0389-B4
Yves Bergeron, Sylvie Gauthier, Thuy Nguyen, Alain Leduc, Pierre Drapeau and Pierre Grondin 1
In this paper, we will illustrate how we have developed and initiated implementation of a management strategy based on the understanding of natural forest dynamics in the black spruce-feathermoss ecoregion of northwestern Quebec. Fire frequency within the study area has varied over the last 300 years, and the fire cycle length is still increasing. Both fire size and severity showed a large range of variability. Together with the surficial geology, the intervals between successive fires on a particular site have a strong influence on forest structure and, to a lesser extent, on tree composition. The presence and abundance of species (birds, vascular and non-vascular plants) change gradually along the time-since-fire sequence, with very few species restricted to a particular stand age. Results have enabled us to propose a number of objectives that can serve as targets at different phases of management planning. At the landscape level, the objectives are aimed at maintaining observed natural diversity resulting from fire variability (size, severity and intervals) whereas at the stand level, we propose that a diversity of treatments should be applied to respect the natural forest dynamics. Briefly, the landscape diversity objectives (proportion of the main type of structure/composition) and the array of possible treatments are established within the 25-year strategic plan. The strategy offers promising results in terms of estimated annual allowable cuts and maintenance of landscape diversity. At the tactical level (five-year plan), zones where the various treatments can be applied are recognized, defining the spatial arrangement objective for the treatments. At the operational level (one-year plan), we discuss the need to develop a guide that allows stand structure to be linked to applicable treatments and facilitates decision making for forest managers in the choice of silvicultural practices.
Forest ecosystem management based on the understanding of natural disturbance regimes has been suggested as a means to maintain biological diversity and productivity in forest systems (Attiwill 1994, Bergeron and Harvey 1997, Angelstam 1998, Bergeron et al. 1999, Bergeron et al. 2002). It is postulated that management strategies aimed at maintaining stand and landscape compositions and structures similar to those that characterise natural ecosystems should be favourable to the maintenance of biological diversity and essential ecological functions (Franklin 1993, Gauthier et al. 1996, Hunter 1999, Mac Nally et al. 2002). Hence, knowledge of characteristics such as age-class distribution, stand composition and structure, and the spatial arrangement of forest stands in natural forest landscapes should be considered as key indicators for the implementation of sustainable forest management.
Recently, we proposed an approach where variation in treatments can be applied to forest to maintain structural and compositional diversity, without lengthening the timber rotation (Bergeron et al. 1999). The main objective of our paper is to show how the knowledge gained on the natural forest dynamics is simplified and transposed into a management strategy. We will illustrate the development and the initial implementation of our management strategy using the example of a pilot study area located in the black-spruce feathermoss ecoregion of north-western Quebec.
The data on fire history reconstruction and forest dynamics were mostly collected in the south-western boreal forest located between 49o 00 to 50o 00' N and 78o 30' to 79o30' W (see Bergeron et al 2001 and Gauthier et al. 2002 for a complete methodology). This reconstruction showed that large areas are still covered by stands that did not burn since a long time period (Fig. 1a). In fact, the compiled age structure of stands shows 57% of the study area is composed of forests that have burned more than 100 years ago, with 20% being older than 200 years. This suggests that under a natural fire regime, the study area had a significant proportion of stands that were exempted from fire for long time periods. Despite the fact thatfire frequency has changed over the last 300 years (Bergeron et al. 2001), the age structure of the forest land base has a certain inertia with regards to changes in the fire cycle. We therefore suggest that the average forest stand age of the studied area (151 years) can be used as a baseline in strategic planning of harvesting activities in order to estimate the desired proportion of each age class to be maintained with different silvicultural treatments (see below and the cohort model in Bergeron et al. 1999, Bergeron et al. 2002).
Whereas the majority of fires are smaller than 1 000 ha (Fig. 1b), these fires are only responsible for a little fraction of the total area burned (Bergeron et al. 2002). Consequently, the large fires (for instance, those over 1,000 ha) are responsible for most of the area burned, and are therefore imposing a given age structure and configuration in the landscape (Weber & Stocks 1998; Johnson et al. 1998).
It must also be recognized that within a single fire the severity is variable leaving patches of green trees following its passage (Ryan 2002, Bergeron et al. 2002, Kafka et al. 2001, Turner and Romme 1994, Van Wagner 1983). In fact, "low severity zones" (including both fire skips and intermittent crown fires) may occupy up to 50 % of a burned area, depending on the type of forest burned and the prevailing weather conditions (Bergeron et al. 2002). The presence of these lightly burned zones suggests that the mortality pattern generated by fire is very distinct from that resulting from conventional forest harvesting.
Many studies on vegetation dynamics have been conducted in the area (Gauthier et al. 2000; Harper et al. 2002; Harper et al. in press; Bergeron et al. 1999). Three different pathways (Fig. 2) have been defined depending on the relative importance of the main three post fire species that are forming the post fire cohort: a jack pine (Pinus banksiana) series, a trembling aspen (Populus tremuloides) series and a black spruce (Picea mariana) series (which is the most common on our study area). The post-fire cohort is defined as cohort 1 (C1). Note that if a fire occurs while the stand is within that stage, a cyclic succession would be observed as it is the case in many boreal regions (Johnson 1992). Given that the fire cycle is relatively long in the region, at many sites, however, the interval between successive fires is longer than the normal longevity of individual trees of the post-fire cohort, allowing these sites to enter into the old growth forest stage (Kneeshaw and Gauthier in press). Under a long fire interval, a change in species composition where the forest is regenerated by a shade intolerant post-fire cohort (Trembling aspen and/or jack pine) or in structure, in stands recolonized by black spruce is expected to occur. These changes reflect the replacement of individuals immediately established following a stand initiating fire and initially constituting the stand canopy (1st cohort) by individuals previously occupying the understory (2nd cohort (C2)). Moreover, in the continued absence of fire, gap dynamics perpetuates the replacement of individuals from these two cohorts by individuals from later cohorts (collectively called 3rd cohort (C3) as individual cohorts gradually become harder to distinguish). These three stages of stand development are characterized by different composition and structure and can serve as a basis for the elaboration of a forest management strategy for our pilot territory (Bergeron et al. 1999). The presence and abundance of many group of species (birds, vascular and non-vascular plants) change gradually along the time-since-fire sequence, with very few species restricted to a particular stand age (Boudreault et al. 2002, Harper et al. in press, Drapeau et al. in press). This suggests that preserving a good proportion of each seral stages (cohorts) could be sufficient to maintain most of the animal and plant diversity.
A management strategy that will maintain a forest mosaic similar to that observed under natural conditions implies: 1) the maintenance of proportions of the area occupied by different stand types (composition and structure) to similar levels to those observed under the natural forest dynamics, 2) the establishment of spatial patterns of harvesting that are resembling those induced by natural disturbances and 3) the maintenance of a variety of disturbance severity within zones of intervention. The natural disturbance and forest dynamics model (Fig.2) can serve as a basis in the conception of such strategy. The implementation of the natural disturbance-based model is currently being tested in a pilot territory. For this pilot territory, the forest management strategy presented in Fig. 3 has been integrated in the different forest management planning levels (Tittler et al. 2001) to assess the feasibility and possible repercussions of the implementation of the disturbance-based management model. At the strategic planning level (25 yrs periods), regional area-based "cohort" objectives for the pilot territory were established at 62% for C1, 21% for C2, and 17% for C3. As suggested by Bergeron et al. (1999), these proportions were established by using the predicted negative exponential age distribution curve (Van Wagner 1978) with an average stand age of 151 years (Bergeron et al. 2001).
In order to compare the effectiveness of different management strategies to achieve the regional "cohort" objectives, a timber supply analysis was carried out for the pilot territory using three different management strategies (Nguyen 2000). The first strategy is an even-aged management system (clear-cut only (CPRS)) while the two others are mixed management systems (clear-cut and partial-cut with 14cm diameter limit (CPPTM14), or clear-cut and partial-cut with 16cm diameter limit (CPPTM16). The results of the timber supply analyses indicate that at the end of the 150 year simulation period, the mixed management systems were closer to meeting the "cohort" area-based objectives than the even-aged management system. The analyses also suggest that the use of a mixed management system would not significantly affect annual allowable cuts.
Because of the size of the fires, large areas are covered by stands with similar age and structure . Old growth forests, i.e., developmental stages where tree replacement of the first cohort begins (see Harper et al. in press)are therefore, extensive and important across the landscape; they provide unique structural features. At the tactical planning level (5 yrs periods), a strategy inspired by those natural patterns was devised for the spatial and temporal distribution of harvesting activities in the pilot territory (Fig.3). It proposes three types of management zones: 1) even-aged management zones where clear-cut harvesting will be predominantly used, 2) uneven-aged management zones where partial harvesting will be predominantly used, and 3) light management zones where non-productive forests predominate (less than 50 m3/ha) and, hence, where harvesting activities will be limited. These zones ranged from 5 000 to 40 000 ha in size.
The implementation phase in the pilot-territory was initiated 3 years ago. It is providing promising results. For instance, preliminary data show that the strategy can be implemented with very little effect on timber supply, while achieving the maintenance of the 3 cohorts in the system. Moreover, preliminary results are also showing that treatments can be developed to maintain the C2 and C3 structure, at least in the short-term. It appears that the available scientific knowledge (i.e. knowledge on the natural ecosystem dynamics) was comprehensive enough to allow the development of the strategy and initiate its implementation. However, this forest management model hypothesizes that partial cutting will maintain or establish ecological attributes associated with older forests. To validate such hypothesis, several partial-cut trials have been or are currently being performed. Data on timber productivity (tree-growth), and biodiversity (plants and animals) are collected before and after interventions and will allow to document and test this hypothesis.
However, there is an urgent need to develop and implement strategies that tackle the conservation of biodiversity across scales from stand level to entire forest management units. The current forest practices, which harvest mostly mature and overmature forest stands will modify considerably the proportions of the land base in C2 and C3, particularly in eastern Canada where they represent large continuous tracks. It is urgent to change our practices even though our knowledge of ecosystems processes and patterns is not completed. It is easier to implement strategies such as the one proposed in this paper while we still have large virgin forests than it is in a context where we have to restore ecosystems as it is presently the case in some parts of Europe (Kuuluvainen et al. 2002).
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Figure 1. Decade of post fire origin of stands (in area percentage; left panel) and fire size distribution (1945-1998; right panel) in the south western section of the Matagami ecoregion.
Figure 2. Simplified natural forest dynamics model proposed for the Matagami ecoregion with the conceptual management strategy. cc: clear cutting; pc: partial cutting; s: selective cutting; p: plantation. BS: Black spruce; TA Trembling aspen; JP Jack Pine.
Figure 3. Initial proposed strategy for the agglomeration of harvesting systems in the pilot territory. Types of management zones: emz (E: even-aged management zone), umz (U: uneven-aged management zone), lmz (L: light management zone). Harvesting blocks within a given management zone: cc (clear-cut harvesting), pc (partial-cut harvesting), nc (no harvesting-unexploitable). One zone of each has been selected for the example to show the treatments possible on each management unit. These units would be treated within 5-10 years. Note that within the management zone, the white areas are those that are non-commercial forest areas.
1 Universite du Quebec, 445 Boulevard de l'Universite, Rouyn-Noranda, Quebec J9X 5E4, Canada. [email protected]