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Effects of drought on the growth and the strategies of survival of Scots pine and pubescent oak in the Valais
In the Rhone-valley the low-elevation forests are changing. While Scots pine (Pinus sylvestris L.) shows high mortality rates, the dissemination of deciduous species and in particular of pubescent oak (Quercus pubescens Willd.) is increasing. We think that land use change and the influence of climate warming are key factor for this change in landscape. In this study we analyse the effect of drought stress on tree growth of pine and oak to rate the potential of the two species to survive in an expected hotter and therefore drier environment.
In this study we want to find out how drought stress affects the growth and the survival of Scots pine and pubescent oak. Therefore we compare oaks and pines growing on irrigation channels (Fig. 1) and in an artificial irrigation experiment with trees on xeric sites. The comparison of watered and un-watered trees allow a) to analyse the effect of drought stress on the growth of oak and pine and b) to draw conclusion on differences in their growth strategy and therefore on the potential of the two species to survive in a drier environment.
Tree growth is analysed on a dendrochronological and wood anatomical level. Because the main factor for survival in xeric regions like the Valais is the maintenance of an effective and safe water-transport system, our studies mainly focus on the wood anatomical analysis of the conducting elements like vessels and tracheids.
In a first approach we analyse the size of the conducting elements on annual basis. As a result we get the mean size of the conducting elements per year separated in earlywood and latewood. These mean values are correlated with monthly or weekly climate data.
In a second approach we analyse the intra-annual growth of pine and oak using the pinning technique. Pinning is the marking of the cambium by injury. So we puncture a small needle through the bark into the cell dividing zone, the cambium. As a reaction of the injury the tissue of the cambial zone build the typical wound tissue (Fig.2). This wound tissue shows us the position of the cambial zone at the time of the pinning, so we can date cell growth in retrospect. Therefore we are able to analyse the climate-growth relationship of cell formation with a high temporal resolution.
The results of the pinning experiment are additionally used to analyse the intra-annual composition of stable isotope (d18O, d13C) to learn more about the effect of drought stress on tree physiological processes.