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 in 2020 through 2021. This paper 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 cactus 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
cacti 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.