Elevational trends in hydraulic efficiency and safety of Pinus cembra roots

In alpine regions, elevational gradients in environmental parameters are reflected by structural and functional changes in plant traits. Elevational changes in plant water relations have also been demonstrated, but comparable information on root hydraulics is generally lacking. We analyzed the hydraulic efficiency (specific hydraulic conductivity ks, entire root system conductance KR) and vulnerability to drought-induced embolism (water potential at 50 % loss of conductivity Ψ50) of the roots of Pinus cembra trees growing along an elevational transect of 600 m. Hydraulic parameters of the roots were compared with those of the stem and related to anatomical traits {mean conduit diameter (d), wall reinforcement [(t/b)2]}. We hypothesized that temperature-related restrictions in root function would cause a progressive limitation of hydraulic efficiency and safety with increasing elevation. We found that both root ks and KR decreased from low (1600 m a.s.l.: ks 5.6 ± 0.7 kg m−1 s−1 MPa−1, KR 0.049 ± 0.005 kg m−2 s −1 MPa−1) to high elevation (2100 m a.s.l.: ks 4.2 ± 0.6 kg m−1 s−1 MPa−1, KR 0.035 ± 0.006 kg m−2 s−1 MPa−1), with small trees showing higher KR than large trees. ks was higher in roots than in stems (0.5 ± 0.05 kg m−1s−1MPa−1). Ψ50 values were similar across elevations and overall less negative in roots (Ψ50 −3.6 ± 0.1 MPa) than in stems (Ψ50 −3.9 ± 0.1 MPa). In roots, large-diameter tracheids were lacking at high elevation and (t/b)2 increased, while d did not change. The elevational decrease in root hydraulic efficiency reflects a limitation in timberline tree hydraulics. In contrast, hydraulic safety was similar across elevations, indicating that avoidance of hydraulic failure is important for timberline trees. As hydraulic patterns can only partly be explained by the anatomical parameters studied, limitations and/or adaptations at the pit level are likely.