Exposure to heat waves and drought improves subsequent leaf heat tolerance for five oak species in southern California.

PREMISE: Given the threats of climate change, we investigated the impact of high temperatures separately and in combination with reduced soil water on leaf heat tolerance for five oaks (Fagaceae) with overlapping ranges in southern California, USA: Quercus agrifolia and Q. chrysolepis (evergreen species), Q. douglasii and Q. lobata (winter-deciduous), and Q. engelmannii (drought-deciduous). METHODS: Leaf heat tolerance was measured in two seasons for Q. agrifolia and Q. engelmannii at the Huntington Botanical Gardens, San Marino, California and for container-grown saplings of three other oak species at Occidental College, Los Angeles, California. At both sites, control plants received regular water, and dry plants received one third that amount. Between measurement periods, plants experienced one or more heat waves (each with three or more consecutive days exceeding 32.2°C). For measuring heat tolerance, leaves were submerged in water baths from 20° to 60°C, then dark-adapted chlorophyll fluorescence (Fv/Fm) was measured to assess damage to photosystem II. RESULTS: Leaf heat tolerance differed due to prior heat exposure, soil moisture, leaf habit, and species identity. Leaves under dry conditions were more heat tolerant than under well-watered conditions, and leaves of evergreen species were more tolerant than those of winter-deciduous and drought-deciduous species, as indicated by values of T50, the temperature at which Fv/Fm was reduced by 50%. CONCLUSIONS: We suggest that prior exposure to dry conditions and heat waves primed these oaks, particularly the evergreen species, to better endure future heat episodes, at least in the short term.