Spruce Biogeoclimatic Zone Research Articles

Spatial scale of stand-replacing forest disturbance influences the amplitude of snowshoe hare population fluctuations in boreal forests of northwest Canada

The natural disturbance model for ecosystem management of timber harvesting promotes the emulation of natural disturbance regimes in the patterns of tree removal. Wildfire is a prominent natural disturbance in boreal forests of western Canada, frequently removing most of the tree canopy from patches of 500–10,000 ha in stand-replacing events. However, fire suppression, coupled with a spatial pattern of timber harvesting dominated by small patch cuts of 10–160 ha, have changed the spatial scale of younger stands away from scales within which boreal organisms evolved. In two regions (Sub-Boreal Spruce biogeoclimatic zone of central British Columbia and Liard Basin of southeast Yukon), we tested the hypothesis that different spatial scales of stand-replacing forest disturbance (wildfire and timber harvesting) result in different amplitudes of change in abundance of snowshoe hare, a keystone boreal forest mammal for which mid-seral stand conditions provide optimal habitat. Landscapes with large patches (>2000 ha) of mid-seral forest following stand-replacing disturbance supported consistently and often significantly more hares, with wider amplitude in cyclic fluctuation, than small patches (20–200 ha) of mid-seral habitat and than mature forest landscapes. Densities of hares high enough to support reproduction by Canada lynx (a specialist hare predator) only occurred in landscapes disturbed at the scale of a moderate to large-sized wildfire (1000 – 10,000 ha). Landscapes unaffected by stand-replacing disturbance for at least 80 years (i.e. mature forests) supported very few hares and without cyclic fluctuations. We recommend that the recent pattern of cutting dominated by small patches (20–200 ha) be shifted to include many larger patches (2000–5000 ha). This can happen with incremental, contiguous patch cutting over a period of years short enough that the completed patch will supply high quality, mid-seral habitat for at least the period of one hare cycle (10 y). In designing relatively large patches, mature green tree retention would be desirable for various values, but would be best as small stands of mature forest dispersed within large patch cuts, similar to the legacy of fire. Silviculture (reforestation and stand tending) should create and sustain a mix of conifer and deciduous regeneration in the mid-seral stands. Emulating spatial patterns of stand-replacing natural disturbance appears necessary to sustain snowshoe hare cycles when most fires are suppressed in intensively managed western Canadian boreal forests.

Diffusion of indigenous fire management and carbon-credit programs: Opportunities and challenges for “scaling-up” to temperate ecosystems

Savanna burning programs across northern Australia generate millions of dollars per year for Indigenous communities through carbon and other greenhouse gas (GHG) markets. In catalyzing Indigenous knowledge and workforce to mitigate destructive wildfires, these programs are considered a success story on a range of social, ecological and economic measures. Scaling-up to temperate ecosystems requires a focus on applying the architecture and governance of these programs, and accounting for fundamental differences in context. We examine the opportunities and challenges in applying the architecture of savanna burning to an Indigenous Fire Management (IFM) program in central British Columbia, Canada (the Chilcotin). The Chilcotin project involves Yunesit’in and Xeni Gwet’in First Nations, and we draw from eight key elements of the Australian savanna burning model to identify a project area that includes Aboriginal title and reserve lands. The area encompasses Interior Douglas Fir (IDF) and Sub-Boreal Pine—Spruce (SBPS) biogeoclimatic zones, or dry forest and grassland ecosystems where low intensity fires are applied by community members to remove forest fuels, with the goal of mitigating wildfires and associated GHG emissions. The multi-decadal intervals between contemporary fires in the Chilcotin region make it challenging to accurately document historical fire location, scale and intensity, and thus to establish an emissions baseline. If this issue can be resolved, the British Columbia Forest Carbon Offset Protocol version 2 (FCOPv2) offers promise for developing verified carbon credits for three reasons: first, carbon (CO2), nitrous oxide (N2O), and methane (CH4), the three main GHG emissions from Indigenous fire management, are included in the protocol; second, credits under FCOPv2 are eligible for either compliance or voluntary markets, offering diversification; and third, a range of activities are eligible under the standard, including fire management and timber harvesting, which offers flexibility in terms of management practices. The Chilcotin project is likely to generate substantial co-benefits related to cultural, health and wellbeing, and livelihood values among First Nations participants. The Australian experience suggests that getting governance right, and building community ownership through “bottom-up” governance, is critical to the success of these programs. From the Australian model, community-based planning, like the Healthy Country Planning approach, can be a positive step to take, engaging community in goal setting for the program to guide and take ownership of its direction.

Lodgepole pine and trembling aspen competition: Neighbourhood studies within 22 to 39 year-old pine plantations of northern British Columbia

Six lodgepole pine plantations located within the Sub Boreal Spruce (SBS) biogeoclimatic zone of the northern interior of British Columbia were chosen to study the impacts of neighbourhood aspen competition on the growth of planted lodgepole pine (22–39 years old). At each site, 30 pine trees (CP) were selected as plot centres across the observed range of aspen competition. Information on the six nearest neighbours (NN) and a variety of competition indices were evaluated using linear and nonlinear models. Competition indices based on diameter or height relative to size were found as the best overall predictors of CP growth. Proximity of NN to the CP was found to be inconsistent as a predictor of growth. Individual pine and aspen NN modelling suggested aspen competition had the greatest influence on CP growth restriction. The results do not support aspen thresholds for optimizing the growth of free growing pine plantations. It is recommended that performance-based standards be developed that account for aspen competition intensity and provide guidance for the management of pine stand productivity.

Lodgepole pine and trembling aspen mixedwoods: Growth and yield within 22 to 39 year old pine plantations of northern interior British Columbia

Six lodgepole pine plantations located in the Sub Boreal Spruce (SBS) biogeoclimatic zone of northern interior British Columbia were chosen to study the impacts of aspen competition on the growth and yield of lodgepole pine. The six sites from four different zonal variants (dw2, dw3, dk1 and mc2) ranged in age from 22 to 39 years. Site preparation treatments occurred on all sites: drag scarification, disking, or a broadcast burn post clear-cut logging. Four of the sites received at least one operational brushing treatment for aspen (chemical or manual). At each site, 30 pine trees (CP) were selected as plot centres to represent the observed range of aspen competition. A series of nested plots up to 7.98 m radius was used to evaluate effects of % pine composition on stand density and stand volume using linear and non-linear models. At five of the six sites total volume was unchanged across the range of pine composition. The growth interaction between aspen and pine was found to be a linear relationship of competitive reduction where pine volume, basal area and merchantable volume was highly sensitive to any increase in aspen density or composition.

A Site Index Model for Lodgepole Pine in British Columbia

A height growth and site index model was developed combining permanent sample plot (PSP) and stem analysis (SA) data from the Sub-Boreal Spruce biogeoclimatic zone of British Columbia, Canada. SA data were screened for indications of early suppression, virtually eliminating that source of bias. Estimation was based on a stochastic di erential equation form of the Bertalan y-Richards growth curve, including environmental and measurement/sampling variation. Methods involve the inclusion of mixed e ects, and obtaining simultaneous maximum likelihood estimates for all the parameters. Di erences in error structure between PSP and SA data were taken into account. The model fits the data well, and compares favorably to existing alternatives.

A Parsimonious Dynamic Stand Model for Interior Spruce in British Columbia

Abstract A whole-stand growth model, Scube, was developed for spruce-dominated, even-aged, natural and planted stands, using permanent sample plot data from the sub-boreal spruce biogeoclimatic zone of British Columbia, Canada. Available data were limited, with no information on mature planted stands or thinning treatments. A parsimonious model design, incorporating knowledge of stand dynamics and experience from other forests, made possible a logical reproduction of observed behavior and plausible extrapolations to other conditions. Scube consists of a system of four differential equations, describing the evolution of four state variables: top height, trees per hectare, basal area, and a stand closure indicator. Concepts of closure and site occupancy drive the behavior of young stands and the response to thinning. Parameters were estimated through numerical optimization of several residual functions, reflecting different projection interval types and weighting strategies. Differences in estimates among the various criteria were small. Calculations are facilitated through expressions that remain invariant in the absence of disturbances. The methods generalize the algebraic difference approach and related forest modeling techniques, linking them to standard dynamical systems theory.

Stable isotope tracing of fertilizer sulphur uptake by lodgepole pine: foliar responses

A plot-scale fertilizer sulphur (S) stable isotope tracer study was established in 2001 in the Sub-Boreal Spruce biogeoclimatic zone in central interior British Columbia where S deficiencies are common in lodgepole pine ( Pinus contorta var. latifolia Engelm. ex S. Wats.) stands. Treatments used operationally realistic applications of 300 kg N·ha–1 as urea and 100 kg S·ha–1 as either sulphate (SO4) or elemental S (S0). δ34S values of fertilizer S differed by >9‰ from pretreatment δ34S values of total S in foliage at the two study sites (5.2‰ and 8.2‰). These differences enabled quantification of fertilizer uptake using isotopic analysis of post-treatment foliar S. Addition of K2SO4 with δ34S of +17.5‰ increased foliar δ34S by 3.5‰ and 6.6‰ at the two sites, respectively, in the year after treatment, indicating fertilizer contributions >40% to foliar total S. For a S0 fertilizer application with a δ34S value of +19.3‰, foliar δ34S increases were smaller but steadily increased, resulting in an average tracer S uptake of ~20% over three years. These results confirmed the more rapid availability of S from SO4-based fertilizers and demonstrated the feasibility of field tracer experiments using stable S isotopes at natural abundance levels.

Density of Fishers in Boreal Mixed-Wood Forests of Northeastern British Columbia

Fishers (Martes pennanti) are medium-sized carnivores of the family Mustelidae that are found in boreal and temperate coniferous and deciduous-coniferous forests across North America (Proulx and others 2004). Fishers are rare, but important members of the forest ecosystems that they occupy, and as mesocarnivores may play an essential role in regulating populations of many mid-sized mammals in these forests (Roemer and others 2009). In British Columbia, Fishers are harvested for their fur and can be legally trapped on registered traplines in the central and northern portions of the province between 1 November and 15 February. Harvest of Fishers has declined considerably in British Columbia over the past 30 y. Causes for harvest declines are unexplained, but may include both declining populations and declining trapper effort. Fishers currently have a provincial status of S2S3 in British Columbia, designating it as a species of ‘‘special concern’’ (BCCDC 2010). To ensure that Fisher harvests are sustainable, wildlife managers and trappers need basic population information. Population density is a key piece of data upon which harvest levels should be based. Because Fishers are secretive and difficult to inventory (Powell and Zielinski 1994), until recently harvest management of Fishers in British Columbia has been largely based on studies from eastern North America, where density estimates range from 50 to 385 Fishers/1000 km (Powell and Zielinski 1994; Fuller and others 2001). Recent work in northcentral British Columbia, however, has demonstrated that the density of Fishers in coniferousdominated western forests are substantially lower (for example, 8.8 Fishers/1000 km, Weir and Corbould 2006), and more conservative harvest management is required relative to eastern Fisher populations. Habitat quality is not uniform throughout British Columbia and, as a result, Fisher density is believed to vary among regions (Weir 2003; Lofroth 2004). This is reflected in the variability of Fisher harvest across the province. The boreal mixed-wood forests of northeastern British Columbia have consistently high harvests of Fishers and are believed to be among the most productive areas for this species in the province (Lofroth 2004). To improve our knowledge of Fisher density in British Columbia, we estimated the density of Fishers in a representative boreal mixed-wood forest landscape in northeastern British Columbia. This information will provide wildlife managers and trappers with better data to evaluate sustainable harvest levels and help facilitate population persistence. Our work builds upon Weir and Corbould’s (2006) estimation of Fisher density in the subboreal spruce forests of north-central British Columbia by applying their methods to Fishers in boreal mixed-wood forests. Because our data was collected, analyzed, and interpreted following methods identical to that of Weir and Corbould (2006), readers are directed to this article for comprehensive details of the capture methodology, density estimation, its biases, and resultant implications for interpretation of the results. Our study area covered 950 km of boreal mixed-wood forests (that is, the moist-warm subzone of the Boreal White and Black Spruce biogeoclimatic zone; DeLong and others 1990) in the Kiskatinaw Plateau and Peace Lowlands ecosections (Demarchi 1995) to the south and west of Dawson Creek, BC (UTM: Zone 10, 674000 E, 6182000 N, NAD83). The climate of the study area is cold and dry, which is typical of continental boreal forests. Mean annual GENERAL NOTES

Verification of a Forest Rating System to Predict Fisher, <em>Martes pennanti</em>, Winter Distribution in Sub-boreal Forests of British Columbia, Canada

This study verified the ability of a forest rating system to predict the winter distribution of Fisher (Martes pennanti) in the Sub-boreal Spruce Biogeoclimatic Zone of central interior British Columbia. Forest polygons (i.e., homogenous areas with similar forest stand characteristics) were classified according to their age and structural development, canopy closure, basal area in mature trees, average tree diameter at breast height, and percentage of shrub cover. Approximately 170 km of transects randomly distributed across polygons were inventoried (snowshoed) from December to February 2005-2008. A total of 278 Fisher tracks were recorded. The observed frequency of Fisher tracks per polygon type was significantly (P < 0.05) different from expected. The majority (245 or 88.1%) of tracks were recorded in excellent- and high-quality polygons corresponding mostly to mixed coniferous stands. On average, these stands were 138.2 years old, and had 54.4% canopy closure, 38.1 m2/ha basal area, 27.8 cm dbh, and 11.4% shrub cover. This study showed that the forest rating system was adequate to predict Fisher winter distribution, and could be used to develop forest management plans that are compatible with the species habitat requirements.

A height-growth and site-index model for interior spruce in the Sub-Boreal Spruce biogeoclimatic zone of British Columbia

Height growth was modelled for spruce-dominated, even-aged stands in the Sub-Boreal Spruce biogeoclimatic zone of British Columbia, Canada, using both stem analysis (SA) and permanent sample plot (PSP) data. The model is based on a stochastic differential equation (SDE) formulation of the Bertalanffy–Richards growth equation. The SDE approach accounts for serial correlation and heterogeneous variance and makes hypothesis testing possible. Statistically significant differences in height–age trends between SA and PSP data were found that may be attributed to bias caused by dominance changes in SA trees. Error structure in SA and PSPs was also significantly different. Combining both data sources in a way that respects these different error structures reduced bias and increased precision. Four parametrizations were tested; the best was a polymorphic version. The final model fit the data well with no appreciable bias over the full range of ages and site qualities. The currently used spruce site-index model was found to underestimate growth and overestimate site index in young stands. The new model can be recommended for height prediction and site-quality assessment in interior spruce.

Factors Affecting Landscape Occupancy by Fishers in North‐Central British Columbia

ABSTRACT To better understand distribution and density of fishers (Martes pennanti) in industrial forests of north‐central British Columbia, Canada, we examined factors affecting the probability of a potential home range being occupied by 10 radiotagged resident fishers in the Sub‐Boreal Spruce biogeoclimatic zone between 1996 and 2000. Percentage of a home range in wetlands and recently logged (within past 12 yr) best predicted likelihood of occupancy by each fisher. Probability of a home range area being occupied by a resident fisher decreased with increasing amounts of wetlands and recent logging present in the area. We estimated that a 5% increase in wetlands or recent logging decreased the relative probability of occupancy of a potential home range by 50%. The accelerated rate of timber harvest in forests affected by mountain pine beetle (Dendroctonus ponderosae) infestations may have substantial implications for the ability of the landscape of central British Columbia to support sustainable populations of fishers.

Identifying the determinants of windthrow damage in wildlife tree patches in the Boreal White and Black Spruce biogeoclimatic zone of northeastern British Columbia

Leaving wildlife tree patches (WTPs) has become a common strategy employed to maintain biodiversity among managed forest ecosystems in British Columbia. High levels of wind damage have been observed in many of these reserves owing to the increased wind loading after harvesting. The ensuing damage disrupts forest management plans and reduces the value of WTPs. The objective of this study was to identify the primary determinants of windthrow in WTPs in the boreal forest of northeastern British Columbia andto suggest management strategies to minimize wind-related damage. Line transects oriented parallel and perpendicular to prevailing and dominant winds across 13 WTP reserves were used to quantify wind-related damage and factors that may contribute to windthrow incidence. The occurrence of windthrow corresponded with the exposure of WTP edges to high velocity winds; common, but lower-velocity winds resulted in little windthrow damage. Edaphic, site, and forest-stand factors appeared to have little influence on the incidence of windthrow in this study as compared to exposure to strong winds. The study suggests that forest managers can reduce the incidence of windthrow in WTPs in the boreal forests of northeastern British Columbia by:(1) creating patches that are elliptically shaped with the long axis in the direction of the dominant winds;(2) reducing wind exposure of susceptible edges; and (3) increasing the size of WTPs.

Riparian alder ecosystems as epiphytic lichen refugia in sub-boreal spruce forests of British Columbia

Alder-dominated riparian forests represent only a small proportion of the landscape in central-interior British Columbia. However, they possess a suite of attributes that may allow them to function as refugia for canopy macrolichens. These include their deciduous habitat, their location in moist nutrient receiving sites, and their distribution as narrow corridors that cross broad regional landscapes. We have examined their potential role as lichen refugia by assessing canopy macrolichen communities in 75 riparian alder forests across a 200 km longitudinal gradient in central-interior British Columbia. Study sites were stratified equally between three climate subzones of the Sub-Boreal Spruce biogeoclimatic zone. Forty-nine macrolichen taxa were observed, including the old-growth indicator cyanolichen species Lobaria scrobiculata (Scop.) DC., L. retigera (Bory) Trevisan, Nephroma isidiosum (Nyl.) Gyelnik, and Sticta limbata (Sm.) Ach. Canonical correspondence analysis identified mean annual temperature, mean annual precipitation, age of adjacent conifer forest, and abundance of large stems (dbh >10 cm) as significant explanatory variables. Regional precipitation gradients explained the exclusion of many lichen species from both the most westerly and most easterly riparian forests, with drier summer conditions and heavy winter snowpack, respectively, being major limiting factors. Lichens preferentially occupied large leaning stems, which may provide greater precipitation interception and continuity of substrate, when compared with smaller upright alder stems. We conclude that alder-dominated riparian forests represent a major refugium for old-growth dependent lichens in British Columbia’s sub-boreal spruce landscapes and as such may provide valuable dispersal corridors between remnant old-growth coniferous forest patches.

Identifying the determinants of windthrow damage in wildlife tree patches in the Boreal White and Black Spruce biogeoclimatic zone of northeastern British Columbia

Leaving wildlife tree patches (WTPs) has become a common strategy employed to maintain biodiversity among managed forest ecosystems in British Columbia. High levels of wind damage have been observed in many of these reserves owing to the increased wind loading after harvesting. The ensuing damage disrupts forest management plans and reduces the value of WTPs. The objective of this study was to identify the primary determinants of windthrow in WTPs in the boreal forest of northeastern British Columbia and to suggest management strategies to minimize wind-related damage. Line transects oriented parallel and perpendicular to prevailing and dominant winds across 13 WTP reserves were used to quantify wind-related damage and factors that may contribute to windthrow incidence. The occurrence of windthrow corresponded with the exposure of WTP edges to high velocity winds; common, but lower-velocity winds resulted in little windthrow damage. Edaphic, site, and forest-stand factors appeared to have little influence on the incidence of windthrow in this study as compared to exposure to strong winds. The study suggests that forest managers can reduce the incidence of windthrow in WTPs in the boreal forests of northeastern British Columbia by: (1) creating patches that are elliptically shaped with the long axis in the direction of the dominant winds; (2) reducing wind exposure of susceptible edges; and (3) increasing the size of WTPs.

Decomposition of pure and mixed foliage litter in a young lodgepole pine – Sitka alder stand in the central interior of British Columbia

A 7-year litterbag experiment examined mass loss dynamics of Sitka alder ( Alnus viridis (Chaix) DC. subsp. sinuata (Regel) A. Löve & D. Löve) and lodgepole pine ( Pinus contorta Dougl. ex Loud. var. latifolia Engelm. ex S. Watson) foliage litter in a young stand dominated by these species in the Sub-Boreal Spruce biogeoclimatic zone of central British Columbia. Although mass loss was initially faster for alder litter, the amounts of mass remaining for alder, pine, and mixed pine–alder (50:50 by mass) litter began to converge after 2 years. Mass loss by the mixed litter differed significantly from that predicted from the behaviour of its pure components, but the mechanisms responsible could not be determined from this experiment. Limit values for the projected maximum extent of mass loss ranged from 63.5% (alder) to 76.9% (pine), with the latter value considerably below estimates reported for lodgepole pine in Scandinavia. Despite these differing limit values, forest floor mass measurements prior to the experiment did not show significantly higher organic matter accumulations under long-established alder clumps on this site.

Site-Dependent Height-Age Models for Paper Birch in British Columbia

In British Columbia, foresters use the height-age models for trembling aspen (Populus tremuloides Michx.) from the neighbouring province of Alberta to predict height and site index for paper birch (Betula papyrifera Marsh.) because height-age models for paper birch in British Columbia do not exist. The aspen model has not been extensively tested for local use on paper birch trees. Consequently, it is not known how well they predict the height growth of site trees. We rectified this situation by developing paper birch height-age models from locally collected stem analysis data for paper birch. Data from 61 plots in the Interior Douglas-Fir, Interior Cedar and Hemlock, and Sub-Boreal Spruce biogeoclimatic zones were fit to a model based on the log-logistic function. Differences in growth patterns between the zones were detected using indicator variables. This height-age model should be used for paper birch in the sampled biogeoclimatic zones in British Columbia because it is the best fitting model. An alternative model is available for sites not in the sampled zones. A comparison of the trembling aspen model and the paper birch model indicates that the difference between the models is small over most of the management age range of paper birch. The exception to this is at young ages and high site indexes, where paper birch is predicted to be taller than trembling aspen.

Lodgepole pine growth as a function of competition and canopy light environment within aspen dominated mixedwoods of central interior British Columbia

Three lodgepole pine and aspen mixedwood sites located in the central interior of British Columbia within the Sub Boreal Spruce (SBS) biogeoclimatic zone were chosen to study the neighbourhood aspen competition and canopy light environment of 14–19-year-old lodgepole pine. All three sites had previously been established as separate research trials designed to explore various silviculture options for controlling aspen competition (aspen brushing, herbicide, thinning and untreated areas). For each site, 33–36 pine trees were selected to represent the observed range of light regimes under the influence of various aspen competition levels. At each sample pine, competition and stand measurements were made and a series of vertical canopy light measurements from the top to the base of the live crown. After an evaluation of a variety of competition indices, the index DRD; sum of the ratio of each of the three nearest neighbour's DBH to the subject pine divided by their distance and, amount of available light at the top of the crown (DIFN t) were found as the best overall predictors of pine stem volume growth. A site specific exponential relationship of relative pine stem volume growth to DRD was found and minimum growth response competition thresholds were determined, which could provide useful targets where maximizing pine volume is intended. Evaluation of both linear and non-linear models of DIFN t versus height growth indicated the response to be linear across the observed range of available light. Implications for management are discussed.

Effects of compaction and water content on lodgepole pine seedling growth

Soil disturbance by heavy machinery usually causes a decrease in porosity and an increase in soil strength, which may limit access to nutrients and compromise seedling survival and growth. This study used a soil strength and a greenhouse experiment to assess the impact of compaction on lodgepole pine ( Pinus contorta Dougl. Ex. Loud. var. latifolia Engelm.) seedling growth and the degree to which soil water influences the effects of compaction. A silt loam soil was collected from a forest landing in the central interior of British Columbia (BC) in the Sub-Boreal Spruce Biogeoclimatic zone. The silt loam soil was used in a soil strength experiment where soil with four water content levels (0.10, 0.18, 0.27, and 0.36 cm 3 cm −3) was packed into 0.21 cm 3 cores with three levels of compaction (74, 79, and 84% of maximum bulk density (MBD)). Soil strength was strongly affected by compaction and water content. In the greenhouse experiment, three water content levels (0.10–0.15, 0.20–0.30, and 0.30–0.35 cm 3 cm −3) and three levels of compaction (67, 72, and 76% of MBD) were applied to soil in pots and 1-year old lodgepole pine seedlings were grown in the pots. Soil strength was highest (1275 kPa) for the high compaction and dry water content treatment in the greenhouse experiment. Though the soil strength for this treatment did not exceed 2500 kPa, the effect of compaction on growth was noticeable, with a decrease in diameter growth, total shoot mass, and new root mass as compaction increased at the dry water content. At dry water content and high compaction, foliar nutrient concentrations were greatest. Generally, water content had a greater impact on seedling growth than did compaction, at the levels of compaction used in this study. This study indicates that if there is a critical value for mechanical impedance of the conifer roots, it likely occurs below 2500 kPa. Our results are consistent with the explanation that soil strength incrementally affects root growth below 2500 kPa for this soil type. Expensive rehabilitation techniques may not be needed on lightly disturbed soils similar to that used in this study if soil water content is high enough throughout the conifer growing season to alleviate the effects of compaction on soil strength.

Growing trembling aspen and white spruce intimate mixtures: Early results (13—17 years) and future projections

Controlled mixtures of trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca [Moench] Voss) were established in 1989 at two locations in the Boreal White and Black Spruce (BWBS) biogeoclimatic zone in northeastern British Columbia. The initial study design of three aspen treatment densities of 0, 5000, and 10000 stems per hectare was expanded by reducing existing densities of aspen on a subset of plots to 1000 and 2000 stems per hectare. A random-coefficients regression model was used to analyze height and diameter growth trends for aspen and spruce 13–17 years after establishment. White spruce grown without aspen had significantly greater rates of height and diameter growth. There were no significant differences in spruce growth between the 5000 and 10000 aspen stems per hectare treatments. Differences in spruce height and diameter growth did not consistently display a pattern of declining growth as aspen density increased from 1000 to 10000 stems per hectare. Aspen responded to aspen density reduction by increased diameter growth of the remaining stems.The Mixedwood Growth Model was used to predict future growth of the experimental stands. Yield projections indicated that a total productivity gain of 21% may be achieved for mixtures compared to a pure spruce scenario. Over the range of conditions studied, spruce comprised approximately 40% of the total volume in mixed stands. These initial results will improve the assessments of the relative contributions that pure- and mixed-species management regimes may offer to achieving forest-level objectives.

Coarse woody debris in the old-growth forests of British Columbia

This paper synthesizes data extracted from the literature and data collected in various studies by the author on the quantity, characteristics, and functional importance of coarse woody debris (CWD) in the old-growth forests of British Columbia (B.C.). There is little agreement in the literature about the minimum diameter of CWD or the number of decay classes recognized. In western North America, five decay classes are commonly used, but recent studies suggest fewer decay classes are preferable. Comparisons among decay classes and biogeoclimatic zones and subzones in B.C. reveal that quantities and volumes are greatest (up to approximately 60 kg/m2 and approximately 1800 m3/ha, respectively), and CWD persists the longest (sometimes in excess of 1000 years) in the Coastal Western Hemlock (CWH) biogeoclimatic zone. The quantity and ground cover of CWD increase with forest productivity. Persistence of CWD has varied from less than 100 to over 800 years in two coastal (CWH and Mountain Hemlock (MH)) and three interior (Interior Douglas-fir (IDF), Interior Cedar–Hemlock (ICH), and Engelmann Spruce – Subalpine Fir (ESSF)) biogeoclimatic zones. Trends in CWD quantity with forest age in managed coastal B.C. forests suggest a U-shaped curve, with greater quantities occurring in recent cutovers than in old-growth forests, and lowest quantities occurring in middle-aged forests. This may be the normal trend in CWD with forest age, with departures from this trend resulting from disturbance- or environment-specific factors. Relatively large amounts of data exist on the characteristics of CWD in the CWH, IDF, ICH, ESSF, and Boreal White and Black Spruce (BWBS) biogeoclimatic zones, but such data for the Coastal Douglas-fir, Sub-Boreal Pine–Spruce, Sub-Boreal Spruce (SBS), and Spruce–Willow–Birch biogeoclimatic zones appear relatively sparse. There have been few studies of the functional role of CWD in B.C. forests, but those studies that have been completed indicate that CWD is an important habitat component for some plant and animal species. A total of 169 plant species, including >95% of all lichens and liverworts, were found to grow on CWD in old-growth forests in the CWH, MH, IDF, ICH, and ESSF biogeoclimatic zones. One third of these species were restricted to CWD. Studies in several biogeoclimatic zones have found that CWD provided preferred habitat for and was associated with higher populations of some small animal species, such as shrews, some voles, and some salamanders, in old-growth forests, but the effects varied with species and biogeoclimatic zone. The nutrient cycling role of CWD is not yet well known, but it currently appears to be relatively insignificant in B.C. old-growth forests. Although it has been considered that CWD could increase mineral soil acidification and eluviation, no evidence for this was found in a study of the CWH, MH, IDF, ICH, ESSF, BWBS, and SBS biogeoclimatic zones. Future studies of the functional role of CWD should consider both scale (square metre vs. hectare) and temporal (changes in CWD with forest age) issues, as studies including these are sparse and both may be important. Key words: biogeoclimatic zones, British Columbia, coarse woody debris, old-growth forests.