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.

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.

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.

X-ray microdensitometry on annually resolved tree-ring samples has gained an exceptional position in last-millennium paleoclimatology through the maximum latewood density (MXD) parameter, but also increasingly through other density parameters. For 50 years, X-ray based measurement techniques have been the de facto standard. However, studies report offsets in the mean levels for MXD measurements derived from different laboratories, indicating challenges of accuracy and precision. Moreover, reflected visible light-based techniques are becoming increasingly popular, and wood anatomical techniques are emerging as a potentially powerful pathway to extract density information at the highest resolution. Here we review the current understanding and merits of wood density for tree-ring research, associated microdensitometric techniques, and analytical measurement challenges. The review is further complemented with a careful comparison of new measurements derived at 17 laboratories, using several different techniques. The new experiment allowed us to corroborate and refresh "long-standing wisdom" but also provide new insights. Key outcomes include (i) a demonstration of the need for mass/volume-based recalibration to accurately estimate average ring density; (ii) a substantiation of systematic differences in MXD measurements that cautions for great care when combining density data sets for climate reconstructions; and (iii) insights into the relevance of analytical measurement resolution in signals derived from tree-ring density data. Finally, we provide recommendations expected to facilitate future inter-comparability and interpretations for global change research.Plain Language Summary Paleoclimatology, the study of how the climate has changed throughout earth history, is an important component of climate change research. The wood density of tree

Climate, stand structure and local site conditions are potentially important determinants of forest dynamics. Understanding the relative contributions of competition and climate to tree growth is critical to project likely stand development under different climate change scenarios. Further, current competition levels and stand structure may reflect legacies of past forest management. Here, we analyze the effects of climate, site conditions and competition on radial growth in three Scots pine plots located along an altitudinal gradient. These stands are subjected to Mediterranean climate with continental influence, i.e., growth is limited by low winter temperatures and dry summer conditions. Current stand structure and retrospective analyses of radial growth (basal area increment, BAI) were used to model changes in tree growth as a function of competition (CI) and climate at an annual resolution. Negative exponential functions characterized the CI-BAI associations, whereas linear mixed-effects models were used to model BAI and to quantify the growth response to climate of trees under low and high competition. Competition effects on growth were steady over time regardless of the elevation and tree age. High competition levels negatively affected growth, with a proportionally stronger influence in suppressed trees than in dominant trees. Sensitivity of tree growth to climate increased with decreasing competition. Altitudinal gradientrelated growth responses to climate were found only for temperature variables. Specifically, growth at high elevations was mainly limited by low winter temperatures, whilst warm spring enhanced growth at middle elevations and late summer temperatures did it at low elevations. Since growth and its sensitivity to climate is more pronounced in low competition trees, we argue that the past management of the forest overrides site conditions and climate effects through the legacies on stand structure and competition. Pro-active forest management practices should be adopted to reduce the vulnerability of previously managed Scots pine forests currently threatened by the predicted warmer and drier conditions.

Warmer and drier climatic conditions are projected for the 21st century; however, the role played by extreme climatic events on forest vulnerability is still little understood. For example, more severe droughts and heat waves could threaten quaternary relict tree refugia such as Circum-Mediterranean fir forests (CMFF). Using tree-ring data and a process-based model, we characterized the major climate constraints of recent (1950-2010) CMFF growth to project their vulnerability to 21st-century climate. Simulations predict a 30% growth reduction in some fir species with the 2050s business-as-usual emission scenario, whereas growth would increase in moist refugia due to a longer and warmer growing season. Fir populations currently subjected to warm and dry conditions will be the most vulnerable in the late 21st century when climatic conditions will be analogous to the most severe dry/heat spells causing dieback in the late 20th century. Quantification of growth trends based on climate scenarios could allow defining vulnerability thresholds in tree populations. The presented predictions call for conservation strategies to safeguard relict tree populations and anticipate how many refugia could be threatened by 21st-century dry spells.

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1. The negative impacts of drought on forest growth and productivity last for several years generating legacies, although the factors that determine why such legacies vary across sites and tree species remain unclear. 2. We used an extensive network of tree-ring width (RWI, ring-width index) records of 16 tree species from 567 forests, and high-resolution climate and normalized difference vegetation index (NDVI) datasets across Spain during the common period 1982-2008 to test the hypothesis that climate conditions and growth features modulate legacy effects of drought on forests. Legacy effects of drought were calculated as the differences between detrended-only RWI and NDVI series (i.e. after removing long-term growth trends) and pre-whitened RWI and NDVI series predicted by a model including drought intensity. Superposed Epoch Analysis (SEA) was used to estimate whether legacy effects differed from random. Finally, legacy effects were related to water balance, growth persistence and variability, and tree species identity. 3. We found a widespread occurrence of drought legacy effects on both RWI and NDVI, but they were seldom significant. According to SEA, first-year drought legacies were negative and different from random in 9% and 5% of the RWI and