HIGH-TEMPERATURE AND DROUGHT STRESS EFFECTS ON SURVIVAL OF PINUS-PONDEROSA SEEDLINGS

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 colon ize drought-prone sites with high soil surface temperatures. Temperatu re profiles were measured with 0.07-mm thermocouples in a sparse ponde rosa pine forest in northern Idaho. The soil surface and the adjacent 5 mm of air reached maximum temperatures exceeding 75 degrees C, well above the lethal temperature threshold for most plants. Air temperatur es 50 mm above the soil surface (seedling needle height) rarely exceed ed 45 degrees C. Pinus ponderosa seedlings that survived maintained ba sal stem temperatures as much as 15 degrees C lower than the surroundi ng air. The apparent threshold temperature at the seedling stem surfac e resulting in death was approximately 63 degrees C for less than 1 mi n. No correlation between seedling mortality and needle temperature wa s found, although some needles reached temperatures as high as 60 degr ees C for periods of less than or equal to 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 t hrough seedling stems may have acted as a heat transfer mechanism, coo ling the stem below the lethal threshold temperature. Heat exchange ca lculations showed that rapid water flow through seedling stems can abs orb sufficient energy to reduce stem temperature by 30 degrees C durin g peak sunlight hours.