Compensatory photosynthesis, water-use efficiency and biomass allocation responses to defoliation of exotic and native bunchgrass seedlings

Background/Question/Methods

Compensatory increases in net photosynthetic assimilation rates (A_net) following herbivory are well-documented in adult rangeland grasses, but have not been quantified in the demograpically critical seedling stage, which may be more sensitive to tissue loss than established plants.  To address this, we twice removed 30% and 70% leaf area of in emergent seedlings of crested wheatgrass (Agropyron cristatum. var. Hycrest II), and the native bluebunch wheatgrass (Pseudoroegneria spicata), and compared A_net, instinsic water-use efficiency (WUEi), the ratio of A_net and stomatal conductance to water vapor (g_s), and aboveground and belowground growth to those in unclipped control plants.  

Results/Conclusions

Compensatory A_net occurred only after the second clipping, roughly one month after the first, and was similar in magnitude and duration between species and clipping  treatments, ca. 26 % higher than control plant A_net for two weeks following clipping.  Despite similar compensatory A_net between species, proportional increases in A_net and g_s  in crested wheatgrass resulted in higher WUE_i integrated across the post-clipping recovery period, while clipping induced lower WUE_i of bluebunch seedlings.  These differences in WUE_i were attributable to differences in root:shoot ratios and root tissue quality (specific root mass), which were lower in the exotic crested wheatgrass.  We concluded that compensatory photosynthesis is an important component of seedling herbivory tolerance, and that observed differences in post-herbivory WUE_i could help in developing stronger seedling selection criteria for conservation and management efforts in water-limited sagebrush steppe ecosystems.