Sierra Nevada subalpine zone
The Sierra Nevada subalpine zone refers to a
The Sierra Nevada subalpine zone occurs between 2,450–3,660 metres (8,000–12,000 ft), and is characterized by an open woodland of several conifer species, including
The marginal conditions make the Sierra Nevada subalpine zone sensitive to environmental changes, such as climate change and pollution. The long-lived nature of the subalpine species make the zone a good study system to examine these effects.
Location
The subalpine zone of the
Climate and physical factors
The climate of subalpine ecosystems is dominated by very long winters and short growing seasons of 6–9 weeks. Temperatures are cool even during the growing season and frost can occur 12 months of the year. Precipitation ranges from 750–1,250 millimetres (30–50 in) per year, which falls mostly as snow during the winter.[2] Temperatures average −11.5 to 1.5 °C (11 to 35 °F) in January and 5.5 to 19.5 °C (42 to 67 °F) in July, with a mean annual temperature around 4 °C (39 °F). Snow depths often exceed 3 metres (10 ft), but average 2 metres (7 ft) by the end of March.[3]
Winds can be high throughout the year and are a major factor limiting plant growth near the upper limit of the subalpine zone (tree line). Wind limits vegetative growth chiefly in two ways: by physically battering plants, including blowing snow and ice, and by increasing evapotranspiration in an environment that is already water-stressed.[4]
Compared to subalpine zones in the Cascade Range, Sierran subalpine experiences less annual precipitation, with a longer drought period during the summer months, but similar temperature ranges throughout the year.[5] Compared to Rocky Mountains subalpine zone, Sierran subalpine experiences a narrower (more mild) range of temperatures and higher annual precipitation, with more winter snow and less summer rain.[6]
Vegetation
Physiognomy
Generally, Sierran subalpine is dominated by woodland, which means the canopy cover averages between 30 and 60% closure (>60 % closure is considered forest). However, some species, particularly in protected sites with deeper soils and reduced wind, form closed-canopy stands. Growth form of trees is also variable; single-stemmed, large individuals are more abundant at lower elevations and protected sites, while multi-stemmed, stunted (
Composition
Shrubs and herbs are usually sparse, but can be common in stands where snow melts earlier in the growing season. Diversity of herbs in the subalpine zone is usually less than lower-elevation zones such as upper and lower montane. Broad classifications of herb and shrub communities can be found in Sawyer and Keeler-Wolf.
The composition of tree species within Sierran subalpine is variable with comparatively high diversity for subalpine.[9] Subalpine stands in the Rocky Mountains, for example, are usually dominated by a single tree species.[6] Stands in the Sierra may be mixed, with up to five species present, or pure, monospecific stands, depending on the range of the species and microsite conditions.
In addition to the species described above,
Adaptations to stressful conditions
The growth form and physiology of subalpine plants is reflective of the stressful environment to which they are adapted. Leaves are very long-lived at this elevation because they are costly to produce and soils are usually nutrient-poor. Since plants ultimately take nutrients such as nitrogen from the soil to produce organs such as leaves, this adaptation provides them an advantage in subalpine soils because their nutrient retention is enhanced. Also, evergreen plants can carry out photosynthesis on periodic warm days during the winter, which is an advantage in a climate with a very short growing season.
Most perennial herbs in subalpine ecosystems have very high root to shoot ratios, or large underground rhizomes, which allow them to store carbohydrates underground during the winter and grow very fast during the short growing season. Shrubs tend to be prostrate and low to the ground. This morphology is advantageous because temperatures near the ground tend to be slightly higher during the day, which helps to maximize photosynthesis, and plants are less exposed to wind close to the ground.[12]
Unlike
The most ubiquitous adaptation of subalpine plants is the ability to perform metabolic activities at extremely low temperatures.[12] Again, without this trait, the growing season would not be long enough to support sustained life. A side effect of this trait is slow growth, even when conditions are good, which may be a major factor in setting the lower limits of subalpine zones. Because subalpine tree species have such slow growth, they are out-competed at lower elevations by trees capable of more rigorous growth, such as red fir.[14] Slow growth, however, may be an adaptation in and of itself in extremely harsh environments as it leads to very long-lived individuals. Many of the tree species in Sierran subalpine are capable of living over 500 years. Whitebark pine has been found to live as long as 800 years, and foxtail pine, which is closely related to bristlecone pine (Pinus longaevis) has been estimated to live 2500–3000 years.[2] Seedling establishment in the harsh subalpine environment is difficult, so evolution has instead favored long-lived individuals that are reproductively active for tens or hundreds of years.
Species interactions
The harsh conditions present in the subalpine zone are sufficient to keep competitive interactions at a minimal level.
There is a mutualistic interaction between
These birds, which use the seeds as a staple food source, cache seeds in the soil and in cracks of rocks. Like most caching species, the nutcrackers store more than they can ever find and eat, so many of the seeds can germinate when conditions are favorable. In the subalpine, this type of seed dispersal may be particularly advantageous because seeds find themselves immediately protected from high winds and low temperatures, and are therefore more likely to succeed during germination.Environmental issues
Climate change
Because the Sierran subalpine is situated in such harsh conditions, i.e. nearly at the limit of tree growth, the ecosystem is particularly sensitive to changes in climate. The long-lived nature of subalpine species plus their relative isolation from civilization make subalpine ecosystems a particularly good study system to examine climate change.[18][19]
The precise elevation of tree line within a given subalpine region has likely fluctuated up- and downslope throughout the history of the Sierra. However, some climatic changes occurring currently in subalpine appear to be unprecedented. Peterson et al.[20] concluded that basal area increment (i.e. growth) increased for all age classes of whitebark pine and lodgepole pine over a 30-year period beginning around 1960. Millar et al.[21] observed increased growth in whitebark pine and accelerated encroachment into snowfields by whitebark pine and western white pine during the 20th century, particularly since 1980. Bunn et al.[22] showed that recent growth rates of subalpine conifers are greater than rates from any other period during the past 1000 years.
Specifically how subalpine ecosystems will respond to warming is yet to be determined. One logical (albeit largely untested) prediction is that plant communities will retreat upslope. If this prediction becomes a reality, it would likely mean the loss of a significant proportion of the subalpine zone that exists today. Since subalpine is the highest zone on many mountain peaks, the migration of subalpine to higher elevations will instead result in a reduction of the zone; in a manner of speaking, species will be pushed off the mountain. In a report sponsored by the California Energy Commission and the California Environmental Protection Agency, researchers simulated the effects of three different warming scenarios on different vegetation types in California over a period of 80–100 years. They concluded that alpine and subalpine vegetation would be reduced by approximately 50-80% of its current total cover.[23]
Pollution
Few studies have addressed pollution in subalpine in the Sierra Nevada, although tree species present there may ultimately prove sensitive to certain atmospheric pollutants. One study from subalpine in the European Alps showed increased levels of atmospheric deposition of nitrogen at some locations.[24] Jeffrey and ponderosa pine are susceptible to ozone pollution in the Sierra Nevada, but mostly at lower elevations.[25] However, another study from the European Alps demonstrated the sensitivity of a native subalpine conifer to ozone pollution[26]
Other environmental issues
The isolation and restricted use of subalpine systems keep them relatively free of recreational impacts. The harsh climate also makes these systems robust against biological invasion and disease. Most
See also
References
- ^ Rundel, P.W.; Parsons, D.J.; Gordon, D.T. (1977). "Montane and subalpine vegetation of the Sierra Nevada and Cascade Ranges". In Barbour, M.G.; Major, J. (eds.). Terrestrial vegetation of California. New York, USA: Wiley. pp. 559–599.
- ^ Terrestrial Vegetation of California(3rd ed.). Berkeley, CA, USA: University of California Press. pp. 460–501.
- ^ JSTOR 1948417.
- ^ ISBN 0-89886-085-7.
- ^ Franklin, C.B.; Halpern, J.F. (2000). "Pacific northwest forests". In Barbour, M.G.; Billings, W.D. (eds.). North American terrestrial vegetation (2nd ed.). Cambridge, UK: Cambridge University Press.
- ^ a b Peet, R.K. (2000). "Forests and meadows of the Rocky Mountains". In Barbour, M.G.; Billings, W.D. (eds.). North American terrestrial vegetation (2nd ed.). Cambridge, UK: Cambridge University Press.
- ^ ISBN 0-943460-25-5.
- JSTOR 1550833.
- ^ ISBN 0-521-55986-3.
- ^ JSTOR 2406295.
- ^ Potter, D.A. (1998). Forested communities of the upper montane in the central and southern Sierra Nevada. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture. pp. 1–319. General Technical Report PSW-GTR-169.
- ^ JSTOR 1550081.
- JSTOR 1550813.
- ISBN 0-520-06922-6.
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- ^ California Climate Change Center (2006). The response of vegetation distribution, ecosystem productivity, and fire in California to future climate scenarios simulated by the MC1 dynamic vegetation model. California Energy Commission and California Environmental Protection Agency. CEC-500-2005-191-SF.
- S2CID 2079960.
- PMID 12676233.
- ^ Manning, W.J. (2005). "Pinus cembra, a long term bioindicator for ambient ozone in subalpine regions of the Carpathian Mountains". Polish Botanical Studies. 19: 59–64.