What does this research mean for the field?

In semi-arid mountain ecosystems, tree growth is primarily constrained by soil moisture availability rather than atmospheric drought, with the latter acting mainly as an additional stressor during combined drought events. Novelty: ClaimNovelty.INCREMENTAL. Consensus alignment: ConsensusAlignment.NEUTRAL.

What question did this study set out to answer?

This research aims to differentiate the impacts of soil and atmospheric drought on tree growth in northeastern Iran using tree-ring and isotopic data.

May 21, 2026Open Access

Tree-ring width and δ18O-derived hydroclimatic reconstructions allow a distinction between soil and atmospheric drought in the Mountain Forests of Northeastern Iran

Key Points

This research aims to differentiate the impacts of soil and atmospheric drought on tree growth in northeastern Iran using tree-ring and isotopic data.
Developed two 200-year chronologies of tree-ring width and δ18O from Juniperus polycarpos in the Hezar Masjed Mountains.
Used multiple regression to reconstruct Standardized Precipitation-Evapotranspiration Index (SPEI07) and growing-season vapor pressure deficit (VPD).
Classified drought events into VPD-only, SPEI-only, and combined drought years to assess their effects on radial growth.
During severe SPEI droughts, tree growth declined significantly, followed by growth reductions in severe combined drought years.
Atmospheric drought alone had a transient effect on tree growth, with its influence weaker than that of soil moisture.
Long-term analysis indicated an increase in the frequency of severe droughts in recent decades, pointing toward heightened climate change impacts.

Abstract

Abstract Iran’s long history of climate-related crises, primarily driven by droughts, has been intensified by ongoing climate change, placing forest ecosystems under increasing hydroclimatic stress. In recent decades, prolonged droughts combined with elevated atmospheric moisture deficits have reduced ecosystem resilience and increased vulnerability to degradation. To better understand long-term drought dynamics and their ecological impacts, we developed two 200-year chronologies (1821–2020) of tree-ring width (TRW) and stable oxygen isotope variations (δ 1 ⁸O) from Juniperus polycarpos in the Hezar Masjed Mountains, northeastern Iran. The δ 1 ⁸O record served as a proxy for atmospheric moisture conditions and was used to reconstruct growing-season (March–September) vapor pressure deficit (VPD). When combined with TRW in a multiple regression framework, this dual-parameter approach enabled reconstruction of the Standardized Precipitation-Evapotranspiration Index (SPEI07), representing cumulative growing-season hydroclimatic conditions related to soil moisture availability. This allows the differentiation of atmospheric and soil drought impacts on tree growth over two centuries. By classifying drought years into VPD-only, SPEI-only, and combined drought events, we found that drought conditions associated with reduced soil moisture availability (SPEI) exerted the strongest constraint on radial growth. Tree growth declined most strongly during severe SPEI droughts, followed by severe combined drought (COMB-D) years, whereas atmospheric drought alone (VPD-D) had a weaker and more transient effect. Growth typically recovered within two years following drought events. Analysis of long-term drought classifications (1821–2020) revealed a shift towards more intense droughts in recent decades, particularly in the frequency of severe VPD and combined drought years. Our findings highlight that tree growth in semi-arid mountain ecosystems is primarily limited by soil moisture availability, with atmospheric drought acting as an additional stressor when coinciding with soil moisture deficits. This study demonstrates the value of combining multiple tree-ring proxies to disentangle drought mechanisms and improve understanding of forest responses to climate change.

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Cite This Study

Foroozan et al. (Tue,) studied this question.

synapsesocial.com/papers/6a0ea14abe05d6e3efb5fccc https://doi.org/https://doi.org/10.1038/s41598-026-52364-3

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