PS 23-68 - Urban heat shifts cavitation resistance of Pseudotsuga menziesii trees in Portland, Oregon

Tuesday, August 13, 2019
Exhibit Hall, Kentucky International Convention Center
Mark DeGuzman1, Jack Aldrich2, Hannah M. Prather1, Vivek Shandas3, Todd N. Rosenstiel4, Kevan B Moffett5 and Aaron Ramirez6, (1)Biology, Reed College, Portland, OR, (2)Biology, California State University, Long Beach, Long Beach, CA, (3)Urban Studies and Planning, Portland State University, Portland, OR, (4)Biology, Portland State University, Portland, OR, (5)School of the Environment, Washington State University Vancouver, Vancouver, WA, (6)Biology and Environmental Studies, Reed College, Portland, OR
Background/Question/Methods

The plasticity of xylem hydraulic traits is a critical knowledge gap in accurately predicting plant responses to climate change. In particular, how warming temperatures affect the vulnerability of xylem to cavitation is unknown. The impacts of climate change on plant traits are difficult to study, owing to a combination of (1) few contemporary analogues for projected future conditions and (2) the expense and complexity of warming experiments. However, urban forests provide an alternative ‘natural experiment’ because many of the conditions predicted for the future already occur in these environments, such as warmer temperatures due to the urban heat island effect. The objective of our study was to characterize the cavitation resistance of common douglas-fir (Pseudotsuga menziesii) trees in ‘warm’ and ‘cool’ sites that differed in mean and maximum temperatures by as much as 5° and 20°F, respectively. We measured cavitation resistance using a standard centrifuge technique. Vulnerability curves were constructed and used to estimate the point of 50% loss in hydraulic conductivity (P50), as well as other xylem hydraulic traits. We also measured dry-season plant water relations during the peak of the 2018 dry season. Warm vs. cool sites were identified using fine-scale surface temperature models and LiDAR-based canopy data.

Results/Conclusions

Cavitation resistance was significantly different across warm and cool sites, with warm sites experiencing a shift towards more resist xylem (P50 difference of > 1 MPa; P = 0.03). However, this shift in cavitation resistance was not sufficient to reduce dry season water stress as indicated by stomatal conductance, minimum seasonal water potentials, and native embolism, which were all less favorable in warm sites (P < 0.05). Our study demonstrates that warmer temperatures can induce plasticity in the xylem hydraulic traits of an ecologically and economically important conifer species, with more resistant xylem as temperature increases. But, these changes do not fully offset the increased stress of these environments. These findings contradict a previous study from Brazil on urban oak trees, which found ‘hydraulic deterioration’ or reduced P50 in warmer urban sites. This may suggest that angiosperm and gymnosperm xylem responds differently to increased temperatures, with important implications for how these taxa will respond to global climate change.