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(Click on this link to listen to an audio version of this blog … Ponderosa pines
The ponderosa pine (Pinus ponderosa) is the most widely distributed pine in North America. It is found from southern Canada down to Mexico, and from the Pacific coast to the Dakotas and the western edges of Nebraska and Oklahoma. It has three geographic varieties: 1. The Pacific ponderosa pine (P. ponderosa var. ponderosa) which grows on the west coast from British Columbia down to near San Diego and over the Continental Divide into Montana and Idaho. 2. The Rocky Mountain (or “Interior”) ponderosa pine (P. ponderosa var scopulorum) which grows in discontinuous populations on isolated mountains and plateaus across Montana into the Dakotas, south across eastern Wyoming and then down into and over the Rocky Mountains in Colorado and New Mexico. And, 3. The Arizona ponderosa pine (P. ponderosa var arizona) which grows primarily in south-eastern Arizona.
The absence of the Interior variety of this tree from large areas east of the Continental Divide is thought to be due to a synergy between two key limiting factors for the species. Growth of very young seedlings requires moisture, and many areas within this potential growth region do not receive predictably significant moisture during the ponderosa pine’s critical seedling growth period (early spring or early summer). Also, the combination of high latitude and high altitude can so compress the length of the potential growing season, that the ponderosa pine is not able to reliably establish itself. It just doesn’t have sufficient time to grow in these truncated, summer-seasonal locations. These two factors have generated the unusual patchwork distribution of the species especially in the northern-most part of its North American distribution.
Photo by R. N. Horne, Wikimedia Commons
Ponderosa pines grow on sites that receive between 11 and 69 inches of precipitation per year. The key to its existence on sites that are drier, as mentioned above, is, at the very least, there has to occasionally be some significant rainfall during the critical, early seedling growth time period. Once established (i.e. when seedlings are more than 100 days old) ponderosa pine seedlings are able to tolerate dry conditions because of their rapidly growing root systems, but for their first three months they are absolutely dependent on rainfall. This tree grows on many types of soils, but, in general, if annual rainfall is low, a more finely textured soil (which will more efficiently hold soil moisture and increase the effectiveness of precipitation) is preferred.
The ponderosa pine is a large tree (tallest recorded specimen was 232 feet tall and the largest recorded diameter at breast height was 8 1/2 feet. Average heights of these trees, though are between 60 and 150 feet). The ponderosa pine has distinctive bark that is black on young trees but yellow-brown and deeply and irregularly furrowed on older trees. Also, if you put your nose very close to one of the deep bark furrows of a ponderosa pine you can distinctly smell the scent of vanilla.
Ponderosa pines have long (5 to 10 inches) yellow-green needles that are clumped into fascicles of two (in the Pacific variety), or three (in the Interior variety) or five (in the Arizona variety). The needles also have a distinctive smell that is dominated by either a turpentine or a citrus scent. In Colorado, these distinctively long needles are a key visual identification feature for ponderosa pines.
Photo by K. Casper, Public Domain
Ponderosa pines are very shade intolerant. This results in many ponderosa pine stands growing as even-aged cohorts without significant seedling regeneration. Competition by other, often more rapidly growing tree species, though, can result in the exclusion of ponderosa pines from a site.
Once past their very delicate early seedling growth period, ponderosa pine seedlings, saplings, pole trees and mature trees are quite resistant to drought. They have extensive root systems with deep, anchoring tap roots and dense, shallow fibrous root systems that extend up to 150 feet around each tree.
Ponderosa pines have a complex relationship with fire. Seedlings are rapidly killed by even low intensity wildfires, but larger, older trees are protected from all but the most intense fires by their thick bark. Further, most of the ponderosa pine’s competing tree species are more sensitive to fire than it is, so fire tends to favor the persistence of ponderosa pines in pure stands. Fire suppression, though, has caused many of these competing species (like Douglas-firs and true firs) to persist and eventually shade out the shade-intolerant ponderosa pines.
Photo by D. Sillman
Ponderosa pines are valuable timber trees that are cut and harvested throughout North America. Often the removal of ponderosa pines leaves behind understory trees (like Douglas-fir, true fir and lodgepole pine) which then become the dominant species of the recovery forest. Natural regeneration of a ponderosa pine stand is a very sporadic occurrence requiring a nearly perfect synchrony of events: there must be an abundance of seed from a heavy seed/cone crop of the previous year that is matched with a sufficient quantity of rainfall for the critical early seedling growth period in the three months immediately after the cessation of freezing temperatures.
In the Rocky Mountains of Colorado ponderosa pine grows in pure stands and also in mixed forests with Rocky Mountain Douglas fir (Pseudotsuga menziesii, var. glauca) , blue spruce Picea pungens), lodgepole pine (Pinus contorta) , limber pine (Pinus flexilis) and quaking aspen (Populus tremuloides).
Male cones. Photo by T. De Gomez, Univ Arizona, Bugwood.org
Ponderosa pine is monoecious (i.e. there is only one type of individual). It bears clusters of small (1/2 inch long), cylindrical, yellow-red to purple, pollen producing (“male”) cones at the bases of its new growth branches and clusters of even smaller (1/4 inch long), egg-shaped, red to bluish, ovulate (“female”) cones near the end tips of these same, new growth branches. Pollen is released in late May to mid-June. The female cones take more than a year after pollination to mature (in August or September of the following year). The cones open on the tree and shed seeds through November. Seeds are shed very close to the parental tree unless the cones are clipped off of their branches and carried off by tree squirrels. I have found ponderosa pine cones from the four trees in my yard some distance down the street and have seen fox squirrels carrying cones along the ground and over the backyard utility wires.
Photo by C. Light, Wikimedia Commons
Each female cone contains about 70 seeds. These seeds are small and are eaten by a variety of birds, insects, chipmunks and tree squirrels. Normal seed production occurs every 2 or 3 years and heavy seed years occur every 8 years or so. Insects (especially the ponderosa pine cone beetle (Conophthorus ponderosaei) and ponderosa pine cone cone-worm (Diaryctria spp.)) damage about 30 to 60% of the seeds in the seed cones. Trees can produce seeds from 7 through 350 years of age. Peak seed production years are between ages 60 and 160 years.
Rabbits and hares and gophers kill seedlings. Browsing by deer, sheep and cattle may stunt trees, and trampling by these larger consumers may also kill seedlings and saplings. Over 100 species of insects are known to attack ponderosa pines including the mountain pine beetle (MPB) (Dendroctonous ponderosae) ( which also attacks lodgepole pines and limber pines). In Colorado 3 to 4 million pine trees (including 70% of the state’s lodgepole pines) have been killed by MPB’s (see Signs of Spring 11, May 13, 2021). Air pollution (especially ozone) damages ponderosa pine needles. There are also many root and bark diseases that affect this tree. Dwarf mistletoe is a frequently damaging epiphytic parasite which causes a 36% mortality in ponderosa pines in northern Arizona.
Photo by D. Sillman
Ponderosa pines have been planted extensively throughout my residential neighborhood here in Greeley. In my relatively small yard, I have four, 50+ foot tall, 18 to 20 inch dbh trees. There are also two stumps from ponderosa pines of a similar size that had been growing very close to the front wall of the house. The yard must have been for many decades a dense, ponderosa pine forest!
All of the intact, yard trees produce abundant cones which are harvested energetically by the local fox squirrels. I have even seen chickarees (“pine squirrels”) up in the trees’ cone-laden branches. The high branches of the ponderosa pine are used as hunting perches and day roosts by red-tailed hawks and great horned owls (although not at the same time, of course!). These trees are a critical, ecological cornerstone of my small, suburban ecosystem.
The ponderosa pine is tree # 32 on the West Campus section of the Campus Arboretum of the nearby University of Northern Colorado. These pines can grow in a wide variety of habitats and under a broad range of conditions as long as their critical, early-seedling stages are carefully nurtured and watered. They are iconic trees of the American West and the mountains!