Productivity of well-watered Panicum virgatum does not increase with CO₂ enrichment

Authors

Philip A. Fay, USDA-ARSFollow
H. Wayne Polley, USDA-ARSFollow
Virginia L. Jin, USDA-ARSFollow
Michael J. Aspinwall, University of Texas at AustinFollow

Date of this Version

10-2012

Citation

Journal of Plant Ecology VOLUME 5, NUMBER 4, PAGES 366-375 OCTOBER 2012 doi:10.1093/jpelrtsOO7

Abstract

Aims Rising atmospheric CO₂ has been shown to increase aboveground net primary productivity (ANPP) in water-limited perennial grasslands, in part by reducing stomatal conductance and transpiration, thereby reducing depletion of soil moisture. However, the benefits of CO₂ enrichment for ANPP will vary with soil type and may be reduced if water limitation is low. Little is known about CO₂ effects on ANPP of Panicum virgatum, a perennial C₄ tallgrass and potential bioenergy crop. We hypothesized that if water limitation is minimized, (i) CO₂ enrichmentwould not increase P. virgatum ANPP because photosynthetic rates of this C₄ grass would not increase and because decreased transpiration at elevated CO₂ would provide little additional benefit in increased soil moisture and (ii) soil type will have little effect on P. virgatum CO₂ responses because of high overall soil moisture.

Methods Growth and leaf physiology of P. virgatum cv. 'Alamo' were studied as plants established for 4 years on silty clay and clay soils along a 250 to 500 J.l1 1-1 gradient in atmospheric C02 located in central Texas, USA. Plants were watered to replace evapotranspiration, fertilized with NO₃NH₄ and P₂O₅ and clipped to standard height during mid-season.

Important Findings ANPP increased through the third year of growth. Soil moisture (0-20 cm), ANPP, tiller numbers and leaf area index were 8-18% higher on the clay than on the silty clay soil. ANPP did not increase with CO₂ except in the planting year. However, biomass removed with clipping strongly increased with CO₂ in years 2 and 3, suggesting that CO₂ enrichment increased the early- to mid-season growth of establishing P. virgatum but not later regrowth or that of fully established plants. Furthermore, CO₂ enrichment differentially affected two components of ANPP in years 2 and 3, increasing tiller mass and reducing tiller numbers. This reallocation of resources in clipped P. virgatum suggested increased meristem limitation of productivity with CO₂ enrichment. CO₂ enrichment had little effect on photosynthesis but increasingly reduced stomatal conductance and transpiration as the plants established. As a result, water use efficiency became increasingly coupled to CO₂ as leaf area increased during establishment. These results suggest that for well-watered and clipped P. virgatum, ANPP differed between soil types, was not affected by CO₂ enrichment when fully established but interacted with clipping to alter allocation patterns during establishment. Soil type effects on ANPP-C02 responses will likely become more apparent when water is more limiting.