Invasion by nonnative plants may have ecosystem-wide effects, altering the decomposition rate of plant material via changes in litter quality or altered environment (abiotic conditions, associated biotic community), or both. Yet, the relative importance of these factors for decomposition rates is not clear. We studied decomposition using the leaves of related shrub species (nonnative Sorbaria sorbifolia and Rosa rugosa, native Rubus idaeus) with comparable physiognomy but different leaf characteristics and origin (alien vs. native) in patches formed by S. sorbifolia and Rubus idaeus in southwestern Finland. Decomposition of cellulose in the topsoils of the patches was also studied. Using litter bags, we found that S. sorbifolia leaf litter decomposed slowest and Rosa rugosa leaves fastest irrespective of patch type. Topsoils in S. sorbifolia patches were richer in carbon, nitrogen, and calcium than those of Rubus idaeus, but these differences did not affect decomposition rates. Very little decomposition appeared to happen during the winter but during the summer, microclimate had minor but significant effects on decomposition rates. Our results highlight the key role of litter source in the decomposition of plant material. Between-patch differences in abiotic conditions appear to play a minor role relative to litter quality.

Nomenclature: False spirea; Sorbaria sorbifolia (L.) A. Braun; rugosa rose; Rosa rugosa Thunb.; red raspberry; Rubus idaeus L.

Management Implications: Decomposition of plant material is crucial for nutrient cycling. Invasive woody species may alter decomposition rates by altering litter quality and by altering the environmental conditions below the plants. We compared the leaf decomposition of a native and two nonnative shrubs in environments created by the native and one of the nonnative shrubs to estimate the relative importance of leaf source (species identity) and environment (microclimatic conditions, soil nutrient levels) for leaf decomposition rates. All shrubs belonged to the same family (Rosaceae) and had comparable physiognomy and leaf biomass production, which helped to minimize the influence of confounding factors. Our results demonstrate how the decomposition rate of leaves depends on species-specific differences in the quality of leaves rather than on the minor differences in the environments created by the patch-forming shrubs. The predominant importance of leaf source has been stressed also in some previous studies, suggesting that our conclusions can be generalized to a wider group of woody species. Although our conclusions can be regarded as preliminary rather than conclusive due to the small number of studied species, our results suggest that evaluations based solely on changes in leaf litter quality (and quantity) may capture the essential changes in leaf decomposition rates after an invasion of a nonindigenous plant. However, our results also indicate that species-specific differences in leaf quality prevent generalizations across all species, so the consequences of plant invasions for decomposition rates should be evaluated at the level of plant species.