• Rocky Mountain Research Station
• 240 West Prospect
• Fort Collins, CO 80526
• (970) 498-1100

Life on the Edge for Limber Pine Seed Dispersal within a Peripheral Population

In the Rocky Mountains, limber pine ranges from southern Canada south to New Mexico and Arizona. As limber pine cones open in late August and early September, some of the seeds fall to the forest floor. At the same time, nutcrackers remove limber pine seeds from cones in trees or from detached cones and place seeds in their pouches for transport. While birds can transport the seeds as far as 22 km, most seeds are carried from a few metres to several kilometres. Once they have reached a site to store the seeds, the birds bury the seeds in soil from 1 to 3 cm deep in groups of 1 to 15 seeds. These sites are returned to for food.

In northern Colorado and southern Wyoming, limber pine ranges from sites at elevation 3,500 m at treeline to as low as 1,600 m on the eastern plans, where it forms small isolated populations on rocky escarpments. These limber pine populations on the plains occur nearly 100 km east of the closest lower-elevation limber pine populations. The environmental conditions on the plains are very different from those in the rest of the limber pine range. These isolated limber pine populations are genetically distinct from the populations of limber pine in the core of its range. The Pawnee population has been shown to have within it subpopulations that are genetically distinct.

The dependence of limber pine on Clark's nutcracker for seed dispersal is well documented from several geographic regions within the core range of limber pine. And nutrcrackers are rarely seen in the Pawnee National grassland. Scientists from the Rocky Mountain Research Station and the University of Colorado conducted a research study at Dave's Draw, a USDA Forest Service Research Natural Area within the Pawnee National Grassland in Colorado. They conducted an inventory of the vegetation, including all trees and all limber pine trees and seedlings. They studied seed caching by nocturnal rodents and trapped nocturnal rodents to identify potential seed-cachers.They conducted a simulated seed-caching experiment to determine whether any of the nocturnal rodent cache types had potential for limber pine recruitment and germination.

This study demonstrated that the principal seed dispersers for limber pine in this isolated population of limber pine are most likely to be nocturnal seed-caching rodents such as deer mice and Ord's kangaroo rats rather than nutcrackers. They showed that nocturnal rodents cache limber pine seeds. They determined the presence and identity of seed-caching nocturnal rodents in the study area of Dave's Draw. Further they showed that the dispersal distances for seeds transported by nocturnal rodents are short and thus could account for genetic substructure at Pawnee. They located cache types made by nocturnal rodents and showed that these caches are not always recovered by rodents -- thus allowing for the possibility of seed germination. Finally they demonstrated that some of the cache types might result in seed germination. They noted that the distribution of seedlings along the escarpment above cone-bearing trees indicates that gravity is not an important mechanism for seed movement. The dynamics of population change in this isolated population are different from those in the core range of limber pine and, withiout the long-distance seed dissemination processes within the core range, each escarpment isolated population may well expand and contract over time.

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