Understory vegetation provides clues to succession in woody weed stands

Abstract

Invasive exotic tree and shrub species (woody weeds) form dense, monospecific stands in many areas of New Zealand. At some sites, the weed dies out naturally and is replaced by native species as succession proceeds, but at others the weed persists indefinitely. The ability to distinguish between these different trajectories is critical to effective weed management, but the conditions that determine successional outcomes remain poorly understood. However, clues to the successional trajectory at any given woody weed site can be found in the understory, because understory plants represent the potential future plant community (in the absence of disturbance). Of key relevance is whether the woody weed species is regenerating under its own canopy, because this enables it to replace individuals as they die, and thus persist as succession proceeds. Conversely, if the understory is comprised entirely of native species, there is potential for the natives to take over the community as the weed dies out. This process is often termed “passive restoration”, because native vegetation is restored without any active management other than (in some cases) the removal of environmental stressors or degrading processes. The likelihood of a native understory developing is affected by site-specific traits such as the natural (historical) vegetation type, proximity to native seed sources, climate, stand age and the presence of herbivores. We present a framework to help land managers use their observations of understory vegetation to assess likely successional trajectories in woody weed stands.