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.