Hydrobiologiu 333: 79-85, 1996. @ 1996 Kluwer Academic Publishers. Printed in Belgium. 79 Decomposition of Eucalyptus globulus leaves and three native leaf species (Alnus glutinosa, Castanea sativa and Quercus faginea) in a Portuguese low order stream Cristina Canhoto & Manuel A. S. Graqa Departamento de Zoologia, Universidade de Coimbra, 3049 Coimbra Codex, Portugal Received 13 September 1995; in revised form 29 February 1996; accepted 25 April 1996 Key words: Leaf decomposition, polyphenols, nitrogen, microbial colonization, Eucalyptus globulus Abstract Leaf decomposition of the exotic evergreen Eucalyptus globulus (eucalyptus), and three native deciduous tree species, Alnus glutinosa (alder), Castanea sativa (chestnut) and Quercus faginea (oak), was compared in a second order stream in Central Portugal. Changes in dry weight, nitrogen and polyphenolic compounds and microbial colonization were periodically assessedfor three months. Negative exponential curves fit the leaf weight loss with time for all leaf species. Mass loss rate was in the order alder (K = 0.0161) > chestnut (K = 0.0079) > eucalyptus (K = 0.0068) > oak (K = 0.0037). Microbial colonization followed the same pattern as breakdown rates. Evidence of fungal colonization was observed in alder after 3 days in the stream, whereas it took 21 days in oak leaves to have fungal colonization. Fungal diversity was leaf species-dependent and increased with time. In all cases, percent nitrogen per unit leaf weight increased, at least, at the initial stages of decay while soluble polyphenolics (expressed as percentage per unit leaf weight) decreased rapidly in the first month of leaves immersion. Intrinsic factors such as nitrogen and polyphenolic content may explain differences in leaf decomposition. The possible incorporation of eucalyptus litter into secondary production in a reasonable time span is suggested, although community balance and structure might be affected by differences in allochthonous patterns determined by eucalyptus monocultures. Introduction In temperate low order streams, food webs rely on allochthonous inputs, mainly autumn-shed leaf litter, as a primary carbon source (Petersen & Cummins, 1974; Boling et al., 1975). Through litter process- ing, a heterogeneous gradient of conditioned detritus is constantly provided to detritivores and gradually con- sumed as they become fully conditioned (Boling et al., 1975; Cummins et al., 1989). Litter decomposition is the result of three more or less distinct and sequential phases that occur over a time span of weeks to months: leaching, microbial colonization, invertebrate feeding and fragmentation (Cummins, 1974). Although ruled by biotic processes (Reice, 1977), litter decomposition is strongly influ- enced by factors such as temperature (Suberkropp et al., 1975), pH (Thompson & Batlocher, 1989), cur- rent velocity and stream sediment particle size (Reice, 1974,1977), water chemistry (Chamier, 1992), hydro- logical regime, stream order (Hill et al., 1992), and, above all, leaf chemical composition (Mellilo et al., 1982; Hill et al., 1992; Merritt & Lawson, 1992). Therefore, breakdown rates may be an useful tool to evaluate the impacts of anthropogenic disturbances in lotic systems (e.g. Benfield et al., 1977; Smock&Mac- Gregor, 1988; Bird & Kaushik, 1992; Ormerod et al., 1993). In 1829, the Australian species Eucalyptus globulus Labill. was introduced to the Iberian Peninsula. Nowa- days, this species is common and widespread, partic- ularly in Central Portugal where monocultures occupy 18.7% of the land area, threefold the value observed 20 years ago. Due to socioeconomic interests, eucalyptus