By Jack Dash

There has been a lot of talk in the news lately about a mass saguaro collapse caused by heat and drought. This claim, and the media storm around it, requires a bit of unpacking.  

The story begins with the fantastic research team at the Desert Botanical Garden in Phoenix who have been surveying the saguaros on grounds at their garden while also seeking information from community members who report damaged or fallen saguaros. What they have found is that the intense heat of years like 2020 and this summer of 2023 are creating detrimental conditions for saguaro survival. Specifically, the intense heat is causing the flesh of the plants to become soft, creating the conditions for structural collapse. High night-time temperatures prevent efficient photosynthesis (cacti mostly photosynthesize at night), while drought stress weakens these plants, making them more susceptible to other issues like disease. 

Urban vs Wild Saguaros

You can imagine that these factors will be particularly prevalent in a place like Phoenix where the urban heat island effect is extreme, and where rainfall is lower than the Tucson area. An inability to tolerate extremes of sunlight and temperature will be even more pronounced in plants which have been recently transplanted and are growing in environments that don’t mimic where they naturally occur. In other words, a bare yard covered in a layer of gravel with pavers and block walls, or a roadway median between four lanes of blacktop, will be challenging areas for saguaros to survive extreme heat waves. 

Inefficient Photosynthesis

The photosynthetic pathway utilized by succulent plants is known as Crassulacean Acid Metabolism (CAM) and is distinguished by the fact that much of the photosynthetic process takes place at night. This pathway works well under high temperatures but begins to break down when evening temperatures are above about 95 degrees Fahrenheit. This has been the case for several weeks in a row in Phoenix where temperatures have rarely dropped below 100. Under these conditions, saguaros and other succulents are not photosynthesizing efficiently and may be building up waste products in their cells. Luckily this has not been the case in the Tucson area. Even through this extreme heat our evening temperatures have tended to drop below 95 degrees. 

Prolonged periods of extreme heat and drought that impact photosynthesis and weaken tissue will provide a vector for other issues like Erwinia bacteria to make an appearance. Additionally, increasing irrigation to these plants may do more harm than good if photosynthesis isn’t occurring efficiently. Moisture builds up in the cells of the plant and causes them to drop heavy limbs or even collapse.  

Looking to the Future

Even outside of urban areas, increased average temperatures combined with intense heat waves and a lack of rainfall will likely spell trouble for saguaros and other desert plants. The nature of these impacts will vary by species and location, so it is important that we begin monitoring and protecting these species now.  

With that said, at the Arizona-Sonora Desert Museum we are currently not observing the phenomena of sudden cactus collapse or limb dropping, nor are we finding obvious signs of damage on large saguaros. It is certainly the case that extreme heat may be negatively impacting our large saguaros, but the types of symptoms that are showing up in Phoenix are not currently appearing at the Desert Museum or in the Tucson Mountains. Of course, if summers like 2020 and 2023 are the new normal, and if even hotter and drier summers can be expected to occur regularly, then we may begin to see this type of damage on grounds here at the Desert Museum.  

Saguaro Recruitment

As we plan for the impacts of climate change on our Museum and the surrounding environment, our focus is less on what this will mean for large saguaros, which have a substantial amount of water holding capacity and resiliency, and more on seedling saguaros. Plants that have germinated in the last 5, 10, or even 20 years and have not built up significant water storage capacity will be much more susceptible to both year-to-year variation and long-term climactic trends. Dessication and death of young saguaros will be exacerbated by possible die-off of important nurse plants like palo verdes, ragweeds, and mesquites that lack the capacity of succulents to store significant amounts of moisture to draw on during periods of drought.  

From our perspective, impacts to saguaro recruitment is the most concerning phenomenon we have observed. Over time it is possible that saguaro populations may be increasingly less able to recruit new generations and a long-term decline in numbers of these plants may occur in our area. 

Climate Change and the Future of Desert Plants

The Sonoran Desert is in the midst of a long-term drought marked by a vicious cycle of unreliable rainfall, increased temperatures, and faster loss of moisture because of the higher temperatures. Though our desert plants are adapted to drought, some of them may reach a point of no return, and our ecosystems may become less productive due to the extreme conditions.  

It is apparent that a primary issue is increasing urbanization and habitat fragmentation in our region, which increases the pace at which climactic change is taking place. A move away from the current layout and materials used in our cities and a shift towards more sustainable materials and practices will be essential if we want to show a true commitment to protecting our iconic Sonoran Desert species such as saguaros.  

Recently, a powerful storm near the Desert Museum decimated many mature saguaros. The damage may be attributed to a microburst, straight line winds, or perhaps tornado-like conditions. Violent and unpredictable storms like this seem to be increasingly frequent (though not entirely out of the ordinary) in our region, most likely due to climate change. This may prove to be yet another threat to saguaro populations—especially in isolated pockets.

For the time being, the Arizona-Sonora Desert Museum will continue to pursue our mission of conservation and education while monitoring our plants so that we can accurately report issues as we see them arising and investigate best practices for mitigating damage resulting from climate change.