Pinus flexilis

Limber pine dominates on dry rocky sites at many elevations (1500-3600m) within its range. It can occur scattered throughout forested regions on more mesic sites, especially in low density, open areas. At higher elevations, Pinus flexilis can define the boundary of the treeline; occurring in high montane forests, often at the timberline (FNA 1993, Schoettle 2004). In these areas (i.e., Utah and the West) it is often very long-lived and slow growing, occurring on dry, harsh sites. In the northern half of its distribution, limber pine is generally found near lower tree line and on dry sites in the montane forests, but between the 45th and 40th parallels, it grows in both lower and upper elevation forests and anywhere in between on dry, windswept sites. Its position gradually shifts upward in more southerly latitudes, so that in southern portions of its distribution, limber pine is more common from upper montane to alpine tree line, with only minor occurrences in the lower forested zones (Burns and Honkala 1990). In some areas, limber pine grows in greater numbers on certain soils, but the relationships vary geographically; but in general, the substrates are Entisols (Burns and Honkala 1990). It grows on a variety of topographies, from gently rolling terrain to cliffs and is most often found on rocky ridges and steep rocky slopes and can survive in extremely windswept areas at both the lower and upper tree line (Burns and Honkala 1990).
Climatic data for actual limber pine habitat are quite scarce, but the general distribution of limber pine in Alberta, Montana, central Idaho, and east of the Continental Divide in Wyoming and Colorado, is in forested areas having a continental climate (Baker 1944). This climate is typified by a relatively small amount of precipitation, with the wettest months during the growing season, very low humidity, and wide annual and diurnal temperature ranges. Winter conditions may be very cold, but relatively dry, and often include rapid fluctuations in temperature associated with chinook winds. Notable exceptions to this distribution are the small populations in eastern Oregon and adjacent Idaho, which lie within the Pacific maritime influence (Baker 1944). In the remainder of its distribution, it grows in climates that tend to have either more evenly distributed yearly precipitation or a winter peak in precipitation along with summer convectional storms (strongly influenced by Pacific maritime weather patterns). Only at its southern limits in the mountains of eastern and southern California does the pine encounter a strong pattern of proportionately high winter precipitation (Baker 1944). The amount of precipitation, however, is relatively smaller than that of the Pacific Northwest.

Limber pine reproduces entirely from seed; it does not layer lower branches in the soil (Daly and Shankman 1985, Tomback 1991, Weisberg and Baker 1995). Seeds are not effectively dispersed by wind as small mammals and birds, especially Clark's nutcrackers and pinyon jays, disperse limber pine seeds (Woodmansee 1977, Lanner and Vander Wall 1980, Lanner 1985, 1996, Tomback 2001). The minimum seed-bearing age of limber pine ranges from 20 to 40 years. There are 2 to 4 years between large seed crops (Krochman and Krochman 1982, Burns and Honkala 1990). Seeds from krummholz trees have low germination potential (Lanner and Wall 1980). Clark's nutcrackers have evolved an important mutualism and are the primary harvester and disperser of its seeds.

Limber pine regeneration on burns is largely from germinants of Clark's nutcrackers seed caches (Woodmansee 1977, Lanner and Vander Wall 1980, Lanner 1985, 1996, Tomback 2001). The birds begin harvesting seeds in late August, while the cones are still green and slightly closed. They remove the cones by pecking them loose, fly them to perches, and peck between the scales to remove the seeds. As cones begin to open on the trees in September, Clark's nutcrackers remove exposed seeds. An individual bird can store as many as 125 seeds in its sublingual pouch, then flies to a cache area and deposits numerous caches from its pouchful of seeds. In a burned-over area in northern Utah, Clark's nutcrackers cached an estimated 12,140 seeds per acre (30,000/ha) in 1 year (Burns and Honkala 1990, Tomback and Linhart 1990).

Mating system: Limber pine seed dispersal by corvids leads to a genetic population structure different from that of wind-dispersed conifers with respect to patterns of gene flow and genetic relationships among neighboring trees. The seed caching by birds influences the distribution, population age structure, and spacing of limber pine. Clusters of seedlings germinating from a single cache may generate multi-stemmed growth forms that contain 2 or more distinct genotypes. A consequence of this growth form is a tendency toward clumped stand structure. Because seeds within an individual cache were often collected from a single parent tree, trees within clumps may be more closely related compared to trees from neighboring clumps (Lanner 1996, Tomback 2001), although multi-stemmed growth is most often a result of apical meristem damage that results in several leaders on an individual tree (Welsh et al. 1987). Tomback and Linhart (1990) found that on 361 limber pine sites in Colorado, 30% showed clumping. Several genetic studies have shown that from 0 to 82% of individuals within limber pine clumps are closely related (Welsh et al. 1987, Burns and Honkala 1990, van Wagtendonk et al. 1998). On the Pawnee National Grassland, clump members were related, on average, as nearly half-sibs. Genetic consequences of this kinship include possible inbreeding. On the plus side, closely related trees within clumps often form roots grafts, which may increase survivorship and fitness of the entire clump (Welsh et al. 1987).

Pollen phenology also influences gene flow. In Colorado, most sites that differ in elevation by more than 1,300 feet (400 m) in elevation do not have overlapping pollination periods, restricting pollination between populations that are widely separated by elevation; however, pollen transfer between intermediate populations and a high level of gene flow via bird-dispersed seeds appear to maintain interpopulation gene flow (Schuster et al. 1989).