Managing second-growth forests as caribou habitat

Habitat management for woodland caribou (Rangifer tarandus caribou) in southeastern British Columbia has generally focussed on protecting old-growth forests from logging. As that strategy becomes more difficult to maintain, biologists are beginning to explore opportunities to manage second-growth stands to provide arboreal lichens and other habitat resources important to caribou. Special harvesting and stand management practices are being developed and formulated into strategies for maintaining caribou populations in managed stands.


Introduction
The woodland caribou of southeastern British Columbia typically winter in old-growth forests, where their primary forage is arboreal lichens.Habitat management for those caribou has generally been directed at trying to protect old-growth forests from logging.As the demand for timber increases, that strategy becomes more and more difficult to maintain.Researchers and managers have begun to ask whether second-growth forests can be managed so that they will function as caribou habitat before they are logged again.
That question has become urgent in some caribou ranges east of Prince George.Spruce bark beetle attacks have resulted in extensive salvage logging over the last seven years.Low-and mid-elevation ranges formerly used by caribou are now in an early serai stage.Because so much of the low-elevation timber has been removed, logging companies are beginning to harvest high-elevation stands formerly considered unprofitable.Wildlife managers are asking how this logging is affecting caribou, and what can be done to make the cut areas into future caribou habitat.To address those questions, the British Columbia Ministry of Environment, in cooperation with the Ministry of Forests and the forest industry, is initiating a research and management project centred east of Prince George.The purpose of this paper is to present some of the ideas that are to be developed and implemented in that project.
Caribou studies in southeastern British Co-Rangifer, Special Issue No. 3, 1990.lumbia (Antifeau 1987;Rominger and Oldemeyer 1989;Scott and Servheen 1985;Simpson 1985;Simpson et al. 1985) have suggested a number of attributes that are important to caribou on winter ranges: standing crop of lichens on accessible portions of trees, arboreal lichens available through litterfall and blowdown, canopy modification of snow depth and snowpack characteristics, availability of vascular forage, and remoteness from vehicle access.
In this paper, I will focus on management practices for stands in which the principal objective is to encourage the development of arboreal lichen forage (mainly Bryoria spp.and Alectoria sartnentosa) while producing merchantable timber.
The habitat management strategies proposed here must be integrated with an overall plan for population management, which includes consideration of direct mortality factors.Providing an extensive habitat base by managing second growth is expected to allow caribou to disperse sparsely over large areas, reducing the risk of mortality from predation and poaching.

Factors limiting arboreal lichen forage in young stands
The development of management strategies for enhancing arboreal lichens should be based on an understanding of the factors that limit their abundance in young stands.

Source ofpropagules
The early establishment of lichens in some young stands is limited by the presence of lichen propagules.Lichen dispersal can be accomplished by spores, soredia, or thallus fragments, but thallus fragments account for nearly all the early colonization of second growth (Stevenson, 1988).
Studies conducted on Vancouver Island, British Columbia have shown that beyond about 400 m from the edge of the mature timber, young twigs in second-growth stands support almost no lichens (Fig. 1; Stevenson, 1988).

Microclimate
Changes in stand structure that occur during succession result in changes in stand microclimate, and those changes affect the suitability of a stand as lichen habitat (Fig. 2; Stevenson and Palmer 1988).

Opportunities to enhance lichen production in young stands
Limitations of lichen forage due to propagule source, stand microclimate, and rates of litterfall and blowdown can be addressed through special management practices.

Maintaining natural dispersal.
There are several approaches to providing mature, lichen-bearing trees as a source of propagules.Diameter-limit logging and the harvesting of very small patches can maintain opportunities for lichen dispersal onto young trees; the tradeoffs associated with those harvesting practices have been discussed elsewhere (Stevenson 1986b).Alternatively, the size and configuration of cutblocks can be planned with consideration to the dispersal limitations of lichens.In a three-pass system (a logging system in which all the merchantable timber within a planning unitis removed in three successive cuts, equally spaced throughout the rotation), it is theoretically possible to maintain a large proportion of the area within 350 m of mature timber during the first half of its rotation.
Patches of unmerchantable timber, such as swamp fringes and inaccesssible areas, contribute to the area that functions as a dispersal source.If the advanced regeneration in the cut areas can be retained during harvesting, lichen recovery will occur more rapidly.Harvesting alternatives such as these require special planning and cooperation between wildlife biologists and foresters.

Inoculating young stands with lichen propagules
The modified harvesting methods disussed above may be feasible where there is still flexibility in harvesting plans, but they cannot be used in places where extensive harvesting has already occurred.To meet the need for lichen propagules in second-growth stands that are distant from mature timber, an inoculation technology is being developed (Palmer 1988).
The method involves first spraying the trees with an adhesive agent, then with mechanically chopped lichens.Handheld equipment has been used to spray trees up to 7.5 m, and aerial application may be practicable.
Modifying stand microclimate to enhance lichen development

Managing for litterfall and blowdown
Litterfall and blowdown are the major sources of arboreal lichen forage for caribou on lowand mid-elevation winter ranges.Thinning dramatically reduces the rates both of litterfall and of tree mortality, which is related to blowdown (Bray and Gorham 1964;Gessel and Turner 1976).Thus, the methods that are used to increase the standing crop of lichens in the canopy could prevent them from becoming accessible.
That problem could be addressed in either of two ways.A harvesting system that leaves a residual stand of older trees will ensure not only a source of lichen propagules, but also a source of litterfall and blowdown over many years.

Conclusions
The special management practices that have

Fig. 1 .
Fig.1.Lichen fragment on two-year-old twigs in second-growth and distance from mature timber in several Vancouver Island studies(Stevenson 1988).

Fig. 2 .
Fig. 2. Hypothetical development of an unmanaged, fully-stocked stand.In the grass-forb stage, the tree seedlings are surrounded by a layer of grasses, herbs and shrubs, which affect the microclimate of the site.The microclimate is more severe than it is within a forest, but less severe than it is over bare grond.Arboreal lichens are generally absent.In the shrub-seedling stage, the trees have grown above the ground vegetation, but the canopy has not yet closed.The crowns of the young trees are exposed to light, and they have an open-grown form.If a dispersal source is present, lichen fragments colonize the branches and trunks on suitable sites.However, the microclimate is still relatively severe, and on some sites, conditions may be too dry or warm for lichen development at the shrub-seedling stage, even though lichens may do well in adjacent mature timber stands.
An important microclimatic limitation to lichen developments believed to occur in fully-stocked stands on moist sites after the time of canopy closure.On those sites, a carefully planned thinning program could be applied to maintain better conditions for lichen development (Stevenson and Palmer 1988).A thinned stand has fewer trees, longer and wider crowns, and maximum foliage biomass located closer to the ground than an unthinned stand.Those differences in stand structure affect stand microclimate.Light intensity and throughfall precipitation in the lower canopy are increased.Temperatures are expected to be intermediate between those of open areas and uncut forests.Summer humidity is expected to be lower in thinned than in unthinned stands.On sites where lichen abundance in the canopy is limited by light, increased stand openness should result in a larger portion of the canopy providing suitable habitat for arboreal lichens.On drier or warmer sites, however, thinning may result in summer temperatures that are too high or humidity levels that are too low for optimum lichen development.Observations of the vertical distribution of lichens in existing stands can be used to predict how lichens will respond to thinning, but those predictions must be refined and tested.
been suggested here are ideas that are being tested for use in an overall management strategy.That strategy will encompass a number of variations.Sites differ in the importance to caribou of various habitat attributes, in climatic limitaions on the potential of the site to produce those habitat attributes, and in the constraints of silviculture and forest economics.The stra-tegy must take into consideration the relationships between management of caribou habitat and populations; for example, practices intended to enhance lichen production may lead to increased disturbance and poaching unless access control is practiced.Potentially, it could provide alternatives to the preservation of old growth as the primary mode of managing habitat for caribou in southeastern British Columbia.