Will environmental changes reinforce the impact of global warming on the prairie–forest border of central North America?

© The Ecological Society of America www.frontiersinecology.org C interiors have biome boundaries that are highly sensitive to climate change. The northern prairie–forest biome border in central North America is such a case; it is ~2700 km in length, extending from northern Alberta, Canada, southeastward across the Canadian prairie provinces, and into the western Great Lakes region of the US (DeFries et al. 2000; Figure 1). Positioned more or less perpendicular to the border is a steep gradient from a prairie climate – featuring frequent droughts, summer heat waves, and a historically high fire frequency – to a forest climate, with rainfall evenly distributed throughout the year and cool summers (Changnon et al. 2002). Although woody expansion into North American grasslands has been documented in the past (Samson and Knopf 1994), it is widely expected that, under a scenario of humaninduced global warming, the prairie biome will shift to the northeast and displace existing forests. The paleoecological record shows that this pattern of biome change occurred during previous climate-warming episodes; during the midHolocene warm period 7500 years before present (ybp), for example, a warmer climate and interactions between climate and fire frequency allowed grassland to replace boreal forests (dominated by jack pine [Pinus banksiana], black spruce [Picea mariana], balsam fir [Abies balsamea], aspen [Populus tremuloides], and paper birch [Betula papyrifera]) as well as hardwood forests (dominated by northern red oak [Quercus rubra], white oak [Quercus alba], sugar maple [Acer saccharum], American basswood [Tilia americana], and elm [Ulmus spp]; Camill and Clark 2000; Umbanhowar 2004). Future projections for a “2 x CO2” climate (ie 560 parts per million [ppm] atmospheric CO2, or twice the preindustrial concentration of 280 ppm) suggest a northeastward shift of biomes and tree ranges of 100–500 km (Lenihan and Neilson 1995; Walker et al. 2002), resulting in the potential loss of forests on 200 000 to 1 million km of land in central North America. The upper estimate is more than twice the size of the state of California. These projections, however, take into account only the climatic envelope within which certain tree species and biomes currently exist. Several other human-induced drivers of change will influence prairie–forest border dynamics, so that the types or magnitudes of future changes may differ from those in the paleoecological record, probably leading to no-analog plant communities (ie there is no past or current community of a similar composition; Williams and Jackson 2007). These drivers include invasive earthworms, tree diseases and pests, changes in dynamics of native insect populations, increasing deer REVIEWS REVIEWS REVIEWS