Effect of Tree-to-Shrub Type Conversion in Lower Montane Forests of the Sierra Nevada (USA) on Streamflow

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.

Ryan Bart, lead author andapostdoctoral researcher at UCSB's Bren School of Environmental Science & Management

• States higher global temperatures and increased levels of disturbance are contributing to greater tree mortality in many forest ecosystems — limiting forest regeneration, leading to vegetation type conversion
• Examines the effects of tree-to-shrub type conversion, in combination with climate change, on streamflow in two lower montane forest watersheds in the Sierra Nevada
• Model results indicated:
  • streamflow may show negligible change or small decreases following type conversion when the difference between tree and shrub leaf areas is small, partly due to the higher stomatal conductivity and the deep rooting depth of shrubs
  • streamflow may increase when post-conversion shrubs have a small leaf area relative to trees
  • vegetation change could have a greater impact on streamflow magnitude than the direct hydrologic impacts of increased temperatures
  • temperature increases may have a greater impact on streamflow timing
• Finds that tree-to-shrub type conversion increased streamflow only marginally during dry years (annual precipitation < 800 mm), with most streamflow change observed during wetter years
• Results underscore the importance of accounting for changes in vegetation communities to accurately characterize future hydrologic regimes for the Sierra Nevada