Field experiments on mechanisms influencing species boundary movement under climate change

Scope: Currently knowledge is weak about which plant species boundaries are determined directly by climate and soil factors, versus which are mediated by biotic interactions, such as exclusion by superior competitors or by pathogens. Under moving climate zones the difference is important. For example, will warm boundaries shrink poleward of their own accord, or only following invasion by other species? What research will most effectively strengthen our understanding of this fundamental issue in ecology? Conclusions: Traditionally the required experiment has been transplant beyond the species boundary, together with density manipulation of putative competitors, herbivores, pathogens or facilitators. But now climate zones are being moved across species distributions, creating a different sort of experiment. In order to learn efficiently from this massive inadvertent experiment, exposure to new climate needs to be combined with experimental manipulation of other factors. Because range extensions are likely to be spatially patchy, these experiments need to be geographically extensive, to be sustained over 2–4 decades, and to be twinned with wide-area population monitoring via drones and other remote sensing. Highest priorities for experimental manipulation would be removal of competition, and provision of seedlings. Highest priorities for locations would be zones where vegetation physiognomy changes, such as woodlands into grasslands, or rainforest into fire-prone forest.