Short-term effects of prescribed burning on radial growth of Douglas-fir trees in south central British Columbia

Abstract

Radial increment of Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) trees in uncut and partially cut and grazed and ungrazed dry cool Interior Douglas-fir forests near Isobel Lake in southern central British Columbia was measured for 2-year periods before and after prescribed burning in burned and unburned (control) areas. Parameters thought to influence the effects of prescribed burning on tree growth – characteristics of the tree and its immediate environment, adjacent surface and ladder fuels, and fire effects – were also measured. Burning caused a decrease in tree radial growth. The proportional change in radial tree growth was not related to degree of cutting or the presence of cattle grazing, but was negatively correlated with preburn radial growth, tree size, and also with some fire effect variables, particularly char height, suggesting that an increasing proportional decline in radial growth was associated with increasing tree size and with increasing fire severity. Soil nutrient (N, S, P, K, Mg or Ca) level was not a significant determinant of tree growth. The best prediction model could only explain 23% of the variation in the change in radial growth. Our results show that the effects of fire on Douglas-fir radial growth are complex and difficult to predict with the variables measured.