Vegetation change along a geothermal stress gradient at the Te Kopia steamfield

Naturally thermotolerant vegetation in New Zealand is concentrated in approximately 39 geothermally heated areas totalling only 580 ha scattered along the 300 km length of the Taupo Volcanic Zone. Relationships between composition and structure of vegetation and environment were analysed at the comparatively large (95 ha) and unmodified Te Kopia geothermal steamfield. Vascular vegetation was mostly influenced by a strong gradient in soil temperature, ranging at 15 cm depth from 15 degrees C to 90 degrees C across the steamfield. Dominant species changed from evergreen, broadleaved forest at the cool end of this gradient to dominance on increasingly hot soils by myrtaceous and epacrid (southern heath) shrubs with small needle-like leaves and shallow roots. While differences in cryptogam composition were mostly associated with spatial variation in soil temperature, they were also significantly influenced by soil acidity and Al concentration. All soils sampled along the gradient had low fertility, high acidity, and high levels of Al, SO4, and Fe. This suggests that vegetation considered to be geothermally influenced should include relatively tall forest and scrub further from fumarole centres than has previously been recognised. Plants of geothermal areas tolerate acid soils with levels of Al that normally would be toxic. Mycorrhizal relationships may play important roles in plant survivorship in this environment by assisting nutrient acquisition. In common with the changes in community characteristics typical of other steep gradients of increasing stress, as soil temperatures increase so canopy height, plant stature, species richness, leaf size, and rooting depth decrease. These distinctive ecosystems provide excellent opportunities for research on plant community response to natural stress.