Thinning

Research occurs across BC’s timber land base to assess the effects of thinning e.g. the removal of trees before rotation age. Thinning is carried out to maintain a healthy forest and to increase the quality and quantity of timber produced. Production of healthy, well-managed forest stands can also improve wildlife habitat for some species.  

Effects of Paper Birch Retention on Douglas-fir Stand Development 19 Years Post Treatment in Southern Interior British Columbia - TR147

Operational birch thinning treatments prescribed to reduce the effects of paper birch (Betula papyrifera Marsh.) and other associated broadleaf trees on the growth performance of planted Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) were followed for 19 years post treatment on four sites in the Interior Cedar–Hemlock biogeoclimatic zone (ICH) of British Columbia. Treatments included 600 and 2500 stems per hectare (sph) of retained birch, as well as untreated controls. These treatments were established at each site, where planted Douglas-fir and retained birch were tagged and repeatedly measured over a 19-year post-treatment period in combination with vegetation assessments and silviculture surveys. Significant early increases in Douglas-fir diameter and crown length associated with birch thinning were not found >5 years post treatment, although Douglas-fir were visually larger through to 19 years. Retained birch diameter and crown size were significantly larger within the 600 sph thinning plots (B600) compared to both the 2500 sph retained birch and the untreated control plots from years 3 to 19. Douglas-fir and retained birch tree volume increments over the 5- to 19-year post-treatment period were 150% and 260% larger, respectively, in the B600 treatment plots than in the controls.

Overstorey Growth Response within a Uniform Shelterwood Silvicultural System Trial in Central British Columbia - TR146

A uniform shelterwood trial in interior Douglas-fir (Pseudotsuga menziesii var. glauca) leading stands, within the Dry Warm Sub-Boreal Spruce biogeoclimatic subzone (SBSdw) in the central interior of British Columbia, was thinned from below twice (1991, 2001) before the final harvest in 2011. Mean merchantable volume and basal area growth rates for all species and the Douglas-fir component were not significantly different among three residual basal area (RBA) treatments (30, 40, and 60 m²/ha) in 10- and 20-year periods from 1990 to 2010 or among five RBA treatments (15, 20, 30, 40, and 60 m²/ha) from 2001 to 2010. For all species and Douglas-fir, merchantable volume growth rate was higher in the 60 m²/ha RBA treatment in the first growth period but was higher in the 30 m²/ha RBA treatment in the second period. The 20-year yield was highest in the 30 m²/ha RBA treatment (102 m³/ha), followed by the 60 m²/ha RBA treatment (97 m³/ha), and they exceeded the other RBA treatments (83–93 m³/ha).

The Economic Implications of Pre-commercial Thinning Treatments in Mixed Western Hemlock and Amabilis Fir Stands in Coastal British Columbia under Varying Cost and Benefit Assumptions and Minimum Harvest Criteria - TR124

To meet the goal of economically valuable timber volume flow over time, silviculture programs apply silviculture treatments such as pre-commercial thinning (PCT) to concentrate growth on higher-value tree species and shorten the time to reach harvestable age. A study was established to examine growth and yield of a range of PCT treatments in a mixed western hemlock (Tsuga heterophylla (Raf.) Sarg.) and amabilis fir (Abies amabilis Dougl. ex Forbes) stand that had regenerated naturally, and at 18 years was characteristically clumpy with density ranging from 0 to 80000 stems per hectare (sph). Using the results from the 15-year post-treatment measurement, future growth and yield of each PCT treatment was projected, and a financial analysis was conducted to compare the costs and benefits of PCT or no treatment. As expected, the results of the financial analysis showed that the site values increased for all the PCT treatments and the control under the scenarios with lower discount rate, lower harvesting costs, and higher log prices.

Effects of Pre-Commercial Thinning on Growth and Diversity of Paper Birch Leading Stands 25 Years Following Thinning - EN123

Current and future challenges for British Columbia forest management include the mid-term timber supply shortages caused in part by numerous natural disturbances. Paper birch and other broadleaf species present a possible solution to supplement volume loss and alleviate support mid-term supply.

Effects of Pre-commercial Thinning after 15 Years on Growth and Yield of Mixed Western Hemlock and Amabilis Fir Stands in Coastal British Columbia - TR119

Dense stands of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and amabilis fir (Abies amabilis Dougl. ex Forbes) are a significant source of timber supply for British Columbia’s coastal forest industry. Following clearcut harvesting, naturally regenerated stands have very high densities, which leads to questions about whether and at what densities these stands should be thinned to meet management objectives. An 18-year-old naturally regenerated mixed species stand in the Callaghan Valley near Whistler, B.C. was studied (Experimental Project 1211). Surveys and destructive sampling were used to examine species composition, density, height, and age of the naturally regenerated stand at 18 years. 

The Effects of Precommercial Thinning on the Growth and Yield of Lodgepole Pine: 25-year Results - EN103

The effects of precommercial thinning 56-year-old lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) are reported 25 growing seasons after treatment. Four post-thinning stand densities of 500, 1000, 1500, and 2000 trees per hectare, plus unthinned controls, were established on plots in central British Columbia. Both individual-tree and per-hectare data were analyzed. With the exceptions of mean height and periodic height growth, thinning had a statistically significant effect on all of the characteristics examined. Although this report provides only short-term information on the effects of thinning on the growth and yield of lodgepole pine, it does indicate the need to optimize individual-tree growth rates with levels of growing stock to maximize yield per unit area.

Can Thinning and Fertilizing Young Lodgepole Pine Mitigate Future Timber Supply Challenges? - EN82

The effects of factorial combinations of post-thinning density (600, 1100, and 1600 stems per hectare [sph]) and repeated fertilization on the growth and development of young lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) were investigated in central British Columbia. After 14 years, the effects of density and multi-nutrient fertilization (applied at 0, 5, and 10 years) on tree height were small relative to the effects on tree radial increment. Individual-tree bole growth was greatest at the lowest stand density, whereas per-hectare volume gains were greatest at the highest residual density and declined with decreasing stand density. Stand volume gains following fertilization were less, in both relative and absolute terms,

A Summary of 10- to 15-year Results from Douglas-fir Thinning Experiments in the British Columbia Interior - TR27

Since 1989, three long-term experiments have been established in the British Columbia interior to determine the effects of precommercial thinning (spacing) on the future growth and yield of interior Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn.] Franco) stands under a variety of age, site, and stand conditions. This report provides a brief summary and overview of the results observed to date, and is intended to acquaint forest managers in the interior with the nature and progress of the trials. Thinning improved individual-tree growth, particularly diameter growth, with the greatest response occurring at the widest spacing. Thinning generally reduced basal area and total volume per hectare, and increased merchantable volume per hectare. The observation periods vary from 10 to 15 years, depending upon the experiment.

Stand-tending and Rehabilitation Treatment Options for 36-year-old, Height-repressed Lodgepole Pine - TR07

This trial is testing stand-tending and rehabilitation (replacement) treatments in a 36-year-old, height-repressed lodgepole pine stand to determine what regime might provide the most cost-effective means of increasing timber productivity. Key among the preliminary (3rd-year) findings for the stand-tending treatments is that fertilization, even in very dense unthinned stands, increased the average annual height growth significantly. Thinning alone produced a diameter-growth response but no height-growth response, and a combination of thinning and fertilization resulted in the largest heightand diameter-growth responses. Although longer-term performance results for the rehabilitation treatments are still to come, the trial findings to date show the potential for growth revitalization using stand-tending treatments in the large areas of height-repressed, wildfire-origin lodgepole pine in British Columbia’s Interior.

A Summary of Early Results from Recent Lodgepole Pine Thinning Experiments in the British Columbia Interior - TR16

Since 1982, 11 long-term experiments have been established in the British Columbia interior to determine the effects of precommercial thinning (spacing) on the future growth and yield of lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) stands under a variety of age, site, and stand conditions. This report provides a brief summary and overview of the results observed to date, and is intended to acquaint forest managers in the interior with the nature and progress of the trials. The observation periods vary from 10 to 20 years, depending upon the experiment.

Thinning and Pruning Coastal Douglas-fir near Chilliwack, B.C.: 8-year Results - EN56

In 1991,a thinning and pruning experiment and pruning severity trial were installed in an 11-year-old coastal Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) plantation near Chilliwack, B.C. The thinning and pruning experiment included three levels of thinning (no thinning, thin to 500 stems/ha, and thin to 250 stems/ha) and three levels of pruning (no pruning, prune 250 stems/ha, and prune all stems/ha) arranged in a 3 ×3 incomplete factorial. Eight-year data showed (1) that thinning and pruning effects were independent, and (2) that thinning had no effect on height growth, increased individual tree diameter and volume, but decreased per hectare total stand volume. Pruning to 3 m reduced height and diameter growth in the first 4 years following pruning, but pruned tree growth was equal to that of unpruned trees from 4 to 8 years. An unreplicated pruning severity trial at the same site examined four levels of crown retention (zero, one, two, four whorls remaining). Mortality exceeded 90% on the zero- and one-whorl treatments, and growth decreased with increasing pruning severity.

Pruning Density and Severity in Coastal Western Hemlock: 4-year Results - EN51

Pruning density is the relative number of trees pruned; pruning severity is the amount of live crown removed. Pruning density and severity experiments were installed in two coastal western hemlock (Tsuga heterophylla [Raf.] Sarg.) plantations, 12 and 13 years old, on Vancouver Island. The pruning density experiment indicated that a single 3-m pruning lift significantly reduced 4-year average diameter by 1.3 cm and height by 0.5 m, regardless of whether all or half the trees were pruned on a plot. Treatments in the pruning severity experiments reflected a range of residual crown lengths: 1.5, 2.5, 3.5, and 4.5 m, plus no pruning (control). After 4 years, there were obvious downward trends in both average diameter and height with increasing pruning severity. Significant growth reduction appeared below a threshold of about 50% retained crown ratio. The most severe pruning treatment (1.5 m) reduced 4-year average diameter by 4.3 cm and height by 1.5 m, compared with the control.

Effects of Thinning and Fertilizing Mixed Western Hemlock–Sitka Spruce Stands - EN28

Limited information exists on the growth and yield of young mixed western hemlock (Tsuga heterophylla) and Sitka spruce (Picea sitchensis) stands on the Queen Charlotte Islands following pre-commercial thinning and fertilization. As a result, research into thinning and fertilization was initiated by the South Moresby Forest Replacement Account research committee. Our research project (Experimental Project [EP] 1097), conducted on Graham Island, investigates tree and stand growth responses to thinning and nitrogen and phosphorus fertilization in young Sitka spruce-western hemlock stands. In this extension note, we describe this project, present the 5-year response results taken from remeasurements after the 1996 growing season, and offer some preliminary conclusions.