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.