A Classification of Geothermal Vegetation of the Taupo Volcanic Zone, New Zealand

Abstract

We undertook stratified random sampling of vegetation and soil chemical fertility and subsurface temperature at 38 sites on 15 geothermal fields in the Taupo Volcanic Zone, central North Island, New Zealand, to develop the quantitative plot-based classification of geothermal vegetation types and to identify the main environmental drivers of vegetation composition. We used the approach of semi-supervised clustering to allow these new data to be incorporated into a pre-existing classification framework of New Zealand’s woody vegetation. We implemented this with the fuzzy classification framework of Noise clustering. Gradients in composition were derived using Detrended Correspondence Analysis ordination and related to soil physical (temperature, surface substrate texture) and chemical (fertility) parameters using correlation. Of 138 vascular plant species, including 58 adventives, 19 moss and nine liverwort species recorded, only seven native species (Kunzea ericoides var. microflora, Leucopogon fasciculatus, Dianella nigra, Pteridium esculentum, Campylopus pyriformis, Leptospermum scoparium, Chiloscyphus novae-zelandiae) were present in more than 20% of plots and only four adventive species (Rubus fruticosus, Axonopus fissifolius, Prunus serrulata, Agrostis capillaris) in more than 5% of plots. Subsurface (10 cm depth) soil temperatures ranged from ambient (7 °C) to near-boiling (98.5 °C) and, on average, were nearly 18 °C above ambient. Classification identified 16 vegetation associations in eight structural classes (two mossfield, three fernland, one treefernland, one grassland, four shrubland, three scrub, one forest, one treeland), almost all dominated by indigenous species and unique to geothermal fields. A substantial number of plots (33) were compositional outliers, i.e., did not conform to any of the associations. Of these, a relatively high proportion were in urban or peri-urban areas with high weed pressure. There appear to be distinct regional patterns, with over half the associations observed on only one or two geothermal fields, and none on all of them. Subsurface soil temperature was the main factor controlling vegetation composition. Vegetation composition was less strongly correlated with soil chemical factors: base saturation, cation exchange capacity, pH, total nitrogen, organic carbon and aluminium levels. Geothermal vegetation in the Taupo Volcanic Zone is floristically very simple, dominated by a small suite of species, and occurs across stress gradients that extend well beyond active geothermal features. Most results from earlier site-specific studies in the region apply generally to geothermal vegetation across the Taupo Volcanic Zone. Geothermal vegetation on slightly warm to warm ground is susceptible to weed invasion, particularly in urban and peri-urban areas where weed pressure is high.