Seasonal differences in green leaf breakdown and nutrient content of deciduous and evergreen tree species and grass in a granitic headwater stream

Litter processing was examined in autumn-winter and spring-summer in a second order stream in Galicia (NW Spain). We compared decay rate and nutrient dynamics of green leaves of several deciduous (riparian: Alnus glutinosa, Betula alba and Populusxcanadensis; terrestrial: Castanea sativa, Quercus robur), and evergreen tree species (terrestrial: Pinus radiata and Eucalyptus nitens), in addition to ray-grass (Lolium perenne). In the autumn-winter period, the decay rates (-k) ranged between 0.0086 degree-days(-1) for poplar, and 0.0019 degree-days(-1) for birch leaves. Alder showed the most rapid breakdown in spring-summer (0.0124 degree-days(-1)), and pine the slowest (0.0016 degree-days(-)1). Deciduous species exhibited general higher processing rates than evergreen species and ray-grass. The initial nitrogen and phosphorus contents were higher in riparian species leaves and ray-grass, being higher in spring (2.28+/-0.14% and 0.24+/-0.04% of nitrogen and phosphorus, respectively) than in autumn (1.88+/-0.36% of nitrogen and 0.18+/-0.03% of phosphorus). A significant correlation coefficient was found only between mean nitrogen leaf packs contents during incubation and decay rates (r=0.61; p=0.012). In deciduous species, processing was faster during the spring-summer than in the autumn-winter period, which may be attributed to the greater nutritional value and less consistency of the leaves during this season. Within evergreen species, pine had a significantly faster processing rate in autumn, attributed in this study to greater physical fragmentation of the needles. Ray-grass and eucalyptus did not exhibit any seasonal differences in processing rate. During the spring-summer period, litterfall inputs are quantitatively less important than during the autumn-winter, but due to high retention and fast breakdown during the spring-summer, green inputs should contribute substantially to nutrient incorporation and cycling in benthic communities.