Fire meets inland water via burned wood: and then what?

Abstract

Wood is a key structural element in aquatic ecosystems. Wood provides habitat complexity, alters geomorphology, retains organic and inorganic material, promotes hyporheic flow, and acts as substrate for biofilms and invertebrates. Fire is an important disturbance promoting wood recruitment into inland waters, but most studies have focused on streams in western North America. Less is known about fire-derived wood dynamics on other continents or in lake environments. Here, we review fire effects on the recruitment, distribution, and function of in-stream wood, with emphasis on a series of studies from the Euro-Mediterranean. The amount of large wood in these streams was low and is expected to decline in the future because of wildfire. Wildfire engendered inputs of wood with low structural complexity, probably reducing habitat heterogeneity for aquatic organisms. Fire also provided wood with greater diameter than wood recruited by other means, but its longevity may be shorter because burned wood was more decayed and less anchored in the channel than unburned wood. Wood delivery processes are important because macroinvertebrate colonization differed between fire-derived wood that fell directly into the river and wood conditioned first on the forest floor. We present a case study describing wood dynamics in a lake after a wildfire in northern Minnesota, USA. In this study, wildfire created an area of lake shoreline with disproportionately more wood than areas unaffected by wildfire. In contrast to streams, burned wood was more complex than unburned wood in the lake system. One of the explanations may be greater scouring, abrasive action by stream flows, which breaks down burned wood faster than in lakes. Given the expected increase in the probability and severity of fire around the world, information in our review can be used to help manage riparian zones of streams and lakes.