Does mixing of senescent and green leaves result in nonadditive effects on leaf decomposition?

Abstract

Most studies of the effects of litter mixing have focused on decomposition in mixtures of senescent leaves of different plant species. In temperate forests, substantial quantities of green leaves fall in spring and summer, and previous research has shown that stream detritivores can benefit from the combined use of senescent and green leaves. We conducted a litterbag experiment to test the hypothesis that mixtures of senescent and green leaves of different species decomposed more rapidly, mediated by increased detritivore colonization, than leaves of each type in isolation. The observed abundances of 2 detritivore species, the amphipod Sternomoera rhyaca and the caddisfly Lepidostoma complicatum, were 3 to 6× higher in mixtures of senescent oak and green alder leaves than expected based on litter patches consisting of each species alone. Lepidostoma complicatum also responded positively to mixtures of senescent oak and green maple leaves, but S. rhyaca did not. These results verify that caddisfly behavior can reflect benefits gained from the combined use of senescent and green leaves. In contrast, the extent of the advantage to amphipods depends on the specific combination of senescent and green leaves. Despite the synergistic effect of mixed senescent oak and green alder leaves on detritivore colonization, observed leaf mass loss in mixtures was not higher than expected. Changes in the chemical properties of senescent oak and green alder leaves in mixtures might decrease feeding rates of detritivores because of preferential or compensatory feeding. Mixtures of senescent and green leaves did not alter leaf decomposition of each component in spring and summer, but might enhance leaf decomposition in autumn and winter by augmenting detritivore populations.