Tissue stable isotope ratios detect relationships among growth, internal water storage and precipitation in giant saguaro occurring across broad spatio-temporal gradients

Background/Question/Methods

Giant saguaro (Carnegia gigantea) is among the most charismatic plants on the planet. These long-lived cacti occur in the driest regions of sub-tropical America and are important foundation species throughout its range. However, the capacity for giant saguaro to serve as foundation species is not only dependent on rare surges of seedling establishment during moist years but survival through long periods of limited precipitation. To better understand relationships between growth / physiology and intra- and inter-annual periods of drought, we conducted three complimentary studies on giant saguaro occurring in the northern Sonoran Desert: 1) evaluated relationships between annual height growth, and δ¹³C signals in the most recent spines across six sites varying in precipitation from 100 to 370 mm over the previous 12 months, 2) measured annual height growth, monthly stem water storage and spine δ¹³C signals over four consecutive years at two sites that varied in winter and summer precipitation, and 3) evaluated the importance of stem water storage by comparing patterns of annual height growth and spine δ¹³C signals with stem water storage over two consecutive years between giant saguaro and co-occurring organ pipe (Stenocereus thurberi) cacti that had on average a five-fold lower water storage capacity.

Results/Conclusions

Across the six sites, ¹³C abundance increased inversely with height growth (R² = 0.38, P < 0.0001). In other words, higher growth rates likely reflect higher stomatal conductance and higher photosynthetic activity (i.e. higher δ¹³C in constitutive CAM plants). Mean height growth and mean δ¹³C both increased linearly with precipitation across all sites (R² = 0.57, and 0.53, for height growth and δ¹³C, respectively). A comparison of two sites over four years also revealed that the site with greater mean annual precipitation had plants with higher growth and higher δ¹³C signals. Interestingly, the wetter site also had plants with considerably more dynamic inter- and intra-annual stem water storage. The importance of stem water storage was further evaluated by comparing saguaro with organ pipe. Over two consecutive years of below average precipitation, internal water storage of giant saguaro never dropped below 30% of maximum where organ pipe dropped to 50% of maximum during the driest periods, suggesting that saguaro is considerably more buffered from the effects of drought than organ pipe. The relatively large volume to surface area ratio of saguaro stems may allow this species to persist under extreme drought conditions that are predicted for the Sonoran Desert.