Abstract
In Aotearoa New Zealand, tussock grasslands below the climatic treeline are largely a result of extensive anthropogenic deforestation, and have been further modified by repeated burning, intensive stock grazing, and the introduction of invasive species. Investigating recovery trajectories of these induced tussock grasslands following the cessation of agricultural practices is crucial to understanding the effects of this land-use change on community composition and species distributions. The increase of invasive exotic herbs and potential establishment of woody species may detrimentally affect native herbs in these communities.
This study investigated community changes and species distributions in recovering tussock grasslands at Motatapu Station, Otago, by resurveying eight historic vegetation transects established before destocking. Further surveys across a broader altitudinal range of 720-1300 m above mean sea level were also conducted to investigate how species ranges might be shifting following destocking. Abundance and dominance of plant groups as well as selected focal species were assessed using measurements of cover and relative cover. Species distributions across the wider elevation range were then compared with altitudinal records from regionally appropriate herbarium specimens to assess key species that may be at risk of disappearing from the study site.
These surveys provided evidence of increasing abundance and dominance of shrubs in these tussock grasslands, as well as increases in dominance of exotic forbs. These changes were accompanied by decreases in abundance and dominance of alpine species, many of which significantly declined in cover or disappeared from the transects. Assessment of the altitudinal distributions of focal species indicated that these changes may be occurring across a wide altitudinal range and extending into the alpine zone, which may threaten alpine herbs with narrow altitudinal ranges. Several species were identified as potentially at risk given such changes, due to their declining status in the resurveyed transects and limited elevation distributions in the extended surveys and regional herbarium specimens.
This study adds to limited knowledge of long-term changes in induced tussock grassland communities below climatic treeline following destocking, while adding valuable context for the wider montane-alpine ecosystem by investigating the state of these communities across a wide altitudinal gradient. In doing so, the vulnerability of alpine herbs to shrub encroachment is examined, highlighting a potential biodiversity trade-off to be considered when promoting recovery of woody communities in tussock grasslands, and also foreshadowing the impacts of contraction in alpine habitats in a warmer future.