Leaf litter breakdown in Patagonian streams:: native versus exotic trees and the effect of invertebrate size

1. Nothofagus native forest in the Southern Andes is being progressively substituted by forestation with rapid growth of the exotic trees, mainly species of Pinaceae. The effect on stream processing dynamics is explored through in situ experiments. 2. The effects of leaf litter quality and macroinvertebrate size on in situ litter breakdown were analysed. Experiments were run in litter bags Which allowed access of macroinvertebrate fauna in streams running through a dense forest of the deciduous Nothofagus pumilio. 3. In Experiment 1, the decay rates of N. pumilio leaves and Pinus ponderosa needles were measured during an autumn-winter period. N. pumilio decayed twice as fast as P. ponderosa (P<0.01). Shredders fed only on N. pumilio leaves. The total abundance of macroinvertebrates colonizing both treatments was similar; however, the biomass was higher in the N. pumilio treatment. Large shredders were only found colonizing N. pumilio leaves. Since no decay due to shredders was observed in P. ponderosa, the presence of macroinvertebrates in these litter bags was related to refuge and feeding on FPOM-biofilm resources. 4. In Experiment 2, N. pumilio leaf litter was exposed in order to allow (open bags) or restrict (closed bags) access of invertebrates. The invertebrate assemblage in open bags showed the similar pattern observed for N. pumilio, in the first experiment. Gathering-collectors Were generally smaller and dominant in number While shredder biomass was higher in open bags as a result of high individual biomass. N. pumilio decayed faster when the whole size spectrum of macroinvertebrates colonized the bags (P<0.01). However, feeding signs of small shredders were observed in closed bags, therefore their role on leaf litter breakdown in Andean streams should not be neglected. 5. The combined results of both experiments lead to the conclusion that the whole litter processing mechanism would be affected as a consequence of the substitution of native forest by exotic pine forestation. Copyright (C) 2002 John Wiley Sons, Ltd.