Physiological responses of Heteromeles arbutifolia seedlings, a California native perennial shrub, during artificial heat waves

Background/Question/Methods

Extreme weather events related to climate change, such as heatwaves, are predicted to increase in California. The impact of an increased frequency of heatwaves on native shrub species in the Mediterranean-type climate of California has not been evaluated. To evaluate whether populations of the widespread California native shrub Heteromeles arbutifolia differed in their responses to heatwaves, we measured responses in water use and photosystem integrity during artificial heatwaves. We collected seed from Northern Coastal (Mendocino County), Northern Inland (Butte County), Southern Coastal (San Diego County), and Southern Inland (San Diego County) sites. We hypothesized that southern sites would better withstand extreme heatwaves because they experience a drier and warmer climate. In addition, we hypothesized inland sites would better withstand extreme heatwaves because they see greater temperature extremes . We used growth chambers to expose half the plants to two artificial heatwaves of 5^oCelsius above average temperatures. Each heatwave lasted 5 days with a 5-day recovery period in between subsequent heatwaves. During the heatwaves we measured responses in stomatal conductance and chlorophyll fluorescence. At the end of the experiment, we measured total projected leaf area and the sapwood to leaf area ratio (i.e. Huber value).

Results/Conclusions

For all four sites stomatal conductance (mmol m^-2s^-1) was 40-48% higher before the heatwave compared to day four of the second heatwave (Northern Coastal: 922.5 ± 115.6, 444.1 ± 51.25, Northern Inland: 982.2 ± 183.1, 399.1 ± 19.56, Southern Coastal 766.7 ± 145.42, 356.0  ± 43.45, Southern Inland 561.7 ± 241.34, 254.8 ± 63.2). On average, northern sites (390.5 ± 24.2) had higher stomatal conductance than southern sites (322.3 ± 21.94). Plants from southern sites had significantly lower (0.011 ± 0.003 m²) total projected leaf area compared to northern sites  (0.015 ± 0.001 m²) (p=0.048). Huber value was significantly higher in plants from southern sites (3.32 ± 0.37) compared to northern sites (2.263 ± 0.27) (p=0.027). Regardless of location, all plants responded to the heatwave treatment. Northern and southern sites differed significantly in their responses, but coastal and inland sites did not. H. arbutifolia exhibits reduced stomatal conductance in response to artificial heatwaves, suggesting that an increase in the frequency and duration of heatwaves will have important impacts on this California native shrub.