Linkage between water stress tolerance and life history type in seedlings of nine chaparral species (Rhamnaceae)

1. Chaparral shrub species along the southwest coast of North America survive wildfire by three different life history types: non-sprouters (NS) are killed by fire and only re-establish through germination of fire-stimulated seeds; facultative sprouters (FS) re-establish by a combination of vegetative sprouting and seed germination; and obligate sprouters (OS) only re-establish by vegetative sprouting because their seeds do not survive fire. 2. Post-fire seedlings of NS and FS re-establish in open canopy gaps, whereas OS seedlings primarily recruit in deep shade during fire-free intervals. 3. It was hypothesized that these life history differences, compared within the same family, Rhamnaceae, would result in differential seedling resistance to water stress such that NS > FS > OS. 4. Seedlings were grown in large pots in a common garden under treatments of sun, shade, irrigation and water stress. Mortality and physiology of seedlings were monitored for about 1 year. 5. In response to the water stress treatment in full sun, life history types were not different in their dry season predawn water potentials, which were extremely negative (< -9 MPa). Water potential of water-stressed shade grown plants were also not different, but were less negative than the sun treatment during the dry season (about -6 MPa). 5. For plants grown in full sun under water stress, NS exhibited 23% mortality, FS 60% and OS 35% mortality, while irrigated controls had greatly reduced mortality across all life histories. Among shade grown plants under water stress, NS exhibited 30% mortality, FS 20% and OS 1% mortality. This pattern of differential mortality was the same for irrigated plants indicating that water stress was not the cause of mortality in the shade. 6. Greater stem and root resistance to water stress induced cavitation were strongly correlated with lower percent mortality during the summer dry season suggesting a link between these traits and fitness. 7. Synthesis. This study finds a link between life history type and water stress tolerance. Consistent with open canopy specialization, NS survived best in full sun. The FS and OS life history type had better survival in the shade. The OS was unexpectedly versatile, surviving well in the sun and shade. The evolutionary history of the life history types is discussed as an important factor in linking water stress tolerance and life history.