Changing landscapes: Compositional and phenological shifts in New Zealand's natural grassland

Abstract

Vegetation in a wide range of ecosystems across the globe is responding to recent anthropogenic climate change. There are two key ecological responses in plants associated with recent anthropogenic climate change: shifts in species’ geographic distributions (range shifts) and shifts in the timing of key life cycle events (phenological shifts). These shifts can lead to temporal and spatial changes in vegetation composition and growth activity and hence ecosystem function. Understanding the patterns and processes of these shifts is crucial for the successful management of natural ecosystems under ongoing anthropogenic environmental change. This thesis investigates recent spatiotemporal compositional and phenological shifts in New Zealand’s natural grassland ecosystems and identifies potential topographical and climatic drivers of these shifts. Three grassland types in New Zealand are investigated (Alpine, Tall Tussock and Low Producing grasslands). They are characterised by high levels of indigenous endemic plant biodiversity and cover a wide elevation range. This thesis primarily utilises remote sensing information for quantifying growth dynamics and vegetation patterns in these grasslands over the last 16 years and across large spatial scales, i.e., the catchment of the river Clutha/Mata-Au River in South Island, New Zealand. Shrub encroachment in grassland ecosystems is a globally observed example of compositional shifts in ecosystems associated with recent anthropogenic climate change. In New Zealand, where extensive area of current grassland habitats exist because of anthropogenic deforestation, shrub encroachment into grasslands has two distinct facets: firstly the invasion of non-native shrub species into native grasslands (i.e., exotic shrub invasion) and secondly the dispersal of native woody and shrub species into native grasslands (i.e., native shrub recovery). Propagule pressure is a measurement of species’ seed source size in neighbourhood of a focal area, and it is a key determinant of the degree to which a location gets colonised by individuals from species present in the neighbourhood. The spatial patterns of potential native and exotic shrub propagule pressure on three grassland types in New Zealand were quantified with the assumption that proximity of higher shrub coverage indicates higher shrub propagule availability. Results show that Alpine grasslands are mostly surrounded by native shrublands, while Low producing grassland are most at risk from exotic shrub invasion from neighbouring areas. High native and exotic shrub propagule pressure does not generally coincide spatially, however, it occurs in very similar climates for Low Producing grassland but not for Alpine and Tall Tussock grassland. The analysis of recent shrub encroachment over the last five years in a tussock grassland area in the central South Island showed a 0.35% year-1 increase in shrub cover in grassland area located in immediate neighbourhood of shrub. Shrub encroachment speed was strongly correlated with shrub cover in the neighbourhood. Recent shrub encroachment into grasslands was most pronounced in areas with neighbouring shrub cover of greater than 40%. A wide range of species and ecosystems worldwide have shown changes in the timing of life cycle events and growing seasons in a direction congruent with recent anthropogenic climate changes. In this study, temporal trends over the last 16 years in the start, peak and end dates of the growing season were analysed using remotely sensed data on the Normalised Difference Vegetation Index (NDVI) in New Zealand’s three main grassland types. Overall, 90% of Alpine, 86% of Tall Tussock and 89% of Low Producing grassland areas showed an advancing start of the growing season over the last 16 years. In these areas start of the growing advanced by 7.2, 6.0 and 8.8 days per decade in Alpine, Tall Tussock and Low Producing grassland, respectively. Only small changes in timing of the end of the growing season were observed in the three grassland types. The length of growing season extended by 3.2, 5.2 and 7.1 days per decade in three grassland types. Landscape topography (elevation and aspect) played an important role in particular in alpine grasslands: the start of the growing season was strongly correlated with elevation (later start with increasing elevation), while the end of the growing season was strongly correlated with aspect (later end of season on more south-facing slopes). The start of season was delayed by 7.5, 5.1 and 3.7 days/100 m elevation increase in Alpine, Tall Tussock and Low producing grassland, separately. The end of season was advanced by 1.7 (Alpine), 1.3 (Tall Tussock) and delayed by 0.3 (Low Producing) days/10-degree-south on the slopes in these three grassland types. The results from this thesis show that recent shrub invasion into New Zealand grasslands is highest near shrub areas once a threshold of shrub cover in the neighbourhood is reached. Shrub encroachment was highest at lower elevations and on north-facing slopes. It also highlighted a measurable shift to an earlier start and extended length of the growing season in New Zealand’s main grassland types over the last 16 years, but the magnitude of these shifts showed considerable geographic variation. Importantly, this study has shown a high degree of topographical control on the timing of the growing in New Zealand’s grasslands with elevation and aspect acting differentially on start and end of the growing season. This highlights the importance of landscape heterogeneity and microclimates for ecosystem responses to climate change. This study shows that remotely sensed data can be successfully used to elucidate ecosystem-level shifts in temporal dynamics and spatial patterns of vegetation growth in grassland ecosystems.