Drought impacts on giant sequoia trees: recent insights and prospects for the future
“Walk in the sequoia woods at any time of year and you will say they are the most beautiful and majestic on earth” – John Muir, 1878
Giant sequoia (Sequoiadendron giganteum) trees are monarchs of the plant kingdom. They are the largest trees on the planet, with the most massive individuals exceeding 7 meters in diameter and 80 meters in height. They are among the oldest living organisms in the world, persisting for more than 3,000 years. And, as John Muir suggested, they are arguably one of the most beautiful and majestic trees anywhere, with enormous fire-scarred orange columns rising up through the forest to support vast sprawling crowns often consisting of hundreds of colossal limbs and reiterated trunks.
Ancient ancestors of the giant sequoia were once widely distributed throughout much of the northern hemisphere hundreds of millions of years ago during the Mesozoic Era. As conditions became cooler and drier their distribution retreated, and the descendants were eventually restricted to about 75 distinct groves along a narrow elevation band in the Sierra Nevada Mountains of California. Their present limited distribution reflects narrow environmental tolerances, high moisture demand, and vulnerability to climate change. Given the current trajectory of global warming, the sustainability of this remarkable species is uncertain.
As a potential preview to the future, the current historic drought in California, now in its’ 4th consecutive year, is hitting the states’ trees and forests hard. Millions of dead or dying pines, firs, and cedars have already succumbed to a combination of increased water stress and insect attacks throughout California. Fortunately, giant sequoia trees do not appear to be dying in higher numbers. However, many are showing signs of severe stress indicated by abnormal levels of foliage and small branch dieback. In the summer of 2015, a team of scientists from UC Berkeley, National Park Service, US Geological Survey, and Carnegie Institution for Science started a project seeking to understand the spatial patterns and causes of crown dieback patterns in giant sequoia trees by linking leaf-level measurements of leaf water status, water content, and other chemical traits with field surveys of tree dieback patterns and airborne spectronomic and LiDAR data collected over the entire landscape.
Leaf water potential measurements showed that most of the giant sequoias in our study were experiencing significant levels of water stress. However, our teams' preliminary results suggest that in the face of severe drought, giant sequoia trees are able to reduce water losses to the atmosphere through strict stomatal regulation of transpiration and foliage shedding. This regulation of water use may allow giant sequoia trees to avoid severe hydraulic failure in large trunks and branches, which represent significant carbon investments. However, while the loss of a large proportion of leaf area may help to minimize water stress, it also reduces the photosynthetic uptake of carbon that can be used for growth, metabolism, and other physiological function.
Our preliminary results show that giant sequoia trees are tough and can withstand droughts, such as the current one in California. While ancient giant sequoia trees have undoubtedly suffered through severe droughts in the past, and are incredibly resilient to such conditions, the past may not be a prelude to the future. All species have a physiological limit. Continually increasing temperatures may push climatic water deficits beyond physiological thresholds for these trees, and at some point they may not be able to maintain a favorable water status as soil water availability continues to decline and evaporative demand continues to increase. Controlled greenhouse experiments examining drought responses in giant sequoia seedlings suggest that the foliar water potentials we measured in mature trees in 2015 approach the minimum levels possible before incurring severe impacts on hydraulic function and physiological performance. It is critical to understand how drought impacts tree physiological functioning in order to predict potential forest responses to a changing climate and to develop effective conservation and management strategies for these ecosystems. Our preliminary research paves the way for further investigations to address the pressing questions facing giant sequoia trees and forests in the future.