USC Dana and David Dornsife College of Letters, Arts & Sciences

Water supply in California’s Sierra Nevada range is primarily determined by seasonal precipitation, which occurs for the most part in the fall and winter seasons. The runoff from this precipitation provides substantial flow during spring snowmelt for animals, plants and humans.  Through various measurements taken by various stations posted throughout the Sierra mountain range, it is evident that annual snowfall and precipitation has decreased and will continue to do so as climate change and anthropogenic influences heat the earth (Hall and Kapnick 2010). The decreases in snowmelt coupled with anthropogenic influences in the Sierras are posing many abiotic and biotic pressures on plants and animals living in the region and many are on the verge of extinction. Home to 135 threatened plant species and providing “as much as 65 percent of California’s water supply”, the Sierra mountain range remains a crucial habitat for humans, animal and plant species.

Decreased snow melt and anthropogenic influences, such as the introduction of non native species to the Sierra Nevada’s water sources, have posed a serious threat to species such as the Sierra Nevada yellow-legged frog or Rana sierrae, which are native to the area. The Rana sierrae relies on perennial sources of water in order to survive into adulthood, but with the introduction of trout, which prey on Rana sierrae in all three stages of their life, into deeper bodies of water, the Rana sierrae has had to rely on inferior and shallower fishless water sources, which often times run dry in the summer due to decreased snow melt and climate change (Mathews and Preisler 2010). As climate change and anthropogenic influences encroach on the Rana sierra’s habitat, they will eventually become extinct in the region and thereby decrease the rich biodiversity.

Unfortunately, other species inhabiting the Sierra Mountains are reacting more severely to increased temperatures than scientists had anticipated. “What surprised us are how sensitive these trees are to short-term changes in climate,” says U.S. Geological Survey’s Phillip van Mantgem, who conducted a study measuring the mortality rate of Sierra’s coniferous species. Beginning in 1983, Mantgem found that by 2007 the death rate had nearly doubled. The conifer’s lower survival rates can greatly be attributed to increased temperatures. The Sierra Forest Legacy notes that during a recent 22-year period, “the Sierra Nevada warmed by 1.8 degrees Fahrenheit”. Despite no apparent fluctuation in the average precipitation during this 22-year period, increased temperatures have led to higher evaporation rates creating dryer soil conditions. The University of California Santa Cruz also conducted a study on the impact of climate change on Sierra’s Pinus contorta and P. jeffreyi seedlings. The study noted a significant decrease in the survival rate of the seedlings – attributed to the seedling’s shallow root system and lower capacity to store water (Alpert 48-49). In addition to decreased irrigation, the conifers also currently suffer from an increased infestation of fungi and insects, which gravitate towards warmer climates. Additionally, the combination of warmer temperature and more dried dead plant species, makes the environment more susceptible to fires, threatening the release of CO² previously stored in the trunks of the trees. This release of CO² will create a positive feedback to the global climate dilemma, sadly leading scientists and conservationist back to where they started.

About the authors: Stephen Holle and Birka Burnison are working towards their bachelor’s degrees in Environmental Studies in the USC Dana and David Dornsife College of Letters, Arts and Sciences.