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Pinus flexilis

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Taxonomy [top]

Kingdom Phylum Class Order Family
PLANTAE TRACHEOPHYTA PINOPSIDA PINALES PINACEAE

Scientific Name: Pinus flexilis
Species Authority: E.James
Infra-specific Taxa Assessed:
Common Name(s):
English Limber Pine, Rocky Mountain White Pine
Synonym(s):
Pinus reflexa (Engelm.) Engelm.
Taxonomic Notes: Two varieties are recognized. The typical variety is widespread in the Rocky Mountains. Pinus flexilis var. reflexa Engelm. is restricted to a few localities in northern Mexico and southwestern USA, and is assessed separately.

Assessment Information [top]

Red List Category & Criteria: Least Concern ver 3.1
Year Published: 2013
Date Assessed: 2011-11-17
Assessor(s): Schoettle, A. & Stritch, L.
Reviewer(s): Thomas, P. & Farjon, A.
Justification:
The assessment of the species as Least Concern is largely determined by the assessment of the much more numerous and widespread var. flexilis :

Pinus flexilis
var. flexilisThere is no question that Limber Pine is experiencing declines due to White Pine Blister Rust (WPBR) and Mountain Pine Beetle (MPB). In areas where WPBR and MPB are both present the decline in population numbers and population resiliency is such that population sustainability over the long-term is predicted to decrease. Work is being conducted to identify seed trees that exhibit some degree of resistance to WPBR. This work is being complicated where populations are also under attack from MPB. Research needs to be initiated into the affects of WPBR and MPB on the mutualism between corvids and Limber Pine in regard to seed dispersal. However, there have been no wide-scale extirpations of Limber Pine populations and the range of the species has not contracted since the last assessment. Therefore, Limber Pine is still evaluated to be of Least Concern. It is recommended that this variety and species be re-evaluated by the Conifer Specialist Group periodically.
Pinus flexilis var.reflexa This is a rare variety limited to five known locations and assessed as Near Threatened (almost qualifies as threatened under criterion B2ab(iii,v)).

Geographic Range [top]

Range Description: Recorded from western North America: Rocky Mountains from Alberta and British Columbia (Canada) to New Mexico and Texas and some isolated localities to the east, mountains of SE California, Nevada, N Arizona, Utah (USA); and extending into northern Mexico in scattered localities in Chihuahua, NE Sonora, Coahuila and Nuevo León. Both the extent of occurrence and the area of occupancy are well in excess of the threatened thresholds.
Countries:
Native:
Canada (Alberta, British Columbia); Mexico (Chihuahua, Coahuila, Nuevo León, Sonora); United States (Arizona, California, Nevada, New Mexico, South Dakota, Texas, Utah)
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: The global population is decreasing.
Pinus flexilis var. flexilis

Populations of Limber Pine occurring in the Great Plains in the states of Colorado, North Dakota, South Dakota, Montana and Nebraska are disjunct and therefore have no gene flow outside their individual population occurrences. These populations are stable except for the population in South Dakota that has been infected by White Pine Blister Rust (WPBR) (Cronartium ribicola). However, these populations are at risk from climate change or some other stochastic event. North Dakota, South Dakota and Nebraska rank their populations as state imperiled (S1) under the conservation ranking system of NatureServe.

From 1985 -1995 a significant drought in California led to high mortality of high-elevation conifers especially Limber Pine. As White Pine Blister Rust moves southward in the Rocky Mountains scientists predict the decline of high-elevation white pines including Limber Pine.  In the southern and central Rocky Mountains populations of Limber Pine and other conifers have and are continuing to die from a massive infestation by mountain pine beetles (MPB) (Dendroctonus ponderosae). Additionally, dwarf mistletoe (DM) (Arceuthobium cyanocarpum) has been noted as causing increased mortality of Limber Pine in some states in the Rocky Mountains and a fungus (Ophiostoma sp.) carried by mountain pine beetles is infecting many populations of Limber Pine.  In Montana, red band needle blight (RBNB) (Dothistroma septospora) has and is causing significant mortality. WPBR, MPB, DM, RBNB and climate change are leading to more isolated populations, adversely affecting gene flow and ultimately genetic diversity. Cumulatively these factors are causing individual Limber Pines and small populations to undergo increased physiological stress which allows for other biotic and abiotic factors that in isolation would not lead to increased mortality to do so. Where WPBR, MPB, and DM are present we are observing isolated cases of population extirpations and an overall decline in Limber Pine populations.

Pinus flexilis var. reflexa
This taxon is much more scattered than var. flexilis which occurs to the north of it, and is restricted to the highest altitudes of the mountains in northern Mexico and the southwest of the USA. Populations are accordingly disjunct and often small
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology:

Pinus flexilis is a conifer tree of the subalpine zone in the Rocky Mountains and the 'Basin and Range' region further west. In many respects its habitat resembles that of Pinus albicaulis, which has a more westerly and northerly distribution. Some isolated populations, here recognized as var. reflexa, occur on high peaks in northern Mexico. In the Black Hills of North Dakota P. flexilis may grow as low as 900 m a.s.l., in the southern Rocky Mountains it can occur at 3,800 m. This species is one of several in the genus Pinus of western North America that can withstand extreme conditions of climate on bare rock or scree without any other vegetation cover. On these sites it occurs either alone or with Pinus albicaulis and Abies lasiocarpa in the northern parts of its range and with Pinus aristata in the SE and P. longaeva in the SW. At lower altitudes it is usually only a minor component of more diverse conifer forest in the south and with Picea engelmannii, Pinus contorta, and Pseudotsuga menziesii in the north. The seeds, which only have rudimentary wings, are mostly dispersed by rodents and birds, of the latter Clark's Nutcracker (Nucifraga columbiana, Corvidae) is the most important vector.

Pinus flexilis var reflexa is limited to subalpine habitats on the highest mountains within its limited range.

Systems: Terrestrial

Use and Trade [top]

Use and Trade: The wood of Limber Pine is not of high quality.  As a timber tree the species is of minor importance due to its occurrence at high altitudes, where it is generally inaccessible and most of the trees do not grow into straight boles. Planted trees have a tendency to become multi-stemmed from a short base and grow rather slowly. Locally, it was heavily used for railroad ties and mine shaft construction in 'pioneer times', but more recently its main use has been as firewood. The species was introduced to Britain in 1851, but has remained rare in gardens there and elsewhere in Europe. In the USA it is more common and several cultivars, most of which have glaucous blue needles and more compact habits, are being grown and are in the horticultural trade, especially Pinus flexilis ‘Vanderwolf’s Pyramid’.
No uses have been recorded for Pinus flexilis var. reflexa. It is probably too rare and remote for it to be of commercial value.

Threats [top]

Major Threat(s):

Pinus flexilis var. flexilis

Limber Pine is highly susceptible to infection and death from the disease white pine blister rust caused by the non-native pathogen Cronartium ribicola.   Limber Pine populations in the northern US Rocky Mountains have been infected by WPBR for over 50 years and mortality is high in many sites.  The disease has continued to spread south and now infects Limber Pine in MT, ID, WY, CO, SD, NM and CA with heavily infected stands in each of these states. New locations of infected stands are being found yearly.  In 2003, Limber Pine in the drainages of the Great Sand Dunes National Park and Preserve were found to be heavily infected with WPBR; this infection centre is over 200 km from the nearest known source of inocula and long distance transport of WPBR spores from California is suspected as the initial source.  The disease was confirmed on Limber Pine in Rocky Mountain National Park in 2010 but infection is still light. Risk analyses suggest that 50% of the 5-needle pine stands in Colorado have average climate conditions that will support the spread of the disease; the remaining habitats are likely to intermittently have years of suitable conditions for the disease.  It is unlikely that extensive Limber Pine forests will escape WPBR infection over time.

Limber Pine has not evolved in the presence of C. ribicola; as a result it only has very low frequencies of resistance to the disease. WPBR kills Limber Pine trees of all ages and young trees are especially susceptible. Even before the disease kills the larger trees it kills the cone bearing branches and severely restricts seed production. WPBR threatens the sustainability of Limber pine populations by compromising the regeneration cycle with high mortality of all aged trees, reduced seed production of the mature trees and high susceptibility of young seedlings and saplings.  These impacts are being seen in many Limber Pine forests now and are expected to expand to other landscapes as the disease continues to spread. Until natural selection results in an increase in durable WPBR resistance, ecosystems impacts will continue. 

Initial modeling suggests that the slow regeneration time, delayed maturation (>50 years for a tree to become reproductive and over 100 years before producing large cone crops), and low initial frequency of heritable resistance in Limber pine will lead to high mortality in native populations for centuries to come.  While this mortality is selection against susceptible individuals, it is likely to reduce populations to below a sustainable threshold. Limber Pine is largely dependent on corvid species to disperse its seeds and studies suggest that once populations are impacted to the point that seed production is very low, the mutualism with the corvids may fail, as the birds do not visit such stands.  Extirpation of populations is likely to occur which will lead to further genetic isolation of remaining populations.

The recent epidemic of Mountain Pine Beetle (MPB) is further threatening the sustainability of Limber Pine populations. Limber Pine is an excellent host for MPB and larger beetle broods are often produced on Limber Pine compared to other pine hosts. MPB generally only attack larger trees (those above 10 cm d.b.h.) and do not attack the smaller advanced regeneration trees.  However, MPB further reduces the seed production of the population and in the presence of WPBR the rust will continue to kill the susceptible seedlings that are able to get established. While MPB epidemics are not unprecedented in Limber Pine forests, the current epidemic is more extensive than in the past and the presence of WPBR severely compromises the recovery capacity of the populations after the MPB epidemic passes. 

In areas of high WPBR incidence, MPB is killing the remaining live reproductive trees which are often those that possess some genetic resistance to the disease. This is setting back the natural selection for resistance in Limber Pine that has already occurred. In those populations that have more recently been invaded by C. ribicola, MPB is killing the seedtrees from which seeds must be collected to test for resistance, slowing the identification of resistance trees and estimates of resistance in populations. MPB in all locations is affecting the availability of seed for collections for restoration plantings.

The extirpation of populations in the high mountain and tree line habitats will likely transition those lands from forests to non-forested landscapes. Limber Pine is often the only tree species that can tolerate those harsh habitats and therefore there will be no replacement tree species to occupy the site. Loss of the trees will also affect snow accumulation and snow melt on the wind-exposed high elevation sites.

WPBR alone is a serious threat to Limber pine populations and will continue to threaten the species for centuries.  The combination of WPBR with the MPB epidemic compromises the regeneration cycle of the pine populations further and reduces the populations’ resiliency to recover from this and any other disturbance.  Extirpation of populations is probable and imminent.

Pinus flexilis var reflexa
WPB does not seem to have reached these isolated southern populations.

Conservation Actions [top]

Conservation Actions: Pinus flexilis var. flexilis.

The decline in population numbers during the naturalization process of Cronartium ribicola is unavoidable yet management can facilitate the recovery from that dip in population numbers.  Increasing the frequency of genetic resistance in the populations is the foundation for recovery.  This can be achieved through combinations of (1) supporting the regeneration cycle by protecting seed trees and stimulating natural regeneration in populations that have some resistance to increase population numbers and promote selection for resistance and/or (2) identifying resistant seed trees, collecting seeds and planting seedlings from those trees.  In areas that are heavily impacted already by WPBR, the seed source is likely to be compromised to the point that out planting resistant seedlings is the only option.  In areas that have not been impacted as heavily (more southern populations), proactive silvicultural treatments to stimulate regeneration by removing competing vegetation or site preparation can be viable options in addition to planting resistant seedlings.  A proactive strategy has been developed for Limber pine and management guides for Limber Pine have been developed for the USFS Rocky Mountain (R2) and Southwestern (R3) Regions and BLM lands in Wyoming. Rocky Mountain National Park is currently developing a management plan to sustain and conserve Limber pine.  To fully implement these plans more work is need to develop appropriate silvicultural prescriptions and identify resistant seed sources.

Low frequencies of genetic resistances to WPBR exist in Limber Pine, however, the geographic distribution of these resistance mechanisms is still not known.  Substantial rust screening efforts for Limber pine began in 2006.  Only populations and ‘families’ from the Southern Rocky Mountains have been screened for resistance and some seed trees with heritable resistance have been identified (even in populations not yet challenged by the rust) and are now being protected from MPB attack with chemical spraying or anti-aggregate pheromone treatment. More resistant seed trees are needed to build seed sources for out planting with sufficient genetic diversity.  Initial studies reveal geographic variation in resistance in the southern Rockies and suggest it varies along environmental gradients. The first tests for several ‘families’ from California and Montana have now begun. Rust screenings of seed trees from range-wide populations are needed as well as research on the mechanisms of resistance to assess their durability and interactions with climatic factors. Tests for the association of resistance traits with other adaptive traits could affect plant performance in a changing climate and needs further work.  Seed transfer guidelines have been developed for Limber pine in the Interior West but are based on inadequate data. Range-wide gene ecology studies are needed to avoid planting errors that will lead to maladaption and failure now and in the future for this long-lived species. This research has begun but needs to be accelerated.

Limber Pine seeds for gene conservation, rust resistance and research have been collected from some populations the Southern Rockies yet still more are needed in this portion as well as the rest of the range.  Restoration seed collections are needed range-wide.  Because current technologies to screen for rust resistance require progeny testing, only seed-producing trees can be tested for rust resistance.  Therefore seeds should be collected from seed trees and populations before MPB kills the mature trees; if not, identifying rust resistant trees from these populations will be delayed for 30-50 years while the advanced regeneration that escapes MPB attack becomes seed producing.  

Some basic information on the regeneration dynamics, fire effects and climate relationships for Limber Pine are still not known.  Little attention was directed to this species in the past yet it is now receiving some research and management focus, though much more work is needed to develop effective prescriptions and to prioritize areas for intervention.   Proactive work has begun and shows promise to mitigate the development of impacts upon invasion of the rust in populations that are still healthy.  Restoration of the heavily impacted stands is more challenging and the outcome is less certain.  Early intervention before the population is reduced and the regeneration cycle is compromised requires a shift in management approach from one of managing under crisis to managing for sustained resiliency – Limber Pine is a good example of where this shift could make a large difference in the future of a species.  The consequences of inaction in the presence of the novel stresses confronting this species are population declines and cascading ecosystem impacts.

Citation: Schoettle, A. & Stritch, L. 2013. Pinus flexilis. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 29 July 2014.
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