Giant Sequoias Challenged by Climate Change

Many things in nature are adapted to certain climate conditions. That said, some are choosier than others. Giant Sequoia trees, the largest living organisms on the planet, need a “Goldilocks” type of environment to grow and reproduce. Visiting Sequoia National Park recently, I had the opportunity to learn more about the climate requirements of these amazing trees.

Needing conditions that are not too hot and not too cold, as well as not too wet and not too dry, Giant Sequoias grow naturally in only one place on Earth. That unique place is a narrow band about 70-miles long on the western slopes of California’s Sierra Nevada Mountains known as the Sequoia Belt. According to the National Park Service (NPS), these trees only grow at elevations between 5,000 and 7,500 feet. Temperatures above 7,500 feet are usually too cold and conditions below 5,000 feet are too dry. Within this limited range, about 75 groves reveal where conditions for the massive trees are just right.

These ideal conditions are produced by a combination of weather and topography. When moisture-laden air from the Pacific runs into the Sierras, it rises and cools. On average, it cools about 3.6°F for every 1000 feet it rises. Since cooler air holds less moisture than warm air, the moisture is dropped as rain and snow over the mountains with precipitation amounts generally increasing with elevation. While California is in the midst of a multi-year drought, the NPS says the area around Giant Forest (elevation 6400 feet) usually gets an average of 44 inches of precipitation a year. It also typically sees only one day a year with temperatures below 10°F.

Compared to other sites within a five mile radius, the average conditions in Giant Forest are perfect for the giant sequoias. Upslope, Emerald Lake (elevation 9200 feet) gets 59 inches in average annual precipitation and sees around ten days a year with temperatures below 10°F. Downslope, Ash Mountain (elevation 1700 feet), only sees about 26 inches of precipitation a year and the temperature reportedly never falls below 10°F.

Given the narrow natural range in which these giant trees are able to thrive, they face serious challenges from climate change. As temperatures increase, more precipitation is coming down in the form of rain instead of snow. This reduces the snowpack and the subsequent spring and summer melt water available to the trees during the region’s dry season. While the resilient mature sequoias – some are over 3000 years old – are not in immediate danger, researchers say these big trees were not made to withstand decades of drought. The younger trees – seedlings and saplings – on the other hand, face a more difficult struggle for survival. The drier conditions make it harder for them to develop robust root systems and also leave them more susceptible to wildfires, which are projected to increase.

Given the impressive age of some of these trees, they must have endured natural climate fluctuations in the past. This time, however, the human caused change is happening very quickly and is forecast to produce conditions unfamiliar to even these ancient giants. According to the US National Climate Assessment, the southwest – which includes California – is normally the hottest and driest part of the country, but climate change is making it even more so. The report shows that 2001 through 2010 was the region’s warmest decade on record with temperatures almost 2°F above historic averages. Looking ahead, it projects continued increases in average annual temperature and a decrease in precipitation. In the meantime, scientists continue to monitor and research the giant sequoias to better understand how they will react to our changing climate and to offer informed recommendations to evolving conservation strategies.

Below is a short video by The Redwoods and Climate Change Initiative (RCCI) on the impact of extended drought on Giant Sequoias. Oh, and in case you were wondering, Giant Sequoias (sequoiadendron giganteum) and Coastal Redwoods (sequoia sempervirens) are closely related, but are two different species.