Summary 



In general, probability of seedling survival increased 

 whenever the frequency of HE's and CE's decreased. It 

 appears likely that CE's caused freezing of plant tissue, 

 which contributed to seedling mortality. For HE's, the 

 damage may have been a direct heat injury that caused 

 death, but it is just as likely that the high temperatures 

 caused dessication of the soil, creating moisture stress 



Table 9— Plantation survival data after 5 years (1982) for lodge- 

 pole pine at Union Pass, WY, on a clearcut with three 

 residue treatments. Hot and cold events are given for 

 July 1979. 



Clearcut 



Broadcast Close 

 burn^ utilization Chips 



Species 



(percentage survival) 

 Lodgepole pine 96 46 26 



Temperature 



(percentage days) 

 Hot events 62 62 10 



Cold events 10 43 



'Temperatures were measured on the burned surface on the broadcast 

 burn, litter surface on the residue removed, and the chip surface on the 

 chips. 



conditions that led to seedling mortahty. Higher radiation 

 loads on the seedlings in the clearcut and above some sur- 

 face treatments, such as the chips, caused the elevated 

 surface and soil temperatures and also greater vapor 

 pressure deficits (increased LE) and thus greater moisture 

 stress in the seedlings. 



Imposing treatment practices to reduce temperature 

 variation must be used with caution. Treatments that have 

 been shown to reduce temperature variation (exposing 

 mineral soil) also increase evaporation and can create a 

 moisture deficiency problem. On sites where high temper- 

 ature is not a problem but moisture is, leaving a mulch 

 (litter) or providing one may reduce moisture losses 

 (Cochran 1969). Another major factor is the condition of 

 planting stock. At Lubrecht, condition of the containerized 

 seedlings was quite variable (Shearer 1985). Seedlings that 

 are already stressed may be more likely to succumb when 

 planted in more stressful situations. Thus, these results do 

 not provide proof of the relationship between temperature 

 and mortality. But they do provide strong evidence that 

 the effects of treatment on site energy balance and ther- 

 mal properties should be evaluated in relationship to the 

 potential for seedling survival. 



CONCLUSIONS AND IMPLICATIONS 

 FOR MANAGERS 



Several points from this and other studies have a bear- 

 ing on surface temperature modification and successful 

 regeneration of stands. 



1. It is crucial to be able to identify potential mortality 

 problems due to temperature and moisture at the time 



silvicultural prescriptions are prepared. If problems are ex- 

 pected, steps can be taken to avoid problems by altering 

 silvicultural and subsequent reforestation practices. 



2. Current reforestation guides indicate that high tem- 

 peratures are a problem on south and west aspects on 

 slopes over 30 percent at lower elevations. In this study, 

 potentially lethal high temperatures were also observed on 

 level ground, on east-facing slopes, and at high elevations 

 on clearcuts in Douglas-fir and subalpine fir habitat types. 

 Potentially lethal cold temperatures were measured on 

 level sites at high and low elevations in both habitat 

 series. The pattern of seedling survival on these sites in- 

 dicates that mortality was in fact related to the hot and 

 cold temperature events. 



3. Results of this study show that shelterwood or partial 

 cuttings with 50 percent or less sunlight transmission to 

 the forest floor significantly reduce the occurrence of high 

 temperatures and low temperatures. Seedling survival was 

 higher in these stands. 



4. Seedbed preparation and residue treatments for slash 

 reduction significantly influence the occurrence of poten- 

 tially lethal seedbed surface temperatures by altering the 

 thermal properties of the surface. In general, practices 

 that increase the thermal conductivity and volumetric heat 

 capacity (thus, V^KC— thermal contact coefficient) of the 

 surface materials will decrease maximum temperatures 

 and increase minimum temperatures. Treatments that ex- 

 pose mineral soil reduce surface temperature variation 

 (decrease maximum and increase minimum) compared to 

 natural litter-covered surfaces. Burned and natural litter- 

 covered surfaces on clearcuts may be equally susceptible 

 to high temperatures, based on results reported here. 

 However, this may vary depending on the thermal proper- 

 ties of the surface materials. 



5. Where it is not practical to alter the surface 

 materials (thermal properties), excessively high and low 

 temperatures can be reduced by providing shade. Leaving 

 enough residues onsite to provide adequate shade protec- 

 tion will reduce temperatures and increase seedling sur- 

 vival. Residues can also conserve moisture at the surface 

 by reducing evaporation. Allowing some vegetation growth 

 (provided moisture is not limiting) will also provide ade- 

 quate shade and thus temperature modification. Seedlings 

 planted on these sites should be positioned to take advan- 

 tage of the modified microsites— on the north or east sides 

 of logs, stumps, branches, rocks, etc. 



6. On sites where soil moisture is more of a problem 

 than is high temperature, mulches (plant litter, artificial 

 materials; dry, loosened soil; etc.) left on the surface can 

 help retard evaporation and thus increase survival. How- 

 ever, these practices must be planned carefully because 

 they will increase surface temperatures. 



7. Frost-pocket problems can be aggravated by over- 

 story and residue treatment. The ability to identify poten- 

 tial frost pockets while developing the silvicultural pre- 

 scription is important. Subalpine fir in drainages or flat 

 areas or Vaccinium caespitosum in the understory are 

 often good indicators of potential frost pockets. 



8. Shade from partial overstories, residue, or understory 

 vegetation can provide varying degrees of protection for 

 seedlings in frost pockets. Only the most frost-resistant 

 species should be used in frost-pocket areas. 



17 



