The Sierra Nevada subalpine zone refers to a biotic zone below treeline in the Sierra Nevada mountain range of California, United States. This subalpine zone is positioned between the upper montane zone (such as red fir forest) at its lower limit, and tree line at its upper limit.
The Sierra Nevada subalpine zone occurs between , and is characterized by an open woodland of several conifer species, including whitebark pine, lodgepole pine, western white pine, mountain hemlock, and Sierra juniper. The vegetation and ecology is determined by the harsh climate, with extensive snow and wind. In addition, soils are thin and nutrient-poor. Due to these harsh conditions, vegetation grows slowly and to reduced heights. In addition, the stressful environment suppress species competition and promotes mutualism.
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 Sierra Nevada occurs between in the southern part of the range and in the north. Because the Sierra is higher in the south, the majority of subalpine occurs in the central and southern portions of the range, south of the Lake Tahoe basin. A few isolated patches occur in the north on mountain peaks higher than .
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 per year, which falls mostly as snow during the winter. Temperatures average in January and in July, with a mean annual temperature around . Snow depths often exceed , but average by the end of March.
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.
Soils are thin, coarse and relatively nutrient-poor, owing to the unproductive climate and repeated glaciation events during the Pleistocene. Moisture retention is usually high, due to the presence of underlying granite bedrock, and soils often become waterlogged early in the growing season. However, because very little precipitation falls during the summer months, soils can dry quickly once snow melts and vegetative growth and reproduction is limited late in the growing season by drought.
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. 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.
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 (krummholz-form) individuals are more abundant near tree line. Herb and shrub-dominated communities also occur, but comprise a small proportion of the total land area within the subalpine zone. Meadows can occur where water is more available.
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. For a fine-scale classification of subalpine meadow communities, see Benedict.
The composition of tree species within Sierran subalpine is variable with comparatively high diversity for subalpine. Subalpine stands in the Rocky Mountains, for example, are usually dominated by a single tree species. 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.