Snow depths and solar radiation before and after partial logging at the Mt. Tom Adaptive Management Trial

Transcription

1 Snow depths and solar radiation before and after partial logging at the Mt. Tom Adaptive Management Trial File Report by Pat Teti, M.Sc. Research Hydrologist B.C. Ministry of Forests and Range Williams Lake, B.C. April 27 th, 2009

2 /2 Project Overview Block is within the Mount Tom Adaptive Management Trial northwest of Wells as shown in Figure 1. In 2000, two snow research plots were established in this block, one on a north facing slope and one on a south facing slope as shown in Figure 2. Each of the two snow plots Figure 1 Location of Block are approximately 8 ha in area consisting of a grid of permanent reference points. Points were established at 10 m intervals along six east-west lines. Each line is 400 m long in the North Plot and 350 m long in the South Plot (Figures 3 and 4). Each point was originally established on a tree and was differentially GPS d in Due to each plot s large size in relation to openings, each plot includes representative areas of openings and unlogged forest. Repeated measures of snow depth were made near each point in each plot in the spring for five years before partial logging ( ) and two years after logging ( ). In order to avoid bias associated with individual trees, snow depths are measured 6 metres due south of each tree. In the summer of 2007, points that had been marked by trees that were logged were reestablished with rebar and bamboo poles. Snow surveys were repeated five or six times between early April and early June of each sample year. Snow surveys consisted of measuring and recording snow depth at every point in each plot, as well as density in several representative snow pits in each plot.

3 /3 Figure 2 Locations of plots in block Hemispherical photos taken before and after partial harvesting (2003 and 2006) were used as the basis for calculating a variety of canopy gap and radiation parameters. These included canopy gap within 30 degrees of the zenith, sky view factor (an index of longwave radiation exchange) and direct and diffuse solar radiation transmittances. These will be compared with peak snow accumulation and snow ablation rates. Pre-logging basal area (prism sweep) surveys were completed for the south plot. Results of Snow Surveys Figures 5 and 6 show time series graphs of average snow depths at the time of each survey. Data points representing sequential surveys are connected by a line and the data for each calendar year are represented with a different symbol. There was high variability in snow depths between years and between plots. Maximum annual observed depths in the North Plot ranged from 35 to 108 cm while they ranged from 71 to 166 cm in the South Plot. The lowest annual maximum occurred in both plots in 2003 and the highest occurred in 2007 in both plots. Maximum annual depths were always higher in the South Plot than in the North Plot but by very different ratios between years. Maximum depth was 2.2 times greater in the South Plot than the north plot in 2005 but only 1.3 times greater in 2004.

4 /4 Figure 3 Locations of snow stakes in the north plot. Circled points indicate trees that were logged in early 2007

5 /5 Figure 4 Locations of snow stakes in south plot. Circled points indicate trees that were logged in early 2007

6 /6 a) b) Figure 5 Snow depth time series for each year in the north plot (a) and each year in the south plot (b).

7 /7 Effects of partial harvesting on snow depths and transmitted solar radiation Two snow depth transects from 2007 are used as examples of the effects of openings on snow accumulation. Figure 6a shows depths in row N-40 in the north plot as measured on March 31 st, Figure 6b shows a transect along row N-80 in the south plot on the same date. These rows are the second rows from the southernmost row in both plots (see Figures 3 and 4). The blue boxes indicate the points which are in small openings that had been logged in early a) b) Figure 6 Snow depth transects along row N-40 in the north plot and N-80 in the south plot on March 31 st, Boxes indicate points located in small openings that were logged in early 2006 Pre-logging transmitted solar radiation was estimated from fisheye canopy photographs taken in 2003 and 2004 at each snow survey point. Post-logging transmitted solar radiation was estimated from fisheye canopy photographs taken at the same points in the summer of Methods of analyzing the photos are described in Teti (2008).

8 /8 Transmitted solar radiation before and after logging are summarized by Sagar (2008). Figure 7 shows calculated transmitted solar radiation along row N-40 of the north plot and row N+80 of the south plot before and after partial logging. Total radiation transmittance, which is the sum of direct and diffuse radiation under clear skies, is estimated to have increased in small openings from about 10 to about 35 megajoules per square metre per day in the north plot, which is on the south facing slope. Total radiation (MJ/(m^2 day) North Plot N +040 prelogging postlogging a) distance (m) b) transmittance South Plot +080 prelogging 0.8 postlogging distance (m) Figure 7 Solar radiation transmittance along row N-40 in the north plot and N-80 in the south plot measured from fisheye photographs taken before and after partial logging References Sagar, R.M Analysis of Fisheye Photography for the Mt. Tom and Blackbear Sites prepared for B.C. Ministry of Forests and Range, Williams Lake, B.C.. 25 p. Teti, Patrick Effects of overstory mortality on snow accumulation and ablation. Mountain Pine Beetle Working Paper Natural Resources Canada. 34p.