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 ****************************************************************************** Dry Interior We can expect the greatest shifts in vegetation species distribution as a result of global warming in the dry interior. Here we can anticipate disappearance of large tracts of forest and the upward and northward expansion of open vegetation rather than simple species replacements as in more moist, forested regions. Reference to the paleoecological record of the southern interior reveals major up-slope shifts in the tree line. Hebda (1983) estimated that the tree line may have been least at about 1500 m (5000') during the warmest and driest part of the Xerothermic Interval. Today the Ponderosa Pine (PP) and Bunchgrass (BG) Biogeoclimatic Zones cover only 10 500 km2 of B.C. A temperature shift of 2ūC - 5ūC could easily convert most of the Interior Douglas-fir zone to PP or BG type of Biogeoclimatic zone. Such a change would add 48 000 km2 to this habitat complex. Indeed, at the northern end of the range of PP and BG, large areas of logged-over second growth pine may convert to open habitats in the near future too. Thus, we could see as much as a four hundred percent increase in the most arid vegetation types. Such a change might look very positive for the many rare species of the dry interior. Indeed it should be so for the common species. Unfortunately, many adventive and weedy species have preferentially found their way, into this habitat; lest we forget the dreaded knapweed!!! It may well be that these vigorous competitors disperse to form a new kind of hybrid interior vegetation. Such will be the case, particularly if open and woodland areas are continuously overgrazed and raw soil turned up as seedbed. One potential area of grave concern is that on southfacing slopes, the adventive-native hybrid grassland developed at lower elevations may merge with dry alpine habitats and facilitate the upward spread of weedy scourges of the overgrazed lowlands. Will the many rare species of the driest southern interior sagelands spread? They may, provided we zealously guard their populations today so that they can eventually form effective dispersal centres. We must resist, as much as possible, urban and agricultural developments at their sites and provide corridors or pathways for them to disperse naturally, especially northward. We may also see the northward spread of Great Basin arid land species of Washington and Oregon, enriching our native flora, provided that there is natural habitat into which they can spread. A further phenomenon we should expect is the development of distinct zones of arid vegetation. The most recent treatment of Biogeoclimatic Zones (Research Branch 1988) recognized two distinct arid land units: the Ponderosa Pine zone and the Bunchgrass zone. Initially, these were not differentiated partly because the climatic gradients in the areas where they occur were so sharp that the zones did not separate distinctly. With increasing warmth and drought, the gradient along the warm dry end of the climate spectrum will likely become gentler and species will find their appropriate position accordingly. We may see an upper elevation grass- and forb-dominated zone, patches of which occur at the upper limits of today's open arid vegetation, a Ponderosa Pine zone, and a well-developed arid or semi desert steppe dominated by Artemisia tridentata Nutt. Certainly Artemisia tridentata was much more abundant in the southern interior of B.C. during the Xerothermic Interval than today (Hebda 1983). The new vegetation assemblages/habitats may provide for the flourishing of species which we cannot guess. We must always remember that species will behave in an individualistic manner according to the fossil record!! New combinations should be expected and indeed are the norm as a result of major climatic shifts.