The influence of irrigation on the growth of Scots pines was investigated by comparing trees growing along an open water channel with trees growing under natural conditions. We focussed on the reaction of pines to sudden changes in water availability due to the cessation of irrigation and on the subsequent adaptation of the trees to the changed site conditions. The study area was located in Valais, an inner-alpine dry valley in Switzerland. The investigations covered the responses of diameter growth to climate and irrigation. We particularly concentrated on resin duct (RD) production as a potential indicator of stress. Irrigation has a balancing effect on radial growth, with irrigated trees showing lower mean sensitivities, lower standard deviations, lower expressed population signal (EPS) values, and higher first-order autocorrelation for the ring-width chronologies than trees of the dry control sites. Irrigation increased the uniqueness of the single trees' radial growth, the RD production in general decreased, and the proportion of RDs in earlywood increased.
The relationships between climate and tree-ring width, as well as RD density, were studied using response function analysis. Although radial growth and RD density are highly positively correlated, the climatic responses were very different. Whereas radial growth responds positively to moist springs and cool summers, RD density is stimulated by hot summer temperatures. The variance explained by climate derived from the regression models is higher for RD density than for tree-ring width and underlines the additional information about climate response to be gained from RDs. Irrigation reduced the response of tree growth to climate and the correlations between radial growth and summer temperature changed from negative to positive.
The cessation of irrigation in 1983 and the resulting sudden reduction in water availability provoked a drastic breakdown of radial growth, but the trees survived. Six years later, the trees seemed to have largely recovered, but the adaptation process was not yet finished, as indicated by the failure of the growth rate to return to levels of the control trees. Although the trees survived the sudden cessation of irrigation, the additional exposure to drought may play an important role as a predisposing factor for plant disease.