The effects of nitrogen and nitrogen + sulphur fertilization on the first-year fascicle weight and foliar nutrient status of immature, thinned lodgepole pine (Pinuscontorta Dougl. var. latifolia Engelm.) were evaluated at seven locations in the interior of British Columbia. The results indicate that sulphur deficiencies, either aggravated or induced by nitrogen fertilization, may limit the responsiveness of lodgepole pine to nitrogen additions. At four locations, combined sulphur and nitrogen additions improved the sulphur status of fertilized trees and significantly increased the weight of fascicles produced during the first year after treatment compared with that achieved with nitrogen alone. Ammonium sulphate was a superior sulphur source compared with elemental sulphur–sodium bentonite prills and also was readily taken up by trees. The oxidation of the elemental sulphur prills was apparently too slow to satisfy sulphur requirements in the first year. Increasing the sulphur application rate from 50 to 100 kg/ha generally gave only small improvements in first-year fascicle weight response despite a significant improvement in foliar sulphur concentration and content. The responsiveness to sulphur fertilization was not consistent across all sites. Further characterization of sites that are responsive and unresponsive to sulphur additions is needed to more accurately diagnose sulphur deficiencies and predict responsiveness to sulphur additions.

The effects of different regimes and frequencies of repeated fertilization (applied periodically and yearly) on the foliar nutrition and growth of young lodgepole pine ( Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) were investigated in central British Columbia. After 12 years, tree and stand growth responses indicated modest positive effects of two applications of nitrogen (N) and boron, with relatively large incremental gains to added sulphur and other nutrients (complete). Despite large increases in effective leaf area index and foliar N concentration relative to the control and periodic treatments, yearly applications of fertilizer were relatively ineffective in stimulating growth. When combined with frequent additions of other essential nutrients, annual fertilization with 50–100 kg N/ha over 12 years (825 kg N/ha in total) produced only slightly more “extra” wood than two applications of the multinutrient, complete fertilizer (400 kg N/ha in total) (19.4 m3/ha vs. 17.2 m3/ha, respectively). The most intensive fertilization treatment (1550 kg N/ha plus other nutrients over 12 years) produced shorter trees and less stand volume increment than periodic fertilization with the complete fertilizer. Treatment-induced changes in stand dynamics and growth allocation, disrupted foliar nutrient balance, and changes in soil biota and understory vegetation community structure may have negatively affected tree growth in intensively fertilized treatment plots.

A study was undertaken to investigate the effects of nitrogen and boron, applied singly and in combination, on the growth of young, thinned lodgepole pine in the interior of British Columbia. Results indicate that subacute boron deficiencies without visible growth disturbance symptoms, may reduce the nitrogen fertilization response potential of lodgepole pine. Combined nitrogen and boron application significantly improved the 3-year mean height and volume increment over that obtained with nitrogen alone. Relatively small boron additions (i.e., 1.5 and 3.0 kg boron•ha−1) to the soil surface elevated foliar boron concentrations and maintained them throughout the 3-year study period. No toxic effects of boron application on foliage were observed. Both 1st-year fascicle weight response and 2nd-year branch elongation proved to be reliable indicators of subsequent stem volume response. The slightly larger stem growth response obtained with a complete fertilizer treatment, when combined with needle weight and foliar analysis data, indicates that nitrogen and nitrogen plus boron treatments may have induced a marginal sulphur deficiency. Because boron deficiency symptoms can develop rapidly following an interruption in boron uptake, and because top dieback can have such an adverse effect on stem quality and value, it is recommended that boron be added to nitrogenous fertilizer when undertaking aerial fertilization projects in lodgepole pine forests where average foliar boron concentrations are below 15 ppm.

The effects of factorial combinations of post-thinning density and fertilization on the growth and development of young lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) were investigated in central British Columbia. The effects of density and fertilization (repeated every 5 years) on tree height were small relative to the effects on stem radial growth. Tree radial growth increased with thinning intensity, whereas per-hectare growth was greatest at the highest residual density. Fertilizer effects varied across the range of residual densities tested. Tree and stand volume gains following fertilization were less, in both relative and absolute terms, at 600 trees/ha than at 1100 or 1600 trees/ha. Vigorous response of understory vegetation to nutrient additions (and strong competition for water and nutrients) may have reduced the effectiveness of fertilization on tree growth at 600 trees/ha relative to higher stand densities. Results indicate that the combined positive effects of thinning and fertilization on the growth of young lodgepole pine will accelerate stand development, thereby shortening technical rotation length. Results also indicate that significant growth gains following fertilization of thinned lodgepole pine will partially compensate for stand volume losses due to thinning. However, fertilization may be less effective at low stand densities, where negative effects of thinning on harvest volume are greatest.

This study was designed to test the hypothesis that large-scale pre-commercial thinning (PCT) to various stand densities, at ages 12-14 years, combined with repeated fertilization, would, over a 10-year treatment period, enhance productivity of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) crop trees. Study areas were located near Summerland, Kelowna and Williams Lake in south-central British Columbia, Canada. Each study area had nine treatments: four pairs of stands thinned to densities of ~ 250 (very low), ~ 500 (low), ~ 1000 (medium) and ~ 2000 (high) stems ha − 1 , with one stand of each pair fertilized fi ve times at 2-year intervals, and an unthinned stand. The very low, low-and medium-density stands were also pruned to a 3-m lift 5 years after thinning. At the tree level, fertilization treatments signifi cantly increased diameter at breast height (DBH), basal area (BA) and volume growth and heavy PCT signifi cantly increased DBH and BA growth. Pruning may mitigate some of the negative stem form and wood quality attributes associated with fast-growing trees without adversely affecting stem growth. At the stand level, PCT to very low and low densities signifi cantly decreased the volume growth compared with high-density stands. The potential benefi cial impacts that PCT and repeated fertilization treatments have for mitigating timber supply shortfalls, as well as potentially minimizing crop tree losses due to mountain pine beetle (Dendroctonus ponderosae Hopk.), are also discussed.