Tracking drought-driven hydraulic impairment in Pinus sylvestris with microtensiometry and dendrometry

Abstract Key message Continuous monitoring of stem water potential and circumference variation reveals a relationship between apparent stem capacitance and Ψ-mapped embolism risk during drought, providing a mechanistic and practical proxy to assess hydraulic safety in mature conifers. Abstract Projected increases in drought frequency and intensity threaten the hydraulic function and survival of mature conifers. However, continuous in-situ monitoring of stem water status remains technically challenging, particularly within forest canopies. We deployed microtensiometers and precision dendrometers in a thinned Pinus sylvestris stand (Sierra Nevada, Spain) to monitor hourly stem water potential ( Ψ STEM ) and stem circumference variation ( SCV ). Stem hydraulic capacitance ( C S ) was derived in situ from SCV– Ψ STEM time series. Embolism risk, PLC (Ψ) , was estimated at diagnostic intervals by mapping in-situ Ψ STEM onto laboratory vulnerability curves. Continuous Ψ STEM closely matched independent leaf pressure-chamber measurements (R² = 0.78) and covaried with sub-daily SCV dynamics, validating both sensors. Midday SCV ( SCV MD ) covaried with midday Ψᴍᴅ (R² = 0.49) and with Ψ -mapped embolism risk ( PLC ( Ψ MD )) (R² = 0.51), indicating that greater shrinkage aligns with more negative tension and higher estimated risk. Across the dry-down, PLC ( Ψ ) indicated rising risk while C S declined; we interpret this as a plausible capacitance–risk linkage given our design. Concurrent eddy-covariance measurements showed late-summer attenuation of canopy latent energy ( LE ), with lower midday peaks and reduced diurnal amplitude-coincident with higher PLC ( Ψ ) estimates and declining C S . Mixed-effects modeling revealed that SCV was jointly driven by Ψ STEM , air temperature, vapor-pressure deficit, relative humidity, and most prominently soil water content. Together, these results demonstrate that non-destructive, high-temporal-resolution sensing resolves diel–seasonal hydraulics and support a capacitance–embolism risk trade-off. We further show that SCV MD provides a practical proxy for hydraulic status where direct tensiometry is impractical, informing physiologically based forest management. Graphical abstract