Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/13/2005
Publication Date: 3/1/2006
Citation: Polley, H.W., Tischler, C.R., Johnson, H.B. 2006. Elevated atmospheric CO2 magnifies intra-specific variation in seedling growth of honey mesquite: an assessment of relative growth rates. Rangeland Ecology and Management. 59:128-134. Interpretive Summary: The shrub honey mesquite is an aggressive invader of rangelands in the southwestern U.S. partly because seedlings of species grow rapidly. The growth of mesquite should be stimulated by the continuing increase in the concentration of carbon dioxide (CO2) gas in the air, but the increase in growth may not be expressed uniformly across mesquite plants that currently differ in growth rate. If CO2 enrichment preferentially stimulates the growth of plants that already grow rapidly, future populations of mesquite plants may contain a larger number of highly invasive individuals. We measured effects of doubling the current CO2 concentration on the growth of seedlings from 14 mesquite families (genetic types). Seedlings from the 14 families varied in size by a factor of almost two after one month of growth. Seedling mass of some families changed little when CO2 was increased. Seedlings of other families increased in size by almost 75% when the CO2 level was doubled. Significantly, CO2 response was not well-correlated with variation in size or in seedling relative growth rate at the current CO2 level. Consequently, CO2 enrichment did not consistently favor the genetic types that currently are the largest or fastest-growing. Our results indicate that rising CO2 could increase the average growth rate of honey mesquite and thereby exacerbate the management challenge of limiting mesquite encroachment, but that the response of mesquite families to CO2 enrichment is not predictable from their growth rate at the current CO2 concentration.
Technical Abstract: The shrub honey mesquite (Prosopis glandulosa var. glandulosa Torr.) readily encroaches into rangelands in the southwestern U.S. that vary in annual rainfall by almost a factor of five (> 200 to 1000 mm), partly because mesquite seedlings grow rapidly. Species that occupy such a wide precipitation gradient frequently include plants that differ genetically in seedling growth rate. Whether atmospheric carbon dioxide (CO2) enrichment affects seedling biomass uniformly across mesquite genotypes or influences the expression of genetic variation in seedling growth remains unresolved. We measured the effects of CO2 enrichment on growth of seedlings derived from 7 adult plants (open-pollinated families) each from rangelands located near extremes of the rainfall gradient occupied by the shrub (an arid rangeland in southeastern New Mexico and a mesic rangeland in central Texas). Growth was measured over days 10 to 30 after emergence on well-watered seedlings in glasshouses at ambient and elevated CO2 concentrations. Seedling biomass on day 30 differed by as much as a factor of two among the 14 families of honey mesquite (0.61 to 1.13 g). Proportional responses of biomass to CO2 enrichment varied from 1.03 to 1.74 among families. Significantly, variation in biomass and in seedling relative growth rate at ambient CO2 was not well-correlated with CO2 response (P > 0.38). Consequently, CO2 enrichment did not consistently favor the largest or fastest-growing genetic types at ambient CO2. Biomass at ambient CO2 was 19% greater, on average, in families from mesic than arid rangelands, but families from extremes of the precipitation gradient did not segregate by seedling size or response to CO2. Our results indicate that selection could augment the mean growth response of honey mesquite to CO2 and thereby exacerbate the management challenge of limiting mesquite encroachment on both arid and mesic rangelands.