Understory development in thinned stands as part of a long-term ecosystem productivity study
Bobsin, Courtney Rose
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Understory species diversity and composition influences forest ecosystems in a myriad of ways including influencing natural tree regeneration and growth, providing food and habitat for wildlife, creating soil stability, changing soil nutrient availability, and more. The understory is both influencing and being influenced by the overstory layer as they shape the outcomes of one another. In the Long-Term Ecosystem Productivity Study based on the Olympic Peninsula, treatments were implemented in 1995 to assess the effects of species composition, management regimes, and levels of down wood retention on soil productivity and vegetation. Early and mid seral treatments included clearcutting the existing second growth (70-year old forest) and replanting with Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plus red alder (Alnus rubra Bong.) and purely Douglas-fir respectively. The late seral treatment included selective thinning of the exiting second-growth forest. Finally, control units were left as is. Three levels of woody debris were left on site in each of these treatments and control plots were left unmanaged. Pre-treatment and multiple post-treatment measurements were taken in 600 understory plots throughout 40 mensuration plots since the study began. I used understory data I collected in 2016 as well as archived data from previous years. The understory species composition, percent cover, and tree seedling and sapling density were sampled at each plot. These variables were analyzed across treatments and over time. Control and late treatments showed an increase in understory species, but with far less overall cover compared to the early and mid treatments over time. Early and mid treatments showed a substantial increase in evergreen woody shrubs and deciduous ferns in the 20 years post treatment, with salal (Gaultheria shallon Pursh) and bracken fern (Pteridium aquilinum (L.) Kuhn) being the dominant species in each group. A natural tree regeneration count showed fewer seedlings and saplings growing in the early and mid treatments compared to the late treatments. The late treatments showed a clear pattern of growth over time where smaller seedlings grew to become larger seedlings by the next measurement. Overall species richness and evenness peaked in 2002 and was followed by a decline in 2016. This indicates certain understory species are outcompeting others for nutrients, space, and light. Stands with high densities of understory cover led to far fewer tree regeneration counts, which will in turn change the future of the overstory layer. The addition of red alder led to greater survival and establishment of Douglas-fir, a highly valued timber product, and fewer western hemlock, a species that often grows in dense patches. This could led to less pre-commercial thinning of western hemlock and the promotion of a target species, overall reducing management costs. When comparing this study to another Long-Term Productivity Study in southwestern Oregon, the results were very similar (Bormann et al., 2015). Early and mid seral treatment units on both sites saw a pulse of understory growth and a decline in species richness 15 years after treatments were implemented. Control and late treatments had increases in cover over time but not nearly as high as the other treatments. This indicates a pattern is occurring across these ecosystems. Overall, when evaluating any given part of this data, the results appear vastly different than when looking across all years, meaning that these ecosystems are dynamic and we cannot extrapolate beyond the timeframe of studies to understand how the forest will change over time. This creates a need for more long-term studies to fully evaluate best management strategies.
- Forestry