Kolb, P.F. and R. Robberecht. 1996. High temperature and drought stress effects on survival of Pinus ponderosa seedlings. Tree Physiology 16:665-672.

Summary. We studied the effects of high temperature and drought on the survival, growth and water relations of seedlings of Pinus ponderosa (Dougl.) Lawson, one of few coniferous tree species that can successfully colonize drought-prone sites with high soil surface temperatures. Temperature profiles were measured with 0.07-mm thermocouples in a sparse ponderosa pine forest in northern Idaho. The soil surface and the adjacent 5 mm of air reached maximum temperatures exceeding 75°C , well above the lethal temperature threshold for most plants. Air temperatures 50 mm above the soil surface (seedling needle height) rarely exceeded 45°C. Pinus ponderosa seedlings that survived maintained base stem temperatures as much as 15°C lower than the surrounding air. The apparent threshold temperature at the seedling stem surface resulting in death was approximately 63°C for less than 1 min. No correlation between seedling mortality and needle temperature was found, although some needles reached temperatures as high as 60°C for periods of 1 min. Surviving seedlings had significantly higher stomatal conductance than seedlings that did not survive until fall. Transpiration rates, calculated from measured needle temperatures, stimulate conductance and evaporative demand, were high (up to 16.7 mmol m-2 s-1 ), indicating that water transport through seedling stems may have acted as a heat transfer mechanism, cooling the stem below the lethal threshold temperature. Heat exchange calculations showed that rapid water flow through seedling stems can absorb sufficient energy to reduce stem temperature by 30°C during peak sunlight hours.

Keywords: heat tolerance, lethal temperature threshold, stomatal conductance, transpirational cooling.

This project was funded by the McIntire-Stennis Cooperative Forestry Research Program through the University of Idaho, College forestry, Wildlife and Range Sciences, Moscow, Idaho. Contribution no. 794 of the Forest, Wildlife and Range Experiment Station, University of Idaho.