The role of native riparian tree species in decomposition of invasive tree of heaven (Ailanthus altissima) leaf litter in an urban stream

Abstract

Increasingly, interactions between human and natural systems centre on the multi-scale restoration of ecosystems. Humans rely on ecosystem services provided by streams, yet human activities degrade water quality worldwide. Re-planting streamside vegetation is a common restoration practice, since trees reduce runoff and stabilize banks. But does riparian tree biodiversity matter? Detrital inputs from riparian vegetation impact in-stream processes, e.g., leaf decomposition. Since the increasing distribution of invasive plant species alters the structure of streamside forest communities, input of invasive litter to streams could alter such processes. We followed decomposition rates of the invasive tree of heaven (Ailanthus altissima, TOH) and 6 native leaf species in an urban stream and complemented this effort with laboratory feeding experiments employing the same treatments and 2 common aquatic detritivores. TOH breakdown was rapid, exceeding native leaf decay. Mixing TOH with native species reduced its decay compared to TOH alone; however, the feeding study demonstrated that detritivores preferred TOH over native species. Subsequent estimates of species-specific structural integrity revealed TOH poorly resisted breakage. The relatively tougher nature of native species may slow TOH breakdown by armouring the invasive litter against the highly variable flow regime characteristic of urban streams. The presence of native riparian tree species may mediate how invasive trees decompose in human-impacted streams.