MODELING THE NUMERICAL RELATIONSHIPS BETWEEN CHRONIC AMBIENT SULFUR-DIOXIDE EXPOSURES AND TREE GROWTH

An exponential growth curve model was developed for Pinus contorta Dou gl. ex Loud. var. latifolia Engelm. x Pinus banksiana Lamb. (lodgepole x jack pine) trees from basal area increment data collected from five ecologically analogous sampling locations (A, to A,) in the vicinity of a sulphur recovery sour gas processing plant emitting sulphur (S) g ases (mainly SO2) in the West Whitecourt study area near the town of W hitecourt in west-central Alberta, Canada. The mean basal area increme nt growth declined by 1.2%, 1.4%, 0.8%, and 0.6% between 1959 and 1981 at sampling locations A(I), A(II), A(III), and A(IV), respectively, i n comparison to the reference sampling location, A(V). Since 1974 ther e has been an increase in wood production at the impacted sites, A(I) to A(IV). This was most likely the result of the significant and progr essive reductions in total sulphur gas emissions from 1963 to 1981, of 58 403 to 6782 t S/year, respectively. A multivariate nonlinear, poly nomial Fourier regression model was applied to explain the relationshi ps between the ambient SO2 exposures at the five sampling locations an d changes in pine tree basal area increment growth. The regression mod el included the ambient SO2 exposure parameters: (1) number of episode s (an episode is equivalent to single or successive occurrences of 0.5 -h mean concentrations of greater than or equal to 10 ppb); (2) cumula tive integral of exposures (concentration with respect to time); and ( 3) peak episodal concentrations. The model parameters were estimated u sing the least squares approach. The MPF regression model captured the actual effects of the episodicity of SO2 exposures on radial tree gro wth of pine species and provided a high degree of forecasting power be cause of the use of the integral of the SO2 exposures. Peak episodal S O2 concentrations or the number of episodes appeared not to play as im portant a role in the model as the integral.