© 1996 Heron Publishing—Victoria, Canada
High temperature and drought stress effects on survival of Pinus ponderosa seedlings
Peter F. Kolb (1) and Ronald Robberecht (2)
1. Department of Forest Resources, College of Forestry, Wildlife and Range Sciences, University of Idaho, Moscow, Idaho 83844,
USA / 2. Department of Range Resources, College of Forestry, Wildlife and Range Sciences, University of Idaho, Moscow, Idaho 83844,
USA / Received July 21, 1995
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 basal 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, stomatal 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.
