INDUCTION OF OVERCOMPENSATION IN THE FIELD GENTIAN, GENTIANELLA-CAMPESTRIS

We present field evidence for the induction of overcompensation, or in creased fruit and seed yield as a consequence of damage, in the grassl and biennial field gentian, Gentianella campestris (Gentianaceae). We compared equally sized clipped and unclipped plants in two populations in central Sweden during three years, 1992-1994, and plants clipped a t different occasions, from 20 June to 2 August. Clipping once, by rem oving half of the biomass, significantly increased fruit production wi thout affecting the number of seeds per fruit or seed mass. The degree of compensation was sensitive to the timing of clipping. Damage induc ed overcompensation only during a restricted inductive time period (IT P) in July. Plants clipped before about 1 July or after about 22 July achieved no overcompensation. The early limit of ITP was presumably de termined by the availability of resources that could be mobilized for regrowth after damage. The late limit, on the other hand, depended pri marily on the differentiation of meristems close to flowering in early August. The effects of clipping varied between years, presumably due to drought in 1994. During 1992-1993, plants consistently overcompensa ted for clipping on 1-20 July, whereas in 1994 only early clipping fro m 1 to 12 July induced overcompensation. In 1994, plants clipped in la te July compensated less well, due to delayed fruit maturation leading to a high proportion of immature fruits at the end of the season. Bec ause of this between-year variation, we used geometric mean fitness to calculate the expected long-term effects of damage over generations. The analysis suggests that the long-term effects can vary from positiv e to negative, depending on the frequency of bad fruiting years. The t ime limits of ITP fit well the hypothesis that predictable damage in J uly may have selected for a capacity of overcompensation in the field gentian. Because the ultimate limits of ITP are set by the length of t he vegetation period, we expect overcompensation in this species to be more common in regions with a longer growing season.