Invasive plant species and litter decomposition: time to challenge assumptions

Invasion of terrestrial ecosystems by non-native plant species is a global environmental concern in light of the ability of some of these species to outcompete endemic species, with attendant implications for plant communities and the multiplicity of organisms that depend on native species for their survival and reproduction. It has further been suggested that non-native species can alter important ecosystem processes, such as nutrient cycling and decomposition, and thereby fundamentally change ecosystem functioning (Liao et al., 2008). Some of the most invasive, non-native plant species studied have indeed been shown to decompose more quickly than native species in the ecosystem (Rothstein et al., 2004; Arthur et al., 2012). This has negative implications for soil carbon (C) stocks and may cause elevated levels of soil nutrients, which, in turn,maymake the site increasingly suitable for invasive species (Ehrenfeld et al., 2001). Faster litter decomposition is a predictable consequence of some of the most successful invasive species, which tend to occupy one end of the leaf economics spectrum, being fast colonizers with rapid growth and little investment in permanent structures (van Kleunen et al., 2010). These same functional traits predispose leaf litter from these plants to be more readily decomposable than litter of slower-growing species with more durable construction (Santiago, 2007). From these observations has grown a generalization that invasive plants decompose more quickly, leading to more rapid cycling of nutrients and release of C in invaded ecosystems. A comprehensive test of this generalization is presented by Jo et al. in this issue of New Phytologist (pp. 115–122) who compare decomposition rates of leaves of 42 native and 36 non-native woody plant species, and those of fine roots of 23 native and 25 non-native species, that occur in temperate deciduous forests in the eastern United States.