Subalpine Forests of Western North America

Subalpine Forests of Western North America
		by *David L. Peterson* National Biological Service
Subalpine forest and meadow ecosystems are important, climatically sensitive components of mountainous regions of western North America (Peterson 1991). Changes in temperature, precipitation, snowpack, storm frequency, and fire all could affect the growth and productivity of these systems, resulting in substantial shifts in the location of ecotones (see glossary) between subalpine and alpine zones and montane and subalpine zones (Canaday and Fonda 1974).
Subalpine forests of western North America provide an excellent opportunity to examine response to past climate variation. Trees in the subalpine zone are frequently more than 500 years old and respond to climatic variations over annual to centuries-long time scales. The magnitude of climatic variation these forests have experienced may be compared with projections of future climate resulting from increased concentration of greenhouse gases. The population dynamics of subalpine tree species can be used to interpret climatic conditions under which trees have regenerated and can indicate how subalpine forest and meadow ecotones changed in the past. Preserved pollen and plant fossils can be used to examine subalpine vegetation distribution during different climatic periods of the Holocene (since the last ice age).

Many subalpine forests in western North America, such as this site in the Olympic Mountains, are currently protected in national parks and wilderness areas. Some of these areas have been experiencing increased tree growth and rapid establishment of young trees during the past century. Courtesy D.L. Peterson, NBS

Recent literature on the potential effects of climate change has focused on changes in the growth and distribution of low-elevation forests (e.g., Woodman 1987; Davis 1989). In western North America, most low-elevation forests are sensitive to soil moisture deficits during relatively dry summers (Peterson et al. 1991; Graybill et al. 1992). Although subalpine forests have been the subject of considerably less study, it appears that snowpack is an important limiting factor to growth, with respect to length of growing season (Graumlich 1991; Peterson 1993). Duration of snowpack also limits seedling establishment in subalpine meadows (Fonda 1976) and after disturbance by fire (Little et al. 1994). Summer temperature also positively affects the growth of mature subalpine conifers (Graumlich 1991; Peterson 1993) and negatively affects the seedlings' survival (Little et al. 1994).

Several reports document recent increases in the growth of subalpine conifer species in western 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 (Pinus longaeva, P. aristata) and limber pine (P. flexilis) 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 suggested 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 efficiency. A more recent examination of these data corroborates the growth increase and restates that carbon dioxide fertilization is the hypothesized cause of the increase (Graybill and Idso 1993). Some disagreement exists about the factors 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. albicaulis) 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 increases (Peterson et al. 1990), 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 America (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 amabilis) and mountain hemlock (Tsuga mertensiana) 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 balfouriana), limber pine, and western juniper (Juniperus occidentalis) in the Sierra Nevada. It is difficult to compare the various studies of tree growth discussed here because the studies employed a diversity of sampling and analytical techniques to evaluate growth patterns.
As noted previously, there are several potential explanations for recent increased growth in subalpine conifers. The possibility of carbon dioxide 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 snowpack duration, which affects length of growing season, are a more likely cause of growth increases. Unfortunately, the long-term relationship 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 somewhat higher now than in the past because of the combustion of fossil fuels. Many subalpine forests are in sites with shallow soils and relatively low fertility, so even a small increase in nitrogen input could have some effect over several 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 abnormal 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 Sierra Nevadas a Mediterranean climate) 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 investigators have concluded that increases in tree establishment are the result of a warmer climate following 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 variation 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), especially in meadows dominated by ericaceous species (species in the heath family such as heather and huckleberries). Much of this establishment is occurring in concavities and other places where snow would normally 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 progressively 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 "contagious 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 (Larix lyallii), and Alaska yellow-cedar (Chamaecyparis 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 subalpine 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 moisture 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 different locations in this region, and there has been little 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 climate, 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 subalpine tree invasions are isolated events or part of a broad pattern in western North America. There are insufficient data from locations other than the Pacific Northwest to speculate about the geographic extent of this phenomenon.
Future Changes
Data on subalpine tree growth for western North America are too sparse to infer that growth increases are a broad regional phenomenon. Additional data from other sites are needed to quantify growth trends in subalpine species. Furthermore, consistent sampling and analytical methods should be applied so that different data sets can be compared.
Sufficient information exists, however, about long-term growth trends and shorter-term response of growth to climate to make some general predictions about potential growth under future climate scenarios. If the climate becomes warmer and drier, as predicted by general circulation models, growth rates of subalpine conifers will probably increase. This growth increase would depend on the seasonality of precipitation. A decrease in snowfall would be particularly beneficial to species such as subalpine fir and Engelmann spruce (Ettl and Peterson 1991; Peterson 1993), although warmer summer temperatures could cause summer soil moisture deficits that would be detrimental to growth. It is unknown how future growth patterns will be influenced by increased concentrations of carbon dioxide. Any potential growth changes must, of course, be considered with respect to the effects of climate change on interspecific competition and disturbance, as well as deposition of nitrogen or other nutrients.
		For further information: David L. Peterson National Biological Service Cooperative Park Studies Unit University of Washington Seattle, WA 98195

References
Becker, M. 1989. The role of present and past vitality of silver fir forests in the Vosges Mountains of northeastern France. Canadian Journal of Forest Res. 19:1110-1117. Briffa, K.R. 1992. Increasing productivity of "natural growth" conifers in Europe over the last century. Pages 64-71 in T.S. Bartholin, B.E. Berglund, D. Eckstein, and F.H. Schweingruber, eds. Tree rings and the environment. Proceedings of the International Dendrochronological Symposium, Lundqua Rep. 34. Lund University, Sweden. Butler, D.R. 1986. Conifer invasion of subalpine meadows, Central Lemhi Mountains, Idaho. Northwest Science 60:166-173. Canaday, B.B., and R.W. Fonda. 1974. The influence of subalpine snowbanks on vegetation pattern, reproduction, and phenology. Bull. of the Torrey Botanical Club 101:340-350. Cooper, C., and J. Gale. 1986. Carbon dioxide enhancement of tree growth at high elevation: commentary. Science 231:859-860. Davis, M.B. 1989. Insights from paleoecology on global change. Bull. of the Ecological Society of America 70:222-228. Ettl, G.J., and D.L. Peterson. 1991. Growth and genetic response of subalpine fir (Abies lasiocarpa) in a changing environment. The Northwest Environmental Journal 7:357-359. Fonda, R.W. 1976. Ecology of alpine timberline in Olympic National Park. Proceedings, Conference on Scientific Res. in National Parks 1:209-212. Franklin, J.F., W.H. Moir, G.W. Douglas, and C. Wiberg. 1971. Invasion of subalpine meadows by trees in the Cascade Range, Washington and Oregon. Arctic and Alpine Res. 3:215-224. Graumlich, L.J. 1991. Subalpine tree growth, climate, and increasing CO₂: an assessment of recent growth trends. Ecology 72:1-11. Graumlich, L.J., L.B. Brubaker, and C.C. Grier. 1989. Long-term trends in forest net primary productivity: Cascade Mountains, Washington. Ecology 70:405-410. Graybill, D.A., and S.B. Idso. 1993. Detecting the aerial fertilization effect of atmospheric CO₂enrichment in tree-ring chronologies. Global Biogeochemical Cycles 7:81-95. Graybill, D.A., D.L. Peterson, and M.J. Arbaugh. 1992. Coniferous forests of the Colorado Front Range. Pages 365-401 in R.K. Olson, D. Binkley, and M. Böhm, eds. Response of western forests to air pollution. Springer-Verlag, New York. Heikkinen, O. 1984. Forest expansion in the subalpine zone during the past hundred years, Mount Baker, Washington, U.S.A. Erdkunde 38:194-202.	Innes, J.L. 1991. High-altitude and high-latitude tree growth in relation to past, present and future global climate change. The Holocene 1:174-180. Jacoby, G.C. 1986. Long-term temperature trends and a positive departure from the climate-growth response since the 1950s in high elevation lodgepole pine from California. Pages 81-83 in Proceedings of the NASA Conference on Climate-vegetation Interactions. Rep. OIES-2. University Corporation Atmospheric Research. Boulder, CO. Kearney, M.S. 1982. Recent seedling establishment at timberline in Jasper National Park, Alberta. Canadian Journal of Botany 60:2283-2287. Kienast, F., and R.J. Luxmoore. 1988. Tree-ring analysis and conifer growth responses to increased atmospheric CO₂ levels. Oecologia 76:487-495. LaMarche, V.C., D.A. Graybill, H.C. Fritts, and M.R. Rose. 1984. Increasing atmospheric carbon dioxide: tree ring evidence for growth enhancement in natural vegetation. Science 225:1019-1021. Little, R.L., D.L. Peterson, and L.L. Conquest. 1994. Regeneration of subalpine fir (Abies lasiocarpa) following fire: effects of climate and other factors. Canadian Journal of Forest Res. 24:934-944. Payette, S., and L. Filion. 1985. White spruce expansion at the tree line and recent climatic change. Canadian Journal of Forest Res. 15:241-251. Peterson, D.L. 1991. Sensitivity of subalpine forests in the Pacific Northwest to global climate change. The Northwest Environmental Journal 7:349-350. Peterson, D.L., M.J. Arbaugh, and L.J. Robinson. 1991. Regional growth changes in ozone-stressed ponderosa pine (Pinus ponderosa) in the Sierra Nevada, California, USA. The Holocene 1:50-61. Peterson, D.L., M.J. Arbaugh, L.J. Robinson, and B.R. Derderian. 1990. Growth trends of whitebark pine and lodgepole pine in a subalpine Sierra Nevada forest, California, U.S.A. Arctic and Alpine Res. 22:233-243. Peterson, D.W. 1993. Dendroecological study of subalpine conifer growth in the North Cascade Mountains. M.S. thesis, University of Washington, Seattle. 66 pp. Rochefort, R.M., R.L. Little, A. Woodward, and D.L. Peterson. 1994. Changes in subalpine tree distribution in western North America: a review of climate and other factors. The Holocene 4:89-100. Rochefort, R.M., and D.L. Peterson. 1991. Tree establishment in subalpine meadows of Mount Rainier National Park. The Northwest Environmental Journal 7:354-355. Woodman, J.N. 1987. Potential impact of carbon dioxide-induced climate changes on management of Douglas-fir and western hemlock. Pages 277-283 in W.E. Shands and J.S. Hoffman, eds. The greenhouse effect, climate change, and U.S. forests. The Conservation Foundation, Washington, DC. Woodward, A., M.B. Gracz, and E.S. Schreiner. 1991. Climatic effects on establishment of subalpine fir (Abies lasiocarpa) in meadows of the Olympic Mountains. The Northwest Environmental Journal 7:353-354.

References

Becker, M. 1989. The role of present and past vitality of silver fir forests in the Vosges Mountains of northeastern France. Canadian Journal of Forest Res. 19:1110-1117.

Briffa, K.R. 1992. Increasing productivity of "natural growth" conifers in Europe over the last century. Pages 64-71 in T.S. Bartholin, B.E. Berglund, D. Eckstein, and F.H. Schweingruber, eds. Tree rings and the environment. Proceedings of the International Dendrochronological Symposium, Lundqua Rep. 34. Lund University, Sweden.

Butler, D.R. 1986. Conifer invasion of subalpine meadows, Central Lemhi Mountains, Idaho. Northwest Science 60:166-173.

Canaday, B.B., and R.W. Fonda. 1974. The influence of subalpine snowbanks on vegetation pattern, reproduction, and phenology. Bull. of the Torrey Botanical Club 101:340-350.

Cooper, C., and J. Gale. 1986. Carbon dioxide enhancement of tree growth at high elevation: commentary. Science 231:859-860.

Davis, M.B. 1989. Insights from paleoecology on global change. Bull. of the Ecological Society of America 70:222-228.

Ettl, G.J., and D.L. Peterson. 1991. Growth and genetic response of subalpine fir (Abies lasiocarpa) in a changing environment. The Northwest Environmental Journal 7:357-359.

Fonda, R.W. 1976. Ecology of alpine timberline in Olympic National Park. Proceedings, Conference on Scientific Res. in National Parks 1:209-212.

Franklin, J.F., W.H. Moir, G.W. Douglas, and C. Wiberg. 1971. Invasion of subalpine meadows by trees in the Cascade Range, Washington and Oregon. Arctic and Alpine Res. 3:215-224.

Graumlich, L.J. 1991. Subalpine tree growth, climate, and increasing CO₂: an assessment of recent growth trends. Ecology 72:1-11.

Graumlich, L.J., L.B. Brubaker, and C.C. Grier. 1989. Long-term trends in forest net primary productivity: Cascade Mountains, Washington. Ecology 70:405-410.

Graybill, D.A., and S.B. Idso. 1993. Detecting the aerial fertilization effect of atmospheric CO₂enrichment in tree-ring chronologies. Global Biogeochemical Cycles 7:81-95.

Graybill, D.A., D.L. Peterson, and M.J. Arbaugh. 1992. Coniferous forests of the Colorado Front Range. Pages 365-401 in R.K. Olson, D. Binkley, and M. Böhm, eds. Response of western forests to air pollution. Springer-Verlag, New York.

Heikkinen, O. 1984. Forest expansion in the subalpine zone during the past hundred years, Mount Baker, Washington, U.S.A. Erdkunde 38:194-202.

Innes, J.L. 1991. High-altitude and high-latitude tree growth in relation to past, present and future global climate change. The Holocene 1:174-180.

Jacoby, G.C. 1986. Long-term temperature trends and a positive departure from the climate-growth response since the 1950s in high elevation lodgepole pine from California. Pages 81-83 in Proceedings of the NASA Conference on Climate-vegetation Interactions. Rep. OIES-2. University Corporation Atmospheric Research. Boulder, CO.

Kearney, M.S. 1982. Recent seedling establishment at timberline in Jasper National Park, Alberta. Canadian Journal of Botany 60:2283-2287.

Kienast, F., and R.J. Luxmoore. 1988. Tree-ring analysis and conifer growth responses to increased atmospheric CO₂ levels. Oecologia 76:487-495.

LaMarche, V.C., D.A. Graybill, H.C. Fritts, and M.R. Rose. 1984. Increasing atmospheric carbon dioxide: tree ring evidence for growth enhancement in natural vegetation. Science 225:1019-1021.

Little, R.L., D.L. Peterson, and L.L. Conquest. 1994. Regeneration of subalpine fir (Abies lasiocarpa) following fire: effects of climate and other factors. Canadian Journal of Forest Res. 24:934-944.

Payette, S., and L. Filion. 1985. White spruce expansion at the tree line and recent climatic change. Canadian Journal of Forest Res. 15:241-251.

Peterson, D.L. 1991. Sensitivity of subalpine forests in the Pacific Northwest to global climate change. The Northwest Environmental Journal 7:349-350.

Peterson, D.L., M.J. Arbaugh, and L.J. Robinson. 1991. Regional growth changes in ozone-stressed ponderosa pine (Pinus ponderosa) in the Sierra Nevada, California, USA. The Holocene 1:50-61.

Peterson, D.L., M.J. Arbaugh, L.J. Robinson, and B.R. Derderian. 1990. Growth trends of whitebark pine and lodgepole pine in a subalpine Sierra Nevada forest, California, U.S.A. Arctic and Alpine Res. 22:233-243.

Peterson, D.W. 1993. Dendroecological study of subalpine conifer growth in the North Cascade Mountains. M.S. thesis, University of Washington, Seattle. 66 pp.

Rochefort, R.M., R.L. Little, A. Woodward, and D.L. Peterson. 1994. Changes in subalpine tree distribution in western North America: a review of climate and other factors. The Holocene 4:89-100.

Rochefort, R.M., and D.L. Peterson. 1991. Tree establishment in subalpine meadows of Mount Rainier National Park. The Northwest Environmental Journal 7:354-355.

Woodman, J.N. 1987. Potential impact of carbon dioxide-induced climate changes on management of Douglas-fir and western hemlock. Pages 277-283 in W.E. Shands and J.S. Hoffman, eds. The greenhouse effect, climate change, and U.S. forests. The Conservation Foundation, Washington, DC.

Woodward, A., M.B. Gracz, and E.S. Schreiner. 1991. Climatic effects on establishment of subalpine fir (Abies lasiocarpa) in meadows of the Olympic Mountains. The Northwest Environmental Journal 7:353-354.

Subalpine Forests of Western North America

Tree Growth

Patterns of Establishment

Future Changes