Limits of thermal and hydrological tolerance in a foundation tree species (Populus fremontii) in the desert southwestern United States

• Published in: The New phytologist 2023-12, Vol.240 (6), p.2298-2311
• Main Authors: Moran, Madeline E., Aparecido, Luiza M. T., Koepke, Dan F., Cooper, Hillary F., Doughty, Christopher E., Gehring, Catherine A., Throop, Heather L., Whitham, Thomas G., Allan, Gerard J., Hultine, Kevin R.
• Format: Article
• Language: English
• Subjects: chlorophyll a fluorescence; Cooling; Evaporative cooling; Genotypes; groundwater dependent ecosystems; Groundwater table; Habitat corridors; Heat resistance; Heat waves; Heatwaves; High temperature; leaf temperature; leaf turgor loss point; Leaves; local adaptation; Locations (working); Metabolism; Photosystem II; Plant species; Populus fremontii; River basins; River flow; Rivers; Safety margins; Stomata; Stomatal conductance; Temperature; Temperature gradients; Temperature tolerance; Thermoregulation; Turgor; Water potential; Water table
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1111/nph.19247

Summary Populus fremontii is among the most dominant, and ecologically important riparian tree species in the western United States and can thrive in hyper‐arid riparian corridors. Yet, P. fremontii forests have rapidly declined over the last decade, particularly in places where temperatures sometimes exceed 50°C. We evaluated high temperature tolerance of leaf metabolism, leaf thermoregulation, and leaf hydraulic function in eight P. fremontii populations spanning a 5.3°C mean annual temperature gradient in a well‐watered common garden, and at source locations throughout the lower Colorado River Basin. Two major results emerged. First, despite having an exceptionally high Tcrit (the temperature at which Photosystem II is disrupted) relative to other tree taxa, recent heat waves exceeded Tcrit, requiring evaporative leaf cooling to maintain leaf‐to‐air thermal safety margins. Second, in midsummer, genotypes from the warmest locations maintained lower midday leaf temperatures, a higher midday stomatal conductance, and maintained turgor pressure at lower water potentials than genotypes from more temperate locations. Taken together, results suggest that under well‐watered conditions, P. fremontii can regulate leaf temperature below Tcrit along the warm edge of its distribution. Nevertheless, reduced Colorado River flows threaten to lower water tables below levels needed for evaporative cooling during episodic heat waves.