Douglas-fir (Pseudotsuga menziesii) is one of the world’s most important timber species. It is often considered the emblematic tree of low-elevation Pacific Northwest forests, but its full native range spans an impressive 36 degrees of latitude (about 2,500 miles) from central British Columbia into central Mexico. Within the Sierra Nevada, Douglas-fir occurs as far south as Yosemite—the southernmost population in the Sierra is found just 25 miles south of the park. This local range limit is intriguing, especially because the conifer species Douglas-fir occurs with in the northern Sierra (think ponderosa pine, incense cedar, and white fir) thrive well south of Yosemite. What is unique about Douglas-fir? An exploratory analysis I have conducted using climate data from PRISM and forest inventory data from FIA and VTM hints at an explanation—one that doesn’t bode well for the fate of these trees as climate continues to change.
This map shows the northern and central Sierra Nevada, and filled symbols represent occurrences of Douglas-fir based on forest inventories and my own observations around Yosemite. The fill color indicates the annual heat moisture index (AHM), which roughly approximates the degree of water limitation at each stand. AHM is calculated as weighted annual average temperature divided by total annual precipitation. It is readily apparent in the map that the most southerly populations of Douglas-fir experience among the highest AHM of all Sierra Nevada Douglas-fir. If the southern populations are at the limits of their drought tolerance, then increased climate warming and drying might be expected to drive Douglas-fir out of the Yosemite region.
But there is hope! Even though the Douglas-fir populations around Yosemite experience high moisture limitation, they may be capable of tolerating drought stress beyond the levels they currently experience. Unfortunately, a recent high-profile global study concluded that most tree populations exist very near the limits of their drought tolerance and have little wiggle room. Whether or not this generalization applies specifically to Douglas-fir in California, however, remains to be explored. Another hope lies in the possibility that tree populations will be able to “migrate,” or disperse seeds into newly-suitable sites outside the species’ historical range, as climate continues to change. A recent study of natural migratory ability in trees, however, concluded that migratory ability is weak and unpredictable; other studies have also drawn very mixed conclusions.
Even if the trees in Yosemite today have little climatic wiggle room and poor migratory ability, there is a potential that their offspring, though evolutionary adaptation, will be able to tolerate more intense drought, perhaps to such a degree (no pun intended) that they will be capable of persistence through continued changes in climate. The potential for evolutionary adaptation depends on the amount of genetic variation present in the populations around Yosemite. Populations at range limits generally have low levels of genetic variation. Whether or not this generalization applies to the trees in Yosemite is completely unknown, despite its critical importance for predicting and responding to the impacts that inevitable climate change will have on the region. And it is just what I have proposed to study in a grant proposal to the National Park Service.