RAPID DECOMPOSITION OF SUMMER-INPUT LEAVES IN A NORTHERN MICHIGAN STREAM

Processing of leaves of five riparian plant species [sugar maple (Acer saccharum), speckled alder (Alnus rugosa), eastern hemlock (Tsuga can adensis), red-stem dogwood (Cornus sericea) and sweet gale (Myrica gal e)] was studied during the summer in a northern Michigan stream. In Ju ne 1992, dried green leaves (similar to 5 g) of Each species were plac ed into coarse-mesh bags and tethered in a riffle. Mass loss and macro invertebrate colonization were measured after 2, 14, 28 and 42 days. I n general, decay rates were fast (k = 0.017 - 0.134), with most specie s losing >80% of mass within 28 days. The order of decomposition (in d eclining rate) was: maple = dogwood > alder > sweet gale = hemlock. Ma croinvertebrate numbers in the leaf packs were highest at 14 days, but densities per unit remaining mass increased steadily during the exper iment. Midge larvae (Diptera: Chironomidae) and net-spinning caddisfli es (Trichoptera: Hydropsychidae) comprised 54% and 44%, respectively, of the macroinvertebrates, which generally lacked typical shredder tax a. Of several measurements of leaf chemistry, toughness and morphology , leaf surface area per unit mass was the best predictor of processing rate. Hemlock and sweet gale may contain secondary compounds that inh ibit decomposition. Leaf processing rates were among the highest obser ved for any North American stream, which may be related to high microb ial activity at summer water temperatures, good nutritional status of fresh leaves, and abundant macroinvertebrates. Summer inputs of leaves to woodland streams are transient but possibly important energy resou rces for some stream organisms.