New Study on Mortality Trends in Southern Sierra Nevada Sugar Pine and other White Pines

Dr. Joan Dudney, our Science Advisor, has recently published a new paper entitled “Compounding effects of white pine blister rust, mountain pine beetle, and fire threaten four white pine species.”

Joan and her co-authors studied long-term mortality trends of four white pine species in Sequoia and Kings Canyon National Parks (SEKI). In SEKI, just as in Tahoe, sugar pine grows at lower elevations, western white pine grows higher up, and whitebark pine occupies the subalpine zone to the treeline.  The rare and beautiful foxtail pine also graces the SEKI high country, but is not found around Lake Tahoe; it prefers to dwell well above 10,000 ft elevation.

A lone foxtail in a stark granite lake basin in southern Sequoia National Park.

Joan and her collaborators discovered very different mortality and recruitment trends between the higher and lower dwelling white pine species in SEKI.  Over a 20-year study period, they found that sugar pine and western white pine were dying off faster than they were regenerating.  The opposite was true for whitebark and foxtail pines.

This graph clearly shows that sugar pine and western white pine are dying faster than they are regenerating.  Whitebark and foxtail pines are faring better.

The fungal pathogen, white pine blister rust, is a major factor contributing to these findings. Blister rust is more prevalent in the lower elevation, warmer regions where sugar pine and western white pine grow. Whitebark and foxtail are less exposed to blister rust in the higher, colder elevations.

Blister rust is not the only cause of mortality, however. Drought, mountain pine beetle and fire also weaken or kill SEKI’s white pines.

In personal communication, Joan explained, “Given our data limitations, it's hard to tease apart the relative importance of these different drivers on mortality… For sugar pine, it's a combination of all factors, including mountain pine beetle, blister rust, fire, and drought. These compounding stressors within a relatively short time interval is making it really hard for sugar pine to sustain its population.”

Joan et al found that 52% of surveyed sugar pines died within the 20 year time frame of the study. Joan expounded on the big picture of what’s happening with the southern Sierra sugar pines by explaining: “When you account for the recruitment – the trees that recruited in as new seedlings in between surveys – the overall population decline was 35%. Losing 35% of the population in 20 years means that in less than 100 years, we could lose ~90% of the population. Given that sugar pine can live to be 400 years old, this is a very fast decline. To make a drastic analogy, it's like losing 90% of the people in your local city by the time you're 25 years old.”

The scientists also found that 13% of western white pines were lost over the course of the study, mainly due to blister rust. A related – and rather concerning – finding of the study was that blister rust is moving up in elevation: sugar pines used to be the most affected, but infections are now increasing more in western white pine and it’s starting to infect whitebark pine. Although this study did not find infected foxtail, scientists did find a few infected foxtail pines outside of the official survey areas in SEKI. So far, foxtail does appear to be more resistant to infection than the other species, but it is still susceptible to the threat of blister rust.

What does this mean for the fate of sugar pine and other white pines in the southern Sierra Nevada and elsewhere?

While the spread of blister rust and its attendant death toll amongst sugar pine and western white pine is certainly alarming, there are many factors at play. Though not a focus of this paper, climate change – which may entail more drought, fire, or even windier or wetter storms – will certainly play a role in the future of our forests. The interaction among drought, blister rust and mountain pine beetle attacks is also interesting: beetles often target trees sickened by drought or blister rust, killing the trees faster than the pathogen or drought would have. Meanwhile, fire can all too easily wipe out large areas of trees – healthy or unhealthy – often resetting the whole ecosystem.

The takeaway? What we know for sure is that our sugar pines and other white pines need as much attention and help as we can afford to give them right now so as to preserve these important species on the landscape. More science, restoration, conservation, awareness and stewardship at all levels – from the individual, to the institutional, to the government level – is needed!

We are grateful to Dr. Joan Dudney and her colleagues for all that they are doing to take the pulse of our forests and keep us all informed on the state of our white pines. It is crucial to stay abreast of the best science and information available to foster healthy forests and a brighter, greener, biodiverse future!

Above photos courtesy of Joan Dunlap