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Increasing temperatures, worsening drought pose threat to saguaros

Desert Cactus Landscape
Posted at 4:48 PM, May 21, 2021
and last updated 2021-06-23 16:04:39-04

ARIZONA — It’s a symbol of Arizona. The saguaro cactus is everywhere.

It beautifully dots our open deserts and stands tall in our city landscapes.

Adapted to withstand heat and drought, saguaros expand and contract like an accordion to store enough water to make it through extended periods without rain.

But the drought Arizona is in now, has been too long and too extreme for some saguaros to handle.

“Everybody thinks of cacti as these great desert plants, but if you desiccate a cactus it shrivels up and dies. They are very good at using very small amounts of water, but if there’s no water for a long period of time, they don’t do so well,” says Larry Venable with Desert Laboratory on Tumamoc Hill in Tucson.

Cactus Drone Gif

Saguaros are about 90% water, so if they get too dehydrated, their structure breaks down and they can collapse.

That’s what happened in the summer of 2020. It was one of the hottest and driest summers on record in Phoenix and many saguaros were left lying dead in our city streets.

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Venable says those saguaros in our urban landscapes are even more vulnerable because their roots are damaged when they’re replanted there. That makes it hard for the plant to get water and oxygen, causing it to slowly decay.

The urban heat island effect is another factor that’s making it more difficult for saguaros to survive in Phoenix.

“The biggest concerns are in the more extreme habitats for the saguaros where it is hotter and drier. We may lose those populations in the western deserts and potentially in the cities where there is a heat effect,” says Don Swann at Saguaro National Park.

That heat island effect means oftentimes in the summer, Phoenix temperatures only drop to the upper 80s or low 90s and that’s not cool enough for a saguaro’s metabolism to work efficiently.

Unlike most plants, saguaros (and other cacti) open tiny pores on their skin at night when temperatures are cooler to take in carbon dioxide. But with these warmer nighttime temperatures we’re now seeing, the amount of carbon dioxide the plant can take in is reduced, so it has a harder time growing and thriving.

So as Phoenix continues to get hotter and drier, will saguaros be able to survive here?

The short answer is, experts just don’t know yet.

Saguaros operate on a much longer timescale than we do with a lifespan between 150 and 200 years. So, we are watching their growth and decline in very slow motion.

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Venable says, “I think there was a Star Trek episode where they ran into an alien species and a second for us was like a year for them. They had to figure out how to communicate with them. Well, a second for a saguaro is like a year for us. And so what’s going on? Are saguaros going away? They are just kind of sitting there and they do for years and years and years.”

With these longer timescales, it has taken generations of scientists working together to understand the life of a saguaro and it will take generations into the future to fully understand how climate change is impacting them.

But, according to Climate Central, we are already seeing changes in their habitat as Arizona becomes a hotter and drier place.

Since the founding of the National Park Service in 1916, the area covering Saguaro National Park has seen an average increase in temperature of 3.5 degrees Fahrenheit.

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That makes it one of the fastest-warming national parks across the country.

Climate projections show Saguaro National Park could bake under nearly four months of triple-digit temperatures a year by 2100.

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Don Swann at Saguaro National Park says they are concerned, but not yet alarmed, about the future of saguaros in this hotter and drier climate.

“Since the mid-1990s, we’ve been in an extended drought. The drought doesn’t tend to affect large saguaros as much as it does the really young saguaros. What we are seeing is very low survival of the small saguaros. They are not entering the population because they are dying before they get large enough to really store water and live during a drought,” says Swann.

Tania Hernandez at the Desert Botanical Garden says the most difficult part in the life of a saguaro is seedling establishment in the soil.

“These little babies are very, very vulnerable,” she explains. “They need very particular conditions to germinate and establish. They need nurse plants like a small tree or bush. That shade these plants provide and the soil underneath them provides excellent conditions for saguaros to establish. But if these plants are affected by climate change, we’re definitely going to see less and less plants establishing, and in many, many years the adult plants are going to die and we’re not going to have new plants to replace them.”

That means saguaros may be at risk in the next century in Phoenix, where we have an urban heat island effect that makes it even more difficult for them to establish and grow.

So, what can we do to ensure their survival?

One solution may be seed harvesting.

“We could try germinating in a greenhouse with a lot of care and then try to introduce them again into the wild,” Hernandez says.

The Desert Botanical Garden has a seed bank with thousands of desert plant species, including the saguaro. They say, “although seed banking is no substitute for protecting the habitat of these plants, the preservation of their seeds acts as an insurance policy to prevent extinction.”

Hernandez also points out that homeowners should be watering their saguaros during periods of drought. Not more often, but more deeply so water gets into the soil and reaches the roots. Make a well around the base of the saguaro and run a hose with a trickle of water for four to six hours once a month.

The other wildcard in all of this is wildfires.

Climate change is causing more frequent and destructive wildfires all across the western U.S. and saguaros are increasingly threatened by them due to invasive species like buffelgrass.

“Fires have been moving downslope more into the desert and those fires have been killing saguaros,” says Swann. “So that’s an immediate concern, and I think one of the things that we can all do right now is to try to reduce the spread of these invasive grasses.”

According to the United States Geological Survey, buffelgrass was brought to Arizona in the 1930s for erosion control, but it started rapidly spreading across our deserts in the 1980s.

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Not only does it steal water from our native desert plants, like saguaros, but it fills in the gaps in our desert landscape, allowing wildfires to spread quickly.

As part of the “Save our Saguaros” effort, the National Parks Service has hosted monthly buffelgrass pulls. Volunteers show up from all over the state to help rid our deserts of this invasive species.

It’s important work, according to Swann, who thinks that wildfires top extreme heat and drought as the biggest immediate threat to our saguaro population across Arizona.