Pinus flexilis James
Limber pine is long-lived, slow-growing, high-elevation tree. Small to medium in size, it often grows on rocky ridges and its form is often irregular. At the lower tree line, it may form either pure stands or mixtures with other conifers. Being relatively shade-intolerant, it is mostly a pioneer species, and maintains its presence only on very severe sites. It provides valuable cover, shelter and nesting opportunities. The large seeds are a nutritious food source for birds, rodents, bears and people. It is useful for reforestation of avalanche paths. It can withstand very windy and dry environments quite well. Its aesthetic value is high (Burns and Honkala 1990; Klinka et al. 2000). White pine blister rust (Cronartium ribicola) is a serious concern for limber pine in Alberta, B.C., and the northern part of Montana. In this area, over 33% of the limber pines are dead and about 75% of the remaining live trees are infected. Further south, the concern gradually lessens, and the disease is largely absent in the southern half of the species’ range (Kendall et al. 1996; Kendall and Schirokauer 1997; Kendall 2001).
Limber pine grows in the Cordilleran, from Alberta and southeastern British Columbia to New Mexico, Arizona, and eastern California. Since it grows at higher altitudes, its distribution is disjunct. Only a small part of the species’ distribution is in B.C. (Little 1971; Burns and Honkala 1990).
Distribution and Protected Areas – from Hamann et.al. 2005
Conservation Status Summary – from Chourmouzis et.al. 2009
“Limber pine has a very limited distribution in British Columbia. The northern edge of its western North American range extends into the southeastern portion of the province, where it is very sparsely distributed in the ESSF, MS, and IDF zones. It is under-protected in all three zones. Field verification is recommended in the IDF zone and in large protected areas that span at least two zones: the IDF/MS zones and the MS/ESSF zones. Considering that there are serious insect and fungal threats to this species, it would be prudent to increase ex situ conservation and implement active management efforts for this species.”
Limber pine produces large seed crops every two to four years. Most of the seeds are wingless and rely on rodents and birds for dispersal. The Clark’s nutcracker (Nucifraga columbiana) caches the seeds in places where little snow accumulates, so their food can be easily accessed. This bird is the most important dispersal agent and may influence the location of limber pine stands more than the pine itself (Burns and Honkala 1990). Trees growing from cached seeds form clusters of related individuals (Carsey and Tomback 1994). The influence of squirrels on seed dispersal was inferred from their influence on the cone and seed traits of limber pine (Benkman 1995).
Several studies have investigated population structure in limber pine. Schuster and Mitton (2000) studied an isolated population in Colorado using 28 allozyme loci. Analysis of needle tissues provided the following estimates for genetic diversity: the average number of alleles per locus (A) was 2.36; the percentage of polymorphic loci (P) was 50%; the expected number of heterozygotes (He) was 0.159. Latta and Mitton (1997) examined genetic differentiation among seven populations in Colorado, using four types of gene marker: mtDNA, cpDNA, allozymes and RAPDs. Both mtDNA and RAPDs indicated a historical division between populations, corresponding to the continental divide, rather than genetic variation associated with an environmental gradient. Four of the nine RAPD loci were significantly more differentiated than allozymes. This difference could not fully be explained by a historical division, and these four RAPD loci appear to be under natural selection. Mitton et al. (2000a) used maternally-inherited mitochondrial DNA to infer the number of glacial refugia for the species’ range. Population differentiation for this marker was large (FST =0.80). They inferred seven glacial refugia, where either existing haplotypes became fixed or new ones originated via mutation. One surprising result was that of the two populations sampled from Alberta, the northernmost contains a unique haplotype and the other a common one (Mitton et al. 2000a). No B.C. populations were included in the study. Constructing phylogeographies based on mtDNA to distinguish between historical scenarios has been difficult because there are so few mitochondrial markers available. However, seven new primers, developed to amplify the b/c intron of the mitochondrial nad-I gene, allow amplification of smaller fragments than before. These fragments can be distinguished on agarose gels, which will make such projects both cheaper and easier in the future (Mitton et al. 2000b). Several studies have also investigated the amount of gene flow. Schuster et al. (1989) studied 8 populations of limber pine across an elevational transect in Colorado. Most sites differing more than 400 m in elevation did not have overlapping reproductive phenology. Ten isozymes were analyzed for the lowest and highest-elevation population. Allele frequencies differed significantly and both populations had ‘unique’ alleles. Population differentiation, estimated by FST =0.022, was low, indicating a high rate of gene flow. They proposed a stepping-stone pollen transfer model between intermediate populations. The resulting estimates for gene flow via pollen correspond to those estimated by paternity analysis (Schuster and Mitton 2000), which estimated mean pollen dispersal distance at 140 m. Latta and Mitton (1997) found that, for seven populations in Colorado, maternally inherited chloroplast DNA was much more differentiated than paternally inherited mitochondrial DNA, indicating that gene flow via pollen is much larger than gene flow via seeds.
Resource management and seed transfer
The primary challenge for genetic resource management of limber pine is white pine blister rust in the northern portion of the range. Rust resistance has been found and bred for in western white pine (P. monticola). A possible management option for limber pine would be to introducing resistance through hybridizing with these rust-resistant western white pine individuals. However, crossability barriers between the species (e.g., Kriebel 1972; Quijada et al. 1997; Bingham 1972) remain a problem. Introducing an exotic genome would also present more risks than identifying resistant individuals within the native species. The latter approach seems preferable, but no efforts have been made to date, likely because of the limited commercial importance of the species. Schoettle and Rochelle (2000) found similar growth for limber pines growing over a range of elevations (1600 to 3300 m), indicating plasticity with regard to the factors changing with elevation. Such plasticity would facilitate the deployment of any rust resistant trees in the field.