Richard Hebda
Botany Unit
Royal British Columbia Museum
Victoria, B.C.   V8V 1X4
Presented at the "British Columbia Native Plants, their current Status and 
Colloquium at Botany Dept., University of British Columbia, May 12, 1990
	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.