Table 8.— Monthly temperature data for October through April, 1976-77' 



Monthly mean temperature' 

 State Oct. Nov. Dec. Jan. Feb. Mar. Apr. 



California 



59.1 



50.9 



41.3 



37.5 



38.6 



43.8 



56.6 





0.5 



0.5 



0.3 



- 1.9 



-4.5 



-2.9 



3.4 



Colorado 



42.0 



32.3 



23.4 



18.9 



28.2 



30.9 



44.1 





-4.1 



-0.7 



-0.9 



-2.7 



0.2 



-0.5 



2.2 



Idaho 



45.4 



36.4 



26.3 



19.1 



30.0 



33.3 



48.4 





-1.9 



1.8 



0.3 



-3.5 



1.8 



-0.6 



4.4 



Montana 



43.6 



31.6 



26.2 



13.5 



32.3 



32.9 



47.4 





2.4 







3.8 



-3.7 



9.5 



4.1 



5.2 



Nevada 



48.9 



41.2 



30.6 



27.3 



36.6 



34.0 



49.2 





-1.4 



2.8 



-0.3 



-1.5 



2.9 



-3.3 



3.1 



Oregon 



48.7 



41.0 



31.7 



25.5 



37.0 



37.0 



48.7 





0.3 



2.9 



-0.3 



-3.5 



1.3 



-1.3 



3.6 



Utah 



49.0 



39.7 



29.0 



25.8 



34.5 



36.1 



52.2 





-2.7 



1.5 



-0.1 



-0.3 



2.8 



-2.2 



3.3 



Wyoming 



42.0 



31.4 



24.2 



16.1 



28.1 



29.5 



44.7 





0.8 







1.3 



-3.5 



4.1 



0.4 



3.7 



'Average mean temperature of the climatic divisions vi/ithin the distributional zone of Artemisia tridentata. 

 ^Upper figure represents the actual monthly mean temperature, °F. Lower figure represents the depart- 

 ure from normal based on the 1941-70 record. 



The monthly mean temperature departure from normal 

 varied little between States (table 8). For most States, October 

 was below normal, November above, December and January 

 below, February above, March below, and April above. 



DISCUSSION AND CONCLUSIONS 



As far as we have been able to determine, the winter injury 

 of wildland shrubs reported here is the most extensive winter 

 injury of indigenous plants ever recorded in the United States. 

 The extensive kill of big sagebrush over large areas represents a 

 significant natural vegetational change. These areas will even- 

 tually return to sagebrush, however, because some young plants 

 survived and seeding will occur from plants in surrounding 

 areas. Although Ceanothus and Arctostaphylos commonly 

 receive winter injury, it is seldom reported. This is the first 

 report of winter injury to most of the other shrub species listed 

 here. 



It is beyond our objective to establish the precise cause of 

 the reported winter injury by an analysis of past weather 

 records. Averaged weather data tend to mask extremes that 

 likely affect plant development and cause injury most directly. 

 Because of the high elevation, distributional pattern of the 

 various shrubs, and remoteness of the sites involved, few 

 weather stations exist that keep complete long-term records for 

 the precise areas where winter injury occurred. 



The weather conditions during the winter of 1976-77 caused 

 winter injury that could have resulted from either freezing or 

 winter desiccation or both. The extremely low precipitation 

 during November through February and resulting low snow 

 cover very likely resulted in frozen soil in those areas where it is 

 normally frozen slightly or not at all. In addition, soil moisture 

 levels were unusually low. The lack of snow and generally 

 above normal temperatures in November could have delayed 



dormancy formation in shrubs. This, followed by continued 

 lack of snow cover and generally below normal temperatures in 

 December, could have resulted in freezing injury. February was 

 above normal in temperature, which could have stimulated 

 growth activity in plants that had broken winter dormancy. 

 During this time, consequential increased transpiration with 

 soils of low available moisture, either due to frozen soil or lack 

 of moisture, could have resulted in desiccation of upper plant 

 parts. Below normal March temperatures followed, and this 

 could also have resuhed in freezing injury to plants that were 

 possibly stimulated to growth activity during February. For the 

 most part these conclusions support those of Schumaker and 

 others (1979) and Hanson and others (1982). 



Although the monthly accumulated percent of normal precip- 

 itation in areas of little or no winter injury was around 20 to 

 45 percent higher than in areas of extensive winter injury, the 

 actual accumulated precipitation was nearly the same. This 

 could indicate that snow cover was actually no greater and 

 plant water deficits were probably just as high as in areas of 

 winter injury. If this is true, it would seem to suggest that an 

 underlying factor leading to winter injury of some of these 

 shrubs, in addition to high water deficits and lack of snow 

 cover, could be less selection for winter hardiness. For example, 

 sagebrush in protected areas of usual snow cover and of the 

 same subspecies as on an adjacent exposed site, may not be 

 selected for winter hardiness to the same extent as their 

 counterpart on the exposed site. This of course assumes other 

 factors are equal. Frost penetration during inversions, for ex- 

 ample, is likely greater in valley bottoms when there is little or 

 no snow cover than on adjacent exposed slopes. Shrubs such as 

 Ceanothus velutinus and Arctostaphylos patula, which are com- 

 monly damaged by winter injury, sprout from lower stems and 

 the root crown and thereby survive without the need for ex- 

 treme winter hardiness. 



15 



