Development of Tree and Understory Vegetation in Young Douglas-Fir Plantations in Western Oregon

The prevalence of young even-aged Douglas-fir (Pseudotsuga menziesii) stands in Oregon and Washington has led to concerns about a landscape being dominated by stands in the stem exclusion phase. In this context, our study documented the development of two aspects important for the diversity of plant and wildlife habitat: tree characteristics and understory vegetation. Using a chronosequence approach, we measured conditions in 39 plantations ranging from 6 to 20 years. Results confirmed intuitive trends, but the quantification indicated that some of these trends develop earlier than commonly assumed. Tree growth in young stands was positively related to stand density, but this trend reversed fairly early. Crown characteristics were influenced very early by stand density, indicating that maintaining a long-lived crown in typical plantations can only be accomplished by lowering stand density through precommercial thinning. Understory herb cover was reduced throughout time, while shrub cover increased. Species compositions were quite complex, with an initial strong presence of invader species and later dominance of species usually associated with mature forests; however, there were many exceptions and early successional species were still present after 20 years. The study showed that this early stage is very complex, that the dynamics vary for different characteristics, and that a finer resolution of the stand initiation stage may be warranted for plantations.