Average tree heights at the Zena Creek study area were greater for all treatments 

 on the east aspect: 



Treatment 



Tree heights 



East 



West 



(Feet) 



Control (no mulch) 

 Glass fiber mulch 

 Polyethylene mulch 

 Simazine 



1.3 

 1.1 

 1.5 

 1.3 



1.3 

 1.0 

 1.2 

 1.1 



Seedlings that received the black polyethylene mulch on the warmer aspect were con- 

 sistently the tallest. Romberger (1963) has suggested that a threshold spring tempera- 

 ture triggers height growth. This increased height can be expected because higher 

 temperatures occur over black surfaces and the threshold temperature is reached 

 earlier in the spring when moisture is readily available. 



These tests at the Boise Basin Experimental Forest attempted to accomplish the 

 following. 



(1) Trace the seasonal soil moisture depletion cycle under: (a) two kinds of 

 mulch--black polyethylene, and glass fiber blanket; (b) weeded mineral soil; and (c) 

 an undisturbed condition (control). 



(2) Measure the summer temperature gradients on a given day above each of the 

 four treatments on both level and sloping ground. 



(3) Measure the seedling survival associated with the treatments, soil moisture, 

 and the air temperature regime. 



Survival results were analyzed for two test areas: (a) An aspect having an azi- 

 muth of 185° and a slope of 35 percent; and (b) an aspect at 223° with a slope of 32 

 percent. 



Undamaged, 2-0, ponderosa pine planting stock with a top:root ratio of 1:2, or 

 better, was planted in mid-May 1964. The mulches were put in place several days later. 

 First-year survival was recorded at 2-week intervals from June 1 to October 15. The 

 weeding treatment was repeated at each of the first five semimonthly survival counts. 

 Annual survival counts were made each fall until 1968. 



Precipitation at Idaho City in June and August of 1964 was more than twice the 

 normal amount; July and September were very close to normal (table 4). 



Soil Moisture and Temperature Relations (Boise Basin) 



Test 5: Soil Moisture and Temperatures Associated With Undisturbed 

 Spots and PitSj With and Without Synthetic Mulches ^ 1964 



12 



