Sierra Nevada subalpine zone
The long-lived nature of the subalpine species make the zone a good study system to examine these effects.
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] 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.
[4] Herb and shrub-dominated communities also occur, but comprise a small proportion of the total land area within the subalpine zone.
Whitebark pine (Pinus albicauls) is possibly the most widespread component of subalpine woodland in the central and northern regions of the Sierra.
[2][10] Western white pine (Pinus monticola) can be found in pure stands, especially on exposed slopes, where snowpack is shorter-lived.
More commonly however, western white pine grows in mixed stands with lodgepole, mountain hemlock, Jeffrey pine (Pinus jeffreyi) and/or red fir (Abies magnifica)[2] Mountain hemlock may be the most common tree species in the subalpine zone, especially in the central and northern Sierra.
This species forms dense, pure stands on protected slopes with moist soil, but can also co-occur with Sierra juniper and whitebark pine.
Mountain hemlock often exhibits two growth forms on the same individual, with one stem upright and several branches at the base extending out along the ground.
[2][9] Sierra juniper is sparse in subalpine, occurring strictly on exposed, rocky slopes, usually among granite boulders.
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.
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.
[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.
The clumpy nature of subalpine vegetation is also in part a manifestation of a positive interaction, whereby individuals increase their fitness by having neighbors that reduce the effects of high wind and cold temperatures.
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.
[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.
Most exotic plants in California are currently restricted to low elevations, although some invasive species may have the potential to reach the subalpine zone.
If climate change does indeed warm regions where the subalpine zone occurs, conditions there may become more conducive to threats such as invasive species as the local ecosystem is disrupted.
