Crown stratification by species in even-aged mixed stands of Douglas-fir – western hemlock

Abstract

The pattern of vertical stratification in mixed Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) and western hemlock (Tsugaheterophylla (Raf.) Sarg.) stands was documented by observing tree interaction patterns on temporary plots in stands between 35 and 80 years old. It was found that Douglas-fir predictably dominated the hemlock. Several possible causes of the stratification were studied. They were differences in ages between species, differences in mortality patterns, and differences in growth rates. Increment borings of trees on sample plots in the 35- to 80-year stands with the observed stratification revealed the overtopping Douglas-firs and suppressed hemlocks to be essentially of the same age. Searches for dead or dying Douglas-fir in groups of mixed stands of 35 to 80 years, 22 to 35 years, and less than 12 years revealed very little Douglas-fir mortality by suppression. Therefore, the possibility of death of intermediate and suppressed Douglas-firs was not an important factor leading to Douglas-firs being found primarily in the upper crown positions. Height and diameter growth patterns of closely growing codominant Douglas-firs, co-dominant hemlocks, and suppressed hemlocks were documented by stem analyses. Douglas-fir became significantly taller than the hemlocks after about 20 years and suppressed competing hemlocks. The possibility of stratification in mixed planted stands is less certain, since only young planted stands were available for study. Management of such mixed stands appears to improve the yield and quantity of wood and reduce or eliminate costly precommercial thinnings.