Understanding vitality decline is essential, as it precedes tree mortality. Anticipating this process requires the identification of its main driving factors, including environmental conditions, stand structure, and species-specific responses to species mixing, whose combined effects remain poorly understood.To address this gap, tree vitality decline in the Iberian pine forests (Pinus halepensis, P. nigra, P. pinaster, P. pinea, P. sylvestris, and P. uncinata) was analysed by developing vitality decline models using a large Spanish National Forest Inventory dataset (∼280000 trees), including 15638 plots in monospecific stands and 5958 plots in 25 representative two-species mixtures spanning all climatic conditions across the Iberian Peninsula.First, we examined monospecific stands to disentangle the role of environmental and endogenous drivers of vitality decline. In these stands, tree vitality decline was mainly driven by temperature and moderated by site quality, with species-specific sensitivities influenced by endogenous factors such as tree size and stand development.Building on these results, we assessed the effect of mixed stands and climatic conditions on vitality decline. The influence of species mixing depended on species composition, suggesting that admixture can either buffer or exacerbate vitality decline depending on asymmetric competition, vertical stand structure, and the proportion of the target species, which in certain mixtures may result in greater conspecific dominance and intensified competitive stress. Climatic conditions, and specifically temperature, strongly influence patterns of tree vitality decline.Overall, tree vitality is shaped by climatic stress, species identity, and forest composition, suggesting that careful selection of species mixtures could enhance forest resilience.

Mixed mountain forests of Norway spruce (Picea abies (L.) Karst.), silver fir (Abies alba Mill.), and European beech (Fagus sylvatica L.) are ecologically important across Europe, providing ecosystem services, supporting biodiversity, and contributing to the bioeconomy as crucial timber sources. Understanding background mortality in these forests is essential to distinguish the effects of this relatively continuous endogenous process from those induced by exogenous disturbances, and to inform sustainable management under changing conditions. To assess how stand density, tree-size dominance, species competition and diversity, site geomorphology, and climate influence background mortality, expressed as the annualised basal area loss rate, we applied generalised linear mixed models. We used tree measurements from 78 plots located in Bulgaria, Bosnia and Herzegovina, Slovenia, Slovakia, Poland, Germany, and Switzerland, inventoried from 1912 to 2016, with most of them first surveyed between 1953 and 1964. At the stand level, mortality increased with stand basal area, greater species diversity, and the interaction between fir dominance and drought. Spruce mortality increased with stand density, species diversity, and fir dominance, indicating a higher probability of spruce mortality under fir dominance pressure. Fir mortality increased with species diversity and mean annual temperature, revealing fir as sensitive to rising temperatures, and with the interaction between fir dominance and drought, with moisture effects varying along the fir dominance gradient. Beech mortality increased with stand density and fir dominance, suggesting that beech suffers more when fir occupies a dominant canopy position. These findings suggest that sustainable management of mixed mountain forests requires targeted silvicultural interventions to regulate stand density, manage species diversity, and limit fir size dominance.

Aim of study: The aim of this study is to improve the estimation of current annual volume increment of forest tree species in Spain. Area of study: The volume increment of the main tree species was studied at national scale using the two most recent data sets of the Spanish National Forest Inventory (SNFI) available. Material and methods: We developed both a tree diameter and height growth models considering the two most recent cycles of the SNFI and used the estimations obtained to calculate the annual volume growth. Diameter, height and volume estimates were validated using Second and Third SNFI cycles that were not been used for modelling. Main results: The developed tree diameter and height growth models indicated a satisfactory model structure. In general, competition, expressed as basal area, negatively impacted tree growth. Although basal area of larger trees had a negative effect, it was modulated by site quality, where better site conditions positively affected growth. Furthermore, tree vitality was found to be an important factor modulating tree growth. Research highlights: The proposed methodology to estimate tree growth considers tree size, competition, site quality and tree vitality and improves the estimates compared to previous methods based on current annual volume increment equations.

Tree mortality is a complex process associated with endogenous factors such as tree size, species composition or stand density, which may interact with each other and/or with exogenous factors such as climate. We studied the influence of these factors on background tree mortality in Scots pine, Norway spruce, and downy and silver birch mixed forests located in Central and Northern European continental (Poland) and boreal (NE Poland, Lithuania, Sweden and Norway) regions. Our main objective was to disentangle how species interactions influence background mortality along a gradient of climate conditions. To achieve this, we developed mortality models using a set of 188405 sample trees in 6840 permanent sample plots, taken from the National Forest Inventories of the four countries. Our results indicate that tree mortality and the effects of competition were species-specific. The competition from pine resulted in higher tree mortality for the pine itself rather than for birch or spruce, and the competition from spruce was usually the greatest, causing higher tree mortality in the three studied species. Temperature modified the effects of competition, always magnifying the effects of intra- or inter-specific competition depending on the species. However, the effect of temperature was complex, as the density-dependent and the density-independent mortality varied in different ways with temperature. In general, mixing pine with spruce and birch increases pine mortality, whereas it favors spruce and birch survival, especially at warmer sites. These findings may be useful for the management of these mixtures in the context of climate change.

Trees often show better growth in mixed as opposed to monospecific neighborhoods as consequence of competition reduction and facilitation between species. However, it can be challenging to discern the effect of these factors as they occur simultaneously, so often they are jointly refereed as complementarity. By using tree growth models based on potential tree growth reduced by a competition modifier and including species mixing effects on both components (potential tree growth and competition modifier), we aim to verify that tree potential growth is modified by admixture, which may provide further information on species interactions and complementarity.We used tree data from the Spanish National Forest Inventory, selecting plots located in two different mixtures, Scots pine - European beech and Scots pine - Oak mixtures, as well as in the corresponding monospecific stands of those species. To analyse whether the species mixture increases the potential basal area growth, we developed linear mixed quantile regressions to model the potential growth, while to analyse the competition reduction we developed linear mixed models. We include the Martonne aridity index as a measure of the site conditions in the models to explore whether climate conditions modify the effects of species mixture. Our results show a slight effect of the mixture on the potential basal area growth of pine while the potential growth of beech clearly benefited from the pine/beech mixtures. For the pine/oak mixtures the potential growth of both species was higher in mixtures than in monospecific stands. Moreover, we found a positive influence of humidity on potential growth for all the species and mixtures, although the magnitude of the effect was less important for beech and more notable in the case of the pine/oak mixtures. We also found that for the studied species, admixed species reduced competition, with lower inter-specific than intra-specific competition effects, and that these effects were mediated by climate conditions.We concluded that the potential tree basal area growth is influenced by the species admixture, which suggest that species mixing effects can be relevant also under low stand densities. Our results highlight the importance of integrating the effect of species interactions on both the potential growth of trees and the competition modifier when the aim of the research is to further our understanding of species interactions.

The increasing disturbances in monocultures around the world are testimony to their instability under global change. Many studies have claimed that temporal stability of productivity increases with species richness, although the ecological fundamentals have mainly been investigated through diversity experiments. To adequately manage forest ecosystems, it is necessary to have a comprehensive understanding of the effect of mixing species on the temporal stability of productivity and the way in which it is influenced by climate conditions across large geographical areas. Here, we used a unique dataset of 261 stands combining pure and two-species mixtures of four relevant tree species over a wide range of climate conditions in Europe to examine the effect of species mixing on the level and temporal stability of productivity. Structural equation modelling was employed to further explore the direct and indirect influence of climate, overyielding, species asynchrony and additive effect (i.e. temporal stability expected from the species growth in monospecific stands) on temporal stability in mixed forests. We showed that by adding only one tree species to monocultures, the level (overyielding: +6%) and stability (temporal stability: +12%) of stand growth increased significantly. We identified the key effect of temperature on destabilizing stand growth, which may be mitigated by mixing species. We further confirmed asynchrony as the main driver of temporal stability in mixed stands, through both the additive effect and species interactions, which modify between-species asynchrony in mixtures in comparison to monocultures. Synthesis and applications. This study highlights the emergent properties associated with mixing two species, which result in resource efficient and temporally stable production systems. We reveal the negative impact of mean temperature on temporal stability of forest productivity and how the stabilizing effect of mixing two species can counterbalance this impact. The overyielding and temporal stability of growth addressed in this paper are essential for ecosystem services closely linked with the level and rhythm of forest growth. Our results underline that mixing two species can be a realistic and effective nature-based climate solution, which could contribute towards meeting EU climate target policies.

National Forest Inventory (NFI) data are the main source of information on forest resources at country and subcountry levels. This chapter explores the strengths and limitations of NFI-derived indicators to assess forest development with respect to adaptation to and mitigation of climate change, that is, the criteria of Climate-Smart Forestry (CSF). We reflect on harmonizing NFI-based indicators across Europe, use literature to scrutinize available indicators to evaluate CSF, and apply them in 1) Switzerland, where CSF is evaluated for NFI records and simulation model projections with four management scenarios; 2) 43 selected European countries, for which the indicators for Sustainable Forest Management (SFM) are used. The indicators were aggregated to composite indices for adaptation and mitigation and to an overall CSF rating. The Swiss NFI records showed increased CSF ratings in mountainous regions, where growing stocks increased. Simulations under business-as-usual management led to a positive CSF rating, whereas scenarios of increased harvesting decreased either only adaptation or both mitigation and adaptation. European-level results showed increases in CSF ratings for most countries. Negative adaptation ratings were mostly due to forest damages. We discuss the limitations of the indicator approach, consider the broader context of international greenhouse gas reporting, and conclude with policy recommendations.

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.

Forest composed of Picea abies L., Abies alba Mill. and Fagus sylvatica L. cover a large area in the European mountain regions and have a high ecological and socio-economic importance as they supply many ecosystems services. Because of climate change, these forests are exposed to warming, and this effect increases with elevation, which may impact their delivery of goods and services. Previous studies did not find significant changes in the overall productivity of these species over the last 30 years, but they observed changes in species competitiveness at the species and tree levels. In this study, we aimed to link previous results on tree, species and stand level growth in spruce-fir-beech mixed mountain forests by analysing species size distribution dynamics under different climate conditions and their effect on stand growth. We developed a matrix model based on data from 76 long-term experimental plots distributed throughout Europe. We used the change in stand basal area to explore whether temperature modifies species size dominances and proportions, whether the temperature effects on changes in species basal area depend on species size dominance, and whether the effect of species size dominance on changes in the stand basal area varies with temperature. Our results showed that annual mean temperature is an important climatic driver of species dynamics in spruce-fir-beech mixed mountain forests, such that stand basal area growth was favored by higher temperatures, particularly due to positive responses of silver fir which were greater than negative effects of temperature on European beech. The high temperatures also favored the size-dominance of silver fir, while European beech tended to have smaller diameters, independent of the temperature. We also found that the identity of the size dominant species also influenced changes in stand basal area, with the highest or the lowest changes when Norway spruce and European beech were the size-dominant species, respectively. Silver fir was less influenced by the identity of the size-dominant species than by temperature.

Crown dimensions such as diameter or crown projection area are important indicators for tree vigour, growing stage or production efficiency and can be used as proxies for the assessment of wood quality, leaf area and leaf biomass, biodiversity, wildlife habitat and disturbance risk to forest stability. Crown allometry can vary with site and competition conditions, but variation patterns are not still well understood. The main aim of this study was to analyze this allometry variation in the most common pine species along the aridity gradient of the Iberian Peninsula, studying the crown response to aridity and to competition, and determining whether the presence of inter-specific competition modifies crown allometry. A set of 40,402 sample trees from 12,283 sample plots, along with the Martonne aridity index as an indicator of climatic conditions, were used to develop allometric models, the parameters of which were generalized by including aridity and competition indicators. The plots were located in monospecific or mixed stands of Pinus sylvestris, Pinus nigra, Pinus pinaster, Pinus pinea and Pinus halepensis. According to our results, both aridity conditions and competition status are important factors modifying the crown allometry of the studied species. Moreover, we found that both size-symmetric competition and size-asymmetric competition should be taken into account when modelling the crown size, both types leading to smaller crown diameter. The crown plasticity of the studied pine species in response to competition presented a low range of variation, the greatest being that of maritime pine, while Scots pine and black pine showed the smallest. The way in which aridity and competition influence crown size differs among the studied species, indicating differences in crown plasticity patterns. Moreover, in humid sites, where competition for light is more important, the crown allometry response to changes in competition is greater, being most evident in P. pinea and P. halepensis. The species mixing effects had less influence on pine crown allometry than the total competition effect, although the effect was always significant. The effect of size-symmetric competition from admixed species was positive or neutral, except for the effect of P. sylvestris on P. nigra crowns. Size-asymmetric competition also plays an important role in the species mixing effects, these effects on intermediate or suppressed trees varying depending on the species mixture. The models developed reveal significant species-specific effects on crown allometry, pointing to the ability of pine species to adapt their crowns to different growing conditions.

Key messageCrown, height and stem allometry vary with stand density and species composition, the plasticity in response to inter- and intra-specific competitions being related to species shade tolerances.AbstractDetermining 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.

National Forest Inventories (NFIs) are the primary source of information to fulfill international requirements, such as growing stock volume. However, NFI cycles may be out of phase in terms of the information required, so prediction techniques are needed. To disentangle the effects of climate and competition on stand productivity and to estimate the volume of stocks at national scale, it is important to recognize that growth and competition are species-specific and vary along climatic gradients. In this study, we estimate the productivity of five pine species (Pinus sylvestris, Pinus pinea, Pinus halepensis, Pinus nigra and Pinus pinaster), growing in monospecific stands or in mixtures along an aridity gradient in the Iberian Peninsula, based on Spanish NFI data. We study the stand volume growth efficiency (VGE), since it allows the comparison of volume growth in monospecific and mixed stands. The results reveal the importance of considering the aridity when assessing VGE. Moreover, it was found that, in general, admixture among pine species leads to modifications in the VGE, which can vary from negative to positive effects depending on species composition, and that this is always influenced by the aridity. Finally, we provide simple growth efficiency models for the studied pines species which are valid for both monospecific and mixed stands along the aridity gradient of the Iberian Peninsula.

The diverse applications of the maximum size-density relationship (MSDR) in monospecific and mixed forests, such as in ecological and economic aspects, lead to continuous advances in our knowledge on this issue. One of the most recent advances was the inclusion of climatic variables in these studies, revealing the variation in MSDR depending on environmental conditions. However, the importance of climatic conditions on the intra- and inter specific variation of MSDR is still poorly understood. The aim of this paper is to explore the dependence of MSDR on climatic conditions for the five principal pine species in the Iberian Peninsula (P. halepensis Mill., P. nigra Arn., P. pinea L., P. pinaster Ait., and P. sylvestris L.) and to analyse the importance of this dependence on the relative carrying capacities in mixed stands. Data from the Third Spanish National Forest Inventory (NFI) were used together with four simple climatic indices calculated from raster maps. Using a quantile regression, a MSDR basic model, relating the maximum number of trees and the quadratic mean diameter, was fitted to the data from the plots in monospecific stands. In a second step, the coefficients of basic model were parameterized as a function of climatic variables. The resulting climate-dependent model was also fitted to the plots. Competition equivalence coefficients (CEC) in mixed stands (i.e. ratio between maximum stand density indices of the two species) were calculated from the resulting models. The differences among species' MSDRS confirm the inter-specific variability of maximum densities and the need for species-specific models. According to the Akaike information criteria, the climate-dependent models, and particularly those dependent on Martonne aridity index, were always better than basic models. Although the higher the aridity the lower the maximum stand density, the influence of climate on the MSDR also varies according to the species considered, this influence being more evident for P. pinaster and P. halepensis, which also display high ecological plasticity. The CEC derived from the basic model for pine-pine mixtures range from 1.10 to 1.70. However, when aridity is considered, these coefficients almost always decrease. Our results highlight the importance of considering environmental variables to better describe and compare the potential density in monospecific and mixed stands and therefore, the utility of species-specific climate dependent models for management decision support.

An increasing amount of research is focusing on comparing productivity in monospecific versus mixed stands, although it is difficult to reach a general consensus as mixing effects differ both in sign (over-yielding or under-yielding) and magnitude depending on species composition as well as on site and stand conditions. While long-term experimental plots provide the best option for disentangling the mixing effects, these datasets are not available for all the existing mixtures nor do they cover large gradients of site factors. The objective of this study was to evaluate the effects and uncertainties of tree species mixing on the productivity of Scots pine–European beech stands along the gradient of site conditions in Europe, using models developed from National and Regional Forest Inventory data. We found a positive effect of pine on beech basal area growth, which was slightly greater for the more humid sites. In contrast, beech negatively affected pine basal area growth, although the effects switched to positive in the more humid sites. However, the uncertainty analysis revealed that the effect on pine at mid- and more humid sites was not-significant. Our results agree with studies developed from a European transect of temporal triplets in the same pine–beech mixtures, confirming the suitability of these datasets and methodology for evaluating mixing effects at large scale.

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.

A growing number of studies provides evidence that mixed-species forests often have higher stand productivity than monospecific forests, which is referred to as overyielding. In this study, we explored how the combination of species and soil conditions affect overyielding in terms of periodic annual volume increment (PAIV) in Dutch forests. We studied Douglas-fir (Pseudotsuga menziesii (Nab.) Franco), common beech (Fagus sylvatica L.), Scots pine (Pinus sylvestris L.), pedunculate oak (Quercus robur L.), and silver birch (Betula pendula Roth) growing in four two species combinations (Douglas-fir-common beech, Scots pine-pedunculate oak, pedunculate oak-common beech, and pedunculate oak-silver birch) from 398 long-term permanent field plots all over the Netherlands. We found that the Douglas-fir-common beech and Scots pine-pedunculate oak mixtures always showed overyielding. This overyielding was largely attributed to the Douglas-fir in the former mixture and to the pedunculate oak in the latter mixture, respectively. In both cases, overyielding was stronger at poor soils than at rich soils. The pedunculate oak-common beech mixtures overyielded at poor soils and underyielded at rich soils, which was attributed to the response of the common beech. Overyielding was not observed for the pedunculate oak-silver birch mixtures, irrespective of soil conditions. The results do not support our hypothesis since overyielding was not always driven by fast-growing light-demanding species. Overyielding was stronger for evergreen-deciduous species combinations, suggesting that differences in leaf phenology are a major driver of overyielding. Secondly, our results imply that overyielding is much stronger at poor soils than at rich soils, which is in line with the prediction of the stress-gradient hypothesis. We conclude that the growth of one species benefits from the admixture species, particularly in evergreen-deciduous species mixtures and that soils affect the extent of overyielding as studied in the Netherlands.

An appropriate, common interpretation of stand structure characteristics is a key element to better understand forest ecosystem ecology and dynamics. Standards for characterizing the structure, dynamics and productivity of even-aged pure stands are well developed, but such harmonized concepts and methods for mixed forest stands are lacking. Here we compile a comprehensive set of measures, indices and methods at stand level to characterize and evaluate mixed stands. The chapter is organized according to the main components of the structure of forest stands; hence it includes (1) the most relevant concepts and approaches to describe stand density as a key component of stand structure; (2) stand species composition indicators and the most common species diversity indices used in the science of forest growth and yield; (3) how to describe tree distribution patterns, including horizontal spatial pattern, species intermingling and vertical spatial pattern, as well as species-specific height growth and canopy space partitioning; (4) ways to characterize tree-size distribution and growth partitioning among trees of different sizes; and (5) site productivity indices and methods for the comparison of productivity in mixed vs. monospecific stands. Finally we discuss some of the methodological and application challenges related to the reviewed indices and methods which require further attention.

The abundance of stems in crowded populations and the subsequent self-thinning is a key issue in forest stand dynamics. However, the mechanisms that control self-thinning are challenging to model. Although some attempts to include climate and structural traits Like specific gravity (SG) are promising, they remain confined to North American species. In this study we aimed to disentangle how SG along with two major abiotic stress tolerances, i.e. shade and drought tolerance, contribute to the maximum density of a forest stand across a climatic gradient in Europe, and thus test the validity of the species-specific trait control over stand density. We propose a modelling approach that incorporates the tolerance to drought and shade in the determination of maximum relative stocking. Here, relative stocking refers to the degree of tree crowding in forest ecosystems. A relative stocking base model where specific wood density is inversely related to stand density is modified, adding normalized indices of drought and shade stress tolerance. We used available species tolerance rankings modulated by stress intensity to analyse the effects of abiotic stress polytolerance or trade-offs in the study area which represent an environmental gradient from Alpine to Mediterranean climate in northern Spain. Results indicated that the role of drought tolerance in controlling maximum stand density is stronger in warmer sites. The simultaneous tolerance to shade and drought results in Less carrying capacity of sites. In those sites where there is no water Limitation but minimum temperature is very Low the tolerance to bending stress (i.e. specific gravity) explains better the maximum tree occupancy.

Model-based inference is an alternative to probability-based inference for small areas or remote areas for which probability sampling is difficult. Model-based mean square error estimators incorporate three components: prediction covariance, residual variance, and residual covariance. The latter two components are often considered negligible, particularly for large areas, but no thresholds that justify ignoring them have been reported. The objectives of the study were threefold: (i) to compare analytical and bootstrap estimators of model parameter covariances as the primary factors affecting prediction covariance; (ii) to estimate the contribution of residual variance to overall variance; and (iii) to estimate thresholds for residual spatial correlation that justify ignoring this component. Five datasets were used, three from Europe, one from Africa, and one from North America. The dependent variable was either forest volume or biomass and the independent variables were either Landsat satellite image bands or airborne laser scanning metrics. Three conclusions were noteworthy: (i) analytical estimators of the model parameter covariances tended to be biased; (ii) the effects of residual variance were mostly negligible; and (iii) the effects of spatial correlation on residual covariance vary by multiple factors but decrease with increasing study area size. For study areas greater than 75 km2 in size, residual covariance could generally be ignored.

Because shrub cover is related to many forest ecosystem functions, it is one of the most relevant variables for describing these communities. Nevertheless, a harmonized indicator of shrub cover for large-scale reporting is lacking. The aims of the study were threefold: to define a shrub indicator that can be used by European countries for harmonized shrub cover estimation using data from their respective national forest inventories (NFIs); to quantify the effects of using different NFI field cover scales; and to establish bridges to facilitate harmonized estimation. Data for shrub species cover from the Third Spanish NFI together with scales for cover assessment from 16 European NFIs were used. The indicator, mean species cover (MSC), was defined for each species and each European forest category. Estimates of MSC calculated using species covers recorded for field plots, with 1% interval widths (MSCobs), were compared with the MSC values that would be obtained for the same data with the different European cover scales (MSCpred). Residuals calculated as differences between MSCobs and MSCpred were analyzed, and a linear mixed model was used as bridging function to adjust predictions and thus further harmonize estimates. Scales with only two or three intervals produced the greatest residuals, while all the other analyzed scales had residuals less than 5%. Most scales, except those most similar to Braun-Blanquet, displayed a tendency to be unreliable for larger covers. The proposed mean species cover indicator provides comparable estimates for shrub communities at large scales. The linear models improved the harmonization of MSC for the scales having two and three intervals.