Abbreviations: CT: current temperature; FT: future temperature; iWUE: intrinsic water use 22 efficiency; Ψ: plant water potential. 23 Abstract 26 Changes in climate can alter the distribution and population dynamics of tree species by 27 altering their recruitment patterns, especially at range edges. However, geographical patterns 28 of genetic diversity could buffer the negative consequences of changing climate at rear range 29 edges where populations might also harbour individuals with

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... -adapted genotypes. 30 Silver fir (Abies alba Mill.) reaches its south-western distribution limit in the Spanish 31 Pyrenees, where recent climatic dieback events have disproportionately affected westernmost 32 populations. We hypothesised that silver fir populations from the eastern Pyrenees are less 33 vulnerable to the expected changing climate due to the inclusion of drought-resistant 34 genotypes. We performed an experiment under strictly-controlled conditions simulating 35 projected warming and drought compared with current conditions and analysed physiology, 36 growth and survival of silver fir seedlings collected from eastern and western Pyrenean 37 populations. Genetic analyses separated eastern and western provenances in two different 38 lineages. Climate treatments affected seedling morphology and survival of both lineages in an 39 overall similar way: elevated drought diminished survival and induced a higher biomass 40 allocation to roots. Increased temperature and drought provoked more negative stem water 41 potentials and increased δ 13 C ratios in leaves. Warming reduced nitrogen concentration and 42 increased soluble sugar content in leaves, whereas drought increased nitrogen concentration. 43 Lineage affected these physiological parameters, with western seedlings being more sensitive 44 to warming and drought increase in terms of δ 13 C, nitrogen and content of soluble sugars. Our 45 results demonstrate that, in Abies alba, differences in the physiological response of this 46 species to drought are also associated with differences in biogeographical history. 47 48 Page 2 of 69 Tree Physiology 49 The increase in temperature recorded during the last decades has the capacity to alter the 50 phenology, growth and biotic interactions of plant species worldwide (Parmesan 2006; 51 Walther 2010). These impacts are expected to be especially evident over relatively small 52 spatial scales in mountain ecosystems, where the elevational ranges and shifts of trees are to a 53 great extent controlled by temperature (Peñuelas and Boada 2003) which declines rapidly 54 with increasing elevation. In addition, temperature changes will be accompanied by an 55 alteration of current precipitation patterns, for instance with a generalised increase of the 56 length and intensity of summer drought in central and southern European regions (Giorgi and 57 Lionello 2008). These climatic alterations are likely to have important consequences for tree 58 species dynamics at local and regional scales (Peñuelas and Boada 2003; Van Mantgem et al. 59 2009; Matías and Jump 2015). 60 61 Changing climatic conditions are especially relevant close to the equatorial limit of 62 species distributions (the rear, or trailing edge), where climatic conditions often correspond to 63 the species drought-tolerance limits (Hampe and Petit 2005). In these locations, even small 64 variations in climate could result in profound demographic effects, such as lower relative 65 fecundity and reduction of local population densities (Case and Taper 2000). Such changes 66 could result in reduced resilience under adverse climate conditions and eventual alteration of 67 species distributions. In the warmest regions of species distributions, the intensity of 68 population responses to drought stress are likely higher than in colder areas, being more prone 69 to local extinction as a consequence of extreme climatic events (Jump et al. 2009; Carrer et al. 70 2010). Alternatively, although the historical isolation and fragmentation of these range edge 71 populations has often resulted in impoverished genetic diversity within populations (Hampe 72