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Climate change (CC) necessitates reforestation/afforestation programs to mitigate its impacts and maximize carbon sequestration. But comprehending how tree growth, a proxy for fitness and resilience, responds to CC is critical to maximize these programs' effectiveness. Variability in tree response to CC across populations can notably be influenced by the standing genetic variation encompassing both neutral and adaptive genetic diversity. Here, a framework is proposed to assess tree growth potential at the population scale while accounting for standing genetic variation. We applied this framework to black spruce (BS, Picea mariana [Mill] B.S.P.), with the objectives to (1) determine the key climate variables having impacted BS growth response from 1974 to 2019, (2) examine the relative roles of local adaptation and the phylogeographic structure in this response, and (3) project BS growth under two Shared Socioeconomic Pathways while taking standing genetic variation into account. We modeled growth using a machine learning algorithm trained with dendroecological and genetic data obtained from over 2600 trees (62 populations divided in three genetic clusters) in four 48‐year‐old common gardens, and simulated growth until year 2100 at the common garden locations. Our study revealed that high summer and autumn temperatures negatively impacted BS growth. As a consequence of warming, this species is projected to experience a decline in growth by the end of the century, suggesting maladaptation to anticipated CC and a potential threat to its carbon sequestration capacity. This being said, we observed a clear difference in response to CC within and among genetic clusters, with the western cluster being more impacted than the central and eastern clusters. Our results show that intraspecific genetic variation, notably associated with the phylogeographic structure, must be considered when estimating the response of widespread species to CC. [ABSTRACT FROM AUTHOR]

Tree diversity and species identity are known to play an important role in modulating forest productivity and its capacity to buffer the effects of climatic events. The FunDivEurope exploratory platform allowed us to analyse this modulating effect in a medium‐term time span, after an abrupt rise to a new stable level of the average summer temperature of ca 2°C, in Mediterranean forests in central Italy. This paper aims to answer the following questions: 1) did increasing temperature and drought events affect the growth of thermophilous deciduous forests? 2) Was this effect buffered in mixed stands compared to monocultures? 3) Did co‐occurring tree species with different ecological characteristics, from more mesophilous to more xerophilous, have different responses? In 2012 and 2017, wood cores were collected from 659 trees in 36 plots representative of thermophilous deciduous forests. The selected tree species were Castanea sativa, Ostrya carpinifolia, Quercus cerris, Quercus ilex and Quercus petraea. In the sampling plots, they were present in pure stands and mixtures from two to four species. After measuring annual rings on cores, chronologies of basal area increment were built, and inventory data were used to estimate tree growth. Results showed a strong reduction of growth, lasting at least 18 years, after the temperature rise. Tree diversity significantly reduced the growth drop after the sudden and stable rise in summer average temperature. Tree mixture effect on growth stability appeared to be dependent on the tree species present in the mixture. Temperature rise and associated drought events, even without changes in rainfall, are one of the main challenges that European forests will face in the current scenarios of climate change. Tree diversity can buffer the effects of climate change over periods of at least 15 years and should be considered in forest management plans. [ABSTRACT FROM AUTHOR]

Long‐term grassland biodiversity experiments have shown that diversity effects on productivity tend to strengthen through time, as complementarity among coexisting species increases. But it remains less clear whether this pattern also holds for other ecosystems such as forests, and if so why.Here we explore whether diversity effects on tree growth change predictably during stand development in Finland's boreal forests. Using tree ring records from mature forests, we tested whether diameter growth trajectories of dominant tree species growing in mixture differed from those in monoculture. We then compared these results with data from the world's longest running tree diversity experiment, where the same combinations of species sampled in mature forests were planted in 1999.We found that diversity effects on tree growth strengthened progressively through time, only becoming significantly positive around 20 years after seedling establishment. This shift coincided with the period in which canopy closure occurs in these forests, at which time trees begin to interact and compete above‐ground. These temporal trends were remarkably consistent across different tree species sampled in mature forests, and broadly matched growth responses observed in the much younger experimental plots.Synthesis. Our results mirror those from grassland ecosystems and suggest that canopy closure is a key phase for promoting niche complementarity in diverse tree communities. They also provide a series of testable hypotheses for the growing number of tree diversity experiments that have been established in recent years. [ABSTRACT FROM AUTHOR]

Questions What are the stand development patterns and successional processes occurring in a Quercus petraea-Fagus sylvatica old-growth forest? Do these two sympatric species have different recruitment and canopy accession strategies? How do they respond to disturbance? Location Old-growth Q. petraea- F. sylvatica forest in Runcu Grosi Natural Reserve, Romania. Methods Increment cores were taken from 110 similarly sized pairs of standing dead and living sessile oak trees, and additionally from the 110 most competitive neighbouring living beech trees, growing in the best-preserved part of the reserve. Release events were detected using the boundary line method. Recruitment and canopy accession strategies were defined using tree juvenile growth rate, overall growth patterns and the presence or absence of a major crown release. Results Sessile oak establishment occurred between 1800 and 1859, while beech have regenerated continuously, with greater in-growth after 1860. Tree recruitment followed species-specific requirements, with most sessile oak having a gap origin (82%) and many beech regenerating in the understorey (41%). Release events occurred continuously during the period analysed (1820-2003), but the percentage of trees per decade that showed releases was <15%. A larger number of releases were recorded in beech compared to sessile oak. Living and dead sessile oak originated under similar conditions, but after the 1900s they experienced different canopy disturbances regimes. Conclusions The investigated stand appears to originate after a large-scale disturbance that promoted sessile oak recruitment. Thereafter, regular small-scale disturbances favoured beech, which became a competitor to sessile oak and achieved faster growth despite a less favourable initial canopy position. Mortality started to occur in oak. Stand maturation and increasing competition could favour the occurrence of large-scale windthrow, which would again promote sessile oak. [ABSTRACT FROM AUTHOR]

[Display omitted] • Climate warming and droughts impact growth in central European oak forests. • Growth shifts in old-growth and managed Quercus petraea declining trees are identified. • Climate sensitivity in managed oak forests is higher than in old-growth oak forests. • Past use legacies and stand structure drive growth sensitivity. • Managed forests are more vulnerable under forecasted climate-warming. Climate warming-related drought could become a major driver of large-scale forest dieback. However, little is known about how past management legacies modulate the climate-growth responses during recent dieback episodes in central European oak forests. Here, we examine the role played by past management –unmanaged old-growth vs. managed forests– in recent tree mortality events occurring in Quercus petraea (Matt.) Liebl. stands across large areas of western Romania. We analyze how stand structure (tree size, competition) and climatic factors (drought, temperature and precipitation) drive tree radial growth patterns in neighboring standing dead and living trees. We analyzed basal area increments (BAI) trends, past management legacies and climate- and drought-growth relationships during the 20th century to distinguish the roles and interactions on recent warming-induced dieback. We observed that temperature rises and changes in atmospheric water demand during growing season let to increasing drought stress during the late 20th century affecting both managed and unmanaged forests. Dead trees from old-growth and managed forests showed lower growth than living trees after dieback onset. In both forests, dead and living trees displayed divergent growth patterns after dry 1980s, indicating that dieback was triggered by severe extreme conditions. Dead trees from managed stands experienced significant stronger growth reductions after 1980s though they experienced less tree-to-tree competition than dead trees in old-growth forest. High stand density negatively drove growth and enhanced climate sensitivity in old-growth stands. Competition acted synergistically with climate warming and drought causing tree mortality regardless of the management legacies in of Q. petraea forests. Our retrospective assessment of growth rates in relation with climate and structure changes offers valuable information for further forest conservation and management decisions of Q. petraea forests. These findings highlight the importance of past uses legacies driving recent forest dieback in temperate oak forests, making them more vulnerable under forecasted climate-warming related droughts in central Europe. [ABSTRACT FROM AUTHOR]