Plant traits in forest understory herbs : a modellling study

Although many ideas exist about the traits that characterize shade-tolerant species from the forest understory, so far no clear and comprehensive can be given. This thesis evaluates the role (plasticity in) shoot-to-plant ratio and an interdependent suite of leaf functional traits play in the tolerance of species for low light conditions. To investigate growth under different light conditions of plants that possess different traits, a model was developed. Besides light availability, nitrogen availability also played a role in the growth of the model plant. A mechanistic approach was followed; plant growth was determined by the allocation to the organs and the costs and benefits this brought about. Either (plasticity in) shoot-to-plant ratio or the suite of leaf functional traits was varied in value. This provided a good way to isolate the role of particular traits and assess the adaptiveness of different values of the trait. The simulations presented in this thesis showed that shade-tolerant species should invest more in aboveground mass as compared to belowground mass and more in leaf functional traits promoting the durability of the leaves for which a trade-off exists with photosynthetic capacity and light capturing area. Furthermore, the model showed that a deciduous forest environment should select for a low plasticity level or rigid allocation strategy. According to the model simulations, a species from a dense (deciduous) forest understory should possess a low SLA (as representative of the suite of correlated traits), a high shoot-to-plant ratio and a low plastic response in shoot-root allocation to changes in light availability. Under better lit conditions, for instance in the forest edge, the model predicted a higher SLA and higher plasticity in biomass allocation to be a suitable adaptation. These predictions are consistent with the results of the experiment described in this thesis. The model and the species characterization can be used to evaluate the effect of increased nitrogen availability on the growth and competition in the understory. Simulations showed that increased nitrogen availability in the understory of deciduous forests would favor competitive species that commonly occur in the forest edge. When light availability is severely reduced later in the growing season or in evergreen forests where light availability is low throughout the season, the competitive species from the forest edge will not be able to penetrate the forest interior.