The effects of forest structure on snow accumulation and melt in south-central British Columbia

To address gaps in our understanding of the interrelationships between snow accumulation, snowmelt, meteorological conditions, and forest cover, a field investigation was undertaken at Mayson Lake and Upper Penticton Creek, in the south-central interior British Columbia. During 1995, 1996 and 1997, snow water equivalent (SWE) was measured at 576 stations over nine sites including three clearcuts, two mature spruce-fir stands, a mature pine stand, a juvenile pine and a juvenile-thinned pine stand, and a juvenile spruce-fir stand. Stand structure was described in detail at each station. Continuous measurements of meterological conditions and snowmelt using lysimeters were made at Mayson Lake in 1995. Peak SWE, taken as that measured on April 1st, was less under forest cover than in the clearcuts, by 32% (at Mayson Lake) and 23% (at Upper Penticton Creek) in the mature spruce-fir, by 14% in the juvenile and juvenile-thinned pine, and by 11% in the mature pine. No difference was found between the juvenile spruce-fir stand and clearcut. At Mayson Lake, average snowmelt rates in the mature spruce-fir, juvenile pine, and juvenile-thinned pine stands were 0.4, 0.8, and 0.9 times those in the clearcut, respectively. At Upper Penticton Creek, average melt rates in the mature spruce-fir and mature pine stands were 0.6 and 0.7 times those in the clearcut, respectively. No differences in melt rate were observed between the juvenile spruce-fir stand and clearcut. Relative to the clearcut, 'recovery' in April 1st SWE, defined as the reduction in peak snow accumulation or average melt rate in a juvenile stand from that in the clearcut towards that in the mature stand (B.C. Ministry of Forests 1999), was 43% in both juvenile pine stands and zero in the juvenile spruce-fir stand. Recovery in average snowmelt rate was 13 and 29% in the juvenile-thinned and juvenile pine stand, respectively, and zero in the juvenile spruce-fir stand. Continuous lysimeter measurements at Mayson Lake showed that maximum daily and average melt rates over the season were similar for the clearcut and both juvenile pine stands. However, on a daily basis, the lysimeters showed a substantially different progression in snowmelt in the juvenile-thinned pine relative to the unthinned stand. Snowmelt began earlier and was more rapid early in the season in the juvenile-thinned stand than in the clearcut and the juvenile unthinned stand. Later in the season, melt rates in the clearcut exceeded those in all other stands. Significant relationships between forest inventory variables and peak SWE and melt were found at the stand rather than plot scale. Standardized ratios of forest to clearcut peak SWE and melt (FOSWE and FOMR) were highly correlated with average stand crown length and basal area, respectively. Crown length explained 73% of the variability in FOSWE among stands and the square root of basal area, 79% of the variability in FOMR. Stand structure also affected below canopy meteorological conditions, particularly wind speed, snow temperature, and air temperature above 0 °C. On average, wind speed was reduced by 30 and 100% relative to the clearcut in the juvenile and mature stands at Mayson Lake, respectively. Prior to the onset of melt, snowpack temperatures in the mature spruce-fir stand were 2 °C colder on average than in the clearcut and juvenile stands. The snowpack became isothermal on the same date at all sites. During the peak melt period of 1995, temperature-index models using daily air temperature above 0 °C and temperature near the base of the snowpack were used to predict daily snowmelt for the stands at Mayson Lake. A radiation budget model, incorporating the FOMR ratios, was used to successfully predict snowmelt measured using lysimeters in these stands. Incorporating the field data into a temperature-index and radiation budget snowmelt model highlighted the importance of quantifying the relationships between forest structure, meteorological conditions, and snowmelt. Improved understanding of the interrelationships between these variables is necessary to validate and improve the performance of snowmelt models over the broad range of forest types found in south-central B.C.Forestry, Faculty ofGraduat