Dry season — wet season volume change of Echinopsis atacamensis ssp. atacamensis (Cactaceae) as proxy for the amount of utilizable water of a stem succulent plant

Echinopsis atacamensis ssp. atacamensis is a columnar cactus from the western slopes of the high Andes of northern Chile. As for other species of the family, water is primarily stored in the succulent cortex of the stems. Repeat observations during the years 2016–2018 include an extremely long dry period and an extremely wet summer showed that the fraction of utilizable water (UW) in the stem is at least 53% of the maximum storage capacity. The observation of longitudinal ruptures in the rib furrows questions the usefulness of saturated water content (SWC) as a measure of water storage capacity.