Study sites and trees
The two study sites are located in oak and pine woodland on the south-facing slope of the main valley of the Wallis, Switzerland. The central Wallis is an inner-alpine valley characterized by a dry climate. This is mainly caused by so-called inner-valley shielding. The valley is positioned SE–NW with regard to the main storm tracks from the West. Mean annual precipitation over the past 20 years was, for both sites, approximately 600 mm per year.
The two sites Salgesch (46°19'27'' N, 7°34'40'' E, 975 m asl) and Jeizinen (46°19'21'' N, 7°43'30'' E, 1270 m asl) had similar oak and pine vegetation which primarily differed in tree height: the site at Salgesch had a shrub-like open canopy vegetation whereas at Jeizinen the same species grew as larger trees. The two sites were 11 km apart.
Pubescent oak (Quercus pubescens) and Scots pine (Pinus sylvestris) were the most abundant tree species and juniper (Juniperus communis L.) was the most abundant woody shrub in the vegetation of this very dry site. Whereas some Q. pubescens were up to 110 years old, 95% of them were younger than 70 years. Most of the dominant P. sylvestris were between 100 and 150 years old (A Rigling, personal communication). The woody species investigated stood in one of the typical patches of trees (32 m2 in area), consisting of 17 Q. pubescens, four P. sylvestris, two Viburnum lantana L. (2 cm in stem diameter), and five J. communis, surrounded by grass and bare rock. Details of the sizes of the investigated trees are listed in Table 1. The soil on this steep south-facing slope (25°) is shallow with a maximum depth of 0.1–0.3 m. It was classified as a rendzic leptosol on solid rock limestone according to the FAO classification system (Rigling et al., 2002). This type of soil generally has a low water-holding capacity. Continuous measurements of the local climate and vegetation started in April 2001 (Fig. 1). For the analyses here, data from 1 January 2002 to 31 December 2004 were used.
At this site, Q. pubescens
and P. sylvestris
were also the most abundant tree species but the deeper soil (about 0.5–1.3 m) had a better water-holding capacity and allowed the trees to grow much taller than at Salgesch (Table 1
). The more moderate humidity conditions (Table 2
) also allowed Norway spruce (Picea abies
) to grow there. These three tree species built an almost closed vegetation layer. The open spots were covered by J. communis
and Juniperus sabina
L. and in the understorey Amelanchier ovalis
L., Prunus mahaleb
L., and Cornus sanguinea
L. No systematic analysis of the age distribution of the three tree species was made, but dendrochronological measurements of selected trees showed that Q. pubescens
and P. sylvestris
were not only larger but also older than at Salgesch. The trees grew in a dip on a steep south-facing slope (
30°). Continuous measurements of the local climate and vegetation started in July 2002. Data from 1 January 2003 to 31 December 2004 were used (Fig. 1
Meteorological data were collected at the site with two similar solar-powered logging and steering-systems (IPS, University of Bern, Switzerland and Markasub AG, Switzerland). The heart of the system was a logger (CR10X, Campell, UK). Details about sensor types and installation are listed in Table 3. In addition to the measurements at the sites, climate data from the nearby national meteorological station at Sion (MeteoSwiss) were used to analyse the climate history of recent years (Fig. 1).
Sampling and analysing stem wood
Wood samples of two individuals of each of Q. pubescens, P. sylvestris, and P. abies (only at Jeizinen) were collected on eight days between April and October 2003 at both sites (Fig. 1; Table 3). The two individuals were a young and an old tree at each site. The wood samples were collected with an increment-punching tool which allowed thin cores of about 3 mm in diameter and about 1 cm in length to be extracted from the stem (Forster et al., 2000). On each sampling day, two cores were taken from each stem. In total, 64 samples were taken from Q. pubescens and P. sylvestris, and 32 from P. abies. All 16 spots to be cored on each stem were marked with paint within a square of about 15x40 cm on the slope-parallel side of the stem before starting the sampling in spring.
For every sample the number of current-year xylem cells were counted (only conifers) and the width of the growing tree ring and the five previous ones were measured. The values of the two cores per date and species were averaged. The information of the chronological samples from the same individual trees led to the corresponding growth curves.
Stem radius changes
Stem radius changes (R) were measured with point dendrometers (ZB01, University of Bern, Switzerland) on seven trees at Salgesch and six trees at Jeizinen (Table 3). The dendrometers were mounted 0.5 m above ground on the upslope (north) of each stem. The electronic part of the dendrometer was mounted on a carbon fibre frame which was fixed to the stem by three stainless steel threaded rods implanted into the heartwood. A sensing rod was slightly pressed against the tree stem by a spring. The contact point of the dendrometer head was positioned 1–6 mm into the bark surface, but still within the outermost dead layer of the bark. The sensitivity of the dendrometers to temperature was negligible because of the temperature-insensitive main parts, the carbon frame, and the electronic transformer (Weggeberpotentiometer LP-10F, Pewatron, Switzerland). Electronic resolution of the dendrometer (in combination with the logger used) was µm.
Deducing tree ring growth and tree water deficit from dendrometer measurements
To estimate intra-annual tree ring growth from R, an algorithm was applied which separated growth from water-storage-related stem radius changes (Zweifel et al., 2005). The growth- and water-related components were found to be the main factors for explaining the course of R (Zweifel et al., 2000, 2001; Daudet et al., 2005). Additional small effects of temperature and xylem-tension-related fluctuations contributed R and were not separable. The water-storage-related stem radius changes were expressed as the difference between the maximum and the actual hydration status of the stem (mainly the bark). This difference was called the tree water deficit according to Hinckley and Lassoie (1981) and Zweifel et al. (2005). It takes into account that all living parts of a tree are hydraulically interconnected and therefore a measured water deficit in the stem takes place in the whole tree.
Calculated physiological measurements
From microclimate and crown-specific parameters, a potential twig transpiration (PET) was calculated according to Zweifel et al. (2002), an adaptation of the Penman–Monteith single leaf model (Penman, 1948; Monteith, 1965). The ratio of measured twig sap flow rates (T) (Dynagage, Dynamax, USA) to estimated potential twig transpiration (T PET–1) was assumed to be a measure of the degree of the down-regulated transpiration and thus for the degree of stomatal aperture of the crown. Since stomatal closure is the strongest limitation for the net CO2 assimilation of trees at dry sites (Hsiao and Acevedo, 1974; Hinckley and Lassoie, 1981), T PET–1 is also a measure of the actual potential photosynthesis and therefore the potential C-income for the tree.
For the analyses, mean daytime (6–20 h) values for T PET–1were used. The 10 min values of the individual twigs were averaged per species. The sap flow gauges and the corresponding microclimate sensors used to calculate T PET–1 were mounted on the trees as listed in Table 3.