Influence of the Mountain Pine Beetle infestation on the water balance of a lodgepole pine stand

The forest canopy has a major impact on the forest water balance through interception of precipitation, tree transpiration and below-canopy evaporation and snow melt. The influence on the loss of needles on the water balance of a mature lodgepole pine stand in central BC (Kennedy Siding site MPB06, Canadian Carbon Project) was evaluated using a daily water balance model and measured data for summer 2006 to fall 2009. Biannual hemispherical photography provided canopy solar radiation transmissivity and longwave view factor data, stand evaporation was measured by eddy correlation methods, and snow water equivalent was obtained at weather stations below the canopy and in a nearby opening. The water balance model was validated on lodgepole pine stands in southern BC. Over three years the diffuse and direct transmissivity of solar radiation increased by 10% and 15%, respectively, and the long wave view factor decreased by 10%. Modelled evaporation and snow data agreed well with the measured values. The site weather record was extended to 8 years using data from the MacKenzie weather station 35 km NW of the site. The water balance was modelled for these 8 years using canopy characteristics for each measurement year to facilitate separation of influences of the inter-annual variability in the weather from changes in canopy characteristics. An increase in energy below the canopy due to needle loss resulted in an increase in soil and understory transpiration that compensated for reduction in transpiration from the forest canopy. Snow melt rates increased by about 10%. On average, transpiration and below-canopy evaporation were 25% of precipitation, interception loss decreased from 15% to 10% of the precipitation, and drainage increased from 60 to 65%. Inter-annual variability in transpiration due to the weather was greater than that due to loss of needles.