Fire - too little or too much?

Forests of jack pine (Pinus banksiana Lamb.) mixed with black spruce (Picea mariana (Mill) B.S.P.), quaking aspen (Populus tremuloides Michx.), and paper birch (Betula papyrifera Marsh.) were a mainstay of the pre-European settlement, upland, southern boreal forests of the Quetico-Superior Ecosystem, and were regenerated by severe crown fires with rotation periods of 50–100 years (Heinselman 1996). These two conifers are adapted to fire via serotinous cones, while Populus tremuloides and Betula papyrifera reproduce after fires by root sprouting, stump sprouting, or long distance seed transport. A second major forest type, white pine (Pinus strobus L.) and red pine (Pinus resinosa Ait.), dominated almost 40% of the townships in northeastern Minnesota in the presettlement era (Friedman and Reich 2005). These species do not have serotinous cones, but are well adapted to surface fires with thick bark and foliage held above the reach of scorching heat. For all these species, fires are a double edged sword; it is necessary for regeneration, but if it occurs too often, young trees will be killed before they reach the age of reproduction, and if it does not occur often enough, then the old trees will die without leaving behind reproduction to continue the forest (Heinselman 1996; Weyenberg et al. 2004). A number of fire sensitive tree species are present that are confined to lakeshore and wetland refuges from fire; the most common include balsam fir (Abies balsamea (L.) Mill), white spruce (Picea glauca (Moench) Voss), northern white cedar (Thuja occidentalis L.), red maple (Acer rubrum L.), and black ash (Fraxinus nigra Marsh.) (Table 1).

In the past century, the interval between fires lengthened markedly from 50–100 years during Heinselman's 1600–1900 study period, to > 700 years since 1910 (Heinselman 1996). This lengthening of the fire cycle is thought to be caused by a warming climate at the end of the Little Ice Age (allowing more humid air masses from the Gulf of Mexico to penetrate further north), fragmentation of the landscape to the south of the BWCAW (interrupting the flow of fire across the landscape), and fire suppression (Johnson 1992; Bergeron and Archambault 1993; Heinselman 1996). During this long period without chronic stand-clearing disturbances, late-successional species that formerly occupied a small proportion of the BWCAW landscape increasingly became the forest dominants (Frelich and Reich 1995). It is clear that a major reintroduction of fire will be necessary, or Pinus dominated forests will decline over time and completely disappear within 150 years (Heinselman 1996; Scheuer et al. 2005). The disappearance of early-successional Pinus forests has also been accelerated by the ‘Big Blowdown’ of 1999, a derecho that leveled about one-fourth of the forests within the BWCAW, which preferentially removed early successional species from the forest canopy (Rich et al. 2007) and placed the canopy stored seedbank of Pinus banksiana near the forest floor, where cones are likely to be consumed by subsequent fires. Such blow downs are predicted to increase with a warmer climate (Trapp et al. 2007).

Table 1.

Tree species native to the BWCAW grouped by response to a warm/wet climate scenario. DED= Dutch elm disease, MPB = mountain pine beetle, ALB = Asian long-horned beetle, EAB = Emerald ash borer, SOD = sudden oak death, HWA = hemlock woolly adelgid, BWA = balsam woolly adelgid, ELB = eastern larch beetle.

Continued.

Even more recently, large-scale fire has returned with the Cavity Lake Fire of 2006 (13,000 ha) and the Ham Lake Fire of 2007 (29,000 ha). It is too early to tell whether this heralds a reversal of the low fire frequencies of the 20th century. Although the warming at the end of the Little Ice Age favored reduced fire frequencies, as warming proceeds, greater evaporation may come to predominate over the effects of moist air masses and lead to an increase in fire frequency. More fires like those of 2006 and 2007 are likely to occur, and with higher blowdown frequencies and the predominance of older forests, these storms and fires will probably hasten the demise of Pinus forests.

Global warming

The BWCAW, and northern Minnesota in general, have a large number of tree species at the edge of their ranges. All of the boreal tree species in the BWCAW are within 300–500 km of the southwestern edge of their ranges. Moreover, for Quetico-Superior forests, the biome boundary with the tallgrass prairie is only 200–350 km to the west. The best available predictions are for increases in summer temperatures of 3–7° C by the end of the 21^st century (Christensen et al. 2007). Rainfall is likely to remain about the same as at present but with increasing variability, creating a warmer and drier climate, although there is also some chance that the future climate will be warmer and wetter (Wuebbles and Hayhoe 2004). In addition, variability in texture, depth, and water holding capacity of soil and topographic features in the BWCAW may allow patches of forest to persist in some areas even if a warm/dry scenario develops.

Thus, a warming climate is likely to cause a biome change - most likely from boreal forest to grassland/savanna and possibly from boreal forest to temperate hardwood forest. Whether and how quickly the existing forest converts to either of these more southerly vegetation types depends on the inertia of the system. The paleoecological record indicates that vegetation of northeastern Minnesota is particularly sensitive to climate change, with little inertia. An episode of warm and dry climate 8000 ybp to 5000 ybp (which was not as warm as the predicted climate for the late 21st century) resulted in northeastward movement of species with high abundances of Quercus and grasses in areas that have boreal forest today. During the last 3000 years (and reinforced during the last 1000 years by episodes of cool climate such as the Little Ice Age), grasses and Quercus declined in the area, and were replaced by the existing boreal forest (Webb et al. 1983). Vegetation near biome boundaries is generally thought to be more sensitive to climate change, and paleohistory shows this is the case for the Quetico-Superior Ecosystem.

Whether transition to savanna or forest occurs, the vegetation of the BWCAW is likely to be in a state of flux for centuries while new species migrate in and develop the relationships among themselves and their environment that will define the future ecosystem. As Heinselman (1996) points out, a warm/dry climate warming scenario would, by 2100, create a climate similar to Iowa that typically supports “tallgrass prairie on the uplands and wet prairie and marsh on poorly drained soils, with scattered oak groves on the uplands and with cottonwood, box elder, willow, elm, and ash along streams”. Bur oak (Quercus macrocarpa Michx.), northern red oak (Q. rubra L.), and northern pin oak (Q. ellipsoidalis E.J. Hill) are already present in low numbers on rocky sites throughout the BWCAW, and Ulmus and Fraxinus are also present, but face other threats discussed below.

A warm/wet scenario would allow for many possibilities for future forests, which is best summarized by dividing tree species into four groups (Table 1). Group 1 includes boreal species with the southern margin of their range in northern or central Minnesota. This group includes Picea mariana, P. glauca, Abies balsamea, Pinus banksiana, P. resinosa, and balsam poplar (Populus balsamifera L.) (Heinselman 1996; Prasad et al. 2007). These species can be expected to decline greatly in abundance or to disappear, depending on the magnitude of warming, based strictly on climate-distribution relations, with premature death of mature trees and lack of regeneration ability to compete with species adapted to warmer conditions probably the actual mechanisms involved. Group 2 includes species present in high numbers in the Quetico-Superior that have ranges extending further south or that can grow on warmer, drier landscape positions than species in Group 1. These species are predicted to remain, possibly in reduced numbers, and the group includes tamarack (Larix laricina (DuRoi)K.Koch.), Betula papyrifera, Populus tremuloides, bigtooth aspen (Populus grandidentata Michx.), Fraxinus nigra, Thuja occidentalis, and Pinus strobus. Group 3 includes species present at low numbers in the Quetico-Superior and elsewhere in the southern boreal forest, but which are limited in terms of northern distribution by the present climate, such as Acer rubrum, Quercus macrocarpa, Q. rubra, Q. ellipsoidalis, yellow birch (Betula alleghaniensis Britton), green ash (Fraxinus pennsylvanica Marsh.), and American basswood (Tilia americana L.). Group 3 species currently dominate small patches that can be expected to expand across the landscape under a warm/wet scenario. Group 4 includes species whose northern range limit is south of the BWCAW but that may migrate in. There are many possibilities, but the most likely species that are closest include cottonwood (Populus deltoides Bartr. Ex. Marsh.), box elder (Acer negundo L.), black willow (Salix nigra Marsh.), sugar maple (Acer saccharum Marsh.), white oak Quercus alba L.), eastern hemlock (Tsuga canadensis (L.) Carr.), and black cherry (Prunus serotina Ehrh.) (Walker et al. 2002; Prasad et al. 2007). Recent experiments in Ontario indicate that Acer saccharum would have no problems other than seed source limitation to invade the southern boreal forest (Kellman 2004). A review of predicted future range maps shows that under a warmer, wetter climate, vegetation similar to that in the region from Upper Michigan to Pennsylvania is likely to develop, although the shallow rocky soils in parts of the BWCAW may limit the success of some of those species (Table 1).

Invasive species

There are no native earthworms in the northern temperate or boreal forests, including the BWCAW (Tiunov et al 2006). European earthworms are invading at this time and are readily able to penetrate the depths of the wilderness since they are used as live fishing bait, and the region has many popular fishing lakes. Earthworms are fundamental ecosystem engineers that change soil structure, seedbeds, nutrient cycles, and the hydrologic cycle. In this region, the invasive earthworm Lumbricus rubellus Hoffmeister, is capable of consuming the forest floor duff layer in all forest types except Picea, Pinus banksiana, and P. resinosa, and the nightcrawler (L. terrestris L.) maintains the litter-free conditions indefinitely, making the soil susceptible to erosion (Hale et al. 2005). Earthworms are particularly abundant in Ca-rich forests dominated by Acer and Tilia (Reich et al. 2005), so it is not clear how intensely they will affect the BWCAW forests as they invade. In Acer-dominated forests, earthworms increase the bulk density of the soil A horizon, and reduce availability of the key nutrients N and P, leading to declines in tree growth rate (Hale et al. 2005). They also open the door for invasive plant species from Europe that coevolved with earthworms, including garlic mustard (Alliaria petiolata (M. Bieb.) Cavara and Grande) and common buckthorn (Rhamnus cathartica L.), and by reducing seedling density also increase the deer:plant ratio and magnify the impact of deer grazing on tree reproduction (Frelich et al. 2006; Heneghan et al. 2006).

Invasive tree diseases and pests Several introduced insect pests and diseases will likely have major impact on temperate and boreal North American tree species over the next several decades. The emerald ash borer (Agrilus planipennis Fairmaire) is a native of Asia and has the capability to cause nearly100% mortality of native ash species over large regions (MacFarlane and Meyer 2005). Fraxinus nigra is abundant within forested wetlands in much of the southern boreal zone, and is considered an important tree for the function of boreal wetlands in eastern North America (Tardiff and Bergeron 1999). The BWCAW is in the high susceptibility zone for Asian longhorned beetle (Anoplophora glabripennis Cano), which can destroy tree species in the genera Acer and Populus (Nature Conservancy 2008). Hemlock woolly adelgid (Adelges tsugae Annand) is a needle sucking insect from Asia that causes close to 100% mortality of Tsuga canadensis (Stadler et al. 2005). It has already caused widespread devastation of forests in the eastern United States and although Tsuga canadensis is currently absent, the BWCAW is predicted to become climatically optimal for hemlock under some global warming scenarios. Thus, the adelgid may prevent Tsuga's future migration into the BWCAW Balsam woolly adelgid (Adelges piceae Ratzeburg) is capable of devastating Abies balsamifera, and is limited by cold winters, but has invaded the southern portion of Maine in recent years due to mild winters (Maine Forest Service 2008). This recent invasion is of concern for the BWCAW because winters in northern Minnesota are projected to become as mild as Maine within several decades. Sudden oak death (Phytopthora ramorum S. Werres) is a foliar and trunk canker disease from Asia first introduced in California several years ago. Tests indicate that Quercus rubra and Q. ellisoidalis are susceptible (U.S. Department of Agriculture, Forest Service 2002; Table 1).

Insects native to Minnesota and other parts of North America can also become invasive in a warmer climate. The mountain pine beetle (Dendroctonus ponderosae Hopkins) has a good chance of making its way to Minnesota from British Columbia, where it has killed 12 million ha of lodgepole pine (Pinus conforta Dougl. Ex Loud.) in recent years (Kurz et al. 2008). With warmer winters, the beetle population may move across the southern margin of the boreal forest, and is capable of killing Pinus banksiana (a close relative of P. contorta) and possibly P. resinosa and P. strobus (Logan 2007). The eastern larch beetle (Dendroctonus simplex LeConte) is a native to Minnesota that has experienced rapid population growth in recent years with mild winters, and has caused significant mortality in Minnesota Larix stands.

Deer Browsing

Currently, grazing by white-tailed deer (Odocoileus virginianus Zimmermann) is not a major problem for tree regeneration in the BWCAW, which has a density of 0.8–1.2 deer per km² (Lenarz 2005), and prior to European settlement in the late 1800s, deer were virtually absent in the region. Regeneration of species such as Thuja occidentalis is abundant, a situation rarely seen 100 km further south where deer populations range from 6–7 per km2 (Lenarz 2005). The deer population in the BWCAW is limited by deep snow and number of severely cold days each winter. Areas south of the BWCAW and much of the eastern U.S. report widespread problems with regeneration of Thuja occidentalis, Pinus strobus, Quercus rubra, and Betula alleghaniensis due to deer grazing (Cornett et al. 2000; Cotè et al. 2004; Table 1); and the BWCAW may experience winter climate conditions similar to those areas within a few decades. A warmer climate will allow a higher deer population in the BWCAW, but it is uncertain how much higher. The BWCAW has a large population of gray wolves (Canis lupus), and it lacks the upland checkerboard of edge habitat that logging has created over most of the landscape, two factors that may work to limit the deer population to lower levels than experienced elsewhere. The next few decades in the BWCAW will be a good test of the hypothesis that high deer populations are facilitated by human-caused landscape fragmentation (Alverson et al. 1988).

Impacts on tree diversity.

For warm/dry global warming scenarios, all existing tree species would be negatively impacted except the three Quercus species. For warm/wet scenarios, six of 22 tree species native to the BWCAW are predicted to be greatly reduced in abundance by warming alone (Abies balsamifera, Picea mariana, P. glauca, Pinus banksiana, P. resinosa, and Populus balsamifera), four additional species may have substantial decreases in abundance (Populus tremuloides, P. grandidentata, Betula papyrifera, and Larix laricina), four additional species may be negatively impacted by increasing deer (Pinus strobus, Betula alleghaniensis, Thuja occidentalis, and Quercus rubra), six potentially impacted by exotic tree pests (Acer rubrum, Populus balsamifera, P. grandidentata, P. tremuloides, Fraxinus nigra, and F. pennsylvanica), and two potentially impacted by exotic disease (Quercus rubra and Q. ellipsoidalis; Table 1). Ulmus americana is a native species that is functionally extinct due to the spread of Dutch elm disease over the last 20 years, although a few individuals remain, and should be able to expand under a warmer climate. This leaves two species with only minor impacts expected under a worst case warm/wet scenario: Tilia americana and Quercus macrocarpa, although there is hope that Sudden oak death and Asian long-horned beetle will be kept at bay, leaving Populus tremuloides, P. grandidentata, Acer rubrum, Quercus rubra, and Q. ellipsoidalis among the species that would be present. With good deer management, four additional species would be gained, and seven more species could migrate in. Thus, with an optimistic scenario, 18 tree species could exist in the BWCAW in the future, although whether these would be in scattered groves on moister sites or patches of contiguous forests would depend on how wet the future climate is.

Wilderness law and management issues

Several management issues mentioned in the impact scenarios have not been debated, have no precedents in the BWCAW, or may conflict with wilderness laws governing wilderness management and/or interpretation of the law by the U.S. Forest Service: (1) chemical and biocontrol treatments for exotic tree diseases and pests; (2) reintroductions of native species extirpated by direct human action, or by indirect, unintentional human action including species removed through exotic pests and diseases; (3) establishment of neo-native species (assisted migration, regionally and locally, into and within the BWCAW); (4) triage of forests dying from invasive pests (i.e., sanitation and fire safety); and (5) loss of endangered species now at the southern edge of their range, as well as non-endangered species whose ranges may no longer include the BWCAW (e.g., Picea mariana and associate wildlife species).

Two other issues have precedents in the BWCAW. Removal of invasive plant species is already allowed and practiced through a joint effort of Friends of the Boundary Waters Wilderness and Superior National Forest (Friends of the Boundary Waters Wilderness 2006). This effort will need to be increased substantially, and would be greatly facilitated by an invasive species buffer zone implemented outside the wilderness. A second issue with precedent is prescribed fire within the wilderness. A prescribed burning program was instituted in response to fire danger created when a massive derecho leveled a great expanse of forest within the BWCAW in 1999. Whether prescribed fire would be allowed for ecosystem management purposes is unclear at present.

Fire and adaptation to climate

Although trees in the past have responded to warming climates mostly by northward migration, which was facilitated by warmer climates at the northern edge of their range and forced by superior competitive ability of more southerly species along the southern margin of their range, it is also possible for trees to adapt genetically to varying temperatures (Davis and Shaw 2001), as evidenced by the remarkable ecotypic variation among populations of widespread tree species. In terms of generation times (but not on an absolute time scale), trees undergo relatively rapid evolution compared to some other organisms because of the intense selection afforded by the large ratio between the number of seeds shed to those that become adult canopy trees. Therefore, it may be possible to allow trees to adapt to a moderate degree of warming by keeping out southern invaders and insuring that sexual reproduction occurs regularly during warming. Southern populations of widely distributed conifers are likely to grow poorly and exhibit heightened mortality with climate warming, largely due to enhanced heat and drought (Reich and Oleksyn 2008), but survival should be sufficient for some selection to proceed. Whether adaptation would occur fast enough to allow these species to persist in the landscape is unknown and uncertain. It is much less likely and perhaps impossible for trees to adapt to large magnitudes of warming that result in a climate suitable for a different biome.

For the BWCAW, reinstatement of fire (or increased natural ignitions due to increased drought during climate warming) would increase Pinus reproduction and, thus, assist their adaptation to warmer temperatures through natural selection as well as prevent takeover by fire sensitive trees in Groups 3 and 4, such as Acer rubrum, which already exists in small numbers (Table 1). Populus would be helped by reinstatement of fire to a limited extent, because much of its reproduction is vegetative, from root sprouts, which limits opportunities to adapt by natural selection; however, it may maintain its populations for far longer on the landscape if disturbance is frequent rather than infrequent. Before undertaking any fire reinstatement program, there are three issues to resolve. First, the debate among scientists and managers over whether to help the existing species adapt, versus allowing or facilitating the movement of new species into the area needs to be resolved. Second, regardless of how this debate is resolved, policies would need to be in place to allow the management strategy to take place. Third, at this point there is a wildland fire-use policy in place which allows natural ignitions to burn under certain conditions, but it is uncertain whether this policy will allow fire to return to its natural role in the BWCAW due to the impacts of decades of fire exclusion, the landscape context in which the wilderness sits, and climate change. If climate becomes warmer and functionally drier, fires could play a larger role in the 21^st than the 20^th century without prescribed burning.

Response to tree pandemics

As previously mentioned, a number of BWCAW tree species could be impacted by pandemics. Should we reintroduce tree species after a pandemic goes through, and should we use the native variety or a new variety with resistance developed through selection or other ways of modifying the genome? Is it more natural to watch how the remaining species respond when one species is removed because of a pest we introduced, or is it more natural to restore the species to the wilderness, even if it has been bred to be resistant? Although it seems that a distinction could be made between species that may go extinct due to an introduced pest or disease (where some might argue that they should probably be restored if possible) and those species reduced to low levels that may be able to respond by natural selection (restoration can perhaps occur by natural processes), there has not been sufficient discussion of these questions nor establishment of policies to deal with the consequences of pandemics.

Another question is whether to treat exemplary stands chemically when a pandemic arrives. This has been done for a few groves of Tsuga canadensis in Shenandoah and Great Smoky Mountain National Parks, where hemlock woolly adelgid is rapidly extirpating the species. Soil treatments with the systemic insecticide, Imidacloprid, are highly effective in eliminating the adelgid for about two years (Cowles et al. 2006). For the BWCAW, this situation may arise for Fraxinus nigra if the rapidly expanding emerald ash borer arrives in the next few decades. Will we choose a representative or exemplary set of ash stands in the BWCAW to treat and preserve a few ‘postage stamps’ of this forest type? One could argue for this for esthetic reasons, preservation of germplasm, the scientific information contained in tree rings, the ecological relationships among tree species, and the relationship of the species to habitat factors.

Unlike previous conservation problems (such as logging and conversion of land to other uses) that were remedied by wilderness protection, just the opposite may occur with this new set of threats to the ecosystem; it will be more difficult to use traditional disease and insect management techniques, such as sanitation and removal of susceptible species to create breaks in transmission, within the wilderness than outside of it. The wilderness may end up with fewer native tree species (i.e., be less natural) than the surrounding forest, an unintended consequence of wilderness laws implemented during times when we as a society believed that natural areas of sufficient size were capable of maintaining themselves.