Armatocereus laetus

Name and identifying features

The accepted name is Armatocereus laetus (Kunth) Backeb. The former combinations Cactus laetus, Cereus laetus and Lemaireocereus laetus belong to the same nomenclatural record. The Loaiza and Roque Gamarra key distinguishes mature plants by 9–13 angular, flattened radial spines, hairy outer perianth parts and fruits with reddish-brown spines. [2-WWW] [Loaiza and Roque Gamarra 2016, POWO record 19746-2, 12 October 2026, p. 38, key]

The studied dry-forest population

Würzberg and colleagues worked in Laipuna Nature Reserve in southern Ecuador, on a plot at 600 m elevation. The forest undergoes a distinct dry season, usually from June to December. Plants identified as A. laetus had a shallow lateral root system there. Permanent forest plots recorded a maximum height of 7 m and a diameter at breast height of 15 cm for this species. These are values from the study site. [Würzberg et al. 2026, pp. 3–5, Table 1]

External sensors were mounted on intact roots 2–6 mm in diameter, at a depth of 5–10 cm. They measured flow through conducting segments close to the small roots absorbing water. Records were taken every 30 minutes from late March 2022 to early September 2023. At least three plants of each of the four species were initially included, but data gaps meant that the presented dataset uses the most complete record from one root per species. [Würzberg et al. 2026, pp. 5–8]

Responding to an isolated rainfall event

Sap flow in the monitored root increased after small December rainfall events of less than 5 mm per day. The authors describe a response within roughly 6 hours. This clear increase preceded the resumption of greater water uptake in the three compared tree species. The cactus root also responded to small rainfall events in July and August, during what is usually the dry season. [Würzberg et al. 2026, pp. 9, 12, Figure 2]

The seasonal uptake maximum occurred early during soil rewetting. Sap flow declined in later months even though the soil was wetter. This pattern links isolated rainfall events with the opportunity to replenish water stored in the stem. The measurement concerns flow in a particular root, expressed per area of its conducting elements. It does not give the total amount of water taken up by the entire plant. [Würzberg et al. 2026, pp. 6–9, 12, Figure 2, Table 2]

Water uptake through day and night

The root also took up water during the day. In January and the early dry season, the daytime peak could exceed the nocturnal one. A clear afternoon decline occurred when soil moisture was highest. The daily uptake rhythm therefore changed with the season. The sap-flow record shows water transport, while the hours of stomatal opening were not measured directly. [Würzberg et al. 2026, pp. 9–10, 13, Figures 3–4]

The authors propose several interacting explanations. Shallow roots permit the use of small rainfall events, while the large stem water reserve may weaken the direct link between uptake and transpiration. They also discuss the formation of small roots after rain and water redistribution between stem tissues. These mechanisms remain interpretations because the experiment did not track fine-root growth or changes in water potential within the stem. [Würzberg et al. 2026, pp. 12–14]

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