Impact of mountain pine beetle
Impact of mountain pine beetle on stand and fuel dynamics in Kootenay National Park
Mountain pine beetle (Dendroctonus ponderosae Hopk., hereafter referred to as MPB) and fire are major natural disturbance agents for lodgepole pine ecosystems in western North America. 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 British Columbia and Alberta. 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 Kootenay National Park. Dr. Terry Shore (Canadian Forest Service, Pacific Forestry Centre), established semi-permanent plots within 6 stands in 1993 to assess the impact of a MPB attack on stand conditions. Five (5) of these stands were relocated and re-measured by PFC staff in October 2003. This provided a unique opportunity to examine the effects of the MPB epidemic on the park’s ecosystems.
Except for the last three years and two fires in 1917 and 1926, located in the Kootenay/Sinclair valley, area burned during the period 1928 to 2002 has been minimal. In 2001 and 2003, fires burned a total of approximately 19,000 hectares, although none of the burned area was in the 1990s outbreak area. The Canadian Forest Service’s Forest Insect and Disease Survey (FIDS) records show that large MPB outbreaks occurred in the 1890s, 1930s, and 1980s/90s. The Dog Lake stands originated from stand replacement fires between 1750-1850 according to a fire history study done in 1988. Lodgepole pine age was estimated by adding ten years to the age determined at the 1.37m coring height (referred to as breast height). Lodgepole pine averaged 108 years old for all five stands. Within these stands some of the lodgepole pines were found to be 110 to 120 years, as well as, others from 65 to 75 years old. The latter trees most likely established following the MPB outbreak in the 1930s. A few older veteran trees were found to be 200 years old, which indicates they most likely established following fires between 1750 and 1850. Other tree species such as Douglas-fir, white spruce, red alder and trembling aspen are present with lodgepole pine in the forest canopy. A stand origin map, developed by Alan Masters as part of his masters thesis at University of Calgary, indicates that Settler Road plots are 1917 and the dog lake plots are split between 1800 to the south and 1820.
MPB-induced mortality influences forest dynamics by thinning the forest from above. The volume and density changes after MPB attack, by tree diameter, indicated that MPB mortality occurred mainly in the larger diameter lodgepole pine. No trees less than 12.5cm dbh were killed by MPB by 1993. Mountain pine beetle was responsible for 69% of all trees killed. The MPB outbreak, ending in 1993, reduced pre-MPB stand lodgepole pine volume by 43%, and live stems by 29%. As lodgepole pine accounted for only 51% of the volume and 46.8% of the trees in the stand, the overall stand-level impact is considerably lower than in a pure lodgepole pine stand. Mountain pine beetle induced mortality reduced stand volume by 21.9% and live stems by 13.6% for all tree species in the stand. Significant variation in MPB’s impact on stand volume and tree density was observed among different stands due to differences in stand structure.
In 2003, 23.7% of the trees that had been standing in 1993 had fallen down. Most trees that had fallen down were killed in the MPB epidemic (75.5%) but a significant component of these fallen trees had been alive at the end of the epidemic (24.5%) indicating further MPB mortality between 1993 and 2003. An increase in the growth rates of the surviving trees resulted in live volume increasing from 209 m3/ha in 1993 to 228 m3/ha in 2003. From 1993 to 2003, standing dead volume (snags) was reduced by 35% due to fall down of dead trees. This result indicates that in 2003 there is a significant number of dead trees still standing that were killed prior to 1993.
Live lodgepole pine density declined by 31% from 1993 to 2003 (219 stems per ha to 151 stems per ha) as a result of snag fall down and additional MPB and other mortality. Live tree density, for all tree species, declined by 16% from 1993 to 2003 (657 stems per ha to 554 stems per ha). Lodgepole pine was the dominant tree species prior to the MPB outbreak, accounting for 47% of the live stems. In 1993, lodgepole pine accounted for 33% of the live stems. In 2003, due to additional mortality from 1993 to 2003, lodgepole pine accounted for only 27% of the live stems.
When non-host tree species are present in the stand canopy, MPB induced mortality of lodgepole pine can advance forest succession towards more long-lived and shade tolerant species such as spruce, Douglas-fir, subalpine fir. These tree species are capable of releasing seed from open cones, which then germinate on the undisturbed forest floor, and within the partial sunlight created by canopy gaps, can successfully establish and grow, eventually reaching the canopy. Non-host conifer tree species that exist with lodgepole pine in the main forest canopy were mainly white spruce, Douglas-fir, and trembling aspen. For all tree species, live tree density, prior to the MPB outbreak was 833 stems per ha, falling to 554 stems per ha, in 2003. In 1993, the proportion of stand density contributed by white spruce, Douglas-fir, and aspen was 32%, 12%, and 6.7%, respectively. In 2003, white spruce, Douglas-fir, and aspen contribution to stand density increased to 50%, 14%, and 8.5%, respectively.
We were unable to assess changes in sapling and seedling density and species proportions from 1993 to 2003 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 2003, pole-sized (greater than 1.5 m in height, but less than 7.5 cm at breast height) tree density averaged 592 stems per ha. Spruce accounted for more than half of this size class (311 stems per ha), with aspen and Douglas-fir accounting for most of the remainder (129 and 125 stems per ha, respectively). Very few pole-sized lodgepole pine were found in the remeasured plots (14 stems per ha). Alder, birch, subalpine fir and larch accounted for the remaining few stems per ha recorded. Seedling (less than 1.5m height) density averaged 1106 stems per ha. Spruce (536 stems per ha) and Douglas-fir (510 stems per ha) account for the vast majority of seedlings, with small amounts of lodgepole pine (46 stems per ha) and subalpine fir (13 stems per ha). Very little regeneration existed under 10cm in height (fewer than 60 stems per ha, all spruce and Douglas-fir). The low numbers 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 recent large-scale fire disturbance may not be conducive to lodgepole pine regenerating in the understory.
In 2003, surface woody fuel loading averaged 3.5 tons per ha and 44 tons per ha for fine (≤7cm diameter) and coarse (>7cm diameter) fuels, respectively. Coarse woody fuel loading included 35% of the standing dead tree volume recorded in 1993 due to snag attrition between 1993 and 2003. If coarse woody fuel loading would have been measured in 1993, it would have been much lower than that estimated for 2003 since the previous MPB outbreak was in the 1930s to early 1940s (based on FIDS records, 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. The separation of live tree crowns by dead trees and the openings created by dead trees falling over has been shown to reduce crowning potential, although the increased surface loading from the downed trees could potentially increase surface fire intensity, thereby increasing the probability of crowning of the remaining live trees. No significant wildfires have occurred in areas affected by MPB during the outbreak in 1993.
Authors: Brad Hawkes, Chris Stockdale, Terry Shore, George Dalrymple, Leo Unger and Steve Taylor
Canadian Forest Service, Pacific Forestry Centre, Victoria, BC
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