A downdraft strikes the desert
The summer storm crossed the western part of the park in the Tucson Mountains. Recorded conditions and radar images indicated a microburst, an intense local downdraft that spreads outwards on reaching the ground. Rapid temperature changes, heavy rainfall and divergent airflow occurred around the damaged saguaros. This flow can load plants intensely within a short time and over a limited area. [Swann et al. 2026]
Data came from a weather station close to the study area and from weather radar. They supported identification of the storm type and direction. Plant damage supplied another line of evidence. Many uprooted saguaros lay in a similar direction, consistent with the storm’s overall movement. Agreement in timing, location and the pattern of fallen trunks linked the weather event to the observed losses. [Swann et al. 2026]
Two surveys reveal the scale of losses
Don Swann’s team began the inventory within days of the storm. Observers walked parallel transects, mapped plants and distinguished uprooting, stem breakage and damage to living individuals. A repeat complete survey was performed in early 2025. The surveys covered a total of 473 ha within the park and on accessible neighbouring private land. [Swann et al. 2026]
The first survey recorded 804 dead saguaros and 214 damaged living plants. Later work added 985 dead and 323 damaged individuals. The combined inventory therefore contained 1789 dead and 537 damaged plants. These are the totals documented within the surveyed area. The full storm footprint extended beyond the accessible sites. [Swann et al. 2026]
Height changes mechanical loading
Large, branched saguaros expose a substantial surface to moving air. Tall, mature plants were particularly affected in this study. To assess the proportion lost, the authors used additional randomly placed plots within the storm area and control plots outside it. Adult mortality was higher in the affected plots than in the controls. [Swann et al. 2026]
The authors related the vulnerability of large plants to wind loading and branch geometry. Root anchorage also depends on the substrate supporting the heavy trunk. Severe damage occurred on relatively deep soils at the mountain base, whereas nearby rocky slopes were less affected. The suggestion that rock crevices improve anchorage remains an explanation requiring direct mechanical tests. [Swann et al. 2026]
Damage can precede death
In 2023, observers recorded leaning plants and crescent-shaped soil fissures at stem bases. These signs accompanied damage to large lateral roots. Some individuals fell later. The repeat survey also found many stems that had died while standing and subsequently snapped. The initial inspection therefore captured only part of the eventual toll. [Swann et al. 2026]
Swann and colleagues proposed that root injury could weaken both anchorage and water uptake. A plant able to draw on its stem reserves for some time could then die gradually. The data confirm that some losses occurred later, while the contribution of this mechanism remains a hypothesis. The authors also considered other possible factors, including drought and lightning damage. [Swann et al. 2026]
A rare event in a long-lived plant’s life
The storm matters because adult plants are replaced slowly. Saguaro enlarges its stem over many years, and successful establishment of new individuals occurs episodically. Losing a tall, flowering plant also reduces the number of tips producing flowers and fruits. Swann’s findings add intense wind and the later consequences of injury to the disturbances shaping population dynamics. [Swann et al. 2026] [Drezner 2014] [Orum et al. 2016]
The study concerns one event in a particular habitat. Its results support including similar storms in monitoring and demographic models. Establishing their frequency and effects across the species’ range requires data from additional places and years. Repeated inspection of damaged plants also separates immediate breakage from losses that emerge over a longer period. [Swann et al. 2026]
Mass mortality of saguaros (Carnegiea gigantea) from windthrow: A rare, demographic disturbance in a desert ecosystem
Swann D. E., Badinger L., Winkler D. E., Davis A. L., Mass mortality of saguaros (Carnegiea gigantea) from windthrow: A rare, demographic disturbance in a desert ecosystem, Ecosphere, 2026, 17(5):e70651, doi: 10.1002/ecs2.70651
An inventory of damage following the 22 August 2023 local downburst in Saguaro National Park and adjoining land. A repeat 2025 survey revealed many additional deaths. The authors analyse plant size, damage type and fall direction, and propose root injury as a hypothesis for delayed mortality.
The keystone saguaro (Carnegiea gigantea, Cactaceae): a review of its ecology, associations, reproduction, limits, and demographics
Drezner T. D., The keystone saguaro (Carnegiea gigantea, Cactaceae): a review of its ecology, associations, reproduction, limits, and demographics, Plant Ecology, 2014, 215(6):581-595, doi: 10.1007/s11258-014-0326-y
A review of saguaro ecology, nurse plants, variation in growth and reproductive maturity, population regeneration and interactions with Sonoran Desert animals. It synthesizes an extensive literature and emphasizes local conditions when interpreting age and population structure.
Saguaro (Carnegiea gigantea) Mortality and Population Regeneration in the Cactus Forest of Saguaro National Park: Seventy-Five Years and Counting
Orum T. V., Ferguson N., Mihail J. D., Saguaro (Carnegiea gigantea) Mortality and Population Regeneration in the Cactus Forest of Saguaro National Park: Seventy-Five Years and Counting, PLoS ONE, 2016, 11(8):e0160899, doi: 10.1371/journal.pone.0160899
Annual observations on six 4-ha plots during 1942–2016, except 1955. The authors describe episodic saguaro regeneration, age- and cover-related survival, and changes in height structure. Germination years of small plants were estimated from local growth data.