Ten-year growth response of 45-year old Pinus banksiana Lamb. to urea fertilization and low thinning

Abstract

The objectives of this investigation were 1) to examine the duration and pattern of jack pine growth response to urea fertilization and low thinning, and 2) to examine methods commonly used in the measurement of fertilization and thinning trials in jack pine. The study area was located near Chapleau, Ontario in a fire-originated jack pine stand. The stand was treated at age 45 with 336 kg urea-N/ha and a low thinning of 20 percent basal area in a 2 X 2 factorial design with four replicates. At stand age 55, 80 trees were felled for stem analysis, representing 5 tree diameter classes within each stand treatment. Volume increment, height increment, measures of form, and local volume equations were determined from the stem analysis data. The local volume equations were applied to diameter frequency distributions at stand ages 45, 50, and 55 to calculate stand volumes at these ages. A growth model was developed to characterize the annual volume growth of individual trees. The model related the annual volume growth of an individual tree to its volume, and the stand volume. Aggregation of the annual volume increments of individual trees allowed annual stand volume increments to be estimated. Individual trees responded to fertilization with greater 10-year volume increment and merchantable height increment. On a stand basis, fertilization resulted in about 20 m[superscript 3] ha[superscript 2] of additional gross volume growth during the ten-year response period, This volume growth response was greatest in the third and fourth years after fertilization. Fertilization appeared to have both a direct effect and an indirect effect. Tlie direct effect was growth response to the improved nitrogen status, and this effect ceased 10 years after treatment. The indirect effect was greater growth due to larger average tree size, and this effect was still evident 10 years after treatment. Thinning had little effect beyond salvaging potential mortality. An examination of methods commonly used to estimate growth responses to silvicultural treatments was made. The results suggest that it is necessary to use treatment-specific and age-specific local volume equations to accurately measure growth response to fertilization.