Rising summer temperatures and extended periods of summer drought are threatening forests globally. Two primary mechanisms underlying tree decline during drought are (1) a loss in the ability to conduct water and (2) insufficient carbohydrate levels to meet metabolic demand. Despite extensive research on how summer drought events affect tree hydraulic and non-structural carbohydrate (NSC) status, potential impacts of warmer and dryer winters have largely been overlooked. Such an effect may be especially relevant for mediterranean-climate forests, in which the period of highest vapor pressure deficit (VPD), summer, is consistently decoupled from the period of highest water availability, winter, often leading to year-round growth. This decoupling suggests that trees may be more sensitive to increased winter than summer temperatures under drought conditions, despite lower absolute winter VPD.
We examined how California’s anomalous 2015 mid-winter drought affected the hydraulic and NSC status of three conifer species by periodically monitoring relative water content (RWC) and soluble/insoluble NSCs over eighteen months. To determine how climatic conditions affected tree hydraulic status, we modeled RWC as a function of recent precipitation and VPD anomaly. We then identified a strong relationship between branch RWC and hydraulic conductivity, a measure that correlates with drought-related stress and mortality, at sub-annual resolution. Finally, we predicted historic changes in hydraulic conductivity at our site for both winter and summer months from 1992 into 2015.
Results/Conclusions
We observed a sharp and unexpected decline in tree hydraulic capacity during California’s anomalous 2015 mid-winter drought that was followed by weakened starch accumulation in the subsequent spring. Despite a relatively low atmospheric vapor pressure deficit (VPD; ~0.8 kPa), three conifer species lost more stem water in mid-winter drought than during the previous summer drought when evaporative demand was over two times higher (~1.8 kPa). We estimate that these losses in stem water content corresponded with a 10-50% loss in hydraulic conductivity from early winter. Moreover, maximum starch accumulation was on average 50% lower following the mid-winter drought than observed the previous year. Seasonal VPD anomaly, not absolute VPD, and recent cumulative precipitation best predicted the hydraulic patterns observed. In comparison to historic climatic events from 1992 to 2015, the mid-winter drought event had among the highest VPD anomaly values and very low precipitation. Our findings suggest a seasonality to tree hydraulic and carbohydrate vulnerability, raising novel physiological and ecological questions about how rising winter temperatures will affect forest vitality.