Giant Rhododendron Tree Rings Reveal Two Centuries of Drying in Southwestern China

High in the Gaoligong Mountains of Yunnan Province, where the eastern edge of the Tibetan Plateau meets the Indian summer monsoon, grows one of the rarest trees on Earth. Rhododendron protistum var. giganteum, famed for producing the largest leaves of any known rhododendron, survives only in a handful of scattered populations along these mist-shrouded ridges. Now, a team of Chinese researchers has coaxed a remarkable secret out of this endangered giant: a continuous, year-by-year record of atmospheric moisture stretching back 221 years, written into the oxygen atoms of its wood. The study, published in Climate Dynamics, provides the first tree-ring oxygen isotope chronology ever built from this species and paints a sobering picture of a region that has been steadily drying since the dawn of the twentieth century.

The research, led by Wanxiong Zhang of Southwest Forestry University and Zhuoya Zhang, with colleagues from the Chinese Academy of Sciences, Fujian Normal University and the Gaoligongshan National Nature Reserve, focused on the cellulose locked inside the tree’s annual growth rings. Cellulose, the structural polymer of plant cell walls, is laid down each growing season and incorporates oxygen from the water the tree takes up and from the leaf water that evaporates through its stomata. Because the heavy isotope oxygen-18 behaves differently from the lighter oxygen-16 during evaporation and condensation, the ratio of these isotopes in tree-ring cellulose acts as a natural archive of the humidity conditions under which each ring formed. In humid air, less water evaporates from the leaf and less isotopic enrichment occurs; in dry air, the opposite happens. The result is a proxy that can be read like a barometer of past atmospheric moisture.

Sampling such a rare and protected species demanded extraordinary care. Working with the reserve administrations of Baoshan and Nujiang, the team obtained cores from living trees without harming them, then cross-dated the rings using standard dendrochronological quality-control procedures to assign each ring a precise calendar year. From the resulting 221-year chronology, spanning 1802 to 2022, the researchers reconstructed May-to-November relative humidity, the period covering the monsoon growing season. The calibration against instrumental records from 1960 to 2022 explained 36.3 percent of the observed variance in relative humidity, a respectable figure for a single-species isotope chronology in a topographically complex mountain environment.

The reconstructed record tells a clear story of long-term change. Wet years cluster predominantly before 1900, while the twentieth century brought a general drying trend that has continued into the present. This finding echoes results from other Himalayan isotope studies, including a 223-year tree-ring oxygen isotope chronology from Nepal that documented increasing aridity over a similar interval. What makes the new record particularly valuable is its location: the Gaoligong Mountains sit at a climatic crossroads where Indian monsoon circulation, Tibetan Plateau dynamics and local orographic effects interact, making the region both hydrologically sensitive and poorly covered by long instrumental observations.

To confirm that the chronology reflects genuine regional climate rather than local noise, the team performed spatial correlation analyses. The reconstructed relative humidity series showed coherent relationships with observed relative humidity, vapor pressure deficit, the standardized precipitation-evapotranspiration index and precipitation across southwestern China and adjacent monsoon-influenced regions. Vapor pressure deficit, the gap between how much moisture the air holds and how much it could hold at saturation, is a key driver of plant water stress, and rising deficits have been implicated in increasing tree mortality across tropical forests worldwide. The fact that the rhododendron isotopes track these regional moisture fields so consistently supports the record’s use as a regional hydroclimatic archive.

A central technical question in tree-ring isotope research is what exactly the signal represents. Oxygen isotopes in cellulose are shaped both by the isotopic composition of the source water taken up by the roots and by evaporative enrichment in the leaves, which is strongly modulated by ambient humidity. Using modeled precipitation isotope data alongside the humidity-related atmospheric variables, the researchers quantified the relative contributions of each pathway. Both made important contributions to the cellulose isotope variability, leading the team to interpret the chronology as an integrated, relative-humidity-sensitive hydroclimatic record that captures atmospheric evaporative demand and source-water isotope changes together. This dual sensitivity, they argue, is precisely what makes the proxy robust: it responds to the full moisture environment of the tree rather than to a single variable in isolation.

The record also carries the fingerprints of the planet’s great ocean-atmosphere oscillations. Spectral analysis of the reconstruction, combined with examinations of sea-surface temperatures and large-scale atmospheric circulation patterns, suggests that May-to-November relative humidity in the Gaoligong region is intermittently modulated by the El Niño-Southern Oscillation and the Walker circulation, the vast east-west overturning of air across the tropical Pacific that shifts rainfall patterns during El Niño and La Niña events. The influence of the Indian Ocean Dipole, by contrast, appears more conditional, depending on the background climate state. This kind of intermittent teleconnection is consistent with earlier work showing that ENSO events disrupt Indian summer monsoon rainfall and that dipole events have been linked to drought episodes in southwestern China, including the severe droughts of 2006 and 2011.

Perhaps the most consequential part of the study looks forward. The team analyzed climate model simulations from the sixth phase of the Coupled Model Intercomparison Project, the model ensemble underpinning the most recent IPCC assessment, under a range of future emissions scenarios. The models consistently indicate an overall decline in May-to-November relative humidity in the region during the twenty-first century, with stronger decreases under higher radiative-forcing scenarios. In other words, the drying trend that the rhododendrons have quietly recorded since 1900 is projected to intensify as greenhouse gas concentrations rise, with the magnitude of future aridification depending directly on humanity’s emissions choices.

For Rhododendron protistum var. giganteum itself, the implications are alarming. The species is already classified as endangered, restricted to fragmented habitat on these mountains, and a companion study by several of the same authors found that rising temperatures have reshaped the tree’s climate-growth relationships and increased its vulnerability. Declining relative humidity and rising vapor pressure deficit compound the physiological stress on a species whose enormous leaves, while spectacular, present a large surface area for water loss. Global syntheses have shown that tropical and subtropical tree mortality rises with atmospheric water stress, and hydraulically vulnerable species have been killed outright by catastrophic droughts elsewhere in the world’s mountains and tropics. A long-term drying trajectory, superimposed on warming, leaves this botanical giant with shrinking margins for survival.

Beyond its significance for conservation, the study demonstrates the power of oxygen isotope dendroclimatology in regions where conventional ring-width records fall short. In humid, montane forests, tree growth is often limited less by water availability than by temperature or light, muting the drought signal in ring widths. Isotope ratios, by contrast, record the evaporative environment directly, regardless of how much the tree grows in a given year. By extending this approach to a new species and a new corner of the monsoon domain, the researchers have added a crucial piece to the puzzle of Asian hydroclimate variability. The full dataset and reconstruction code have been made publicly available in the Zenodo repository, allowing other scientists to scrutinize, reuse and extend the record. As climate change accelerates across the world’s mountains, the silent testimony of these ancient rhododendrons, and the 221-year moisture history they preserve, may prove essential for anticipating what the coming century holds for the water supplies, forests and extraordinary biodiversity of southwestern China.

Subject of Research: Tree-ring oxygen isotope reconstruction of 221 years of relative humidity variability in the Gaoligong Mountains, China

Article Title: Tree-ring oxygen isotopes reveal a 221-year hydroclimatic history for the endangered Rhododendron protistum var. giganteum in the Gaoligong Mountains

Article References: Zhang, W., Ge, H., He, Y., Xu, C., Wang, J., Fang, K., An, W., & Zhang, Z. (2026). Tree-ring oxygen isotopes reveal a 221-year hydroclimatic history for the endangered Rhododendron protistum var. giganteum in the Gaoligong Mountains. Climate Dynamics, 64(11), Article 458. https://doi.org/10.1007/s00382-026-08425-4

Image Credits: AI Generated

DOI: 10.1007/s00382-026-08425-4

Keywords: tree rings, oxygen isotopes, paleoclimate, relative humidity, drought, Rhododendron protistum var. giganteum, Gaoligong Mountains, monsoon, ENSO, Indian Ocean Dipole, CMIP6, climate change