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Key message: Reconstruction of needle dynamics reveals prolonged drought legacy effects on crown condition that represent early warnings of drought-induced dieback in Scots pine. Understanding the mechanisms of drought-induced forest dieback and tree mortality is a priority for predicting forest responses to climate change. However, long-term information on drought impacts on crown dynamics and how it relates to tree water and carbon economy is still lacking. Comparing declining and non-declining Scots pines at the same site, we quantified primary and secondary growth and intrinsic water-use efficiency (WUEi). Further, we reconstructed the needle production and the annual total number of needles using the Needle Trace Method. Here, we reconstructed the apical needles through the assessment of 2460 longitudinal sections. We fitted partial least squares regression models to assess climate influence on primary and secondary growth and needle dynamics, and then linear mixed models using climate covariates and discerning between health status in the 1975–2016 period. Finally, we analyzed drought legacy effects applying a Bayesian hierarchical framework. We detected the highest differences between health tree classes in the annual total number of needles during the warm-dry 1980s. Declining trees responded more negatively to climate than non-declining conspecifics and showed a higher variability of drought legacy effects. We found significant drought legacy effects in the annual total number of needles up to 3 years after a drought. The warm, dry 1980s, that preceded the dieback, showed the highest δ13C values since the 1970s. Declining trees showed higher WUEi than non-declining trees from the 2000s onwards. Our study sheds light on the major role of needle dynamics in dieback episodes and illustrates how past drops in needle production may be interpreted as early warnings of drought-induced dieback in Scots pine. [ABSTRACT FROM AUTHOR]

Key message: Tree-ring growth of pollarded narrow-leaved ash trees in Central Spain reveals traditional management cessation in 1970 and property-specific management patterns. Tree pollarding was a dominant management strategy of European forests for centuries creating open agroforestry landscapes with important cultural and environmental values. This traditional practice has been widely abandoned in last decades with a subsequent impact in terms of biodiversity and cultural loss. Central Spain hosts the largest and best-preserved area of pollarded narrow-leaved ash (Fraxinus angustifolia Vahl.) woodlands in Europe. The main aim of this research is to obtain rigorous historical records of pollarding frequency to get adequate information for traditional ash management. We used dendrochronological techniques to evaluate temporal changes of pollarding frequency and rotation length. We analysed the stand level synchrony and the effect of land property on pollarding activity from 322 trees growing in eight pollard stands in Central Spain. Pollarding events were unequivocally identified at tree level by a characteristic change in growth pattern. We identified 2426 tree-level pruning events with the first event dated in 1777. Historical pruning recurrence ranged between 5 and 10 years with higher pollarding frequency on private lands. Pruning events within each site were synchronous, suggesting the existence of a rotational schema within each stand. Pruning frequency decreased drastically in the 1970s matching with the depopulation of rural areas and the general abandonment of traditional practices. Pollarding practices have recovered in recent decades although with lower intensity and lacking the synchronic historical patterns. Providing technical and economic support to make this traditional activity profitably would have strong environmental revenue due the multiple ecosystem services provided by pollarded ashes. [ABSTRACT FROM AUTHOR]

Legacies of past climate conditions and historical management govern forest productivity and tree growth. Understanding how these processes interact and the timescales over which they influence tree growth is critical to assess forest vulnerability to climate change. Yet, few studies address this issue, likely because integrated long-term records of both growth and forest management are uncommon. We applied the stochastic antecedent modelling (SAM) framework to annual tree-ring widths from mixed forests to recover the ecological memory of tree growth. We quantified the effects of antecedent temperature and precipitation up to 4 years preceding the year of ring formation and integrated management effects with records of harvesting intensity from historical forest management archives. The SAM approach uncovered important time periods most influential to growth, typically the warmer and drier months or seasons, but variation among species and sites emerged. Silver fir responded primarily to past climate conditions (25–50 months prior to the year of ring formation), while European beech and Scots pine responded mostly to climate conditions during the year of ring formation and the previous year, although these responses varied among sites. Past management and climate interacted in such a way that harvesting promoted growth in young silver fir under wet and warm conditions and in old European beech under drier and cooler conditions. Our study shows that the ecological memory associated with climate legacies and historical forest management is species-specific and context-dependent, suggesting that both aspects are needed to properly evaluate forest functioning under climate change. [ABSTRACT FROM AUTHOR]

Background: Equatorward, rear-edge tree populations are natural monitors to estimate species vulnerability to climate change. According to biogeographical theory, exposition to drought events increases with increasing aridity towards the equator and the growth of southern tree populations will be more vulnerable to drought than in central populations. However, the ecological and biogeographical margins can mismatch due to the impact of ecological factors (topography, soils) or tree-species acclimation that can blur large-scale geographical imprints in trees responses to drought making northern populations more drought limited. Methods: We tested these ideas in six tree species, three angiosperms (Fagus sylvatica, Quercus robur, Quercus petraea) and three gymnosperms (Abies alba, Pinus sylvestris and Pinus uncinata) by comparing rear-edge tree populations subjected to different degrees of aridity. We used dendrochronology to compare the radial-growth patterns of these species in northern, intermediate, and southern tree populations at the continental rear edge. Results and conclusions: We found marked variations in growth variability between species with coherent patterns of stronger drought signals in the tree-ring series of the southern populations of F. sylvatica, P. sylvestris, and A. alba. This was also observed in species from cool-wet sites (P. uncinata and Q. robur), despite their limited responsiveness to drought. However, in the case of Q. petraea the intermediate population showed the strongest relationship to drought. For drought-sensitive species as F. sylvatica and P. sylvestris, southern populations presented more variable growth which was enhanced by cool-wet conditions from late spring to summer. We found a trend of enhanced vulnerability to drought in these two species. The response of tree growth to drought has a marked biogeographical component characterized by increased drought sensitivity in southern populations even within the species distribution rear edge. Nevertheless, the relationship between tree growth and drought varied between species suggesting that biogeographical and ecological limits do not always overlap as in the case of Q. petraea. In widespread species showing enhanced vulnerability to drought, as F. sylvatica and P. sylvestris, increased vulnerability to climate warming in their rear edges is forecasted. Therefore, we encourage the monitoring and conservation of such marginal tree populations. [ABSTRACT FROM AUTHOR]

Letter to the Editor [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]

Climate warming is predicted to amplify drought stress. Thus, it is important to understand how coexisting plant species respond to severe droughts. Here we study how seven Mediterranean woody plant species with different evolutionary history and functional characteristics ( Pinus halepensis Mill., Juniperus phoenicea L. , Pistacia lentiscus L., Rhamnus lycioides L., Rosmarinus officinalis L., Genista scorpius (L.) DC., and Globularia alypum L.) responded to a severe winter drought during 2011-2012 in Spain. The study site is located in the Valcuerna valley, Monegros desert, northeastern Spain. We evaluated how the drought affected the annual growth-ring formation of the species by using dendrochronology and quantified the intensity of drought-induced defoliation and mortality and compared it between species and groups of species with different evolutionary history. Radial growth of all species was strongly reduced by the 2012 drought. The pre-Mediterranean species ( P. halepensis, J. phoenicea, P. lentiscus and R. lycioides) reduced growth more than the Mediterranean species ( R. officinalis, G. scorpius and G. alypum). Defoliation was significantly higher in pre-Mediterranean than in Mediterranean species. When species were analyzed separately we found that P. halepensis was the species with the highest growth reduction but J. phoenicea was defoliated more severely and showed higher mortality rates as a consequence of drought. In the case of the Mediterranean shrubs, drought-induced mortality was only noticeable in R. officinalis. Drought impacted growth of all species but this did not induce mortality in all of them. Growth reduction was dependent on evolutionary history. However, functional characteristics of the species such as leaf stomatal regulation and root architecture may be more important than evolutionary history on explaining drought-induced mortality. Indeed, species with shallow root systems such as J. phoenicea and R. officinalis were the most adversely affected by the drought. [ABSTRACT FROM AUTHOR]

Humans have historically played a critical role in the management of Mediterranean-type ecosystems (MTEs) through traditional fire use. Although chestnut forests are widespread across the Mediterranean Basin, little is known about their historical fire regimes. Our goal here is to generate testable hypotheses about the drivers of fire regime dynamics in chestnut dominated ecosystems. To examine anthropogenic fire management we selected two sites in Spain that have similar biophysical characteristics but divergent levels of economic development and fire management policies. Fire regime-landscape feedbacks were characterized through a pilot dendroecological study, official fire statistics, aerial photography and forest inventory data. Our results suggest that fire incidence in both sites has increased since the pre-industrial era but fire season, fire size, and forest structure have changed to a greater extent in the more developed site. These changes are probably driven by the decline in annual anthropogenic burning of litterfall by local communities at the more developed site during the non-vegetative season. [ABSTRACT FROM AUTHOR]