Farwell Canyon Research Project

Transcription

1 T E C H N I C A L R E P O R T 1 Stand Dynamics after Partial Cutting in Dry Douglas-fir Forests in Central British Columbia Farwell Canyon Research Project 216 1

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3 Stand Dynamics after Partial Cutting in Dry Douglas-fir Forests in Central British Columbia Farwell Canyon Research Project Michaela J. Waterhouse and Nola M. Daintith

4 The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the Government of British Columbia of any product or service to the exclusion of any others that may also be suitable. Contents of this report are presented for discussion purposes only. Funding assistance does not imply endorsement of any statements or information contained herein by the Government of British Columbia. Uniform Resource Locators (urls), addresses, and contact information contained in this document are current at the time of printing unless otherwise noted. ISBN Print version ISBN Digital version Citation Waterhouse, M.J. and N.M. Daintith Stand dynamics after partial cutting in dry Douglas-fir forests in central British Columbia: Farwell Canyon Research Project. Prov. B.C., Victoria, B.C. Tech. Rep Prepared by Nola Daintith and Michaela Waterhouse B.C. Ministry of Forests, Lands and Natural Resource Operations Cariboo Region, Williams Lake, B.C. Copies of this report may be obtained, depending upon supply, from: Crown Publications, Queen s Printer 2nd Floor, 563 Superior Street Victoria, BC v8w 9v For more information on other publications in this series, visit hfdcatalog/index.asp 216 Province of British Columbia When using information from this report, please cite fully and correctly.

5 ABSTRACT The Farwell Canyon project was established within two Douglas-fir (Pseudotsuga menziesii) stands in the Very Dry Mild Interior Douglas-fir (IDFxm) biogeoclimatic subzone in the Cariboo Region, British Columbia in 21. The project goals were to improve forage for wildlife and livestock (i.e., increase vascular plant cover), improve the growth of the residual stand by reducing inter-tree competition, shift the plant community composition to one that is more typical of open forest condition, and improve the resiliency of the stand to catastrophic fire. From a timber management perspective, the goal was to increase individual tree growth by logging and thinning while maintaining overall stand-level growth. To achieve these goals, treatment combinations of modified logging, pre-commercial thinning, and burning were applied to return the forest to a more open condition that is typical of Douglas-fir forest adjacent to grassland in the IDFxm. Four treatments were applied to one or both blocks: 1. No-treatment areas were established to serve as untreated controls for demonstration and comparison purposes. 2. The logging treatment used a fellerbuncher and grapple skidder combination to apply a BDq approach that left a residual stand basal area of about 15 m 2 /ha (B = residual stand basal area; D = largest-diameter trees; q = diminution quotient). The merchantable utilization was reduced to 12.5 cm diameter at breast height (dbh) for Douglas-fir. 3. The logging treatment was followed by manual thinning of juvenile stems (< 12.5 cm dbh). 4. On block 2, the logged-thinned-burned treatment was logged and thinned as noted above and then underburned to remove accumulated slash. Assessment of stand structure, growth, and regeneration 1 14 years after harvesting in the pilot project provided trends and probable outcomes of the management options tested. The modified logging treatment, based on the prescribed BDq values, successfully created and maintained an open forest stand condition for 1 years. However, growing space has started to infill with Douglas-fir regeneration, particularly in the past 5 years (28 213), and this may increase dramatically with the 214 germinant crop. The growth and yield results show that the logging and logging-thinning prescriptions that were tested provided a benefit to individual tree growth as well as stand volume growth. However, the residual basal area target of 15 m 2 /ha was set too low because Prognosis BC model projections showed that relatively long cutting cycles of 4 7 years are needed to meet the chosen management target. It would appear that if the goal of harvesting ingrown Douglas-fir stands is to return them to a more open stand structure with fewer, larger trees without sacrificing merchantable volume growth, then basal area must be greater than 15 m 2 /ha, and there must be density management (thinning) in the smaller diameter classes. iii

6 ACKNOWLEDGEMENTS We gratefully acknowledge the work of Ordell Steen, who envisioned the project and worked with Rick Dawson and Harold Armleder (British Columbia Ministry of Forests, Lands and Natural Resource Operations [FLNRO] staff), Ken Day (University of British Columbia Alex Fraser Research Forest), and Rider Cheyne (Riverside Forest Products Ltd., now Tolko Industries Ltd.) to make it a reality. The timber harvesting treatments tested in the pilot project were made possible through the support and co-operation of Tolko Industries Ltd. and the Cariboo-Chilcotin Resource District. This project has been funded by the B.C. Terrestrial Ecosystem Restoration Project, the Habitat Conservation Trust Fund, and FLNRO under Experimental Project We would like to thank the numerous FLNRO staff who assisted with the project since its establishment; Amanda Nemec, who provided statistical analyses for this report; and three anonymous reviewers who provided helpful and insightful comments, which substantially improved this report. iv

7 CONTENTS Abstract... Acknowledgements... Introduction... 1 Study Area... 2 Methods... 3 Statistical Design and Treatment Layout... 3 Treatments... 4 Control... 4 Logged Logged-thinned... 6 Logged-thinned-burned... 6 Stand Structure and Regeneration Sampling... 7 Stand Growth and Yield Sampling... 7 Data Analysis... 8 Stand structure and regeneration... 8 Stand growth and yield... 8 Results and Discussion... 9 Stand Structure and Regeneration... 9 Seedfall and Germination after a Bumper Seed Year Stand Growth and Yield Quadratic mean diameter Residual basal area Merchantable volume Management Implications The value of harvesting with modifications The value of thinning The value of burning Conclusion... 2 Literature Cited APpendices 1 Project timeline Mean density of live Douglas-fir trees per hectare by location, year, treatment, and layer Stand and stock tables for each block and treatment in 22 and tables 1 Basal area, diminution quotient, merchantable volume, and stem density pre-harvest, prescription target, and post-harvest anova design to compare tree density among the control, logged, and logged-thinned treatments Layer densities of Douglas-fir compared among treatments using analysis of variance, based on the Poisson model Quadratic mean diameter, basal area, and merchantable volume immediately after harvest and 1 years later by block and harvesting treatment iii iv v

8 Figures 1 Layout of treatment areas and sampling units in block Layout of treatment areas and sampling units in block Pre-harvest stand in 21 and no-harvest control in Logged treatment post-harvest 21 and Logged-thinned treatment in 22 and Logged-thinned-burned treatment in 22 and Density of live Douglas-fir trees by layer in the no-treatment controls from 23 to Density of live Douglas-fir trees by layer in the logged treatments from 23 to Density of live Douglas-fir trees by layer in the logged-thinned treatments from 23 to Density of live Douglas-fir trees by layer in the logged-thinned-burned treatment from 23 to Density of germinants by substrate and treatment in 214 in the control, logged, logged-thinned, and logged-thinned-burned treatments in Quadratic mean diameter of all stems 2. cm by treatment in block 1 and block Basal area of all stems 2. cm by treatment in block 1 and block Merchantable volume of trees 17.5 cm dbh by treatment in block 1 and block Merchantable volume of trees 12.5 cm dbh by treatment in block 1 and block vi

9 INTRODUCTION Timber supply shortfall over the next 6 years is of great concern in several central interior British Columbia Timber Supply Areas (TSAs) due to the high mortality of lodgepole pine (Pinus contorta) forests caused by mountain pine beetle (Dendroctonus ponderosae) and rapid clearcut harvesting of those forests over the past 2 years. Prior to the shift to pine salvage, uneven-aged Douglas-fir stands in the Interior Douglas-fir (IDF) biogeoclimatic zone were a major contributor to timber supply, particularly in the Williams Lake TSA, and the forest industry is in the process of transitioning back to harvesting those forest types. Douglas-fir stands are managed through a single-tree selection silvicultural system based on cutting roughly 5% of the stand basal area through all size classes 17.5 cm dbh (Steen 25 1 ). Pre-commercial thinning of smaller stems (> 1.3 m tall and < 7.5 cm dbh) is required only when maximum density exceeds 1 stems per hectare (sph). The Farwell Canyon project was established in 21 to pilot some refinements to the single-tree selection system in order to return ingrown forests to a more productive, resilient state. The trial provides a timely opportunity to assess the longer-term effects of partial cutting, pre-commercial thinning, and underburning treatments on stand structure and timber productivity. Uneven-aged Douglas-fir forests have many sizes of trees of varying ages and densities as a result of the ecological adaptations to climate and soil, and interactions with fire and insects, such as western spruce budworm (Choristoneura occidentalis) and Douglas-fir bark beetle (Dendroctonus pseudotsugae). Historically, fires were more frequent, and they controlled tree density, especially in the smaller size classes, which created forests dominated by larger, older trees with open understoreys (Iverson et al ; Daniels 25; Steen and Armleder 28). Western spruce budworm can also cause substantial mortality in the understorey tree layers, and strongly affects growth rates in the residual trees (Axelson et al. 215). Geographically broad outbreaks have occurred every 32 years, with durations of 15 years in the Cariboo Region, at least as far back as the early 16s (Axelson et al. 215). High stand densities, resulting from lower fire frequency, have reduced individual tree growth rates, increased the risk of crown fire, lowered forage production, and reduced understorey tree vigour (Steen and Armleder 28). In the very dry mild subzone (xm) within the IDF biogeoclimatic zone, understoreys are often dense (frequently > 2 sph of Douglas-fir between 2.5 and 12.5 cm dbh), and trees have high height-to-diameter ratios, negligible leader growth, low leaf area and live crown (Steen 25), and generally poor condition. These small stems often form a closed canopy, especially in the space between clumps of larger trees, which are typically heavily fire scarred. Larger trees are commonly 2 years old, while the oldest trees are 45 years old (Axelson et al. 215). The smaller trees are predominantly years old, with few trees < 5 years old (Steen 25). The Farwell Canyon project was established within two Douglas-fir stands in the IDFxm in the Cariboo Region to improve forage for wildlife and live- 1 Steen, O.A. 25. Establishment report and working plan for Farwell Canyon study: managing ingrown Douglas-fir stands for biodiversity, forage and timber. B.C. Min. For. Range, Williams Lake, B.C. Unpubl. rep. 2 Iverson, K.E., R.W. Gray, B.A. Blackwell, C. Wong, and K.L. MacKenzie. 22. Past fire regimes in the Interior Douglas-fir, dry cool subzone, Fraser variant (IDFdk3). Prepared for Lignum Ltd., Williams Lake, B.C. Unpubl. rep. 1

10 stock (i.e., increase vascular plant cover), improve the growth of the residual stand by reducing inter-tree competition, shift the plant community composition to one that is more typical of open forest condition, and improve the resiliency of the stand to catastrophic fire (Steen and Armleder 28). From a timber management perspective, the goal was to increase individual tree growth by logging and thinning while maintaining overall stand-level growth (Steen and Armleder 28). To achieve these goals, treatment combinations of modified partial cutting, pre-commercial thinning, and burning were applied to return the forest to a more open condition that is typical of Douglas-fir forest adjacent to grassland in the IDFxm. Two study blocks were partially cut using the single-tree selection silvicultural system guided by a residual basal area target of 15 m 2 /ha a target that was thought would maintain, or at least not substantially reduce, stand-level growth. A post-harvest, pre-commercial thinning treatment removed trees < 12.5 cm dbh with poor vigour and form, and those that were damaged by harvesting and growing too closely together. Pre-commercial thinning activities in conjunction with harvesting have the potential to improve individual tree and merchantable stand growth (Marshall et al. 25; Johnstone and van Thienen 26). Underburning was done on part of one block to reduce the volume of thinning and logging slash in order to encourage establishment of native grassland species, reduce wildfire hazard, and create fuel conditions where open stand conditions could be maintained by repeated burning. Steen and Armleder (28) found that the logging treatment, without thinning or burning, was enough to set the forest on a path to meet the intended objectives within the first 5 years post-harvest. Vascular plant cover increased, grassland grasses became more common, forage production increased, weeds were not a significant concern, and the density of Douglas-fir regeneration remained low. Steen and Armleder (28) were concerned that ingress and growth of the regeneration over the subsequent 3-year period would necessitate further treatments to maintain the open stand condition. This report describes effects of the treatments on the overstorey tree growth and the changes in stand structure over the 1 14 years since treatment. It also documents the abundant Douglas-fir cone crop in 213 and subsequent production of germinants in 214; this influx of regeneration may have long-term effects on stand structure and growth. STUDY AREA The study was conducted on two blocks near Farwell Canyon, south of Riske Creek, on the Chilcotin Plateau, in central British Columbia. Both blocks are adjacent to open grasslands and are in the IDFxm biogeoclimatic subzone. Block 1 is 44.2 ha, and block 2 is 71.5 ha. Prior to treatment, both sites supported Douglas-fir stands consisting of 4 5 Douglas-fir sph that were > 4 cm dbh and occurred as widely spaced individuals or in small groups of three to five trees separated by a very dense sub-canopy of small (mostly cm dbh) Douglas-fir trees. There were very few recently established tree seedlings in 21, possibly because the stand was fully occupied. The undergrowth vegetation consisted primarily of mosses (especially Pleurozium 2

11 schreberi and Hylocomium splendens), with few grasses or shrubs. Directly beneath the canopy of groups of large (> 4 cm dbh) trees, small trees were typically absent, and the undergrowth included an increased cover of grasses. METHODS Statistical Design and Treatment Layout The Farwell Canyon project was established as a pilot study. The two blocks are representative of the Farwell Canyon project area, but results cannot necessarily be applied to a broader geographic area of the IDFxm in the Cariboo Region without confirmation through other studies. The study is a randomized block design with two replicate blocks and three treatments: control (no treatment), logged, and logged-thinned. An additional treatment (loggedthinned-burned) was added to block 2 to gather preliminary information on vegetation and regeneration after burning (Steen and Armleder 28). Wildlife-tree patches were established in each block prior to harvesting and were used as the no-harvest controls. The patches were visually assessed to ensure that they were representative of the pre-treatment condition. Preharvest, the blocks were cruised using a grid of plots set up at 1-m line intersections. After harvesting, four areas (8 8 m) (i.e., logged treatment) were established in each block, at points centred on randomly selected cruise plots. The areas were field checked to ensure that they were representative of the site in terms of stand structure, soil type, moisture, and vegetation, and had relatively uniform slope and slope position. If these criteria were not met, another point was randomly selected or the area was moved 4 m along the cruise line. The rest of each block was thinned after harvesting, and in block 2, a portion of the thinned area was underburned. Figures 1 and 2 show the location of the treatment areas in each block. LEGEND Block boundary Road/landing Fence Growth and yield plot Control Logged Logged-thinned Wildlife Tree Patch Regeneration subplot W N C E S m Sign FIG E 1 Layout of treatment areas and sampling units (subplots) in block 1. 3

12 LEGEND Block boundary Control Road/landing Logged Fence Logged-thinned Growth and yield plot Logged-thinned-burned N Regeneration subplot W E S 2 m Layout of treatment areas and sampling units (subplots) in block 2. Treatments (a) C Control No-treatment control areas contained large-diameter, older trees as scattered individuals or in small groups, and dense understorey layers of Douglas-fir saplings and poles. Figure 3a and b show the pre-harvest stand in 21 and no-harvest control in 212, respectively. (b) Pre-harvest stand in 21 (a) and no-harvest control in 212 (b) (photos taken at different points). Logged Partial cutting was planned using the BDq approach associated with single-tree selection harvesting. The residual basal area target (B) was set to a minimum to maintain stand-level growth (Steen and Armleder 28). The largest-diameter trees included in the cut (D) were 62.5 cm dbh; larger trees were left for their biodiversity and historic value. The low diminution quotient (q) values ( ) were intended to intensify the harvest in the smaller diameter classes. Harvesting was aided by marking trees to leave on 5% of block 1 and 1% of block 2. In 21, the logged treatment was harvested mostly with a fellerbuncher, but a few of the largest trees were hand-felled. Whole trees were moved with grapple skidders for processing on landings. The logged treatment included two modifications to practices that were considered standard in 21. The 4

13 first modification was to lower the diameter limit of harvested trees from 17.5 to 12.5 cm dbh. This was done to increase the number of small trees harvested and reduce the dense sub-canopy layer as the fellerbuncher moved through the stand. In addition, the inclusion of these small trees in the harvest allowed a greater number of larger trees to be retained while cutting the same total volume. The second modification was to increase the mortality of juvenile trees (< 12.5 cm dbh) by hot-sawing (leaving the fellerbuncher saw running and open) while accessing merchantable stems. Physically damaged juvenile trees were slashed following logging, as part of normal practices. Table 1 shows the pre- and post-harvest basal area, merchantable volume, and stems/ha for all species (live and dead potential 12.5 cm dbh), and the target values by block. Pre-harvest data were collected from a grid of variableradius cruise plots (block 1: n = 27; block 2: n = 46). Post-harvest, marked leave trees 12.5 cm dbh were tallied by size class in variable-radius plots within marked parts of each block (block 1: 13 ha; block: 2 48 ha), and the trees > 1.3 m tall and < 12.5 cm dbh were counted, by size class, in the original fixed-area plots (3.99-m radius) that were established at cruise plot centres. The postharvest stand structure was very close to the intended structure (Table 1). Figure 4a and b show the stand structure post-harvest in 21 and 1 years after treatment in 212, respectively. TABLE 1 Basal area, diminution quotient (q), merchantable volume, and stem density pre-harvest, prescription target, and post-harvest Block Treatment Basal area (m 2 /ha) q Volume (m 3 /ha) Stems/ha ( 12.5 cm dbh) Stems/ha (> 1.3 m tall and < 12.5 cm dbh) 1 Pre-harvest Target post-harvest Post-harvest Pre-harvest Target post-harvest Post-harvest (a) (b) FIGURE 4 Logged treatment post-harvest 21 (a) and 212 (b) (photos taken at different points). 5

14 Logged-thinned This treatment formed the matrix treatment and was applied to > 5% of both blocks. The logging was followed by motor-manual thinning of non-merchantable stems. The objective of the thinning treatment was to retain only desirable saplings with good form and vigour with a high potential for growth response. Saplings were cut if they had < 3% live crown, sweep, galls, scars, cankers, or breakage. The minimum inter-tree distance was set at 1.5 m. Thinning was completed in March 22 (Figure 5a). In 23, the density of all trees < 12.5 cm dbh was 36 sph on block 2, which was close to the estimate of 335 sph using the thinning criteria (Steen 25). Postthinning stem densities were not measured on block 1 but were estimated to be 93 sph. Figure 5b shows the stand 1 years post-treatment. (a) 5 (b) Logged-thinned treatment in 22 (a) and 212 (b) (photos taken at different points). Logged-thinned-burned In May 22, part of the logged-thinned treatment area in block 2 was underburned to remove accumulated slash (Figure 6a). Fine-fuel consumption was nearly continuous, with 85% ground coverage. Due to high moisture levels in the forest floor, little forest floor was consumed, other than surface litter. Most of the forest floor was surface-charred, and some large (> 3 cm diameter) pieces of woody debris were completely consumed. Figure 6b shows the stand 1 years post-treatment in 212. (a) 6 (b) Logged-thinned-burned treatment in 22 (a) and 212 (b) (photos taken at different points). 6

15 Stand Structure and Regeneration Sampling Stand Growth and Yield Sampling For the stand structure and regeneration component of the study, sampling units consisted of a cluster of five 3.99-m subplots. Three sampling units (per block) were located in wildlife tree patch control areas one or two sampling units per patch depending on patch area; three sampling units were located in the logged treatment units one in each of three units selected at random from the four logged units; and three sampling units were installed at random locations (i.e., randomly selected points on the cruise-survey grid) in the logged-thinned area of each block. In block 2, additional sampling units were installed along three strips of burned ground, which were separated by unburned skid trails. The locations of the sampling units and subplots are shown in Figures 1 and 2. All subplots were located inside fenced areas to eliminate the effect of cattle grazing. In each 3.99-m radius subplot, the number of live trees by species was counted in four layers: layer 1: stems 12.5 cm dbh; layer 2: stems 7.5 to < 12.5 cm dbh; layer 3: stems > 1.3 m in height and < 7.5 cm dbh; and layer 4: stems 1.3 m in height (including germinants). In block 2, data were collected in the subplots in 23, 26, 28, and 213. In block 1, data collection began in 26 in the logged-thinned treatment and then expanded to include all the treatments in 28 and 213 (Appendix 1). In September 214, germinants within each subplot were counted by seedbed type (humus, mineral, rotten wood, and moss) using a set of five micro-plots (1 m 2 ). The micro-plots were located at each subplot centre and at 2 m from the centre in four cardinal directions. Micro-plots were marked with 3-cm flagged wire pins. In conjunction with the density counts in 213, the number of well-spaced trees was counted in each subplot. The well-spaced survey followed methods for uneven-aged interior Douglas-fir (Weaver 213). The minimum height was set at 1 cm, and the counted well-spaced trees were potential crop trees. In 213, many of the layer 3 trees were in poor condition due to repeated defoliation by spruce budworm, and were not counted as well-spaced. The cone crop was visually rated in June 213 (Eremko et al. 1989). In July 213, five seedfall traps (.5.5 m, metal mesh covered) were placed in the three logged-thinned treatment plots in each trial block. Seed was collected from the traps, then counted and cut (potentially viable or not) in May of 214. In each block, growth and yield plots were established in larger control areas (two plots in block 1 and three plots in block 2), in each of the four logged treatment areas, and in three locations within the logged-thinned treatment (Figures 1 and 2). One of the plots in block 2 was in the logged-thinnedburned treatment, but for the purposes of growth and yield, it was treated as a logged-thinned plot because it had similar stand structure. Provisional growth and yield plot locations were randomly selected from all original cruise plots and points midway between cruise plots along each cruise line. Each provisional plot was visited on-site to ensure that the stand structure of the plot was representative of the treatment area and that the site features were relatively uniform (Steen 25). In all, 19 plots were installed across both blocks, resulting in two to four plots per treatment on each block. Establishment of growth and yield plots was consistent with the procedures for natural stands outlined in Forest Inventory and Monitoring Program Growth and Yield Standards and Procedures, 21 (Province of British Columbia 21). Plots were established post-harvest, and all stems 2 cm dbh were tagged; data were collected in either December 21 January 22 or 7

16 September October 22 (Appendix 1). On block 1, plot size was.4 ha in the control and.8 ha in the logged and the logged-thinned treatments. On block 2, plot size was.4 ha in the control and logged treatments, and.8 ha in the logged-thinned treatment. Subplots (.1 ha) were established at the centre of each sample plot to measure stems 2 to < 4 cm dbh. Growth and yield plots were remeasured in 212 (1 11 years after establishment), and each plot tree was re-assessed for species, dbh, total height, and crown ratio. Data Analysis Stand structure and regeneration Analysis of variance was used to compare the 213 regeneration counts and the 214 germinant counts by layer among treatments. Prior to analysis, count data were summed over the five subplots (and five micro-plots) comprising each sampling unit. Sources of variation in the sampling unit totals (for a given year and layer) are listed in Table 2. Each count (sampling-unit total) was assumed to have a Poisson distribution with mean μ ijk, given by: log (μ ijk ) = μ + τ i + β j + τβ ij + ε ijk where i denotes treatment (control, logged, or logged-thinned), j is the block (1 or 2), and k is the sampling unit (1, 2, or 3) within a given block and treatment unit; μ is a constant (intercept), τ i is the fixed treatment effect, and β j, τβ ij, and ε ijk are the random (independent and normally distributed) effects of block, block treatment, and sampling unit, respectively. All analyses were conducted using PROC GLIMMIX in SAS/STAT. 3 TABLE 2 anova design to compare tree density among the control, logged, and logged-thinned treatments Source of variation Degrees of freedom Fixed or random Block (B) 1 Random Treatment (T) 2 Fixed B T 2 Random Plot (B T) 12 Random Total 17 Stand growth and yield The 1-year growth and yield data were modelled in Prognosis BC to predict treatment effects on stand growth. Prognosis BC is an individual-tree, distance-independent, growth and yield model that can be used to simulate the development of multi-layered Douglas-fir stands over a specified period (ESSA Technologies Ltd. 29). It has been calibrated for use in uneven-aged Douglas-fir stands in the IDFdk3 in the Cariboo Region. The Farwell Canyon blocks are classified as zonal IDFxm sites, but both are very close to the transition to the IDFdk3 subzone. For the purpose of Prognosis modelling, the project blocks were approximated by the IDFdk3/5 site series based on slope, aspect, and plant association (Steen and Coupé 1997). The 22 and 212 dbh measurements were used to calculate a 1-year diameter increment, which is used by the model to localize the predicted growth of stems with dbh 7.5 cm. This localization allows for unique variations in site characteristics that are not represented in the model parameters (ESSA Technologies Ltd. 29). 3 SAS/STAT software, Version 9.3 of the SAS System for Windows Copyright SAS Institute Inc. SAS and all other SAS Institute Inc. product or service names are registered trademarks or trademarks of SAS Institute Inc., Cary, N.C., USA. 8

17 RESULTS AND DISCUSSION Stand Structure and Regeneration In the control treatment on block 2, the total density of Douglas-fir trees (all layers combined) increased from 23 to a peak in 26 (372 sph), then dropped to a 1-year low in 213 (268 sph) (Figure 7). The density of layer 1 trees (> 12.4 cm dbh) remained steady over the study period (Figure 7). The density of layer 2 trees ( cm dbh) declined substantially from 26 (813 sph) to 213 (467 sph), while the decline in layer 3 (> 1.3 m tall up to 7.5 cm dbh) was even steeper from 212 to 57 sph (i.e., 76% decline). By 213, stems per hectare were equally divided between the four size classes, with infill occurring in layer 4 (trees < 1.3 m tall) between 26 and 213. In block 1, the total tree density in the control in 213 was similar to that in block 2; however, block 1 had many more layer 4 trees and fewer layer 2 and 3 trees. Appendix 2 presents a summary of the mean density and standard deviation of live Douglas-fir trees by layer, treatment, and year in each block. 6 5 Control Layer 1 Layer 2 Layer 3 Layer 4 Density (sph) blk 2 26 blk 2 28 blk blk 2 28 blk blk 1 FIG E 7 Density (stems per hectare [sph]) of live Douglas-fir trees by layer in the no-treatment controls from 23 to 213. The logged treatment on block 2 had a larger proportion of layer 3 and 4 trees than layer 1 and 2 trees, in all years (Figure 8). Block 1 had a much lower density of layer 2 and 3 trees than layer 1 or 4, and was dominated by layer 4 trees in 28 and 213. In block 2, there was a drop (22%) in layer 2 trees and steep decline (about 6%) in layer 3 trees between 26 and 213 (Appendix 2). There was a strong influx of layer 4 trees into both blocks between 28 and 213. In both the logged-thinned and logged-thinned-burned treatments, the stands in block 2 were composed primarily of layer 1 trees in 23 (Figures 9 and 1) (Appendix 2). There was a decline in layer 1 trees from 23 to 26 (14% in logged-thinned and 17% in logged-thinned-burned), most likely due to bark beetles and fire scorching. In layers 2 and 3, trees density was notably low or trees were absent throughout the 1-year study period. Total densities rose with ingress of regeneration in both treatments by 26, then in 28 declined substantially in the burned treatment and slightly in the thinned treatment. There was a second influx of ingress into the regeneration layer in 9

18 6 5 Logged Layer 1 Layer 2 Layer 3 Layer 4 Density (sph) blk 2 26 blk 2 28 blk blk 2 28 blk blk 1 FIG E 8 Density (stems per hectare [sph]) of live Douglas-fir trees by layer in the logged treatments from 23 to Logged-thinned Layer 1 Layer 2 Layer 3 Layer 4 Density (sph) blk 2 26 blk 2 28 blk blk 2 26 blk 1 28 blk blk 1 FIG E 9 Density (stems per hectare [sph]) of live Douglas-fir trees by layer in the logged-thinned treatments from 23 to Layer 1 Layer 2 Layer 3 Layer 4 Logged-thinned-burned Density (sph) blk 2 26 blk 2 28 blk blk 2 FIG E 1 Density (stems per hectare [sph]) of live Douglas-fir trees by layer in the logged-thinned-burned treatment from 23 to

19 both treatments between 28 and 213. In both treatments, total tree density reached a maximum in 213, approximately double that of any previous year (Appendix 2). The density of trees in layer 4 was considerably higher in block 1 than in block 2 by 213, perhaps because more growing space was available due to lower initial tree densities in layers 2 and 3. As of 213, the species composition of the forest on the blocks was dominated by Douglas-fir (96.1%), with a minor component of aspen, pine, and spruce. Most of the aspen occurred in the logged-thinned-burned treatment, where it made up 11.3% of the total stems in layer 4. In 213, for each layer, there were significant (α =.5) differences in Douglas-fir density among the control, logged, and logged-thinned treatments (Table 3). Due to the harvesting in 21, layer 1 continued to have a lower density in the logged or logged-thinned treatments than in the control treatment. Due to the thinning in 22, both layers 2 and 3 continued to have lower densities in the logged-thinned treatment than in either the control or logged treatments despite substantial mortality caused by spruce budworm in those two treatments. TABLE 3 Layer densities of Douglas-fir compared among treatments using analysis of variance, based on the Poisson model. Treatments (in a given row) that share the same superscript letter are not significantly different. Treatment Variable Year Control (log[mean]) Logged (log[mean]) Loggedthinned (log[mean]) Prob F Layer a 1.98 b 1.84 b.3 Layer a 1.55 a.21 b.1 Layer a 2.94 a 2.9 b <.1 Layer a 4.8 b 4.19 b.3 Germinant a 3.92 b 4.62 c <.1 Layer 4 densities were substantially higher in the logged and loggedthinned treatments than in the no-harvest control. This differs from another study in the Interior Douglas-fir biogeoclimatic zone in British Columbia where there was no significant difference in either ingress or advanced regeneration density among no-harvest controls (37 m²/ha) and stands cut to 15, 2, and 25 m²/ha of basal area after 1 years (Bealle Statland and Johnstone 24). In their study, ingress was abundant ( sph in the treatments) but was mostly less than 3 cm tall. In 213, despite the high total stem densities (> 2 sph) in all treatments, the well-spaced density of crop trees was < 7 sph in the logged-thinnedburned treatment and the logged-thinned treatment on block 1 (Appendix 2). Densities in the other treatment units were marginally > 7 sph, which is the minimum well-spaced density required to consider a stand stocked. The low well-spaced counts reflect the clumpy distribution of Douglas-fir and the lack of crop trees in layers 2 and 3 caused by either mechanical thinning or spruce budworm damage. The minimum survey height was set at 1 cm in our study, and captured many new ingress trees in layer 4. The number of well-spaced trees would have been even lower if the standard 4 cm survey height had been used. 11

20 Over the last 1 years, the stands were dominated by layer 1 Douglas-fir trees, but the densities in the non-merchantable layers have shifted in response to the initial treatments in 21 and 22, and the spruce budworm infestation. In the controls, tree mortality in layers 2 and 3, and small leaf area on survivors due to spruce budworm, were sufficient to allow limited establishment of regeneration. Many of the newly established layer 4 trees are quite small and may not continue growing if the larger trees recover their foliage. The surviving layer 2 and 3 trees are in poor condition, and they continue to contribute to the risk of a catastrophic fire, compete with larger trees for resources, and occupy growing space that would otherwise be available for vascular plants. Steen and Armleder (28) proposed that the harvesting treatment on its own was enough to create open stand conditions. This is certainly true for block 1, where the density of small trees (> 1.3 m tall and < 12.5 cm dbh) was low pre-harvest (184 sph) and probably sufficient for block 2, especially when combined with the later mortality caused by spruce budworm. However, this open stand condition is in the process of reversing because there has been good establishment of layer 4 regeneration. Thinning made no difference to layer 4 densities after 1 years, in 213; however, germinant densities in 214 were significantly higher in the logged-thinned treatment than in the logged treatment. The density of the layer 4 trees may fluctuate over time depending on weather, seedfall, and other factors. For example, in the logged-thinnedburned treatment, there was a substantial decline in the density of layer 4 trees between 26 and 28, possibly due to exposure to frost and/or desiccation generated by the very open conditions. The growth rate of the layer 4 trees is uncertain; very slow growth of recruits in partially cut Douglas-fir forests has been reported elsewhere (Bealle Statland and Johnstone 24). Seedfall and Germination after a Bumper Seed Year In June 213, the cone crop on the co-dominant Douglas-fir trees was visually rated as heavy on both blocks using the method described by Eremko et al. (1989). Potential viability of the mature seed was 63.4% (based on cutting tests). The potentially viable seed rain was 1.41 million seeds/ha (SD =.61 million seeds/ha) on block 1 and 2.8 million seeds/ha (SD =.64 million seeds/ha) on block 2. Douglas-fir cone crops are expected every 2 1 years (Eremko et al. 1989) but are extremely variable in time and space (Owens 1973; El-Kassaby and Barclay 1992). In the Sub-boreal Spruce zone to the east of the Farwell Canyon project, good cone crops in Douglas-fir forests (i.e., > 5 viable seeds/ha) occurred six times in a 17-year period. 4 Given the amount of new regeneration on the study blocks between 28 and 213, there must have been some years with reasonable seed production despite the foraging by spruce budworm. In 214, there were many new germinants in the treatments, ranging from a low of 3 sph in the controls to a high of 37 sph in the loggedthinned-burned treatment in block 2 (Figure 11). There was no significant difference (p =.36) between the number of germinants in the loggedthinned and logged-thinned-burned treatments on block 2 when densities were compared. When the three replicated treatments were compared, the control, logged, and logged-thinned treatments were significantly different from each other 4 M. Waterhouse, FLNRO, Williams Lake, B.C., unpubl. data. 12

21 35 Mineral soil Humus Moss Wood 3 Density (sph) Control Logged Logged-thinned Logged-thinnedburned FIG E 11 Density (stems per hectare [sph]) of germinants by substrate and treatment in 214 in the control (n = 3), logged (n = 3), logged-thinned (n = 3), and logged-thinned-burned (n = 15) treatments in 214. (Table 3), with the greatest density of germinants in the logged-thinned treatment. The differences were most likely due to the amount of available growing space, light, and moisture, and the type of seedbed. The controls in 26 had 53% moss cover, while the other treatments had < 17% moss cover (Steen and Armleder 28). Moss is not a productive seedbed due to its sensitivity to desiccation (Burton et al. 2). Most of the germinants recorded in 214 occurred on a humus seedbed within the three logged treatments (Figure 11). If a small percentage of the germinants survive, they could be a significant input into the regeneration understorey, and could expedite the return to closed stand conditions. Stand Growth and Yield Growth and yield plot data collected in 22 and 212 were compiled using Prognosis BC. Compiled stand variables are shown in Table 4. Appendix 3 provides further details (by 5 cm es) for each site, treatment, and assessment year. Prognosis BC reports the quadratic mean diameter (QMD), which is the diameter of the tree of mean basal area, and which gives greater weight to larger trees in the stand. The estimation procedure for merchantable log volumes in Prognosis BC uses equations developed by Kozak (1988) (ESSA Technologies 211). Merchantable volume is based on a stump height of 3 cm, top diameter of 1 cm, and minimum dbh of 17.5 cm (the normal utilization limit for Douglas-fir). Immediately after harvest, the QMD of the control and logged treatments was similar on each block (Table 4). In comparison, the QMD of the loggedthinned treatment was substantially higher. Even though the logged treatment used hot-sawing to reduce the number of small-diameter stems, the diameter distribution after treatment was similar to that on the control, with most stems in the 5 and 1 cm es (Appendix 3). The loggedthinned treatment shifted the diameter distribution so that most stems were in the 15 2 cm es on block 1 and the 2 25 cm es on block 2. Ten years later, QMD on the control treatments on both blocks showed a steep upward trend due to significant mortality in the 1 cm dbh 13

22 TABLE 4 Quadratic mean diameter (qmd), basal area, and merchantable volume (based on a stump height of 3 cm, top diameter of 1 cm, and minimum dbh of 17.5 cm) immediately after harvest and 1 years later by block and harvesting treatment Block Treatment qmd (cm) Prognosis BC compilation of 22 post-harvest G&Y a plot data Basal area (m 2 /ha) Merchantable volume (m 3 /ha) qmd (cm) Prognosis BC compilation of 212 G&Y plot data Basal area (m 2 /ha) Merchantable volume (m 3 /ha) 1 Control Logged Logged-thinned Control Logged Logged-thinned a G&Y: growth and yield. classes. On block 1, a similar upward trend in QMD was observed on the logged treatment due to mortality in the 5 1 cm es. The post-harvest data showed changes in stand basal area and merchantable volume in response to the two logging treatments (logged and logged-thinned) (Table 4). On block 1, the average residual basal area between the two logging treatments was 17 m 2 /ha compared to 43 m 2 /ha on the control, resulting in approximately 4% basal area retention (assuming that the control plots were representative of the pre-harvest stand structure). On block 2, 47% of the basal area was retained when the logged and logged-thinned treatments were averaged. On blocks 1 and 2, respectively, 46% and 58% of the merchantable stand volume was retained in the two logging treatments. Over the 1-year assessment period, basal area and merchantable volume generally increased on the two logging treatments (Table 4), with the exception of the logged treatment on block 2, where Douglas-fir bark beetle killed some large-diameter trees (Appendix 3). In the logged growth and yield plots, 42% and 32% of the trees on block 1 and 2, respectively, died in the 1 years following harvest. This was considerably higher than mortality reported on the logged-thinned plots, where only 11% of the trees on block 1 and none of the trees on block 2 died. Most of the mortality occurred in the suppressed and intermediate layers, particularly in the 1 cm diameter class, as a result of continual spruce budworm infestation, but a few large-diameter trees were killed by Douglas-fir bark beetle (logged treatment on block 2). The 1-year mortality rate was greatest on the control, where approximately 6% of trees on both sites died. On block 2, most mortality was reported in the smaller diameter classes ( 1 cm), and this resulted in a reduction in basal area from 34 m 2 /ha to 29 m 2 /ha (Table 4). Similarly, on block 1, many small-diameter trees died in the 1-year assessment period, but the loss of a number of large-diameter trees to Douglas-fir bark beetle was the the primary contributor to the downward trend in basal area and merchantable volume (Appendix 3). The 212 plot data were modelled in Prognosis BC for a period of 8 years ( ), and growth projections are presented for three stand variables: quadratic mean diameter, basal area, and merchantable volume. In Figures 12 15, the 22 and 212 data points are based on measured tree data. 14

23 Quadratic mean diameter Prognosis BC predicted that over the 8-year simulation period ( ), the QMD of the residual stand on both blocks will increase at a faster rate in the two logging treatments than in the control (Figure 12a and b). On block 1, the QMD on the logged and logged-thinned treatment will increase at a similar rate, whereas, on block 2, the QMD will increase at a much greater rate on the logged-thinned treatment than on the logged treatment. Based on the 212 data, the model predicted a slowdown in the rate of increase of QMD on all treatments on block 1 and on the control treatment on block 2 after the steep upward trend in the first 1 years after harvest. The rate of mortality caused by spruce budworm in all treatments, except the logged-thinned treatment on block 2, would not be predicted by the model. If mortality in the smaller diameter classes has stabilized since 212, QMD could be expected to increase as predicted by the model and as shown in Figure 12a and b. Control Logged Logged-thinned (a) 5 4 Block 1 (b) 5 4 Block 2 QMD (cm) QMD (cm) Year Year FIGURE 12 Quadratic mean diameter (qmd) of all stems 2. cm by treatment in block 1 (a) and block 2 (b). Residual basal area On both control units, the residual basal area decreased significantly over the 1-year assessment period (Table 4; Figure 13a and b), as already described. After this initial decline, the modelled residual basal area showed only a small increase over the next 8 years, indicating that perhaps high levels of inter-tree competition inhibit basal area growth. In 212, approximately 2% of the basal area (6 m 2 /ha) in the control units was in the 5 15 cm es, or in stems < 17.5 cm dbh, which is typically the utilization limit for Douglas-fir (Appendix 3). Immediately after harvesting, the residual basal area on block 1 ranged from 16 m 2 /ha on the logged-thinned treatment to 18 m 2 /ha on the logged treatment. On block 2, residual basal area varied considerably between the logged-thinned (11 m 2 /ha) and logged (21 m 2 /ha) treatments. Regardless, at the end of the 1-year assessment period, basal area had equalized on the two treatments on block 1 (Figure 13a) and was showing a trend toward equalization on block 2 (Figure 13b) due to mortality in the logged treatments and growth in the logged-thinned treatment. Over the 8-year simulation period, residual basal area on both blocks increased at approximately the same rate on both logging treatments and at a considerably higher rate than on the control. Despite the differences in residual basal area between the blocks in 15

24 Control Logged Logged-thinned (a) Basal area (m 2 /ha) 45 4 Block Year (b) Basal area (m 2 /ha) 45 4 Block Year FIGURE 13 Basal area of all stems 2. cm by treatment in block 1 (a) and block 2 (b). the two logging treatments, the model predicted that, over the simulation period, residual basal area growth would be comparable on both blocks, and basal area would be approximately 28 m 2 /ha at the end of the 8-year period. Based on the distribution of the residual basal area in the logged treatments in 212, most basal area growth will be largely on trees 17.5 cm dbh because only 1 2 m 2 /ha of basal area remains in the 5 15 cm es. The exception to this is the logged treatment on block 2, where 5 m 2 /ha of residual basal area remains in the 5 15 cm es (Appendix 3). Merchantable volume On block 1, harvesting removed approximately 54% of the merchantable volume, and resulting residual volumes were roughly comparable on the two logging treatments (Table 4). Prognosis BC projections showed merchantable volume increasing at the same rate on the two logging treatments, and at a higher rate than on the control. At the end of the 8- year simulation period, the difference between merchantable volume on the logging treatments and the control was less than 4 m 3 /ha (Figure 14a). Control Logged Logged-thinned (a) Merchantable volume (m 3 /ha) Block Year Merchantable volume (m 3 /ha) Block Year FIGURE 14 Merchantable volume of trees 17.5 cm dbh by treatment in block 1 (a) and block 2 (b). (b) 16

25 Determining the appropriate cutting cycle in a partial-cutting silvicultural prescription is an important management decision. The cutting cycle must be long enough to allow sufficient volume to accrue for an economical harvest and not too long to result in significant growth reduction in the understorey stems. In order to provide context for the modelling results, a target merchantable volume similar to that of the original stand was chosen to represent a reasonable harvest opportunity based on harvesting 5% of the volume at the next entry. For block 1, this target was 2 m 3 /ha, and the model predicted that this would be exceeded after 6 years on the logged-thinned treatment and 7 years on the logged treatment. Based on 12.5 cm dhb utilization, the target would be reached in 6 years on both treatments (Figure 15a). Control Logged Logged-thinned (a) Merchantable volume (m 3 /ha) 3 Block (b) Merchantable volume (m 3 /ha) 3 Block Year Year FIGURE 15 Merchantable volume of trees 12.5 cm dbh by treatment in block 1 (a) and block 2 (b). On block 2, harvesting reduced merchantable volume by approximately 25% on the logged treatment and by almost 55% on the logged-thinned treatment. By the end of the 1-year assessment period, however, merchantable volume was similar on the two harvesting treatments (Table 4; Figure 14b). Merchantable volume on the logged treatment increased at a rate similar to that on the control over the 8-year simulation period. On the logged-thinned treatment, however, the Prognosis BC simulation of merchantable volume showed a considerably higher rate of growth compared to the other treatments. If the merchantable volume target were set at 14 m 3 /ha on block 2, the next harvest entry would be reached in about 4 years on the loggedthinned treatment and 7 years on the logged treatment (Figure 14b). When the utilization limit is reduced to 12.5 cm dbh, the next harvest entry is predicted to still be around 4 years on the logged-thinned treatment but 6 years on the logged treatment (Figure 15b). Management Implications The value of harvesting with modifications The modified logging treatment, based on the prescribed BDq values, successfully created open-forest stand conditions (Steen and Armleder 28), and this treatment was still in an open condition in 214. This was due to harvesting within the cm layer (1.4% and 21.4% of the stand basal area in block 1 and 2, respectively) and killing many sub-merchantable (< 12.5 cm dbh) trees with the fellerbuncher, in combination with post-harvest slashing of damaged trees (54% and 7% reduction on block 1 and 2, respectively). The stands continued to open after 17