Mapping Tumamoc Hill

By Aaryn Olsson

Sometimes you’ve got to do it the hard way in order to do it the easy way later. This winter we’re mapping buffelgrass on Tumamoc Hill the hard way by walking 50 miles back and forth across its volcanic slopes in a tight grid pattern. While many Tucsonans hike the zigzagging road up to the summit, this isn’t that kind of a hike. We’re following north-south and east-west grid lines 25 meters apart over the bulk of the 870 acre property to map the density of buffelgrass across Tumamoc Hill in 2023.

This image shows Aaryn’s tracks as he walks the east slope of Tumamoc Hill.

This ground truth map will serve a number of purposes. First, it gives an honest assessment of the current status of the buffelgrass invasion on Tumamoc Hill. Knowing the locations and densities of buffelgrass is essential for planning treatments and assessing the effectiveness of past treatments. Second, having a reliable ground truth map enables us—and you—to map buffelgrass the easy way—that is, from the comfort of your own home or office. So what’s the easy way?

Google Earth screenshot showing an aerial view of A Mountain and Tumamoc Hill.
Google Earth screenshot showing dense buffelgrass on the western slope.

Simply put, the easy way is outlining buffelgrass patches on satellite imagery or aerial photography using Google Earth. We have been experimenting with a variety of methods with very promising results and believe that almost anybody can be trained to see buffelgrass in aerial photography and estimate its density on a computer screen. However, we need a ground truth dataset for training mappers and evaluating their accuracy. Most exciting, the Tumamoc Hill ground truth dataset can be used as a launch pad for releasing a whole team of mappers to apply their training to map other areas of our imperiled saguaro-studded Sonoran Desert ecosystems.

Look for the dead palo verde tree in the foreground.
This saguaro is burnt along the length of its trunk and trinchera.
This photo shows the extent of the buffelgrass invasion on Tumamoc Hill.

While walking Tumamoc Hill, I snapped some photos, shown above. If they are alarming to you, join the club. They’re alarming to us, too. This is the likely future for large swaths of our scenic Arizona Upland (the region of the Sonoran Desert dominated by saguaros and palo verde trees). We are witnessing the transformation of these highly diverse cactus forest ecosystems into large, continuous buffelgrass monocultures. Scattered throughout the buffelgrass are dead and dying palo verdes, struggling to compete for sufficient water in a dense network of shallowly rooted buffelgrass plants. Even though adult saguaros may persist in dense stands of buffelgrass (though they are still extremely vulnerable to buffelgrass-fueled fires), a lack of young saguaros suggests that the next generation will be much smaller than the current one.

In places like Tumamoc Hill, the Santa Catalinas, and the Rincon Mountains, large continuous buffelgrass patches like this have persisted for decades and continue to expand, especially during wet summers like we had in 2021 and 2022.

That’s one reason why Tumamoc Hill is a special focus of the Desert Museum. Tumamoc Hill is a strategic location for treating buffelgrass not just because of its cultural and historical significance, but also because its buffelgrass-covered slopes loom over downtown Tucson, reflecting sunlight all across the Tucson valley 365 days of the year. Eradicating buffelgrass on Tumamoc Hill would be a big psychological win for Sonoran Desert conservation, galvanizing future investments and community action. It would give us hope that the future of the saguaro-studded slopes and bajadas of the Arizona Upland is not an irreversible transition into a dense flammable grassland, and that, together, we can preserve this uniquely biodiverse desert landscape for centuries to come.

We are still hoping that this ecosystem transformation is not unstoppable. We are taking action now to push the transformation tide back a little, protecting the native ecosystem we know and love by removing buffelgrass, raising awareness in our community, and making ground truth assessments. But we need to ground our hopes and our actions in reality. With these maps, our actions will have greater impact, our removal efforts will be more effective, and our hope for the future of this beautiful desert will grow.

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