Niche modelling suggests low feasibility of assisted gene flow for a Neogene relict tree, Castanea sativa Mill.

Abstract: As many tree species populations are being degraded by climate change, adaptive conservation, and forest management, such as assisted gene flow (AGF), can provide the genetic variation needed to adapt to climate change. The core of this strategy is to assist the adaptation process in populations at risk of climate maladaptation by introducing individuals with beneficial alleles to cope with expected climate changes. Castanea sativa Mill. (sweet chestnut) is an essential component of natural forests in the Mediterranean and Caucasian regions, with a long history of cultivation. Current climate change may seriously threaten the long-term persistence of the species, particularly in the Caucasus region, where the largest range reductions are predicted. Here, we used Species Distribution Models (SDMs) to assess the feasibility of AGF in European and Caucasian populations of Castanea sativa. Bioclimatic variables for present (1981–2010) and future (2071–2100) conditions were obtained from the CHELSA climate database. The final models of future species ranges were averaged across three climate models (IPSL-CM6A-LR, MPIESM1-2-HR and UKESM1-0-L) and three climate change scenarios – SSP1-2.6, SSP3-7.0 and SSP5-8.5. There are marked differences in the climatic niches of the Iberian, Alpine-Apennine, Balkan, and Caucasian populations, with significant implications for AGF. The most suitable European areas for the Caucasian populations were found only in the Adriatic region. The Iberian populations were not compatible with the predicted future climate in the Caucasus in any of the scenarios tested. Suitable areas for Alpine-Apennine populations within the AGF strategy were predicted in the Colchic lowlands, the eastern Pontic mountains and the Hyrcanian forests in the SSP1-2.6 and SSP3-7.0 climate change scenarios. In contrast, the Balkan populations would be compatible at most with the western Pontic mountains and, to a lesser extent, with the Hyrcanian forests. According to the most damaging climate scenario SSP5-8.5, the potential of AGF in the Caucasus with Alpine-Apennine and Balkan populations could be very limited. Our study showed limited applicability of AGF for Castanea sativa between the European and Caucasian populations due to low climate match. Genomic modelling is needed to fully assess the feasibility of this strategy in the species.