Climate Change Impact on Grasslands & Savannas
Ecosystem function and species composition of grasslands and savanna are most likely to be impacted by changes in precipitation and by warming in temperate regions while in tropical systems, CO2-fertilization and fires will be very important controlling factors. Unfortunately, there are very few studies that assess ecosystem responses to these various factors and experiments on warming, rainfall change or atmospheric CO2 level are virtually absent in savannas. Most ecosystem studies are confined mainly to temperate grasslands (IPCC, 2007).
The IPCC (2007) describes the following impacts:
- Rainfall change and variability is very likely to lead to a reduction in cover and productivity in the southern African savanna in response to the observed drying trend of about 8 mm/yr since 1970.
- Large-scale changes in savanna vegetation cover may also result in a feedback to regional rainfall patterns. Modeled removal of savannas from global vegetation cover has larger effects on global precipitation than for any other biome and, in four out of five savannas studied globally, modeled savanna-grassland conversion resulted in 10% lower rainfall, suggesting positive feedback between human impacts and changing climate.
- Canadian grasslands stored roughly five times as much carbon in a year with 30% higher rainfall, while a 15% rainfall reduction led to a net carbon loss. Similarly, Mongolian steppe grassland switched from carbon sink to source in response to seasonal water stress.
- Trees and shrubs show higher CO2 responsiveness than do herbaceous forms. Savannas may thus be shifting towards greater tree dominance as atmospheric CO2 rises, with diminishing grass suppression of faster-growing tree saplings.
- Climate change impact studies for savanna and grassland fauna are few. The proportion of threatened mammal species may increase to between 10% and 40% between 2050 and 2080.
According to Scheiter and Higgins (2009), African savannas are characterized by C4-grasses and C3-trees. An increase in CO2 might favor trees over grasses due to potentially larger benefit that C3-plants would gain over C4-plants (IPCC, 2007). On the other hand, an increase in temperature would increase rates of C4-photosynthesis, C3-photorespiration and evaporative demand. Each of these temperature-driven factors might directly (or indirectly by promoting fire) favor grasses (Scheiter & Higgins, 2009).
The authors presented a new vegetation model, the adaptive dynamic global vegetation model (aDGVM) specifically developed for tropical vegetation, to simulate the vegetation patterns of Africa in the year 2100 under projected increasing CO2 levels. Simulations under elevated temperature and atmospheric CO2 concentrations predicted longer growing periods, higher allocation to roots, higher productivity, more biomass and a dramatic shift toward tree dominated biomes. As shown in the figure below, the model predicts that large parts of today’s savannas will be replaced by deciduous woodlands under elevated CO2 conditions. It is predicted that 34.6% of today’s grasslands are transformed into savannas and 45.3% of today’s savannas are transformed into deciduous woodlands. The fraction of deciduous woodlands is predicted to increase by 13.2% to 26.9% while the savanna biome is predicted to decrease by 6.1% to 19.7%. The total biomass stored in each of the biomes increases, with high relative changes in grasslands and savannas (by 256% and 241%, respectively). The absolute changes are 420 Pg for savannas, deciduous woodlands and evergreen forests. These changes t ogether imply that the total carbon stored in trees in Africa increases from 74.9 Pg in 2008 to 151.7 Pg in 2100, a difference of 76.8 Pg. (Note: 1 Pg = 1 billion metric tons)
The authors are also careful to note that their study considers the influence of climate on vegetation while it does not account for the fact that vegetation modifies the climate. It is known that the feedbacks between climate and vegetation might significantly influence vegetation. According to the IPCC (2007) modeled removal of savannas from global vegetation cover has larger effects on global precipitation than for any other biome and, in four out of five savannas studied globally, modeled savanna-grassland conversion resulted in 10% lower rainfall, suggesting positive feedback between human impacts and changing climate.
According to the IPCC (2007) the proportion of threatened mammal species may increase to between 10 and 40% between 2050 and 2080 due to the changing savanna and grassland regimes in Africa. Changing migration routes especially threaten large, hoofed animals and their predators. Observed population declines in three African savanna hoofed species suggest that summer rainfall reductions could result in their local extinction if climate change trends continue.
According to The State of the Birds 2010 Report on Climate Change (2010): “Climate change is expected to exacerbate declines in birds that already have declining populations, and several now-common birds will probably be added to concern lists in the near future unless additional conservation measures are taken.” Grasslands in the Uninted States are expected to get warmer and drier which will impact many bird species. For example, the desert southwest and northern Mexico Chihuahuan Desert grasslands may become too warm and dry for bird species that typically winter there. “Six species stand out as especially vulnerable. Sharp-tailed Grouse and Lesser and Greater prairie-chicken are less likely than other grassland birds to move in response to changing conditions because they are closely tied to their leks where males display to attract females. Wilson’s Phalarope, Bobolink, and Dickcissel are long-distance migrants that may not be able to adapt quickly enough to changing conditions. Although most grassland bird species appear able to move in response to environmental changes, Christmas Bird Count data show that grassland birds were the only group of birds that failed to shift north during the past 40 years in response to warmer winter weather. Perhaps they did not shift because the quality of remaining grasslands in the north is too poor to sustain additional birds” (Ibid).