The rise and fall of Leptospermum laevigatum: plant community change associated with the invasion and senescence of a range‐expanding native species

Abstract

AbstractQuestionShrub invasions often alter ground layer vegetation but the permanence of these changes is sometimes unclear. We asked: (i) do affected communities recover to their former state when invading species decline; (ii) do plant species density and composition change through the successional sequence of shrub invasion, dominance and decline; and, if yes, (iii) is change dependent on the scale of observation?LocationYanakie Isthmus, Wilsons Promontory National Park, southeastern Victoria, Australia.MethodsWe use a nested quadrat approach along a chronosequence of invasion states (uninvaded, invaded, senescent) to document composition and diversity of coastal grassy woodland in response to invasion by the range‐expanding native shrub Leptospermum laevigatum (Myrtaceae) across multiple spatial scales. Non‐metric multidimensional scaling and species–area curves were used to assess changes in vegetation and the ability of invaded communities to return to ‘uninvaded’ states after the senescence of Leptospermum.ResultsShrub invasion did not lead to a decline in total species density. However, there was a significant shift in species composition, suggesting species replacement. Canopy gaps created by Leptospermum senescence had significantly higher species density at a large scale of observation (256 m2), but not at any of the smaller scales. Species composition within Leptospermum gaps was initially highly variable and significantly dissimilar to that prior to gap creation. With increasing time‐since‐gap creation, floristic composition became less varied and more similar to the uninvaded state, but the oldest gaps were still compositionally different to the uninvaded state. These results were coupled with decreases in light, soil nitrate and soil moisture following invasion, and an increase in those variables following the creation of tree‐fall gaps.ConclusionsCommunity composition was positively influenced by the creation of gaps within an otherwise shrub‐dominated ecosystem. Gaps represent the removal of potential inhibitory influences created by the dominant species and permit the return of the system to that more similar the uninvaded state. Gaps also increased heterogeneity in the landscape that promoted species diversity in an otherwise uniform landscape. These patterns were scale‐invariant.