The Sierra Nevada is a mountain range in California’s central valley. It gets its name from the Spanish word that means a snowy range. The mountain is located in several climate zones with different types of vegetation. Learning about them helps in understanding various species that compose the vegetation. It also guides through the drivers of species distribution in the mountain range. The paper discusses changes in vegetation that occur on the ascent of the western slope and descent of the eastern slope of the mountain range. It also discusses the primary drivers of species distribution noting where some of the common tree species widely spread in the mountain region.
The Western Foothill Woodland and Chaparral zone lies at the bottom of the slope. It is along the Central Valley boundary (Dolanc et al. 3). The region is full of grassland savanna, oak woodland and chaparral vegetation. Oak forests include blue oak (Quercus douglasii) and gray pine (Pinus sabiniana). With the rise of the altitude, the grassland increases on the mountain range. At the same level, hardwoods like blue oak woodland and Blue oak-foothill pine reduce in area. Above this level, Ponderosa pine decreases while Montax Hardwood and Douglas-Fir increase in area (Dolanc et al.).
The next region of the western slope is the Lower Montane Zone. It begins at 600 meters elevation (Fites-Kaufman et al. 456). Forests dominate in this area. The Ponderosa pine (Pinus ponderosa) from the mixed conifer forest is dominant and the most broadly distributed vegetation in this field. Although the Jeffrey Pine predominantly grows on the eastern slope, the tree is also found on this western side. The zone also contains Sugar Pine, White Fir, California black oak and Incense-cedar. Black oak grows in the lower levels of this area. At the elevated standard of the region, White-mixed conifer (Abies concolor) grows. Other vegetation in this high region includes Giant sequoia (Sequoiaadendron gigainteuin). The zone is also inhabited by Canyon live oak that grows deep in the canyons (Fites-Kaufman et al.).
The region that follows in elevation is the Upper Montane Forest Zone. It begins at the altitude of 1500 meters above the sea level (Fites-Kaufman et al. 456). It links the Subalpine Zone to the Montane Zone. Trees such as Red fir and Lodgepole pine (Pinus contorta subsp.murrayana) are the dominant vegetation. The zone also contains Western juniper (Fites-Kaufman et al.). Other plants found here include meadows and Montane chaparral.
The Subalpine area is next in elevation after the Upper Montane region. It stands at 3,000metres above the sea level (Fites-Kaufman et al. 456). Jeffrey pine grows in this region. It may occur with Lodgepole pine and Mountain Hemlock or alone in small scattered paths. It maintains a tree form of growth until replaced by Whitebark pine or Foxtail pine on the rocky ridges (Fites-Kaufman et al.). Compared to other forest zones, like upper and lower Montane, this area has less forest vegetation because of the harsh climate with extensive wind and snow. The vegetation growing here includes the Western white pine. The plant may occur in little pure stands or with Lodgepole pine, Jeffrey pine, Mountain Hemlock and Red fir. It grows to the near height of tree line where the Whitebark pine or Foxtail pine replaces it (Fites-Kaufman et al.). Another species dominating in the zone is Mountain hemlock (Tsuga mertensiana) which grows in elevated regions of the area. Whitebark pine is also common in this zone. Other trees growing here are Foxtail pine and Limber pine. However, it is so often mixed with another common species, such as Whitebark pine and Sierra juniper.
The Alpine Zone stands above the Subalpine Forest in elevation. It starts at 3000 meters (Fites-Kaufman et al. 456). There are no trees in this region. The lack of vegetation is the result of harsh climatic conditions like long, cold and snowy winters. The only vegetation that is widespread in this area is the Macrolichen flora (Dolanc et al.).
When descending the Eastern slope, the vegetation zones are the same but only in a descending order. However, this slope is at a more elevated angle than the western slope because of less precipitation in this region. At the highest level is the Alpine Zone. It stands at 3700 meters. Just like in the Alpine Zone on the western slope, there is no vegetation in this region (Fites-Kaufman et al. 456).
The Subalpine Forest Zone follows the Alpine Zone in height. It stands at 3200 to 3700 meters (Fites-Kaufman et al. 456). The dominant vegetation changes with height and species composition differs from slopes at the crest. Red fir is common in this region. Mountain hemlock, Western white pine and Whitebark pine also grow in this area. The zone also contains Foxtail and Limber pines, yet at a lesser percentage than in the west area, with Limber pine growing in a scattered form. It occupies steep, eroded and nutrient- poor sites near tree lines (Fites-Kaufman et al.). The next zone in depression from the Subalpine region is the Upper Montane Forest Zone. It stands at 2700 to 3200 meters elevation (Fites-Kaufman et al. 456). The zone comprises several common vegetation species such as Lodgepole pine and Jeffrey pine. The region is also inhabited by Red fir that grows in a scattered manner compared to its composition on the western slope (Fites-Kaufman et al.).
The Lower Montane Zone is the next region. It stands at 2100 to 2700 meters (Fites-Kaufman et al. 456). Jeffrey pine is dominant in this region. It combines with Ponderosa pine in the Kern River Basin. Unlike the western slope that has an abundant Sugar pine vegetation, on the eastern slope it is rare and almost absent. Incense cedar that is frequent on the western slope is less frequent on this side of the hill. In this zone, common vegetation consists of Chaparral species. It is less at higher levels of the region but crosses over to the Juniper Woodland zone at the lower end of the mountain range. Meadows and herbaceous dominated sites are present in this region.
At the lowest level of the eastern slope, there is the Pinyon-Juniper Woodland Zone. It extends down to 1500 meters. It harbors Sierra junipers and Pinyon pines of the single-leaf variety. It also contains big sagebrush (Artemisia tridentata) vegetation. The region also harbors Jeffrey pines.
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The vegetation in the Sierra Nevada keeps undergoing changes. It is attributable to several drivers. The first factor is the climate of the Sierra Nevada. The forest structure is linked to the climate of the mountain range. In the western slope it is Mediterranean with wet winters and dry summers. The amount of rainfall rises with the level of elevation reaching about 2000 mm a year (Fites-Kaufman et al. 457). From the peak, the precipitation decreases towards the crest area. The western slope’s Subalpine zone experiences warm temperatures. It explains why the Zone has a large and increased number of small trees in the meadows. The temperature and the climate contribute to the growth of this type of vegetation. However, some conditions limit the survival of young trees. These include low moisture of the summer soil (Fites-Kaufman et al.).
Forest fires are another cause of vegetational changes in the Sierra Nevada. They mostly occur during drought years from midsummer till the end of the fall. Thanks to these fires, there is an increase in tree density in the mid-elevation forests. It is attributable to the fact that fires are more frequent at low elevations. As a result, more trees are growing at upper elevation levels. The vegetation pattern of nonconiferous forest such as hardwoods and Chaparral has also been attributed to the forest fires. The Montane chaparral grows in mid and higher elevations because forest fires are rare at these levels (Fites-Kaufman et al.).
Logging/ timber harvesting is another driver of the distribution of vegetation species. Humans cut down these trees for both commercial and subsistence use. It has primarily led to a reduction in the area of mixed conifer forest. Although all mixed conifer species are affected by logging, the species that undergo the greatest decline are Ponderosa and Sugar Pines. It is because these plants have a higher commercial value as compared to other trees such as firs.
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Weeds and pathogens are another driver of vegetation patterns in the Sierra Nevada. These pathogens such as White Pine blister rust (Cronartium ribicola) and bark beetles brought about by climate changes kill trees such as Western White pine and Whitebark pine. The most affected vegetation by the bluster rust is the Sugar Pine (Dolanc et al.). Also thanks to timber harvesting, these weeds and pathogens enter the stumps of the trees and spread to other ones.
Ozone contamination from upwind valley locations is another cause of the changes in the vegetation of the Sierra Nevada. It has increased the mortality of Jeffrey pines. The plant is sensitive due to its low photosynthetic threshold and therefore shows broad foliar injury at current ozone parts in the Eastern and Western zones of the Sierra Nevada.
Grazing as led to the loss of Q.douglassi such as Blue oak woodland and Foothill pine (Dolanc et al.).
The rapid population growth of El Dorado County has also contributed to the distribution of species in the Sierra Nevada (Dolanc et al.). Several houses of a low-density category have been built. Consequently, it has affected the vegetation at the foothill woodland zone. Chaparral and Montane Hardwood are the most affected as they have been cut down and replaced by buildings.
Other causes of change in vegetation are wind and avalanche. Thinned vegetation and forest edges are vulnerable to the wind. The wind affects a significant portion of Upper Montane vegetation. In their turn, avalanches are common in Red fir, Lodgepole pine and Subalpine forests. They frequently occur forming avalanche paths and destroying trees along the paths (Fites-Kaufman et al. 461).
The Sierra Nevada and its variety of vegetation are an indispensable part of ecological structure. However, it is clear from the paper that its vegetation keeps changing. Mostly affected are large trees that are declining at higher and faster rates. They are being replaced by smaller plant types, albeit temporarily. However, unless the conditions such as natural fires, pathogens and climatic conditions change for better, then these replacements may lead to a permanent displacement of the trees. It would amend the vegetation of the Sierra Nevada mountain range completely. However, there is still hope. Human controlled drivers such as logging, population growth and actions that lead to global warming and consequently affecting the climate should be controlled. With determination then we can save the Sierra Nevada’s vegetation.
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