500 years of forest growth dynamics in the high mountain valley of Avers, Switzerland

Context and aim: This thesis is a subproject conducted as part of the broader research project “History of climate, population, land use and landscape in the region of Avers” at WSL. The project results will lay the foundation for developing appropriate adaption to the current forest management concepts in Avers. The aim of this thesis is to analyze forest (i) stand and (ii) growth dynamics in Avers. (iii) Climatic and (iv) non-climatic, site-specific factors influencing tree growth are also analyzed. The results are discussed in relation to the region’s past land-use history. Methods: Features and increment cores of 892 trees along seven altitudinal gradients were collected, their ring widths measured and crossdated. Several tree-ring chronologies were established using regional curve standardization (RCS) and spline detrending methods. 30-year splines with a 50% frequency cutoff were used for moving-correlation analysis between the annual summer temperature and radial growth since 1930. Linear mixed effects (LME) models were calculated to assess non-climatic and site-specific effects on tree growth. Results: (i) Apart from the stand of Croeterwald, the age distribution shows widespread establishment of larch (Larix decidua) and stone-pine (Pinus cembra) during the 20th century. (ii) Old-growth (monumental) trees of different stands, elevation and species show similar growth trends during the past 500 years. Since the 1980s, low-elevation stone-pine show a decreasing growth trend, while both high- and low-elevation larch show a pronounced growth increase. (iii) A singularly important factor influencing tree growth are summer temperatures. The sensitivity of trees towards the latter has been undergoing an upward-shift on the altitudinal gradient since the 1980s. (iv) LME models highlighted the pioneer character of larch, as well as the importance of low crown coverage for the establishment of regeneration. Conclusion: Complete or near abandonment of forest pasture in Capetta- and Hohenhauswald has led to a widespread establishment of trees during the 20th century. Ongoing climate change is leading to an upward shift in temperature sensitivity of both larch and stone-pine along the altitudinal gradient. This particularly impedes the growth of low-elevation stone-pine. Three measures should be applied to increase tree and stand resilience: (1) Tree density should be reduced around monumental trees to avoid increased competition for light, water and nutrients. (2) Thinning interventions in Hohenhauswald will allow recently established trees to serve as stabilizing elements. (3) Between the monumental trees in Croeter- and Hohenhauswald, new regeneration should be initiated and favored. Keeping open but sustainable structures allows grass species to develop, which is important for pasturing and biodiversity. Further, it maintains the forests’ touristic and cultural historic value and safeguards their uniqueness for future generations