Natural disturbance and old-forest management in the Alberta Foothills1 by Richard L. Bonar2, Hugh Lougheed3 and David W. Andison4
There is no widely accepted definition of old growth for west central Alberta. The Foothills Model Forest used tree species composition and time since major disturbance to more broadly define old forest and a stochastic model to project levels of old forest across a landscape. Historically, the simulated “natural” forest landscape was, at any one time, mostly covered by young forest due to active fires. Areas of mature and old forest were, and likely always have been, in the minority. There were even rare times historically when virtually no old forest existed over vast landscapes, and what little did persist occurred in small, isolated patches. The single greatest human influence on old growth in the Alberta Foothills appears to have been successful fire control, which has produced forests today that are on average older than would be expected under natural conditions. Managers of both protected areas and working forests are implementing or developing strategies to restore forests to more natural conditions, and at the same time managing old forest to ensure that it remains a part of current and future forest landscapes. We describe an old-forest analysis and strategy recently incorporated into a new Forest Management Plan for the Weldwood of Canada Limited Forest Management Area. Traditional attitudes toward old forest and its role in highly dynamic landscapes are being revisited on the path to a consistent and broadly supported old-forest strategy. Key words: old growth, management, natural disturbance, Alberta Il n’existe aucune définition généralement acceptée des forêts anciennes pour le centre-ouest de l’Alberta. La Forêt modèle de Foothills utilise la composition en essences et le temps écoulé depuis une importante perturbation pour définir généralement les forêts anciennes et applique un modèle stochastique pour déterminer l’évolution de la part des forêts anciennes dans un paysage. En raison des feux de forêt, le paysage forestier « naturel » simulé était toujours dominé par de jeunes forêts. Les étendues de forêts mûres et anciennes constituaient et ont sans doute toujours constitué une faible part du territoire. À de rares occasions par le passé, de vastes paysages ne contenaient pratiquement pas de forêt ancienne, et les rares peuplements anciens qui persistaient étaient petits et isolés. La plus importante influence humaine sur les forêts anciennes des contreforts des Rocheuses albertaines semble avoir été la suppression des feux, laquelle a produit des forêts dont l’âge moyen est aujourd’hui plus grand que ce à quoi on se serait attendu dans des conditions naturelles. Les gestionnaires des zones protégées et des forêts exploitées élaborent ou appliquent des stratégies visant à rétablir les conditions naturelles des forêts tout en aménageant les forêts anciennes pour s’assurer qu’elles continuent de faire partie intégrante des paysages forestiers actuels et futurs. Nous décrivons une analyse et une stratégie de forêt ancienne récemment intégrées au nouveau plan d’aménagement de la zone d’aménagement forestier de Weldwood of Canada Limited. En vue d’établir une stratégie cohérente de gestion des forêts anciennes qui jouit d’un appui généralisé, on révise les attitudes traditionnelles à l’égard des forêts anciennes et de leur rôle dans des paysages très dynamiques. Mots clés : vieilles forêts, aménagement, perturbation naturelle, Alberta
Introduction There is no widely accepted definition of old growth for the Rocky Mountains and Foothills Natural Regions (Beckingham et al. 1996) in west central Alberta. Popular descriptions are most often associated with tree age and tree size, but there are widely varying perceptions about how old, or large, trees in a stand have to be before people conRichard L. Bonar sider them to be old growth. Scientific descriptions are more consistent and focus on structural 1Paper
presented at the “Old-growth Forests in Canada: A Science Perspective” Conference, October 14–19, 2001, Sault Ste. Marie, ON.
2Rick Bonar, Chief Biologist, Weldwood of Canada Limited, 760 Switzer Drive,
Hinton, Alberta T7V 1V7. E-mail: rick_bonar@weldwood.com 3Weldwood of Canada Limited, 760 Switzer Drive, Hinton, Alberta T7V 1V7. 4Bandaloop Landscape-Ecosystem Services, 3426 Main Avenue, Belcarra, B.C. V3H 4R3.
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Hugh Lougheed
David W. Andison
characteristics. Old growth is typically described as the last stage of stand development, where trees reach large size for the species and site, and stands develop age-related structural characteristics such as dead or declining trees, logs, canopy gaps, and multiple canopy layers (Mehl 1992, Kneeshaw and Burton 1997, Wells et al. 1998, Lee et al. 2000). People generally have an opinion about old growth and how it should be managed. The overwhelming social demand for
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Fig. 1. Foothills Model Forest.
old growth results in a key question for forest managers: how much old growth should there be in managed forests? As do other jurisdictions in boreal Canada, Alberta requires Forest Management Plan (FMP) projections of age-class distributions over a planning horizon of 150–200 years (Alberta Land and Forest Service 1998). In the past, this was mainly used to demonstrate the degree to which the idealized goal of a flat, stable ageclass distribution was being achieved (Burton 1996), synonymous with a sustainable supply of fibre. More recently, age-class projections are being used to demonstrate what amounts of ecologically important seral stages, such as old growth, are being maintained. Targets for age-class or seral-stage percentages have, until now, been single number minima or maxima (e.g., British Columbia Ministry of Forests and British Columbia Ministry of Environment Lands and Parks 1995). Single numbers are simple to integrate into planning as goals in spatial or non-spatial planning models. However, recent evidence suggests that age-class distributions are historically highly variable (Romme 1982, Turner and Dale 1991, Payette 1993), making the question of “how much old growth” more complex. Weldwood systematically approached the problem of “how much old growth to manage for” on their 1 000 000-hectare Forest Management Area (FMA) in west central Alberta. The first step was to understand the natural range of variability in ageclass distributions (including, but not limited to, old forest) through the Foothills Model Forest natural disturbance research program. Different means of integrating this new knowledge were explored and evaluated. Finally, a new model was chosen and implemented to develop an old-forest strategy as a component of a new Forest Management Plan for the FMA. This paper describes the research program and the Weldwood management planning application.
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Foothills Model Forest The Foothills Model Forest (FMF) is the largest Model Forest in Canada. Its 2 750 000-ha area includes the Weldwood FMA, Jasper National Park, Willmore Wilderness Area, and several smaller provincial protected areas and provincial Forest Management Units (Fig. 1). The region spans several ecological zones including Alpine, Subalpine, Upper Foothills, Lower Foothills and Montane (Beckingham et al. 1996). Elevation ranges from 3600 m in the Rocky Mountains to 850 m along the Athabasca River in the Lower Foothills. Continuous forest cover starts at about 1800 m and covers about 90% of the land. Coniferous stands are predominant at higher elevations, with increasing amounts of deciduous species as elevation decreases. The main coniferous species are lodgepole pine (Pinus contorta), upland spruces (Engelmann spruce (Picea engelmannii), white spruce (P. glauca), and hybrids), black spruce (P. mariana), and firs (subalpine fir (Abies lasiocarpa), balsam fir (A. balsamea), and hybrids). Less common species include whitebark pine (Pinus albicaulis) and alpine larch (Larix lyalli) near the timberline, Douglas fir (Pseudotsuga menziesii) in Montane forests, and tamarack (Larix laricina) in forested wetlands. The main deciduous species are trembling aspen (Populus tremuloides), balsam poplar (P. balsamifera), and paper birch (Betula papyrifera). Most of the stands in the Foothills Model Forest originated from stand-replacing forest fires.
Methods Natural disturbance research In co-operation with Weldwood and Jasper National Park, the Foothills Model Forest initiated an integrated disturbance research program in 1996 (Andison 2000a). The main goal was to determine historical, “natural” disturbance patterns in the
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Foothills Model Forest of all types, and at all spatial and temporal scales. The focus of the early research was directly related to the management issue of how much old forest is appropriate to manage for over the long term. The scientific question undertaken to address this management issue was “What are the natural, historical levels of all seral-stage percentages over the last 200–300 years?” A detailed description of the data and methods used to reconstruct the historical range of variability for different seral stages on the Weldwood FMA was described in Andison (1998). Briefly, Weldwood was in the fortunate position of having a pre-industrial stand-origin map developed from 1960–1963 using age-based polygons mapped on 1:31 000 aerial photographs taken in 1955. Actual fire years were determined by counting tree rings back to the most recent fire scar on veteran trees at fire boundaries (Weldwood of Canada Limited, unpublished data). Although samples sizes were not controlled, ages were taken for several trees per sample site, which were regularly spaced along fire boundaries (Jack Wright, personal communication). Data were summarized in 10-year age classes and polygon boundaries were adjusted based on results of the field analysis. Additional fieldwork was conducted from 1996–1997 to determine ages of approximately 40 000 ha of stands > 150 years since fire, which were lumped into a single age class in the original Weldwood map inventory (Andison 2000b). These data were used as a basis for stochastic disturbance modelling scenarios using LANDMINE, a cellular automaton Monte Carlo disturbance spread model (Andison 1996, 1998; Andison and Marshall 1999). Based on the original formulation of Clarke et al. (1994), LANDMINE is similar to models developed by Baker et al. (1991) and Mladenoff et al. (1993). It uses a dispersal algorithm to spread fires from one pixel to another so fire movement responds probabilistically to fuel type and topography. The simulations created multiple landscape possibilities based on probabilistic levels of fire frequency and size derived empirically from the raw stand-origin data. Fire ignition probabilities were based on 10 years of historical lightning frequency data derived from lightning location positioning devices owned by the province of Alberta (Alberta Sustainable Resource Development, unpublished data), and fire spread probabilities were established using both fuel-type “scores” of relative flammability, and a topographic model, which determined probabilities of burning on various slopes (Andison 1998). The model was calibrated to fire shapes and numbers and sizes of unburned patches, and to allow for different fire frequencies and sizes between the three main ecological zones on the landscape (Andison 1998). Each step in LANDMINE is stochastic, so the model never “burns” the same way twice but over the long term, results are consistent with internally defined probabilities. Sixteen spatial strata were used for this model scenario, representing combinations of four broad age classes (seral stages): young, pole, mature, and old; and four forest-types: pinedominated, spruce-dominated, hardwood-dominated, and mixedwood (Table 1). The distinction between the mature and old stages was the age at which stands feature the structural characteristics (large trees, increasing numbers of dead and declining trees, logs, canopy gaps) associated with the last stage of stand development. Seral stage age separations were assigned subjectively by local experts and will be confirmed with future research. For pine types, independent work conducted
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Table 1. Age-classes by forest types used to summarize the LANDMINE simulations for the Weldwood FMA age-class variability Age range in years by forest type strata Seral stage Young Pole Mature Old
Pine and spruce 1–40 41–100 101–180 > 180
Hardwood and mixedwood 1–20 21–80 81–120 >120
by Weyerhaeuser Company in the same forest region corroborated the estimates by showing that stands > 180 years old contained old-forest structural characteristics (Morgantini and Kansas 2003). The model used a digital version of the stand-origin map as of 1950 as the starting point. This year was selected to represent the approximate onset of significant harvest activity and effective fire suppression on the FMA. The model performed 100 consecutive 20-year runs, which formed the raw data set for the analyses to follow. At the end of each run, the percent area in each of the 16 strata was recorded at different landscape sizes. The maximum landscape size was limited to the total area of the natural sub-region in the study area. Smaller landscapes were created by halving the size of the original landbase to a minimum of 30 000 ha (representing the size of a large operating area). The Upper Foothills natural sub-region was the largest area, so data were summarized at five scales starting at 480 000 ha, and then at 2400 00, 120 000, 60 000, and 30 000 ha. Four scales were possible for each of the Lower Foothills and Subalpine Subregions beginning at 240 000 ha. Information on burning rate, or percent total area disturbed per time period for the model, was calculated by creating mathematical functions using seven 20-year “roll back” estimates based on the 1950 landscape. Rolling back works by “peeling off” the area of the youngest age class and assuming that underneath are areas from all other older age classes in proportion to the area of those age-classes on the landscape today (for details, see Andison 2000b). Fire size functions were developed using historical fire size data (Andison 1998). Simulation results were compared to landscape composition in 1950 (precommercial landscape) and 1998 (current managed landscape). The very low historic harvest levels and fire suppression before 1950 probably did not markedly influence the development of the 1950 landscape. Summaries of the model runs were developed to determine the range of natural variation (RNV), defined as the maximum and minimum limits of an age-class proportion over periods of many centuries. Variation in age-class proportion over time reflecting variation in disturbance rates was also determined to represent distributions of age-class proportions within RNV (Fig. 2). Weldwood Forest Management Plan Concurrent with the initiation of the FMF natural disturbance research program, Weldwood began preparation of a new Forest Management Plan for the FMA portion of the FMF. The FMP is a strategic plan prepared every 10 years and includes a 180-year forecast of expected forest dynamics in response to a proposed harvest level. The analysis used WOODSTOCK (Remsoft 1998) coupled with timber yield curves (Weldwood of Canada Limited, unpublished data) to develop the forecast. Stand
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Fig. 2. Simulated historical proportion of the percentage of spruce forest that is old (>180 years) for the Lower Foothills and Subalpine Natural Subregions, Weldwood Forest Management Area. For example in the Lower Foothills, 0–2% old spruce occurs about 35% of the time, and old spruce is always in the range of 0–40%. The actual old spruce proportion in 1950 and 1998 is shown for reference.
polygons were assigned to a forest type and allocated for timber harvest or protection. We used WOODSTOCK and yield curves to forecast, separately, future age for both allocation categories. Yield curves were extended to account for age-related stand development and transitions in protected areas. Results were summed to provide projections for the entire landbase and each subcategory (Weldwood of Canada Limited 2000). The management objective was to remain within RNV at three different scales: 1) the entire FMA, 2) each Natural Subregion, and 3) 38 Natural Variability Units (NVU; each about 30 000 ha in size). The LANDMINE model output was summarized by each one of these three scales, resulting in more than 200 RNV estimates for combinations of forest age class, type and landscape size.
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Results Natural disturbance research In general, RNV suggested by the LANDMINE simulations decreased as landscape size and elevation increased and this relationship was consistent across all age-classes and forest types (for detailed results see Andison 2000b). For example, at the FMA scale, old-forest RNV was 0.1–32.7%, but old-forest RNV increased to 0–70.0% at the NVU scale for some strata. Aside from one age class in the Lower Foothills landscape, the 1950 and 1998 age-class distributions were within RNV suggested by the simulations for all combinations of age class, forest type, and landscape size (Andison 1998). The single exception was in the Lower Foothills, where old mixedwood forest in 1998 was more prevalent than expected based on RNV from the sim-
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Table 2. Distribution of patch sizes for the three youngest age classes for three natural subregions for the Weldwood FMA (Andison 1997) Percent area by size range in hectares Natural Subregion Lower Foothills Upper Foothills Subalpine
< 40
40–79
80–199
200–599
600–1999
2000–4999
5000–9999
> 10 000
4 2 2
4 2 2
9 5 6
22 12 9
43 11 13
18 15 20
0 8 14
0 45 34
Fig. 3. Forecast old-forest proportion over 180 years on the Weldwood Forest Management Area (FMA). The simulated range of natural variation is shown in the shaded area, and the median is shown as a horizontal line.
Fig. 4. Forecast proportion over 180 years of old forest for the spruce forest type in the Upper Foothills Natural Subregion, Weldwood Forest Management Area. The simulated range of natural variation is shown in the shaded area, and the median is shown as a horizontal line.
ulation results. The Lower Foothills historically experienced the highest burning rates and the 1998 age-class distribution may have been related to a combination of fire suppression and harvest avoidance over a 50-year period. Mixedwood stands are common in the Lower Foothills and these stands were infrequently harvested until markets for trembling aspen were developed in the early 1990s. Reconstruction of the disturbance history showed that although fires were highly variable in time and space there were
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general landscape-specific patterns. Fire frequency was highest in the Lower Foothills and lowest in the Subalpine Subregions, producing landscapes that had different probabilities of old forest. For example, the probability of having more than 20% of the spruce-dominated forests older than 180 years of age was only 5% in the Lower Foothills, but was 32% in the Subalpine Subregion (Fig. 2). Of the four seral stages, the old-forest stage had the strongest tendency toward low percentages of landscape occurrence, although it still had the rare potential to comprise significant percentages of smaller landscapes. Fire behaviour and resulting landscape patterns also showed high levels of variation. Most of the area disturbed was accounted for by large disturbances, as represented by patch sizes for the three youngest age classes (Table 2). Old-forest patch size occurred over a range from small patches of unburned forest that survived in burned areas through to larger areas that had not burned for a long time. The position of old forest within landscapes was also highly variable. The simulations and the empirical evidence both suggested that old forest in the Foothills Model Forest was highly dynamic in space and time (Andison 2000b). At times, old forest in the simulation exercise was rare or absent over large landscapes. A preliminary analysis showed that old forest was statistically more likely to occur in complex terrain, west to north aspects, and toe-slopes (Andison 1997), but in general there was only weak evidence supporting the concept that old forest might exist as refugia that repeatedly escape fires in the landscape (Camp 1995). The more likely explanation is that patches of old forest escaped fire mainly by chance. Weldwood Forest Management Plan At the FMA scale, old-forest amount overall remained within simulated historic RNV and increased over the forecast period (Fig. 3). At other scales, the current forest was within RNV for all combinations except for the mixedwood and hardwood old seral stages in the Upper Foothills and Lower Foothills Natural Subregions, where the current forest had more old forest than RNV. In these areas, fires and logging in the last 110+ years did not combine to levels of disturbance sufficient to overcome the effect of a huge pulse of a single age class from 1880s fires. The forecast showed that old spruce forest would exceed RNV after about six decades (Fig. 2). This was because both upland and lowland spruce forests tended to be located in areas that will be protected from fire but not harvested (such as riparian zones). All other categories remained within RNV over the forecast period. At the NVU scale, the results suggested significant levels of variability in amount of old forest over the forecast period (e.g., Fig. 5). At any point in time, some units would have relatively high amounts of old forest and others would have relatively low amounts. This closely tracks natural distribution of old forest, except for the Subalpine Natural Subregion. Fires were less frequent in the Subalpine Subregion and under natural dis-
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Fig. 5. Forecast proportion of old forest over 180 years in each 30 000-ha Natural Variability Unit (NVU; n = 38) in the Subalpine, Upper Foothills, and Lower Foothills Natural Subregions, Weldwood Forest Management Area. Each data point represents one NVU. The simulated range of natural variation is shown in the shaded area, and the median is shown as a horizontal line.
turbance regimes it was possible, but rare, for NVU-sized landscapes to be mostly old forest.
Discussion The Foothills Model Forest natural disturbance research did not address the question of whether or not old forest was ecologically necessary. The simple answer is perhaps not, because in the study area at present there are no known species that are found only in old forest and old forest is sometimes very rare. However, we may yet discover old-forest obligate species, many species reach their highest abundance in old forest, and others require specific structural characteristics associated with old forest such as large trees and logs, so old forest is certainly ecologically important. Further, the overwhelming social demand for old forest provides a powerful incentive for managers to conserve the old-forest resource. What we do know from the research is that, to be consistent with historical patterns of landscape dynamics, it would be inappropriate to manage for a fixed percentage of old forest in Foothills Model Forest landscapes. Rather, managers seeking to approximate natural variability should plan for a range in old-forest proportion over time. Managers should also be wary of plans that tend to fix the spatial position of old forest, which are unlikely to be successful over the long term in a highly dynamic disturbance regime. Comparison of the natural dynamics of old forest with current management practices showed a number of differences between natural and managed forests. With the single exception noted above, current landscapes were within historic ranges. In general, however, current management practices would tend to reduce variability if applied over long periods of time. The simulation results showed that the amount of old forest on the FMA would remain within RNV and above the longterm median. This addresses a primary conservation objective to maintain old forest in the managed forest. Remaining above the median may satisfy social demands for old forest and also provides a safety margin to buffer the effects of unforeseen events such as a large forest fire. Alternatively, there may be opportunities for increased harvest rates while still conserving old forest at acceptable levels within RNV. If the proposed harvest schedule is applied, future old forest will exist mainly in the 28% of the FMA that is currently protected from all types of disturbance. Options to introduce
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more disturbances in these areas and to increase old-forest representation in other areas are being investigated. Ultimately, we hope to plan for a dynamic old-forest resource that moves around the landscape, and allow a combination of “natural” and “managed” old forest. For managed old forest, options being considered include prescriptions to manage for old-forest structural characteristics in some stand types. In this paper, we report results based on old-forest quantity. Work is underway to determine structural and spatial characteristics of natural old forest, such as patch size and landscape position, and to compare these characteristics to current and forecasted managed forests. Although natural old forest is often found in small isolated patches remaining after a fire event, it also occurs in large patches that have escaped fire for a long time. Understanding this variability will help managers decide how the oldforest resource should be patterned in managed forests.
Conclusions The natural variability approach to the question “how much old growth should there be in managed forests?” described in this paper represents a major departure from other plans in Alberta and is a significant step forward in the effort to manage more in keeping with natural patterns. This alone is a noteworthy accomplishment, but there are at least two additional advantages of the natural variability approach to age-class management. First, it allows for the risk of fire, insect or disease outbreaks, or a change in the plan. Establishing and planning for single targets of old forest (in particular) is not likely to be successful simply because the odds are heavily in favour of either a natural event, or a change in management policy to negatively affect the projections. By using a range as the target, and projecting that, in most cases, we are beyond the median of that range, we have a significant buffer for unforeseen events that will still allow us to stay within the natural range. The second benefit of using ranges of natural variability, as presented here, is a quantifiable, defensible ecological foundation for monitoring at multiple scales. Both biodiversity monitoring and local-level indicator programs have adopted seralstage percentages as indicators across most of Canada. Weldwood has already integrated this indicator into its planning and management vocabulary, and has also established the natural baseline from which to set management targets.
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Acknowledgements Financial and in-kind support was provided by Foothills Model Forest sponsors Weldwood of Canada Limited, Jasper National Park, Alberta Sustainable Resource Development, and Canadian Forest Service, plus other Natural Disturbance Research Program partners. We thank all of the people who participated in the Research Program and preparation of the Weldwood Forest Management Plan. Additional information for the Natural Disturbance Research program is posted on the Foothills Model Forest Web site, www.fmf.ab.ca, and information for the Weldwood FMP is posted at www.hintonforestry.weldwood.com.
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