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 scatt ered 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 ge othermal steamfield. Vascular vegetation was mostly influenced by a st rong gradient in soil temperature, ranging at 15 cm depth from 15 degr ees 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 (southe rn heath) shrubs with small needle-like leaves and shallow roots. Whil e differences in cryptogam composition were mostly associated with spa tial variation in soil temperature, they were also significantly influ enced by soil acidity and Al concentration. All soils sampled along th e 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 fro m fumarole centres than has previously been recognised. Plants of geot hermal 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. I n common with the changes in community characteristics typical of othe r steep gradients of increasing stress, as soil temperatures increase so canopy height, plant stature, species richness, leaf size, and root ing depth decrease. These distinctive ecosystems provide excellent opp ortunities for research on plant community response to natural stress.