Life History Type and Water Stress Tolerance in Nine California Chaparral Species (rhamnaceae)

Chaparral species of California, USA, exhibit three life history types in response to fire: non-sprouters (NS), facultative sprouters (FS), and obligate sprouters (OS). Adult non-sprouters are killed by fire; thus populations reestablish only through firestimulated seed germination and seedling recruitment. Facultative sprouters reestablish by both vegetative sprouting and seed germination. Obligate sprouters reestablish only by vegetative sprouting and do not recruit seedlings post-fire. Previous data suggest that post-fire NS and FS seedlings reestablish as open-canopy gap specialists, whereas OS seedlings primarily reestablish in deep shade during fire-free intervals. Their non-refractory seeds are killed by fire. We hypothesized that these differences in life history, compared within the same taxonomic group, would result in a range of relative resistance to water stress such that NS . FS . OS. To test our hypothesis, we estimated resistance to water stress using resistance to xylem cavitation (the water potential at 50% loss in hydraulic conductivity; W50) for stems and roots in nine species of the family Rhamnaceae: Ceanothus megacarpus, C. crassifolius, and C. cuneatus (NS); C. spinosus, C. oliganthus, and C. leucodermis (FS); and Rhamnus ilicifolia, R. crocea, and R. californica (OS). Stems of NS species displayed greater resistance to cavitation (W501⁄4 8.386 0.47 MPa) compared to both the FS (W501⁄4 5.076 0.55 MPa) and OS species (W50 1⁄4 5.99 6 0.38 MPa), whereas FS and OS species were not different. For roots, the general pattern was the same, but roots were generally less cavitation resistant than stems. A hydraulic model predicted that water uptake in OS species was limited by extensive cavitation in vulnerable root xylem, consistent with a reliance on deep soil water. Water uptake in cavitation-resistant NS species was most limited by soil hydraulic resistance, consistent with maximizing extraction of shallow soil water. These results suggest a link between life history and water stress tolerance in chaparral.