Forest health challenges in the Rocky Mountain West

Background/Question/Methods

The Rocky Mountains are notably biodiverse, in large part because of their expansive and varied forest communities. In Rocky Mountain Futures, we ambitiously assessed emerging threats to all biodiversity, plants and animals alike, including forest communities. For forest communities, we described the effects of exotic plants, pests, and disease, and especially the invasive pathogen Cronartium ribicola which causes white pine blister rust; fire suppression; and, historical outbreaks of native insects, such as mountain pine beetle (Dendroctonus ponderosae). We repeated predictions that a century of fire exclusion and logging had created conditions eminently suited to large-scale pine beetle outbreaks. And lastly, we pointed out that global warming would provide additional challenges. Little did we know that in the decade to come warming trends, fire, beetles, and invasive disease would synergistically interact to provide serious challenges to the long-term health and biodiversity of Rocky Mountain forests.

Results/Conclusions

During the past decade, general forest health has declined precipitously in the Rocky Mountains, with major changes in forest structure and historical disturbance patterns. Warming trends and drought have increased rates of tree mortality in general, caused massive tree die-offs in the southern Rocky Mountains, and fostered upsurges of western bark beetles including unprecedented outbreaks of mountain pine beetles. Reduced snowpack and changes in the timing of snow melt have altered local hydrology and water availability, leading to drought stress in many forest communities. Furthermore, warming trends and drier conditions are generating fire regimes characterized by regionally-synchronized, high-intensity fires. Invasive plants, pests, and diseases have further altered forest community structure; and, distributional shifts in response to climate warming are predicted for all forest trees. Whitebark pine (Pinus albicaulis), an ecologically important component of high-elevation forests in the central and northern Rocky Mountains, illustrates how several of these factors may interact and significantly alter forest communities and ecosystem function. Whitebark pine has experienced widespread declines from white pine blister rust, recent and historical mountain pine beetle outbreaks, and fire suppression practices. The loss of functional whitebark pine communities not only reduces forest diversity, it alters local stream flows and impacts the numerous animals that feed on whitebark pine seeds, including Clark’s nutcrackers and grizzly bears, or live in whitebark pine forests. In 2011, whitebark pine was designated a candidate species for listing as Endangered or Threatened by the U.S. Fish and Wildlife Service.