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Many studies show that mixed species stands can have higher gross growth, or so-called overyielding, compared with monocultures. However, much less is known about mortality in mixed stands. Knowledge is lacking, for example, of how much of the gross growth is retained in the standing stock and how much is lost due to mortality. Here, we addressed this knowledge gap of mixed stand dynamics by evaluating 23 middle-aged, unthinned triplets of monospecific and mixed plots of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) repeatedly surveyed over 6–8 years throughout Europe. For explanation of technical terms in this abstract see Box 1. First, mixed stands produced more gross growth (+10%) but less net growth (−28%) compared with the weighted mean growth of monospecific stands. In monospecific stands, 73% of the gross growth was accumulated in the standing stock, whereas only 48% was accumulated in mixed stands. The gross overyielding of pine (2%) was lower than that of beech (18%). However, the net overyielding of beech was still 10%, whereas low growth and dropout of pine caused a substantial reduction from gross to net growth. Second, the mortality rates, the self- and alien-thinning strength, and the stem volume dropout were higher in mixed stands than monospecific stands. The main reason was the lower survival of pine, whereas beech persisted more similarly in mixed compared with monospecific stands.Third, we found a 10% higher stand density in mixed stands compared with monospecific stands at the first survey. This superiority decreased to 5% in the second survey.Fourth, the mixing proportion of Scots pine decreased from 46% to 44% between the first and second survey. The more than doubling of the segregation index (S) calculated by Pielou index (S increased from 0.2 to 0.5), indicated a strong tendency towards demixing due to pine. Fifth, we showed that with increasing water supply the dropout fraction of the gross growth in the mixture slightly decreased for pine, strongly increased for beech, and also increased for the stand as a whole. We discuss how the reduction of inter-specific competition by thinning may enable a continuous benefit of diversity and overyielding of mixed compared with monospecific stands of Scots pine and European beech., The study received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No952314 and under the Marie Skłodowska-Curie grant agreement No 778322. The first author also wishes to thank the German ScienceFoundation (Deutsche Forschungsgemeinschaft) for funding the project “Structure and dynamics of mixed-species stands of Scots pine and European beech compared with monospecific stands; analysis along an ecological gradient through Europe” (# DFG PR 292/15-1). Felipe Bravo is grateful for Funds by the Junta de Castilla y León through the projects “CLU-2019-01 and CL-EI-2021-05 - iuFOR Institute Unit of Excellence” of the University of Valladolid and the co-financing by the European Regional Development Fund (ERDF “Europe drives our growth”). Miren del Río thanks for the support by the Spanish Ministerio de Ciencia e Innovación (# PID2021-126275OB-C21/C22). Dejan Stojanović thanks the Ministry of Education, Science and Technological Development of the Republic of Serbia for funding. Tzvetan Zlatanov thanks the Ministry of Education and Science of the Republic of Bulgaria (# DO1-405/18.12.2020 LTER-BG).

13 Pág., Forest stand and environmental factors influence soil organic carbon (SOC) storage, but little is known about their relative impacts in different soil layers. Moreover, how environmental factors modulate the impact of stand factors, particularly species mixing, on SOC storage, is largely unexplored. In this study, conducted in 21 forest triplets (two monocultures of different species and their mixture on the same site) distributed in Europe, we tested the hypothesis that stand factors (functional identity and diversity) have stronger effects on topsoil (FF + 0–10 cm) C storage than environmental factors (climatic water availability, clay + silt content, oxalate-extractable Al–Alox) but that the opposite occurs in the subsoil (10–40 cm). We also tested the hypothesis that functional diversity improves SOC storage under high climatic water availability, clay + silt contents, and Alox. We characterized functional identity as the basal area proportion of broadleaved species (beech and/or oak), and functional diversity as the product of broadleaved and conifer (pine) proportions. The results show that functional identity was the main driver of topsoil C storage, while climatic water availability had the largest control on subsoil C storage. Functional diversity decreased topsoil C storage under increasing climatic water availability, but the opposite was observed in the subsoil. Functional diversity effects on topsoil C increased with increasing clay + silt content, while its effects on subsoil C were negative at increasing Alox content. This suggests that functional diversity effect on SOC storage changes along gradients in environmental factors and the direction of effects depends on soil depth., The European Union and the National Fund for Scientific Research (Fonds de la recherche scientifique) of Belgium supported this research as part the ERA-Net SUMFOREST project REFORM–Mixed species forest management. Lowering risk, increasing resilience (www.reform-mixing.eu). The lead author was further supported by Université catholique de Louvain for covid-19-related delays of his PhD project. Some of the triplets were selected, established and measured with funding from the EuMIXFOR FP1206 Cost Action project. The Polish State Forests Enterprise also supported one of the Polish co-authors (Grant No: OR.271.3.15.2017). The Orléans site, OPTMix was installed thanks to ONF (National Forest Service, France), belongs to research infrastructure ANAEE-F; it is also included in the SOERE TEMPO, ZAL (LTSER Zone Atelier Loire) and the GIS Coop network.

Process-based models and empirical modelling techniques are frequently used to (i) explore the sensitivity of tree growth to environmental variables, and (ii) predict the future growth of trees and forest stands under climate change scenarios. However, modelling approaches substantially influence predictions of the sensitivity of trees to environmental factors. Here, we used tree-ring width (TRW) data from 1630 beech trees from a network of 70 plots established across European mountains to build empirical predictive growth models using various modelling approaches. In addition, we used 3-PG and Biome-BGCMuSo process-based models to compare growth predictions with derived empirical models. Results revealed similar prediction errors (RMSE) across models ranging between 3.71 and 7.54 cm2 of basal area increment (BAI). The models explained most of the variability in BAI ranging from 54 % to 87 %. Selected explanatory variables (despite being statistically highly significant) and the pattern of the growth sensitivity differed between models substantially. We identified only five factors with the same effect and the same sensitivity pattern in all empirical models: tree DBH, competition index, elevation, Gini index of DBH, and soil silt content. However, the sensitivity to most of the climate variables was low and inconsistent among the empirical models. Both empirical and process based models suggest that beech in European mountains will, on average, likely experience better growth conditions under both 4.5 and 8.5 RCP scenarios. The process-based models indicated that beech may grow better across European mountains by 1.05 to 1.4 times in warmer conditions. The empirical models identified several drivers of tree growth that are not included in the current process-based models (e.g., different nutrients) but may have a substantial effect on final results, particularly if they are limiting factors. Hence, future development of process-based models may build upon our findings to increase their ability to correctly capture ecosystem dynamics.

Heterogeneity of structure can increase mechanical stability, stress resistance and resilience, biodiversity and many other functions and services of forest stands. That is why many silvicultural measures aim at enhancing structural diversity. However, the effectiveness and potential of structuring may depend on the site conditions. Here, we revealed how the stand structure is determined by site quality and results from site-dependent partitioning of growth and mortality among the trees. We based our study on 90 mature, even-aged, fully stocked monocultures of Scots pine (Pinus sylvestris L.) sampled in 21 countries along a productivity gradient across Europe. A mini-simulation study further analyzed the site-dependency of the interplay between growth and mortality and the resulting stand structure. The overarching hypothesis was that the stand structure changes with site quality and results from the site-dependent asymmetry of competition and mortality. First, we show that Scots pine stands structure across Europe become more homogeneous with increasing site quality. The coefficient of variation and Gini coefficient of stem diameter and tree height continuously decreased, whereas Stand Density Index and stand basal area increased with site index. Second, we reveal a site-dependency of the growth distribution among the trees and the mortality. With increasing site index, the asymmetry of both competition and growth distribution increased and suggested, at first glance, an increase in stand heterogeneity. However, with increasing site index, mortality eliminates mainly small instead of all-sized trees, cancels the size variation and reduces the structural heterogeneity.Third, we modelled the site-dependent interplay between growth partitioning and mortality. By scenario runs for different site conditions, we can show how the site-dependent structure at the stand level emerges from the asymmetric competition and mortality at the tree level and how the interplay changes with increasing site quality across Europe. Our most interesting finding was that the growth partitioning became more asymmetric and structuring with increasing site quality, but that the mortality eliminated predominantly small trees, reduced their size variation and thus reversed the impact of site quality on the structure. Finally, the reverse effects of mode of growth partitioning and mortality on the stand structure resulted in the highest size variation on poor sites and decreased structural heterogeneity with increasing site quality. Since our results indicate where heterogeneous structures need silviculture interventions and where they emerge naturally, we conclude that these findings may improve system understanding and modelling and guide forest management aiming at structurally rich forests., This study was supported by the ERA-Net COFUND programme SUMFOREST, with the national funding organization Federal Ministry of Nutrition and Agriculture (REFORM, grant #2816ERA02S). The project has also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No952314 and under the Marie Skłodowska-Curie grant agreement No 778322, and the 2017 - 2018 Belmont Forum and BiodivERsA joint call for research proposals, under the BiodivScen ERA-Net COFUND programme, with the national funding organization Federal Ministry of Education and Research (grant #16LC1805B). The German Science Foundation funded the project “Structure and growth of mixed Scots pine/European beech stands compared with pure stands analyzed along an ecological gradient through Europe” (grant #PR 292/15-1). H.P. and E.U. were additionally supported by the Bavarian Ministry of Nutrition, Agriculture and Forestry through the projects “Pine-spruce mixed stands in Bavaria” (grant #7831-20339-2012) and “Maintenance of the long term experimental plots in Bavaria (W07, #7831-26625-2017). M.d.R. and R.R.-P. were supported by the projects CLU-2019-01 - iuFOR Institute Unit of Excellence” of the University of Valladolid and VA183P20–SMART, funded by the Junta de Castilla and Leon (Spain) and co-financed by the European Union (ERDF “Europe drives our growth”). J.C. was supported by the National Agency of Agricultural Research (Project No. QK21020307). K.B. was supported by the Polish Government MNiSW 2018–2021 Matching Fund No. 117/H2020/2018. M.M. was supported by the Estonian Research Council grant (PRG1586) and Estonian University of Life Sciences projects number P180024MIME and P200029MIME. R.S. was supported by the Slovak Research and Development Agency project No. APVV- 19-0035.

Aboveground biomass quantification is essential for determining carbon stocks in forests. Multiple tree biomass models are available, but estimations can be biased outside the fitting range. This is due to the lack of data for larger trees, mainly because of the cost and time required. This study proposed a methodology based on tree crown biomass ratio (crown biomass: total aboveground biomass) modelling. The original data used in the existing biomass models in Spain have been notably extended by the inclusion of stem data from First Spanish National Forest Inventory and other databases, covering better tree size variability. The analysis of the crown biomass ratio against tree size (d2h), allowed us to distinguish three different patterns: an increasing pattern, a constant one, and a decreasing pattern. A new system of biomass models was fitted simultaneously by species, including a model for crown biomass ratio according to the identified pattern, a stem biomass model, and a total aboveground biomass model. Using this methodology, models were fitted for the 29 most important species in Spain. The fitted models result in more accurate and unbiased predictions for stem biomass, and realistic estimations for the crown biomass. This methodology means more robust and flexible biomass estimations with the possibility of using different data sources. The absence of crown information is not an obstacle because this component is a percentage of total aboveground biomass. Moreover, determining the crown biomass ratio pattern allows improving the accuracy of tree biomass estimation beyond the range of tree sizes (2–70 cm) for which these models were fitted., This research was funded by Ministry of Science, Innovation and Universities, grant number AGL2017-83828-C2-1-R; Ministry of Agriculture, grant number EG17-042-C02-02; INIA grant number IMP-2018-004-C02-02, and Horizon2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 77832 (CARE4C).

1 Pág. Centro de Investigación Forestal (CIFOR), The REMAS project SOE3/P4/E0954 is co-financed by the Interreg Sudoe Program through the European Regional Development Fund (ERDF).

espanolEl contexto actual de cambio global obliga a realizar una selvicultura adaptativa en todo tipo de masas forestales, pero la particular situacion de miles de hectareas de repoblaciones protectoras, en las que una gestion inadecuada compromete la estabilidad de las masas, las debe convertir en asunto prioritario. Su edad, aunque algo alejada de los turnos habituales, no es impedimiento para la aplicacion de cortas de regeneracion que, ademas, pueden potenciar la naturalizacion y resiliencia de estas repoblaciones. Sin embargo, esta opcion apenas se ha aplicado en las miles de hectareas de pino resinero (Pinus pinaster Ait.) situadas al sur del Sistema Central. Por ello, en este trabajo se ha estudiado un canton de repoblacion de pino resinero situado en Fuencaliente (Ciudad Real), con cierto grado de diversificacion en elsubpiso (distintas especies de Quercus mediterraneos y madrono), que fue cortado a hecho en dos tiempos en 2012. Previamente a las cortas, se distinguieron tres tipos de masa (pinar puro, y dos grados de mezcla con frondosas), instalandose 78 parcelas de regeneracion repartidas equilibradamente entre los trestipos. Estas parcelas se han inventariado anualmente durante 7 anos, clasificando el regenerado de las especies arboreas en cuatro clases de altura, y midiendose diferentes parametros ecologicos asociados. Laregeneracion del pino puede considerarse exitosa tanto en cantidad como en distribucion, asociandose positivamente con veranos frescos y humedos. No obstante, no se ha promovido una diversificacion especifica apreciable, pues los Quercus solo regeneran bien donde eran abundantes ya antes de las cortas EnglishThe actual context of global change necessary leads to the application of adaptive silviculture techniques in forests. Nevertheless, the particular situation of several protective reforestations, in which an inadequate management jeopardizes its stability, makes them a priority nowadays. Although its ages are still far from rotation ages, regeneration fellings may enhance naturalization and resilience of these reforestations. However, this solution has been scarcely developed in the maritime pine (Pinus pinaster Ait.) forests located at the south of the Central System mountain range. Thus, this work studied a reforested maritime pine stand with variable presence of broad-leaved species under cover located in Fuencaliente (Ciudad Real). This stand, cut following a seed-tree method in 2012, presented before the cuttings three types of forest structure: pure pine forest and two mixlevels. 78 circular plots were installed to monitor pine and Quercus regeneration (classified in four height classes) and several ecological parameters. Pine regeneration could be considered as successful in both density and spatial distribution, and was positively associated with humid and warm summer. Specific diversification, however, was not promoted enough,as Quercus regeneration was abundant only in plots with significant densities of adult trees of these species

La gran superficie ocupada en España por repoblaciones protectoras con edades cercanas al turno exige abordar urgentemente su regeneración. El temperamento de luz de Pinus pinaster Ait. obliga a aplicar cortas continuas, justificadas selvícolamente pero que provocan impacto paisajístico y riesgo de erosión. Buscando conseguir regeneración suficiente y diversificación estructural y específica pero minimizando riesgos, se han instalado dos dispositivos experimentales en repoblaciones de Pinus pinaster de 50-65 años de edad situadas en Arbancón (Guadalajara) y Fuencaliente (Ciudad Real), sobre los que otoño de 2017 se cortó a hecho por bosquetes circulares (entresaca por bosquetes) de tamaños reducidos; en Fuencaliente también se ejecutaron cortas a hecho en dos tiempos sobre grandes superficies. Un año después de las cortas, los primeros resultados muestran que en Arbancón se ha conseguido regeneración suficiente en bosquetes con diámetros iguales a 1,5 y 2,5 veces la altura dominante de la masa: densidades medias en otoño de alrededor de 4500 pies ha-1 uniformemente repartidas, supervivencia estival del 67 % y nueva regeneración otoñal, con valores significativamente superiores a los de las zonas sin tratar). En Fuencaliente (bosquetes con diámetros entre 2 y 3,5 la altura dominante), la inadecuada ejecución de las cortas mecanizadas provocó remoción y compactación excesivas en el suelo, a pesar de lo cual en el otoño siguiente se alcanzan densidades de regeneración medias de 1500 pies ha-1 (en bosquetes) y de 2300 pies ha -1 (cortas a hecho en dos tiempos). En ningún caso se han apreciado signos de erosión.

11 Pág., Biomass models are key to the correct quantification of aboveground biomass and carbon stocks. Most of the current available sets of biomass models were developed for adult trees although very few have been employed to accurately estimate biomass during the first years of reforestations, despite the increasing number of reforestations over recent years. In this study, species-specific and generalized models for aboveground biomass of single trees have been developed for the main 14 species used in reforestations in Spain. A comparison with the existing biomass models at individual tree level for larger trees of these species was also conducted to confirm the hypothesis that specific models are required for the first years of the reforestation. A new set of biomass models was fitted simultaneously per species, based on three different independent variables – root-collar-diameter, total height and crown projection area- and their combinations. The fitted models provide more accurate and unbiased predictions of aboveground biomass in the first years after reforestation when compared with previously existing models for adult trees. The generalized models can be applied to reforestations when the main species is unknown or for mixed plantations where species are not individualized. The set of models provided can be used in a wide variety of surveys and monitoring conditions depending on the available data, from classical field inventories to more novel approaches such as airborne LiDAR data or aerial photographs., This research was funded by Ministry of Science, Innovation and Universities, grant number AGL2017-83828-C2-1-R; Ministry of Agriculture, grant number EG17-042-C02-02; and INIA grant number IMP-2018-004-C02-02.

14 Pág., While the impacts of forest management options on carbon (C) storage are well documented, the way they affect C distribution among ecosystem components remains poorly investigated. Yet, partitioning of total forest C stocks, particularly between aboveground woody biomass and the soil, greatly impacts the stability of C stocks against disturbances in forest ecosystems. This study assessed the impact of species composition and stand density on C storage in aboveground woody biomass (stem + branches), coarse roots, and soil, and their partitioning in pure and mixed forests in Europe. We used 21 triplets (5 beech-oak, 8 pine-beech, 8 pine-oak mixed stands, and their respective monocultures at the same sites) in seven European countries. We computed biomass C stocks from total stand inventories and species-specific allometric equations, and soil organic C data down to 40 cm depth. On average, the broadleaved species stored more C in aboveground woody biomass than soil, while C storage in pine was equally distributed between both components. Stand density had a strong effect on C storage in tree woody biomass but not in the soil. After controlling for stand basal area, the mixed stands had, on average, similar total C stocks (in aboveground woody biomass + coarse roots + soil) to the most performing monocultures. Although species composition and stand density affect total C stocks and its partitioning between aboveground woody biomass and soil, a large part of variability in soil C storage was unrelated to stand characteristics., The European Union and the National Fund for Scientific Research (Fonds de la recherche scientifique) of Belgium supported this research as part the ERA-Net SUMFOREST project REFORM–Mixed species forest management. Lowering risk, increasing resilience (www.reform-mixing.eu). The lead author was further supported by Université catholique de Louvain for covid-19 related delays of his PhD project. Some of the triplets were selected, established and measured with funding from the EuMIXFOR FP1206 Cost Action project. The Polish State Forests Enterprise also supported one of the Polish co-authors (Grant No: OR.271.3.15.2017). The Orléans site, OPTMix was installed thanks to ONF (National Forest Service, France), belongs to research infrastructure ANAEE-F; it is also included in the SOERE TEMPO, ZAL (LTSER Zone Atelier Loire) and the GIS Coop network.

Establishing mixed-species stands is frequently proposed as a strategy to adapt forests to the increasing risk of water scarcity, yet contrasted results have been reported regarding mixing effects on tree drought exposure. To investigate the drivers behind the spatial and temporal variation in water-related mixing effects, we analysed the δ13C variation in 22-year tree ring chronologies for beech and pine trees sampled from 17 pure and mixed pine–beech stands across a large gradient of environmental conditions throughout Europe. In the pure stands, average δ13C values were lower for beech (−27.9‰ to −22.2‰) than for pine (−26.0‰ to −21.1‰), irrespective of site conditions. Decreasing SPEI values (calculated over June to September) were associated with an increase in δ13C for both species, but their effect was influenced by stand basal area for pine and site water availability for beech. Mixing did not change the temporal constancy of δ13C nor the tree reaction to a drought event, for any of the species. While the mixing effect (Δ δ13C = δ13C pure stands − δ13C mixed stands) was on average positive for beech and non-significant for pine across the whole gradient, this effect strongly differed between sites. For both species, mixing was not significant at extremely dry sites and positive at dry sites; on moderately wet sites, mixing was positive for beech and negative for pine; at sites with permanent water supply, no general patterns emerge for any of the species. The pattern of mixing effect along the gradient of water availability was not linear but showed threshold points, highlighting the need to investigate such relation for other combinations of tree species., The networking in this study has been supported by COST Action FP1206 EUMIXFOR.

One of the characteristics of great relevance in the wood quality in Spanish coniferous forests due to growth conditions is the presence of corewood. This wood area presents properties and characteristics inferior to those in outerwood, which are more appropriate for certain uses. In plantations with short rotations, the percentage of juvenile timber is higher, and the profitability of the wood can be affected due to its unsuitability for high added value applications, such as structural. This work focuses on determining whether the use of models of current annual increase in height can provide suitable estimations of the year or transition zone between corewood and outerwood. This knowledge will contribute to analyze the expected yield of quality wood. The material studied is from Pinus sylvestris L. (Scots pine) and Pinus nigra Arnold subsp. nigra (black pine) pure stands. Densitometry results showed that the corewood–outerwood transition year could not be predicted by tree height functions.

Mixed forests are suggested as a strategic adaptation of forest management to climate change. Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) are tree species of high economic and ecological value for European forestry. Both species coexist naturally in a large part of their distributions but there is a lack of knowledge on the ecological functioning of mixtures of these species and how to manage such stands. This paper analyses these species’ intra- and inter-specific competition, including size-symmetric vs. size-asymmetric competition, and explore the effect of weather conditions on tree growth and competition. We studied basal area growth at tree level for Scots pine and Norway spruce in mixed versus pure stands in 22 triplets of fully-stocked plots along a broad range of ecological conditions across Europe. Stand inventory and increment cores provided insights into how species mixing modifies tree growth compared with neighbouring pure stands. Five different competition indices, weather variables and their interactions were included and checked in basal area growth models using a linear mixed model approach. Interspecific size-asymmetric competition strongly influenced growth for both tree species, and was modulated by weather conditions. However, species height stratification in mixed stands resulted in a greater tree basal area growth of Scots pine (10.5 cm2 year−1) than in pure stands (9.3 cm2 year−1), as this species occupies the upper canopy layer. Scots pine growth depended on temperature and drought, whereas Norway spruce growth was influenced only by drought. Interspecific site-asymmetric competition increased in cold winters for Scots pine, and decreased after a drought year for Norway spruce. Although mixtures of these species may reduce tree size for Norway spruce, our results suggest that this could be offset by faster growth in Scots pine. How inter-specific competition and weather conditions alter tree growth may have strong implications for the management of Scots pine-Norway spruce mixtures along the rotation period into the ongoing climate change scenario.

Centro de Investigación Forestal (CIFOR), Mixed-species stands have been found to be more productive than would be expected from the performance of their component species in monocultures due to facilitation and complementarity between species, although these interactions depend on the combination of species present. Our study focuses on monospecific and mixed-species stands of Scots pine and Norway spruce using 20 triplets established in nine countries along a climatic gradient across Europe. Differences in mean tree and stand characteristics, productivity and stand structure were assessed. Basal area increment in mixed stands was 8% higher than expected while volume increment was only 2% greater. Scots pine trees growing in mixed-species stands showed 11% larger quadratic mean diameter, 7% larger dominant diameter, 17% higher basal area and 25% higher stand volume than trees growing in monospecific stands. Norway spruce showed only a non-significant tendency to lower mean values of diameters, heights, basal area, as well standing volume in mixtures than monocultures. Stand structure indices differed between mixed stands and monocultures of Scots pine showing a greater stratification in mixed-species stands. Furthermore, the studied morphological traits showed little variability for trees growing in monospecific stands, except for diameter at breast height, crown length and crown length ratio. For trees growing in mixed stands, all the morphological traits of the trees were identified as different. Some of these morphological traits were associated with relative productivity. Nevertheless, relative productivity in mixed-species stands was not related to site conditions., We thank national funders of REFORM project (Spanish Ministry of Science and Innovation: PCIN2017-026; Research Council of Lithuania: S-SUMFOREST-17-1) and those funders for the support to non-participants in the REFORM project (Denmark: Contract between Danish Ministry of Environment and Food and Department of Geosciences and Natural Resource Management of UCPH; Estonia: Estonian University of Life Sciences projects number P180024MIME and P200029MIME; Poland: EU CARE4C project (GA 778322) supported by the Polish Government MNiSW2018-2021 matching fund (W117/H2020/2018); Slovakia: APVV-15-0265) that allowed the establishment and measurement of the triplets. A grant for a research stay was received by RR-P from the CARE4C project (Carbon smart forestry under climate change, H2020-MSCA-RISE-2017)., 13 Pág.

16 Páginas., Background: National and international institutions periodically demand information on forest indicators that are used for global reporting. Among other aspects, the carbon accumulated in the biomass of forest species must be reported. For this purpose, one of the main sources of data is the National Forest Inventory (NFI), which together with statistical empirical approaches and updating procedures can even allow annual estimates of the requested indicators. Methods: Stand level biomass models, relating the dry weight of the biomass with the stand volume were developed for the five main pine species in the Iberian Peninsula (Pinus sylvestris, Pinus pinea, Pinus halepensis, Pinus nigra and Pinus pinaster). The dependence of the model on aridity and/or mean tree size was explored, as well as the importance of including the stand form factor to correct model bias. Furthermore, the capability of the models to estimate forest carbon stocks, updated for a given year, was also analysed. Results: The strong relationship between stand dry weight biomass and stand volume was modulated by the mean tree size, although the effect varied among the five pine species. Site humidity, measured using the Martonne aridity index, increased the biomass for a given volume in the cases of Pinus sylvestris, Pinus halepensis and Pinus nigra. Models that consider both mean tree size and stand form factor were more accurate and less biased than those that do not. The models developed allow carbon stocks in the main Iberian Peninsula pine forests to be estimated at stand level with biases of less than 0.2 Mg∙ha− 1. Conclusions: The results of this study reveal the importance of considering variables related with environmental conditions and stand structure when developing stand dry weight biomass models. The described methodology together with the models developed provide a precise tool that can be used for quantifying biomass and carbon stored in the Spanish pine forests in specific years when no field data are available., This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Centro de Investigación Forestal (CIFOR), Species complementarity by morphological and physiological trait differences could cause distinct temporal and spatial use of resources. Accordingly, mixed stands may enhance production, biodiversity and/or provide a better adaptation to future climate conditions. We aim to identify species differences in intra-annual stem radial variation patterns, and to recognize species-specific responses to contrasting weather conditions at key intra-annual growth phases. Stem radial variation was recorded from high temporal resolution point dendrometers (2012–2014) installed on twelve dominant maritime pine and Pyrenean oak trees in two mixed stands in central Spain. Species differences in stem radial variation were analyzed by synchrony statistics, intra-annual pattern modelling, and evaluating the dependence of main intra-annual growth phases on climate conditions. Both species showed important differences on intra-annual radial increment pattern despite general stem radial variation synchrony. Radial increment onset was earlier for maritime pine during the spring and rainy autumns. Species-specific responses to weather indicate that stem radial variation increased with autumn temperature in maritime pine, but decreased in Pyrenean oak trees. However, summer vapor pressure deficit may reduce stem radial variation for maritime pine. Therefore, maritime pine would adapt more efficiently to warmer temperatures associated with climate change, although summer water stress may reduce this competitive bonus., This work was developed into the project “Mixed-species forest management. Lowering risk, increasing resilience (REFORM)” funded by national partners under the framework of Sumforest ERA-net, Spanish projects PIN-2017–026 and PIN-2017–027. It was also supported by the Spanish Ministry of Economy and Competitiveness, through the research projects AGL2014-51964-C2-1-R and AGL2014-51964-C2-2-R, and SIMWOOD: Sustainable Innovative Mobilisation of Wood (7FP grant agreement no 61376). The main author’s work was funded by a partnership grant from the University of Valladolid and Banco Santander, and by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS)., 10 Pág.

Centro de Investigación Forestal (CIFOR), Despite growing evidence that diverse forests play an important role in ecosystem functioning, ensuring the provision of different ecosystem services, whether such diversity improves their response to drought events remains unclear. In this study, we use a large tree-ring database from thirty case studies across nine European countries and eleven species, covering from Mediterranean to hemiboreal forests, to test if the growth response to site specific drought events that occurred between 1975 and 2015 varied between mixed and monospecific stands. In particular, we quantify how stands resist those specific drought events and recover after them, thus analyzing their resilience. For each drought event and forest stand we calculated resistance, recovery, resilience and relative resilience and we related the variation in these indices between monospecific and mixed stands with type of admixture, tree species identity, site aridity gradient, stand basal area and stand age. We found a large variability among case studies, even for those that share similar species composition and have similar climates. On average, mixed stands showed higher resistance, resilience and relative resilience to drought events than monospecific stands. However, the beneficial effect of mixtures could not be generalized, being greatly modulated by the type of admixture and tree species identity, and depending on site water supply and stand characteristics, such as basal area and age. The increase in resilience in mixtures compared with monocultures was greater on the conifer-broadleaved admixtures, and to a lesser extent in the broadleaved-broadleaved combinations. The observed response patterns to drought largely varied among the eleven studied species, thus revealing the importance of functional traits for understanding a species’ response to drought across its distribution range. Along the site aridity gradient, resilience and relative resilience to drought increased in drier sites for both monospecific and mixed stands, with an observed trend towards higher resilience in mixed stands in the drier and hotter sites. Our results confirm the complexity of the relationships found of resistance, recovery, resilience and relative resilience with drought when comparing pure vs mixed stands., The study was supported by the ERA-Net SUMFOREST project REFORM “Mixed species forest management. Lowering risk, increasing resilience” (PCIN2017-026) and Marie Sklodowska Curie Actions-RISE , CARE4C . Special thankW117/H2020/2018s to Nuria Simón and Guillermo Madrigal for their assistance building up the common database. All coauthors thank their national funding institutions for supporting the establishment, measure and analysis of core data in the studied plots. Research at the Polish case studies was additionally supported by the Ministry of Science and Higher Education of the Republic of Poland (No W117/H2020/2018 )., 15 Pág.

Since ancient times, Mediterranean pine forests have been habitat for human activity, providing a wide range of goods such as timber, seeds, resin and derived products. Among them, tar and resin have played an historical role on the interaction between human activity and forests. In Spain, the resin played an important role in the economic and social development in rural areas during 20th century. But after 1980, resin production plummeted and the virtual disappearance of resin tapping caused the abandonment of traditional forest activities and the subsequently losses of ecosystem forest services (provision, regulation and cultural). This paper deals with some of the ecosystem services provided by resin tapped pine forests and shows how the abandonment of this traditional forestry activity would lead to a loss of social welfare beyond the economic activity. Among these ecosystem services, special attention is paid to the biodiversity of the pine forests. For that purpose, a stratified vegetation sampling was conducted in the leading resin-tapping Spanish region. Ecological analysis was therefore compared with the social preferences for several attributes associated to resin-tapped pine forests in Spain, including the biodiversity of flora., Financial support from the SUDOE Interreg IV B – UE/EU FEDER/ERDF program (project SUST-FOREST, ref. SOE2/P2/E261) is acknowledged. This paper is also an accidental contribution of M.S. and S.M. to the project StarTree (funded by the European Union through the European Commission's FP7 Cooperation Work Programme).

13 Pág. Instituto de Ciencias Forestales (ICIFOR), The forest floor C stock needs to be accurately estimated in order to quantify its contribution to nutrient cycling and other ecological processes as well as for reporting purposes under international agreements. Hence, a modelling approach was used which involved testing three different types of models (GLM, GAM and random forest) to determine which one provided the best estimates of forest floor C stocks. The dataset employed contained over 1650 observations from different available sources embracing different climatic, topographic and biotic variables to be tested in the model. The approach that provided the best estimation of forest floor C stock was the random forest method, with forest type, latitude, altitude, canopy cover, mean summer temperature, annual accumulated temperature, summer precipitation, water deficit and the normalized difference vegetation index (NDVI) as covariates. To obtain a robust forecast, several iterations of the model were performed to estimate forest floor C stocks from the mean of the predictions. The model estimated a forest floor C stock of 0.148 ± 0.081 Pg, equivalent to a biomass of 0.381 ± 0.214 Pg, for a wooded area of almost 184,000 km2 in peninsular Spain and the Balearic Islands. The predictions were also presented in the form of a map showing the spatial distribution of the forest floor C stock. The results revealed a mean forest floor C stock of 8 Mg C ha-1 for Spanish forests and identified differences between coniferous (10.1 Mg C ha-1) and hardwood forests (6.3 Mg C ha-1)., This study was conducted under the ‘Encomienda MAPAMA-INIA: Scientific support for the generation of forest information’ (EG17-042-C02-02 project) and the INIA project ‘Sistemas forestales ante el cambio global’ (IMP2018-004-C02-02). We would like to thank Dr. Gregorio Montero for the concept and development of the idea for this study, Dra. Miren del Río for her helpful comments and suggestions for improving the manuscript and Adam Collins for the language revision of the manuscript.

Determining the way in which variability in tree allometry is modulated by intra- and inter-specific competitions in different species and stand compositions is of particular interest for forest modelling and practice. In this study, we explore this variability by developing models for tree crown diameter, total height and diameter at a height of 4 m, which include intra- and inter-specific competition terms. More than 19,000 Scots pine, silver fir, sessile oak and European beech trees from 4711 sample plots belonging to the Spanish National Forest Inventory were included in the study, covering both monospecific and two species mixed stands in Northern Spain. Trees growing under conditions of high competition displayed narrower crowns, greater heights and less taper for a given tree diameter, the plasticity in crown and height in response to intra-specific competition being related to species shade tolerance. The inter-specific competition effect on crown diameter and height was related to the difference in shade tolerance between the two species of the mixture, while stem taper did not exhibit this pattern. These results suggest that trees in mixed stands indeed show a modified allometry, which might be related to complementary resource acquisition strategies. The large variability observed in tree allometry indicates the need to consider both intra- and inter-specific competitions in allometric models., They also thank the Spanish Ministry of Economy and Competitiveness for funding the research project “Mixed Forest complexity and sustainability: dynamic, silviculture and adaptive management tools” (AGL2014-51964-C2-2-R).

Past failures of monocultures, caused by wind-throw or insect damages, and ongoing climate change currently strongly stimulate research into mixed-species stands. So far, the focus has mainly been on combinations of species with obvious complementary functional traits. However, for any generalization, a broad overview of the mixing reactions of functionally different tree species in different mixing proportions, patterns and under different site conditions is needed, including assemblages of species with rather similar demands on resources such as light. Here, we studied the growth of Scots pine and oak in mixed versus monospecific stands on 36 triplets located along a productivity gradient across Europe, reaching from Sweden to Spain and from France to Georgia. The set-up represents a wide variation in precipitation (456–1250 mm year−1), mean annual temperature (6.7–11.5 °C) and drought index by de Martonne (21–63 mm °C−1). Stand inventories and increment cores of trees stemming from 40- to 132-year-old, fully stocked stands on 0.04–0.94-ha-sized plots provided insight into how species mixing modifies stand growth and structure compared with neighbouring monospecific stands. On average, the standing stem volume was 436 and 360 m3 ha−1 in the monocultures of Scots pine and oak, respectively, and 418 m3 ha−1 in the mixed stands. The corresponding periodical annual volume increment amounted to 10.5 and 9.1 m3 ha−1 year−1 in the monocultures and 10.5 m3 ha−1 year−1 in the mixed stands. Scots pine showed a 10% larger quadratic mean diameter (p p p p p p p

13 Pág. Instituto de Ciencias Forestales (ICIFOR), Mixed forests are usually associated with higher aboveground carbon storage compared to the corresponding monocultures but information on the impact of tree species mixing on soil organic carbon (SOC) is still limited. Yet, maximizing SOC storage is crucial for ecosystem C sequestration and many other ecosystem services. This study used a triplet approach (ie. two-species mixed stand and respective pure stands at the same site) to assess the impact of tree species identity and mixing on SOC storage in eight pine-oak, eight pine-beech and five beech-oak triplets in Europe. We sampled the forest floor (FF) and 0–40 cm in the mineral soil per 10 cm interval. For each triplet type, we fitted basal area (BA) proportion of one component species (for species identity) and a BA-based plot-level True Shannon Diversity index (for species mixing) as explanatory variables for SOC stocks in linear mixed effects models, which included stone content and plot BA as covariates, and site as a random intercept. Considering the total soil depth (FF + 0–40 cm), species identity effect on SOC stocks was only significant for pine-beech and pine-oak triplets but explained more variability in SOC stocks than species mixing across triplet types. Species mixing effect was not significant for any triplet type in the total soil depth. While species identity consistently drove SOC storage in the topsoil layers across triplet types, species mixing explained more variability in SOC stocks in the deeper soil layers except for pine-oak triplets. The results showed that species identity is a stronger driver of SOC storage than species mixing. While tree species identity effect was strongly related to a conifers vs broadleaves signature, the drivers behind mixing effects remained elusive. The results suggest that targeted selection of tree species could better enhance SOC storage in European forests than a mere increase in species richness., The European Union and the National Fund for Scientific Research (Fonds de la recherche scientifique) of Belgium supported this research as part the ERA-Net SUMFOREST project REFORM – Mixed species forest management. Lowering risk, increasing resilience (www.reform-mixing.eu). Some triplets were selected, established and measured with funding from the EuMIXFOR FP1206 Cost Action project. The Polish State Forests Enterprise also supported one of the Polish co-authors (Grant No: OR.271.3.15.2017). We are grateful to Karine Henin for her key role in the laboratory analyses of the samples. We appreciate the technical assistance by Benjamin Willems for soil sampling across Europe. We would like to thank Thomas Pérot for the dendrometric measurements in the Orléans sites. Finally, we are debtful to the two anonymous reviewers who contributed to improving a previous version of this manuscript.

Biomass equations are needed to correctly quantify harvestable stock and biomass for sustainability efforts in forest management, but this kind of information is scarce in Ethiopia. This study sought to develop biomass models for five of the most common native tree species in the Chilimo dry afro-montane mixed forest in the central highlands of Ethiopia: Allophyllus abyssinicus, Olea europaea ssp.cuspidata, Olinia rochetiana, Rhus glutinosa and Scolopia theifolia. Comparison with generalized models was intended to show the greater accuracy of the specific models. A total of 90 trees from different diameter classes were selected, felled and divided into different biomass compartments. Biomass equation models were fitted using joint-generalized least squares regression to ensure the additivity property between the biomass compartments and total biomass. These were the first models developed for these species in African tropical forests. Models were including diameter at breast height and total height as independent variables, obtaining more accurate biomass estimations using these models than from generalized models. Fitted models are reliable for estimating aboveground biomass in the Chilimo forest and for more general application in similar forest types. Model applicability for biomass or carbon estimation is high within forest inventory data contexts., Instituto Universitario de Gestión Forestal Sostenible

Forest inventory data, observational studies (mensurative and manipulative), and planted experiments provide the main data sources for the study of mixed forests. Demonstrative experiments such as marteloscopes are also basic to both research and training. Under the umbrella of the EuMIXFOR project, several research efforts have been conducted to further our knowledge as regards the functioning and development of mixed forests. Within this project, information from different forest research experiments at European level focusing on mixed forests has been compiled in order to identify the current state of the art. In the case of the European beech and Scots pine mixture, a gradient of triplets of monospecific and pure plots of these species has been established across Europe. Common establishment and measurement protocols have been developed in order to harmonize comparisons and analyses. Data sharing is also an objective; therefore, data from the triplet study of the European beech and Scots pine mixture are now available. A common dataset with the National Forest Inventory data from some European countries has been used for the study of mixtures.

Over recent decades, forest management in Europe has increasingly moved towards the emulation of natural dynamics. Natural dynamics in beech-oak forests leads to the formation of monospecific beech stands, the oak usually being excluded or restricted to sites with poor growing conditions. However, beech is more vulnerable than oak to drought and high temperatures. In this study, we examine whether climate change could attenuate the dominance of beech and improve the competitive capacity of oak in an old-growth temperate forest located in the “Picos de Europa” National Park, northern Spain. We used a dendroecological approach to reconstruct the competitive capacity of beech and oak and developed a projection for the 21st century based on forecasted climate conditions under three different emission scenarios. Beech is the dominant tree species in the studied forest, where the disturbance regime has favored the replacement of oak by beech. In general oaks are older than beeches and most of the small trees are beeches. Our results show that this substitution process may weaken due to the vulnerability of beech to warmer and drier conditions. Climate change will benefit oak growth over beech over the course of the 21st century, as was observed in the late 20th century. However, the natural gap dynamic benefits beech due to its greater shade-tolerance. Therefore, if the resilience of the ecosystem is to be increased, management strategies favoring oak regeneration are necessary given the better adaptation of oak to climate change., This project has been funded by the National Parks Autonomous Agency of the Spanish Ministry of Agriculture, Fisheries, Food and Environment through the project 979S/2013, by the Autonomous Community of Madrid through the project P2013/MAE-2760, by the project AGL2016-76769-C2-1-R of the Spanish Ministry of Economy, Industry and Competitiveness and through a PhD fellowship of the Spanish Ministry of Education, Culture and Sports. JJC acknowledges the support of the project “Fundiver” (CGL2015-69186-C2-1-R, Spanish Ministry of Economy).

International audience

The importance of shrub formations in the Mediterranean area both in terms of area occupied and carbon sequestered by them is currently being recognized. However, due to the lack of suitable models to estimate biomass accumulation and growth rate in this region, the carbon sequestered by these formations is not included when computing the total carbon stocks in aboveground biomass in Mediterranean forest ecosystems, according to the IPCC guidelines. The aim of the present study is to develop equations to predict biomass accumulation and growth rate for the main shrub formations in the region of Andalusia (Southern Spain), using the fraction of canopy cover (FCC m ) and the average height of the shrub formations (H m ) as predictors. To build these models, more than 800 plots were inventoried using destructive sampling; the mean value found in the region for biomass accumulation and annual growth rate being 16.73 Mg ha−1 and 1.14 Mg ha−1 year−1, respectively. Heathers and big-size Cistaceae formations were the ones that presented the highest values of biomass accumulation (24.99 and 21.01 Mg ha−1, respectively), while the highest values for annual growth rate were achieved by Leguminous gorse shrubs and, again, big-size Cistaceae bushes (1.49 and 1.64 Mg ha−1 year−1, respectively). The carbon content for the main shrub species and formations in the area was also obtained. The developed models provide the opportunity to estimate shrub carbon stocks in Mediterranean forest management from easy-to-obtain variables, namely FCC m and H m . Moreover, the used shrub formations classifications and model structure allow their applicability to compute biomass accumulation and growth rate at regional and national level using as input data from the Spanish National Forest Inventory.

We review the main effects of forest management by comparing the effects of silvicultural systems (even-aged vs. uneven-aged stands, coppice systems, agroforestry systems), silvicultural options (thinning, rotation period, species composition), afforestation, harvesting, fire impact or effects of shrub layer on carbon sequestration in the Mediterranean area., Instituto Universitario de Gestión Forestal Sostenible

There is increasing evidence that species diversity enhances the temporal stability (TS) of community productivity in different ecosystems, although its effect at the population and tree levels seems to be negative or neutral. Asynchrony in species responses to environmental conditions was found to be one of the main drivers of this stabilizing process. However, the effect of species mixing on the stability of productivity, and the relative importance of the associated mechanisms, remain poorly understood in forest communities. We investigated the way mixing species influenced the TS of productivity in Pinus sylvestris L.;Fagus sylvatica L. forests, and attempted to determine the main drivers among overyielding, asynchrony between species annual growth responses to environmental conditions, and temporal shifts in species interactions. We used a network of 93 experimental plots distributed across Europe to compare the TS of basal area growth over a 15-year period (1999–2013) in mixed and monospecific forest stands at different organizational levels, namely the community, population and individual tree levels. Mixed stands showed a higher TS of basal area growth than monospecific stands at the community level, but not at the population or individual tree levels. The TS at the community level was related to asynchrony between species growth in mixtures, but not to overyielding nor to asynchrony between species growth in monospecific stands. Temporal shifts in species interactions were also related to asynchrony and to the mixing effect on the TS. Synthesis. Our findings confirm that species mixing can stabilize productivity at the community level, whereas there is a neutral or negative effect on stability at the population and individual tree levels. The contrasting findings regarding the relationships between the temporal stability and asynchrony in species growth in mixed and monospecific stands suggest that the main driver in the stabilizing process may be the temporal niche complementarity between species rather than differences in species’ intrinsic responses to environmental conditions. © 2016 The Authors. Journal of Ecology © 2016 British Ecological Society

Radial increment analyses allow us to determine tree responses to weather and tree competition, thus enabling the development of management strategies for adapting forest stands to forecasted climate change scenarios. In this study, the responses of pine-oak mixed stands (Pinus pinaster Ait.;Quercus pyrenaica Willd.) to thinning treatments were assessed at two sites in central Spain with contrasting drought conditions. Inter- and intra-annual radial increments were recorded every two weeks from dendrometer band measurements, using a Latin square design. Each site consisted of a control (unthinned) and two thinning intensities moderate (25% of pine basal area removed) and heavy (40% of pine basal area removed). Thinning effects were monitored over three years of different weather conditions (2010–2012), with an extreme drought event in 2012. Linear mixed models were fitted to analyze weather and thinning effects on the inter- and intra-annual radial increment rates. Additionally, the sum of two logistic functions was used to estimate cumulative radial increment patterns. Heavy thinning resulted in the greatest annual radial increment for pine, regardless of site and year, even during the 2012 drought, when compared to densely stocked forest stands. The model results indicated that heavy thinning positively affected the intra-annual pattern, increasing inflection point and spring and autumn asymptotes. Thinning also had a slight positive effect on inter- and intra-annual radial increment in oak, although this was not clear during extreme drought in 2012. The radial increment rate was related to weather variables, which reflected light (positive for solar radiation) and water availability dependence (increased with precipitation, but decreased with vapor pressure deficit) for both species. Besides water status, air humidity (positive relation with air temperature and relative humidity) also affected oak. Reducing competition by thinning stimulated radial increment in oak-pine mixed stands, which may provide an adaptive forest management tool for reducing tree vulnerability to extreme droughts, which are linked to climate change. © 2017 Elsevier B.V.

In the Mediterranean area, the role of forest as carbon sinks is particularly significant since usually ecosystem services provided by forests are frequently of greater value than their direct productions. In this chapter, how carbon sequestration changes over time and with different management regimes in Mediterranean pine forests are presented. The information come from a number of sources including: (i) carbon stock estimates under different management plans using a chronosequence trial in Pinus sylvestris forests; (ii) simulations based on the process model 3-PG of the effect of different thinning regimes on Pinus pinaster biomass under a climate change scenario; (iii) a comparison of the effect of different age structures in Pinus pinea forest using the PINEA growth model which includes the biomass allocated in cones and considers the different wood uses; and finally, (iv) a model for estimating coarse woody debris.

Biomass and leaf area equations are often required to assess or model forest productivity, carbon stocks and other ecosystem services. These factors are influenced by climate, age and stand structural attributes including stand density and tree species diversity or species composition. However, such covariates are rarely included in biomass and leaf area equations. We reviewed the literature and built a database of biomass and leaf area equations for 24 European tree species and 3 introduced species. The final dataset contained 973 equations. Most of the equations were site-specific and therefore restricted to the edaphic, climatic and stand structural conditions of the given site. To overcome this limitation, the database was used to develop regional species-specific equations that can be used in a wide range of stands and to quantify the effects of climate, age and stand structure on biomass or leaf area. The analysis showed considerable inter- and intra-specific variability in biomass relationships. The intra-specific variability was related to climate, age or stand characteristics, while the inter-specific variability was correlated with traits such as wood density, specific leaf area and shade tolerance. The analysis also showed that foliage mass is more variable than stem or total aboveground biomass, both within and between species, and these biomass components have contrasting responses to age and changes in stand structure. Despite the large number of published equations, many species are still not well represented. Therefore, generic equations were developed that include species-specific wood density instead of species identity. Further improvements may be possible if future studies quantify the stand structure of individual tree neighbourhoods instead of using the stand means for all trees sampled with the given stand. © 2017 The Authors

Competition with neighboring trees of different species can affect crown size and shape. However, whether intra-specific differences in crown characteristics in mixed stands compared to pure stands are dependent on site conditions remains poorly understood. We used terrestrial laser scanning (TLS) to examine the differences in Fagus sylvatica crown characteristics at four sites, each of which contained pure stands of F. sylvatica and their mixture with Pinus sylvestris. These sites covered the area where the mixture occurs in Europe from south to north, representing a gradient of F. sylvatica productivity, defined as the mean increment of annual volume growth in pure F. sylvatica stands. Despite the large range in productivity, F. sylvatica trees in mixtures had larger crowns regardless of site conditions, with a higher proportion of their crown volume in the lower canopy compared to trees in pure stands. Larger crown volumes were related to higher live crown ratios and greater crown expansion, depending on the site. The magnitude of the mixing effect was variable among the crown characteristics evaluated, but overall our findings provide evidence that for a given species combination and density, the effect of mixture increased in the two most productive sites. TLS-derived novel crown metrics revealed that the mixing effect was affected by productivity, which was not captured by traditionally measured crown variables. © 2017 Elsevier B.V.

Regardless of their geographical location , forests play an important role in CO2 fixation. Carbon stored in terrestrial ecosystems is distributed among three compartments: living plant biomass (stem, branches, foliage, roots), plant detritus (fallen branches and cones, forest litter, tree stumps, tree tops, logs) and soil (organic mineral humus, surface and deep mineral soil). Trees acquire energy for their living structures through photosynthesis, which requires CO2 captured by stomata in the leaves. Part of the captured CO2 is used to create living biomass, while the remainder is released back into the atmosphere through autotrophic respiration. When leaves or branches die and decompose, they increase soil carbon and also release a small amount into the atmosphere through heterotrophic respiration.

Key message This data set provides unique empirical data from triplets of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) across Europe. Dendrometric variables are provided for 32 triplets, 96 plots, 7555 trees and 4695 core samples. These data contribute to our understanding of mixed stand dynamics. Dataset access at http://dx.doi.org/10.5061/dryad.8v04m. Associated metadata available at https://metadata-afs.nancy.inra.fr/geonetwork/apps/georchestra/? uuid=b3e098ca-e681-4910-90990e25d3b4cd52&hl=eng.; Key message :This data set provides unique empirical data from triplets of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) across Europe. Dendrometric variables are provided for 32 triplets, 96 plots, 7555 trees and 4695 core samples.These data contribute to our understanding of mixed stand dynamics.Dataset access at http://dx.doi.org/10.5061/dryad.8v04m.Associated metadata available at https://metadata-afs.nancy.inra.fr/geonetwork/apps/georchestra/? uuid=b3e098ca-e681-4910-90990e25d3b4cd52&hl=eng.

Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) dominate many of the European forest stands. Also, mixtures of European beech and Scots pine more or less occur over all European countries, but have been scarcely investigated. The area occupied by each species is of high relevance, especially for growth evaluation and comparison of different species in mixed and monospecific stands. Thus, we studied different methods to describe species proportions and their definition as proportion by area. 25 triplets consisting of mixed and monospecific stands were established across Europe ranging from Lithuania to Spain in northern to southern direction and from Bulgaria to Belgium in eastern to western direction. On stand level, the conclusive method for estimating the species proportion as a fraction of the stand area relates the observed density (tree number or basal area) to its potential. This stand-level estimation makes use of the potential from comparable neighboring monospecific stands or from maximum density lines derived from other data, e.g. forest inventories or permanent observations plots. At tree level, the fraction of the stand area occupied by a species can be derived from the proportions of their crown projection area or of their leaf area. The estimates of the potentials obtained from neighboring monospecific stands, especially in older stands, were poorer than those from the maximum density line depending on the Martonne aridity index. Therefore, the stand-level method in combination with the Martonne aridity index for potential densities can be highly recommended. The species' proportions estimated with this method are best approximated by the proportions of the species' leaf areas. In forest practice, the most commonly applied method is an ocular estimation of the proportions by crown projection area. Even though the proportions of pine were calculated here by measuring crown projection areas in the field, we found this method to underestimate the proportion by 25% compared to the stand-level approach. The networking in this study has been supported by COST Action FP1206 EUMIXFOR. All contributors thank their national funding institutions to establish, measure, and analyze data from the triplets. The first author thanks the Austrian Science Fund, which supported his work under project number P24433-B16. We also want to thank the two anonymous reviewers for their constructive criticism.

Thinning, as a forest management strategy, may contribute towards mitigating climate change, depending on its net effect on forest carbon (C) stocks. Although thinning provides off-site C storage (in the form of wood products) it is still not clear whether it results in an increase, a reduction or no change in on-site C storage. In this study we analyze the effect of thinning on C stocks in a long-term experiment. Different thinning intensities (moderate, heavy and unthinned) have been applied over the last 30 years in a Scots pine (Pinus sylvestris L.) stand, with a thinning rotation period of 10 years. The main C compartments were analyzed above and belowground tree biomass, deadwood, forest floor and upper 30-cm of the mineral soil and tree biomass removed in thinning treatments. The results revealed that unthinned stands had the highest C stocks with 315 Mg C ha−1, moderate thinning presented 304 Mg C ha−1 and heavy thinning 296 Mg C ha−1, with significant differences between unthinned and heavily thinned stands. These differences were mainly due to C stock in live biomass, which decreased with thinning intensity. However, soil C stocks, forest floor and mineral soil, were not influenced by thinning, all of the stands displaying very similar values 102–107 Mg C ha−1 for total soil; 15–19 Mg C ha−1 for forest floor; 87–88 Mg C ha−1 for mineral soil). These results highlight the sustainability of thinning treatments in terms of C stocks in this pinewood afforestation, and provide valuable information for forest management aimed at mitigating climate change. © 2014, Springer Science+Business Media Dordrecht.

We compiled a global database for leaf, stem and root biomass representing c. 11 000 records for c. 1200 herbaceous and woody species grown under either controlled or field conditions. We used this data set to analyse allometric relationships and fractional biomass distribution to leaves, stems and roots. We tested whether allometric scaling exponents are generally constant across plant sizes as predicted by metabolic scaling theory, or whether instead they change dynamically with plant size. We also quantified interspecific variation in biomass distribution among plant families and functional groups. Across all species combined, leaf vs stem and leaf vs root scaling exponents decreased from c. 1.00 for small plants to c. 0.60 for the largest trees considered. Evergreens had substantially higher leaf mass fractions (LMFs) than deciduous species, whereas graminoids maintained higher root mass fractions (RMFs) than eudicotyledonous herbs. These patterns do not support the hypothesis of fixed allometric exponents. Rather, continuous shifts in allometric exponents with plant size during ontogeny and evolution are the norm. Across seed plants, variation in biomass distribution among species is related more to function than phylogeny. We propose that the higher LMF of evergreens at least partly compensates for their relatively low leaf area : leaf mass ratio. © 2015 New Phytologist Trust.

An important aspect of sustainable forest management is to assess the impact of forest operations on ecosystem services. This work analyzed the long-term effect of two standard thinning regimes on carbon stocks both in tree biomass and soil compartments as well as its effect on soil condition. The target population was a 90-year-old Scots pine (Pinus sylvestris L.) stand in southwestern Europe. Soil condition was measured as dry mass of the forest floor and concentration of carbon, nutrients in the forest floor as well as exchangeable cations, effective cation exchange capacity, pH and bulk density in the mineral soil. Repeated thinnings from below in a southern European population of Scots pine led to a reduction in current on-site carbon stock in tree biomass of 28% in moderately thinned stands (D grade average residual basal area of 65-79% relative to the control plots) which was consistent with an observed loss of volume production. However, the inclusion of the amount of carbon exported off-site with harvested biomass reduced the decrease in stock to 4.8%. Nutrient concentrations in the forest floor increased in moderate thinned stands (P, K, Mg and Mn) or were unchanged (C, N and Fe). The selected thinning regime did not alter mineral soil condition. A decreasing pattern of Ca and Mn stocks with depth was consistent with a high reduction of nutrient concentration of elements and higher bulk density with depth. However K, Mg and Na showed stable stocks across depths because of a much smaller reduction of nutrient concentrations in deeper layers relative to the surface layer. We hypothesized that this stable pattern with soil depth was due to leaching. The sustainability of forest thinning is a trade-off between loss of standing biomass and increasing stand stability as long as other indicators, like soil condition, do not significantly change. © 2015 Elsevier B.V.