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Emerging Stress and Relative Resiliency of Giant Sequoia Groves Experiencing Multiyear Dry Periods in a Warming Climate

  • Author(s): Su, Yanjun;
  • Bales, Roger C;
  • Ma, Qin;
  • Nydick, Koren;
  • Ray, Ram L;
  • Li, Wenkai;
  • Guo, Qinghua
  • et al.

The relative greenness and wetness of Giant Sequoia (Sequoiadendron giganteum, SEGI) groves and the surrounding Sierra Nevada, California forests were investigated using patterns in vegetation indices from Landsat imagery for the period 1985-2015. Vegetation greenness (normalized difference vegetation index) and thus forest biomass in groves increased by about 6% over that 30-year period, suggesting a 10% increase in evapotranspiration. No significant change in the surrounding non-grove forest was observed. In this period, local temperature measurements showed an increase of about 2.2 °C. The wetness of groves (normalized difference wetness index) showed no overall long-term trend, but responded to changes in annual water-year precipitation and temperature. The long-term trends of grove greenness and wetness were elevation dependent, with the lower rain-snow transition elevation zone (1700 – 2100 m) marking a change from an increasing trend at lower elevations to a decreasing trend at higher elevations. The 2011-2015 drought brought an unprecedented drop in grove wetness, over five times the 1985-2010 standard deviation; and wetness in SEGI groves dropped 50% more than in non-grove areas. Overall, the wetness and greenness of SEGI groves showed a larger response to the warming climate and drought than non-grove areas. The influence of droughts on the wetness of SEGI groves reflected effects of both the multi-decadal increase in forest biomass and the effects of warmer drought-year temperatures on the evaporative demand of current grove vegetation, plus sufficient regolith water storage of rain and snowmelt to sustain that vegetation through seasonal and multi-year dry periods.

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