Paleohistory of Whitebark Pine

 

Burgeoning interest in whitebark pine because of mounting threats to its survival has catalyzed a veritable cottage industry of scientific research. As a result, publications reporting on the paleohistory, current ecology, and future prospects of this tree species are accumulating at an exponential rate, aided by a well-mapped distribution, easily studied specimens, durable pollen and macrofossils, and increasing availability of research funds. Of relevance here, we probably know more about the past abundance and distribution of whitebark pine than we do about virtually any other North American grizzly bear food, much less the distribution and abundance of grizzlies themselves.

Aside from the intrinsic uncertainties of any prehistorical reconstructions, one thing we know for sure. Given the fundamentals of bear nutrition and energetics, whitebark pine seeds were a potentially important food for grizzly bears wherever the two species overlapped in time and space. With that as a premise, it is instructive to look at what we know about the relative abundance of whitebark pine in the northern US Rocky Mountains going back to the Last Glacium Maximum of approximately 21,000 years ago.

The map at right features a modeled distribution of whitebark pine, produced by David Roberts and Andrea Hamann, set in western North America roughly 21,000 years ago at the peak of the last ice age. Everything white is ice. Darker shades of green correspond with higher odds that whitebark pine would have been present, at least according to the joint results of two widely-used and accepted General Circulation Models (GCMs). Allowing for the vagaries of models, the results point to one relatively robust conclusion. Whitebark pine was probably much more widespread during the ice ages compared to our present and previous warm intervals, which is not surprising given that this species occupies such a cold dry niche.

However, a notable proviso arises from critically examining the current distribution of whitebark pine and the known distribution of whitebark pine macro-fossils. Despite a suitable climate, whitebark pine probably never occupied the central and southern US Rocky Mountains. Instead, there is evidence that limber pine (Pinus flexilis) and perhaps various species of bristlecone pines occupied this climate space, which is consistent with the occurrence of these species in current-day niches to the south much like those occupied by whitebark pine to the north. The orange dashed line at right is a speculative delineation between where whitebark pine was probably predominant, to the north and west, and where limber pine replaced it, to the south and east. Likewise, whitebark pine was probably never present in Alaska/Beringia and, if any stone pine were present, it was probably one of the Eurasian stone pines, most likely P. pumila.

The Last Glacial Maximum

Yellowstone During the Last 15,000 Years

Thanks to a comparative abundance of ponds and peatlands, the post-ice-age history of vegetation in the Yellowstone ecoregion is probably one of the best-studied anywhere on Earth, largely owing to the efforts of Cathy Whitlock and her many graduate students. Fortunately for my purposes here, pollen from white (hapoxylon) pines is well-preserved. Unfortunately for my purposes, the pollen of whitebark pine can't be distinguished from the pollen of limber pine, which is also common in the Yellowstone area. That aside, a relative complete picture of vegetation changes in this ecoystem has emerged.

The graphic at left is my attempt to synthesize information from a number of publications reporting the results of research done at numerous sites. The basic notion is that each timeline represents changes in vegetation from older, at left, to more recent, at right, and at different elevations, from highest at top to lowest at bottom. Whitebark pine is graphically shown in red. Major transitions in vegetation are denoted by corresponding dates, in white. Dominant tree species are denoted by their 4-letter acronyms: PIEN Picea engelmannii; PIAL  Pinus albicaulis;  PICO Pinus contorta; and PSME Pseudotsuga menziesii. The bottom-most graph shows a reconstruction of temperatures based on measurements obtained from Greenland ice cores. The vertical orange bars correspond with periods of epic drought, most notably the Altithermal labeled as "Holocene Climatic Optimum."

 

One obvious prefatory point needs to be made. The mountains and highlands of Yellowstone and the Northern Continental Divide were buried under deep icecaps up until roughly 14,000 years ago, with the maximum extent of ice occurring around 15,000-19,000 years ago. But shortly after ice-melt, a predictable early pattern unfolded at all elevations, coincident with rapid global climate warming. Barring the relatively harsh and sterile Central Plateaus, tundra rapidly transitioned to an Englemann spruce parkland, with whitebark pine arriving shortly after. This early succession of tree species reflected the dispersal capacity of each. Spruce seeds spread rapidly on the wind, which made spruce a pioneer species shortly after the melt of glaciers in most parts of North America. Whitebark pine seeds are likewise widely dispersed, but not by wind, rather as a result of caching by the Clark's nutcracker (Nucifraga columbiana). At the peak of Holocene warmth, during the Altithermal, whitebark pine was in decline, and lodgepole pine (P. contorta) ascendant.

The figure at right, produced by Virginia Iglesias and Cathy Whitlock, complements the information in the synthetic diagram above. The various trend lines represent variation in abundance of dominant conifers in the Yellowstone ecosystem, together with indicators of wildfire activity (in orange, second from top) as well as winter humidity (black squiggly line at very top). As in the diagram above, I've also indicated the Holocene periods of peak warmth and drought at orange vertical bars.

The consensus of all this seems to be that abundance of spruce peaked earliest, between roughly 12,000 and 14,000 years ago, followed by a peak in abundance of whitebark pine between 6,000 and 14,000 years ago. Thereafter, both these cold-tolerant species steadily declined at the same time that the more heat- and drought-tolerant lodgepole pine and Douglas-fir (Pseudotsuga menziesii) steadily increased in abundance.

From a bear's point-of-view, Yellowstone was richest in pine seeds thousands of years ago, and has become steadily more impoverished in that regard. As important as pine seeds have been to grizzly bears during recent decades, they were probably even more important in the millennia between deglaciation and the height of the Altithermal. Or, put another, way, whitebark pine seeds have probably been an important food for Yellowstone's grizzlies throughout the Holocene.