Can trait plasticity across a pedo-climatic gradient predict future-climate suitability in non-native tree species?

Phenotypic plasticity describes the ability of a species to adjust its phenotype in response to environmental variation. While functional trait plasticity is well documented for several native European tree species (e.g., Fagus sylvatica, Quercus petraea), much less is known about the capacity of non-native species to adjust to contrasting pedo-climatic conditions. This knowledge gap is increasingly relevant because species originating from drought-prone regions are receiving growing attention as potential candidates for assisted migration strategies aimed at mitigating climate-change impacts on forests.To evaluate trait plasticity and drought tolerance in non-native tree species, we take advantage of a multi-site plantation established in 2012 along a hydroclimatic gradient spanning Switzerland and Germany. For each species, individuals from the same source population were planted after 2–3 years of nursery growth at each site in a randomized block design. The study sites differ strongly in long-term precipitation (1984–2024), with the Swiss site Mutrux being the wettest (MUT, 1324 mm), followed by the German sites Schmellenhof (SCH, 750 mm), Grossostheim (GRO, 635 mm) and Oldisleben (OLD, 481 mm). In addition, sites vary in subsurface properties and soil texture, which ultimately modulate plant-available water.Across 2024 and 2025, we monitored key meteorological variables (air temperature, precipitation, relative humidity and photosynthetically active radiation), soil water potential and soil temperature, and we used phenocams to assess the timing of phenological events over the growing season. In summer 2024, we sampled branches from five non-native species (Tilia tomentosa, Fagus orientalis, Abies bornmuelleriana, Cedrus libani and Tsuga heterophylla) and a native reference species (Quercus robur in GRO and SCH, and Quercus petraea in MUT and OLD). We assessed predawn leaf/needle water potential and collected vegetative material to examine the hydraulic, morphological and anatomical properties of photosynthetic organs. In summer 2025, we revisited the same individuals to determine the turgor loss point (TLP) of leaves and needles as a functional proxy for drought tolerance. We also use tree growth (diameter and height) and vitality inventories as traditional indicators of species performance.We hypothesize that species showing greater trait plasticity across sites represent better candidates for assisted migration, as they may display higher adaptive potential to local pedo-climatic conditions.