Assessing blue oak (Quercus douglasii) growth resilience to extreme drought across fine-scale topography in California

Abstract

Extreme hot temperatures and low precipitation were observed in California, especially during the past two decades. The combination of the two factors increase the frequency and intensity of drought events across the state. Many forest ecosystems within the state, including the drought-tolerant blue oak (Quercus douglasii)-dominated savannas and woodlands, have exhibited signs of drought stress and mortality. Despite some literature addressing the role of topographic position in mediating climate-tree growth relationships, few studies have focused on blue oak drought responses across topoclimate gradients. We studied regional climate trends in Kern County and topoclimate trends across five topographic positions, using data from NOAA Climate at a Glance and USGS Basin Characterization Model. Tree-ring chronologies were developed from 81 blue oak individuals sampled from five positions, which were selected as representative within the localized landscape-scale range of water year mean climatic water deficit. Climate response analyses were conducted to quantify the relationship between seasonal climate variables and tree growth. To characterize tree growth response to extreme drought events in the 20th and 21st century, nine drought resilience indices were calculated, examining tree growth before, during, and after droughts. Our results verify the positive trend of temperature, particularly summer minimum temperature, over the past 35 years, while the precipitation had no significant change throughout the period. Precipitation appears to be the most dominant climatic factor for blue oak growth. However, temperature has become increasingly influential in recent years. Blue oak growing in drier sites had higher tree growth reduction, high recovery, long recovery period and lower drought resilience. Although blue oak in wetter sites had higher drought resilience, the increasing temperature influence on tree growth, as well as the disproportionate temperature rise at higher elevation, may increase the drought severity in wetter sites and vulnerability of these populations in future decades.