FOREST HEALTH: Bark Beetles Add More Stress to BC’s Ponderosa Pine Ecosystems
by Janice Hodge, RPBio and Mike Fenger, RPF

Forest insects, diseases, and wildfires are natural disturbance agents that cycle and shape forest stand structure and composition. Their role has remained unchanged, however, the frequency, duration, and intensity of some of these disturbances has been modified due to a number of factors, one of which is climate change. In BC climate change is undoubtedly partially responsible for the unprecedented mountain pine beetle (MPB) outbreak. While much of the focus of this outbreak is on the lodgepole pine ecosystem and forestry dependent communities in the core of the outbreak, there are other issues or concerns that receive less attention. One such concern is the effect of this MPB outbreak on the ponderosa pine ecosystem. 

A little known or perhaps overlooked fact is that MPB successfully attacks other pines, including ponderosa pine. This type of pine grows along the dry valley bottoms and slopes of the southern interior forests of BC (see distribution map). Historically, these low density park-like forests experienced frequent stand-maintaining fires, approximately every 15-25 years. Fire was the main disturbance agent with occasional localized mortality of stressed trees caused by either western pine beetle (WPB), and/or MPB, and/or red turpentine beetle. Ponderosa pine is the most drought-resistant and fire-adapted conifer, which requires open conditions for seedling establishment. The heavy seeds from the large cones do not disperse far from the seed tree. Good cone crops come every 4 -5 years and seedlings need open conditions. Once established, ponderosa pine provides structure which is extremely valued by many species. 

In 2005, the provincial peak (red attack) year of the MPB outbreak, pockets of MPB-killed ponderosa pine were recorded in portions of the southern interior. By 2006 approximately 45,000 ha of MPB-killed ponderosa pine were recorded on the leading edge of the outbreak, and in 2007 the area infested almost doubled to 83,000 hectares. Modeling projections indicate peak MPB years of 2007 onwards for BC’s southern interior. As the MPB continues to spread southward from central BC, the likelihood of more ponderosa pine mortality over the next few years is high. In addition to the loss of habitat due to MPB, urban expansion, conversion of ponderosa pine forests to vineyards, and a shift from open grown ponderosa pine to higher density shade tolerant species such as Douglas-fir (due to fire exclusion) have placed forest-dependent wildlife at risk. 

Ponderosa pines live 300-400 years and begin to develop excellent critical habitat features at about 100 years of age. Physical attributes of both mature and old or dead ponderosa pine trees (wildlife trees) provide nesting, roosting, and feeding for 52 species that depend on them for part of their life cycle requirements. There are 13 species of birds of prey (owls and hawks), 10 species of woodpeckers, 8 species of bats, 11 other bird species, and 5 mammal species in the ponderosa pine zone that use ponderosa pine as well as Douglas-fir and riparian black cottonwood. There are several forest-dependent species at risk with the white-headed woodpecker most strongly restricted to mature and old growth ponderosa pine for nesting and a source of seeds. Though more elastic in tree selection, Lewis’s woodpecker, Flammulated owl, Western Screech Owl, and Williamson’s sapsucker are other listed species that use ponderosa pine as habitat. 

In the short term, leaving as many dead trees standing where safe to do so will mitigate the impacts for some wildlife species. There is no way, however, to create a supply of mature ponderosa pine in the mid- term to replace stands that will be lost because there are too few mid-seral pine stands. 

The longer view indicates there may be significant opportunity for improvement to BC’s ponderosa pine forest ecosystem. Climate change will create warmer, drier conditions to which ponderosa pine is well adapted. Climate envelope forecasts indicate that suitable conditions for ponderosa pine will be present in northern BC - Prince George and Fort St. John - 60 years hence. Given its resistance to drought, there is an opportunity for silviculturalists to expand the range of ponderosa pine by prescribing it to sites for which it is suitable. This may be a prudent strategy, which must take into consideration potential changes in the range and behaviour of biotic and abiotic agents resulting from climate change. Even though ponderosa pine is not valued as an economic species, it may well be the tree of choice to hedge for an uncertain future.