Water stress tolerance and life history in Rhamnaceae seedlings

Chaparral shrub species survive wildfire by three different life histories: non-sprouters (NS) are killed by fire and reestablish only through germination of fire-stimulated seeds; facultative sprouters (FS) reestablish by vegetative sprouting and seed germination; and obligate sprouters (OS) only reestablish by vegetative sprouting.  Post-fire seedlings of NS and FS reestablish in open canopy gaps, whereas OS seedlings primarily recruit in deep shade during fire-free intervals.  We hypothesized that these life history differences, compared within the Rhamnaceae, would result in differential resistance to water stress such that NS>FS>OS.  Seedlings were grown in large pots in a common garden and water stress and shade treatments were applied.  In response to the water stress treatment in full sun, life history types were not different in their predawn water potentials, which were extremely negative (<-9 MPa).  Water potential of water stressed shade grown plants were also not different, and were less negative than the sun treatment (about -6 MPa).  For sun treated plants, NS exhibited 23% mortality, FS 60%, and OS 35% mortality, while irrigated controls had lower mortality across all life histories.  In the shade, NS exhibited 30% mortality, FS 20%, and OS 1% mortality, and irrigated control mortality was not different indicating that water stress did not cause mortality in the shade.  Consistent with their open gap specialization, NS and FS performed best in full sun.  The OS life history type was unexpectedly versatile performing well in the sun and shade.