Gelfand, I. and G.P. Robertson
Presented at the All Scientist and GLBRC Sustainability Meeting (2009-05-05 to 2009-05-07 )
The increase in atmospheric CO2 concentration is proposed to have an effect on both the hydrological cycle and ecosystem productivity. The physiological response that drives these effects is believed to be an increase in carbon uptake either by (a) stronger CO2 gradient between the stomata and the atmosphere or by (b) an increase of Rubisco activity. The (a) scenario will lead to increase of water use efficiency (WUE) in plants. However, the evidences of such WUE increase is mostly based on modeling studies, and short duration or step-like increase in CO2 concentration (e.g. free-air CO2 enrichment) experiments.
Understanding of the consequences of increases in atmospheric concentration of CO2 for ecosystem functioning and productivity is critical when assessing anthropogenic impacts on climate change. Current assumptions of the increasing ecosystem productivity and WUE are driving optimistic predictions of the possible increase in crop yields and therefore increase in food security, as well lesser limitation of the fresh water resources due to a decrease in evapotranspirational water loss.
We hypothesize that the increase in atmospheric CO2 concentration will have no positive effect on ecosystem productivity and WUE. To check this hypothesis, we used long term ecological research (LTER) data, collected between 1989 and 2007 (~33 ppm atmospheric CO2 increase) from the agricultural LTER site at Kellogg Biological Station (KBS). The datasets includes meteorological, agricultural, and soil CO2 flux data. Additionally, we will use historical changes in 13C/12C ratio in plants to calculate changes in intrinsic WUE. Changes in intrinsic WUE, if any, will provide evidence of physiological forcing on the hydrological cycle due to continues, and not step-like increase in atmospheric CO2 concentration as predicted previously.
Preliminary analysis of the LTER dataset shows significant increase of soil CO2 flux over last ~20 years. Additionally, we found a positive increase of the maximum air temperature and precipitation on the site; gravimetric soil moisture (GWC) shows no increase over the studied years. Finally, we are showing increasing trends of ecosystem productivity in agricultural, as well, sucessional ecosystems.
These results suggest a positive feedback between atmospheric CO2 increase, ecosystem productivity, and soil CO2 fluxes. This positive effect rejects our hypothesis of no CO2 fertilization effect on plant productivity. The lack of trend in GWC over the last 17 years reveals the need in different methods of analysis for changes in WUE. The further work on isotopic analysis of the archived plant samples will answer the question if intrinsic WUE increases together with increase in atmospheric CO2.
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