aquatic taxa community composition; (2) taxa and functional diversities; and (3) trait affinities. We conducted a field experiment using pieces of freshly-cut wood for which we first manipulated the burn status (burned, unburned). We then manipulated conditioning status by placing pieces directly into streams (no conditioning), leaving other pieces in streams for a year (water conditioning), or on the forest floor for a year before submergence (soil conditioning). Analyses included distance based redundancy analysis and linear mixed-effects modeling. Our results demonstrated that changes in wood quality resulting from fire may not alter per se the structure of macroinvertebrate communities. Conditioning status, however, had significant effects on taxonomic composition, taxa and functional diversities, and trait affinities of wood invertebrate communities. The terrestrial legacy of soil conditioning was clearly important in structuring macroinvertebrate assemblages. Furthermore, taxonomic and functional patterns of stream macroinvertebrate colonization were not substantially different between burned and unburned wood, even after a year of incubation in the stream or on the forest floor. This is an important finding for the research topic on how wildfire structures aquatic communities. This information can function as a guide for post-fire stream and riparian management operations taking ecosystem function into account.