Could knotweeds invade mountains in their introduced range? An analysis of patches dynamics along an elevational gradient.
The highly invasive knotweeds (Reynoutria spp.) are still infrequent in mountain regions. Despite their current low abundance, they may represent a significant threat for high elevation ecosystems if their population dynamics remain as aggressive as in lowlands during their range expansion to higher elevation. The aim of this study is to assess the knotweed's invasion potential in mountainous regions by studying patch dynamics along an elevational gradient (between 787 and 1666 m a.s.l.) and by reviewing existing literature on their presence and performance in mountains. The outlines of 48 knotweed patches located in the French Alps were measured in 2008 and in 2015 along with biotic, abiotic and management variables. Based on these variables, knotweed's cover changes and patch density were predicted using mixed models. Results showed that elevation has no effect on knotweeds dynamics along the studied elevational gradient. It appeared that the local expansion of knotweed patches is essentially controlled by the patches' initial size and the distance to roads and rivers, i.e. to obstacles and sources of disturbance. Shade and patches' size also impact knotweed patch density, probably through an effect on the species' clonal reproduction and foraging strategies. Interestingly, patches seemed insensitive to the gradient of mowing frequency sampled in this study (between zero and five times per year). All evidences indicate that the knotweed complex is able to colonize and thrive in mountains areas. However, due to the particularities of its spatial dynamics, adequate and timely actions could easily be undertaken to prevent further invasion and associated impacts and reduce management costs.