PARENT SESSION
Oral Session # 77: Plant Ecology VI: Temperature and Water Stress.
Presiding: C Ford
Thursday, August 7. 1:30 PM to 5:00 PM, SITCC Meeting Room 102.

Freezing limits the distribution of Malosma laurina in the chaparral of southern California.

Pratt, R Brandon¹, Ewers, Frank², Lawson, Michael¹, Jacobsen, Anna¹, Davis, Stephen¹, ¹ Pepperdine University, Malibu, California² Michigan State University, East Lansing, Michigan

ABSTRACT- We examined the freezing response of two co-occurring evergreen chaparral shrubs, Malosma laurina and Rhus ovata. M. laurina leaves die at temperatures ≤−6^oC whereas R. ovata leaves do not. The death may be caused by direct damage to cells or from desiccation following freezing-induced cavitation of stem xylem. We tested three hypotheses: 1) freezing-induced cavitation causes the damage seen in M. laurina; 2) M. laurina exhibits cavitation at higher temperatures than R. ovata; and 3) M. laurina seedlings are more frost sensitive than R. ovata seedlings. To test our hypotheses we simulated natural freeze-thaw cycles in a freezing chamber on branches ≥1 m in length followed by measurements of the temperature at 50% cell death (LT₅₀'s) of leaves and the percentage loss of hydraulic conductivity (PLC) of stems. Additionally, we measured dark-adapted fluorescence (F_v/F_m) of seedlings in the field during freezing events. The LT₅₀ of leaves was −10^oC±0.23 for R. ovata and −6^oC±0.17 for M. laurina. Stems of both species displayed high PLC post-freezing to ≤−6^oC (PLC=92%±2.6 for R. ovata and 90%±4.2 for M. laurina) compared to unfrozen control stems (PLC=58%±5.2 for R. ovata and 45%±7.6 for M. laurina). M. laurina seedlings displayed freezing damage at −4^oC (F_v/F_m=0.441±0.074; n=12) compared to unfrozen seedlings at a control site (F_v/F_m=0.826±0.004; n=12) and all seedlings were killed by a −6^oC freeze. No freezing injury was observed in R. ovata seedlings. We conclude that M. laurina is co-limited by direct freezing damage to leaves and high PLC of stem xylem at temperatures ≤−6^oC. R. ovata leaves are more tolerant of freezing than M. laurina and can survive at high PLC in the field (native PLC in November=84%±3.9). M. laurina seedlings are more vulnerable death from freezing explaining their absence from colder microsites where R. ovata often dominates.

Key words: cavitation, embolism, chaparral, freezing