Limber pine (Pinus flexilis) is a five needle subalpine tree found in the mountains and foothills in the western United States. It is named for the flexibility of its branches that can literally be tied in a knot. It competes poorly with other tree species and typically grows on dry sites where few other trees can survive. Living in such harsh conditions, limber pines tend to grow very slowly. The limber pine closely resembles the higher elevation growing whitebark pine in appearance and habitat. Where their ranges overlap the only way to distinguish the two is through the differences in cones, however only limber pine occurs within Craters of the Moon.
In addition to the many stresses from the harsh conditions found in limber pine habitats, there are many new issues currently affecting the health of limber pines in the western United States which raise concern for conservation of this species. These issues include white pine blister rust, dwarf mistletoe and mountain pine beetles. The greatest threat to the limber pine is the introduced white pine blister rust which causes sporulating cankers resulting in the loss of cone production and eventual death of a tree. White pine blister rust has had the greatest affect in northwestern Montana where one third of limber pine stands are dead with the remaining 75% of the trees left infected.
At Craters of the Moon, limber pine is the dominant tree at the higher elevations above 5,500 feet where it grows in scattered open stands on both lava flows and cinder fields. The flexibility of the branches is especially beneficial here as it must withstand the high winds in the summer and heavy snowfall in the winter.
Limber pines provide cover and food for many wildlife species including many birds, mule deer and red squirrels. One bird, the Clark’s nutcracker, plays a significant role in the regeneration of limber pine. Each nutcracker gathers and caches thousands of the large high-fat, high-protein limber pine seeds in the ground each summer. The nutcracker does not recover all its cached seeds and some seeds left in the ground will germinate and become seedlings far from the parent tree.
The National Park Service (NPS) monitors the limber pine at Craters of the Moon to evaluate the ecological health of this iconic species. The NPS regularly surveys limber pine stands to detect mountain pine beetles, mistletoe, and white pine blister rust and determine how infection rates change over time. Dwarf mistletoe is a native parasitic plant common in limber pine growing in the area surrounding the loop road. When combined with drought and other stressors it can weaken and even kill trees. In 2006, white pine blister rust was found in the monument on a small number of limber pines for the first time. Since then, there have been no blister rust outbreaks in the limber pine stands. Mountain pine beetles are native insects causing widespread losses of whitebark and lodge pole pines in other areas of the western US but have only been observed in small numbers at Craters of the Moon.