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A long-term study of the post-harvest population dynamics, development, and emergent characteristics of mature Douglas-fir leave trees on sub-boreal sites in Central Interior BC
Jull, Michael J.
Interior Douglas-fir (Pseudotsuga menziesii var. glauca) plays a unique ecological role in the sub-boreal forests of Central British Columbia. Douglas-fir in this region typically occurs as a secondary or minor component of mixed-species even- or two-aged stands of pine, spruce, and/or hardwoods. The thick barked Douglas-fir is more fire-resistant than other sub-boreal tree species, and can grow to very large sizes (40-50 m and 100-300 cm dbh) and great ages (300 to 500+ years). It is an important and distinctive element of stand-level biodiversity in the Sub-boreal Spruce (SBS) zone (Rogers and Hawkins, 2003; Whittaker, 1999; Lousier and Kessler, 1999). The importance of Douglas-fir retention in SBS ecosystems has been recognized in government policy for the last decade (BC Provincial Government, 2005; BC Ministry of Forests, 1999; BC Ministry of Forests, 1995). In Central Interior SBS forest types where Douglas-fir (or ?fir?) occurs naturally, post-harvest retention of dispersed mature fir leave-trees is now a common element of operational planning and harvesting. However, there also have been numerous, albeit anecdotal, reports of heavy attrition of fir leave-trees after harvesting on these sites (including standing mortality and windthrow). This fir mortality has been a major cause of concern to field foresters in the region. As a result, there is widespread uncertainty about the long-term outcomes of these Douglas-fir retention treatments, and the fate of fir leave-tree cohorts. The apparent phenomenon of Douglas-fir leave-tree mortality and attrition after harvesting raises two fundamental questions: 1) Why is it happening, and what mechanisms are contributing to it? 2) What is the long-term fate of Douglas-fir leave-tree populations in harvested areas and resultant second-growth stands? The first question was the focus of a recent UNBC study. Bruce Rogers and Dr. Chris Hawkins (publications in prep.) examined potential causes and site factors that may contribute to observed mortality of standing Fir leave-trees in SBS cutblocks. In his recently completed Masters thesis research, Rogers (2006) surveyed a large number of recent SBS cutblocks with fir retention. He found highly variable rates of standing tree mortality (ranging from <5% to >75% but with a mean of about 10-20%); he also found also a lack of conclusive evidence linking any readily-identifiable site factors to rates of leave tree mortality. Rogers looked further, focusing on the physiological water relations of individual fir leave-trees. He found that: (1) leave trees showed distinct signs of water (drought) stress in the first year or so following harvest of the surrounding stand, at levels that are felt induce tree mortality; but (2) leave trees showed significantly lower levels of water stress after 5 years, suggesting adaptation over time. However, Rogers could not examine longer-term trends in fir leave-tree response after harvest, as this question was beyond the scope of the study design. Our proposed study focuses on this second question, and examines the medium- to longer-term (one decade or more) development of mature Douglas-fir leave-tree cohorts retained following timber harvesting in several sub-boreal study areas in Central Interior BC (SBSdw1, SBSmk1, SBSwk1 subzones). Our overall approach will be conceptually similar to Busby et al (2006) who examined the fates of live trees retained 9 to 18 years after logging in cutblocks in western Oregon. However, in our study we have not used the aerial photo interpretation which is a major element of the Oregon study; instead, we are using direct ground-based repeated-measures observations of leave trees over time, in study populations that have been tracked and monitored since shortly after the initial harvest treatment. Data for this proposed study will be gathered from an established network of long-term monitoring sites (8 existing sites plus 2 new sites) which include a net ...
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