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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. [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]

Key message: Warming will induce an upward displacement of Scots pine, but this can be partially mitigated by maintaining a more intense land use. Context: Scots pine is currently declining in most inner alpine sectors of southern Europe. The relative contribution of climate, land use change, and disturbances on the decline is poorly understood. What will be the future distribution of the species? Is vegetation shifting toward oak-dominated forests? What is the role of extreme drought years? Aims: The aims of the study were to determine drivers of current distribution of Scots pine and downy oak in Aosta valley (SW Alps), to extrapolate species distribution models to year 2080 (Special Report on Emissions Scenarios (SRES) A1B), and to assess the correlation between pine vitality after the extreme droughts of 2003 and 2006, and modeled longterm vegetation changes. Methods: Ensemble distribution models were created using climate, topography, soil, competition, natural disturbances, and land use. Species presence was derived from a regional forest inventory. Pine response to drought of 2003-2006 was assessed by Normalized Difference Vegetation Index (NDVI) differencing and correlated to modeled cover change between 2080 and present. Results: Scots pine and downy oak were more likely to occur under higher climatic aridity. Scots pine was also associated to higher wildfire frequency, land use intensity, and lack of competition. In a warming scenario, pine experienced an elevational displacement. This was partially counteracted if no land abandonment was hypothesized. Downy oak cover increased in all scenarios. Short- and long-term drought responses of pine were unrelated. Conclusion: Warming will induce an upward displacement of pine, but this can be partially mitigated by maintaining a more intense land use. The drought-induced decline in pine vitality after extreme years did not overlap to the modeled species response under climate warming; responses to short-term drought must be more thoroughly understood in order to predict community shifts. [ABSTRACT FROM AUTHOR]

Large crown fires in the Aosta Valley mostly occur in south-facing slopes where xeric conditions favour the presence of highly flammable conifer stands dominated by Pinus sylvestris. North-facing slopes are less affected by the occurrence of wildfires but the availability of large fuel amounts and prolonged drought periods can raise the fire risk also in these areas. Although stand-replacing wildfires affected about 20% of the total burned forest area from 1989 to 2017, the scarce fire adaptations of Pinus sylvestris pose serious problems for the regeneration of this species and a suitable post-fire management strategy is thus required to enhance this process. Long and mid-term forest regeneration dynamics of Pinus sylvestris were explored using both field surveys and remote sensing techniques in 12 stand-replacing wildfires occurred between 1989 and 2006 in the Aosta Valley. Tree regeneration at different development stages was sampled during 2017 and its fractional cover was also assessed. Time series of Landsat derived spectral vegetation indices were employed to track forest regeneration over time and temporal trajectories were modelled using robust regression methods to assess the magnitude and the direction of spectral changes throughout the years. [ABSTRACT FROM AUTHOR]