Future Prospects of Whitebark Pine as Bear Food
The figure at right speaks to a stark and, at this point, unavoidable reality. It shows projected temperatures (top) and precipitation (bottom) for the Yellowstone ecosystem, employing three different scenarios that make different assumptions about future social, economic, and physical changes of relevance to climate change (ea_rcp85 is the most pessimistic, ea_rcp45, the most optimistic). Even the most optimistic shows major warming. The most pessimistic projects monumental change. Regardless of the scenario, comparatively large increases in temperature will offset the more modest projected increases in precipitation to yield ever more widespread, long-lasting, and severe drought.
The upshot for whitebark pine, a denizen of cold environments, is bleak, in fact catastrophic. The effects will be direct as well as indirect. The direct effects will be through surpassing physiological optima and even tolerances. The indirect effects will be legion, including the promulgation of lethal insects, devastating fires, and the up-elevation dispersal of more drought- and warmth-tolerant competitors. I provide details below, as well as a concluding comment on what is almost certainly misplaced if not inane optimism on the part of some researchers and managers.
The Disappearing Climate Envelope
There is a veritable cottage industry of scientific activity devoted to turning out projected changes in species distributions under different climate change scenarios. Perhaps because whitebark pine is such an interesting case, it has been the subject of multiple independent modeling efforts, as many as, if not more than, devoted to any other species I know of. According to my latest count, there are at least 10 different publications reporting projected changes in the distribution of whitebark pine, including efforts that have focused exclusively on this species, as well as efforts that have addressed whitebark pine as part of an ensemble.
The first projection was reported in a 1991 publication by Bill Romme and Monica Turner. In it, they forecast an approximate 90% loss of whitebark pine in the Yellowstone ecosystem during the next 100 years, well pre-dating the major losses that happened 10-20 years after publication of their paper (see Temporal variations). Despite ever more sophisticated methods, all of the subsequent modeling exercises have produced essentially the same result. A map modified from one of the more recent endeavors, by Tony Change and Andy Hansen, is immediately above. The different colors denote the probability that whitebark pine will be present at three future time steps. Warmer colors represent a higher probability, cooler colors, a lower one, and no color at all, a near-zero probability. You can see the shadow of Yellowstone National Park's boundary in the middle. The upshot is that by 2099 the only places in the Yellowstone ecosystem where there will likely be even a modest chance of whitebark pine surviving is on the Beartooth Plateau, in the Teton and Wind River Ranges, and a few isolated places elsewhere in the Absarokas..
Notably, all of these projections have been based largely, if not solely, on a reckoning of suitable climate, i.e., the "climate envelope." Proximal prospective causes of decline are usually not addressed, which matters, because tree species are rarely killed outright by climate. Instead, individual trees are rendered more vulnerable to any number of proximal lethal agents. In the case of whitebark pine, these include the non-native fungal disease white pine blister rust (Cronartium ribicola), native Rocky Mountain pine beetles (Dendroctonus ponderosae), wildfire, and competition at stand initiation from lower-elevation competitors such as lodgepole pine (Pinus contorta) and Douglas-fir (Psudotsuga menziesii).
Bark Beetles Rampant
As I describe under Temporal variations, mountain pine beetles have emerged as probably one of the most severe threats to whitebark pine. Bark beetles lay eggs under the bark of mature or near-mature trees, after which the resulting larvae kill the host tree by girdling it; i.e., by eating through the entire circumference of the life-giving cambium that trees depend upon for transport of water and minerals from the roots, and carbohydrates from the leaves. However, bark beetle larvae can't survive to inflict damage if temperatures either drop rapidly to low levels during the fall (say, -30 degrees F in November) or are cold enough, for long enough, during winter for temperatures beneath the bark to drop below -40 degrees F. Cold temperatures are critical to shielding whitebark pine from outbreaks of mountain pine beetles, to which they have little natural resistance, especially given that extensive lower-elevation lodgepole pine forests surrounding most whitebark pine are a near-inexhaustible reservoir of beetles.
Mountain pine beetles have already had devastating effects on whitebark pine in the Yellowstone ecosystem. A survey conducted during 2009 showed that nearly half of all whitebark pine forests exhibited severe mortality from an outbreak of bark beetles that began around 2000. Some recent research published by Polly Buotte and her co-authors clarified what was going on with climate during and before the outbreak. There had not been as favorable a period of warm dry conditions in the whitebark pine zone since at least 1950. She was also able to develop a simulation model that allowed her to project what would likely happen with beetle outbreaks at high elevations as the climate continues to warm. Her results are summarized immediately above in the figure at left and map at right.
The figure shows the results of Polly's simulation model (the black line and associated variability of results in orange and gray), both backcast and forecast, along with observed historical climate suitability for beetles (the red line). The historical average is shown as a vertical black line projected out to 2100. The inset figure above shows the observed extent of beetle-kill among whitebark pine (red dots), as well as modeled predictions (black line and gray bounds). These results show, first, that climate suitability was at sustained high levels during the outbreak, unlike any time in the previous 40 years; and, second, that suitability of climate for beetles is likely to steadily increase to sustained levels much like those that typified the 2000-2009 outbreak. Importantly, the kind of warmth observed during 2000-2009 allowed, not only for beetle larvae to survive over-winter, but also for beetles to complete more than one generation in a single summer season. The results will be chronically fatal to any whitebark pine tree big enough to be beetle-fodder.
The map above right translates the modeling results into spatial form, reckoned as the probability that beetle-caused mortality of whitebark pine would occur in any given area during an approximate 30-year period. Red denotes a probability nearing certainty. As you can see, even as early as 2040, a mere 20 plus years hence, there will likely be no refuge from bark beetles for whitebark pine in the Yellowstone ecosystem. Regardless of the nature and degree of heroic restoration efforts (see at bottom), any whitebark pine that survive to be pole-size trees will likely fall prey to beetles.
Inasmuch as wildfire often benefited whitebark pine historically, this will probably no longer be the case. Under "normal" conditions, wildfires disproportionately kill shade-tolerant competitors such as subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea englemannii), and create an environment favorable for the establishment of less shade tolerant whitebark pine germinating from seeds cached by Clark's nutcracker. Without fire, whitebark pine would be slowly replaced over the centuries by spruce and fir. But, despite this, mature whitebark pine are not particularly fire tolerant. They are killed by fire at near the same rate as spruce and subalpine fir.
The upshot is that if wildfire becomes more frequent, at the same time that climate change increasingly favors invasion by lower-elevation shade intolerant and fire-adapted species such as lodgepole pine and Douglas-fir, fire may actually hasten the demise of whitebark pine. The regenerative advantage of whitebark pine would be lost, at the same time that mature trees capable of surviving for centuries would be brought to an early demise.
Virtually every prognosis of wildfire extent and frequency in the western United States during the next several-hundred years has shown that both will increase. Another consistent results has been that fire severity will lessen in most places as increasingly frequent burns preclude the accumulation of large fuels that would otherwise sustain hotter wildfires. Moreover, recent research has suggested that high-elevation habitats such as those typical of whitebark pine will not be exempt from these patterns, at the same time that climate will increasingly favor lodgepole pine and Douglas-fir in places previously occupied by whitebark pine. All of this holds true for modeling specific to the Yellowstone ecosystem.
Perhaps the most apocalyptic research was reported in 2011 by Tony Westerling and his co-authors, who projected that forests would essentially disappear from most of Yellowstone to be replaced by steppe, primarily because fires would be so frequently that they would preclude successful regeneration and recruitment of trees. More recent research by Jason Clark, using a more sophisticated simulation model, shows the same trend, but with some twists. His results are summarized in the figure at left immediately above. The top graphs show simulated trends in extent of forest cover paired with graphs beneath showing trends in the extent of burned area, comparing simulations that projected historical conditions, at left, with projections assuming a climate approximately 5 degree F warmer and 5% wetter, at right.
Forest cover will likely decline substantially, coincident with larger and more frequent fires. So Clark's results were much the same as Westerling projected. But the twist is that Clark forecast that Douglas-fir would increase and that lodgpole pine would decline, primarily because Douglas-fir is better adapted to cooler frequent fires, which debars a need to reproduce and recruit at an otherwise hectic rate. But, of relevance here, whitebark pine is more like lodgepole pine that it is like Douglas-fir. In fact, whitebark pine is even more vulnerable than lodgepole pine to the scenario painted by Jason Clark given that it takes much longer for whitebark pine to mature and produce cones.
The implication? Future wildfires will likely hasten the demise of whitebark pine rather than foster its persistence in an increasingly hostile world.
Research into and writings about prospects for and methods of restoring whitebark pine have proliferated since the early 1990s. The genesis of this effort was early losses of whitebark pine to white pine blister rust and resulting concerns about the species' acute vulnerability to this non-native fungal pathogen. This legacy has continued to powerfully shape perspectives of researchers and managers and keep blister rust at the center of attention. But, as I describe here and under Temporal variations, blister rust is just one of several severe threats to the persistence of whitebark pine. All but blister rust tier back to climate change, including physiological constraints and tolerances; increasing competition at stand initiation from drought- and warmth tolerant species previously confined to lower elevations; increasingly frequent and extensive wildfires; and increasingly common and unmitigated outbreaks of mountain pine beetles.
Given this plethora of severe threats, I am frequently struck by the extent to which otherwise intelligent scientists are afflicted by glaring blind spots in their framing of research and deliberations over restoration. They will consider blister rust, but ignore changes in fire behavior and the threat posed by lower-elevation competitors. They will consider fire, but then continue to ignore competitors and, more surprising yet, mountain pine beetles. They will consider pine beetles, but ignore fire. And so on ad nauseum.
The upshot is a widespread tendency to overstate the future prospects of whitebark pine and restoration efforts organized around over-coming blister rust. To be sure, blister-rust-resistant strains of whitebark pine have been found and propagated, increasing the odds of survival among such stock by 2- to 5-fold. But virtually every other consideration is more-or-less swept under the rug. Never mind that odds of planting blister rust resistant strains in typically roadless rugged terrain sufficient to sustain ecological function is close to nil in the first place. What then about the fact that currently hospitable sites will become increasingly less so, climatically and because of invasion by lodgepole pine and Douglas-fir? What then about bark beetles, which will predictably kill any trees that reach or approach maturity, probably before or soon after they can produce cones? What about the increasingly frequent wildfires that will likely burn the places you've planted with blister-resistant stock, again, before or soon after the trees produce cones?
Some people claim that all will be well with whitebark pine because it survived, diminished, but still comparatively abundant during the Altithermal--the warmest driest period of the Holocene predating our current extremes (see Paleohistory). But there are several problems with this claim. First, reconstructions of whitebark pine abundance are based almost wholly on pollen (see left), which is indistinguishable from that of limber pine. It is likely that the warmth-adapted limber pine increased in abundance during the Altithermal and accounted for an increasing fraction of the hapoxylon (i.e., white pine) pollen during that period.
But more problematic, these claimants assume that, aside from climate change, the world was pretty much the same then as now, especially when it comes to factors that threaten whitebark pine. But it wasn't. Lodgepole pine and Douglas-fir were not as abundant (see Temporal variations), which means that they were probably not as much the competitors then as now. With less lodgepole pine, the reservoir of bark beetles was also probably less, and, moreover, the paleohistory of bark beetles in the northern Rockies has not been clarified. For whatever reason, fires did not apparently increase in prevalence much during the Altithermal. So, much of the context within which whitebark pine lived was probably more favorable then compared to what is likely to unfold in the future. Certainly blister rust was not around.
I find the impulse towards optimism to be understandable. But, optimism founded on seemingly willful distortion of inconvenient truths is a poor basis for any projections and related planning for likely future scenarios. I love whitebark pine. I have a deep emotional connection to the tree itself and the places it grows. But that does not make me any more tolerant of platitudes or palliatives.
Barring a few enclaves, whitebark pine will almost certainly be lost in the northern US Rocky Mountains and, with that, it will be, to all intents and purposes, forever lost as a food of grizzly bears in this part of the world. Wishful thinking will not make it otherwise.