Invasive trees show only weak potential to impact nutrient dynamics in phosphorus-poor tropical forests in the Seychelles

1. Some invasive alien plants accelerate nutrient turnover in the ecosystem because the litter they produce has a high specific leaf area (SLA), high concentrations of nutrients and low concentrations of lignin and polyphenolics, and therefore decomposes rapidly. These litter properties are typical of plants from nutrient-rich but not nutrient-poor ecosystems, and we therefore hypothesize that species that successfully invade nutrient-poor ecosystems might not exhibit them. We tested our hypothesis in a nutrient-poor tropical forest on a granitic island in the Seychelles. 2. Leaf litter properties and annual decomposition rates of six native and six invasive woody species were compared in a field experiment. Each species group included two pioneer, two understorey and two canopy species. We also compared soil fertility under trees of Cinnamomum verum (dominant alien canopy tree), Falcataria moluccana (alien pioneer N-fixing tree) and Northea hornei (dominant native canopy tree), measuring total nutrient concentrations and nutrient absorption by ion exchange resins. 3. Within the ecological groups of pioneer and canopy species, litter of invasive species had higher mean values of SLA (120% higher) and leaf N concentration (80% higher), and litter decomposition was 40%-80% faster (rate k) than for native species; however, there were no such differences in the group of understorey species. Soils sampled under the three tree species differed little in nutrient concentrations. 4. The results indicate that even on the very nutrient-poor soils of the granitic Seychelles, some pioneer invasive species produce more decomposable litter and therefore have the potential to alter rates of nutrient cycling. However, the small differences in soil fertility beneath native and invasive trees suggest that impacts of invasive species on nutrient cycling are more complex and less predictable in nutrient-poor ecosystems, where several nutrients may be co-limiting, and native and alien species coexist.