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 fire-stimulated 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; Psi(50)) for stems and roots in nine species of the family Rhamnaceae: Ceanothus megacarpus, C. crass folius, 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 (Psi(50)=-8.38 +/- 0.47 MPa) compared to both the FS (Psi(50=-)5.07 +/- 0.55 MPa) and OS species (Psi(50) =-5.99 +/- 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 INS 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.