xImplicatins of Rising Atmospheric CO2 and Climate Change for Grazinglands Dr. Jack Morgan, plant physiologist, USDA-ARS Crops Research Laboratory, Fort Collins, CO
Atmospheric CO2 levels remained essentially constant from 0 AD to about 1900 at around 280 ppm. It then shot up to 380 ppm over the last century. In the same time span, methane gas has climbed from 800 parts per billion (ppb) to over 1900 ppb. While, nitrous oxide has increased from 270 ppb to 340 ppb after being fairly constant over the first 1900 years.
The direct responses of plants to increased atmospheric CO2 is for the cool season photosynthesizers to be favored over the warm season photosynthesizers. Water use efficiency (WUE) of cool season plants will increase far more than WUE of the warm season plants. Cool season plant production will increase by 40 percent. Meanwhile, warm season plants will show little yield response to increased atmospheric CO2. Grassland CO2 enrichment experiments are being conducted around the world on such diverse grass types as Colorado shortgrass, Kansas tallgrass, New Zealand temperate pasture, Swiss calcareous grassland, and BioCON, synthetic plant communities, at Minnesota.
In the Colorado shortgrass steppe, a subshrub, fringed sagebrush (Artemisia frigida), increased greatly between 1997 and 2001. It is very responsive to atmospheric CO2 enrichment, soil water, and nitrogen use efficiency (NUE) compared to its rivals.
When two different grasslands, tallgrass prairie and shortgrass steppe, are looked at, different things occur. When CO2 levels are elevated, the shortgrass steppe has greater relative increases in plant production over that of when its plants are growing under current ambient CO2 levels. WUE increases among the cool season plants, this allows them to grow more mass and this production increases most in a dry year. On tallgrass prairie, mostly a warm season grass plant community, production increases are muted under CO2 enrichment and occur only in dry years and at levels much lower than on the shortgrass steppe with its mostly cool season plant community.
Although there is more plant biomass under CO2 enrichment, there is less N concentration in the plant tissue, thus less crude protein in them. However, CO2-induced reductions in N is off-set by increases in total nonstructural carbohydrates (TNC). (Ed. note: Total non-structural carbohydrates (TNC) is the sum of total glucose, free fructose and free sucrose in plant tissue found within each plant cell. They are not of the cell wall, hence nonstructural. They are highly digestible and provide the ruminant their main energy source.) To some extent, the lowering of nitrogen in the plant tissue while increasing TNC would go towards creating forage that is more in line with the dietary needs of a ruminant.
Under CO2 enrichment of the air, legumes fix more nitrogen and thus yield much more forage under those conditions. Non-leguminous forbs have even higher production as they become more efficient in water usage and respond greatly to increased soil water if it occurs under a CO2 enriched environ.
In a Swiss pasture experiment on a white clover, perennial ryegrass pasture where ambient CO2 and elevated CO2 at 600 ppm were used to observe differences in growth under two different N fertilizer treatments and cutting frequencies, white clover yield was enhanced 25% under high CO2, regardless of N fertility or cutting. Meanwhile, perennial ryegrass growth response to elevated CO2 was greater at high versus low N treatment. In mixed swards, white clover percentage of the stand increased under elevated CO2. Perennial ryegrass exhibited N deficiency symptoms at high CO2 with no additional N.
WVU Extension gives three guidelines for pasture forage quality for Northeast US conditions:
- Energy intake first limiting nutrient for grazers
- Managing Pasture height and forage NDF critical
- Maintain adequate legumes (reduces NDF and increases forage nutrient value)
Compare these guidelines with the implications of rising CO2 for Northeast US pastures:
- Greater productivity
- Shift in competitiveness (favoring legumes, other forbs)
- Forage quality stays same, maybe improves
It is quite likely pastures in the Northeast will become more productive since they are cool season plant communities. Since legumes are favored under heightened atmospheric CO2, it may be easier to improve legume content in our pastures than heretofore. Also, more TNC will be in our forage. This would lessen the need for energy supplements to utilize the high protein found in our pasture forages, which may be dampened slightly under an enriched CO2 environ.
In review, warmer temperatures mean:
- Longer growing season (possible disproportionate change in spring green-up and fall senescence; Skinner. 2007. Ag. & Forest Met.)
- Altered phenology (Ed. note: annually recurring life cycle events, such as flowering),
- More pests - diseases and insects
- Differential species responses (diverse & individualistic species responses; C4 > C3); Warm season grasses likely to move north in abundance.
- Desiccation due to warming (Wang. 2005. Climate Dynamics)
- Altered hydrologic cycle: warm atmosphere holds more water vapor; intense precipitation events (more snow in winter), timing altered, droughtier growing season
In the Boston Area Climate Experiment experiment, above-ground net primary production (Editor’s note: All plant production above the ground surface; no stubble left as with hay harvest or grazing animals) was measured on an old field plant community (forbs and grasses primarily).
Four warming treatments were used to simulate different global warming scenarios: no change, low temperature increase above ambient air temperature, medium, and high (4o C increase). Infrared heaters were used to increase air temperatures. Three precipitation regimes were used: low - rain out shelters remove half of the incoming precipitation, ambient, and high precipitation - 150% of ambient (sprinkle the captured water from the low treatment immediately onto the rain addition treatment).
Two years of data were presented at the Ecological Society of America 2010 Annual Meeting, 2008-2009. Precipitation and year interact with warming effects on herbaceous biomass production. In 2008, a wet year (in comparison to 2009) the rain out (drought) treatment (50% of the growing season’s rainfall) actually increased plant production from the no change heat treatment to the 4o C higher heat treatment.
While the ambient and the 150% rainfall above ambient treatments actually decreased plant production once the optimum temperature increase was reached. The ambient rainfall peaked at the medium heat treatment and the wet peaked at the low heat treatment. Both treatments then retreated in plant production at higher temperatures. 2009 appears to be a year of less precipitation than 2008. With the drought treatment in 2009, net primary production declined steadily from no change in temperature to temperatures 4º C above ambient air temperature.
The ambient rainfall and 150% of rainfall treatments tended to rise in production slowly from no temperature change to 4º C above ambient air temperature. It appears the extra water was useful in offsetting the increased evapotranspiration caused by rising air temperatures. Therefore, the two higher moisture treatments, produced twice as much biomass as the drought treatment (50% of the growing season’s rainfall) at 4º C above ambient air temperature.
In another experiment done in Belgium , three years of climate warming in experimental grasslands reduced WUE and biomass production (H.J. DeBoeck. 2008. Biogeosciences 5:585-594). Here a monoculture, a 3-species mix, and 9-species mix were studied to see how a 3º C increase over ambient air temperatures would affect production. They received equal amounts of water. Nine different monocultures, nine 3-species mixtures, and six 9-species mixtures (spatial arrangement differences, not 54 different plant species in various combinations) were planted to get 24 different grassland communities.
The overall result of higher air temperatures was to decrease biomass production by 29%. Soil drying was a significant factor in reduced productivity. The pasture species used were cool season plants: 3 grasses - orchardgrass, tall fescue, and perennial ryegrass; 3 legumes - white clover, alfalfa, and birdsfoot trefoil, and 3 non-leguminous forbs commonly found in European pastures - daisy, common sorrel, and narrow-leaf (buckhorn) plantain.
(Ed. note: Interestingly, species richness at the nine species level was detrimental when reading the full paper by DeBoeck. I quote: “Our data suggest that warming could cause a significant non-transient decline of primary production in many temperate grasslands through increased heat and drought stress, and that such a negative impact may not necessarily be alleviated at higher species richness.” Also: “More biomass was produced in mixtures (3-species) than in monocultures, in line with similar experimental studies (Hector et al., 1999; van Ruijven and Berendse, 2005), while productivity differences between 3- and 9-species were small or absent as predicted from theory (De Boeck et al., 2006b).”)
Another study (Nolan et al. 2001. Ann. Bot. 88:713-724) found that warmer temperatures and cattle grazing enhance white clover content of Irish pastures. Sheep grazing these same pastures led to a decline in white clover at the highest temperature. Mixed (grazers) was a combination of cattle and sheep.
In the Prairie Heating and CO2 Enrichment (PHACE) study at Cheyenne, Wyoming (summer, 2008), two variables were used. CO2 levels were 385 ppm (ambient air concentration) and 600 ppm. Temperatures were ambient air temperature and +1.5/3.0º C day/night. In this study, they found CO2 increases WUE and this off-sets soil desiccation due to moderate warming. The combination of higher CO2 (600 ppm) in the air and warmer temperatures resulted in the same forage production as the control over a 4-year period.
In summary, by itself, rising levels of atmospheric CO2:
- Increase plant productivity (more so under dry conditions)
- Alter species interactions (favor cool season grasses, legumes, forbs)
- Forage quality: negative impacts in semi-arid range; neutral or mixed effects in mesic pastures
- Increase/decrease plant productivity (soil moisture key)
- Alter plant competitive relationships (management for favored species may change; warm-season grasses, legumes?)
- Extend growing season
- CO2 and warming already affecting competitive interactions
- Alterations in precipitation amount and distribution will be key in outcome
- Monitoring/managing for forage quality critical