Geographic versus environmental space: Patterns of potential native and exotic woody propagule pressure on New Zealand's indigenous grasslands

Abstract

Indigenous grassland ecosystems worldwide are increasingly subject to shrub encroachment. A key factor determining encroachment patterns is the availability of shrub propagules in the areas surrounding the grasslands. We here provide a multi-scale spatial analysis of the geographic distribution (geographic space) and the climatic conditions (environmental space) of potential native and exotic shrub propagule pressure for New Zealand’s main grassland types. We show that alpine grasslands are most at risk from native, and low-producing grasslands are most at risk from exotic shrub propagule pressure. Inferred spatial patterns of potential propagule pressure differ between the local, landscape and regional scale.