Increasing air humidity influences hydraulic efficiency but not functional vulnerability of xylem in hybrid aspen

Climate models predict greater increases in the frequency than in the amount of precipitation and a consequent rise in atmospheric humidity at high latitudes by the end of the century. We investigated the responses of hydraulic and relevant anatomical traits of xylem to elevated relative humidity of air on a 1-yr-old coppice of hybrid aspen (Populus×wettsteinii) growing in the experimental stand at the Free Air Humidity Manipulation site in Eastern Estonia. The hydraulic conductivity of stems was measured with a high pressure flow meter; artificial cavitation in the stem segments was induced by the air injection method. Specific conductivity of xylem decreased from 4.42 in the control to 3.94kgm^-1s^-1MPa^-1 in the humidification treatment, while the trend was well correlated with increasing wood density. Humidified trees exhibited smaller leaf area at the same xylem cross-sectional area, resulting in 34% higher average Huber values compared to the control. Control and humidity-treated trees differed by neither native embolism level nor susceptibility to dehydration-induced cavitation. Increasing atmospheric humidity reduces the hydraulic efficiency of hybrid aspen trees expressed on a xylem area basis and causes substantial changes in resource allocation between photosynthetic and water transport tissues. This climate trend does not influence stem vulnerability to cavitation.

Reliability of dose volume constraint inference from clinical data

Dose volume histogram points (DVHPs) frequently serve as dose constraints in radiotherapy treatment planning. An experiment was designed to investigate the reliability of DVHP inference from clinical data for multiple cohort sizes and complication incidence rates. The experimental background was radiation pneumonitis in non-small cell lung cancer and the DVHP inference method was based on logistic regression. From 102 NSCLC real-life dose distributions and a postulated DVHP model, an 'ideal' cohort was generated where the most predictive model was equal to the postulated model. A bootstrap and a Cohort Replication Monte Carlo (CoRepMC) approach were applied to create 1000 equally sized populations each. The cohorts were then analyzed to establish inference frequency distributions. This was applied to nine scenarios for cohort sizes of 102 (1), 500 (2) to 2000 (3) patients (by sampling with replacement) and three postulated DVHP models. The Bootstrap was repeated for a 'non-ideal' cohort, where the most predictive model did not coincide with the postulated model. The Bootstrap produced chaotic results for all models of cohort size 1 for both the ideal and non-ideal cohorts. For cohort size 2 and 3, the distributions for all populations were more concentrated around the postulated DVHP. For the CoRepMC, the inference frequency increased with cohort size and incidence rate. Correct inference rates  >[Formula: see text] were only achieved by cohorts with more than 500 patients. Both Bootstrap and CoRepMC indicate that inference of the correct or approximate DVHP for typical cohort sizes is highly uncertain. CoRepMC results were less spurious than Bootstrap results, demonstrating the large influence that randomness in dose-response has on the statistical analysis.

Impact of elevated atmospheric humidity on anatomical and hydraulic traits of xylem in hybrid aspen

This study was performed on hybrid aspen saplings growing at the Free Air Humidity Manipulation site in Estonia. We investigated changes in wood anatomy and hydraulic conductivity in response to increased air humidity. Two hydraulic traits (specific conductivity and leaf-specific conductivity) and four anatomical traits of stem wood-relative vessel area (VA), vessel density (VD), pit area and pit aperture area-were influenced by the humidity manipulation. Stem hydraulic traits decreased in the apical direction, whereas branch hydraulic characteristics tended to be greatest in mid-canopy, associated with branch size. A reduction in VD due to increasing humidity was accompanied by a decrease in vessel lumen diameter, hydraulically weighted mean diameter (Dh), xylem vulnerability index and theoretical hydraulic conductivity. VA and Dh combined accounted for 87.4% of the total variation in kt of branches and 85.5% of that in stems across the treatments. Characters of branch vessels were more stable, and only the vessel-grouping index (the ratio of the total number of vessels to the total number of vessel groupings) was dependent on the interactive effect of the treatment and canopy position. Our results indicate that the increasing atmospheric humidity predicted for high latitudes will result in moderate changes in the structure and functioning of the hybrid aspen xylem.