Horsetail plants push evaporation to extremes, creating stem water with oxygen isotope signatures so wild they outstrip anything else on Earth and mimic space rocks.
Horsetail plants along riverbanks brew up stem water with oxygen isotope ratios, unique chemical signatures, so extreme they rival extraterrestrial samples, researchers report in a study published December 2025 in the Proceedings of the National Academy of Sciences. This discovery expands Earth’s known isotopic range fivefold and refines models for plant water evaporation, with ripple effects for reconstructing ancient climates from fossils.
Horsetails, or Equisetum, are ancient survivors from the Devonian era, resembling segmented green pipes that dot wetlands and river edges worldwide. These vascular plants pull water from roots to tips, losing most through stomata — tiny pores along their jointed stems. While evaporation concentrates heavier oxygen forms in plant water, horsetails amplify this due to their tall, leaky structure.
The team sampled smooth horsetail (Equisetum laevigatum) from New Mexico’s Rio Grande floodplain, measuring triple oxygen isotopes in stem water from base to tip. The signatures grew increasingly skewed upward, shattering records for terrestrial materials.
This isotopic “distillation” stems from continual water loss along the meter-long stem, like a chain of evaporating lakes. Using a hybrid model, the researchers pinpointed a key evaporation factor, explaining odd signatures in desert life.
The findings show the drastic changes that happened in a common ancestor before the genus spread, sparing some core functions despite ditching photosynthesis needs. “The Equisetum data extend the known oxygen isotope range of samples from Earth and our entire solar system by a factor of five,” the authors note.
Phytoliths — silica deposits in the stems — showed mismatched signatures, blending early mild and late intense water as the plant grew. This mismatch hints that fossil phytoliths might mislead paleoclimate reads, urging caution. “This new value is required to explain the low Δ′17O values previously measured in desert plants and animals and is critical when using fossil samples for paleoclimate reconstruction,” the team adds.
Beyond botany, these extremes spotlight horsetails as models for evaporation in arid spots, aiding paleoclimate tweaks from fossils. As climates shift, such insights could reveal how ancient plants weathered dry spells, informing today’s ecosystems.
Citations: Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2507455122
