RESPONSE OF CHAPARRAL SHRUBS TO BELOW-FREEZING TEMPERATURES - ACCLIMATION, ECOTYPES, SEEDLINGS VS. ADULTS

Leaf death due to freezing was examined for four, co-occurring species of chaparral shrubs from the Santa Monica Mountains of southern Calif ornia, Rhus laurina (= Malosma laurina), R. ovata, Ceanothus megacarpu s, and C. spinosus. Measurements were made on seedlings vs. adults for all species, and for Rhus spp. in winter vs. summer, and at a warm vs . a cold site. We used four methods to determine the temperature for 5 0% change in activity or cell death (LT50) of leaves: (1) electrical c onductivity (electrolyte leakage into a bathing solution), (2) photosy nthetic fluorescent capacity (Fv/Fm), (3) percentage of palisade mesop hyll cells stained by fluorescein diacetate vital stain, and (4) visua l score of leaf color (Munsell color chart). In all four species seedl ings were found to be more sensitive to freezing temperatures than wer e adults by 1 degrees-3 degrees C. For adults the LT50 ranged from -5 degrees C for Rhus laurina in the summer to -16 degrees C for Rhus ova ta in the winter. The LT50 of R. ovata located at a colder inland site was 4 degrees C lower than R, ovata at the warmer coastal site just 4 km apart, suggesting ecotypic differences between R. ovata at the two sites. Both R. laurina and R. ovata underwent significant winter hard ening. At the cold site, R. ovata acclimated by 6 degrees C on average , while R. laurina acclimated by only 3 degrees C. These results were consistent with species distributions and with field observations of d ifferential shoot dieback between these two congeneric species after a natural freeze-thaw event in the Santa Monica Mountains.