The conifer forests of California’s Sierra Nevada are slowly climbing the mountains. As the climate warms, several tree species are establishing themselves at higher elevations where temperatures remain cooler. While this upslope migration shows that forests can adapt to changing conditions, scientists warn that many species may eventually run out of suitable habitat.
A study published in the journal California Fish and Game found that climate change is shifting the distribution of several key conifer species in the Sierra Nevada.
Among the species moving upslope are lodgepole pine, red fir, and western white pine.
These high-elevation conifers are adapted to cool temperatures. As the climate warms, the conditions they depend on are gradually moving higher up the mountainsides.
By comparing historical forest surveys with modern observations, researchers found that these tree species have shifted their average elevation by nearly 500 feet (150 meters) over the past 80 years across much of the Sierra Nevada, from north of Lake Tahoe to the southern portion of the range.
The movement reflects successful adaptation to a changing climate. Unlike animals, trees cannot simply relocate. Instead, forests gradually expand into new areas as seeds establish the next generation under favorable conditions. (Related: Why Sierra Nevada Forests Are Becoming Climate Mismatched)
Over the past century, temperatures in the Sierra Nevada have increased by approximately 2.34°F (1.3°C). As lower elevations become warmer, seedlings are increasingly establishing themselves farther upslope where cooler conditions still exist.
The shift raises an important concern. Mountains become progressively smaller toward their peaks, leaving less land available at higher elevations. As more plants and animals migrate uphill in response to warming temperatures, competition for suitable habitat is expected to increase.
For species already living near the highest elevations, there may eventually be nowhere left to migrate if warming continues.
Not every Sierra Nevada tree species has responded in the same way. Douglas fir, for example, has shown relatively little upslope movement compared to lodgepole pine, red fir, and western white pine. Researchers suggest that differences in drought tolerance, temperature sensitivity, and life history may explain why some species are shifting more rapidly than others.
This study documented one of the earliest large-scale examples of Sierra Nevada forests responding to climate change by moving upslope. More recent research has found that, in many lower-elevation forests, climate is now changing faster than trees can adapt. Instead of simply shifting uphill, some conifer forests are experiencing vegetation climate mismatch, where mature trees remain but are no longer able to regenerate successfully under today’s warmer, drier conditions.
Together, these studies illustrate two stages of forest response to climate change. Where conditions allow, forests slowly migrate to higher elevations. Where warming outpaces that migration, forests may gradually transition to different types of vegetation better suited to the new climate.t smaller and smaller as you move up.
Species will eventually begin to die off with nowhere else to go.
Other tree species in the Sierra Nevada are more resilient to global warming, like the Douglas fir. Although the Douglas fir trees haven’t seen a discernible shift up the mountainsides, researchers wonder how long it will be before these trees too begin an exodus away from hotter temperatures.
Rather than leave it until it’s too late to save these vital tree species, researchers emphasize the need to curb the manmade problems associated with global warming sooner rather than later.
References
Wright, D.H., Nguyen, C.V., and Anderson, S. (2016). Upward shifts in recruitment of high-elevation tree species in the northern Sierra Nevada, California. California Fish and Game 102(1):17-31; 2016
Study: Sierra Nevada forests shifting to higher elevations as temperatures warm, Mercury News, May 25, 2016.
