Compensatory Responses of Two Melampyrum Species After Damage

Compensatory responses after damage were studied in two closely related annual hemiparasites of the genus Melampyrum. We followed the growth and the seed set of individuals subjected to clipping of the main stem, defoliation and removal of flowers and unripe fruits. Defoliation resulted in decreased seed set and growth. Despite the hemiparasitic life-style of the species, the leaves are thus important, either for photosynthesis or because transpiration makes possible the acquisition of resources from a host. There was a trade-off between early and late season reproduction: when all the flowers or unripe fruits were removed in early season, the plants enhanced their seed production in late season, compensating totally (in Melampyrum pratense) or partially (in Melampyrum sylvaticum) for the loss of the seeds from early season flowers and fruits. This compensation was mainly brought about through changes in the abortion regimes of reproductive structures, not through regrowth. Thus, flower production substantially outstripped the ability of the plants to provision all their seeds. After main stem clipping the branches produced more fruits and seeds per flower in M. sylvaticum and fewer in M. pratense than the branches of the control plants. The timing of the damage determined the compensation mechanism in M. sylvaticum: plants compensated by regrowth (increased flower number) after main stem clipping in May and by reducing seed abortion after main step clipping in June. A large part of the findings can be accounted for in terms of resource allocation and the timing of the damage. The different responses to main step clipping, however, suggest that differences in the architecture of the species are also important in compensatory responses.