GLOBAL CHANGE AND BRITISH COLUMBIA NATIVE FLORA Richard Hebda Botany Unit Royal British Columbia Museum Victoria, B.C. V8V 1X4 Presented at the "British Columbia Native Plants, their current Status and Future Colloquium at Botany Dept., University of British Columbia, May 12, 1990 ****************************************************************************** Future of British Columbia's Flora and Vegetation Let us examine the possible consequences of global warming on British Columbia's flora. I cannot discuss all of the possible effects because we have little knowledge for many areas and because the analysis would be extremely lengthy. I will focus on four broad vegetation assemblages where there will be significant impacts and where we can make predictions of what might happen with considerable degree of confidence. The four categories are: Alpine Tundra (= Alpine Tundra Biogeoclimatic Zone or AT), Ponderosa Pine and Bunchgrass Biogeoclimatic Zones (PPBG, these are combined in this paper even though recently separated from each other, Research Branch 1988), wetlands and shoreline vegetation. Alpine Tundra The alpine zone and adjacent subalpine openings face serious changes in the near future. Today alpine tundra occupies approximately 150 000 km2 which represents 16 percent of the province's land area. The alpine zone harbours many plant species, which grow in no other zone. Large areas of alpine tundra occur along the Coast Mountains, Rocky Mountains, and in the northern half of the province. In the north, where the treeline is low enough, and in the highest mountains, alpine habitats may be continuous or nearly so. However, many alpine areas, especially in the south are discontinuous and cover small isolated areas. This component of the alpine tundra is most at risk from global warming. Let us turn to paleoecological studies as predictive tools for changes in the extent of the alpine. Several studies (see Clague and Mathewes 1989) in British Columbia identified that timberlines were significantly higher during the first half of the Holocene than today. I estimate the potential rise in timberline to have been in the range of 100 - 200 m, (see also Clague and Mathewes 1989). Since Alpine zones are topographically top-limited even a slight climb of the timberline dramatically reduces the area for alpine habitats. This "cone" effect would work as follows. Suppose you have an ideal conical peak 2300 m high with 25û slopes and suppose the alpine zone begins today at about 2000 m. A rise of timberline by 100 m would reduce the alpine area by 56%. (surface area of a cone = ¹r x r2 + h2; r is radius of cone base, h is height of cone). Proportionately small increases in timberline reduce alpine habitat dramatically. What impact might we expect at the species level? For many species there will only be a reduction in area of distribution and perhaps isolation of populations leading to potential increases in speciation. Notably, many species survived the xerothermic interval intact to give us the alpine flora that we know today. Unfortunately, because there have been few paleoecologic studies in alpine sites we do not know in any detail how the alpine flora and zone were affected by the warmer and presumbably drier xerothermic climate. On the Brooks Peninsula, in one of the most moist and equable climatic regimes in British. Columbia, alpine and subalpine species did survive (Hebda in press). Their survival was likely the result of unique microclimatic conditions. Indeed plants of alpine settings may survive by clinging to unforested cliffs or exposed outcrops below timberline. However, we must expect some loss of populations and perhaps some species if global warming occurs. B.C.'s Alpine Tundra Biogeoclimatic Zone contains 106 taxa listed as rare by Straley et al. (1988). Many of these also occur in other zones, but many grow only in the AT. Often the taxa listed for other zones occur only in openings. These taxa, especially those of southern B.C. alpine habitats, are at greatest risk. Their populations should be identified and monitored for non-climatic disturbances to assist them to survive until, and perhaps persist through, the predicted climatic maximum. Further, we must double our efforts to document species occurrence in the alpine and understand the factors controlling their distribution. We must examine our parks, reserves, and other "set-asides" and assess whether we have protected areas large enough, especially in the southern part of the province, so that some AT will survive. Fortunately, the human impact on alpine habitats remains relatively low, but it is increasing.