Thanks to Sallie Sprague for alerting me (through an Long Term Ecological Research – LTER project I am on) to an interesting new paper that has been published in the April 2007 issue of New Phytologist.
The article is
William J. Parton, Jack A. Morgan, Guiming Wang, Stephen Del Grosso
Projected ecosystem impact of the Prairie Heating and CO2 Enrichment experiment New Phytologist (OnlineEarly Articles). doi:10.1111/j.1469-8137.2007.02052.x
The abstract reads
“The Prairie Heating and CO2 Enrichment (PHACE) experiment has been initiated at a site in southern Wyoming (USA) to simulate the impact of warming and elevated atmospheric CO2 on ecosystem dynamics for semiarid grassland ecosystems.
The DAYCENT ecosystem model was parameterized to simulate the impact of elevated CO2 at the open-top chamber (OTC) experiment in north-eastern Colorado (1996–2001), and was also used to simulate the projected ecosystem impact of the PHACE experiments during the next 10 yr.
Model results suggest that soil water content, plant production, soil respiration, and nutrient mineralization will increase for the high-CO2 treatment. Soil water content will decrease for all years, while nitrogen mineralization, soil respiration, and plant production will both decrease and increase under warming depending on yearly differences in water stress. Net primary production (NPP) will be greatest under combined warming and elevated CO2 during wet years.”
Model results are consistent with empirical field data suggesting that water and
nitrogen will be critical drivers of the semiarid grassland response to global change.”
This paper demonstrates that the effect of climate variability and change on the grassland must consider much more than the global average surface temperature, and even the local temperature. The paper also emphasizes the need to consider CO2′s biogeochmical climate forcing, as well as its radiative forcing. The need for better understanding of this complexity was emphasized in the 2005 National Research Council Report “Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties” which has been discussed many times on Climate Science.
The news release on this paper, however, inserts the customary attribution to “global warming”. No where in the Parton et al paper is this term even used! The paper reports on the effect of perturbations of different environmental stressors on the grassland including local air temperature increases, but it does not (and cannot) relate directly to global average climate system heat changes (i.e. “global warming”).
The news release reads as follows (and except for the alluding to “global warming”) is a good report.
“Global warming will have mixed effects on eastern Colorado’s grasslands April 23, 2007
New research results from Colorado State University suggest that the effects of rising atmospheric concentrations of carbon dioxide and global warming will lead to an increase in grass production and a decline in forage quality for grasslands of eastern Colorado and Wyoming.
Study results suggest that both elevated CO2 and warming will increase grass production but the quality of the vegetation will decrease due to lower nitrogen concentration in the forage. William Parton, researcher from Colorado State’s Natural Resource Ecology Laboratory, or NREL, and researcher Jack Morgan from the U.S. Department of Agriculture’s Agricultural Research Service, USDA ARS, studied the effects of warming, increased CO2 levels and the combination of both factors on eastern Colorado grasslands to predict how global warming will affect these ecosystems.
Warming can have both positive and negative impacts on plant production
Results revealed that elevated atmospheric CO2 levels always increase grass growth; however, warming can have both positive and negative impacts on plant production.
‘The potential impact of elevated CO2 levels on Colorado and Wyoming grasslands is mixed since grass production will likely increase while digestibility of forage and cattle weight gains will likely decrease,’ Parton said. ‘Increased air temperatures will have a mixed impact with plant production increasing in wet years and decreasing in dry years.’
These predictions are based on results from an ecosystem model developed using data from locally observed climatic change experiments that will continue during the next five to 10 years.
Experiments combined with ecosystem models
In this experiment, Colorado State and ARS scientists were able to use empirical knowledge from relatively short-term experiments combined with ecosystem models to predict long-term ecosystem responses to the effects of global warming. The scientists used results from a five-year-long CO2 enrichment experiment conducted in northern Colorado to test CO2 impacts in the Daycent ecosystem model. They also used field data from the experiment site, Prairie Heating and CO2 Enrichment or PHACE, located in southern Wyoming that will continue for the next five to 10 years.
‘One of our biggest challenges is how to interpret relatively short-term experiments and predict the long-term global warming consequences on grasslands,’ Morgan said. ‘By taking the results from our field experiments and applying computer models tested using the observed field data, we are able to extrapolate beyond our short-term experiments into the future.’
Critical matter for livestock and native prairie animals
The scientists observed that doubling CO2 levels caused strong and consistent increases in grass growth which was due to improved water-use efficiency. Under the elevated CO2 levels, it was also found that plant nitrogen content was declining in native grasslands. This is a critical matter for livestock and for native animals that have grazed these prairies for thousands of years. Increased CO2 dilutes nitrogen concentration in grazing vegetation. Animals require sufficient forage protein nitrogen to sustain normal weight gains.”