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Interior West — Our Living Resources 



employed a diversity of sampling and analytical 

 techniques to evaluate growth patterns. 



As noted previously, there are several poten- 

 tial explanations for recent increased growth in 

 subalpine conifers. The possibility of carbon 

 dio.xide fertilization has been supported by 

 experimental studies (Graybill and Idso 1993). 

 but is extremely difficult to demonstrate for 

 mature trees in the field. Increased temperature 

 is another potential cause, but its relationship 

 with growth is correlative and also difficult to 

 demonstrate for mature trees. Changes in snow- 

 pack duration, which affects length of growing 

 season, are a more likely cause of growth 

 increases. Unfortunately, the long-temi rela- 

 tionship of snowpack to tree growth has not 

 been adequately investigated because snowpack 

 data are often difficult to obtain. 



Fertilization through nitrogen deposition 

 could be another cause of growth increases. 

 Although nitrogen deposition is relatively low 

 in western North America, it is probably some- 

 what higher now than in the past because of the 

 combustion of fossil fuels. Many subalpine 

 forests are in sites with shallow soils and rela- 

 tively low fertility, so even a small increase in 

 nitrogen input could have some effect over sev- 

 eral decades. Finally, the growth increases may 

 simply be the result of normal forest stand 

 dynamics because relatively little is known 

 about the growth and ecological characteristics 

 of subalpine forest ecosystems. Although the 

 observed increases appear abnomial compared 

 to lower elevation species, they may in fact be a 

 normal phenomenon that reflects the natural 

 range of variation in growth of subalpine 

 species. Growth response to climate or other 

 factors likely varies considerably by region 

 (e.g., the Rocky Mountains have a continental 

 climate, the Siena Nevadas a Mediterranean cli- 

 mate) and by microsite (north aspect versus 

 south aspect). 



Patterns of Establishment 



Recent increases in tree establishment in 

 subalpine meadows have been documented in 

 mountainous regions throughout western North 

 America (Rochefort et al. 1994). Most locations 

 show an expansion of the forest margin after 

 1890. with establishment peaks between 1920 

 and 1950. Additional establishment peaks have 

 been identified on a local basis. Most investiga- 

 tors have concluded that increases in tree estab- 

 lishment are the result of a warmer climate fol- 

 lowing the Little Ice Age (Franklin et al. 1971; 

 Kearney 1982: Heikkinen 1984; Butler 1986). It 

 is unclear if establishment patterns signify a 

 long-term directional change or short-term vari- 

 ation in relatively stable ecotones, regardless of 

 the potential causes. 



Most studies on subalpine tree establishment 

 have been conducted in the Pacific Northwest in 

 British Columbia in Canada and Washington 

 and Oregon (Woodward et al. 1991; Rochefort 

 et al. 1994) where tree invasion in subalpine 

 meadows is widespread. Trees in this area are 

 rapidly becoming established (Rochefort and 

 Peterson 1991; Woodward et al. 1991), espe- 

 cially in meadows dominated by ericaceous 

 species (species in the heath family such as 

 heather and huckleberries). Much of this estab- 

 lishment is occuiTing in concavities and other 

 places where snow would nomially accumulate 

 and inhibit germination and survival (personal 

 observation). As trees become established, tree 

 clumps act as black bodies to increase the 

 absorption of radiation, snowmelt occurs pro- 

 gressively earlier, tree canopies intercept (and 

 allow sublimation of) snow, and tree survival 

 adjacent to the tree clump is further enhanced. 

 This progression of events is termed "conta- 

 gious dispersion" (Payette and Filion 1985). 



Eight separate studies in the Pacific 

 Northwest have documented large increases in 

 populations of subalpine fir {Abies lasiocarpa). 

 Pacific silver fir. mountain hemlock, subalpine 

 larch {Lari.x lyallii). and Alaska yellow-cedar 

 iChainaecyparis nootkatensis). All these species 

 experienced increases in establishment between 

 1920 and 1950. This was generally a period of 

 lower snowpacks. which probably allowed 

 seedlings to become established during a longer 

 growing season. Winter precipitation limits sub- 

 alpine tree growth and establishment in the 

 Pacific Northwest, which has a maritime climate 

 with wet winters and dry summers; high summer 

 temperature can also limit tree establishment 

 because shallow-rooted seedlings are subject to 

 soil moisture stress (Little et al. 1994). 



Increases in establishment of three species 

 have been documented in the Sierra Nevada and 

 White Mountains of California: foxtail pine, 

 lodgepole pine, and bristlecone pine. Soil mois- 

 ture stress is clearly a limiting factor in this area, 

 which is dominated by a Mediterranean climate 

 with very dry summers. Temporal patterns of 

 establishment are inconsistent among the differ- 

 ent locations in this region, and there has been lit- 

 tle documented establishment during the past 20 

 years. 



Studies conducted in the Rocky Mountains 

 have documented increases in subalpine tree 

 establishment for subalpine fir, lodgepole pine, 

 and Engelmann spruce (Picea engelmannii). 

 This region is dominated by a continental cli- 

 mate, with low precipitation and cold winters. 

 Temporal patterns of establishment were more 

 consistent in the Rocky Mountains, especially 

 during 1940-50, a period with a warmer, wetter 

 climate. 



It is unclear whether observations of sub- 



