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European beech (Fagus sylvatica L.) forests have recently experienced severe diebacks that are expected to increase in future. Oriental beech (Fagus sylvatica spp. orientalis (Lipsky) Greut. & Burd) is a potential candidate for assisted migration (AM) in European forests due to its greater genetic diversity and potentially higher drought resistance. Yet AM entails not only benefits, but also risks, and it is therefore important to monitor the progression of introduced (sub)species. Here, we demonstrate the potential of leaf spectroscopy to replace resource-intensive genetic analysis and field phenotyping for the discrimination and characterization of these two beech subspecies. We studied two European beech forests, one in France and one in Switzerland, where Oriental beech from the Greater Caucasus was introduced over 100 years ago. During two summers (2021, 2022), we measured leaf spectral reflectance, leaf morphological and biochemical traits from genotyped adult trees. Subspecies prediction models were developed separately for top-of-canopy leaves (amenable to remote sensing) and bottom-of-canopy leaves (easier to harvest) using partial least squares discriminant analysis (PLS-DA) and different sets of spectral predictors. Morphological, biochemical and spectra-derived leaf traits indicated that Oriental beech trees at the sites studied were characterized by higher lignin and nitrogen per unit leaf area than European beech, suggesting more protein-rich leaves on a per-area basis. The model based on top-of-canopy leaf reflectance spectra in the short-wave-infrared region (SWIR I: 1450–1750 nm) most accurately distinguished Oriental from European beech (BA = 0.86 ± 0.08, k = 0.72 ± 0.15), closely followed by models based on SWIR II, and on spectra-derived traits (BA ≥ 0.84, k ≥ 0.67). This study provides a proof-of-principle for the development of spectroscopy-based approaches when monitoring introduced species, subspecies or provenances. Our findings hold promise for upscaling to large forest areas using airborne remote sensing.

Assisted migration programs, introducing new better adapted species at critical locations in our forests, have the potential to mitigate the adverse effects of climate change through the increase of forest diversity and resilience. While such measures entail ecological risks related with invasiveness of exotic species or outbreaks of new diseases, introducing close relatives of native species or populations from different parts of the species range is seen as the ecologically safer option. However, due to the similar appearance of closely related species, monitoring based on the external phenotype becomes difficult and leaves genetic screening as the only reliable, yet expensive option, limiting our ability to monitor large geographic areas. Reflectance spectroscopy has emerged as an important tool to assess plant functional trait distributions and taxonomic diversity, representing a rapid, scalable and integrated measure of the plant external and internal phenotype.Here, we examine the potential of leaf-level reflectance spectroscopy to discriminate between the subspecies European beech (Fagus sylvatica L.), and Oriental beech (Fagus sylvatica spp Orientalis (Lipsky) Greut. & Burd), which has been proposed as a potential candidate for assisted migration in European forests due to its greater genetic diversity and potentially higher drought tolerance. We investigated two European beech forests in France and Switzerland where Oriental beech from the Caucasus was introduced over 100 years ago next to European beech. Over the summers of 2021 and 2022, we measured leaf spectral reflectance and leaf morphological and biochemical traits from previously genotyped adult trees.Using least squares discriminant analysis (PLS-DA), we found that leaf spectral reflectance allowed the accurate discrimination of the two beech subspecies. In particular, we found that the short-wave-infrared (SWIR) region between 1450-1750 nm from top-of-canopy leaves provided the most accurate subspecies discrimination (BA = 0.86±0.08, k = 0.72±0.15). To provide a mechanistic basis of our findings, we estimated a suite of leaf traits based on spectra-derived indices and standard field and lab protocols. Phenotyping confirmed significant subspecies differences between traits that are known to govern light-plant interactions in the SWIR, including lignin, nitrogen in proteins, leaf mass per area and leaf thickness.Our study provides a basis for crown-level subspecies classifications from airborne or satellite-based imagery in the genus Fagus. Our findings provide an important starting point for the interpretation of variability in tree crown reflectance and the superior discrimination capacity we found for leaves at the top as compared to leaves at the bottom of the canopy, holds promise for the upscaling of the method using remote sensing.

The benefits and risks of human-aided translocation of individuals within the species range, assisted gene flow (AGF), depend on the genetic divergence, on the rate and direction of hybridization, and on the climate transfer distance between the host and donor populations. In this study, we explored the use of Oriental beech (Fagus sylvatica subsp. orientalis), growing from Iran to the Balkans, for AGF into European beech populations (F. sylvatica subsp. sylvatica) that increasingly suffer from climate warming. Using samples from natural populations of Oriental and European beech and microsatellite loci, we identified 5 distinct genetic clusters in Oriental beech with a divergence (FST) of 0.15 to 0.25 from European beech. Using this knowledge, we traced the origin of 11 Oriental beech stands in Western Europe established during the 20th century. In two stands of Greater Caucasus origin, we found evidence for extensive hybridization, with 18% and 41% of the offspring having hybrid status. Climate data revealed higher seasonality with warmer and drier summers across the native Oriental beech sites in comparison to the planting sites in Western Europe. Accordingly, we found that bud burst of Oriental beech occurred four days earlier than in European beech. Overall, our results suggest that AGF of Oriental beech could increase the genetic diversity of European beech stands and may foster introgression of variants adapted to expected future climatic conditions. Our study showcases the evaluation of the benefits and risks of AGF and call for similar studies on other native tree species., Forest Ecology and Management, 531, ISSN:0378-1127, ISSN:1872-7042