Adjustment to the light environment in small-statured forbs as a strategy for complementary resource use in mixtures of grassland species.

Background and Aims The biological mechanisms of niche complementarity allowing for a stable coexistence of a large number of species in a plant community are still poorly understood. This study investigated how small-statured forbs use environmental niches in light and CO2 to explain their persistence in diverse temperate grasslands. Methods Light and CO2 profiles and the corresponding leaf characteristics of seven small-statured forbs were measured in monocultures and a multi-species mixture within a biodiversity experiment (Jena Experiment) to assess their adjustment to growth conditions in the canopy. Key Results Environmental conditions near the ground varied throughout the season with a substantial CO2 enrichment (> 70 mu mol mol(-1) at 2 cm > 20 mu mol mol(-1) at 10 cm above soil surface) and a decrease in light transmittance (to < 5% deep in the canopy) with large standing biomass (> 500 g d. wt m(-2)) in the multi-species assemblage. Leaf morphology, biochemistry and physiology of small-statured forbs adjusted to low light in the mixture compared with the monocultures. However, the net carbon assimilation balance during the period of low light only compensated the costs of maintenance respiration, while CO2 enrichment near the ground did not allow for additional carbon gain. Close correlations of leaf mass per area with changes in light availability suggested that small-statured forbs are capable of adjusting to exploit seasonal niches with better light supply for growth and to maintain the carbon metabolism for survival if light transmittance is substantially reduced in multi-species assemblages. Conclusions This study shows that adjustment to a highly dynamic light environment is most important for spatial and seasonal niche separation of small-statured forb species in regularly mown, species-rich grasslands. The utilization of short-period CO2 enrichment developing in dense vegetation close to the ground hardly improves their carbon balance and contributes little to species segregation along environmental niche axes.