PLANTS AS MOSAICS - LEAF-LEVEL, RAMET-LEVEL, AND GENDER-LEVEL VARIATION IN THE PHYSIOLOGY OF THE DWARF WILLOW, SALIX-ARCTICA

1. Plant ecophysiological investigations Often fail to account for age-specific, tissue- or organ-specific, or life-history-specific effects on overall performance. To address this issue, aspects of gas exchange and water relations of a deciduous and dioecious dwarf willow, Salix arctica, were examined. The degree of physiological variation that occurred as leaves aged was investigated, within and among vegetative and reproductive ramets of a plant's canopy, and between the sexes. 2. Significant differences were observed in rates of photosynthetic carbon assimilation, dark respiration and stomatal conductance, as well as in bulk tissue water relations of leaves at different ages, between reproductive and vegetative ramets within an individual, and between male and female individuals. Developing leaves had lower rates of carbon assimilation (A) and stomatal conductance (g), but higher rates of dark respiration (R(d)), higher levels of total non-structural carbohydrates (TNC), higher levels of foliar nitrogen and greater variation in leaf nitrogen concentration. Compared with mature leaves, developing leaves also displayed greater variation in osmotic, elastic and turgor properties as well as greater sensitivity to both tissue water deficits and increased leaf-to-air vapour pressure differences (DELTAw). 3. Reproductive ramets, those which produced both leaves and catkins, had significantly higher A, g and R(d), as well as higher levels of TNC, but a lower tolerance to both tissue water deficits and increased DELTAw when compared with vegetative ramets. The enhanced rates of gas exchange in reproductive ramets may have resulted from a greater sink strength produced by the presence of catkins. 4. These results suggest that physiological variation at several different levels of organization can be quite significant. Accounting for such variation is important: (a) if accurate extrapolations of leaf- or shoot-level performance to whole-plant canopies are to be successful, (b) if an adaptive explanation for variation in physiological properties within a canopy in terms of maximizing whole-plant performance is desired, (c) if a detailed understanding of the magnitude of individual-level and population-level variation is desired, and (d) if the goal is to be able to construct a detailed picture of what contributes to whole-plant performance, or (e) to characterize accurately a species' ecophysiological breadth within or across environments that are heterogeneous in both time and space.