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Competition can intensify the struggle for resources among plants, affecting forest growth and dynamics. The intensity and mode of competition – asymmetric vs. symmetric – can change along environmental gradients and with time, impacting the response of plants to their environment, but this is seldom explicitly considered in management plans. In this study, we aim to (i) disentangle the main environmental and tree-related factors that affect post-fire Pinus halepensis sapling growth; (ii) determine which mode of competition characterizes the species post-fire regeneration; and (iii) elucidate if the mode of competition changes with time, and along climatic gradients. We sampled 148 P. halepensis saplings located in 15 sites affected by wildfires between 1987 and 2013 in Catalonia (NE Spain). We measured their radial growth at the base, and we identified their competitive environment by locating and measuring all the trees in a 3-meter-radius from our target saplings. We modelled the effect of tree size, age, climate, and competitive environment on the post-fire regeneration growth following the neighborhood theory of forest dynamics and using model comparison and information criteria to address our research questions. We used a modification from the Hegyi index to determine the prevalence of symmetric vs. asymmetric competition, and we assessed the support for models in which competition was allowed to vary with age and precipitation. The best model showed that competition, precipitation, age, and target size influenced the radial growth of P. halepensis post-fire regeneration (Akaike weight 0.66, R2 = 0.52). We found evidence that P. halepensis mode of competition is generally asymmetric but can change along a precipitation gradient: it shifts from asymmetric in drier sites to almost fully symmetrical as precipitation increases. Our findings have implications for the management of post-fire P. halepensis stands, enabling the adaptation of silvicultural prescriptions to the climatic environment. In a context of increasing water scarcity, adaptive silviculture is fundamental to foster the resilience of pinewoods to future climatic and disturbance regimes. • We studied the role of drought and competition on post-fire pine growth. • We show evidence for asymmetric competition in pine growth. • Precipitation had a positive effect on growth but changed the competition mode. • Intraspecific pine competition becomes less asymmetric in more humid sites. • Management should adapt to changes in competition mode along precipitation gradients. [ABSTRACT FROM AUTHOR]

• Low intensity single-tree selection cuttings generally increased tree growth. • Soil moisture increased by 10% after treatments, mostly in the growing season. • Best tree growth models included data on management, competition and soil moisture. • Close-to-nature forestry appears as a promising tool to reduce drought vulnerability. Given the rising frequency of drought events, close-to-nature principles appear to be a suitable management option for promoting more adapted forests by increasing diversity and vitality (e.g., enhancing tree growth), while preserving soil moisture. In this study, we evaluate individual tree growth from selected trees (including future crop trees) and soil moisture responses to close-to-nature silvicultural treatments in sub-humid Mediterranean mixed forests in Catalonia (Northeastern Spain). We relied on 18 permanent sample plots (i.e., 9 paired-plots: managed / control) representing a competition intensity gradient and different forest types (dominated by either Quercus ilex , Castanea sativa , Quercus pubescens/petraea , Pinus pinea and Pinus sylvestris). From them, 8 plots were subject to intense monitoring in terms of intra-annual tree radial growth, monitored by manual band dendrometers, and hourly soil water content. Basal area increment, and annual and seasonal (May, June, July) radial increment rates were computed as a proxy of tree growth for all plots and for those plots with dendrometers, respectively. We conducted Spearman correlation analyses between tree growth, competition intensity, soil water content and climatic variables to detect significant relations between forest management and variables of interest (e.g., basal area increment, radial increment rates, soil water content). To investigate further into the effect of this silviculture on tree growth, we used mixed-effects models using as predictors variables related to competition intensity and forest management (dummy variable) in combination with monthly soil water content variables. We observed that basal area increment and radial increment rates had a positive (albeit weak) correlation with forest management, while competition intensity had a negative correlation with both variables. The models were not capable of detecting clear species-specific tree growth responses to competition. Compared to control plots, on average, the treatments boosted soil water content by about a 10% in managed plots in Q. ilex , P. sylvestris and P. pinea stands, and this increase was particularly significant during the growing season. Moreover, the best-fitted model predicted seasonal radial increment rates using not only forest management and competition variables, but also monthly soil water content factors. Our results suggest that close-to-nature-forest management may reduce the vulnerability to drought by increasing tree vitality and soil moisture, at least in the short term (<3 years). [ABSTRACT FROM AUTHOR]

Highlights • Oak resprouts compete with Pinus nigra saplings regenerated after fire. • The sensitivity of P. nigra to neighbors varied with pine size and water availability. • Competitive effects depend on the relative size of pines to oak resprouts. • Early-dispersed seedlings in open sites can grow faster and mitigate late competition. Abstract The persistence of non-serotinous pines in Mediterranean forests can be threatened by climate-mediated changes in fire regimes that may favor the dominance of resprouters or other fire-adapted species. Recovery of non-serotinous pines after large wildfires is often determined by their ability to grow under the canopy of promptly established resprouters. Mechanisms of facilitation or competition between resprouters and pines will thus have a profound effect on forest dynamics. We examined here the effect of neighboring oak resprouts on Pinus nigra Arn. ssp. salzmannii saplings 18 years after a wildfire. We determined the net outcome of interactions between oaks and pines and how they vary with the life stage and size of the interacting plants or the environmental conditions. We did not find any net facilitative effects of oaks on pine sapling growth. The sensitivity of pines to neighbors varied markedly with pine size, and to a lesser extent, with water availability during the growing season. Our findings suggest a self-reinforcing hierarchical process by which early-dispersed seedlings growing in low-competitive microsites can grow faster, mitigating neighboring competition in the later stage of canopy closure. These results entail a potentially critical role of management practices to promote post-fire recovery of non-serotinous pines under expected changing conditions of disturbance regimes. [ABSTRACT FROM AUTHOR]

The maximum size-density relationship (MSDR) reflects the boundary site occupancy and the self-thinning line for a given species, being a useful tool in forestry. Studies focusing on the MSDR often do not cover the whole distribution of the studied species, which results in different boundaries for a given species in different regions. A common MSDR is lacking for the increasingly demanded large-scale studies. However, this information is important where silvicultural responses must be prioritized among monospecific stands or where comparisons among maximum and relative stand densities between and within species are required. For the purposes of this study, we used data from 9911 sample plots located in Scots pine and European beech monospecific stands. Both of these species are of considerable importance and widely distributed throughout Europe. The data came from National or Regional Forest Inventories of five European countries (Austria, Germany, France, Spain and Poland) and therefore were distributed across a wide range of climatic conditions. The main aim of this study was to determine whether the MSDR of these species depends on environmental variables and to develop a MSDR model for each species that explain this variability along a climate gradient. The resulting models showed that both parameters of species boundary lines were climate-dependent, but that the pattern of variation differed between species. Hence, the higher the humidity, the steeper the MSDR (more negative exponent) and the higher the intercept for beech, while in the case of pine, the higher the humidity, the straighter the MSDR and the lower the intercept. According to these models, the stand density indices, for a reference diameter of 25 cm, varied with the humidity in a different way for each species. Consequently, the ratio between the two species increases with humidity, although it also depends on stand diameter. These results are in accordance with the yield level theory and could contribute to the development of more precise silvicultural guidelines and growth models based on the self-thinning line. Moreover, they are of particular importance in the discussion of growth and the effects of mixing on mixed species stands. [ABSTRACT FROM AUTHOR]

A random intercept for each triplet and year for Quercus robur and Quercus petraea (top) and Pinus sylvestris (bottom) showing the inter-annual variation in diameter increment on different sites at the logarithmic scale in mm. [Display omitted] • Tree growth reactions to climate are highly site-specific within a bioclimatic zone. • Local (micro)-site conditions outweigh large-scale climatic patterns. • Tree growth reactions to climate are dependent on relative tree size. • Growth sharply declines with increasing potential evapotranspiration in June. • Species identity and mixture effects clearly differ between years at the same site. Quercus robur / Quercus petraea and Pinus sylvestris are widely distributed and economically important tree species in Europe co-occurring on mesotrophic, xeric and mesic sites. Increasing dry conditions may reduce their growth, but growth reductions may be modified by mixture, competition and site conditions. The annual diameter growth in monospecific and mixed stands along an ecological gradient with mean annual temperatures ranging from 5.5 °C to 11.5 °C was investigated in this study. On 36 triplets (108 plots), trees were cored and the year-ring series were cross-dated, resulting in year-ring series of 785 and 804 trees for Q. spp. and P. sylvestris , respectively. A generalized additive model with a logarithmic link was fit to the data with random effects for the intercept at the triplet, year and tree level and a random slope for the covariate age for each tree; the Tweedie-distribution was used. The final model explained 87 % of the total variation in diameter increment for both tree species. Significant covariates were age, climate variables (long-term mean, monthly), local competition variables, relative dbh, mixture, stand structure and interactions thereof. Tree growth declined with age and local density and increased with social position. It was positively influenced by mixture and structural diversity (Gini coefficient); mixture effects were significant for P. sylvestris only. The influence of potential evapotranspiration (PET) in spring and autumn on tree growth was positive and non-linear, whereas tree growth sharply decreased with increasing PET in June, which proved to be the most influential month on tree growth along the whole ecological gradient. Interactions of PET with tree social position (relative dbh) were significant in July and September for Q. spp. and in April for P. sylvestris. Interactions of climate with density or mixture were not significant. Climatic effects found agree well with previous results from intra-annual growth studies and indicate that the model captures the causal factors for tree growth well. Furthermore, the interaction between climate and relative dbh might indicate a longer growth duration for trees of higher social classes. Analysis of random effects across time and space showed highly dynamic patterns, with competitive advantages changing annually between species and spatial patterns showing no large-scale trends but pointing to the prevalence of local site factors. In mixed-species stands, the tree species have the same competitivity in the long-term, which is modified by climate each year. Climate warming will shift the competitive advantages, but the direction will be highly site-specific. [ABSTRACT FROM AUTHOR]

In the Mediterranean, non-serotinous pinewoods are suffering an increasing occurrence of high-severity crown fires that usually drive vegetation shifts to fire-adapted communities and a decrease in pine-dominated area. Here we used a case-study approach on a large area dominated by Pinus nigra Arn. ssp. salzmannii burned in 1998 to gain further understanding of the relative importance of different factors related to local topography (elevation, aspect, slope, curvature), pre-fire vegetation (land-use history, canopy cover) and fire behavior (burn severity, presence of unburned patches) as drivers of post-fire regeneration dynamics. The results find that pine shows locally resilient responses driven mainly by factors related to fire effects (presence of unburned patches) and the characteristics of the pre-fire vegetation (i.e. stable forest areas). When fire-induced changes from pine dominance to other types of vegetation occurred, landscape 15 years post-fire was dominated by woody vegetation, with some rare grassland communities emerging under very specific conditions (mountain ridges, hilltops and rocky sites). Conversion from forest to shrubland occurred mainly in the most xeric sites (south-facing areas, in some cases with steep slopes) and areas dominated by young pine stands prior to the fire. We found manageable factors such as the pre-fire structure and composition of the vegetation strongly determine the occurrence of post-fire regeneration trajectories dominated by tree species regeneration. This knowledge can be used to define preventive management strategies oriented to direct regeneration dynamics in anticipation of fire occurrence. At landscape level, managing forest fuels to favor the occurrence of unburned patches and modify their spatial distribution along the burned landscape will favor a more resilient pine response. At stand level, adjusting silvicultural interventions to favor the natural establishment of late-successional tree species will favor post-fire oak regeneration. [ABSTRACT FROM AUTHOR]

Evidence of tree regeneration failure of some species in the Iberian Peninsula forests warns us about the impact that the global change may exert on the preservation of Mediterranean forests, such as we know them. Predictions agree about an exacerbation of the summer drought there, acknowledged as the main limiting factor for the recruits’ survival. On the other hand, many studies have also proved the relevant role that local heterogeneity has over the spatial distribution of forest species recruitment by providing safe sites. Therefore, to unravel how climate interacts with local factors over juveniles’ performance seems crucial for the design of successful management strategies that allow facing the global warming. Here, we surveyed the natural recruitment of four dominant tree species in seven mountainous regions in the Iberian Peninsula, along entire elevational ranges as surrogates of their climatic ranges. Two of them have alpine and temperate distributions with populations at their rear edge in the Spanish mountains: Fagus sylvatica and Pinus uncinata ; and the other two have a genuine Mediterranean distribution: Quercus ilex and Pinus nigra . Our main goal was to analyze for each species the effect of climate, local factors ( i.e. light availability, stand structure and ground cover) and the interactions among them to identify the main drivers leading the regeneration process, assessed in terms of presence, abundance and mean annual growth of juveniles. The results showed different environmental factors determining the recruitment patterns of each species. Nevertheless, they highlighted the pervasive role exerted by both climate and fine scale factors, particularly the co-occurring vegetation on recruits’ abundance, and the light availability on their growth. Moreover, we found some interactions among annual mean temperature and local factors, suggesting that climate and local heterogeneity act hierarchically, i.e. the local conditions may mitigate or exacerbate the impact of climate on juveniles. These results advocate for further research to increase our knowledge on the complex net of interactions among factors involved in recruitment at different scales, which in turn should be taken into account and incorporated in forthcoming management strategies. [ABSTRACT FROM AUTHOR]

The progressive abandonment of traditional forest management over the last few decades has led to significant densification processes in most Mediterranean pine stands. In parallel, some of these stands have also shown tree-species diversification processes, the occurrence of which is considered essential for future adaptability and resilience to change. Here we aim to gain further understanding of the main factors driving these diversification processes via a case-study approach using the long-term-managed black pine ( Pinus nigra Arn. ssp. salzmannii ) forests of the Catalan Pre-Pyrenees (NE Spain). For this purpose, we sampled 155 plots distributed in 8 different stands and analyzed the role played by a number of microsite factors and stand attributes (including canopy openness and heterogeneity) on the abundance of seedlings ( h < 1.3 m) and saplings ( h > 1.3 m; dbh < 7.5 cm) of the main tree-species in the area (i.e. black pine, evergreen oak and marcescent oaks). Results revealed ongoing black pine recruitment limitation processes mainly associated to the high canopy cover of the overstory and the increasing abundance of shrubs, which may compete with pines for light resources. In contrast, we found that current environmental and stand-level conditions favor the progressive advance of the recruitment of evergreen and marcescent oaks, which are able to establish successfully under the dominant pine canopy. However, in the absence of canopy openings, light levels may not allow the established oaks (in particular the evergreen Quercus ilex ) to grow and progress to higher developmental stages. Our findings bring deeper insight into the role of stand-level factors regulating species diversification, and can be used by forest managers to adjust their practices (e.g. by modifying the spatial and temporal patterns of silvicultural treatments such as thinnings or selection cuttings) in order to favor this natural process and increase stand resilience. [ABSTRACT FROM AUTHOR]

Browsing damage is among the most determinant factors that limit the establishment of tree seedlings in forests. In some areas, this process leads to massive mortalities that can reduce or even completely prevent the regeneration of some tree species. Mediterranean mountain forests have undergone during the last decades important changes in land-uses that have significantly altered the type and abundance of herbivore populations. In this study we assessed the impact of current grazing conditions in forest regeneration using a set of experimental plantations established in the Eastern Pyrenees in areas visited by domestic livestock (cattle and horses) and wild ungulates (mainly roe deer and chamois). We analyzed during 4 years the role of seedling species and size, mesohabitat (elevation and type of forest cover) and microhabitat (herbaceous cover, distance to shrub, and light availability) on the browsing-induced mortality of more than 500 seedlings of Pinus sylvestris , Pinus uncinata , Betula pendula and Abies alba , the most common tree species in the study area. Browsing-induced mortality for the three conifer species was much lower (<15%) than the one observed for B. pendula (>40%) and depended on both microhabitat – mainly on the distance to protective elements such as shrubs; and mesohabitat, with an interaction between the elevational belt (site) and the type of forest cover (gaps vs . understory). In the subalpine belt, browsing on A. alba and P. uncinata was higher during summer at plots located in the forest understory whereas, during winter, it was higher at plots located in gaps. The study shows that both mesohabitat and microhabitat can exert an effect on the patterns of plant damage by herbivores, providing useful information to adapt forest management in areas particularly exposed to them. [ABSTRACT FROM AUTHOR]

• The study based on 90 mature Scots pine stands along a productivity gradient across Europe. • Growth partitioning became more asymmetric and structuring with increasing site quality. • Mortality eliminated predominantly small trees with increasing site quality. • We found the highest size variation on poor sites and the lowest on rich sites. • As a result stand structure became more homogeneous with increasing site quality. 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. [ABSTRACT FROM AUTHOR]

• Patterns of mixing effects on pine/beech growth-climate relationships were analyzed across Europe. • Tree growth-climate relationships were driven by the regional climate conditions. • Differences in climate-growth relationships between pure and mixed beech stands were evidenced in the driest climates. • On average, mixing had no significant effect on resistance to drought events. • Growth reduction during drought events was lower in mixed compared to pure pine stands in sites with higher water balance in autumn. Increasing species diversity is considered a promising strategy to mitigate the negative impacts of global change on forests. However, the interactions between regional climate conditions and species-mixing effects on climate-growth relationships and drought resistance remain poorly documented. In this study, we investigated the patterns of species-mixing effects over a large gradient of environmental conditions throughout Europe for European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.), two species with contrasted ecological traits. We hypothesized that across large geographical scales, the difference of climate-growth relationships and drought resistance between pure and mixed stands would be dependent on regional climate. We used tree ring chronologies derived from 1143 beech and 1164 pine trees sampled in 30 study sites, each composed of one mixed stand of beech and pine and of the two corresponding pure stands located in similar site conditions. For each site and stand, we used Bootstrapped Correlation Coefficients (BCCs) on standardized chronologies and growth reduction during drought years on raw chronologies to analyze the difference in climate-tree growth relationships and resistance to drought between pure and mixed stands. We found consistent large-scale spatial patterns of climate-growth relationships. Those patterns were similar for both species. With the exception of the driest climates where pure and mixed beech stands tended to display differences in growth correlation with the main climatic drivers, the mixing effects on the BCCs were highly variable, resulting in the lack of a coherent response to mixing. No consistent species-mixing effect on drought resistance was found within and across climate zones. On average, mixing had no significant effect on drought resistance for neither species, yet it increased pine resistance in sites with higher climatic water balance in autumn. Also, beech and pine most often differed in the timing of their drought response within similar sites, irrespective of the regional climate, which might increase the temporal stability of growth in mixed compared to pure stands. Our results showed that the impact of species mixing on tree response to climate did not strongly differ between groups of sites with distinct climate characteristics and climate-growth relationships, indicating the interacting influences of species identity, stand characteristics, drought events characteristics as well as local site conditions. [ABSTRACT FROM AUTHOR]

Highlights: [•] We assess the influence of microsite factors on seedling growth and mortality. [•] Short periods of extreme climate have a strong impact on seedling mortality. [•] In the subalpine belt, this effect is mediated by the facilitative role of shrubs. [•] In the montane belt, competition exerted by herbs increases seedling mortality. [•] Mortality is higher for seedlings at the extreme of their distributional range. [ABSTRACT FROM AUTHOR]

Abstract: Predicting light availability in forest understory is a key step in the modeling of forest dynamics. Aspect is often the main source of environmental variation in mountain forests, and so aspect-induced differences in crown geometry and transparency can be expected to affect light transmission through the canopy and modify understory light predictions. To gain a better understanding of the effects of aspect on the light interception capacity of forest trees, we determined crown allometry and crown openness (CO) in 120 adult trees of three common conifer species in the Pyrenees (mountain pine, Scots pine and silver fir), sampled in eight montane-subalpine forests that each included two contrasting aspects. CO was calculated from digital photographs and several crown outlines were automatically determined to prevent user bias. We also calculated crown irregularity as the difference between crown surface area for the tightest and loosest outlines. Predictions of understory light availability obtained from a forest dynamics model were compared with actual values obtained from 115 hemispherical photographs. Crown length and CO varied across species following previous rankings of shade tolerance. Both pines had longer and wider crowns in the north aspect, whereas fir crown geometry was not affected by aspect. CO depended largely on the method chosen for determining crown outline, ranging from 0.10 to 0.56, and the tightest outline provided the best predictions of understory light (slope=0.89, R 2 =0.46). CO was also significantly affected by tree size and plot aspect (p <0.001): crowns in southern-oriented plots were more open for mountain pine and silver fir, whereas Scots pine showed the opposite trend. Predictions of understory light were significantly improved when crown geometry and openness were allowed to vary according to plot aspect (slope of the regression: 0.95, R 2 =0.50). Our results suggest that aspect should be explicitly considered when modeling ecological processes and dynamics in mixed mountain forests given its influence on both abiotic conditions and crown responses to them. [Copyright &y& Elsevier]

• Belowground biomass equations for 14 juvenile tree species. • Estimators of root biomass based on tree sapling height, diameter, aboveground biomass. • Broadleaf species with higher root biomass for a given dimension. • Trend of increasing relative belowground biomass with increasing light availability. • Height to diameter ratio (HD ratio) negatively correlated to relative belowground biomass. Just as the aboveground tree organs represent the interface between trees and the atmosphere, roots act as the interface between trees and the soil. In this function, roots take-up water and nutrients, facilitate interactions with soil microflora, anchor trees, and also contribute to the gross primary production of forests. However, in comparison to aboveground plant organs, the biomass of roots is much more difficult to study. In this study, we analyzed 19 European datasets on above- and belowground biomass of juvenile trees of 14 species to identify generalizable estimators of root biomass based on tree sapling dimensions (e.g. height, diameter, aboveground biomass). Such estimations are essential growth and sequestration modelling. In addition, the intention was to study the effect of sapling dimension and light availability on biomass allocation to roots. All aboveground variables were significant predictors for root biomass. But, among aboveground predictors of root biomass plant height performed poorest. When comparing conifer and broadleaf species, the latter tended to have a higher root biomass at a given dimension. Also, with increasing size, the share of belowground biomass tended to increase for the sapling dimensions considered. In most species, there was a trend of increasing relative belowground biomass with increasing light availability. Finally, the height to diameter ratio (H/D) was negatively correlated to relative belowground biomass. This indicates that trees with a high H/D are not only more unstable owing to the unfavorable bending stress resistance, but also because they are comparatively less well anchored in the ground. Thus, single tree stability may be improved through increasing light availability to increase the share of belowground biomass. [ABSTRACT FROM AUTHOR]

• Spatial and temporal variation of fuel characteristics are assessed for vegetation strata. • Overstory and understory fuel characteristics are poorly correlated. • Forest flammability across Mediterranean landscapes relates to climate conditions. • The rate of change of fuel characteristics was weakly affected by climate over the last 25 years. • Fuel management policies should include species-specific information for each stratum. Mediterranean forests are prone to fire and highly diverse in environmental conditions, species composition, and forest structure. While much is known about fire hazard and forest responses to fire in Mediterranean climate regions, our understanding of the spatial distribution of fuel characteristics and their dynamics remains incomplete. We hypothesized that fire hazard in heterogeneous Mediterranean regions is primarily explained by the variation in fuel loading and flammability characteristics of the understory layer across the landscape and over time. We used the Spanish National Forest Inventory data to estimate overstory and understory fuel characteristics, compare their spatial and temporal variations across Catalonia (NE Spain) over the last 25 years, and assess the role of climate conditions in explaining spatial distribution and dynamics of fuel characteristics. Our results showed that fuel characteristics strongly depend on the species composition of the overstory and understory layers. Fuel characteristics of both vegetation layers were significantly different and poorly correlated with each other. Although both strata showed significantly different rates of change in most fuel characteristics, the average changes were, in general, very small for the time window considered. Forests in the warmest and driest environments showed the highest flammability characteristics in both overstory and understory layers in contrast to mountain communities. We did not find, however, a remarkable effect of climatic conditions on the rates of change of fuel characteristics during the study period. Altogether, these findings confirm the importance of considering the spatial variation of fuel characteristics of different vegetation strata separately and provide critical information on forest vulnerability to develop appropriate fire mitigation policies in Mediterranean ecosystems. [ABSTRACT FROM AUTHOR]