Abstract
Shrub encroachment in grassland ecosystems is an increasingly pressing issue globally and locally in New Zealand. New Zealand’s grassland ecosystems, specifically tussock grasslands, are also experiencing woody vegetation change from both native and exotic shrub species. Tussock grasslands are important landscape features as they offer a range of essential ecosystem services, in particular, clean freshwater for downstream users is a key hydrologic service provided by indigenous snow tussock grasslands. Increased water yield has been attributed to low evapotranspiration (ET) rates as tussock plants have the ability to reduce transpiration under water limited conditions. It is unknown whether shrub species have the same ability to repress transpiration as tussock and, therefore, may reduce water yields from tussock dominated catchments.
Research was undertaken at Glendhu, Eastern Otago, to assess the potential impacts on water yield associated with the woody vegetation change from snow tussock (Chionochloa rigida) to mānuka (Leptospermum scoparium) that is occurring in the catchment. Water loss, in terms of evapotranspiration, was compared between mānuka and snow tussock. Meteorological equipment, including automatic weather stations and eddy covariance towers, were deployed for a nine-month sampling period, from late spring 2016 through to winter 2017, to collect environmental inputs for ET equations and actual evapotranspiration data respectively. Mānuka evapotranspiration rates were 2.4 mm day-1 compared to snow tussock average ET rates of 1.2 mm day-1. Across all evapotranspiration methods, mānuka was found to have higher ET rates than snow tussock vegetation which, potentially, could have a significant impact on the water yield from the catchment. A 30% cover of mānuka in the catchment could result in a decline in annual catchment water yield of at least 4%. Results from this research indicate that woody vegetation at Glendhu will likely have adverse implications for the hydrological services associated with tussock headwater catchments.
Three management options are considered to address the implications of woody vegetation change in tussock dominated catchments; 1) ‘do nothing’ and allow the vegetation change to continue, which may result in a compositional shift in vegetation cover of these catchments towards a mosaic of exotic and native species, and a loss of ecosystem services associated with indigenous land cover, 2) partially manage grasslands to preserve values associated with indigenous vegetation and reduce the occurrence of exotic species, and 3) actively manage the grasslands through removal of native and exotic shrub species to preserve the ecosystem values associated with tussock vegetation, specifically the hydrological services. Implementation of the second management option is recommended to move towards restoring these grasslands to the native shrub and forest vegetation that would have once dominated the mid-altitudinal slopes of Glendhu, prior to anthropogenic disturbance. Findings offered from this research may help in defining future management agendas in tussock grasslands which will be dependent upon the vegetation that is deemed to hold the highest societal values.