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
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.