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 ****************************************************************************** GLOBAL CHANGE To understand the potential impact of global change on the native flora and vegetation, we must understand clearly the environmental effects of global change. For the purposes of this paper I will address the subject of global change as global warming. Schneider (1989) gives, in an eminently readable manner, an introduction, description and discussion of all major aspects of global warming. Global warming has been predicted as a result of heat buildup in the atmosphere because so called "greenhouse gases" (such as carbon dioxide, methane, and, Chlorofluorocarbons), whose production has dramatically increased by activities of post-industrial society, are transparent to incoming short-wave solar radiation, but absorb outgoing long-wave outgoing infra-red radiation. In the popular literature this is referred to as the "Greenhouse Effect", for, like the glass of a greenhouse the gases trap heat in the atmosphere. Such a "Greenhouse Effect" is not a speculative theory. It has been observed on the planet Venus where high concentrations of carbon dioxide and other gases result in a surface temperature much greater than would be expected from the position of the planet with respect to the sun (Schneider 1989). On earth various degrees of climatic change are predicted depending on assumed scenarios of greenhouse gas build up and global circulation models. Considerable debate rages over the degree to which our climate will change, but more and more there is a consensus that change will occur and indeed is occurring already (Kerr 1990). For example, there is a real observed warming of 0.5ūC in the global average temperature in the last 100 years. I will not discuss the merits of various models in this paper; rather I will consider the impact of moderate warming scenarios predicted by models. The major effects of climatic change as far as our native species concerned are likely to be: 1) warming, 2) drying, and 3) sea-level rise. The degree of expected global warming varies according to the model used, and indeed is an issue of great controversy. Assuming that only carbon dioxide concentration doubles from pre-industrial levels at some time in the next century, we might realistically expect an average global mean annual surface temperature increase in the range of 2-5ūC. Warming will not occur uniformly over the globe; rather certain regions, notably continental heartlands like our Canadian prairies, possibly the interior of B.C., and mid to high latitudes, will warm up more than other areas. Taking this into consideration, we can expect most of British Columbia to warm up at least the predicted 2ūC-5ūC. A 2ūC-5ūC change will have major impact on the flora. A lowering of global temperatures of 3ūC-5ūC was sufficient to bring on Pleistocene glaciations with devastating effects on British Columbia's flora. The now well-demonstrated early Holocene warming or Xerothermic Interval about 10 000 - 6 000 years ago in B.C. saw temperatures about 2ūC warmer and was associated with vegetation considerably different than today. An increase of 2ūC, from 0ūC to 2ūC cold-season-mean temperature would result in a shift from broadleaf deciduous to broadleaf evergreen vegetation in other parts of the world (Wolfe 1978). In British Columbia one can get a sense of the potential effects of the predicted temperature rise by examining the mean annual temperatures for the Biogeoclimatic Zones (see Kragina et al. 1982). For example, most of the Interior Douglas-fir Biogeoclimatic Zone (IDF) would shift to climatic conditions of the Ponderosa Pine-Bunchgrass Zone (PPBG) (or current equivalents in Research Branch 1988). The IDF has a Mean Annual Temperature of 4-8ūC; PPBG has a Mean Annual Temperature of 6-10ūC. Changing weather patterns and warming temperatures will bring increasing dryness to certain parts of the globe. Manabe and Wetherald (1986) prepared a map of percent-soil-moisture change resulting from a doubling of carbon dioxide climatic scenario. Much of British Columbia can expect a 30-40% decrease in soil moisture. For extremely moist regions, such as the windward side of the Coast Mountains, the effect on vegetation may not be great, as there is a surfeit of moisture. However, even here wetlands will likely be significantly affected. In other areas where moisture is already in short supply, the effect will be dramatic. Perhaps most noticeable will be the disappearance of trees from marginally forested areas. The third major effect to consider is sea-level rise. In general, ocean levels are expected to rise about 1 m (0.5-1.5m) over the next century (Schneider 1989). The rise will be tempered or exacerbated by regional factors such as tectonic uplift or subsidence. In British Columbia both subsidence and uplift have been recorded over the past 5 000 years (Clague et. al. 1982). Relative sea level on parts of the west side of Vancouver Island is dropping (Hebda and Rouse 1979), so the 1 m global rise could be tempered. In the Strait of Georgia, sea level has remained more or less stable (Clague et. al 1982), so a 1 m rise would have a greater net effect. In any scenario the rise in sea level over the next 100 - 150 years would be at a rate greater than seen in several thousand years. Sea-level rise would have its greatest impact on coastal morphology and especially estuarine habitats and species.