MERISTEM ACTIVITY AND BIOMASS PRODUCTION AS RESPONSE MECHANISMS IN 2 FOREST HERBS

Actaea spicata and Geranium sylvaticum are deciduous rosette herbs, fr equently coexisting in nutrient-rich forests of boreal Sweden. A. spic ata is restricted to this habitat whereas G. sylvaticum has a wider ec ological amplitude. To explore the different distributions of these sp ecies we studied the growth mechanisms by which they responded to nutr ient and light supply. Plants raised from seeds were grown under four combinations of light and nutrient supplies and growth was followed fo r 20 weeks. Growth responses were measured as biomass accumulation and miristem activity, i.e. leaf development by shoot meristems. Biomass accumulation and meristem activity were affected by nutrient and light treatments. However, A. spicata responded more to nutrients, whereas G. sylvaticum responded more to light, and low resource supply shorten ed the time during which shoot meristems of A. spicata developed leave s without a corresponding effect on biomass accumulation. In G. sylvat icum, the periods of leaf development and biomass accumulation were th e same, but a relationship between high development rates and peak all ocation of biomass to shoot meristems indicated a storage function of these during leaf development. We conclude that the inflexible growth of A. spicata, mainly affected by nutrient supply, makes it less compe titive in open habitats and restricts it to shaded, nutrient-rich habi tats. G. sylvaticum should be more successful in habitats subjected to unpredictable disturbances, due to a flexible growth morphology and a dynamic function of meristems that buffers against variation in nutri ent supply and facilitates positive light responses. Meristematic data should be included in interspecific comparisons on growth responses t o different resources supplies. Unless species have the same growth me chanism, it is not satisfactory to study biomass-related traits alone, as internal constraints concerning plant development might confuse th e interpretation.