Biome transition in a changing world: from indigenous grasslands to shrub-dominated communities

Shrub encroachment in grassland environments is observed in many regions worldwide. However, in New Zealand, there is no consensus on the trend and magnitude of this phenomenon, and we lack empirical data to determine what environmental variables may promote shrub invasion. Here, we present a comprehensive study evaluating shrub cover change in a tussock water catchment in eastern Otago, New Zealand. Specifically, we aim to quantify shrub cover change in the catchment between 1980 and 2015, to identify the shrub species involved and to determine the environmental variables that promote shrub cover change in the studied area. Using aerial photographic records over a 35-year period, we found a 29% increase in shrub cover (from 10.5% in 1980 to 39.5% in 2015 – a 3.8-fold increase in 35 years). According to ground truthing, this shrub expansion was mainly associated with an increase in mānuka (Leptospermum scoparium) cover. Using model selection with Akaike Information Criterion, we found that the best model explaining shrub cover change in the studied tussock catchment included multiple environmental variables. Among these, initial shrub cover and elevation negatively impacted shrub cover increase within the study area, whereas slope had a positive in uence, especially on the north- and east-facing aspects. Overall, shrub cover change was mostly observed in low-elevation gullies (< 500 metres elevation) with steep slopes, where mānuka is known to have optimal growth conditions. However, further shrub cover expansion may be slow as most of the available space is now restricted to poorly drained spurs at higher elevation (> 650 metres elevation).