Understanding how different life history types are physiologically divergent is important for understanding their evolution and ecology. This study tested for a link between vascular physiology and life history type in chaparral shrubs. Chaparral shrub species along the southwestern coast of North America survive wildfire by three different life histories. Non-sprouters (NS) are killed by fire and reestablish exclusively through germination of fire-stimulated seeds, facultative sprouters (FS) reestablish by a combination of vegetative sprouting and fire-stimulated seeds, and obligate sprouters (OS) reestablish exclusively by vegetative sprouting because their seeds do not survive fire. Non-sprouters and FS establish seedlings in the open canopy post fire environment, whereas OS establish seedlings in the shady understory of the mature chaparral canopy. Seedlings of nine species (Rhamnaceae) representing three each of the different life history types were grown in deep containers in a common garden under treatments of sun and shade. Cavitation resistance of stems and roots were measured, as well as hydraulic conductance using a high-pressure flow meter for all organs, and a vacuum technique was used to measure conductance of fine and woody roots. We predicted that NS would exhibit greater cavitation resistance and hydraulic efficiency than the sprouting species, and that FS species would be more efficient than the shade tolerant OS species.
Results/Conclusions
Non-sprouters had the greatest cavitation resistance and greatest hydraulic conductance at the whole seedling level, whereas FS and OS were not different. Comparing hydraulic conductance across major organs (from fine roots to leaves) showed that the hydraulic system was well coordinated. At the whole seedling level, the root system was more of a bottleneck than the shoot system. This pattern was consistent with high resistance extraxylary pathways in roots and differences in root architecture. The results suggest a link between xylem hydraulics and life history type in chaparral seedlings.
