The objective of this thinning and fertilization experiment (Experimental Project [EP] 1097) was to investigate tree and stand growth responses to thinning, and to nitrogen and phosphorus fertilization in a mixed western hemlock-Sitka spruce ecosystem in the Queen Charlotte Islands, British Columbia. The experiment was established before the 1991 growing season. This report presents the 5-year response from remeasurements taken after the 1996 growing season. Two different fertilization levels (no application; application of 250 kgN/ha + 100kgP/ha) and five different thinning levels (no thinning; thin to 1600, 1200, 800, or 400 stems per hectare) were used in an incomplete factorial experiment in a completely randomized block design.
Thinning improved diameter growth for hemlock and spruce. Individual tree height was generally not affected by thinning; however, trees in the lowest diameter class grew faster. On average, trees in thinned plots were larger in diameter, slightly taller, and grew faster in diameter than trees in unthinned plots. However, thinning has caused only a small increase in individual tree volume growth to date. Fertilization increased diameter and volume growth by about 20%, but had little effect on height growth.
On a per hectare basis, thinning and fertilization did not have an effect on top height growth. Volume growth was lower in thinned stands. Although thinning increased volume growth per tree (by shifting growth to the remaining trees), this increase was not great enough to offset the decrease in the initial stem count that resulted from thinning. Thus, after five years, volume growth in thinned plots is lower than in unthinned plots.
While fertilization increased tree volume growth, the effect was not significant. As fertilizer effects are usually immediate and not long lasting, it is unlikely that increased gains in tree volume growth will be observed in the next five years. In mixed-stand conditions, spruce and hemlock showed a similar magnitude of response to thinning and fertilization. In pure-stand conditions, however, spruce appeared to respond slightly better than hemlock.
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Updated October 24, 2008