EFFECTS OF ELEVATED CO2 ON GROWTH RESPONSES OF HONEY MESQUITE (PROSOPIS GLANDULOSA) SEEDLINGS FROM SITES ALONG A PRECIPITATION GRADIENT

Authors: Derner, Justin; Tischler, Charles; Polley, Herbert; Johnson, Hyrum

Submitted to: Seed and Soil Dynamics in Shrubland Ecosystems Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 12/15/2003
Publication Date: 2/1/2004
Citation: Derner, J.D., Tischler, C.R., Polley, H.W., Johnson, H.B. 2004. In: Hild, A.L., Shaw, N.L., Meyer, S.E., Booth, D.T., McArthur, E.D., editors. Proceedings of the Seed and Soil Dynamics in Shrubland Ecosystems, August 12-16, 2002, Laramie, Wyoming. p. 156-160.

Interpretive Summary: Land managers need to understand whether plant species vary in their responses to rising atmospheric carbon dioxide (CO2) concentrations, and how those responses vary in different environments. Honey mesquite (Prosopis glandulosa) is a noxious shrub that has invaded many former grasslands in the southwestern US, with one ecotype that grows in wet environments and another in dry environments. An experiment was conducted to determine how these two shrub ecotypes respond to increased levels of CO2. Carbon investment was initially allocated aboveground to develop photosynthetic area (leaves) for honey mesquite seedlings, and then allocated later to root systems. Both ecotypes responded similarly to elevated CO2 with ample soil water, suggesting that this species will continue to be problematic for land managers across many environments.

Technical Abstract: Atmospheric carbon dioxide (CO2) enrichment may exaggerate intraspecific variation, but this possibility has been little studied. We collected seeds from two honey mesquite (Prosopis glandulosa var. glandulosa) trees at each of three sites along an east-west precipitation gradient in Texas: Marlin (93.2 cm annual precipitation), Menard (62.0 cm) and Bakersfield (36.6 cm) to test the hypothesis that CO2 enrichment would differentially affect plant responses along this gradient. Seeds of similar mass were planted in 0.05 m X 1.60 m pots filled with sandy loam soil, with pots randomly assigned to either ambient or elevated CO2 in a glasshouse setting. Seedlings were well watered and fertilized daily with full strength Hoagland's solution. Seedlings were destructively harvested at 8 and 16 days post emergence. For both harvest dates, aboveground variables were greater for plants from the wettest site than in either of the drier sites. CO2 enrichment did not affect above- or belowground variables at the 8-day harvest. In contrast, root volume, number of root tips, root mass, leaf area, leaf mass, total plant mass and root:shoot ratios were enhanced with CO2 enrichment at the 16-day harvest in seedlings from all three sites. The absence of significant interactions between CO2 and sites along the precipitation gradient suggests that genotypes of the invasive shrub honey mesquite will respond similarly to CO2 enrichment regardless of precipitation at the site of origin. Whether this genetic potential is displayed under field conditions, however, is uncertain as existing environmental constraints may influence responses to CO2 enrichment.