My colleagues and I study the history of desert rainfall using the chemistry of saguaro spines.
Saguaros add new growth at the very top of the central stem and arms from June into about October. We can therefore read a plant’s physiological and environmental history by sampling its spines from the top to the bottom.
A healthy saguaro holds its spines throughout its life, meaning spines at the bottom of a big saguaro can be more than 100 years old — ideal for building long climate records.
By analyzing 853 spines collected from five different saguaros on Tumamoc Hill, we reconstructed the rainfall record of the last 30 years. We calibrated our cactus-spine record by comparing it to the instrumental weather record and found we can clearly identify El Niño winter rains just from our cactus record.
We also use spine chemistry to assess a saguaro’s health over its lifetime as the climate changes. We track the plant’s water balance and photosynthesis because these measurements can reveal significant water stress in healthy-looking saguaros.
In addition, our regional monitoring program studies spines from saguaros in different climates across the desert to understand how a warming climate will affect saguaros.
We are now analyzing columnar cactus spines from Bolivia. The region lacks long-term instrumental climate records, so a cactus-spine index of El Niño rainfall will be valuable.
