Tolerance of an annual hemiparasite, Euphrasia stricta agg., to simulated grazing in relation to the host environment

Many annual plants are adapted to man-made habitats, such as grazed and mown meadows, because they are poor competitors and hence confined to relatively open habitats with low vegetation. In order to survive in grazed and mown habitats, they should also have evolved tolerance to apical damage. We studied the tolerance of eyebright (Euphrasia stricta agg.), a hemiparasitic annual, to varying intensities of apical damage (0%, 10% and 50% cutting) in a common garden experiment in two consecutive seasons with Trifolium repens and Festuca rubra as available hosts. The host environment was manipulated by mowing or by leaving the surrounding vegetation intact. Because root hemiparasitic plants poach resources from their host plants, while the hosts are simultaneously also potential competitors for light, mowing of the surrounding vegetation may have both negative (less host resources available) and positive (reduced competition) effects on hemiparasites. In the present study, neighbourhood mowing tended to improve the growth and fruit production of apically damaged eyebright plants. In the first study year, apically damaged (10% cut) plants produced more branches, higher shoot biomass and more fruits compared to intact plants when the surrounding vegetation was mown. No such overcompensation was observed either in an unmown neighbourhood or in the second year. The plants tolerated more extensive damage less well, and all of the 50% cut plants died in the second year in the unmown neighbourhood. The poor survival of these plants was likely to be due to the unfavourable combination of heavy damage, summer drought and competition with the surrounding vegetation. The present study suggests that Euphrasia, and possibly also other annual meadow herbs, may tolerate relatively well minor apical damage, while more extensive damage is likely to adversely affect their performance. The results also support a compensatory continuum in which grazing tolerance varies from undercompensation (damage decreases plant performance) in unfavourable conditions to overcompensation (damage increases plant performance) in favourable growing conditions. While tolerance to apical damage may contribute to the success of these species, they are poor competitors, and their long-term persistence may hence notably depend on the fact that grazing, and mowing suppress stronger competitors by maintaining low vegetation.