Understanding the physiology and ecology of shade tolerance

Shade suppression is a common stress for understory juvenile trees. Different physiological and ecological strategies in response to shade suppression have been identified. Empirical work reveals a life history trade-off between growth rate in high light and survival in shade existent in a many forest communities across geographical boundaries. The major aims of this thesis are therefore to evaluate the contribution of different physiological mechanisms to juvenile shade tolerance, and to demonstrate particularly which mechanism(s) can explain the trade-off between growth rate in high light and survival in shade.;Measurements on 5 species in temperate New Jersey forests show shade tolerant and intolerant species, as well as deciduous and coniferous species, generally differentiate in a lot of photosynthesis and respiration features. This difference in photosynthesis and respiration patterns comes from contribution of different adaptive strategies in leaf structure and physiology. Most of the photosynthesis and respiration changes with light dynamics can be explained with changes in SLA, leaf area per unit leaf mass, or LMR, leaf mass per unit leaf area. SLA (or LMR) is proven an important characteristic for plant's fast response to light changes. Evidence in support of the net energy gain hypothesis predictions is found in this study, yet still limited and even with contrary phenomena observed. No relation between shade tolerance and leaf respiration down-regulation has been found.;Carbohydrate storage has been proven a critical component in the life history of shade-tolerant tree species. Measurement results strongly support the storage hypothesis that shade tolerance is determined by an allocational trade-off between non-structural storage and structural growth. This hypothesis was validated for both coniferous and deciduous species. Total nonstructural carbohydrate (TNC) storage is a key functional trait which provides a mechanistic explanation for the trade-off manifold of species specific differences in growth and survival. Nonstructural carbohydrate storage may be a more important suppression deterrent measure for deciduous species as they usually store more TNC than coniferous species. The results also highlight the importance of using effective TNC, the difference between healthy and newly dead saplings or branches, as the index of carbon storage.;Two simple carbon budget analysis combining respiration plasticity and carbohydrate storage furthermore illustrates that shade tolerant species survive better than intolerant species under shade as a result of its pre-shade nonstructural carbohydrate storage and fast respiration reduction response under shade.;A theoretical model with carbon storage allocation was established. Analysis shows that diameter growth is negatively affected by carbon storage when the size of a tree is small; but this negative effect almost diminishes when a tree's diameter or carbon storage is big enough. Carbon storage enhances the survival time under negative net carbon gain, while the time for a tree or tree cohort to reach canopy is independent of carbon storage except for early years. Carbon storage was mathematically proven to enhance the survival fraction under negative net carbon gain. However, the survivorship enhancement saturates when allocation to non-structural carbon storage is big, indicating the existence of an optimal strategy on allocation between structural growth and non-structural storage.;An increase of foliar nitrogen per unit leaf mass (Nm) with decreasing light irradiance has been found for all the 5 species in this study. However, the result presented in this paper show no relationship between plant N content, allocation and shade tolerance.