While changing precipitation patterns can have a significant impact on stream flows in the Sierra Nevada mountains, a new study by UC Santa Barbara researchers indicates that shifts in vegetation type resulting from warming and other factors may have an equal or greater effect.
Their findings appear in the journal PLOS One.
"We found that vegetation change may have a greater impact on the amount of stream flow in the Sierra than the direct effects of climate warming," said lead author Ryan Bart, a postdoctoral researcher at UCSB’s Bren School of Environmental Science & Management.
Exacerbated by climate and drought, fires such as the 2013 Rim Fire in Yosemite National Park can destroy entire stands of forest, which may not return.
Because the future composition of shrub lands and the distribution of shrub species in the Sierra Nevada is unknown, the researchers examined stream flows under multiple possible scenarios of vegetation-type conversion in two Sierra Nevada watersheds.
While some forest-to-shrub land conversion scenarios resulted in higher stream flow, depending on factors such as the size and area covered by shrub leaves relative to tree leaves, Bart noted that a shrub-dominated landscape would not necessarily result in more water in stream.
"Shrubs are adept at pulling water out of the soil, so that in some cases, a decent-sized shrub may use just as much water as a much taller tree.
It is only when shrubs are much smaller than trees that we see less water used by vegetation and thus more stream flow."
"The results underscore the importance of accounting for changes in vegetation communities to accurately characterize future stream flow for the Sierra Nevada."
Effect of Tree-to-Shrub Type Conversion in Lower Montane Forests of the Sierra Nevada (USA) on Streamflow.