Applying the stress-gradient hypothesis to curb the spread of invasive bamboo.
The stress-gradient hypothesis (SGH) provides a conceptual framework for explaining how environmental context determines the nature of biotic interactions. It may be also useful for predicting geographic variability in the effect of management interventions on biological invasions. We aimed to test hypotheses consistent with the SGH to explain context dependency in bamboo invasion of secondary forests in Japan, and establish a predictive understanding of forest management impacts on invasion. We use a priori physiological knowledge of invasive giant bamboo, Phyllostachys bambusoides, to generate hypotheses consistent with the SGH. We modelled variation in giant bamboo occupancy within 810 secondary forest plots across the broad environmental gradients of Japan using a national vegetation database. Consistent with the SGH, we find that the effect of tree canopy cover on bamboo occupancy depends on interactions between solar radiation and mean annual temperature. In cool regions with high solar radiation-stressful conditions for bamboo-shade cast by dense canopies facilitates invasion. However, in warmer regions that are more benign, dense canopies tend to inhibit spread via competition for light, space and other resources. Synthesis and applications. We provide evidence that the stress-gradient hypothesis can inform practical recommendations for invasive species control. We characterised geographic variability in the effect of forest thinning, a widespread management intervention used to enhance forest biodiversity, on the risk of bamboo spread into secondary forests in Japan. Thinning forest canopies to increase understorey light radiation should limit bamboo spread in cooler regions, while tree planting to increase canopy shade should limit bamboo spread in warmer regions.