Our Liviiii; Rcsnuici'S — Intcnur West 



il5 



climate change has focused on changes in the 

 growth and distribution of low-elevation foiests 

 (e.g.. Woodman 1487; Davis 1989). In western 

 North America, most low-elevation forests are 

 sensitive to soil moisture deficits during rela- 

 tively dry summers (Peterson et al. 1991; 

 Grayhill et al. 1992). Although subalpine 

 forests have been the subject of considerably 

 less study, it appears that snowpack is an impor- 

 tant limiting factor to growth, with respect to 

 length of growing season (Graumlich 1991; 

 Peterson 1993). Duration of snowpack also lim- 

 its seedling establishment in subalpine mead- 

 ows (Fonda 1976) and after disturbance by fire 

 (Little et al. 1994). Summer temperature also 

 positively affects the growth of mature sub- 

 alpine conifers (Graumlich 1991; Peterson 

 1993) and negatively affects the seedlings" sur- 

 vival (Little et al. 1994). 



Several reports document recent increases in 

 the growth of subalpine conifer species in west- 

 em North America (Innes 1991) as well as 

 recent increases in the abundance of subalpine 

 conifer populations at several locations. This 

 article reviews recent reports of changes in the 

 growth and distribution of subalpine conifers in 

 western North America and discusses some 

 possible causes. 



Tree Growth 



The first prominent report of a recent 

 increase in growth of subalpine coniferous 

 species was published by LaMarche et al. 

 ( 1984). who reported dramatic increases in the 

 growth rate of bristlecone pine {Piiuis longaeva. 

 P. aristata) and limber pine (P. fle.xilis) in 

 California and Nevada. The extreme age of 

 these trees, combined with the fact that radial 

 growth has increased since 1850. makes this a 

 particularly interesting result. The authors sug- 

 gested that elevated levels of carbon dioxide 

 associated with fossil fuel combustion may 

 enhance the growth and productivity of these 

 trees, perhaps through increased water-use effi- 

 ciency. A more recent examination of these data 

 corroborates the growth increase and restates 

 that carbon dioxide fertilization is the hypothe- 

 sized cause of the increase (Graybill and Idso 

 1993). Some disagreement exists about the fac- 

 tors causing the growth increase and whether 

 the increases in growth found in these studies 

 (which included sampling of strip-bark trees) 

 are representative of the populations as a whole 

 (Cooper and Gale 1986). 



A subsequent study of basal area growth 

 trends of lodgepole pine {P. contorta) and 

 whitebark pine {P. albicaiilis) at sites above 

 3.000-m elevation in the east-central Sierra 

 Nevada of California also revealed that a high 

 proportion of trees has had recent growth 



Miiny .subalpine forests in western 

 Nortli America, sucti as ttiis site in 

 (he Olympic Mountains, are cur- 

 rently protected in national parks 

 and wilderness areas. Some of 

 these areas have been experiencing 

 increased tree growth and rapid 

 establishment of young trees dur- 

 ing the past century. 



increases (Peterson et al. 199(J), with the onset 

 of the increase normally between 1850 and 

 1900. as found by LaMarche et al. (1984). 

 Growth was particularly rapid during the past 

 30 years or so. 



There are other reports of recent growth 

 increases in subalpine conifers of western North 

 Ameiica (Innes 1991). Jacoby (1986) found 

 radial growth increases in lodgepole pine in the 

 San Jacinto Mountains of southern California, 

 but did not identify a strong causal factor 

 despite detailed climatic analysis. Graumlich et 

 al. (1989) found increases in the growth and 

 productivity of Pacific silver fir (Abies ama- 

 hilis) and mountain hemlock (Tsiif^a iiicrteu- 

 siana) in the Cascade Mountains of Washington 

 State, and suggested that these trends were 

 related to increased temperature. 



Recent growth increases have also been 

 reported in European conifers (Innes 1991), 

 such as Norway spruce (Picea abies; Kienast 

 and Luxmoore 1988; Briffa 1992) and silver fir 

 (Abies alba: Becker 1989), although these 

 species are generally found below the subalpine 

 zone. Both increased carbon dioxide (Kienast 

 and Luxmoore 1988) and temperature (Becker 

 1989; Briffa 1992) have been suggested as 

 potential causes for increased growth. 



Not all studies of subalpine conifers have 

 found recent increased growth, however. 

 Graumlich (1991). for example, did not find 

 increased radial growth in foxtail pine (Pinus 

 balfoiirianci), limber pine, and western juniper 

 (Jiiniperiis occidentalis) in the Sierra Nevada. It 

 is difficult to compare the various studies of tree 

 growth discussed here because the studies 



