Progress 04/04/05 to 12/31/10
Outputs
OUTPUTS: Our experiments were designed to test the potential to enhance understory structure and diversity in dense, second-growth forests by altering overstory structure, and to assess the consequences of retaining, rather than removing trees felled during thinning. In the Pacific Northwest, there has been a long history of similar studies of vegetation response to structural manipulations in young coniferous forests. These range from studies of plantation stands thinned for silvicultural objectives (growth of crop trees; Doerr and Sandburg 1986, Alaback and Hermann 1988, Bailey et al. 1998, Thomas et al. 1999, Lindh and Muir 2004), to those in which thinning prescriptions incorporate -- or emphasize -- ecological objectives (e.g., enhancing habitat complexity or accelerating forest structural development; Thysell and Carey 2001, Fahey and Puettmann 2007, 2008, Wilson and Puettmann 2007, Ares et al. 2009, 2010, Davis and Puettmann 2009, Wilson et al. 2009). We completed two experiments to support three principal studies. The first, the "Main Restoration Experiment," compared vegetation responses to experimental treatments (control, thinned, and gap creation) at each of three locations. Within the context of this design, we established a smaller study that explored the responses of bryophytes to these treatments and how these responses are mediated by substrate (decayed wood and forest floor). We refer to this as "The Bryophyte Study." A second experiment, implemented at a single location, was designed to assess the consequences for vegetation of retaining vs. removing trees following thinning. We refer to this as "The Coarse Woody Debris (CWD) Experiment." PARTICIPANTS: Charles B. Halpern, Research Professor, University of Washington School of Forest Resources. Douglas G. Sprugel, Professor, University of Washington School of Forest Resources. Kelsey Ketcheson, graduate student, University of Washington School of Forest Resources. Shelley A. Evans, research coordinator, University of Washington School of Forest Resources. TARGET AUDIENCES: Target audiences include the Seattle Public Utilities Watershed Management Division, foresters, land managers, researchers, educators, students, and the general public. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Our analyses point to several consistent trends: 1. For most measures of vegetation response, effects of treatments were not evident until the third growing season. Had there been greater initial plant cover, first-year responses are likely to have been dominated by declines. 2. The magnitude of response to treatments varied markedly within sites. Variation in soil properties may explain these differences. In both thinned and gap treatments, there were strong positive correlations between local density of Thuja plicata and changes in plant cover and richness. The factors that enhance Thuja establishment and growth may similarly affect understory responses to thinning. Regardless of the mechanism(s), local density of Thuja may be a useful indicator of the potential for understory response to stand manipulation. 3. Variation among sites was greater than variation within sites. The same factors that lead to withinâsite variation may be responsible for the large differences among sites. However, there are potentially other differences that separate the sites. We considered several factors that might contribute to this variation. Site differences were driven to a greater degree by the responses of forest species. The potential for significant variation among sites, or for significant site x treatment interactions among ostensibly similar forests, has important implications for broader application of these treatments. Short-term responses to thinning and gap creation can be unpredictable, even in forests in which pre-treatment structure is simple. 4. Gap creation has a stronger effect on plant cover than it does on diversity. Effects were manifested primarily within the physical openings of the gaps, although gap influence extended into adjacent-forest for some measures. Similar localization of gap response has been observed in previous studies, even with significantly larger gaps. Gap creation resulted in a greater mean increase in cover, and in a broader distribution of local responses. Changes in species richness also suggest stronger responses to gap creation than to thinning, but treatment effects were confounded by other sources of variation within sites. Despite random assignment, gap treatments tended to be assigned to experimental units with higher Thuja density, thus to sites with greater potential for response. In contrast, at similar Thuja density, gap creation and thinning had similar effects on species richness. Greater heterogeneity of resource availability or physical environments is thought to promote greater diversity of species with differing resource or environmental requirements. Although the spatial distribution of light (or soil moisture) may be more heterogeneous or complex in thinned treatments, there was no indication in the short term that this resulted in greater diversity of species, or of groups of species with differing environmental or resource requirements. Both treatments resulted in an increase in species number and in the equitability of species' abundance -- responses that were further enhanced by variation in site characteristics.
Publications
- Halpern, C.B., Sprugel, D.G., Ketcheson, K., and Evans, S.A. 2010. An Ecological Restoration Experiment in the Cedar River Municipal Watershed, Final Report. Available online at: http://faculty.washington.edu/chalpern/CRW_final_report_2010.pdf, link verified 10/26/2011.
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Progress 10/01/07 to 10/01/08
Outputs
OUTPUTS: This study contributes to a broader upland restoration program and habitat conservation plan in the City of Seattle's Municipal Watershed. The primary goals of forest restoration are to accelerate in dense young stands, the development of structures characteristic of late-seral forests, to improve habitat complexity (particularly for species associated with late-seral forests), and to enhance biological diversity. We are studying vegetation responses to three experimental treatments: thinning to increase spatial heterogeneity, creation of canopy gaps, and a control. An additional, smaller experiment utilizes the same thinning treatment, but varies whether felled trees are removed or left in place. During the past year we: (1) completed summaries of first-year responses of vascular plants and bryophytes, and changes in the light environment; (2) submitted a manuscript on changes in forest structure and light availability in response to simulated thinning treatments; and (3) published a manuscript on the importance of forest floor substrates for the abundance and performance of understory herbs and shrubs. PARTICIPANTS: Not relevant to this project. TARGET AUDIENCES: Not relevant to this project. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Experimental treatments: First-year responses of understory vegetation to experimental thinning and gap creation were limited, despite major increases in available light. This reflects the depauperate nature of the understory prior to treatment. In contrast, alder seedlings established in abundance, particularly within the centers of gaps. Simulation models: In combination, stem maps, simulated thinning, and a light model provide a useful planning tool for predicting initial effects of silvicultural manipulations on forest structure and light distribution. Substrate effects: Of 29 species tested, 62% showed significant positive associations with forest floor and 21% with decayed wood. However, species showed similar overall performance on these substrates. Substrate associations may reflect differences in dispersal, germination, or early survival and not differences in the performance of established plants.
Publications
- Six, L. J., and C. B. Halpern. 2008. Substrate effects on distribution, biomass allocation and morphology of forest understory plants. Botany 86:1133-1142.
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Progress 10/01/06 to 10/01/07
Outputs
This study supports a broader upland restoration program and habitat conservation plan in the City of Seattle's Municipal Watershed. The primary goals of restoration in the watershed are to (1) accelerate development of structures characteristic of late-seral forests, (2) improve habitat complexity, particularly for species associated with late-seral forests, and (3) enhance biological diversity. Our restoration experiment focuses on treatments intended to enhance the abundance and diversity of understory plants in dense, hemlock-dominated forests. The primary design consists of three treatments replicated four to five times at each of three sites: (1) a control (no harvest); (2) a thin that removes 30% of the original basal area by retaining a proportion of the largest trees and randomly located clumps of subcanopy trees; and (3) a gap that is 20 m in diameter. An additional experiment utilizes the same thinning treatment, but varies whether cut trees are removed or
retained. During the past year we completed post-treatment sampling of understory vegetation (vascular plants and bryophytes) and transmitted light (via hemispherical photography). Data have been entered and proofed. In addition, using detailed stand maps and a light-attenuation model we completed a simulation study of changes in forest structure and light in response to a broad diversity of thinning approaches. The results of these simulations were presented at the Annual Meeting of the Ecological Society of America, and will be submitted for publication in the coming year.
Impacts
Ability to predict effects of thinning on stand structure and light availability may be useful in designing silvicultural treatments to achieve particular ecological outcomes. Simulation models revealed some surprising results: at similar levels of retention, random thinning, thinning from below, and random thinning of only sub-canopy stems produced similar distributions of light despite contrasting effects on forest structure. A "structured ecological" thin in which the largest trees were retained but smaller trees were removed in groups produced the most variable light distribution and the most clumped spatial structure. Tree removal via gaps produced the broadest light distribution. These simulations provide insights into the ways in which different types of stand manipulations produce different forest structures and potentially different ecological outcomes.
Publications
- No publications reported this period
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Progress 10/01/04 to 10/01/05
Outputs
There has been growing recognition that many of the species and ecological processes found in older forests are lacking or are compromised in young and intensively managed stands. Dense, young forests are characterized by low levels of light, poor understory development, limited regeneration of shade-tolerant tree species, and reduced abundance of wildlife. As societal perceptions of the value of public lands have changed, emphasis has shifted toward the role of forests in supporting biological diversity and ecosystem services (e.g., water production and carbon sequestration). Resource managers, faced with the challenge of restoring these ecological values to extensive tracts of dense, young forest, are considering thinning. We are working with staff from Seattle Public Utilities Watershed Management Division to design a forest restoration experiment in the Cedar River Municipal Watershed. The experiment will support a broader upland restoration program under the
Watersheds Habitat Conservation Plan. The primary goals of restoration are to (1) accelerate development of structures characteristic of late-successional forests, (2) improve habitat complexity, particularly for species associated with late-seral forests, and (3) enhance biological diversity. To guide the experimental design, preliminary measurements were made to examine relationships among forest structure, light availability, and abundance of understory plant species. An intensive sample of forest structure and understory composition was completed at the three sites selected for experimental manipulation. At each site, a 60 x 200-400 m belt transect was established. Within each of these, all live and dead stems 1.4 m tall were mapped and measured for diameter. A series of understory transects was used to obtain a continuous sample of ground conditions (litter and coarse woody debris, surface hydrology), bryophyte cover, and density of plant species (herbs, shrubs, trees <1.4 m
tall). Data have been entered and proofed and preliminary summaries/analyses have been completed. Tasks for the coming year include (1) further analysis of the relationships between forest structure, light, and understory development, and (2) completion of the experimental and sampling designs.
Impacts
work in progress
Publications
- No publications reported this period
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