Are drought-resistant non-native species adapted to Central European cold winter?

The increasing frequency and intensity of severe droughts linked to climate change has recently led to significant regional dieback of certain forest species, even at the core of their geographical distribution. The introduction of non-native species, i.e. by assisted migration, from regions with climatic conditions similar to those expected for the target area, could complement other management strategies to maintain sustainable ecosystem services in the future. However, not only drought but also frost is a key physiological stressor shaping trees species distribution. It is therefore essential to assess to what extent the introduced species can withstand cold temperatures and potential extreme frost events. Buds which comprise the leaf primordia are the most vulnerable part of the tree to winter frost damage and different species have evolved contrasting morphological and biochemical strategies to achieve varying degrees of  cold-hardiness in winter.

To assess whether, and to what degree, buds from non-native species can withstand severe frosts compared to the phylogenetically related native-species, we sampled branches from five non-native species (Tilia tomentosa, Fagus orientalis, Abies bornmuelleriana, Cedar libani, and Tsuga heterophylla) planted in Switzerland in 2012 and, when present, branches from native species phylogentenically close (Tilia cordata, Fagus sylvatica, Abies alba, and Quercus petraea) growing in the close proximity of the experimental stand. Since cell tolerance to cold temperature is a dynamic trait, reaching a minimum value in deep winter and increasing in spring, we repeated the sampling in January and February 2025 and combined it with an artifical hardening and dehardening treatment (3 days at -4°C and +15°C, respectively). Shortly after sampling, or after the hardening/dehardening treatment, we exposed the buds to different freezing temperatures (-8, -15, -2, -25, -30, and -35°C) and measured cold hardiness of each species using the electrolyte leakage method. The hardening and dehardening treatments aimed to determine the species-specific maximum frost resistance as well their capacity to loose freezing resistance when exposed to a warm spell, which can be critical in the study climate. Additionally, to assess the depth of winter dormancy across all studied species, we placed twigs from each species in a growth chamber set at +20°C, and monitored the time to budbreak and the subsequent phenological development.

We hypothesize that (i) native species are better adapted to withstand extreme frost events compared to the non-native counterparts, and that (ii) non-native species will exhibit higher phenological plasticity by tracking earlier warming at the start of the season, allowing them to take advantage of the favourable growing conditions but potentially exposing them to frost. In contrast, native species, being adapted to the local climate, may escape the frost damage through a later phenological development (temporal frost exclusion) but might be less responsive to take advantage of extended favourable growing season induced by climate warming.