Geothermal vegetation types of the Taupō Volcanic Zone

TR 2015/11
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Report: TR 2015/11

Author: M C Smale, N B Fitzgerald (Landcare Research)

About this report

Stratified random sampling was undertaken in March–June 2012 for the Waikato and Bay of Plenty regional councils, in order to develop a robust, objective classification of geothermal vegetation types in the Taupō Volcanic Zone, and to understand the main environmental drivers of vegetation composition.

Vegetation composition patterns were examined at 38 sites on 15 geothermal fields in the zone. A total of 138 vascular plant species was recorded, including 58 adventives (non original, exotic), 19 moss and nine liverwort species. Only seven native species were truly widespread (>20 per cent of plots); only four adventive species were at all widespread (>5 per cent of plots). Of 10 threatened species known to occur in geothermal areas, six were recorded from the plots. Apart from very widespread Kunzea ericoides var. microflora, none occurred in more than 10 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. Soil chemistry varied considerably amongst plots, and to some extent, localities. On average, soils were strongly acidic, with moderate levels of organic carbon and Olsen phosphorus and moderate cation exchange capacity, but low base saturation and low levels of total nitrogen, calcium, magnesium, potassium and sodium. Most had high levels of aluminium and sulphur.

Classification identified 16 vegetation types in eight structural classes (two mossfield, one grassland, three fernland, four shrubland, three scrub, one treefernland, one treeland, one forest), all but one dominated by indigenous species, and 14 of them unique to geothermal fields. A relatively high proportion of these were in urban or peri-urban areas with high weed pressure.

Subsurface (10cm depth) soil temperature was the main factor related to vegetation composition. Vegetation composition was secondarily related to soil chemical factors.

A range of communities mostly unique to geothermal fields occurs across the spectrum of soil temperatures present, most of them – including two that are widespread beyond them – on the extensive slightly heated soils surrounding geothermal features, with a small number on the smaller areas of markedly heated soils.

Active weed control is necessary to maintain the integrity of geothermal vegetation on slightly warm to warm ground in urban or peri-urban areas where weed pressure is high. Ten sites on five geothermal fields have significant weed problems, six (Arawa Park Racecourse, Arikikapakapa Golf Course, Cemetery Reserve, Kuirau Park, Old Government Gardens, Whakarewarewa) of them in Rotorua city. The others are Parimahana Scenic Reserve (Kawerau), Karapiti (Wairakei), Tikorangi North (Tikorangi), and Waiotapu Thermal Wonderland (Waiotapu).

Distinctive geothermal vegetation occurs across a wide range of soil temperatures, so protection and management should ensure that the full gradients of composition and environment are protected and managed to prevent their degradation.

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Contents
  Summary 
1 Introduction and background
2 Methods
2.1 Sampling
2.2 Data analysis
2.3 Soil analysis
3 Results
3.1 Soil chemistry
3.2 Subsurface soil temperature
3.3 Flora
3.4 Vegetation classification
3.5 Relationships between vegetation and environment
3.6 Cross-reference with earlier classifications
4 Conclusions
5 Recommendations
6 Acknowledgements
7 References
  Appendix 1 - Common and scientific names of plants used in text
  Appendix 2 - Random plots by location and geothermal field in the Taupō Volcanic Zone
  Appendix 3 - Sites excluded from random plot generation
  Appendix 4 - Soil chemical analyses and mean soil temperatures (10cm depth) of random plots in the Taupō Volcanic Zone
  Appendix 5 - Vascular plant species recorded during geothermal vegetation sampling in the Taupō
  Appendix 6 - Bryophytes recorded during geothermal with vegetation sampling in the Taupō Volcanic Zone
  Appendix 7 - Environmental weeds (Howell 2008) encountered at geotehrmal sites in the Taupō Volcanic Zone
  Appendix 8 - Cross reference between Merrett & Clarkson (1999) and this classification
  Appendix 9 - Cross-reference between Fitzgerald & Smale (2010), excluding wetland types and bare ground, and this classification
  Appendix 10 - Cross-reference between Wildland Consultants (2011), excluding wetland types and bare ground, and this classfication
  Appendix 11 - Cross-reference between outlier and transitional plots and Wildland Consultants (2011) associations
  Appendix 12 - Individual and organistations acknowledged for access permission