This project is designed to determine whether early stand management has foreclosed options for older stands to develop all desired structural components.
The first two decades are a very dynamic period in Douglas-fir plantations when components considered critical habitat for several species, such as crown structure and understory vegetation, are changing quickly and are very responsive to manipulations. This study explores alternative management approaches in young plantations to minimize negative aspects of the stem exclusion phase. Specifically, an Observational study quantified how various stand structural components are influenced by stand density over time in young Douglas-fir plantations. As a follow-up, a Manipulative study was setup to determine whether density management can slow down or reverse undesirable trends in young plantations.
Using a chronosequence approach, stands for the Observational study were selected in winter 2003 along an age continuum between 6-20 years old in three Oregon Department of Forestry districts (Astoria, Forest Grove, and West Oregon). Plots were placed along transects to ensure coverage of low, medium, and high densities (gap, transition, matrix). Tree characteristics and understory vegetation were measured in spring and summer. Data analysis began in late fall and preliminary results were summarized for presentation at the Young Stand Management Workshop (November 25, 2003). The workshop was organized to gain insight from folks in different agencies on the issue of managing young Douglas-fir stands for structure. Preliminary data were also presented at the State Forests Conference (March 10-12, 2004) by Klaus Puettmann (Principal Investigator).
Data from the retrospective analysis indicate that some trends in stand development 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 live crown in typical plantations can only be accomplished by lowering stand density through pre-commercial thinning. Understory herb cover was reduced over time, while shrub cover increased. Species compositions were quite complex, with an initial strong presence of invasive 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. These results of the retrospective analysis show that this early stage is very complex and the dynamics vary for different characteristics.
The retrospective showed that any gaps or openings in young plantations may provide for a diversity of within-stand conditions that may affect the role and impact of the stem exclusion phase on development of stand composition and ecosystem functions. Even if gaps are created over time due to various mortality agents, it appears that stand modifications of standard management operations are necessary to ensure gaps that have fully developed shrub, herb, and hardwood vegetation layers.
The results of the Observational study, findings from other recent studies and discussions with department personnel determined that gaps (low density areas) can provide opportunities to maintain or enhance biodiversity in Douglas-fir plantations. The specific objective of the Manipulative study was to document development of natural gaps and compare them with managed gaps, i.e., gaps treated to maintain or enlarge their size. In keeping with the goal to avoid, rather than reverse undesirable trends (e.g., crown recession) in stand development, 10 to 13-year old stands studied in the Observational study were used in the Manipulative study. Thus, we followed individual gaps and documented 20 to 24 gaps within a stand (one stand per department district) in each of the three districts (described above). Gaps were either left untreated or enlarged by cutting trees a 20 ft buffer around the gap. Plot installation occurred in winter 2004 and tree characteristics and understory vegetation were measured in spring and summer, 2004-2005. Initial data indicate that conditions in gaps prior to treatments were very similar and future differences between treatments can likely be attributed to management efforts. Periodic (2 to 4 year interval) re-measurements are planned through 2014.
In addition, questions were raised whether standard plantation management practices provide for gaps or whether additional management to create gaps was needed. As a follow up, we initiated a gap inventory study with the objective to provide baseline information about diversity of conditions (i.e., area in gaps versus fully stocked areas) created by standard management operations in plantations on Oregon Department of Forestry land. The investigation indicated that gaps make up a minor proportion of Douglas-fir plantations on Oregon Department of Forestry land. Even if gaps are created over time due to various mortality agents, it appears that stand modifications of standard management operations are necessary to ensure gaps that have fully developed shrub, herb, and hardwood vegetation layers. In comparison, gaps that appear or are enlarged in later developmental stages would have reduced understory vegetation, as a consequence of early stand development, as new invasion would be required to provide desirable stand structural components. In any case, concerns about the total plantation area not growing crop trees would need to be assessed before management actions occur.