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Abstract
Fibreglass soil moisture units and thermistors were used to record changes in soil moisture and temperature to a depth of 5 ft under a stand of 30-year-old radiata pine in Kaingaroa State Forest, in 1956–8. Weekly readings were made and changes related to rainfall and air temperatures.
During the summer, this soil (Taupo silty sand) dried out to a greater depth than Rotomahana shallow sandy loam in which an earlier study was carried out. However, moisture tensions rose to wilting point only in soil that had been “bleached” by intense fungal activity. “Bleaching” is associated with a complete canopy closure and a build up of litter. These same factors restrict the availability of moisture in the main rooting zone. In a drought the combined effect may be to restrict tree growth.
Soil temperature changes were similar to those found previously. Surface soil temperatures were associated with day to day changes in air temperature, but at a depth of 5 ft only seasonal changes occurred, and these lagged behind changes in air temperature by several weeks.
Available evidence suggests that, during the winter, day length and not air or soil temperature limits the growth of radiata pine in Kaingaroa State Forest.