Meristem activity and biomass production as response mechanisms in two forest herbs.

Actaea spicata and Geranium sylvaticum are deciduous rosette herbs, frequently coexisting in nutrient-rich forests of boreal Sweden. A. spicata is restricted to this habitat whereas G. sylvaticum has a wider ecological amplitude. To explore the different distributions of these species we studied the growth mechanisms by which they responded to nutrient and light supply. Plants raised from seeds were grown under four combinations of light and nutrient supplies and growth was followed for 20 weeks. Growth responses were measured as biomass accumulation and meristem 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 shortened the time during which shoot meristems of A. spicata developed leaves without a corresponding effect on biomass accumulation. In G. sylvaticum, the periods of leaf development and biomass accumulation were the same, but a relationship between high development rates and peak allocation 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 competitive in open habitats and restricts it to shaded, nutrient-rich habitats. 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 nutrient supply and facilitates positive light responses. Meristematic data should be included in interspecific comparisons on growth responses to different resource supplies. Unless species have the same growth mechanism, it is not satisfactory to study biomass-related traits alone, as internal constraints concerning plant development might confuse the interpretation.