Fertilization and Thinning Effects on a Douglas-Fir Ecosystem at Shawnigan Lake: 32-Year Growth Response

The Canadian Forest Service’s Shawnigan Lake Project (SLP) main experiment was established in 1971–1972 to study the effects of fertilizing and thinning 24-year-old coastal Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) on a dry and nutrient-poor site (Site index 25 m at 50 years breast height age). It consists of three levels of thinning and five levels of fertilization, including the controls. The thinning levels (single entry) are: no thinning (T0), 1/3 basal area removed (T1), and 2/3 basal area removed (T2). The fertilization levels are: no fertilization (F0), application of 224 kg N/ha once (F1) or thrice (F1-1-1), and application of 448 kg N/ha once (F2) or twice (F2-2). The second application of fertilizer was done in 1981 and the third in 1990. Each thinning and fertilization treatment combination consists of two or four 0.0405-ha plots, totalling 36 plots. These plots were remeasured nine times over a 32-year period. Plot average and per-hectare tree data were pooled by treatment and analyzed separately for the entire stand and crop trees. Crop trees (or prime trees) represented the largest 250 trees per ha at time of treatment that were still alive 32 years after treatment. The treatment effects were compared using the fixed-effects model, for 32-year periodic annual increment (PAI) and yield 32 years after treatment, for various stand attributes. The stand attributes included quadratic mean DBH (QMD), arithmetic average height (AAH), and total and merchantable volume. The analysis results were generally consistent with our expectations. The results showed the following:

1. Cumulative mortality over the 32-year period since the initial treatment was highest in the T0 and T1 treatments, and negligible in the T2. The main cause of mortality continues to be suppression, although snow damage was also prevalent. Thus, thinning can be beneficial by capturing the volume from trees that would otherwise have been lost to mortality, if the thinned fibre can be used.

2. Fertilization increased stand and crop tree diameter and height PAI in both thinned and unthinned stands, with greatest growth in the T2F2-2 treatment.

3. Thinning also increased quadratic mean DBH (QMD) and arithmetic average height (AAH) periodic annual increment (PAI), and shifted stand basal area to fewer, larger-diameter trees. However, part of this increase in QMD and AAH is due to the arithmetic increase in average diameter or height resulting from the removal of smaller-than-average trees from thinning and mortality.

4. Heavy thinning increased the live crown length, and fertilization slightly decreased the live crown length. That is, the amount of clear bole is reduced by the heavy thinning. If the management objective is lumber production, thinning may have to be combined with pruning to reduce the size and number of knots on the bole.

5. Fertilization increased production of stand and crop tree total and merchantable wood volume fibre in both thinned and unthinned stands, with the greatest growth in the F2-2 treatment. For example, the total volume PAI responses relative to F0 ranged from 26% (F1) to 55% (F2-2). However, the F1 treatment was the most efficient in terms of total volume PAI per kg N/ha.

6. Thinning did not have a significant effect on stand total and merchantable volume production, although heavy thinning significantly increased crop tree total and merchantable volume PAI. However, the stand volume results are confounded by post-treatment volume differences, the slower growth of more numerous smaller trees in the unthinned plots, and the faster growth of fewer larger trees in the thinned plots. The ranking of total volume/ha at 32 years was T0 > T1 > T2, although the gap between total volume in the thinned and the unthinned stands is narrowing.

These 32-year results added to the information base supporting informed forest management decision-making in coastal British Columbia, and contributing to the Canadian Wood Fibre Centre’s initiative of promoting research on the effect of silviculture and stand dynamics on fibre attributes. These results may be extended to similar dry and nutrient-poor sites on the coast and will also contribute to our knowledge of the response of coastal Douglas-fir stands to thinning and fertilization. To confirm the conclusions to date, and to provide more complete plot histories, continued remeasurement of the SLP plots is recommended.