Effect of herbivory on the growth and photosynthesis of replanted Calligonum caput-medusae saplings in an infertile arid desert

Replanted Calligonum caput-medusae saplings in the Tarim River watershed face short-term and frequent herbivory by goats, which can result in either growth inhibition or stimulation. The effects of herbivory on shrub saplings are unclear. We simulated herbivory with clipping to test two hypotheses. We hypothesized that (1) moderate herbivory may positively affect replanted shrub saplings due to overcompensatory growth and compensatory photosynthesis and that (2) high amounts of defoliation may change water availability and impair photosynthesis and growth of saplings. We applied four defoliation treatments (0, 30, 50, and 70 %) to 2-year-old C. caput-medusae saplings to test the effects of herbivory. Moderately defoliated (~30 %) saplings grew faster and had higher photosynthetic performance than controls; however, defoliation of 50 % or more reduced growth due to undercompensatory photosynthesis and reduced water availability. Non-photochemical quenching by thermal dissipation provided photoprotection when absorbed light energy used in PSII photochemistry was inhibited, reducing excess excitation energy and allowing saplings with high amounts of defoliation to maintain adequate photosystem functioning. This suggests that moderate herbivory of replanted shrubs used as forage in arid ecological restoration projects is feasible, but that uncontrolled grazing should be forbidden.