TALKING POINTS: The impacts of summer heat waves and drought on saguaro cactus

August 28, 2023

Prepared by KIMBERLIE MCCUE, PH.D., CHIEF SCIENCE OFFICER, KEVIN R. HULTINE, PH.D. DIRECTOR OF RESEARCH and TANIA HERNANDEZ, PH.D., NEW WORLD SUCCULENTS CACTUS SCIENTIST

No feature of the Phoenix metropolitan area and surrounding desert is more recognizable than the giant saguaro cactus that often stands over 30 feet in height and can live for over a century.

Saguaros have evolved over thousands of years to cope with the harsh desert climates where temperatures can soar above 100⁰ F with little or no rain falling for several months.

Yet even these iconic desert dwellers have their tolerance limits to drought and heat exposure. The record July temperatures of 2023 in Phoenix coupled with the delayed arrival of monsoon rains has triggered a surge of fallen saguaros around Phoenix.

When drought and heat waves become extreme, high nighttime temperatures force saguaros to use more water from their succulent stems while they conduct photosynthesis, and the extreme daytime temperatures damage the photosynthetic tissues that gives saguaro stems their green appearance. The hot and dry conditions cause the stems to literally wilt, similar to how leaves wilt when drought-stressed. However, unlike the leaves on other plants, the saguaro’s green stems are also responsible for maintaining the structural integrity that allows saguaros to grow tall despite carrying the weight of stored water in their succulent tissues.

Under prolonged heat stress and drought, the wilted stems first drop their arms to the ground. As stress persists, the majestic stems often snap at or near the base and topple over, marking the end of the plant’s life. Unfortunately, the largest, most majestic plants are most at risk due to the shear mass of their upright stems.

While it is still too early to know the extent to which this July’s heat wave will impact the vast population of saguaros in the Phoenix Valley, the excessively hot and dry summer of 2020 offers important clues. Repeated censuses of saguaro plants located at the Desert Botanical Garden show that annual mortality rates have increased 3% above mortality rates previous to 2020.

In 2022 the Garden launched the Saguaro Census, engaging members of the community to make and share observations of saguaros in the urban environment of metro Phoenix. In 2023, the second year of the census, our Saguaro Census volunteers made repeat observations of saguaros first observed in 2022. These repeat observations revealed uncertainty of the condition of 19% of the re-observed plants and that 10% were definitely in worse condition than in 2022. This is concerning since most of the saguaros in the Phoenix Valley are mature to old specimens, without a young cohort growing in the city to replace them.

As Phoenix summer temperatures persistently increase due to the expansive (and expanding) urban heat island coupled with climate change, it appears likely that recent impacts of heat exposure and drought on saguaro cactus and other native plants will be repeated in the future.

Residents of Phoenix are invited to share information with Desert Botanical Garden scientists on saguaros that have died during the summer of 2023.

Reports received through the end of August document 85 saguaro deaths, thus far. By filling out this short form to report saguaro deaths, the community can provide valuable information for understanding the impact of extreme conditions on saguaros in the Phoenix region.