Photo by L. Drake
On our drives and hikes through Yellowstone National Park we saw (as I mentioned last week) two wolves (pictured at a distance to the left), one bald eagle, a dozen sandhill cranes, a hundred elk, and a couple of hundred bison. The most abundant large organism we saw, though, numbered in the millions. We drove past and hiked through vast mountainsides covered with lodgepole pines. These pines were noteworthy not only for their incredible abundance but also for their remarkable uniformity. It was like someone had used CGI technology to replicate a tall, thin, astonishingly straight, incredibly uniform looking pine tree thousands and thousands of times over. These vast tree armies reminded me of the orc or elf or dwarf armies in the various Lord of the Rings movies. They were the most natural looking, unnaturally repetitive set of trees I think that I ever seen!
Photo by B. Olsen, Flickr
The lodgepole pine (Pinus contorta) is found broadly across western North America from the Pacific Ocean’s coast to the northern parts of the Rocky Mountains. There are four subspecies of P. contorta: the “beach pine” (P.c.bolanderi) of northern California, the “shore pine” (P.c.contorta) of northern California up into Alaska, the “tamarack pine” (or “Sierra lodgepole pine”) (P.c. murrayana) of the mountains of southern California and Nevada up the Sierra Nevada into the Cascades), and the “Rocky Mountain lodgepole pine” (P.c.latifolia) which we saw so abundantly in Yellowstone.
Many of the shore species grow in forms that fit the “contorta” species name: they are short, twisted, stunted, shrub-like trees. The Sierra lodgepole pine and the Rocky Mountain lodgepole pine, though, can reach substantial heights (130 to 160 feet) and girths (over six and a half feet dbh). The species designation “contorta,” though, is said to be based on the characteristically twisted needles seen on all of the sub-species. Lodgepole pines can live for up to 400 years if conditions are stable.
Photo by D. Sillman
Fire is an important environmental factor especially for the Rocky Mountain lodgepole pine. The mature tree is actually quite susceptible to fire-kill because of its thin bark, but its seed bearing cones are designed to only open after they have been heated to 113 to 140 degrees F. The lodgepole pines begin to make cones when they are six or ten years of age, and these cones (and their seeds) can accumulate for many decades on the forest floor. So when a lodgepole pine forest burns (which historically happens every 100 to 300 years) a new forest of pine seedlings quickly springs into existence. This is why lodgepole pine forests are typically so evenly aged (and so remarkably uniform in appearance!). The succession sequence in these fire driven forests, then, is short, rapid and focused on the tree species that is best adapted to the high altitude, short growing seasons, and relatively dry conditions of these sites: the lodgepole pine.
Native Americans of the Great Plains traveled great distances to the Rocky Mountains to gather lodgepole pine logs for their tipis. A typical tipi would use more than a dozen, 15 to 18 foot pine poles to support its buffalo skin encasement. The narrow diameter and straight growth aspect of these poles and the low density (and light weight) of the wood made them ideal structural supports for the frequently moved tipis.
Photo by D. Sillman
One of our hikes in Yellowstone was around Ice Lake in the northwestern section of the park. Ice Lake was the site of a severe forest fire in 1988 that destroyed its mature lodgepole pine forest. The logs from this burned forest still litter the surrounding landscape, and lay sun-bleached and slowly decomposing on the dry forest floor. A number of the logs have been actively split (by bears, perhaps, seeking grubs?) and their torn and shredded woody materials have been mixed in with the slowly accumulating needles from the new pines. After 28 years of growth the new pines are about 20 feet tall and 4 or 5 inches in chest high diameter. There were also a few standing,
Photo by D. Sillman
older trees that somehow survived the 1988 fire. Pine cones were seen on the older trees and also on many of the young, re-growth trees. They were also seen in the growing mass of dry material that was accumulating on top of the sandy soil. Although a few understory plants were observed (especially “fireweed” (pictured to the left)) in the incompletely shaded forest floor, most of the plant growth in this forest were lodgepole pine seedlings, saplings and pole trees. The scattered herbaceous plants provide food for grazers (like elk and deer), but they will soon be shaded out by the coalescing pine canopy.
Photo by D. Sillman
A similar burn area in between Yellowstone and the Grand Tetons was described in a recent New York Times article (September 13, 2017). This lodgepole pine forest burned seventeen years ago (in 2000). The pre-fire forest was 200 years old and, so, had a substantial cone and seed reserve. The post-fire response was robust with 32,000 lodgepole pine seedlings per hectare densely filling the burned area. This recovery forest, though, did not have its ecologically expected 100 to 300 years to grow and accumulate new cones and seeds. Instead, because of ongoing climate change and the associated elevated temperatures and reduced moisture (especially reduced snowfall) a part of this young forest re-burned in 2016. Because of the very short time interval between forest fires, this forest had a very small cone and seed reserve and re-generated a very sparsely treed pine forest (only 400 lodgepole pine seedlings per hectare (1.25% of the original post-fire forest)). These scattered pine trees will never completely shade their forest floor and, so, will be under intense competition from aspens and a wide variety of herbs and grasses. It is possible that this more sparsely treed forest will not be sustainable or stable. It is, in fact, possible that these more frequent burnings could lead to the extinction of the lodgepole pine!
Lodgepole pines make up 80% of the trees in Yellowstone National Park. The transition of this pine forest into one dominated by aspens or some other deciduous species would radically change the ecological dynamics of the entire Greater Yellowstone Ecosystem. Every organism from marmots and elk to grizzly bears would be affected. We’ll just have to wait and see if the pine forests find some new way to survive.