Some monocarpic plants show surprisingly varying responses to natural as well as simulated grazing. In favourable conditions injured plants may grow bigger and produce more fruits and viable seeds compared to their uninjured counterparts. Such overcompensation, however, is associated with potential costs. Usually, grazed plants are delayed in flowering and fruit maturation. The delay may further increase a risk of frost injury before seed maturation in the early autumn. We tested the effects of artificially advanced night frosts on the compensatory capacity of the monocarpic herb Erysimum strictum in a common garden experiment. Two simulated herbivory treatments were applied by means of 25% apical clipping in early vs. late June. Frost treatment was applied in a climate chamber in late August – early September, about three weeks before the first natural frost period.
Apically damaged plants not exposed to frost treatment produced 1.9 – 2.6 times more total biomass and 2.5 times more fruits than intact plants. Frost treatment reduced plant performance by 35 – 48 %, but in contrast to our expectation there was no significant difference between intact and apically damaged plants in response to early frost. In spite of the delay in phenology, compensatory regrowth did not increase the risk of frost injury. We conclude that, while early night frosts imply a potential risk to monocarpic herbs recovering from herbivory, possibly other sub-optimal conditions, such as drought in late summer, may provide a greater threat for early-flowering meadow and grassland plants recovering from grazing. Possibly multiple selective forces and environmental risk factors operate together in the evolution of grazing tolerance associated with flowering phenology, plant growth strategy and architecture.