The Iconic Saguaro Cactus Across its Landscape

Abstract

The saguaro cactus (Carnegiea gigantea) is a long-lived columnar cactus that is among the most well-studied plants in the world. Long-term research indicates that saguaro establishment is generally episodic and strongly influenced by precipitation and temperature. Drought conditions can reduce survivorship of recently germinated saguaros up to 100%. Long-term drought will likely cause saguaro populations to decline as older or vulnerable saguaros die without new cohorts replacing them. However, the relative importance of local habitat variability and the coinciding drought impacts on establishing populations remain largely unex-plored. We modeled saguaro establishment in response to drought in various habitat and soil types at Saguaro National Park. Recent, severe drought coincided with drastic declines in saguaro establishment that has seen few saguaros establishing in recent years. Overall, saguaro establishment was best explained by the interaction of drought and habitat type. The best model identified bajada and foothill plots as responding somewhat similarly to drought regardless of severity but foothill plots outperformed bajada plots regardless of drought severity. Furthermore, the predicted number of saguaros to establish in bajada or foothill plots dropped to near zero under the most severe drought but remained higher in slope plots, suggesting that the most suitable habitat type for establishing sagua-ros has shifted during the recent drought. These results reveal that saguaro establishment strongly correlates with drought but that the impact of drought varies with local habitats and their associated physical characteristics. Overall, our study shows that incorporating the range of local variability in how plants may respond to past climate conditions is important for predicting future response to climate change. The following image gallery is meant to illustrate the demographic complexity of saguaros and how the interactive dynamics of desert climate with landscape position influence population regeneration trends in Saguaro National Park. All photographs in this section are provided by authors of papers in our scientific journals and are used by permission.

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VOLUME 99 • NUMBER 3 • JULY 2018
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© 2018 The Authors. The Bulletin of the Ecological Society of America, published by Wiley Periodicals, Inc., on behalf of the Ecological Society of America.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
July 2018 329Photo Gallery
THE ICONIC SAGUARO CACTUS ACROSS ITS LANDSCAPE
D. E. Winkler , J. L. Conver , T. E. Huxman, and D. E. Swann
PHOTO GALLERY
The saguaro cactus (
Carnegiea gigantea
) is a long- lived columnar cactus that
is among the most well- studied plants in the world. Long- term research indicates
that saguaro establishment is generally episodic and strongly influenced by precip-
itation and temperature. Drought conditions can reduce survivorship of recently
germinated saguaros up to 100%. Long- term drought will likely cause saguaro
populations to decline as older or vulnerable saguaros die without new cohorts
replacing them. However, the relative importance of local habitat variability and
the coinciding drought impacts on establishing populations remain largely unex-
plored. We modeled saguaro establishment in response to drought in various
habitat and soil types at Saguaro National Park. Recent, severe drought coincided
with drastic declines in saguaro establishment that has seen few saguaros estab-
lishing in recent years. Overall, saguaro establishment was best explained by the
interaction of drought and habitat type. The best model identified bajada and
foothill plots as responding somewhat similarly to drought regardless of sever-
ity but foothill plots outperformed bajada plots regardless of drought severity.
Furthermore, the predicted number of saguaros to establish in bajada or foothill
plots dropped to near zero under the most severe drought but remained higher
in slope plots, suggesting that the most suitable habitat type for establishing sagua-
ros has shifted during the recent drought. These results reveal that saguaro estab-
lishment strongly correlates with drought but that the impact of drought varies
with local habitats and their associated physical characteristics. Overall, our study
shows that incorporating the range of local variability in how plants may respond
to past climate conditions is important for predicting future response to climate
change. The following image gallery is meant to illustrate the demographic com-
plexity of saguaros and how the interactive dynamics of desert climate with land-
scape position influence population regeneration trends in Saguaro National Park.
All photographs in this section are provided
by authors of papers in our scientific
journals and are used by permission.
All copyrights reserved.

Photo Gallery
330 Bulletin of the Ecological Society of America, 99(3)
Photo 1. Saguaro National Park’s landscape spans at, sandy bajadas to rolling alluvial foothills
(pictured above in the Rincon Mountain District of the Park) to steep, rocky slopes higher in the
mountains. All of these habitats are home to the iconic saguaro cactus (
Carnegiea gigantea
). Photo by
D. E. Winkler.

July 2018 331Photo Gallery
Photo 2. Adult saguaro can reach heights of 18 m, easier to spot among the Sonoran Desert’s
landscapes than recently established, juvenile saguaros that remain <0.1 m for the rst ca. 10 years of
life. Photo by D. E. Swann.

Photo Gallery
332 Bulletin of the Ecological Society of America, 99(3)
Photo 3. The effects of prolonged drought on saguaro populations have led to declines in establishment
at lower elevations. Instead, saguaros have been establishing (though still rarely) at higher elevations
where the terrain is rocky and microclimates have lower evaporative demand and water may remain
longer. Photo by J. L. Conver.

July 2018 333Photo Gallery
Photo 4. Similar aged saguaros can often be found in groups where nurse trees once stood. Nurse
debris often remains in place for years after the nurse died. The exact mechanisms that facilitate the
nurse–protégé relationship between saguaros and their nurses remain poorly understood. Photo by D.
E. Winkler.

Photo Gallery
334 Bulletin of the Ecological Society of America, 99(3)
Photo 5. Another example of a saguaro grouping around their dead nurse mesquite tree (
Prosopis
velutina
). Creosote bush (
Larrea tridentate
) and shhook barrel cactus (
Ferocactus wislizenii
) are also
pictured. Photo by D. E. Winkler.

July 2018 335Photo Gallery
Photo 6. Saguaros appear opportunistic, growing on cliffs and into bedrock cracks by taking advantage
of limited water and nutrient resources likely present in these microclimates as pictured above. Photo
by D. E. Swann.

Photo Gallery
336 Bulletin of the Ecological Society of America, 99(3)
Photo 7. Although saguaro can be found in many microclimates across Saguaro National Park,
they are most often found in upland areas where trees likely manipulate the water table by accessing
groundwater. Photo by D. E. Winkler.

July 2018 337Photo Gallery
Photo 8. The controls on saguaro phenology are thought to be primarily determined by winter
precipitation. That said, these prolic seed producers are capable of producing ca. 10 million seeds per
year but the chances of one seed establishing into a mature plant are incredibly low. Photo by D. E.
Winkler.

Photo Gallery
338 Bulletin of the Ecological Society of America, 99(3)
Photo 9. Saguaro spines (as well as their height) can be used to age plants but their primary function
is to protect them from animal damage. In doing so, saguaros can store water in their eshy ribs when
precipitation is plentiful. Photo by J. L. Conver.

July 2018 339Photo Gallery
Photo 10. Saguaros are so long- lived (>150 years) that the individuals may be slow to respond to
climate change. Population regeneration has declined and will likely continue to do so but, as is the
nature of this species with historically episodic recruitment, it remains to be seen when the next good,
wet year for reestablishment might be. Photo by J. L. Conver.
These photographs illustrate the article “The interaction of drought and habitat explain space–
time patterns of establishment in saguaro (
Carnegiea gigantea
)” by Winkler et al., published in
Ecology
99(3), https://doi.org/10.1002/ecy.2124