Freezing Tolerance Impacts Chaparral Species Distribution in the Santa Monica Mountains

A shift in chaparral species composition occurs from coastal to inland sites of the Santa Monica Mountains of southern California. Past studies have attributed this pattern to differential adaptations of chaparral species to gradients in moisture and solar radiation. We examined an alternate hypothesis, that shifts in species composition from coastal to inland sites is a result of differential response to freezing and the interactions of freezing with drought. Coastal sites rarely experience air temperatures below 0 °C whereas just 5 to 6 km inland, cold valleys experience temperatures as low as -12 °C. Seasonal drought can last 6 to 8 months and may extend, on rare occasions, into the month of December, coincidental with the onset of winter freeze. Either water stress or freezing, by independent mechanisms, can induce embolism in stem xylem and block water transport from soil to leaves, leading to branchlet dieback or whole shoot death. Water stress in combination with freezing may enhance xylem embolism formation. Postfire seedlings are especially vulnerable because of greater tissue sensitivity to freezing injury, diminutive roots that preclude access to deep soil moisture or resprout success, and greater exposure to nighttime radiation freezes after canopy removal by fire. Ceanothus megacarpus, C. spinosus, and Malosma laurina dominate the non-freezing landscape of coastal exposures, whereas C. crassifolius and Rhus ovata dominate inland cold air drainages. Because C. spinosus and M. laurina form lignotubers they can persist immediately upslope of cold valleys through vegetative resprouting after periodic freeze-induced death of shoots. Because C. megacarpus and C. crassifolius are not capable of vegetative resprouting (non-sprouters after shoot death by freezing), they are either eliminated from cold inland sites (C. megacarpus) or are adapted to freezing in combination with drought (C. crassifolius). In some cases, stems are more susceptible to freezing induced dysfunction than leaves (R. ovata) whereas in other species, both stems and leaves are extremely resistant to freezing-induced dysfunction, even when partially dehydrated 160 Davis, S.D. et al. (C. crassifolius). In the case of M. laurina, both stems and leaves are susceptible to freezing with total shoot dieback at -6 °C. However, stunted individuals of M. laurina may persist at -9 °C sites in the Santa Monica Mountains through repeated resprout success. We conclude that a suite of factors, such as resprout success, the susceptibility of leaves and stem xylem to freezing injury, and the interactions of drought and freezing, contribute to the final distribution patterns of chaparral species in the Santa Monica Mountains.