Using foliar variables to predict the response of lodgepole pine to nitrogen and sulphur fertilization

Six-year basal area responses to nitrogen (N) fertilizer, alone and in combination with sulphur (S), in 31 lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) stands in the interior of British Columbia were compared with pre-treatment measures of foliar N and S and first-year increases in fascicle mass to determine the utility of these variables for predicting growth response and for determining appropriate fertilizer prescriptions. Results indicate that pre-fertilization levels of foliar N or inorganic sulphate-S (SO4) may be more reliable than the fascicle mass screening method for predicting whether or not a significant stemwood response will occur following N fertilization. When combined, N and SO4 levels in unfertilized foliage explained 68% of the variation in relative basal area response to fertilization with N alone. Foliar nutrients were also useful for determining whether or not lodgepole pine would respond incrementally to S when added in combination with N. Stands in which pre-fertilization foliar SO4 was <60 mg/kg and N/S ratio was >13 did not respond significantly to N alone but always responded significantly to N + S. Conversely, a foliar SO4 level >60 mg/kg combined with a N/S ratio of <12 always resulted in a favourable response to N with no incremental benefit of added S.