The effect of commercial thinning on the growth and yield of interior Douglas-fir (EP1184)

Introduction

This study was established in 1994 in cooperation with Weldwood Canada Ltd. and the Cariboo Forest Region. The site is located in a 65-year-old, naturally regenerated mixed stand of Douglas-fir, lodgepole pine, white spruce and paper birch on TFL #5, north of Quesnel. The site is within the moist hot SBS biogeoclimatic zone. 

Objectives 

To determine the effects of different levels of residual stand density on the growth, development and yield of commercially thinned Douglas-fir.

Treatments 

The study consists of three replicate blocks of three residual (post-thinning) densities (400, 600, and 800 trees per hectare) plus an unthinned control. The thinning method used was a "thinning from below" or "low thinning", wherein the lower crown densities were removed. The selection of leave trees was based on size (dominants and codominants favoured), species (Douglas-fir was favoured), health and vigour. 

Each of the four treatments is randomly assigned to three treatment plots in a randomized complete block design. Control plots are 30m x 30m (0.09 ha) with a 15 m unthinned buffer surrounding plot boundaries. Thinned plots vary from 0.20 to 0.30 ha depending on post-thinning density, and consist of either 12 rows of 12 trees (400 and 600 trees per hectare) or 14 rows of 14 trees (800 trees per hectare). 

All thinnings were yarded to the road-side by horses in order to minimize damage to the residual stand. 

Growth data (diameter at breast height, total height, height to live crown, crown width, tree form and damage) are collected from this installation every 5 years.