EuMIXFOR Introduction: integrating scientific knowledge in sustainable management of mixed forests

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European Mixed Forests: definition and research perspectives

peer-reviewedAim of study: We aim at (i) developing a reference definition of mixed forests in order to harmonize comparative research in mixed forests and (ii) briefly review the research perspectives in mixed forests. Area of study: The definition is developed in Europe but can be tested worldwide. Material and methods: Review of existent definitions of mixed forests based and literature review encompassing dynamics, management and economic valuation of mixed forests. Main results: A mixed forest is defined as a forest unit, excluding linear formations, where at least two tree species coexist at any developmental stage, sharing common resources (light, water, and/or soil nutrients). The presence of each of the component species is normally quantified as a proportion of the number of stems or of basal area, although volume, biomass or canopy cover as well as proportions by occupied stand area may be used for specific objectives. A variety of structures and patterns of mixtures can occur, and the interactions between the component species and their relative proportions may change over time. The research perspectives identified are (i) species interactions and responses to hazards, (ii) the concept of maximum density in mixed forests, (iii) conversion of monocultures to mixed-species forest and (iv) economic valuation of ecosystem services provided by mixed forests. Research highlights: The definition is considered a high-level one which encompasses previous attempts to define mixed forests. Current fields of research indicate that gradient studies, experimental design approaches, and model simulations are key topics providing new research opportunities.The networking in this study has been supported by COST Action FP1206 EuMIXFOR

The 2018 European heatwave led to stem dehydration but not to consistent growth reductions in forests

Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes

EuMIXFOR Introduction: integrating scientific knowledge in sustainable management of mixed forests

No abstrac

Mixing has limited impacts on the foliar nutrition of European beech and Scots pine trees across Europe

Tree species-mixing has been suggested as one option to counteract the adverse effects of global change on tree mineral nutrition, yet the effect of mixing on nutrient availability remains poorly documented. We therefore analyzed the current foliar nutrient (N, P, K, Ca, Mg) quantities and ilr balances (isometric log transformed ratios between elements or groups of elements) for 261 European beech and 248 Scots pine trees from 15 sites, each consisting of one beech-pine mixed stand and the respective monocultures, across a gradient of environmental conditions in Europe. We hypothesized an overall positive effect of mixing on tree foliar nutrient content, and that this mixing effect would be stronger on nutrient-poor sites. Using linear mixed models and multivariate linear regression models, we first tested for the effects of species (beech/pine) and composition (pure/mixed) across all sites; we then investigated whether the species-mixing effect was related to site fertility. The nutrient composition of beech leaves and pine needles differed significantly for all ilr balances. For both species, significant mixing effects were detected for some nutrients and ilr balances; those effects, however, could not be consistently related to contrasted nutrient composition between species. For most nutrients and ilr balances, the mixing effect was influenced by the site nutritional status, but the pattern differed from expectation: absence or minor differences between monocultures and mixtures at the lower end of the chemical fertility gradient, and maximum differences in rich soils. The contrasting foliar nutrient composition of pine and beech trees and the site nutrient status only partly explained the mixing effects on tree mineral nutrition. Our results claim for a better understanding of nutrientrelated mechanisms associated with complementarity and points towards the need to further expand the existing frameworks to account for the multivariate nature of tree nutrition

Mixing has limited impacts on the foliar nutrition of European beech and Scots pine trees across Europe

The mineral nutrition of major broadleaved and coniferous tree species is deteriorating in Europe. In that context, species-mixing could be an efficient management tool to improve stand nutrition through effects of species interactions on the availability, uptake or use efficiency of nutrients. We analyzed the current foliar nutrient (N, P, K, Ca, Mg) quantities and balances of 260 beech and 248 pine trees from 15 pure and two-species stands across a gradient of environmental conditions throughout Europe. We hypothesized an overall positive effect of mixing on tree nutrition, and that this mixing effect would be stronger on nutrient-poor sites. Using linear mixed models, we first tested for the effects of species (beech/pine) and composition (pure/mixed) across all sites; we then investigated whether mixing was related to site fertility. The composition of beech leaves and pine needles differed significantly for all balances. For both species, significant mixing effects were detected for some nutrients and balances; those effects could however not be related to interspecific differences in foliar nutrient composition. For most nutrients and balances, the mixing effect was affected by the site nutritional status; however, the magnitude of this effect was low, and no consistent pattern could be detected. The contrasting foliar elemental composition of pine and beech trees, and the difference in nutrient status between sites proved insufficient to explain the mixing effects on tree nutrition. While our results suggest limited impact of mixing for those two species, they claim for a better understanding of nutrient-related mechanisms associated with complementarityMiškų ir ekologijos fakultetasVytauto Didžiojo universiteta