Table 4— Average minimum surface temperatures (°F) by month for 1978, 1979, and 1980 for 

 tfie overstory removal treatments at the Lubrecht study site. Data from the litter 

 surface were used for all treatments. Values for a month (months are not compared) 

 having different letters are significantly different at the 0.05 level. 



— — — — 1 



Overstory 



treatment 



May 



June 





July 



Aug. 



Sept. 



Oct. 









1978 











Uncut 



34b 



39b 





43b 



41b 



37b 



27b 



1 InHorctnrv/ romr\\/al 

 iwi 1 lUv^i oiui y iciiiuvcti 





oou 





41 h 



H 1 U 





OOU 





Shelterwood 



36b 



40b 





44b 



41b 



38 b 



26b 



Clearcut 



26a 



30a 





35a 



34a 



28a 



12a 









1979 











Uncut 





o / D 









o / C 



ODC 



Understory removal 



34b 



36b 









33b 



26b 



Shelterwood 



34b 



36b 









36bc 



28bc 



Clearcut 



25a 



29a 









25a 



17a 









1980 











Uncut 





44b 





45c 



40c 



38b 



30b 



Understory removal 





47b 





45cd 



38b 



39b 



28b 



Shelterwood 





47b 





47d 



40bc 



39b 



31b 



Clearcut 





40a 





38b 



32a 



33a 



23a 



or greater were measured from April (12 days after snow- 

 melt) into October. Maximums on the understory and 

 uncut treatments rarely exceeded 122 °F. Figure 3 is 

 representative of the patterns observed for most summer 

 months over the 3 years. 



The degree of overstory removal also significantly af- 

 fected minimum surface temperature (table 4). Removal of 

 up to about 50 percent of the overstory (understory re- 

 moval and shelterwood) did not cause minimum tempera- 

 tures to fall lower than in the uncut stand. However, 

 removal of all the overstory caused minimum tempera- 

 tures to be significantly lower than on the shelterwood, 

 understory removal, or uncut treatments for all months. 



Minimum temperatures on the clearcut in 1978 were as 

 low as 18 °F in June, 25 °F in July, and 21 °F in August. 

 By September nearly every night was below 32 °F, 

 with a low of 10 °F. Temperatures below 32 °F were not 

 observed on the shelterwood or uncut treatments, and 

 only occasionally on the understory removal treatment 

 in June, July, or August (fig. 4). 



The same pattern seemed to exist in 1979, but instru- 

 ment failure precluded records in July and August. The 

 summer of 1980 was warmer; no temperature below 32 °F 

 was recorded in July. 



Hot event days, when the temperature exceeded the 

 lethal threshold, occurred on the clearcut and shelterwood 

 treatments from May through October on 14 to 37 percent 

 of the days (table 5). The probability of an HE was greater 

 on the clearcut, but the difference is not significant. No 

 he's occurred on the understory removal treatment, and 

 only three were measured on the uncut control over the 3 

 years. These results suggest that the potential for seedling 

 mortality due to high temperatures is quite high on the 

 clearcut and shelterwood treatments, but almost nonexist- 

 ent in the uncut stand and on the understory removal 

 treatments. 



0£ 



.CP 



Q. 

 2 



< 

 o 



Z) 

 Z 



20 



JGUST DAY 



Figure 4 — Minimum daily ground surface 

 temperatures in August 1980 at Lubrectit for ttie 

 uncut, understory removal (ur), and clearcut (cc) 

 treatments. Temperatures in the stielterwood 

 (sw) were the same as the understory removal. 

 The surface condition was litter. 



On the surface of the clearcut, cold event days— when 

 the temperature dropped to the lethal threshold of 23 °F 

 or below— occurred on 16 to 26 percent of the days in May 

 through September the first 2 years after treatment. Dur- 

 ing the third year none occurred on the clearcut. During 

 the 3 years following treatment, only one CE occurred on 

 the uncut and understory removal treatments and none on 

 the shelterwood treatment. Most of the CE's occurred in 

 May and September (75 to 80 percent), but a significant 

 number occurred in June, July, and August. Like the 

 he's, the results show that a potential for seedling mor- 

 tality due to low temperatures exists on the clearcut 

 treatment. 



7 



