Enhanced photosynthesis and flower production in a sagebrush morphotype associated with animal burrows

Two morphotypes of the evergreen shrub Artemisia tridentata Nutt. ssp. wyomingensis occur in the Shirley Basin of central Wyoming (USA), one of which was associated exclusively with Mima-like mounds generated by animal burrowing activity. Measured on a particularly dry year according to a 34-year precipitation record, plants growing on mounds (M) versus inter-mound locations (IM)were taller with greater leaf biomass and leaf area per unit ground area, and had over 90% of all inflorescences. As a result, the landscape consists of a patchy distribution of reproductive islands (approximately 20-40 m-2 in size) separated by a mean distance of approximately 30 m. In addition, greater photosynthesis per unit leaf area occurred for M plants when ephemeral leaves dominated total leaf area in spring and early summer, as well as during short time periods (< 3 days) following sporadic rainfall events in summer when only perennial leaves were present. As a result, estimated total annual carbon gain was 41% greater for M plants from May to mid-June, but was not significantly different from IM plants for the remainder of the season, resulting in a total summer carbon gain that was 14% greater in M plants. Stomatal and nonstomatal conductances to CO2 uptake were also greater for the ephemeral leaves of M plants, along with lower internal CO2 concentrations (193 ± 4 μl l-1 vs. 209 ± 8 μl-1, respectively). M plants also maintained higher xylem water potentials throughout most of the growth season (-1.1 ± 0.1 SD MPa in May, declining to -4.4 ± 0.3 SD MPa in August), along with higher water use efficiencies (photosynthesis/transpiration). M and IM soils did not differ significantly in total organic or nitrate contents, although leaf nitrogen content was higher in M plants when photosynthesis was also greater. Photosynthesis in M plants also responded more positively to afternoon showers greater than about 7 mm compared to IM plants. Thus, improved water and nutrient relations was associated with enhanced photosynthetic carbon gain in M plants, enabling greater flower production. Moreover, morphotypic plasticity coupled with the effects of animal burrows may have substantially increased sexual reproductive success in A. t. wyomingensis.