Tracking The Saguaro Cacti Decline

IRA FLATOW: One of the most iconic symbols of the Southwest is the saguaro cactus. You know the one. The big, tall cactus with branching arms that has a “G” in its name that you don’t pronounce. Well, the saguaro are the most studied variety of cactus, and yet there’s still much we don’t know about them.

Once a decade, researchers from the University of Arizona survey plots of roughly 4,500 saguaro to assess the health of the species. And this year, there was a record low number of new cacti growing, the fewest since they started decadal surveys in 1964.

Wow. What’s driving this decline? Joining me now to talk more about the state of the saguaro cacti and their role in the Sonoran Desert ecosystem is my guest Peter Breslin, postdoctoral researcher at the University of Arizona’s Desert Laboratory in Tumamoc Hill, based in Tucson.

Welcome to Science Friday.

PETER BRESLIN: Thanks, Ira. It’s great to be here.

IRA FLATOW: Tell us about that survey. Were you surprised at what you found?

PETER BRESLIN: We were surprised, especially by, really, the very low number of small or new recruit– we call them new recruits like they’re new to the population. So the small saguaros, really, there were only 33 that we were able to find. And that’s out of a total population of more than 4,000 saguaro plants.

So we thought, because of the drought and because of other factors, such as changing climate, that there would be very few new saguaros. The whole Sonoran Desert runs on a couple of different cycles of precipitation– summer, we call them the monsoons, and then winter, rain. So it’s a biseasonal rain pattern. And that has been disturbed over the past 20 years or so.

And when you put that together with the drought, you get conditions that probably just are not conducive for the saguaros to establish.

IRA FLATOW: Is there any invasive species that might be driving them out?

PETER BRESLIN: Yes. It seems as if the colonization by non-native grass– it’s called buffelgrass– is excluding a lot of new saguaro recruitment in the population. Because what we’re finding is a very strong negative correlation between the buffelgrass colonization and where the new saguaros are. They’re not colonizing where the buffelgrass has started to take over the landscape on the Hill.

IRA FLATOW: Now, the population is not evenly distributed, right, across the landscape? Do you know why some places are better than others for the cacti to thrive?

PETER BRESLIN: Well, that is the $64,000 question. The current research project that we’re doing– and the publication that we hope to get out of it– is exactly to answer that question. Why are saguaros establishing where they are? And where are the conditions where the saguaros are avoiding establishment also?

And our hope is that we can take that information and make it useful for people who are doing saguaro restoration projects and other management, such as the folks at Saguaro National Park.

IRA FLATOW: I understand that they have a really interesting growth pattern. Tell me more about how saguaros grow.

PETER BRESLIN: I really just find it fascinating that their life strategy, so to speak, is to grow very, very slowly for the first 10 to 20 years of their establishment on the landscape. They really are in no hurry whatsoever. Sometimes as slow as a centimeter a year of new growth in their protected habitat.

And then, once they get to a certain height– sometimes when they’re above a vegetation canopy and they get more sun or other times, just apparently randomly, they really take off and they grow a lot more quickly. And then they reach flowering height, maybe they start to put arms out– and keep in mind, this is after about 70 years a lot of the time– they slow way down. It’s not a linear growth pattern. It’s got this growth spurt that happens for a few decades there, and then they slow down again.

IRA FLATOW: Yeah. How long did they live, in general?

PETER BRESLIN: It varies, of course, depending on all kinds of things. But the amazing thing about saguaros is that they live to usually about 150 years old or 200 years old. And so we humans, we have a 60-year data set from these repeat surveys on Tuma. We call it a long-term data set. But 60 years, something that germinated in 1964 might not even be flowering yet 60 years later. And so it’s not even half the lifespan of a typical saguaro.

So we try to think in saguaro time, not human time, when we’re trying to understand these plants.

IRA FLATOW: When some plants go away or they die, there’s an effort to replant them. Can you replant more cacti?

PETER BRESLIN: You can. It’s a difficult restoration feat to accomplish. I know that the Tucson Audubon Society is sponsoring a major replanting of saguaros in one of the big fire areas near Tucson. And it’s an awesome project. We’re all very excited about it. I’ll be interested to see the survivorship after 10, 20 years, if I’m still around.

IRA FLATOW: When you say awesome, what kind of scale are you talking about?

PETER BRESLIN: I think the current plan is to put 4,000 saguaros out into this burn area. And one of the weird things about it, too, is that they’re actually having difficulty obtaining that many saguaros from cultivation. There’s an unexpected saguaro gap from local nurseries or other sources.

IRA FLATOW: So how dire is the situation here in terms of the future of the saguaro cacti?

PETER BRESLIN: Well, I’m not sure. If you ask a saguaro, they might say, no, I’m fine.

[LAUGHTER]

They’re very resilient. This is a desert-adapted species, and they’re really tough. So you have decades and centuries of adaptation. On the other hand, we have a lot of rapid change going on in precipitation, temperature, the buffelgrass situation. And the rapid change comes up against the slow pace of the Sonoran Desert.

So we just need to stay vigilant, I believe. Over the next 30 to 50 years especially, we’ll have a much clearer picture of what’s coming down the pike.

IRA FLATOW: Wow. That’s very interesting news. Thank you for sharing it with us.

PETER BRESLIN: Sure. Thank you for having me.

IRA FLATOW: You’re welcome. Peter Breslin, postdoctoral researcher, University of Arizona’s Desert Laboratory in Tumamoc Hill, based in Tucson, Arizona.

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