J92 



Global Climate Chani>e — Our Living Resnuive.'i 



Potential 



Impacts of 



Climate 



Change on 



Nortli 



American 



Flora 



by 

 Larry E. Morse 

 Lynn S. Kiitner 



The Nature Conservancy 



John T. Kartesz 



North CaroUna Botanical 



Garden 



Climate change is a natural phenomenon that 

 has occurred throughout the history of the 

 earth. The frequency and magnitude of climate 

 change have varied substantially during and 

 between glacial periods, and temperatures on 

 both global and local scales have been both sub- 

 stantially warmer and colder than present-day 

 averages (Ruddiman and Wright 1987; Pielou 

 1991; Peters and Lovejoy 1992). While poten- 

 tial magnitudes of local and global climate 

 change are of concern, it is the predicted rate of 

 temperature change that poses the greatest 

 threat to biodiversity. The ability of species to 

 survive rapid climate changes may partially 

 depend on the rate at which they can migrate to 

 newly suitable areas. 



In the next few centuries climate may 

 change rapidly because of human intluences. 

 The concentrations of "greenhouse"" gases in the 

 atmosphere are being altered by activities such 

 as carbon dioxide emission from burning fossil 

 fuels. Models of climate change (IPCC 1990. 

 1992) predict an increase in mean global tem- 

 perature of about 1.5-4.5°C ( 2.7-8. Vf) in the 

 next century. Temperature changes suggested 

 by general circulation models would present 

 natural systems with a wanner climate than has 

 been experienced during the last I ()().()()() years. 

 While this would be a substantial change from 

 the current climate, the rale of climate change is 

 the greatest determinant of the impact on bio- 

 logical diversity. Future climate change due to 

 human inlluences could occur many times 

 faster than any past episode of global climate 

 change (IPCC 1990, 1992; Schneider et al. 

 1992). 



The strong association between distributions 

 of plant species and climate suggests that rapid 

 global climatic changes could alter plant distri- 

 butions, resulting in extensive reorganization of 

 natural communities (Graham and Grimm 

 1990). Climate changes could also lead to local 

 extirpations of plant populations and species 

 extinctions. The effects of global climate 

 change are likely to vary regionally, depending 

 on factors such as proximity to oceans and 

 mountain ranges. Alteration of the amount and 

 timing of precipitation and evaporation would 

 affect soils and habitats; freshwater ecosystems 

 are likely to be vulnerable to these changes in 

 hydrology (Carpenter et al. 1992). Even minor 

 fluctuations in the availability of water can rad- 

 ically affect habitat suitability for many wetland 

 plant species. Rapid, large-scale shifts in tem- 

 perature, precipitation, and other climate pat- 

 terns could have broad ecological effects, pre- 

 senting major challenges to the conservation of 

 biodiversity. 



Analysis of Potential Effects 



An analysis conducted by The Nature 

 Conservancy on the potential effects of climate 

 change on the native vascular tlora of North 

 America (Morse et al. 1993) provides a prelim- 

 inary assessment of patterns of plant species" 

 vulnerability. For this preliminary analysis, we 

 made several simplifying assumptions about the 

 relationships between plants and climate to esti- 

 mate the viable climate "envelopes'" for each of 

 over 15.000 native vascular plant species in 

 North America recognized in the checklist by 

 Kartesz (1994). 



The principal assumptions are that climate 

 determines the range of plant species; mean 

 annual temperature adequately approximates 

 climate; species distribution appears to be in 

 equilibrium with present climate; and a species" 

 cunent climate envelope is equivalent to its tol- 

 erance of climate variation. Together, these 

 assumptions state that the current distribution of 

 each species is greatly influenced by climate 

 and that temperature adequately represents cli- 

 mate. 



Clearly, each of the above assumptions are 

 not actually met for all native vascular plant 

 species. For example, precipitation and soil 

 moisture are extremely important determinants 

 of range limits in some regions. These simpli- 

 fied temperature envelopes, however, allow the 

 initial identification of broad patterns of 

 species" vulnerability to cliinate change. 



In the analysis, the mean temperature was 

 uniformly increased in 1°C (1.8°F) increments 

 up to an increase of 20°C (36°F) above current 

 mean annual temperatures (Fig. 1). Many 

 species would be vulnerable to climate change 

 in all scenarios of uniform temperature 

 increase. With a mean global warmina of 3°C 



100- 



6 8 10 12 14 

 Temperature increase (°C) 



Fig. 1. Tlie proportion of native vascular plant species that 

 were entirely out of their climate envelopes as a function 

 of the increase in temperature above mean annual temper- 

 ature. Three methods were used to determine climate 

 envelopes (A. B, C). 



