Leaf- and crown-level adjustments help giant sequoias maintain favorable water status during severe drought

Drought is expected to become an increasingly important stressor on forests globally, and understanding the physiological mechanisms driving tree drought response is essential for developing effective mitigation and conservation measures for these ecosystems. In 2014, during California's 2012-2016 “hotter” drought in which higher temperatures exacerbated the effects of low water availability, many giant sequoia trees in the Sierra Nevada mountains exhibited foliage dieback at levels previously unreported. We hypothesized that this apparent drought-induced foliage dieback was associated with spatial patterns of site water balance and consequently tree water status and physiology. As part of an ongoing collaborative project aimed at understanding and mapping giant sequoia drought response and vulnerability at leaf to landscape scales, in 2015, 2016 and 2017 we climbed and measured leaf-level water status and physiology of mature and seedling giant sequoia trees at sites exhibiting low and high average foliage dieback in Giant Forest, Sequoia National Park. We also compared our 2015-2017 measurements with similar measurements made in giant sequoias prior to the commencement of the 2012-2016 drought. We found that during the drought, leaf water potentials of both mature and seedling sequoias were as low or lower than any previous measurements, and leaf water potentials of some seedlings were as low as the tops of mature trees. In contrast to our expectations, we found similar water potentials in both foliage dieback classes, although there was a high degree of within-site variability and in some measurement periods trees and seedlings growing in flat or meadow-side topographic positions had higher water potentials than those growing on mid- or upslope positions with presumably less favorable water supply. Leaf-level adjustments included multiple mechanisms to reduce water loss and resist the effects of desiccation at both seasonal and annual time scales, including stomatal closure, redistribution of leaf water to less mobile storage, and a shift in leaf carbon fractions to build tougher, more drought-resistant foliage. Our results suggest that giant sequoia employs a drought avoidance water regulation strategy and leaf-level adjustments are sufficient to maintain relatively isohydric water potentials above critical thresholds under most conditions. Because of the high severity of the 2012-2016 drought, however, the capacity for leaf-level compensation in some giant sequoias was overwhelmed and both leaf- and crown-level adjustments were necessary to maintain favorable water status to protect whole-tree hydraulic functioning. Additional research aimed at understanding and monitoring the consequences of future hotter droughts is necessary in order to determine the long-term persistence of giant sequoias within their current geographic distribution.