Climate change is having a dramatic effect on pine forests in the low, south-facing regions of the Chur Rhine valley and in Domleschg. With a two-degree rise in the annual average temperature and the same level of precipitation as 50 years ago, Scots pines need more water than is available and are currently dying off. If the forest is to continue to carry out its various roles, young trees must replace the ones that are dying. A key challenge is to determine which locations still have a sufficient level of soil moisture for Scots pine seeds to germinate and for their fragile seedlings to grow.
Between 2009 and 2014, WSL carried out an experiment in collaboration with the Office for Forestry and Natural Hazards (Amt für Wald und Naturgefahren) in canton Graubünden, in which it tested the germination and development of Scots pines and spruce trees in the Rhine and Rhone valleys, as well as those from continental areas of eastern Europe and the Mediterranean region. “By focusing on germination, we wanted to find out whether seeds from dry areas were better suited to the future climate of the Rhine valley than native ones,” says Barbara Moser from the Disturbance Ecology research group. The team sowed seeds on south-facing woodland areas with different soil characteristics situated less than 1000 meters above sea level, and altered the soil moisture using small roofs that provided varying degrees of shelter from rain.
Spring weather conditions are decisive
The most decisive factor in the successful germination of seeds and the development of seedlings was the spring weather conditions. In 2013, these were wet; not only did significantly more young trees survive than in the dry spring of 2011, but they also had five times as much biomass two growing seasons later. Dry summers decreased the growth and proportion of young trees germinated in a wet spring only slightly. With sufficient rainfall in spring, Swiss seedlings did better than those from other countries; however, in the dry conditions of 2011, all the seedlings grew slowly and produced very little biomass.
Barbara Moser draws two conclusions from the experiment: With occasional damp springs and disturbances that bring light into the forest, native Scots pines should rejuvenate regularly, even in very sunny areas, and help to ensure the continued survival of these forests. Scots pine regeneration is threatened only in soils that retain very little water. With consistent spring rain, Scots pines from already dry regions of eastern and southern European regions are unlikely to represent an alternative to native pines. However, should it become so warm by the end of the 21st century that almost every spring is dry, Moser recommends that other species are tested. (Reinhard Lässig, Diagonal 1/16)