Decoupled responses of native and exotic tree diversities to distance from old‐growth forest and soil phosphorus in novel secondary forests

Abstract

AbstractQuestionsHow (de)coupled are native and exotic tree diversities in their relationships with local soil conditions and landscape configurations? Can (de)coupled diversity–environment relationships be used to manage native and exotic species separately to minimize unintended impacts on one another?LocationThe tropical city‐state of Singapore, Southeast Asia.MethodsIn two types of novel, exotic‐dominated secondary forests regenerating from land abandonment, we surveyed both native and exotic trees ≥5 cm diameter‐at‐breast height (DBH) in 97 plots and correlated taxonomic and functional diversities to soil fertility (nitrogen, phosphorus, and potassium) and landscape configuration gradients (distance to old‐growth forests and patch area).ResultsNative and exotic tree species had unshared or decoupled responses to distance to old‐growth forest and soil phosphorus. The taxonomic and functional diversities of native, but not exotic, species declined rapidly with increasing isolation from old‐growth forests. Exotic, but not native, species richness increased with soil phosphorus levels. Neither native nor exotic diversities were associated with soil nitrogen, potassium, and patch area.ConclusionsNative and exotic tree species could be managed separately without unintended impacts on one another because of their decoupled responses to distance from old‐growth forest and soil phosphorus. The native–distance‐to‐old‐growth relationship suggests that novel secondary forests adjacent to old‐growth forests can accrue native species with minimal intervention, while enhancement plantings of old‐growth native species should target more isolated patches. The exotic‐phosphorus relationship suggests that restoring soils to historically‐low phosphorus levels can help to reduce exotic diversity, without negative impacts on native diversity. Our study highlights the importance of understanding (de)coupled species or species‐group responses to identify optimal management solutions with minimal non‐target effects.