Photo by C. Schnepf, Univ of Idaho, Bugwood.org
(Click on the link to listen to an audio version of this blog … Lodgepole pine
When we were in Yellowstone a couple of years ago, the most abundant large organism I saw numbered in the millions. They were not bison or moose or grizzly bears, but, instead, they were the vast numbers of lodgepole pines that filled almost all of the mountainsides around us. 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, incredibly straight, uniform looking pine tree thousands and thousands of times over to make a vast lodgepole pine “army.” They reminded me of the orc or elf 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!
The lodgepole pine (Pinus contorta) is found across a broad stretch of western North America from the Yukon down to Baja California and from the coast of the Pacific Ocean up over the Rocky Mountains all the way to the Black Hills of South Dakota. There are estimated to be 15 million acres of lodgepole pine forest in the United States and 50 million acres more in Canada. This broad range and extensive distribution is clear evidence of this species wide range of tolerance to environmental variables. It has, in fact, the broadest tolerance to environmental factors of any North American conifer!
Photo by JSayre64, Wikimedia Commons
Lodgepole pines grow on almost every type of soil (from rocky gravels to organic-rich mucks). They can be found on almost any type of topography (from slopes, to basins to steep ridges). They grow in environments that receive as little as ten inches of rain per year and also in places that get over two hundred inches of rain per year. Although they are very intolerant of shade, lodgepole pines are often found growing not only in dense, pure stands but also intermixed with very wide varieties of other tree species.
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) that is found in the mountains of southern California and Nevada and on up the Sierra Nevada into the Cascades, and the “Rocky Mountain lodgepole pine” (P .c. latifolia) which we saw so abundantly in Yellowstone and which is also an important species here in our Colorado forests.
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.
Male cones. User:Geographer, Wikimedia Commons
Lodgepole pines are monoecious. This means that there is only one type of individual and that these individuals have both male and female flowers. The male flowers are yellow-orange and form on catkins at the bases of new shoots on the older, lateral branches in the lower crown. The female flowers are reddish-purple that grow in whorls of two to five on the tips of the main branches of the upper crown. This spatial arrangement of flowers helps to insure that a given tree is unlikely to pollinate its own female flowers.
Pollen is released from mid-May to mid-July depending on elevation and climate, and seed cones mature between August and October more than a year after pollination has occurred. Seeds are small and very abundant. Good seed years are seen each year or every two or three years. Most of the seeds, though, are locked up in serotinous cones which can persist on the branches of the pines for decades. Squirrels and a few species of birds (like the red cross-bill) are able to break into intact cones to harvest seeds. Other rodents are known to eat lodgepole pine seeds from fallen cones which may open if the conditions of the forest floor are relatively damp.
Photo by NPS. Public Domain
Fire is an important environmental factor 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. 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 trees and 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.
Seedling density, amount of sunlight and presence of competing plants (especially grasses) greatly affect the growth rates of new lodgepole pine forests. In low density seedling stands, the saplings may reach 18 feet of height after 20 years, while in high density seedling stands 20-year old saplings may only be 10 feet tall.
Lodgepole pines form taproots in favorable soils but more typically have very shallow root systems. This causes the trees to be quite vulnerable to wind throw especially on exposed mountain ridges.
Mountain Pine Beetle, Photo by S. Clarkson. Wikimedia Commons
Mountain pine beetles (Dendroctonus ponderosae) causes extensive damage to lodgepole pines as do several other type of bark beetles. Fungal diseases, epiphytic parasites (like dwarf mistletoe (Arcenthobium americanus), porcupines and pocket gophers may also cause extensive damage to lodgepole pine stands.
Native Americans of the Great Plains traveled great distances to the Rocky Mountains to gather lodgepole pine logs for their tepees. A typical tepee would use more than a dozen, 15 to 18 foot long 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 tepees.
On one of our hikes in Yellowstone back in 2017 we walked around Ice Lake in the north-central section of the park. Ice Lake was the site of a severe forest fire in 1988 that destroyed its mature lodgepole pine forest. The charred logs from this burned forest still litter the surrounding landscape and lay sun-dried 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 almost 30 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, 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”) 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 R. Routledge, Sault College, Bugwood.org
A similar burn area in between Yellowstone and the Grand Tetons was described in a 2017 New York Times article (September 13, 2017). This lodgepole pine forest burned seventeen years before (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 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 sparse 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 less dense 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!
Interestingly, lodgepole pines are being widely sold as landscaping trees and also as Christmas trees! My daughter and son-in-law bought a living lodgepole pine for their Christmas tree this year and now have it overwintering in their backyard. Later this spring we will plant it somewhere that it will get as much sunlight as their densely-treed yard will allow. Also, the lodgepole pine growing in the arboretum over on the Northern Colorado University campus (tree 35 on the West Campus Loop) looks healthy and solid! Greeley must be a good, low elevation environment for the species!
Great article. Where can I buy 100 seedlings?