Increasingly frequent episodes of drought combined with high heat in the southwestern U.S. may test the limits of plant tolerance to heat and water stress. We investigated the leaf heat tolerance of two native California tree species, Quercus agrifolia (coast live oak) and Q. engelmannii (Engelmann oak) growing in irrigated and unirrigated locations at the Huntington Botanical Gardens in southern California. The species co-occur in California, although Q. agrifolia, an evergreen, is widespread with a relatively large range and Q. engelmannii is drought-deciduous and infrequent with a more restricted range. Our goals were to evaluate the combined effects of heat and water stress on leaf heat tolerance throughout the hot, rainless summer of 2022 to test the hypotheses that 1) leaves of Q, agrifolia would tolerate higher temperatures and greater drought and 2) that leaves of both species would become more heat tolerant as summer progressed. Our approach was to assess leaf heat tolerance using changes in the quantum efficiency of photosynthesis as indicated by the ratio of variable to maximal chlorophyll fluorescence, Fv/Fm. Measurements were made in April, July, and September after timed immersion of excised, bagged leaves in water baths of increasing temperatures (24 ℃ - 60 ℃) were used to calculate the temperature at which Fv/Fm was 0 for 50% of the leaves (T50). Stomatal conductance to water vapor was measured for the same trees to estimate the amount of water stress trees were experiencing. There were no significant differences in stomatal conductance due to species, water treatment, or their interaction in spring, but in late summer decreases were significantly greater for Q. engelmannii than for Q. agrifolia. There were no significant differences between the two species in T50 at any sampling time. However, water treatment had a greater impact on leaf heat tolerance, with irrigated plants having T50 values ranging from 54.1 - 54.2 ℃ and unirrigated trees from 55.2 - 56.4℃. Seasonality also influenced heat tolerance, with the variance in Fv/Fm and differences between irrigated and unirrigated trees decreasing as heat and water stress increased. In conclusion, these data support the growing evidence that environmental conditions are more important than interspecific differences in determining leaf-level responses to high temperatures (Lancaster and Humphreys, 2020). Further, we suggest that trees that have endured prolonged or repeated water stress may may have greater leaf heat tolerance than do well-watered trees, with possible management implications for urban forests.
