Severely burned wood from wildfires has low functional potential in streams.
More frequent and more severe forest fires mean more severely burned wood in streams. Instream wood provides habitat complexity, alters geomorphology, retains organic and inorganic material, promotes hyporheic flow and acts as substrate for biofilm and invertebrates. However, little is known about instream wood functions in fire-prone biomes and how they combine with wood burning levels to guide postfire management efforts. Over 3 years, we tracked instream wood following forest fires in central Portugal. We examined for the first time the influence of burning level, season and a large suite of driving factors on the likelihood of four different functions having primary ecological consequences-retention of organic matter, serving as substrate for aquatic biota, being key pieces forming wood jams and deflecting flow including pool habitat formation. One of the main ecological functions of wood in rivers is to provide substrate for vegetation, periphyton, biofilm and oviposition. Our results show that this can be negatively affected when the wood is severely burned. Except for jam formation, the probability of each stream wood function changed markedly with season and the probability of non-function was nearly twice as high in the Euro-Mediterranean dry season than in the wet season. More anchored and decayed wood increased the probability of all functions, whereas the effect of submergence depended on the function. Challenging the 'size paradigm' that larger sized pieces provide more function, our data suggest that the effect of size is function-specific. Synthesis and applications. We show how postfire stream restoration success can be maximized by selecting the most appropriate wood and taking advantage of attribute-function relationships. We urge managers to refrain from removing wood or to selectively remove the most severely carbonized only, allowing the persistence of great potential to provide substrate for stream biota. The non-attraction of severely burned wood as substrate can be compensated for by other wood with attributes enhancing this function, such as wood deeper within the bankfull area, and with large diameters. These results will help to inform successful management, as is increasingly asked from restoration ecology.