The experimental design consists of six treatments, each 13 ha in size, replicated at six locations (blocks) in western Washington and Oregon. Treatments represent strong contrasts in retention level (15–100% of original basal area) and pattern (trees dispersed vs. aggregated in 1-ha patches) in mature Douglas-fir (Pseudotsuga menziesii) forests.
Six study areas (blocks) were selected to represent a diversity of physical environments and forest types at low to moderate elevations (210–1700 m) in western Washington and Oregon. Three are in the Cascade Range and one is in the Black Hills of southwestern Washington; two are in the Cascade Range of southwestern Oregon. Blocks represent several forest zones (defined by different climax species), but Douglas-fir (P. menziesii) is the dominant tree species throughout (see Maguire et al., 2007). 13-ha experimental units were established on gentle to steep slopes (4–66%) in upland areas of relatively homogeneous mature forest. Perennial streams, wetlands, roads, and existing harvest units were avoided. Past management activities varied among blocks: three had no previous activity, one was salvage-logged, one was thinned, and one regenerated naturally after clearcut logging ~65 years earlier.
The experiment is a randomized complete block design, containing six treatments: (1) unharvested control - a reference for assessing responses to harvest and natural temporal variation; (2) 75% aggregated retention - all merchantable trees (>18 cm dbh) were harvested from three 1-ha circular gaps (56.4 m radius; 25% of the treatment unit); (3) 40% aggregated retention - five circular 1-ha aggregates were retained (40% of the treatment unit); all merchantable trees in the surrounding matrix (“clearcut areas”) were harvested; (4) 40% dispersed retention - dominant and co-dominant trees were retained in an even distribution throughout the treatment unit; in each block, the basal area retained was equal to that retained in the corresponding 40% aggregated treatment; (5) 15% aggregated retention - two circular 1-ha aggregates were retained (15% of the treatment unit); all merchantable trees in the surrounding matrix were harvested; (6) 15% dispersed retention - dominant and co-dominant trees were retained in an even distribution throughout the treatment unit; in each block, the basal area retained was equal to that retained in the corresponding 15% aggregated treatment.
A permanent grid (8 × 8 or 7 × 9, with 40-m spacing) was established in each experimental unit to systematically sample most biological responses. For some studies, additional transect- or gradient-based designs were established in a subset of forest aggregates to test for the presence, depth, or magnitude of edge effects that could affect their ability to serve as refugia for organisms sensitive to disturbance or environmental stress. Details on field and analytical methods, timing and duration of sampling, and subsampling of blocks or treatments are available in the primary publications referenced in the cited reference below.
Aubry, K. B., M. P. Amaranthus, C. B. Halpern, J. D. White, B. L. Woodard, C. E. Peterson, C. A. Lagoudakis, and A. J. Horton. 1999. Experimental design of the DEMO project: A study of varying levels and patterns of green-tree retention in harvest units in Oregon and Washington. Northwest Science 73 (Special Issue):12-26.
Keith B. Aubry, Charles B. Halpern, Charles E. Peterson, Variable-retention harvests in the Pacific Northwest: A review of short-term findings from the DEMO study, Forest Ecology and Management, Volume 258, Issue 4, 30 July 2009, Pages 398-408, ISSN 0378-1127, http://dx.doi.org/10.1016/j.foreco.2009.03.013. (http://www.sciencedirect.com/science/article/pii/S0378112709001753)
Halpern, C.B., and D. McKenzie. 2001. Disturbance and post-harvest ground conditions in a structural retention experiment. Forest Ecology and Management 154:215-225.
Halpern, C. B., D. McKenzie, S. A. Evans, and D. A. Maguire. 2005. Early responses of forest understories to varying levels and patterns of green-tree retention. Ecological Applications 15:175-195.
Halpern, C. B., J. Halaj, S. A. Evans, and M. Dovciak. 2012. Level and pattern of overstory retention interact to shape long-term responses of understories to timber harvest. Ecological Applications 22:2049-2064.
For descriptions of sampling methods for different vegetation strata, see the following:
Dovciak, M., C. B. Halpern, J. F. Saracco, S. A. Evans, and D. A. Liguori. 2006. Persistence of ground-layer bryophytes in a structural retention experiment: initial effects of level and pattern of retention. Canadian Journal of Forest Research 36:3039-3052.
Evans, S. A., C. B. Halpern, and D. McKenzie. 2012. The contributions of forest structure and substrate to bryophyte diversity and abundance in mature coniferous forests of the Pacific Northwest. The Bryologist 115:278-294.
Maguire, D. A., D. B. Mainwaring, and C. B. Halpern. 2006. Stand dynamics after variable-retention harvesting in mature Douglas-fir forests of western North America. Allgemeine Forst und Jagdzeitung 177:120-131.
Maguire, D. A., C. B. Halpern, and D. L. Phillips. 2007. Changes in forest structure following variable-retention harvests: target and non-target effects. Forest Ecology and Management 242:708-726.
Halpern, C. B., S. A. Evans, C. R. Nelson, D. McKenzie, D. A. Liguori, D. E. Hibbs, and M. G. Halaj. 1999. Response of forest vegetation to varying levels and patterns of green-tree retention: An overview of a long-term experiment. Northwest Science 73 (Special Issue):27-44.
Urgenson, L. S., C. B. Halpern, and P. D. Anderson. 2013a. Twelve-year responses of planted and naturally regenerating conifers to variable-retention harvest in the Pacific Northwest, U.S.A. Canadian Journal of Forest Research 43:46-55.
Urgenson, L. S., C. B. Halpern, and P. D. Anderson. 2013b. Level and pattern of overstory retention influence rates and forms of tree mortality in mature, coniferous forests of the Pacific Northwest, U.S.A. Forest Ecology and Management 308:116–127
