Compensatory photosynthesis, water-use efficiency, and biomass allocation of defoliated exotic and native bunchgrass seedlings

Authors: Hamerlynck, Erik; SMITH, BRENDA - Oregon State University; Sheley, Roger; Svejcar, Anthony

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2015
Publication Date: 4/6/2016
Publication URL: http://handle.nal.usda.gov/10113/62968
Citation: Hamerlynck, E.P., Smith, B., Sheley, R.L., Svejcar, A.J. 2016. Compensatory photosynthesis, water-use efficiency, and biomass allocation of defoliated exotic and native bunchgrass seedlings. Rangeland Ecology and Management. 69(3):206-214. doi: 10.1016/j.rama.2015.12.007.

Interpretive Summary: Seedling performance of desirable native species is important to the successful conservation and restoration of sagebrush steppe, yet how seedlings physiologically respond to many environmental stresses in these water limited systems is not well known. To address this, we subjected seedlings of crested wheatgrass, a widely planted exotic grass, and a native grass, bluebunch wheatgrass to increasingly severe levels of defoliation and measured their leaf-level photosynthetic and whole plant biomass allocation responses to tissue loss. We concluded that photosynthesis in response to simulated herbivory is likely an important part of seedling grazing tolerance, and the different ways herbivory affected water use help explain why the two species differ in their ability to compete as seedlings with invasive annual grasses. Managers should consider taking steps to control seedling herbivory to enhance seedling success of bluebunch wheatgrass.

Technical Abstract: Compensatory increases in net photosynthetic assimilation rates (Anet) following herbivory are well-documented in adult rangeland grasses, but have not been quantified in bunchgrass seedlings, which may be more sensitive to tissue loss than established plants. To address this, we twice removed 30% and 70% leaf area of seedlings of crested wheatgrass (Agropyron cristatum. var. Hycrest II), and the native bluebunch wheatgrass (Pseudoroegneria spicata), and compared Anet and aboveground and belowground growth of these to unclipped control plants. Compensatory Anet occurred only after the second clipping, roughly one month after the first, and was similar in magnitude and duration between species and treatments, ca. 26 % higher than control plant Anet for two weeks following clipping. Despite similar compensatory Anet between species, increases in Anet were more proportional with increased stomatal conductance to water vapor (gs) in crested wheatgrass. This resulted in higher intrinsic water use efficiency (WUEi = Anet/gs) integrated across the post-clipping recovery period compared to WUEi of bluebunch seedlings, which declined with clipping. These differences in WUEi were attributable to differences in root:shoot ratios and root tissue quality (specific root mass), which were lower in crested wheatgrass. We concluded that compensatory photosynthesis is an important component of seedling herbivory tolerance, and that observed differences in post-herbivory WUEi could help improve management strategies by informing seedling selection criteria to help develop methods aimed at minimizing impacts of herbivory during the seedling stage.