The overarching goal of my dissertation is to explore the potential and limits of a trait-based approach to plant ecology. Together, the different studies presented here address two explicit and implicit foundational assumptions underpinning the trait-based approach: (1) that the correlation patterns and biological significance of traits transfer across scales and (2) that the phenotypic complexity of plants can accurately be synthesized into a few meaningful traits to study their ecology. Moreover, the last chapter focuses on a third key assumption: (3) that traits are strong predictors of plant performance (Shipley et al. In Press). I examine these assumptions by exploring multivariate patterns of phenotypic variation and integration across different ecological scales (e.g., individuals, populations, species) while explicitly considering the phenotypic complexity of trees, both in terms of their multidimensional and integrated nature. Two themes thus permeate this body of work: scales and phenotypic complexity. Much of what we know about the relationships among key traits comes from species-scale studies. Trait variation at smaller scales are often interpreted in the context of these interspecific relationships, but it is not clear that interspecific patterns observed at global scales apply to smaller scales. Moreover, although plants are complex, integrated organisms with intricate relationships among their traits, single traits are often studied and interpreted without considering the rest of the phenotype. Yet, examining individual traits outside of their phenotypic context might provide limited insight or be misleading. To address these shortcomings, this body of work examines multidimensional patterns of trait variation and correlation across ecological scales. It uses (1) a set of six ecophysiological leaf traits from mature trees in a lowland tropical rainforest, and (2) a set of twenty leaf, root, stem, branch and whole-plant ecophysiological traits from deciduous saplings in a temperate forest. The combination of our findings point to three main conclusions: (i) local interspecific and intra-population trait integration structures differ from each other and from the global interspecific patterns reported in the literature, such that global-scale interspecific patterns cannot readily be transferred to more local scales; (ii) considering the complexity of the plant phenotype provides better insights into ecological patterns and processes than what we can learn from considering individual or a handful of traits; and (iii) traits strongly affect individual plant performance, although there is no relationship between a species' trait correlation structure and its environmental niche, which suggests that there are multiple alternative optimal phenotypes in a given environment.

Understanding the ability of species to respond to climate change is essential for prediction of their future distribution. When migration is not adequate, reaction via phenotypic plasticity and/or genetic/epigenetic adaptation is necessary. The main aim of this study is to determine mechanisms of response to climate change in dominant grass species Festuca rubra. The study used reciprocal transplant experiment with growth chambers simulating different climatic conditions. Original localities in western Norway represent factorially crossed gradients of temperatures and precipitations, thus it was possible to study the effect of temperature and moisture separately as well as combined. In first part of the experiment, plastic responses were separated from genetic differentiation. To do this, plants with different genotypes from original localities were transplanted to growth chambers set to simulating temperature and moisture course in the four extreme localities (wettest and driest combined with warmest and coldest). After five months, ecophysiological photosynthetic-related traits were measured. These traits are important for species ability to adapt and maintain high fitness and thus they are essential for plants function. Specifically, it was net photosynthetic rate (PN), fluorescence of...