Medium‐term effects of active management on the structure of mature Douglas‐fir (Pseudotsuga menziesii) stands

Abstract

Citation: Williams, N., and M. Powers. 2019. Medium-term effects of active management on the structure of mature Douglas-fir (Pseudotsuga menziesii) stands.Abstract. We developed an observational study to (1) examine differences in the structure of mature Douglas-fir stands representing thinned, structural retention harvest and unmanaged conditions, and (2) assess the extent to which active management in mature stands expedites development of old-growth structure, relative to two old-growth indices. Time since treatment averaged 38 and 22 yr in thinned and retention harvest conditions, respectively, and stand age ranged from 106 to 193 yr when sampled. Differences in stand structure were apparent between all three management conditions, with attributes associated with stumps, live tree diameter diversity, understory vegetation, and vertical foliage structure being the strongest individual drivers of these differences. Mean old-growth index scores were lower than expected of true old-growth for each of the management conditions examined here, but were not significantly different between thinned and unmanaged stands. Our results suggest thinning in mature stands may promote certain elements of old-growth structure, including foliage height diversity, but also indicate the need for provisions against the depletion of dead wood resources. Stumps are an understudied dead wood structure, and in actively managed mature stands, our results imply that stumps may provide a degree of functional substitution where down log availability is low.

Partial Retention of Legacy Trees Protect Mycorrhizal Inoculum Potential, Biodiversity, and Soil Resources While Promoting Natural Regeneration of Interior Douglas-Fir

“…Total ingress of natural regeneration increased with tree retention in the range of harvesting treatments, in keeping with our first hypothesis. This agrees with many studies showing increased recruitment and regeneration density with retention harvesting (Coates, 2002;Newsome et al, 2010;Gottesman and Keeton, 2017) or thinning (Kuehne and Puettmann, 2008;Roberts and Harrington, 2008;Shatford et al, 2009;Dodson et al, 2012;Olson et al, 2014;Gauthier and Tremblay, 2019; Williams and Powers, 2019). The regeneration response we observed with partial overstory retention was associated with changing resource levels, providing better growing conditions for leave trees and natural regeneration.…”

“…The regeneration response we observed with partial overstory retention was associated with changing resource levels, providing better growing conditions for leave trees and natural regeneration. For example, partial removal of neighboring trees increased light and water availability and this would have promoted greater seed production of leave trees (Reukema, 1982;LePage et al, 2000;Claveau et al, 2006;Kuehne and Puettmann, 2008;Roberts and Harrington, 2008;Devine and Harrington, 2009;Shatford et al, 2009;Reich et al, 2012;Urgenson et al, 2013;Zhou et al, 2016;de Montigny et al, 2018;Willis et al, 2018;Williams and Powers, 2019). Additionally, soil disturbance resulting from harvesting itself produced a range of suitable substrates for seedling establishment (LePage et al, 2000;Roberts and Harrington, 2008;Shatford et al, 2009;Dodson et al, 2012).…”

Trade‐offs among management objectives in mature Douglas‐fir forests of the Pacific Northwest

“…Recent evaluation of drought responses in 60-to 75-year-old coast Douglas-fir implies that the benefits of single-entry thinning may be substantially diminished within two decades of treatment due to increases in stand density following thinning (Elfstrom & Powers, 2023). The current structure of our thinned stands is indicative of similar post-treatment regrowth by the time of the 2001 drought (Williams & Powers, 2019a, 2019b. These findings suggest that differences in treatment timing relative to the 2001 drought event may explain why trees in retention harvest stands showed increased drought resistance relative to trees in both thinned and unmanaged stands, while trees in thinned stands did not show improved drought responses relative to trees in unmanaged stands.…”

“…We used a randomized complete block design with eight study blocks (or replicates), each containing one stand from each of the three management conditions under analysis (see below). We used Landtype Association (LTA) (Ecoshare, 2016) and geographic proximity to delineate study blocks (see Williams & Powers, 2019a, for more information on site selection). For the drought adaptation component of this study, we collected cores from trees in all eight study blocks.…”

“…With the exception of drought resistance and drought resilience, data used in this trade-off analysis have been published in previous studies of management effects on stand structure (Williams & Powers, 2019a), carbon storage (Williams & Powers, 2019b), and forest birds (Williams et al, 2021). We briefly summarize these data below for the five forest management objective categories in this trade-off analysis, focusing on the specific variables employed in the trade-off analysis.…”