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A central paradigm in comparative ecology is that species sort out along a slow-fast resource economy spectrum of plant strategies, but this has been rarely tested for a comprehensive set of stem traits and compartments. We tested how stem traits vary across wood and bark of temperate tree species, whether a slow-fast strategy spectrum exists, and what traits make up this plant strategy spectrum. For 14 temperate tree species, 20 anatomical, chemical, and morphological traits belonging to six key stem functions were measured for three stem compartments (inner wood, outer wood, and bark). The trait variation was explained by major taxa (38%), stem compartments (24%), and species within major taxa (19%). A continuous plant strategy gradient was found across and within taxa, running from hydraulic safe gymnosperms to conductive angiosperms. Both groups showed a second strategy gradient related to chemical defense. Gymnosperms strongly converged in their trait strategies because of their uniform tracheids. Angiosperms strongly diverged because of their different vessel arrangement and tissue types. The bark had higher concentrations of nutrients and phenolics whereas the wood had stronger physical defense. The gymnosperms have a conservative strategy associated with strong hydraulic safety and physical defense, and a narrow, specialized range of trait values, which allow them to grow well in drier and unproductive habitats. The angiosperm species show a wider trait variation in all stem compartments, which makes them successful in marginal- and in mesic, productive habitats. The associations between multiple wood and bark traits collectively define a slow-fast stem strategy spectrum as is seen also for each stem compartment.

Functional biogeography has advanced the field of functional ecology into a more spatially[sbnd]predictive science. However, freshwater plants are still underrepresented in these trait[sbnd]based advancements. Here, we argue that there is a need for developing a functional biogeographical framework for freshwater plants and initiate global mapping efforts focusing on the form and function of freshwater plants. Specific attention should be given to (1) the placement of freshwater plants in the global plant trait space and show how this placement links to global trait[sbnd]environment relationships; (2) the theoretical framework for major structural trait[sbnd]trait correlations based on the physical constraints in aquatic ecosystems; (3) the evolutionary and environmental drivers underlying the global distribution of inter[sbnd] and intra[sbnd]specific variation in different life forms; and (4) the level of equilibrium between spatial and temporal trait[sbnd]environment relationships in freshwater plants. By putting freshwater plants in the context of these spatial aspects, we could advance our understanding of freshwater plant adaptations and responses to environmental gradients, and thereby facilitate predicting the consequences of global changes for freshwater ecosystem functions and services.

Functional biogeography has advanced the field of functional ecology into a more spatially[sbnd]predictive science. However, freshwater plants are still underrepresented in these trait[sbnd]based advancements. Here, we argue that there is a need for developing a functional biogeographical framework for freshwater plants and initiate global mapping efforts focusing on the form and function of freshwater plants. Specific attention should be given to (1) the placement of freshwater plants in the global plant trait space and show how this placement links to global trait[sbnd]environment relationships; (2) the theoretical framework for major structural trait[sbnd]trait correlations based on the physical constraints in aquatic ecosystems; (3) the evolutionary and environmental drivers underlying the global distribution of inter[sbnd] and intra[sbnd]specific variation in different life forms; and (4) the level of equilibrium between spatial and temporal trait[sbnd]environment relationships in freshwater plants. By putting freshwater plants in the context of these spatial aspects, we could advance our understanding of freshwater plant adaptations and responses to environmental gradients, and thereby facilitate predicting the consequences of global changes for freshwater ecosystem functions and services.

The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis-based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.