The saguaro cactus (Carnegiea gigantea) is a keystone species that grows only in the Sonoran Desert of northern Mexico and the southwestern United States. The USGS’ Southwest Biological Science Center is working to better understand the species’ biological features, its role as a keystone species that supports greater than 100 other plant and animal species, and its responses to climate change across the Sonoran Desert.
Saguaros in a changing climate
Research on saguaros over the past century has revealed much about the plant’s growth, reproduction, populations. Saguaro National Park was established (1933) and expanded (1995) to “preserve the exceptional growth thereon of various species of cacti and the prime Sonoran desert habitat that the saguaro lives in.” Saguaros provide habitat for sensitive cactus ferruginous pygmy-owl and endangered lesser long-nosed bat. Due to the integrated threat of climate change and invasive grasses, Saguaro National Park was named one of the country’s 25 most imperiled national parks in 2009.
A 2018 study examined saguaro population regeneration across the National Park and its variable terrain that stretches from low-elevation sandy bajadas in the valleys to higher-elevation rocky slopes (pictured right). The study revealed that saguaro establishment park-wide is strongly positively correlated with the Palmer Drought Severity Index (PDSI), a standardized index of dryness that spans from -6 (dry) to +6 (wet). In other words, when the index is less than -2, few saguaro seedlings endure. Saguaro establishment surged during the period of high annual precipitation in the 1980s but has slowed since, coinciding with a prolonged drought. In the current drought, lower precipitation has been exacerbated by higher temperatures that dry soils and may prevent younger plants from reaching a size where they are able to store sufficient water to survive. Interestingly, the 2018 study also found that some saguaros have established in recent years in very rocky areas, suggesting that they may be able to take advantage of water captured in rock cracks.
Read the full study and National Park Service report:
Winkler, D. E., Conver, J. L., Huxman, T. E., and Swann, D. E. 2018, The interaction of drought and habitat explain space–time patterns of establishment in saguaro (Carnegiea gigantea): Ecology, v. 99 no. 3, p. 621-631, https://doi.org/10.1002/ecy.2124
Swann, D.E., Winkler, D.E., Conver, J.L., and Foley, T. 2021, Climate change and other factors influencing the saguaro cactus: Intermountain Park Science, https://www.nps.gov/articles/000/climate-change-and-other-factors-influencing-the-saguaro-cactus.htm
Saguaro flowers move with the season
Saguaros produce large, white, fragrant flowers near their crown during April–June, which bloom at night and close the following day.
Over a four-year period, Research Ecologist Daniel Winkler at the Southwest Biological Science Center, in partnership with Saguaro National Park, used historic and current data collected by National Park Service staff, USGS scientists, and community scientist volunteers to understand a phenomenon that was first observed in 1924, but not described, in which saguaro cacti produce flowers in a radial pattern. Their findings revealed that blooms emerge in the east at the beginning of their reproductive season, and continue to flower in a counterclockwise pattern around their stems — the only known flowering plant to do so.
A key component to collecting data was the use of a collapsible flag pole (referred to as the "world's largest selfie stick"; pictured left) to photograph flowers throughout the spring.
This research team will now study the full, 5-year dataset to examine the environmental triggers that initiate flowering and fruiting. The team will also identify potential influences from future climate change that may lead to impacts on pollinators and the many other species dependent on saguaro fruits for their own survival.
Read the full study: Foley, T., Swann, D.E., Sotelo, G., Perkins, N., and Winkler, D.E., 2021, Asynchronous flowering patterns in saguaro cacti (Carnegiea gigantea): Ecosphere, v. 12, no. 12, e03873, p. 1-14, https://doi.org/10.1002/ecs2.3873.
Saguaros as a keystone species in water-limited systems
The impacts of climate change on the iconic saguaro cactus remain poorly understood but climate is known to directly affect regeneration and establishment, and probably explains the reduced recruitment of saguaros in the park during the past two decades. Saguaros are well-known for having “nurse trees”; that is, for being part of a nurse-protégé relationship with desert trees that provides protection from extreme heat and cold (see photo right). The relationship is poorly understood, but it seems possible that climate change could have indirect effects on saguaros by altering the nurse-protégé relationship.
Scientists at the Southwest Biological Science Center are actively working with biologists at Saguaro National Park and the Sonoran Desert Network to uncover the relationships between saguaros and their nurses. Preliminary evidence suggests that saguaros shift their water-use strategies throughout the year and may benefit from water passed from nurse tree roots into shallow surface soils during periods of drought through a process called hydraulic redistribution.
The necessity of understanding the influence of environmental drivers on saguaro distributions is nothing new; scientists have been studying the species and its complicated relationship with water for more than a century. The USGS’ collaboration with Saguaro National Park will likely produce a greater understanding of this iconic species and the fascinating ways it interacts and persists in water-limited Sonoran Desert ecosystems.
Landscape-scale perspectives in most areas of the National Park illustrate the importance of nurse trees, especially at lower elevation in the park. Foothills paloverde (Parkinsonia microphylla) and velvet mesquite (Prosopis velutina) trees act as 'nurse trees' to young saguaros, providing protection from cold in winters and shade in summer, thereby enabling a stronger chance of saguaro survival during early growth. For example, according to NPS and USGS research in the park, in 1935 paloverde and mesquite trees were scarce since they were harvested during that time period. Repeat photography approximately 75 years later in 2010 at the same location shows the return of those tree species.
For more information about the Southwest Repeat Photography Collection, see: https://www.usgs.gov/centers/southwest-biological-science-center/science/southwest-repeat-photography-collection
All USGS and non-USGS funded publications that relate to this research:
Foley, T., Swann, D.E., Sotelo, G., Perkins, N., and Winkler, D.E., 2021, Asynchronous flowering patterns in saguaro cacti (Carnegiea gigantea): Ecosphere, v. 12, no. 12, e03873, p. 1-14, https://doi.org/10.1002/ecs2.3873.
Swann, D.E., Winkler, D.E., Conver, J.L., and Foley, T. 2021, Climate change and other factors influencing the saguaro cactus: Intermountain Park Science, https://www.nps.gov/articles/000/climate-change-and-other-factors-influencing-the-saguaro-cactus.htm.
Winkler, D., and Brooks, E., 2020, Tracing extremes across iconic desert landscapes—Socio-ecological and cultural responses to climate change, water scarcity, and wildflower superblooms: Human Ecology, v. 48, no. 2, p. 211-223, https://doi.org/10.1007/s10745-020-00145-5.
Mohan, J.E., Wadgymar, S.M., Winkler, D.E., Anderson, J.T., Frankson, P.T., Hannifin, R., Benavides, K., Kueppers, L.M., and Melillo, J.M., 2019, Plant reproductive fitness and phenology responses to climate warming: Results from native populations, communities, and ecosystems—Chapter 3, in Mohan, J.E., ed., Ecosystem Consequences of Soil Warming—Microbes, Vegetation, Fauna and Soil Biogeochemistry: Academic Press, p. 61-102, https://doi.org/10.1016/B978-0-12-813493-1.00004-1.
Rodríguez‐Buriticá, S., Winkler, D.E., Webb, R.H., and Venable, D.L., 2019, Local temporal trajectories explain population‐level responses to climate change in saguaro (Carnegiea gigantea): Ecosphere, v. 10, no. 8, e02844, https://doi.org/10.1002/ecs2.2844.
Winkler, D.E., Conver, J.L., Huxman, T.E., and Swann, D.E., 2018, The iconic saguaro cactus across its landscape: Bulletin of the Ecological Society of America, v. 99, no. 3, p. 329-339, https://doi.org/10.1002/bes2.1401.
Winkler, D.E., Conver, J.L., Huxman, T.E., and Swann, D.E. 2018, The interaction of drought and habitat explain space–time patterns of establishment in saguaro (Carnegiea gigantea): Ecology, v. 99 no. 3, p. 621-631, https://doi.org/10.1002/ecy.2124.
Winkler, D.E., Conver, J.L., Huxman, T.E., and Swann, D.E., 2017, The interaction of drought and habitat explain space–time patterns of establishment in saguaro (Carnegiea gigantea): Ecology, v. 99, no. 3, p. 621-631, https://doi.org/10.1002/ecy.2124.
Conver, J.L., Foley, T., Winkler, D.E., and Swann, D.E., 2017, Demographic changes over >70 yr in a population of saguaro cacti (Carnegiea gigantea) in the northern Sonoran Desert: Journal of Arid Environments, v. 139, p. 41-48, https://doi.org/10.1016/j.jaridenv.2016.12.008.
- Overview
The saguaro cactus (Carnegiea gigantea) is a keystone species that grows only in the Sonoran Desert of northern Mexico and the southwestern United States. The USGS’ Southwest Biological Science Center is working to better understand the species’ biological features, its role as a keystone species that supports greater than 100 other plant and animal species, and its responses to climate change across the Sonoran Desert.
Saguaros in a changing climate
Sources/Usage: Public Domain.Steep, rocky slopes in Saguaro National Park near Tucson, AZ provide habitat for saguaros and may buffer populations from the negative effects of drought. Photo courtesy of Daniel Winkler, USGS SBSC. Research on saguaros over the past century has revealed much about the plant’s growth, reproduction, populations. Saguaro National Park was established (1933) and expanded (1995) to “preserve the exceptional growth thereon of various species of cacti and the prime Sonoran desert habitat that the saguaro lives in.” Saguaros provide habitat for sensitive cactus ferruginous pygmy-owl and endangered lesser long-nosed bat. Due to the integrated threat of climate change and invasive grasses, Saguaro National Park was named one of the country’s 25 most imperiled national parks in 2009.
A 2018 study examined saguaro population regeneration across the National Park and its variable terrain that stretches from low-elevation sandy bajadas in the valleys to higher-elevation rocky slopes (pictured right). The study revealed that saguaro establishment park-wide is strongly positively correlated with the Palmer Drought Severity Index (PDSI), a standardized index of dryness that spans from -6 (dry) to +6 (wet). In other words, when the index is less than -2, few saguaro seedlings endure. Saguaro establishment surged during the period of high annual precipitation in the 1980s but has slowed since, coinciding with a prolonged drought. In the current drought, lower precipitation has been exacerbated by higher temperatures that dry soils and may prevent younger plants from reaching a size where they are able to store sufficient water to survive. Interestingly, the 2018 study also found that some saguaros have established in recent years in very rocky areas, suggesting that they may be able to take advantage of water captured in rock cracks.
Read the full study and National Park Service report:
Winkler, D. E., Conver, J. L., Huxman, T. E., and Swann, D. E. 2018, The interaction of drought and habitat explain space–time patterns of establishment in saguaro (Carnegiea gigantea): Ecology, v. 99 no. 3, p. 621-631, https://doi.org/10.1002/ecy.2124
Swann, D.E., Winkler, D.E., Conver, J.L., and Foley, T. 2021, Climate change and other factors influencing the saguaro cactus: Intermountain Park Science, https://www.nps.gov/articles/000/climate-change-and-other-factors-influencing-the-saguaro-cactus.htm
Saguaro flowers move with the season
Saguaros produce large, white, fragrant flowers near their crown during April–June, which bloom at night and close the following day.
Sources/Usage: Public Domain.Southwest Biological Science center research ecologist Daniel Winkler photographs the top of a saguaro using the "world's largest selfie stick" to collect phenology data for a study that examined saguaro flowering patterns. Over a four-year period, Research Ecologist Daniel Winkler at the Southwest Biological Science Center, in partnership with Saguaro National Park, used historic and current data collected by National Park Service staff, USGS scientists, and community scientist volunteers to understand a phenomenon that was first observed in 1924, but not described, in which saguaro cacti produce flowers in a radial pattern. Their findings revealed that blooms emerge in the east at the beginning of their reproductive season, and continue to flower in a counterclockwise pattern around their stems — the only known flowering plant to do so.
A key component to collecting data was the use of a collapsible flag pole (referred to as the "world's largest selfie stick"; pictured left) to photograph flowers throughout the spring.
This research team will now study the full, 5-year dataset to examine the environmental triggers that initiate flowering and fruiting. The team will also identify potential influences from future climate change that may lead to impacts on pollinators and the many other species dependent on saguaro fruits for their own survival.
Read the full study: Foley, T., Swann, D.E., Sotelo, G., Perkins, N., and Winkler, D.E., 2021, Asynchronous flowering patterns in saguaro cacti (Carnegiea gigantea): Ecosphere, v. 12, no. 12, e03873, p. 1-14, https://doi.org/10.1002/ecs2.3873.
Saguaros as a keystone species in water-limited systems
Sources/Usage: Public Domain.Similar aged saguaros can often be found in groups where nurse trees once stood. Nurse debris remains in place for years after the nurse died. Photo courtesy of Daniel Winkler, USGS SBSC. The impacts of climate change on the iconic saguaro cactus remain poorly understood but climate is known to directly affect regeneration and establishment, and probably explains the reduced recruitment of saguaros in the park during the past two decades. Saguaros are well-known for having “nurse trees”; that is, for being part of a nurse-protégé relationship with desert trees that provides protection from extreme heat and cold (see photo right). The relationship is poorly understood, but it seems possible that climate change could have indirect effects on saguaros by altering the nurse-protégé relationship.
Scientists at the Southwest Biological Science Center are actively working with biologists at Saguaro National Park and the Sonoran Desert Network to uncover the relationships between saguaros and their nurses. Preliminary evidence suggests that saguaros shift their water-use strategies throughout the year and may benefit from water passed from nurse tree roots into shallow surface soils during periods of drought through a process called hydraulic redistribution.
The necessity of understanding the influence of environmental drivers on saguaro distributions is nothing new; scientists have been studying the species and its complicated relationship with water for more than a century. The USGS’ collaboration with Saguaro National Park will likely produce a greater understanding of this iconic species and the fascinating ways it interacts and persists in water-limited Sonoran Desert ecosystems.
Sources/Usage: Public Domain.A repeat photographic series of Saguaro National Monument (now Saguaro National Park) East, Rincon Mountain District. SBSC Southwest Repeat Photography Collection stake (camera point) s0013. The earliest photo, courtesy of the National Park Service (NPS; public domain image) was taken in 1935 of the abundant 'cactus forest' of saguaros (Carnegiea gigantea). The later photograph by Dr. Raymond Turner, USGS, was taken in 2010 at this same location shows the return of those tree species.
Landscape-scale perspectives in most areas of the National Park illustrate the importance of nurse trees, especially at lower elevation in the park. Foothills paloverde (Parkinsonia microphylla) and velvet mesquite (Prosopis velutina) trees act as 'nurse trees' to young saguaros, providing protection from cold in winters and shade in summer, thereby enabling a stronger chance of saguaro survival during early growth. For example, according to NPS and USGS research in the park, in 1935 paloverde and mesquite trees were scarce since they were harvested during that time period. Repeat photography approximately 75 years later in 2010 at the same location shows the return of those tree species.
For more information about the Southwest Repeat Photography Collection, see: https://www.usgs.gov/centers/southwest-biological-science-center/science/southwest-repeat-photography-collectionAll USGS and non-USGS funded publications that relate to this research:
Foley, T., Swann, D.E., Sotelo, G., Perkins, N., and Winkler, D.E., 2021, Asynchronous flowering patterns in saguaro cacti (Carnegiea gigantea): Ecosphere, v. 12, no. 12, e03873, p. 1-14, https://doi.org/10.1002/ecs2.3873.
Swann, D.E., Winkler, D.E., Conver, J.L., and Foley, T. 2021, Climate change and other factors influencing the saguaro cactus: Intermountain Park Science, https://www.nps.gov/articles/000/climate-change-and-other-factors-influencing-the-saguaro-cactus.htm.
Winkler, D., and Brooks, E., 2020, Tracing extremes across iconic desert landscapes—Socio-ecological and cultural responses to climate change, water scarcity, and wildflower superblooms: Human Ecology, v. 48, no. 2, p. 211-223, https://doi.org/10.1007/s10745-020-00145-5.
Mohan, J.E., Wadgymar, S.M., Winkler, D.E., Anderson, J.T., Frankson, P.T., Hannifin, R., Benavides, K., Kueppers, L.M., and Melillo, J.M., 2019, Plant reproductive fitness and phenology responses to climate warming: Results from native populations, communities, and ecosystems—Chapter 3, in Mohan, J.E., ed., Ecosystem Consequences of Soil Warming—Microbes, Vegetation, Fauna and Soil Biogeochemistry: Academic Press, p. 61-102, https://doi.org/10.1016/B978-0-12-813493-1.00004-1.
Rodríguez‐Buriticá, S., Winkler, D.E., Webb, R.H., and Venable, D.L., 2019, Local temporal trajectories explain population‐level responses to climate change in saguaro (Carnegiea gigantea): Ecosphere, v. 10, no. 8, e02844, https://doi.org/10.1002/ecs2.2844.
Winkler, D.E., Conver, J.L., Huxman, T.E., and Swann, D.E., 2018, The iconic saguaro cactus across its landscape: Bulletin of the Ecological Society of America, v. 99, no. 3, p. 329-339, https://doi.org/10.1002/bes2.1401.
Winkler, D.E., Conver, J.L., Huxman, T.E., and Swann, D.E. 2018, The interaction of drought and habitat explain space–time patterns of establishment in saguaro (Carnegiea gigantea): Ecology, v. 99 no. 3, p. 621-631, https://doi.org/10.1002/ecy.2124.
Winkler, D.E., Conver, J.L., Huxman, T.E., and Swann, D.E., 2017, The interaction of drought and habitat explain space–time patterns of establishment in saguaro (Carnegiea gigantea): Ecology, v. 99, no. 3, p. 621-631, https://doi.org/10.1002/ecy.2124.
Conver, J.L., Foley, T., Winkler, D.E., and Swann, D.E., 2017, Demographic changes over >70 yr in a population of saguaro cacti (Carnegiea gigantea) in the northern Sonoran Desert: Journal of Arid Environments, v. 139, p. 41-48, https://doi.org/10.1016/j.jaridenv.2016.12.008.
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