Tree water deficit estimated by measuring water-related changes in stem radius (ΔW) was compared with tree water deficit estimated from the output of a simple, physiologically reasonable model (ΔW_E), with soil water potential (Ψ_soil) and atmospheric vapor pressure deficit (VPD) as inputs. Values of ΔW were determined by monitoring stem radius changes with dendrometers and detrending the results for growth. We followed changes in ΔW and ΔW_E in Pinus sylvestris L. and Quercus pubescens Willd. over 2 years at a dry site (2001–2002; Salgesch, Wallis) and in Picea abies (L.) Karst. for 1 year at a wet site (1998; Davos, Graubuenden) in the Swiss Alps.
The seasonal courses of ΔW in deciduous species and in conifers at the same site were similar and could be largely explained by variation in ΔW_E. This finding strongly suggests that ΔW, despite the known species-specific differences in stomatal response to microclimate, is mainly explained by a combination of atmospheric and soil conditions. Consequently, we concluded that trees are unable to maintain any particular ΔW. Either Ψ_soil or VPD alone provided poorer estimates of ΔW than a model incorporating both factors. As a first approximation of ΔW_E, Ψ_soil can be weighted so that the negative mean Ψ_soil reaches 65 to 75% of the positive mean daytime VPD over a season (Q. pubescens: ∼65%, P. abies: ∼70%, P. sylvestris: ∼75%). The differences in ΔW among species can be partially explained by a different weighting of Ψ_soil against VPD. The ΔW of P. sylvestris was more dependent on Ψ_soil than that of Q. pubescens, but less than that of P. abies, and was less dependent on VPD than that of P. abies and Q. pubescens. The model worked well for P. abies at the wet site and for Q. pubescens and P. sylvestris at the dry site, and may be useful for estimating water deficit in other tree species.