A study on thinning Douglas-fir trees in Western Washington found that leaving larger trees after thinning led to greater volume growth per hectare. The researchers thinned the trees at different times and found that the volume growth varied depending on the stand structure. They discovered that density indexes that prioritize larger trees showed the closest relationship to volume growth. This suggests that thinning to a specific basal area density can increase volume growth, especially if larger trees are retained and given time to recover.