Lethal surface temperatures and their interpretation for Douglas-fir

The concept is developed that direct measurement of physicalfactors at the microsite level might provide better criteria thangross features of the landscape for judging whether Douglas-fir clear-cut areas will restock naturally. Large scale sampling of micrositetemperatures vas considered as most promising of possible criteriafor initial work.
The percentages of surfaces that exceeded 125°, 138°, and 150°F.on eight Douglas-fir clearcuts in western Oregon were estimatod bythe use of temperature pellets during 1954. By September 50.5 per-cent of north slopes and 86.5 percent of south slopes had exceeded138°F.
Microsite descriptions of the 100 points sampled on each clear-cut revealed that most microsites remaining below 138°F. were inperpetual shade. Others were on steep northerly aspects, or in shadeof dense or lcw vegetation. Vegetation giving intermittent shadeprovided little protection against high surface temperatures.
The meaning of various levels of temperature was tested by plac-ing the temperature pellets near stems of Douglas-fir seedlings andrelating heat injury with melting of pellets. The 138°F. pellet wasbest related to mortality, but the thermal death point in the fieldseems to range from 125°F. to more than 1500F.
Why some seedlings did not die at very high surface tenperatureswas the subject of laboratory studies. Seedlings were exposed in atemperature gradient under tungsten lamps at various ages and dura-tion of exposure. Seedbeds tested were peat moss, yellow mineralsoil, and quartz sand. Time-temperature relationships at which mor-tality occurred were significantly different for each material.Seedbed material and duration of exposure were major factors, but theeffect of seedling age up to three months wee not significant.
Exposure of seedling stems to water bath temperatures under drycondition, was accomplished with a new apparatus. This gave a tine-temperature relationship at which mortality occurred, but with lower
corresponding values than those in dry materials tested.
Heat—lamp tests on 49 materials indicated that there is probablya complete spectrum of such time—temperature curves. Seedling mor-tality occurred as low as 122°F. at four hours exposure to water bathtemperatures and as high as 176°F. at 15 minute exposure in whitesand, a range of 54°F. Little evidence was seen of a strong displayof natural resistance of individual seedlings to heat injury. In-stances of resistance to high surface temperatures in the fieldcould be explained adequately by short duration of exposure or seed-bed materials having protective qualities.
FUrther studies showed that transpiration had no measurableeffect on killing temperatures. Thermocouple implantations in seed-ling stems indicated no biological mechanism that might keep thestems cooler than surrounding surface temperatures. The seedlingstem appeared to have temperature characteristics no different thanmeasured inside a plastic tube of similar size filled with water.
The 138°F. temperature pellet was indicated to be a reliableindicator of probable heat mortality on clearcuts, provided durationsof exposure and seedbed materials were near the average listed forthe eight clearcuts in this study.
The high proportions of south slopes that exceed 1250F., usuallyover 90 percent, is information of interest to forest managers. Theimplications that older seedlings may die of heat damage, and thatheat killing occurring late in summer may be confused with droughtkilling, is of interest to researchers. Of theoretical interest arethe conclusions that thermal death points vary with the type ofmaterial that surrounds the seedling, that heat flaw in calories in-stead of actual temperature is probably the correct measurement unitto express heat injury, and that relatively more heat is transferredat the same temperature ,into a seedling in organic materials than inmineral soils.