Impact of mountain pine beetle

Impact of Mountain Pine Beetle on Stand and Fuel Dynamics in Waterton Lakes National Park

Executive summary

Mountain pine beetle (Dendroctonus ponderosae Hopk.), hereafter referred to as MPB, and fire are major natural disturbance agents for lodgepole pine ecosystems in Waterton Lakes National Park.

A three-year research project, funded by Forest Innovation Investment British Columbia (FII), was established in 2001 by fire and bark beetle researchers at the Pacific Forestry Centre (PFC), Canadian Forest Service, Victoria, BC to determine the impact of MPB on stand and ecosystem dynamics across a range of biogeoclimatic zones, stand conditions, and fire regimes in BC and AB. In 2003, additional funding was provided by the Government of Canada through the Mountain Pine Beetle Initiative (MPBi), a program administered by Natural Resources Canada, Canadian Forest Service to complete the research in Waterton Lakes National Park (WLNP). The BC-wide MPB outbreak of the early 1980’s spilled over the Rocky Mountains, affecting several locations in AB, including WLNP. This provided a unique opportunity to examine the effects of the MPB epidemic on the park’s ecosystems. Dr. Ben Moody, formerly with the Canadian Forest Service, Northern Forestry Centre, established semi-permanent plots within 5 stands in 1981, with re-measurements until 1983, to assess the impact of a MPB attack on stand conditions. These stands were relocated and re-measured by PFC staff in October 2002.

MPB-induced mortality influences forest dynamics by “thinning the forest from above” (killing the larger diameter lodgepole pine). Most of the lower-elevation lodgepole pine stands in WLNP, date from stand-replacement fires during the period 1875 to 1896 as a single seral component. These stands, in terms of their age, basal area, and density in 1977, were highly susceptible to MPB. The most recent MPB outbreak in WLNP started in 1977 and lasted until 1983. The difference between the oldest and youngest lodgepole pine trees within each stand varied from 29 to 86 years, indicating a long regeneration period for lodgepole pine after fire, as well as, the possibility of a limited number of low to moderate intensity surface fires occurring after the last stand-replacement fire, that would release some additional seed from serotinous cones, as well as, provide a seedbed for lodgepole pine establishment. Non-beetle-host conifer tree species that exist with lodgepole pine in the main forest canopy were mainly Douglas-fir, white spruce, and subalpine fir. The non-host tree species were found to be younger on average than the lodgepole pine indicating that these tree species established after lodgepole pine.

Rob Watt (Waterton Lakes National Park senior warden) conducted an assessment of lodgepole pine mortality in 1981, reporting an average mortality of 49.5% (range 14.5%- 76.0%) across a wider range of stands than were remeasured. The volume and density results, by tree diameter, indicated that MPB mortality occurred mainly in the larger diameter lodgepole pine. Remeasurements in 2002 confirmed that mountain pine beetle caused significant mortality in all tree size classes, while Ips spp beetle was restricted to the smaller size classes, specifically the 15-25cm trees. Mountain pine beetle was responsible for 94% of all trees killed. The MPB outbreak, ending in 1983, resulted in an average 67% volume loss, and 46.7% loss of stems, although there was significant variation due to differences in stand structure.

In 2002, 52% of the trees that had been standing in 1983 had fallen down. Most trees that had fallen down were killed in the MPB epidemic (80%) but a significant component of these fallen trees had been alive at the end of the epidemic (20%) indicating further mortality between 1983 and 2002. Although surviving trees demonstrated an increase in growth rates from1983 to 2002 (reduced competition for above- and below-ground resources) live volume remained the same as in 1983, (1983 95.3 m³/ha vs. 2002 94.4 m³/ha: statistically insignificant) as the fall-down of beetle killed trees caused many living trees to be knocked down or created opportunities for other forest pathogens to kill additional trees. From 1983 to 2002, standing dead volume (snags) was reduced on average by 70% due to fall down of dead trees. This indicates that in 2002 there is a significant number of dead trees still standing that were killed prior to 1983 separating the live tree crowns. This separation of live tree crowns by dead trees and the openings created by trees that have fallen over has been shown in Yellowstone National Park to reduce crowning potential, although the increased surface loading from the downed trees could potentially increase surface fire intensity increasing the probability of crowning the remaining live trees.

Live pine density continued to decline by 33% (527 to 354 stems per ha) as a result of fall down and additional mortality. Some of the reduction in pine density was offset by the increased growth rates observed in other tree species. Lodgepole pine was the dominant species prior to the MPB epidemic, accounting for 89% of the live stems. At the end of the epidemic (1983), lodgepole pine had declined in dominance to 73%. The additional mortality of pine in the subsequent 19-year period (to 2002) further reduced pine to only 59% dominance. Other tree species in 2002 were white spruce (15%), subapine fir (10%), Douglas-fir (8%), balsam poplar (5%) and trembling aspen (2%). Live tree density was drastically reduced over the period 1980 to 2002, falling from 1089 to 520 stems per ha, with an increase in the dominance of non-host species. In 1980, non-host trees accounted for only 11% of trees, increasing to 41% in 2002. There were no non-host species in the smallest size class of overstory trees (10cm) immediately prior to the epidemic, but in 2002 they account for nearly half of these smaller trees, likely the result of rapid growth in the advanced regeneration component of the pre-disturbance stand. In 2002, non-host tree species dominated the larger diameter size classes (> 27.5 cm).

We were unable to assess changes in sapling and seedling density and species proportions from 1983 to 2002 as regeneration was not originally sampled. A detailed stem analysis involving destructive sampling to infer prior regeneration levels was not desirable to maintain the integrity of the permanent plots. In 2002, pole-sized (greater than 1.5 m in height, but less than 7.5 cm at breast height) tree density averaged only 170 stems per ha, while seedling density averaged 2261 stems per ha. Lodgepole pine, subalpine fir and aspen were the most common pole-sized species. Subalpine fir dominated seedling composition under 0.5 m in height with an average density of 1280 seedlings per ha. Douglas-fir was the next most common seedling species averaging 662 stems per ha, of which half were less than 10cm in height. Subalpine fir and Douglas-fir are shade tolerant and can successfully regenerate and grow under a tree canopy. There were no lodgepole pine seedlings found in any of the stands sampled, although a few were observed during hikes in the surrounding areas. The lack of lodgepole pine seedlings may be due to the dominance of closed cones because stand replacement fires are the most common type of fire disturbance; the absence of a suitable seedbed without fire disturbance, as well as, climatic conditions that may not be conducive to lodgepole pine regenerating in the understory. The low sapling and moderate seedling densities, as well as, the lack of lodgepole pine saplings and seedlings, indicates that the filling in of canopy gaps (created by MPB induced mortality of dominate and co-dominate lodgepole pine) with non-host species will take longer than if there were more saplings available for release.

In 2002, surface woody fuel loading averaged 6.8 tons per ha and 42 tons per ha for fine (≤7cm diameter) and coarse (>7cm diameter) fuels, respectively. Coarse woody fuel loading included 77% of the standing dead tree volume recorded in 1983 due to snag attrition between 1983 and 2002. If coarse woody fuel loading would have been measured in 1983, it would have been much lower than that estimated for 2002 since the previous MPB outbreak to 1977-1983 was probably the 1930s to early 1940s (based on preliminary analysis by Dr. Rene Alfaro, Canadian Forest Service, Pacific Forestry Centre, Victoria, BC) such that most of the fallen dead trees from those decades would not have remained on the forest floor surface due to 40-50 years of decomposition. No significant wildfires have occurred in mountain pine beetle affected areas in Waterton Lakes National Park since the collapse of the outbreak in 1983.

Executive Summary taken from draft internal Canadian Forest Service File Report, dated July 31, 2004
Authors: Brad Hawkes, Chris Stockdale, George Dalrymple, Leo Unger and Steve Taylor
Canadian Forest Service, Pacific Forestry Centre, Victoria, BC