Woody shrubs in chaparral shrub lands are adapted to winter-wet, summer-dry Mediterranean-type climates. However, the Great California Drought of 2012-2016 caused plant mortality throughout many ecosystems in California. The main goal of our work was to associate species-specific mortality and survival with physiological mechanisms, in order to predict future plant responses to extreme droughts. We set up replicated permanent plots at three altitudes along a chaparral–desert ecotone, characterized by a unique mixture of chaparral and desert plant species. Drought survival traits included stomatal conductance responses to leaf water potential, leaf water potential at turgor loss point (ΨTLP), leaf hydraulic conductance, deciduousness, presence of photosynthetic stems, leaf carbon isotopes, rooting depth, and leaf cuticular conductance (gmin). We also installed automatic dendrometer bands in the largest trees to record growth following extreme drought.
Most evergreen species originated in chaparral, while drought- and winter-deciduous species were of desert origin. Within deciduous species, five had green photosynthetic stems. Deciduous leaf phenology was related to presence of photosynthetic stems, high values of leaf gmin, high values of ΨTLP, and low values of carbon isotopic composition, indicating lower long-term integrated water use efficiency (WUE). Plants with photosynthetic stems had leaves with high specific leaf area and low WUE that have short lifespans, but photosynthesize intensely during the short growing season. Plants with high leaf hydraulic conductance had high ΨTLP, indicating trade-off between hydraulic efficiency and drought tolerance in leaves. Our results indicate that plants use a broad suite of drought survival traits in the Chaparral-desert ecotone to respond to drought and plants with photosynthetic stems do not greatly rely on leaf performance for surviving drought periods.