A leader temperature based hazard rating system for the white pine weevil under present and climate change conditions

• Main Author: Sieben, Brian G.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/12315

The white pine weevil, Pissodes strobi Peck, is a pest of young spruce, plantations in British Columbia which seriously hampers forest regeneration. Weevils kill year old spruce leaders resulting in growth reductions and poor stem form. As accumulated heat is required for completion of the weevil's life cycle, a degree-day based hazard rating system for the weevil could be utilised by field foresters preparing silvicultural prescriptions, to assess the potential weevil hazard and to include the hazard in reforestation decisions. A degree-day based weevil hazard rating system was developed by determining a method to adjust air temperature data to reflect leader temperatures, by determining methods to calculate degree-days from climate station data, and by determining methods for spatially extrapolating such data. The literature indicates that weevil development in a leader requires 785 degreedays above 7.2°C in the interior of British Columbia. This published heat sum value may be in error and it is suggested that the true heat sum requirements for the weevil in the interior of British Columbia may be 700 degree-days above 8.9°C. The microclimate of a spruce leader was studied to determine the difference between leader and air temperatures in the open and the shade. On clear days, instantaneous leader temperatures were found to be up to 7°C warmer than air temperatures. On a daily average basis, shaded leaders were found to approximate the air temperature, while exposed leaders were up to 2.6°C warmer than the air temperature. The reflectivity o f interior spruce, Picea glauca x engelmannii, leader bark was measured by wavelength and the direct and diffuse albedo was calculated to be 0.43 and 0.25 respectively using atmospheric conditions representing Vancouver for solar elevations representing midday. The amount of needle shading on a spruce leader was found to vary by solar elevation using a ray-tracing simulation. An energy balance model was developed to predict spruce leader temperatures given the amount of needle shading, the bark albedo, air temperature, solar radiation, long-wave radiation, wind speed and humidity values. The energy balance model was run using thirty years of hourly climatic data from climate stations throughout British Columbia to determine the spatial variability in the monthly leader to air temperature offsets. The topoclimate of the Ryan River Valley in the Coast Range of British Columbia, in the coast interior transition zone, was studied to the examine the influence of slope position and aspect on degree-day accumulation. It was found that nocturnal inversions resulting from cold air drainage led to decreased degree-day accumulations on the valley floor relative to the mid slope. Degree-day accumulations above 7.2°C during the period of May-September 1995 for the valley bottom, north aspect mid-slope and south aspect mid- slope were 779, 930 and 1006 degree-days. On clear days, daily mean air temperatures were found to be up to 5.1 °C cooler on the valley floor relative to the warmer thermal belt on the mid slope. Due to cold air drainage, daily minimum leader temperatures on the valley floor were up to 9.4°C less mid slope sites. On clear days, south aspect daily mean air temperatures were approximately 0.5°C warmer than the north aspect. Topographic shading at the beginning and end of the growing season led to a reduction in leader temperatures on the north aspect relative to the south aspect. A weevil hazard rating system was developed for the interior of British Columbia that used three hazard classes that reflect climatic variability. Four iterative approaches were used to develop the hazard rating system. Initially, the range of climate station heat sums in each biogeoclimatic variant was utilised. The second iteration utilised degreeday lapse rates for each biogeoclimatic subzone and was tested in British Columbia as well as the Mackenzie Basin. The third iteration utilised a regression equation of latitude, longitude, and elevation to predict degree-day accumulations using a geographical information system. The fourth iteration of the weevil hazard rating system was developed for the Mackenzie River Basin of western and northern Canada as part of Environment Canada's Mackenzie Basin Impact Study. Monthly mean temperatures were extrapolated throughout the Mackenzie Basin in a geographical information system using 1951-80 temperature normals, a digital elevation model and radiosonde derived lapse rates. Daily temperature data were generated from monthly temperature means using a cosine curve to allow the calculation of degree-days to determine the weevil hazard rating for the region. The hazard rating system was used to indicate the potential range of the weevil under climate change scenarios. The simulations indicated that with climate warming, the white pine weevil could potentially expand its range northward in latitude and upward in elevation resulting in almost all spruce in British Columbia and the Mackenzie Basin becoming susceptible to weevil damage.