Plant Responses to Anomalous Heat and Drought Events in the Sonoran Desert.

A shift to greater aridification in dry regions of the world is ongoing and rapidly increasing in intensity, including in the biodiverse Sonoran Desert of the Southwest United States and northern Mexico. In addition to experiencing over two decades of drought, the Sonoran Desert is facing anomalous heat events that are increasing in frequency, evidenced in a record hot and dry period from 2020 to 2021. This article evaluates the impacts of the 2020-2021 region-wide heat and drought event at three scales: (1) a landscape level assessment of ecosystem stress across the entirety of the Sonoran Desert based on precipitation and temperature data from meteorological stations and a satellite-derived vegetation health index (VHI), (2) assessments of stress on iconic columnar cacti and succulent trees, and (3) mechanistic plant responses to extreme heat and drought, and secondary biotic stressors from insect attacks. 2020 was the hottest and driest year since 1980 across the Sonoran Desert region, and vegetation health, determined from VHI, was also near its lowest point. Field-based assessments of columnar cacti across the Sonoran Desert revealed high levels of acute plant stress, including cactus scorching, defined by rapid onset of discolored photosynthetic tissue that leads to permanent photosynthetic dysfunction and increased plant mortality. Tissue scorching corresponded with a three-fold increase in mortality of giant cactus species across the region relative to background levels following 2020-2021. Likewise, repeated plant health surveys show a persistent legacy of the 2020-2021 anomaly, resulting in a marked reduction in the current health and survival of the iconic giant saguaro (Carnegiea gigantea) in the northern Sonoran Desert. This multi-scale assessment of previously anomalous heat and drought events on succulent desert plants shows landscape-wide impacts that could fundamentally reshape populations of these keystone species and the communities that depend on them.