Quantitative Phenology and Water Relations of an Evergreen and a Deciduous Chaparral Shrub

Quantitative phenology and pre—dawn xylem pressure potentials (XPP) of an evergreen shrub, Ceanothus megacarpus, and a deciduous shrub, Salvia mellifera, were compared on pairs of plants in shared microsites in southern Californian chaparral for 2 yr. Shoot elongation of both species occurred at essentially the same time during both growing seasons. Leaf maturation also occurred at essentially the same time for both species during the 1977—1978 season, but commenced ≈1 mo earlier on S. mellifera than on C. megacarpus during the 1978—1979 season. No substantial differences in phenology were found among leaves of C. megacarpus canopy, subcanopy, or short shoots. On S. mellifera, however, leaf maturation was more rapid and was completed earlier in the season on flowering canopy shoots than on nonflowering canopy and subcanopy shoots, and although no mature leaves on flowering canopy shoots persisted beyond June during either year, a substantial number of mature leaves on subcanopy shoots remained well into their second growing season. Leaf curling, associated with changes in XPP, was significant in both species, and allowed rapid adjustment of leaf area without the abscission or production of leaves. Flowering and seed production occurred early in the growing season for C. megacarpus and late in the growing season for S. mellifera. This difference in flowering phenology corresponded to the more rapid vegetative development of S. mellifera flowering canopy shoots. In both species subcanopy or short shoots made up a much larger fraction of the total number of shoots per plant, and consequently contributed relatively more to whole—plant leaf phenology and annual shoot production, than did canopy shoots. There were only subtle differences between species in the timing of increases in whole—plant mature leaf area (MLA). Maximum rates of increase of whole—plant MLA occurred in both species when soil temperature (ST) was ≈13.5°C and XPPs were ≈—0.60 MPa, and these rates declined sharply at STs above 15° and XPPs below —1.5 MPa. Ceanothus megacarpus and S. mellifera had nearly indistinguishable XPPs throughout both years. Leaf shedding by S. mellifera was more related to photoperiod and flowering than to low XPP, and was clearly not a successful drought—avoiding mechanism. Like some evergreen species, S. mellifera tolerated long periods of extremely low XPP. Important differences between the species were that over a 12—mo period the integrated total of MLA—days for S. mellifera was 73% of that for C. megacarpus, and leaves produced during the current growing season contributed 71% of the total MLA—days in S. mellifera and only 37% in C. megacarpus. The differences between the results of this study and several accepted generalizations regarding growth and water relations of deciduous and evergreen chaparral shrubs may be due to the observation of these growth forms in different microsites and the lack of truly quantitative phenological measurements in previous studies.