The Pfynwald research platform

Results from experiments in dry areas with a long series of in-situ measurements are necessary for a better understanding of the medium- to long-term effects of drought periods on forests.

As the largest contiguous pine forest in Switzerland, the Pfyn forest in Canton Valais (46° 18' N, 7° 36' E, 615 m asl) offers the best conditions for such measurements. In light of this, a WSL research team set up a long-term experiment that is to last 30 years in this forest.  The average temperature here is 10.6°C (average 1995-2014), the yearly accumulated precipitation is 575 mm (average 1995-2014). The pines in the middle of the forest are about 120 years old and 11 m high. The test area comprises 876 trees over a surface of 1.2 ha divided into 8 plots of 1000 m² each (Figure 4). Between April and October, four of these plots are irrigated by a sprinkler system which provides an additional 600 mm of water annually. In the other four plots, the trees grow under natural, hence relatively dry conditions.

Almost immediately after measurements began in 2003, a considerable increase in the production of mycorrhiza fruit bodies was noticeable in the irrigated subplots. After a one-year lag, the pines have been forming wider tree rings and longer needles than before since 2004. In the following years, the length of the height and branch shoots and the stand density increased as well. From the summer 2006, irrigation has also increased root growth and led to more biomass, especially in the fine roots (Brunner et al. 2009). The growth period of irrigated trees was extended by 2 -5 weeks (Eilmann et al. 2010).

Over the entire experimental period from 2003 to 2019, the irrigation experiment allowed us to track the recovery trajectories in trees and in the whole ecosystem released from the natural dry conditions. The monitoring data over 16 years showed that irrigation improved the soil water availability, and 120-year-old Scots pine trees were able to regain vigor by increasing shoot length, needle length and leaf area, and by decreasing crown transparency.

We detected rapid and stronger responses from above-ground tree traits compared to below-ground tree traits. The altered above-ground traits during the initial years of irrigation increased the water demand. Trees adjusted by increasing root biomass during the later years of irrigation, resulting in an increased survival rate of Scots pine trees in irrigated plots. However, after reaching the peak in 2006, the magnitude of the impact of irrigation on a number of tree-related variables declined in the following years. This declining trend after reaching the peak may indicate that the water supply at a constant rate over the years did not meet the progressively increasing water demand from increasing vegetation activities.

The results indicate that increased water availability in the long term changed tree and ecosystem properties in a way that a new balance between soil water availability and water demand is reached, which changed the boundary conditions of the ecosystem. The irrigation also stimulated foliar decomposition rate at ecosystem level, biomass of fungal fruit body, and abundance of regeneration in broadleaved tree species. However, irrigation did not promote the regeneration of Scots pine trees, which are reported to be vulnerable to extreme droughts (Bose et al. 2022).

Earthworms in drought sleep: Due to climate change, dry periods are getting longer and longer, which is putting a strain on soil organisms. A long-term study in the Pfynwald forest shows just how much. (Kanal 9, 9.06.2022).

Michèle Ursprung

Dry forest soils have a negative effect on climate change because earthworms in particular can no longer contribute well to fixing carbon in the soils, explains material cycle expert Frank Hagedorn in Dlf. Earthworms decompose less leaf litter during drought. In the long term, less carbon and thus also climate-impacting CO2 would be stored in the soil.

Krauter, Ralf, Deutschlandfunk | 01. 06. 2022, 16:47

A report by [w] wie Wissen about the 'Wood Wide Web' in the Pfynwald, with Martina Peter and members of her team (Das Erste, 26.09.2020).

What does it sound like in the ground? What does the forest sound like in a dry summer? Eco-acousticians like the Swiss sound researcher Marcus Maeder describe nature in a completely new way with the help of sound recordings. Sound becomes an indicator of an ecosystem. "Einstein" shows what opportunities ecoacoustics opens up.

The pine forests in the Valais are threatened. In some regions, up to 40 percent of the trees have died during the last two years. This is because it has been particularly hot and dry. This reduces the protection that the affected forests offer against rockfall and avalanches.