Saguaros & Climate Change

Researchers are studying saguaros in Cascabel to learn how the plants might respond to climate change. Dr. Kevin Hultine, from the Desert Botanical Garden in Phoenix, AZ, contacted Sweetwater Center for help finding a suitable research site in our area. Kevin and Cindy Salo selected an area and marked suitable plants in November 2013.

Kevin and his colleagues at other institutions are studying saguaros growing across a range of climates. The team has six sites in the Sonoran Desert, from a dry site in northwest Mexico to Cascabel, on the wettest edge of saguaro’s range. Understanding how well saguaros grow in each area will allow researchers to predict how their growth might change as climates change.

The research team is measuring 13C, a stable carbon isotope, in saguaro spines to determine how well the plants are growing at each site. As a stable isotope, this form of carbon isn’t radioactive and doesn’t decay. About 1% of carbon is 13C, which researchers can measure in plants. The vast majority of carbon on Earth is 12C.

Plants build sugars, carbohydrates, and the other things they need from carbon, which they take in through their stomates as CO2. Unfortunately, when CO2 moves in, water moves out through the stomates. Losing too much water, or not being able to take enough up through their roots, stresses plants and slows their growth and other processes.

Plants take up 12C faster than 13C. However, when plants are water stressed, they take up relatively more 13C. Measuring changes in the ratio of 13C to 12C in saguaro spines tells researchers how water stressed the plants were when the spines were formed. Water stress affects all plant processes, including growth.

Date image courtesy Kevin Hultine.

The research team illustrated this by comparing how much saguaros grew in height each year with the ratio of 13C to 12C in spines from the same year. On the graph to the right, the saguaros in Cascabel, the wettest site, grew fastest. Those at the driest two sites were most water stressed and grew slowest.

The horizontal and vertical lines at each point on the graph show how much the growth rate (horizontal lines) and 13C:12C compared to the standard (vertical line) varied among the plants measured at each site.