PARENT SESSION
Contributed Oral Session 40: Forest Understories
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 514 A, Level 5, Palais des congrès de Montréal
Patterns in understory vegetation distributions across gap openings in variable density thinned forest stands in western Oregon.
Fahey, Robert*,1, Puettmann, Klaus1, Berryman, Shanti1, 1 Oregon State University, Corvallis, OR
ABSTRACT- Canopy gap formation may be an important tool in creation of understory habitat heterogeneity in thinned second-growth (40-50 year old) forest stands in western Oregon, but the opportunities and implications of this practice are not well understood. Four stands were treated with a variable density thinning regime, which included gap openings, intended to accelerate development of late-successional habitat, as a part of the Bureau of Land Management's Density Management Study (DMS). Our study investigates the role of gap openings in the creation of understory heterogeneity in vegetation communities seven years post-treatment at the DMS sites. Understory vegetation distributions and light conditions were characterized along north-south transects through two different gap sizes (0.25 and 1 acre) and into the surrounding forest matrix. Findings were related to the gap-partitioning and gap-size partitioning hypotheses, which state that variation in resource levels across gaps may result in formation of distinct areas of vegetation composition in different positions relative to gap openings and that this effect will differ depending on gap size. Preliminary results indicate that understory vegetation composition (multiple response permutation procedure; p = <0.0001, A = 0.05) and species richness (ANOVA, F4,24 = 9.27, p = 0.0001) differed statistically among positions along transects. Our findings indicate a strong division between the plant communities present in gap interior positions and those in edge and forest matrix locations as determined by non-metric multidimensional scaling (NMS) ordination. Early seral and invasive species occupied gap interior positions, while forest understory and late-successional species were more strongly associated with gap edge and forest matrix positions (according to both NMS and indicator species analysis). Gap-partitioning seems to be occurring in these communities over this fairly short time frame. These results support the view that gap formation may be enhancing understory heterogeneity in these stands, and may have implications for maintenance of plant species diversity and future development of late-successional habitat features in these stands.
Key words: canopy gaps, understory vegetation, thinning, gap partitioning