Grasslands - Mountains and Plains: The Ecology of Wyoming Landscapes - page 108

of increased plant water stress and possibly less water
for irrigation. Forage could be reduced for large mam-
mals of any kind. Further changes could occur if warm-
season grasses or shrubs become more common, which
have lower nutritive value than do cool-season grasses.
For such reasons, the entire food web could change,
belowground and above. 95
notably, however, research in mixed-grass prairie
near cheyenne indicates that increased atmospheric
carbon dioxide, while sometimes increasing the rate
of photosynthesis, has the additional effect of reduc-
ing transpiration in some prairie plants. More water is
left in the soil for a longer time. A team of colorado
and Wyoming scientists led by Agricultural Research
Service ecologist Jack Morgan conducted this research,
known as the Prairie Heating and carbon enhance-
ment experiment (PHAce). in replicated plots they
increased levels of atmospheric carbon dioxide while
simultaneously warming the plant canopy and soil.
With higher amounts of carbon dioxide, the soil did
not dry as rapidly as in control plots, and plant water-
use efficiency was enhanced. this result is counter-
intuitive, because if carbon emissions cause warming,
there should be higher rates of evapotranspiration,
causing more frequent periods of plant water stress.
However, as Morgan and his associates concluded, “in
a warmer, co 2 -enriched world, both soil water content
and productivity in semi-arid grasslands may be higher
than previously expected.” they added that, “elevated
co 2 can completely reverse the desiccating effects of
moderate warming.” 96
the PHAce study documented other changes as
well, including a rise in the importance of fungi, com-
pared to bacteria, and an increase in the carbon-to-
nitrogen ratio of the soil. Along with more water later in

the season, such changes caused differences in nutrient
cycling and forage quality on the plots. 97 For example,
fringed sagewort, a small native shrub that is avoided
by livestock, had much greater biomass than in the
control plots; and there is evidence that less nutritious
warm-season grasses will also become more common. 98
Research is under way to determine whether invasive
plants will be favored by elevated carbon dioxide as well
as the nitrogen added by present-day levels of atmo-
spheric deposition.
Warming may also increase the rate of decomposi-
tion of soil organic matter, at least during times when
the soil is moist, such as in the spring and on warm win-
ter days. the overall effect could be a gradual decline
in the carbon stored in the soil with the concomitant
release of more carbon dioxide to the atmosphere,
potentially aggravating global warming at a time when
carbon sequestration is an important management
objective. Moreover, soils also could become more erod-
ible if plant cover declines; subtle changes could reduce
the capacity of grasslands to sustain grazing by live-
stock. Animal scientists are already considering how the
livestock industry might adapt its practices.

thus, the results of carbon dioxide enrichment and
climate warming are more complicated than poten-
tially causing higher rates of photosynthesis. if water
stress develops quickly, or continues during extended
droughts, there will be no enhanced photosynthesis of
the mixed-grass prairie, at least with the native plants
that dominate the area now. the climate of the future
may favor novel combinations of more drought-tolerant
species. could this possibility be in the future for all
kinds of ecosystems in the region? Will the ecosystems
provide the services that many would like to have?