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Irrigation Experiment Pfynwald
Switzerland, the temperature increase in the late 20th and beginning
of the 21st century was more than twice the global average. Climate
models predict a further increase during the 21st century and
beyond. In correspondence with the warming-induced increase in evaporation, a
change in water supply is expected with increased frequency of summer
heatwaves, but also an increase in frequency and intensity of precipitation
events with strong surface runoff, probably further enhancing drought stress
for plants. Drought periods severely affect forest productivity, decrease tree
vigor and reduce tree growth. Drought is an important trigger for forest decline,
mortality and decline-induced vegetation shifts worldwide and in forest
ecosystems in inner-Alpine dry valleys such as the Valais (Rigling et al. 2013).
The impact assessment of drought and drought release on forests is
challenged by the complexity of these ecosystems and the longevity of the
trees. Hence, large-scale drought experiments and long-term field measurements
of plant water stress in young and mature forests are needed. Recently,
Leuzinger et al. (2011) discussed the need of global change experiments and
their restrictions with respect to i)
degree of complexity depicted, ii)
time scale, and iii) spatial
coverage. It seems that many experiments overestimate the net ecosystem
responses because of a too short time scales or due to the young age of investigated
trees. Hence, larger-scale and fully coupled field experiments with longer time
scales and mature trees are needed to study the impact of drought and drought
release on forest ecosystems and their net responses.
To study the performance of mature Scots pine (Pinus sylvestris L.) under chronic drought conditions in comparison to their immediate physiological response to drought release, a controlled long-term and large-scale irrigation experiment has been set up in 2003. The experiment is located in a xeric mature Scots pine forest in the Pfynwald (46º 18' N, 7º 36' E, 615 m a.s.l.) in one of the driest inner-Alpine valleys of the European Alps, the Valais (mean annual temperature: 9.2°C, annual precipitation sum: 657 mm, both 1961-1990). Tree age is on average 100 years, the top height is 10.8 m and the stand density is 730 stems/ha with a basal area of 27.3 m2/ha. The forest is described as Erico Pinetum sylvestris (Dobbertin et al. 2010) and the soil is a shallow pararendzina characterized by low water retention (Brunner et al. 2009). The experimental site (1.2 ha; 800 trees) is split up into eight plots of 1'000 m2 each (Fig. 3). During April-October, irrigation is applied on four randomly selected plots with sprinklers of 1 m height at night using water from an adjacent water channel. The amount of irrigation corresponds to a supplementary rainfall of 700 mm/year. Trees in the other four plots grow under naturally dry conditions. Soil moisture has been monitored since the beginning of the project at 3 soil depths (10, 20 and 60 cm) (Fig. 4). The duration of the irrigation experiment is 20 years, hence it will end in 2022.
Almost immediately after the start in 2003, the production of mycorrhiza fruit bodies started to increase (Hutter et al. in prep.). With a delay of one year, in 2004 tree-ring width and needle length increased, whereas shoot growth remained stable until 2005 and the stand leave area index increased by 20% until 2006 (Dobbertin et al. 2010). Irrigation increased the fine roots’ biomass during the season, but surprisingly no effect was detectable between years until 2005 (Brunner et al. 2009). A significant shortening of the growth period by 2-5 weeks in the non-irrigated trees in comparison to the irrigated trees was observed (Fig. 4) (Eilmann et al. 2010). Based on an initial analysis of δ13C in wood, we conclude a tight coupling between wood formation and freshly produced assimilates and low availability of old stored carbohydrates for trees exposed to chronic drought (Eilmann et al. 2011). Data from the irrigation experiment Pfynwald have also been considered for two studies comparing hydraulic traits from Scots pine across Europe (Martinez-Vialta et al. 2009, Sterck et al. 2012). The ratio of leaf- to sapwood-area was found to be one of the rare traits that consistently reflected drought conditions at the very site. Since 2003, yearly crown assessments and continuous measurements of soil moisture, soil and air temperature and precipitation have been conducted. Continuous sap flow and diameter growth (point dendrometers) has been measured on six trees (three control and irrigated trees respectively) since spring 2010. Resin flow and intra-annual diameter growth (band dendrometers) of 60 trees was measured in 2011 and will be repeated in 2013/14. In addition, soil respiration was measured in 2003-2005 and has been repeated since 2012. This year, investigations on the tree hydro-system using wood anatomy (P. Fonti & G. von Arx, WSL) have started. In 2013/14 leaf gas exchange and hydraulic measurements are conducted in collaboration with M. Mencuccini (University of Edinburgh/University of Barcelona)(Fig 5).
The irrigation treatment will be stopped on sub-plots of 10 x 25 m within each of the irrigated plots in October 2013 to simulate the ecosystem response and resilience to drier conditions. The irrigation treatment on the remaining plots will continue. Hence three treatments will be compared and analyzed: i) control trees growing under local dry climate, ii) irrigated trees, and iii) trees exposed to irrigation-stop (Fig. 3).
We acknowledge the long-term support by HYDRO Exploitation SA in Sion.