Photo by M. Barrie, Wikimedia Commons
(Click on the link to listen to an audio version of this blog … The limber pine
There are, depending on where exactly your cut-off point is between trees and shrubs, about fifty species of trees that are native to Colorado. I have specifically written about four of these before (Rocky Mountain maple, blue spruce, eastern cottonwood, quaking aspen). This post is the first of a series about several of the other important, native Colorado tree species. This week we talk about the limber pine!
The limber pine (Pinus flexilis) is a remarkably tolerant tree that is capable of growing under a wide range of conditions. It has the broadest natural elevation range of any Rocky Mountain tree species (it grows from just under 3000 feet to almost 13,000 feet!) and will thrive in a great variety of soils and under a wide range of moisture regimes. It is very slow growing but is very long lived. Many limber pines, including several in Colorado, are over 1000 years old and one, in Alberta is quite possibly over 3000 years old!
Photo by Farmartin, Wikimedia Commons
A few of the pine trees in my neighborhood here in Greeley are limber pines. Local tree nurseries advertise their availability and adaptability to suburban landscapes. Over in the campus arboretum of the University of Northern Colorado Tree #3 on the West Campus Loop is a limber pine. These neighborhood and campus trees are all tall, straight and densely branched. You have to go an hour or so west, up to the ridges of the Rocky Mountain National Park, to see the more archetypical form of this tree.
On those rocky, almost soilless ridges are the stunted, windblown, twisted and often very ancient limber pines that demonstrate why this tree has the species name “flexilis.” Branches are twisted into curling masses, and trunks snake along the rocky surface and rise only just high enough to send up clusters of needles. A tree might be hundreds of years old but only rise to a height of three or four feet. They cling tenaciously to the rocks and survive!
Limber pine is one of the white pines. It has the characteristic “white pine” clusters of five needles in each of its fascicles. Its dark, blue-green needles are long (2 1/2 to 3 ½ inches), smoothed edged, pointed and curved. Needles persist on a tree for five to six years. The limber pine’s bark is light gray and smooth on young trees but gets dark brown and increasingly furrowed on older trees. It can be found on its mountain slopes growing intermixed with white-bark pine or lodgepole pine or a number of other species, but frequently it is found in pure stands growing in places where other tree species could not survive.
The limber pine’s natural range is down the backbone of the Rocky Mountains from Alberta and southwestern British Columbia to New Mexico. This range extends both east (into the Dakotas and western Nebraska) and west (into Arizona and eastern (and also southwestern) California). The limber pine’s very flexible branches enable it to handle the substantial weight of winter snow and also bend itself (sometimes literally into knots!) as it deflects the force of the often powerful prevailing winds.
Limber pine trees are monoecious. This means that there is only type of individual and that it has both male and female flowers. The male flowers are small and red and are found on lower branches of the tree (a typical arrangement in monecious trees that helps to prevent self-fertilization). These male flowers release pollen in June and July. Female flowers are located in large (3 to 6” long), woody cones. These female flowers are concentrated in the upper branches of the tree (again, to minimize the chance that pollen released from a tree will be wind-blown into that tree’s own female flowers). The female cones close after pollination, but fertilization does not occur until more than a year after pollination has occurred.
Photo by T. De Gomez, Univ. of Arizona, Bugwood. org
Immature cones are green and as they ripen they turn yellow and then brown. It takes two years for the seeds to form in the cones. When the cone is fully mature, it opens and drops its seeds directly to the ground under the parent tree. Prior to this fully ripened opening, however, many birds and mammals break into the still-closed cone in order to consume the large, nutritious limber pine seeds.
Red squirrels, northern flickers and mountain bluebirds all feed on limber pine seeds. The bird that is most closely associated with this process, however, and that may be most important in disseminating not only limber pine seeds but also the seeds of other high elevation, white pine species (like the white bark pine (P. albicaulis), the foxtail pine (P. balfouriana), the bristlecone pine (P. longaeva) and the western white pine (P. monticola)) and also lower elevational pines (like the pinyon pine) is the corvid called “Clark’s nutcracker” (Nucifraga columbiana).
Photo by Wing-Chi Poon, Wikimedia Commons
Clark’s nutcracker was first described in 1805 by William Clark (co-leader of the Lewis and Clark Expedition). It is a corvid slightly larger than a jay but smaller than a crow. It is found in the mountains of the western United States from 3000 to 12,000 feet elevation. Year-round, Clark’s nutcracker relies on pine nuts for most of its food. It consumes fresh nuts gleaned out of mature cones and digs caching trenches in the soil where it buries substantial amounts of pine seed for off-season consumption. It can carry up to 125 pine seeds at a time in its sublingual pouch, and, yearly, each nutcracker buries many thousands of pine seeds in its food caches.
Preferred locations for a nutcracker’s seed caches are wind-swept ridges that have southern exposures. These sites are unlikely to remain covered very long with deep snow even during a heavy snowfall winter. The nutcracker, then, is able to retrieve its stored seeds throughout the winter season. These ridges are also the ideal habitat for the establishment of limber pine stands!
The ability of a Clark’s nutcracker to remember the location of caches is extremely impressive! Relying on spatial cues of surrounding features of the landscape, the nutcracker can, at need, very reliably find most of its seed caches. The caches that are not found then have the opportunity to germinate. Limber pine seedlings are often found growing in dense clumps due to their origins from abandoned nutcracker seed caches!
Limber pine seedlings vigorously root down even into extremely rocky soil. Their large tap root anchors them securely and makes them quite wind-firm even in the very thin soils of their mountain ridge habitats. A mycorrhizal fungus (Gomphidius smithii) is quite important for seedling and tree viability. When limber pines are grown in loamy, tree nursery soils they form discrete root balls. The dispersed rooting in their more natural environments, however, makes tree transplantation in field very difficult.
Photo by USDA, Public Domain
Limber pines are not only very slow growing but are also very intolerant of shade. If they become established in a site in which other tree species can also grow, then they are almost always shaded out by the faster growing tree species. Their frequent occurrence in pure stands reflects their ability to tolerate environmental conditions that would kill almost any other species of tree.
Limber pines are not a commercially important tree species. Their slow growth rates and twisting, tortuous growth patterns generates a wood product that has few uses other than firewood. Some limber pines are grown for Christmas trees and for landscaping trees, but, again, their slow growth rate makes the profit margins of these types of enterprises quite narrow.
Photo by NPS, Public Domain
Limber pines are extremely important in their natural ecosystems, though. They provide cover for numerous species of birds and mammals on their wind-swept ridges and generate food for many species of birds, rodents, bears and people. Native Americans and early European pioneers relied on limber pine (and other pine species) seeds for a substantial component of their year-round diet.
The thin bark of young limber pines makes them quite susceptible to fire. Insect damage (including widespread damage by the mountain pine beetle (Dendraotonus ponderosae) (see Signs of Spring 11, May 13, 2021) and budworms (Choristoneura lambertiana ponderosa)) and fungal diseases (including the extremely widespread, exotic invasive white pine blister rust) are killing limber pines all across their broad, North American range. Several types of mistletoe (a parasitic epiphyte) are also commonly found encasing and physiologically strangling the limbs of limber pine.
The limber pine is a tree of the high mountains that is also capable of growing in very domesticated settings. It generates irreplaceable habitats on its bare rocky ridges and generates significant amounts of food for wildlife. It, like most of pines of North America, is under assault from insects, fungi and climate change. Hopefully, it will survive.