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Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in Southwestern Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (d18O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical-chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate-growth relationships and lower d18O than ND trees. The uncoupling between iWUE and d18O, together with the positive correlations between P and K leaf concentrations and d18O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies. [ABSTRACT FROM AUTHOR]

Warming-related growth decrease on southern Fagus sylvatica forests has been observed in different regions; however, whether it is a generalized fact or not remains unclear. Here we investigate the geographical pattern on growth response of the southwestern European beech forests to the warming climate shift which started in the 1980s. We sampled 15 beech forests (215 trees) across four climatically contrasting regions (Mediterranean, Pyrenean, low- and high-elevation Atlantic areas) near the southern distribution limit of the species in the Iberian Peninsula. Dendrochronological analyses were carried out to evaluate the growth of European beech since the 1950s. Growth responses quantified as pointer years, abrupt growth changes and long-term growth trends were compared between periods (before and after the 1980s climate shift), geographical regions and tree sizes. Analyses of the studied variables indicated a growth decrease in basal area increment after the climate shift in three of the four studied regions. Pyrenean stands were not negatively influenced by the climate shift, although an increase in the frequency of negative abrupt growth changes was also found there. Growth after the climate shift presented divergent patterns depending on the geographical region. Although Mediterranean and Atlantic stands presented different indicators of constrained growth, Pyrenean stands showed rising long-term growth trends. Such results suggest that regional characteristics differentially determine the growth response of the southern European beech forests to recent warming periods. Iberian beech forests located at the Pyrenees would benefit from forecasted warming conditions, whereas Atlantic and Mediterranean forests would be more prone to suffer warming-related growth decline. [ABSTRACT FROM AUTHOR]

• Silver fir forests show different degrees of dieback in the western Pyrenees. • Dieback and growth decline were caused by late-summer drought stress. • The Vaganov-Shashkin (VS) model simulated silver-fir growth and identified its climatic constraints. • Simulated growth series showed a decoupling with respect the climate baseline. • The VS model allows identifying vulnerable forests and early signals of dieback. Assessing tree growth patterns and deviations from expected climate baselines across wide environmental gradients is fundamental to determine forest vulnerability to drought. This need is particularly compelling for the southernmost limit of the tree species distribution where hot droughts often trigger forest dieback processes. This is the case of some silver fir (Abies alba) populations located in southwestern Europe (Spanish Pyrenees) which present ongoing dieback processes since the 1980s. We sampled 21 silver fir stands showing different dieback intensity, assessed using defoliation levels, quantified their growth patterns and characterized their responses to climate. Then, we assessed growth deviations from climatic predictions using the process-based Vaganov-Shashkin (VS) growth model. The forests showing most intense dieback, i.e. highest defoliation levels, were mainly located in low-elevation sites of the western Pyrenees. Trees in these stands displayed the lowest growth rates and the highest year-to-year variability in growth and their growth was limited by late-summer evaporative demand. In eastern and central Pyrenees, we detected a mild growth limitation by low soil moisture during the late growing season and positive growth recovery in recent years with respect to a climate baseline. Decreasing growth trajectories were the most common pattern, while rising trends were common in stands with low dieback in eastern and central Pyrenees. Our results portend systematic spatial variability of growth trends across the Pyrenean silver fir populations forming the south-western distribution limit of the species in Europe. Decoupling of growth between eastern and western populations observed in the recent decades suggests contrasting vulnerability to climate change, and more importantly, the decoupling of growth patterns in western clusters could be used as an early-warning signal of impending dieback. Consequently, we foresee future dieback events to have more detrimental effects in the western compared with the eastern Pyrenees. [ABSTRACT FROM AUTHOR]

• Droughts and frosts impact growth in species southernmost distribution limits. • Droughts and frosts impair Silver fir and European beech radial growth. • Silver fir growth is more affected by drought than by late frosts. • European beech growth is more affected by late frosts than by drought. • We could not find interactive effects of drought and late frosts on growth. Climate warming has lengthened the growing season by advancing leaf unfolding in many temperate tree species. However, an earlier leaf unfolding increases also the risk of frost damage in spring which may reduce tree radial growth. In equatorward populations of temperate tree species, both late frosts and summer droughts impose two constraints to tree growth, but their effects on growth are understudied. We used a tree-ring network of 71 forests to evaluate the potential influence of late frosts and summer droughts on growth in two tree species that reach their southern distribution limits in north-eastern Spain: the deciduous European beech (Fagus sylvatica L.) and the evergreen Silver fir (Abies alba Mill). The occurrence of late frost events and summer drought was quantified by using a high-resolution daily temperature and precipitation dataset considering the period 1950–2012. Late frosts were defined as days with average temperature below 0 °C in the site-specific frost-free period, whereas drought was quantified using the 18 month-long August Standardized Precipitation Evapotranspiration Index (SPEI). The growth of European beech and Silver fir was reduced by the occurrence of both late frost events and summer drought. However, we did not find a significant interaction on growth of these two climate extremes. Beech was more negatively impacted by late frosts, whereas Silver fir was more impacted by summer drought. Further studies could use remote-sensing information or in situ phenological records to refine our frost index and better elucidate how late frosts affect growth, whether they interact with drought to constrain growth, and how resilience mechanisms related to post-frost refoliation operate in beech. [ABSTRACT FROM AUTHOR]

• Climate warming may favor treeline advances while threatening rear-edge tree populations. • Summer precipitation influences growth in mountain pine rear-edge populations. • Populations subjected to contrasting environmental conditions vary in leaf morphology. • Soil nutrients and microbial community structure vary along treeline ecotones. • Interactions between tree features and soil conditions indicate complex plant-soil feedbacks. Alpine treelines are expected to shift upward due to climate warming, whereas warmer conditions can have negative impacts on forests located near the xeric, equatorward limit of the distribution of tree species (rear edge). We compare tree populations forming the distribution limits of mountain pine (Pinus uncinata) in north-eastern Spain: two cold-limited alpine treeline populations, and a rear-edge drought-prone stand. In the treelines, trees were sampled in three altitudinal belts to evaluate within-site variations considering the forest, the transitional ecotone and the treeline. Tree growth was markedly higher in one of the treeline sites (Tesso), particularly in the forest belt, as compared to the other treeline (Las Cutas), and to the warmest rear-edge stand. Tree growth in the rear-edge population depended on June precipitation. Trees presented also comparatively smaller leaves with higher Specific Leaf Area (SLA) there than in the two treelines. Regarding the differences between altitudinal belts within the treelines, treeline soils showed lower N concentrations and a lower content of sand than in forests. Soil microbiota was dominated by bacteria in the treeline and by fungi in the forest, although with differences between sites. Soil characteristics and microbial composition were strongly related, whereas its relationship with tree growth and functional traits was less clear. The marked differences in leaf traits and growth response to climate found when comparing treeline and rear-edge sites highlights the biogeographical uniqueness of sites forming the equatorward distribution limit. The greater tree growth in the Tesso treeline as compared to Las Cutas indicate that interactions between climate and physical and chemical properties of the soil influence tree growth and its interactions with soil microbial communities in the treeline. A better understanding of plant-soil feedbacks may help to understand the future dynamics of tree populations forming the species' climatic and geographic limits of distribution. [ABSTRACT FROM AUTHOR]

• The seasonal growth of Pinus uncinata forests was affected by snow dynamics. • Snow dynamics exerted strong influence on soil temperature. • Soil temperature has a key role in Pinus uncinata wood formation. Snow dynamics are key to understanding tree growth in mountain forests and future response to climate change. However, precise monitoring of microclimate conditions and variables related to tree growth and functioning are lacking. To advance on those issues, snow cover and microclimate conditions, tree phenology, xylogenesis, intra-annual radial growth and the concentration of sapwood and needle non-structural carbohydrates were intensively monitored in four Pinus uncinata forests along an altitudinal gradient over three years in a Pyrenean valley (NE Spain). Snow dynamics exerted strong influence on soil temperature and moisture, particularly before and during the early growing season. Soil temperature was the most relevant microclimate variable during the overall xylogenesis, mainly influencing the production of mature tracheids. Large snow accumulation resulted in later snow depletion and a consequent delay in soil warming onset. Low soil temperatures in the spring, related to prolonged snow persistence, retarded cambial reactivation and led to lower growth rate. Despite strong spatial variability among plots, wood production was determined by snow dynamics in three out of the four studied plots. This study highlights the major role played by early and late growing season soil temperatures on radial growth of mountain conifers. The results of this study suggest that a future shallower and more transitory snowpack in the studied forests, together with warmer soil and air temperatures, may increase radial growth and productivity of similar mid-latitude, young mountain forests. [ABSTRACT FROM AUTHOR]