Eucalyptus plantations affect fungal communities associated with leaf-litter decomposition in Iberian streams

Abstract

The replacement of diverse deciduous forests by eucalyptus plantations changes the timing, quality and quantity of litter inputs to streams, which has the potential to affect the activity of decomposers and thus ecosystem functioning. Here, we compared (a) the decomposition rate of alder and oak leaves incubated in deciduous and eucalyptus streams in Spain and Portugal, (b) the activity (fungal biomass and sporulation) and diversity (species richness and Pielou's evenness index) of the associated fungal communities and (c) changes in N and P content of leaves. Alder and oak leaves decomposed at similar rates in both stream types and countries, with the exception of oak leaves in the Spanish eucalyptus stream, which decomposed faster than in the corresponding deciduous stream or in the Portuguese eucalyptus stream. This difference was attributed to physical fragmentation due to flooding and not to forest cover. Higher nitrogen and phosphorus content and higher fungal biomass and sporulation were generally found on leaves from eucalyptus rather than from deciduous streams. The higher fungal activity in eucalyptus streams was attributed to higher water temperature and benthic organic matter storage. The Spanish eucalyptus stream had higher species richness of aquatic hyphomycetes than the deciduous one (27 vs. 20) while in Portugal the opposite was true (16 vs. 20). Fungal community evenness was significantly higher on alder leaves in eucalyptus than in deciduous streams. The community structure (MDS analysis) discriminated both stream types in Portugal much better than it did in Spain. At least for Portugal, differences between stream types can be explained by higher litter diversity in deciduous than in eucalyptus streams. In conclusion, stream fungal communities in Portugal were more affected by eucalyptus plantations than in Spain. In both countries, fungal diversity and activity were more affected by eucalyptus plantations than decomposition rates of submerged litter. We suggest therefore that, to mitigate the effect of eucalyptus plantations, deciduous trees could be planted on the river banks or, preferably, riparian strips of native vegetation should be left unmodified.