They generally leave the best growing trees and remove the smaller diameter ones, which are often 5 to 10 inch diameter on the truck. Just guessing - the timing of the first commercial thinning in fir (on the Westside) somewhere around 25 years from the planting, but it may occur much later than that. It all depends on how fertile the soil is and how fast the trees are growing. Eleven to twelve inch diameter trees (at breast height) are ready to thin if originally planted at the DNR standard spacing of 12 - 13 feet. Subsequent thinnings often occur as the stand ages. The stand in question looks like it is somewhere around 35 - 40 years old to me. I'll bet cougforester could give you a much better answer.