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Potential Impacts of Climate Change on North American Flora


by
Larry E. Morse
The Nature Conservancy
Lynn S. Kutner
The Nature Conservancy
John T. Kartesz
North Carolina 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 substantially warmer and colder than present-day averages (Ruddiman and Wright 1987; Pielou 1991; Peters and Lovejoy 1992). While potential 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 influences. 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 temperature of about 1.5-4.5°C (2.7-8.1°F) in the next century. Temperature changes suggested by general circulation models would present natural systems with a warmer climate than has been experienced during the last 100,000 years. While this would be a substantial change from the current climate, the rate of climate change is the greatest determinant of the impact on biological diversity. Future climate change due to human influences 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 distributions, 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 radically affect habitat suitability for many wetland plant species. Rapid, large-scale shifts in temperature, precipitation, and other climate patterns could have broad ecological effects, presenting 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 flora of North America (Morse et al. 1993) provides a preliminary assessment of patterns of plant species' vulnerability. For this preliminary analysis, we made several simplifying assumptions about the relationships between plants and climate to estimate 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' current climate envelope is equivalent to its tolerance of climate variation. Together, these assumptions state that the current distribution of each species is greatly influenced by climate and that temperature adequately represents climate.
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 simplified temperature envelopes, however, allow the initial identification of broad patterns of species' vulnerability to climate change.
Fig. 1. The 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 temperature. Three methods were used to determine climate envelopes (A, B, C).
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 warming of 3°C (5.4°F), 7% to 11% of 15,148 native vascular plant species in North America (about 1,060 to 1,670 species) could be entirely out of their climate envelopes. These species would thus be vulnerable to extinction unless they can migrate rapidly enough or can persist despite climate change. In comparison, about 90 plant species in North America are believed to have gone extinct in the last two centuries (Russell and Morse 1992).

Rarity and Vulnerability
Of the native vascular plant species studied, about 4,100 (27%) are considered rare by The Nature Conservancy (see article by Stein et al., p. 399, for definitions of ranking system for rarity). These species occur at fewer than 100 sites or are comparably vulnerable. Our analysis shows that these rare plants are likely to be further affected by climate change. In this analysis, about 10%-18% of the rare species would be vulnerable to a mean 3°C (5.4°F) temperature increase. In contrast, only 1% to 2% of the common species appear vulnerable under these conditions. These results imply that numerous rare vascular plant species could be additionally threatened by climate change. Early warnings of species' vulnerability to a rapidly changing climate might allow the development and implementation of new conservation strategies before a crisis occurs, thus improving the success rate for the protection of rare plants while minimizing the cost.

Regional Patterns of Vulnerability
Fig. 2. The proportion of species that would be out of their climate envelope in each state or province with a +3°C (+5.4°F) temperature change.
Based on the uniform 3°C (5.4°F) mean increase in temperature used for this preliminary climate change impacts analysis, there appear to be regional patterns to the proportion of potentially vulnerable species in each state or province (Fig. 2). In this initially simplified analysis, the southeastern states have the highest percentage of species out of their climate envelopes, while the Great Plains states and provinces may experience proportionally fewer species losses. The relatively high proportion of species vulnerability in the Southeast may be due in part to the presence in state floras of Appalachian Mountain species at their southern range limits. Many of these species are already rare in states along their southern range limits and are likely to be lost from the local floras if the climate warms.
Global warming models, however, suggest that the temperature and precipitation changes in the interior of the continent will be far greater than in coastal regions. In the Great Plains, some models suggest increases in summer temperatures by 4-7°C (7.2-12.6°F), accompanied by dramatic decreases in precipitation. Future analyses that incorporate regional changes in climate projected by models will further refine our understanding of regional patterns of plant species' vulnerability to climate change.

Dispersal and Persistence of Vascular Plants
The survival of species during periods of changing climate will be determined in part by their abilities to disperse to new sites or to persist in place. For this analysis, a dispersal-ability scale was used to assess the potential for different species to migrate. The scale is based on characteristics important to species mobility such as pollination mechanisms, dispersal mechanisms, reproductive characteristics, degree of self-compatibility, growth form, trophic type, and number of populations. Biological factors likely to increase species mobility include wind pollination, at least partial self-compatibility, dispersal of propagules by wind or birds, and a short generation time. Characteristics such as dependence on specific pollinators (e.g., yucca and yucca moth), dispersal by ants, or a long generation time reduce the chances for successful rapid dispersal and establishment. By using these criteria, most of the species studied appear to have an intermediate dispersal potential.
Fig. 3. The proportion of species on the dispersal-ability scale that are out of their climate envelopes (vulnerable) or in their climate envelopes (not vulnerable) with a +3°C (+5.4°F) temperature change. Full data were available for 8,668 species.
The species in this analysis that would be vulnerable in a +3°C (5.4°F) climate appear to have characteristics that limit long-distance dispersal (Fig. 3). This suggests that the plants potentially most vulnerable to climate change may be those forced to adapt in place to new conditions. In general, rare plants and narrow endemics will be particularly endangered by climate change. These plants often have restricted ranges, a reduced seed source, and may depend on specific microclimatic conditions for survival. Rare plants would thus potentially have trouble migrating to comparable new sites, regardless of their ability to disperse. For example, Boott's rattlesnake-root (Prenanthes boottii) and mountain avens (Geum peckii), endemic to alpine habitats in the northeastern United States, would be particularly sensitive to global warming.

Migration Rate

During the warming at the end of the last glacial period, plant migration rates, as calculated from the fossil pollen record, ranged from about 5 to 150 km (3-90 mi) per century (Shugart et al. 1986). Human-caused climate change may occur at rates more than five times faster than any changes since the last glacial maximum, including the period of most rapid deglaciation (Overpeck et al. 1991). Various studies have suggested that such rapid climate changes would require shifts of plant ranges of up to 500 km (300 mi) within the next century, exceeding the known rates of migration for many plant species (Davis 1984; Davis and Zabinski 1992).
Since species respond individually to climate change, migration rates will vary within and among natural communities. It is unlikely that entire biological communities would move together in response to climate changes (Graham and Grimm 1990). Some plants may respond rapidly to changes; others may survive for several generations in place or persist as long-lived clones despite significant climate change. The fossil record provides evidence of decade- or even century-long time lags in species migration (Davis 1989). The process of changing community composition in response to climate change has been documented in the fossil record through the disassociation and reassembly of plant and animal taxa (Graham and Grimm 1990). This variation in species assemblages displays the transitory nature of former as well as existing and future community types.
Temperature extremes and changes in the frequency and severity of local disturbances may have a greater influence on the survival of plant species at particular locations than small shifts in the average climate. More frequent droughts, fires, and pest and pathogen outbreaks are predicted to act in conjunction with climate change to significantly transform the landscape (Peters 1992). This prediction is supported by paleoecological evidence that altered disturbance regimes can intensify the effects of climate change on plants and increase the amount of overall vegetational change (Davis 1989).

Threats by Weedy Exotics
With global climate change, some exotic weeds may be favored over native species. Many weeds are able to expand relatively quickly, posing serious threats to existing species and overall biodiversity (Schwartz 1992). Many weedy species are widespread, prolific, fast-growing annuals capable of establishing in disturbed habitats and are often favored by disturbances. Climate-induced changes could expose native plants to non-native competitors for the first time (Peters 1992), stressing the balance established between native plants and their habitat. Exotic weeds may become a greater problem in the management of many preserves and natural areas.

Landscape Fragmentation
The potentially rapid rates of warming, combined with habitat loss and fragmentation from human development, suggest that many species will not adjust as successfully to climate change as in the past. Most native plant species exist in a highly fragmented landscape that further separates appropriate habitat patches, increasing the dependence of many species on relatively rare events of long-distance dispersal. Furthermore, species often must disperse across hostile habitats, including roads, cities and suburbs, and farmland (Peters 1992). Finally, plants would need to establish themselves in landscapes where many of the open or disturbed areas have been colonized by aggressive weedy exotics.

Climate Change and Conservation Planning

Rapid climate change could place novel demands and constraints on plant species conservation. Vulnerability to climate change could affect selection and design of new preserves and management procedures in existing preserves, especially in southern or low-elevation portions of species' ranges. Management of species threatened by climate change could involve restoration and transplantation of species among preserves or into new locations. Actions such as removal of exotic species or hydrological controls may not be qualitatively different than those that are currently required of land managers, but climate change may increase the intensity and frequency of threats from exotic species, drought, and fire. In view of the unpredictable and potentially devastating effects of global climate change on species' viability and distribution, conservation strategies such as propagation of critical species outside of their natural range to provide materials for reintroductions are likely to become increasingly important to preserve biological diversity.
For further information:
Larry E. Morse
The Nature Conservancy
1815 N. Lynn St.
Arlington, VA 22209
References
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