Modeling tree water deficit from microclimate: an approach to quantifying drought stress

Tree water deficit estimated by measuring water-related changes in stem radius (DeltaW) was compared with tree water deficit estimated from the output of a simple, physiologically reasonable model (DeltaW(E)), with soil water potential (Psi(soil)) and atmospheric vapor pressure deficit (VPD) as inputs. Values of DeltaW were determined by monitoring stem radius changes with dendrometers and detrending the results for growth, We followed changes in DeltaW and DeltaW(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 DeltaW in deciduous species and in conifers at the same site were similar and could be largely explained by variation in DeltaW(E). This finding strongly suggests that DeltaW, 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 DeltaW. Either Psi(soil) or VPD alone provided poorer estimates of AWthan a model incorporating both factors. As a first approximation of DeltaW(E), Psi(soil) can be weighted so that the negative mean Psi(soil) reaches 65 to 75% of the positive mean daytime VPD over a season (Q. pubescens: similar to65%, P abies: similar to70%, P sylvestris: similar to75%). The differences in DeltaW among species can be partially explained by a different weighting of Psi(soil) against VPD. The DeltaW of P. sylvestris was more dependent on Psi(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.

Interpretation of fluid inclusions in quartz deformed by weak ductile shearing: reconstruction of differential stress magnitudes and pre-deformation fluid properties

A well developed theoretical framework is available in which paleofluid properties, such as chemical composition and density, can be reconstructed from fluid inclusions in minerals that have undergone no ductile deformation. The present study extends this framework to encompass fluid inclusions hosted by quartz that has undergone weak ductile deformation following fluid entrapment. Recent experiments have shown that such deformation causes inclusions to become dismembered into clusters of irregularly shaped relict inclusions surrounded by planar arrays of tiny, new-formed (neonate) inclusions. Comparison of the experimental samples with a naturally sheared quartz vein from Grimsel Pass, Aar Massif, Central Alps, Switzerland, reveals striking similarities. This strong concordance justifies applying the experimentally derived rules of fluid inclusion behaviour to nature. Thus, planar arrays of dismembered inclusions defining cleavage planes in quartz may be taken as diagnostic of small amounts of intracrystalline strain. Deformed inclusions preserve their pre-deformation concentration ratios of gases to electrolytes, but their H2O contents typically have changed. Morphologically intact inclusions, in contrast, preserve the pre-deformation composition and density of their originally trapped fluid. The orientation of the maximum principal compressive stress (σ1σ1) at the time of shear deformation can be derived from the pole to the cleavage plane within which the dismembered inclusions are aligned. Finally, the density of neonate inclusions is commensurate with the pressure value of σ1σ1 at the temperature and time of deformation. This last rule offers a means to estimate magnitudes of shear stresses from fluid inclusion studies. Application of this new paleopiezometer approach to the Grimsel vein yields a differential stress (σ1–σ3σ1–σ3) of ∼300 MPa∼300 MPa at View the MathML source390±30°C during late Miocene NNW–SSE orogenic shortening and regional uplift of the Aar Massif. This differential stress resulted in strain-hardening of the quartz at very low total strain (<5%<5%) while nearby shear zones were accommodating significant displacements. Further implementation of these experimentally derived rules should provide new insight into processes of fluid–rock interaction in the ductile regime within the Earth's crust.

Metamorphosis of human lumbar vertebrae induced by VEPTR growth modulation and stress shielding

INTRODUCTION Distraction-based spinal growth modulation by growing rods or vertical expandable prosthetic titanium ribs (VEPTRs) is the mainstay of instrumented operative strategies to correct early onset spinal deformities. In order to objectify the benefits, it has become common sense to measure the gain in spine height by assessing T1-S1 distance on anteroposterior (AP) radiographs. However, by ignoring growth changes on vertebral levels and by limiting measurement to one plane, valuable data is missed regarding the three-dimensional (3D) effects of growth modulation. This information might be interesting when it comes to final fusion or, even more so, when the protective growing implants are removed and the spine re-exposed to physiologic forces at the end of growth. METHODS The goal of this retrospective radiographic study was to assess the growth modulating impact of year-long, distraction-based VEPTR treatment on the morphology of single vertebral bodies. We digitally measured lumbar vertebral body height (VBH) and upper endplate depth (VBD) at the time of the index procedure and at follow-up in nine patients with rib-to-ileum constructs (G1) spanning an anatomically normal lumbar spine. Nine patients with congenital thoracic scoliosis and VEPTR rib-to-rib constructs, but uninstrumented lumbar spines, served as controls (G2). All had undergone more than eight half-yearly VEPTR expansions. A Wilcoxon signed-rank test was used for statistical comparison of initial and follow-up VBH, VBD and height/depth (H/D) ratio (significance level 0.05). RESULTS The average age was 7.1 years (G1) and 5.2 year (G2, p > 0.05) at initial surgery; the average overall follow-up time was 5.5 years (p = 1). In both groups, VBH increased significantly without a significant intergroup difference. Group 1 did not show significant growth in depth, whereas VBD increased significantly in the control group. As a consequence, the H/D ratio increased significantly in group 1 whereas it remained unchanged in group 2. The growth rate for height in mm/year was 1.4 (group 1) and 1.1 (group 2, p = 0.45), and for depth, it was -0.3 and 1.1 (p < 0.05), respectively. CONCLUSIONS VEPTR growth modulating treatment alters the geometry of vertebral bodies by increasing the H/D ratio. We hypothesize that the implant-related deprivation from axial loads (stress-shielding) impairs anteroposterior growth. The biomechanical consequence of such slender vertebrae when exposed to unprotected loads in case of definitive VEPTR removal at the end of growth is uncertain.