Forecasted increase drought frequency and severity may drive worldwide declines in forest productivity. Species-level responses to a drier world are likely to be influenced by their functional traits. Here, we analyse forest resilience to drought using an extensive network of tree-ring width data and satellite imagery. We compiled proxies of forest growth and productivity (TRWi, absolutely dated ring-width indices; NDVI, Normalized Difference Vegetation Index) for 11 tree species and 502 forests in Spain corresponding to Mediterranean, temperate, and continental biomes. Four different components of forest resilience to drought were calculated based on TRWi and NDVI data before, during, and after four major droughts (1986, 1994-1995, 1999, and 2005), and pointed out that TRWi data were more sensitive metrics of forest resilience to drought than NDVI data. Resilience was related to both drought severity and forest composition. Evergreen gymnosperms dominating semi-arid Mediterranean forests showed the lowest resistance to drought, but higher recovery than deciduous angiosperms dominating humid temperate forests. Moreover, semi-arid gymnosperm forests presented a negative temporal trend in the resistance to drought, but this pattern was absent in continental and temperate forests. Although gymnosperms in dry Mediterranean forests showed a faster recovery after drought, their recovery potential could be constrained if droughts become more frequent. Conversely, angiosperms and gymnosperms inhabiting temperate and continental sites might have problems to recover after more intense droughts since they resist drought but are less able to recover afterwards.

The rise in atmospheric CO 2 concentrations (Ca) has been related to tree growth enhancement and increasing intrinsic water-use efficiency (iWUE). However, the extent that rising Ca has led to increased long-term iWUE and whether climate could explain deviations from expected Ca-induced growth enhancement are still poorly understood. The aim of this research was to use Ca and local climatic variability to explain changes during the 20th century in growth and tree ring and needle d 13 C in declining and nondeclining Abies alba stands from the Spanish Pyrenees, near the southern distribution limit of this species. The temporal trends of iWUE were calculated under three theoretical scenarios for the regulation of plant-gas exchange at increasing Ca. We tested different linear mixed-effects models by multimodel selection criteria to predict basal area increment (BAI), a proxy of tree radial growth, using these scenarios and local temperature together with precipitation data as predictors. The theoretical scenario assuming the strongest response to Ca explained 66-81% of the iWUE variance and 28-56% of the observed BAI variance, whereas local climatic variables together explained less than 11-21% of the BAI variance. Our results are consistent with a droughtinduced limitation of the tree growth response to rising CO 2 and a decreasing rate of iWUE improvement from the 1980s onward in declining A. alba stands subjected to lower water availability.

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AimsThe combined effects of changes in climate and land use on tree mortality and growth patterns have rarely been addressed. Relict tree species from the Mediterranean Basin serve as appropriate models to investigate these effects, since they grow in climatically stressed areas which have undergone intense cover changes. The aim is to use climate, aerial photographs, stand structure and radial-growth data to explain the mortality and historical patterns of growth of Abies pinsapo in the area where this relict species was first protected.Location Sierra de las Nieves, West Baetic Range, southern Spain. MethodsWe assessed variations of tree cover in A. pinsapo forests through image analyses of aerial photographs spanning the last 50 years. We sampled 31 stands to assess current altitudinal patterns of forest structure and mortality. We evaluated the relationships between radial growth and regional climate using linear models in three sites at different elevations. ResultsRegional warming and a decrease in precipitation were detected. Forest tree cover increased at all elevations from 1957 until 1991, but it afterwards decreased below 1100 m. Currently, the likelihood of tree mortality increases downwards and is associated with dense, closed stands with a low living basal area. In contrast to previous droughts, a sharp synchronized reduction in tree growth, not fully accounted for in linear climate-growth models, occurred at low elevations in 1994-95, but not upwards. It was preceded by a weakening of the negative association between low-elevation growth and water deficit since the late 1970s. ConclusionsThe intense densification of A. pinsapo forests following strict protection measures in the late 1950s enhanced the vulnerability of climate-sensitive A. pinsapo forests to recent drier conditions. Such abrupt land-use changes help to explain recent patterns of mortality and growth decline in low-elevation A. pinsapo forests.

Radial growth and xylogenesis were studied to investigate the influence of climate variability and intraspecific competition on secondary growth in Abies pinsapo Boiss., a relic Mediterranean fir. We monitored the responses to three thinning treatments (unthinned control -C-, 30% -T30- and 60% -T60- of basal area removed) to test the hypothesis that they may improve the adaptation capacity of tree growth to climatic stress. We also assessed whether xylogenesis was differentially affected by tree-to-tree competition. Secondary growth was assessed using manual band dendrometers from 2005 to 2007. In 2006, xylogenesis (phases of tracheid formation) was also investigated by taking microcores and performing histological analyses. Seasonal dynamics of radial increment were modeled using Gompertz functions and correlations with microclimate and radiation were performed. Histological analyses revealed it as fundamental to calibrate the dendrometer estimates of radial increment and to establish the actual onset and end dates of tracheid production. The lower radial-increment rates and number of produced tracheids were observed in the trees subjected to high competition in the unthinned plots. The growing season differed among the plots, and its duration ranged from an average of 78 days in unthinned plots to 115 days in thinned ones (T60). Variations in the beginning of the growing season (13 April to 22 May) and earlywood-latewood transition (early August) were mainly determined by the temperature pattern, while the onset and the end of the growing season were related to both annual precipitation and tree-to-tree competition. The tracheid-formation phases of radial enlargement and cell-wall thickening showed similar patterns in the trees from thinned and unthinned plots subjected to low and high competition, respectively, but the mean number of tracheids in each phase was always higher in the trees from the thinned plots. The reduction of competition through thinning induced a longer growing season and enhanced the radial growth in A. pinsapo.

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought-prone areas, tree populations located at the driest and southernmost distribution limits (rear-edges) should be particularly threatened. Here, we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris; and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear-edges of the continuous distributions of these tree species. We used tree-ring width data from a network of 110 forests in combination with the process-based Vaganov-Shashkin-Lite growth model and climate-growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear-edge. By contrast, growth of high-elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of -10.7% and -16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear-edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear-edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions.

Global climate change is expected to further raise the frequency and severity of extreme events, such as droughts. The effects of extreme droughts on trees are difficult to disentangle given the inherent complexity of drought events (frequency, severity, duration, and timing during the growing season). Besides, drought effects might be modulated by trees’ phenotypic variability, which is, in turn, affected by long‐term local selective pressures and management legacies. Here we investigated the magnitude and the temporal changes of tree‐level resilience (i.e., resistance, recovery, and resilience) to extreme droughts. Moreover, we assessed the tree‐, site‐, and drought‐related factors and their interactions driving the tree‐level resilience to extreme droughts. We used a tree‐ring network of the widely distributed Scots pine ( Pinus sylvestris ) along a 2,800 km latitudinal gradient from southern Spain to northern Germany. We found that the resilience to extreme drought decreased in mid‐elevation and low productivity sites from 1980–1999 to 2000–2011 likely due to more frequent and severe droughts in the later period. Our study showed that the impact of drought on tree‐level resilience was not dependent on its latitudinal location, but rather on the type of sites trees were growing at and on their growth performances (i.e., magnitude and variability of growth) during the predrought period. We found significant interactive effects between drought duration and tree growth prior to drought, suggesting that Scots pine trees with higher magnitude and variability of growth in the long term are more vulnerable to long and severe droughts. Moreover, our results indicate that Scots pine trees that experienced more frequent droughts over the long‐term were less resistant to extreme droughts. We, therefore, conclude that the physiological resilience to extreme droughts might be constrained by their growth prior to drought, and that more frequent and longer drought periods may overstrain their potential for acclimation.

46Scots pine forests subjected to continental Mediterranean climates undergo cold winter 47 temperatures and drought stress. Recent climatic trends towards warmer and drier 48 conditions across the Mediterranean Basin might render some of these pine populations 49 more vulnerable to drought-induced growth decline at the southernmost limit of the 50 species distribution. We investigated how cold winters and dry growing seasons drive 51 the radial growth of Scots pine subject to continental Mediterranean climates by relating 52 growth to climate variables at local (elevational gradient) and regional (latitudinal 53 gradient) scales. Local climate-growth relationships were quantified on different time 54 scales (5-, 10-and 15-days) to evaluate the relative role of elevation and specific site 55 characteristics. A negative water balance driven by high maximum temperatures in June 56 (low-elevation sites) and July (high-elevation sites) was the major constraint on growth, 57 particularly on a 5-to 10-day time scale. Warm nocturnal conditions in January were 58 associated with wider rings at the high-elevation sites. At the regional scale, Scots pine 59 growth mainly responded positively to July precipitation, with a stronger association at 60 lower elevations and higher latitudes. January minimum temperatures showed similar 61 patterns but played a secondary role as a driver of tree growth. The balance between 62 positive and negative effects of summer precipitation and winter temperature on radial 63 growth depends on elevation and latitude, with low-elevation populations being more 64 prone to suffer drought and heat stress, whereas high-elevation populations may be 65 favoured by warmer winter conditions. This negative impact of summer heat and 66 drought has increased during the past decades. This interaction between climate and site 67 conditions and local adaptations is therefore decisive for the future performance and 68 persistence of Scots pine populations in continental Mediterranean climates.