Two morphotypes of the evergreen shrub Artemisia tridentata Nutt. ssp. wyomingensis occur in the Shirley Basin of central Wyoming ͑USA͒, 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 ͑M͒ versus inter-mound locations ͑IM͒ 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 ͑~20-40 m -2 in size͒ separated by a mean distance of~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 ͑ Ͻ 3 days͒ 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 CO 2 uptake were also greater for the ephemeral leaves of M plants, along with lower internal CO 2 concentrations ͑193 Ϯ 4 l l -1 vs. 209 Ϯ 8 l l -1 , respectively͒. M plants also maintained higher xylem water potentials throughout most of the growth season ͑-1.1 Ϯ 0.1 SD MPa in May, declining to Ϫ 4.4 Ϯ 0.3 SD MPa in August͒, along with higher water use efficiencies ͑photosynthesis/transpiration͒. 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.
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