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 exchang e and water relations of a deciduous and dioecious dwarf willow, Salix arctica, were examined. The degree of physiological variation that oc curred as leaves aged was investigated, within and among vegetative an d reproductive ramets of a plant's canopy, and between the sexes. 2. S ignificant differences were observed in rates of photosynthetic carbon assimilation, dark respiration and stomatal conductance, as well as i n bulk tissue water relations of leaves at different ages, between rep roductive and vegetative ramets within an individual, and between male and female individuals. Developing leaves had lower rates of carbon a ssimilation (A) and stomatal conductance (g), but higher rates of dark respiration (R(d)), higher levels of total non-structural carbohydrat es (TNC), higher levels of foliar nitrogen and greater variation in le af nitrogen concentration. Compared with mature leaves, developing lea ves also displayed greater variation in osmotic, elastic and turgor pr operties as well as greater sensitivity to both tissue water deficits and increased leaf-to-air vapour pressure differences (DELTAw). 3. Rep roductive ramets, those which produced both leaves and catkins, had si gnificantly 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 exchan ge in reproductive ramets may have resulted from a greater sink streng th produced by the presence of catkins. 4. These results suggest that physiological variation at several different levels of organization ca n be quite significant. Accounting for such variation is important: (a ) if accurate extrapolations of leaf- or shoot-level performance to wh ole-plant canopies are to be successful, (b) if an adaptive explanatio n for variation in physiological properties within a canopy in terms o f maximizing whole-plant performance is desired, (c) if a detailed und erstanding of the magnitude of individual-level and population-level v ariation 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 withi n or across environments that are heterogeneous in both time and space .