Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-continental tree-ring width database from sites where both dead and living trees were sampled (2970 dead and 4224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event. We observed a decrease in radial growth before death in ca. 84% of the mortality events. The extent and duration of these reductions were highly variable (1-100 years in 96% of events) due to the complex interactions among study species and the source(s) of mortality. Strong and long-lasting declines were found for gymnosperms, shade- and drought-tolerant species, and trees that died from competition. Angiosperms and trees that died due to biotic attacks (especially bark-beetles) typically showed relatively small and short-term growth reductions. Our analysis did not highlight any universal trade-off between early growth and tree longevity within a species, although this result may also reflect high variability in sampling design among sites. The intersite and interspecific variability in growth patterns before mortality provides valuable information on the nature of the mortality process, which is consistent with our understanding of the physiological mechanisms leading to mortality. Abrupt changes in growth immediately before death can be associated with generalized hydraulic failure and/or bark-beetle attack, while long-term decrease in growth may be associated with a gradual decline in hydraulic performance coupled with depletion in carbon reserves. Our results imply that growth-based mortality algorithms may be a powerful tool for predicting gymnosperm mortality induced by chronic stress, but not necessarily so for angiosperms and in case of intense drought or bark-beetle outbreaks.

Summary1. Some disturbances can drive ecological systems to abrupt shifts between alternative stages (tipping points) when critical transitions occur. Drought-induced tree death can be considered as a nonlinear shift in tree vigour and growth. However, at what point do trees become predisposed to drought-related dieback and which factors determine this (tipping) point? We investigated these questions by characterizing the responses of three tree species, silver fir (Abies alba), Scots pine (Pinus sylvestris) and Aleppo pine (Pinus halepensis), to a severe drought event. 2. We compared basal area increment (BAI) trends and responses to climate and drought in declining (very defoliated and dying) vs. non-declining (slightly or not defoliated) trees by using generalized additive mixed models. Defoliation, BAI and sapwood production were related to functional proxies of tree vigour measured at the onset and end of the drought (non-structural carbohydrate concentrations, needle N content and C isotopic discrimination, presence of wood-inhabiting fungi). We evaluated whether early warning signals (increases in synchronicity among trees or in autocorrelation and standard deviation) could be extracted from the BAI series prior to tree death. 3. Declining silver fir and Scots pine trees showed less growth than non-declining trees one to three decades, respectively, before the drought event, whereas Aleppo pines showed growth decline irrespective of tree defoliation. At the end of the drought period, all species showed increased defoliation and a related reduction in the concentration of sapwood soluble sugars. Defoliation was constrained by the BAI of the previous 5 years and sapwood production. No specific wood-inhabiting fungi were found in post-drought declining trees apart from blue-stain fungi, which extensively affected damaged Scots pines. Declining silver firs showed increases in BAI autocorrelation and variability prior to tree death. 4. Synthesis. Early warning signals of drought-triggered mortality seem to be species specific and reflect how different tree species cope with drought stress. Highly correlated declining growth patterns during drought can serve as a signal in silver fir, whereas changes in the content of sapwood soluble sugars are suitable vigour proxies for Scots and Aleppo pines. Longer growth and defoliation series, additional vigour parameters and multi-species comparisons are required to understand and predict drought-induced tree death.

Aim The intensity and frequency of drought have increased considerably during recent decades in some Northern Hemisphere forested areas, and future climate warming could further magnify drought stress. We quantify how forests resist drought events and recover after them, i.e. we determine their growth resilience. Location North America and Europe. Methods We use a large tree‐ring database to study how drought influences forest growth resilience. We selected 775 tree‐ring width chronologies and studied the occurrence of years with extremely dry conditions (low soil moisture and/or high evaporative stress; hereafter ‘drought’) in these forests. For each drought in each forest we calculated three indices that represent different components of growth resilience to drought: resistance (Rt), recovery (Rc) and resilience (Rs). We related the variation in these indices with geographical, topographic, climatic and ecological conditions from each region. Results The three components of forest growth resilience were interrelated. Resistance and recovery were negatively related, and both were positively and nonlinearly related to resilience. Drought resistance increased with latitude, soil moisture and slope, whereas drought recovery decreased with latitude, soil moisture and summer normalized difference vegetation index. Drought resilience increased with elevation and decreased with the variation in soil moisture. Temperate broadleaf forests from wet regions showed a greater growth resistance (e.g. north‐eastern USA, central Europe) while conifer forests from dry to semi‐arid regions (e.g. south‐western USA, southern Europe) presented a greater growth recovery. Main conclusions The geographical patterns of growth resilience indices confirm the existence of different strategies among forests to cope with droughts, depending on the biome, the tree species and the prevailing climatic conditions. Geographical patterns in soil moisture availability tend to override species‐specific responses to drought.

Drought-triggered declines in forest productivity and associated die-off events have increased considerably due to climate warming in the last decades. There is an increasing interest in quantifying the resilience capacity of forests against climate warming and drought to uncover how different stands and tree species will resist and recover after more frequent and intense droughts. Trees form annual growth rings that represent an accurate record of how forest growth responded to past droughts. Here we use dendrochronology to quantify the radial growth of different forests subjected to contrasting climatic conditions in Spain during the last half century. Particularly, we considered four climatically contrasting areas where dominant forests showed clear signs of drought-induced dieback. Studied forests included wet sites dominated by silver fir (Abies alba) in the Pyrenees and beech (Fagus sylvatica) stands in northern Spain, and drought-prone sites dominated by Scots pine (Pinus sylvestris) in eastern Spain and black pine (Pinus nigra) in the semi-arid south-eastern Spain. We quantified the growth reduction caused by different droughts and assessed the short-and long-term resilience capacity of declining vs. non-declining trees in each forest. In all cases, drought induced a marked growth reduction regardless tree vigor. However, the capacity to recover after drought (resilience) at short-and long-term scales varied greatly between declining and non-declining individuals. In the case of beech and silver fir, non-declining individuals presented greater growth rates and capacity to recover after drought than declining individuals. For Scots pine, the resilience to drought was found to be lower in recent years regardless the tree vigor, but the growth reduction caused by successive droughts was more pronounced in declining than in non-declining individuals. In the black pine forest an extreme drought induced a marked growth reduction in declining individuals when accounting for age effects on growth rates. We demonstrate the potential of tree-ring data to record short-and long-term impacts of drought on forest growth and to quantify the resilience capacity of trees.

International audienceThe seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associated canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is < 2000 mm yr(-1) (water-limited forests) and to radiation otherwise (light-limited forests). On the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration, respectively. Consequently, light-limited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. First-order control by precipitation likely indicates a decrease in tropical forest productivity in a drier climate in water-limited forest, and in current light-limited forest with future rainfall < 2000 mm yr(-1)

It is generally accepted that animal heartbeat and lifespan are often inversely correlated, however, the relationship between productivity and longevity has not yet been described for trees growing under industrial and pre-industrial climates. Using 1768 annually resolved and absolutely dated ring width measurement series from living and dead conifers that grew in undisturbed, high-elevation sites in the Spanish Pyrenees and the Russian Altai over the past 2000 years, we test the hypothesis of grow fast—die young. We find maximum tree ages are significantly correlated with slow juvenile growth rates. We conclude, the interdependence between higher stem productivity, faster tree turnover, and shorter carbon residence time, reduces the capacity of forest ecosystems to store carbon under a climate warming-induced stimulation of tree growth at policy-relevant timescales.

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.

We studied the growth response to drought of a Quercus faginea Lam. stand in a xeric site in NE Spain, that experienced an intense defoliation in 1993–94. This event coincided with very low precipitation from November to February, the period when total monthly precipitation exceeds evapotranspiration. We evaluated the effects of November–February precipitation (recharge precipitation, RP) on internode length, radial growth, and wood anatomy. Quercus faginea showed reduced longitudinal and radial growth during the years with low RP, and most sampled trees did not produce latewood in 1993–94 but showed wide earlywood vessels. We observed the reverse for years with a high RP. Radial growth was enhanced by increased precipitation during January and May of the growth year. If severe droughts become more frequent, due to a greater climatic variability, extensive dieback of marginal Q. faginea populations may be expected.