Modelling forest ecosystems: a crossroad between scales, techniques and applications

When organizing this special issue, we (the guest editors) decided to carry out a quick overview of current trends in forest modelling. To do so, we used Clarivate’s Web of Science to carry out a search in the top five journals with “Forestry” as their unique category (Forest Ecology and Management, Tree Physiology, Trees – Structure and Function, Canadian Journal of Forest Research, International Journal of Wildland Fire), and two journals specialized in ecological modeling (Environmental Modelling and Software and Ecological Modelling). We searched for papers containing the same keywords as the papers published in the present article collection on modelling forest ecosystems. We limited our search to the last three complete years in the database (2017 and 2019). As several hundred papers were obtained, we made our personal selections as our search was not intended to be exhaustive, only to provide a sense on what is going on nowadays in the forest ecological modelling community (Table 1)

Combining innovative mulches and soil conditioners in mountain afforestation with ash (Fraxinus excelsior L.) in the Pyrenees (NE Spain)

Aim of study: To assess the effectiveness for improving early seedling performance of the individual and combined application of (i) various doses of an innovative soil conditioner including polyacrylamide-free super-absorbent polymers, fertilizers, root precursors and humic acids; and (ii) innovative mulches based on renewable-biodegradable or recycled raw materials. The assessment was carried out in comparison with reference (commercial) soil conditioners and mulches.Area of study: Upper montane afforestation site located at 1,430 m altitude in the southern Pyrenees (NE Spain).Material and methods: We studied the effect of 15 treatments (various combinations of soil conditioners and mulches) on mountain ash (Fraxinus excelsior L.), testing survival, diameter and height growth and water and nutrient status during two growing seasons (2014-2015). We also assessed mulch durability during 2014-2016.Main results: The innovative soil conditioner improved diameter and height seedling growth (92% and 72% respectively) and water and nutrient status. The 40 g/seedling dosage was more cost-effective than the 20 and 80 g/seedling doses. The new formulation performed better in general than the commercial formulation. Mulches led to slight gains compared to control seedlings, and there were no major differences between the mulch models. The combined application of soil conditioners and mulches was not of particular interest.Research highlights: Soil conditioners consisting of synergic mixtures of water super-absorbent polymers, fertilizers, root growth precursors and humic acids can improve early seedling performance in coarse-textured, stony soils in montane conditions. Small mulches may be only of limited interest as long as weed competitiveness is poor.Keywords: ecotechnology; groundcovers; reforestation; seedling performance; restoration; water super-absorbent polymer; weed.Abbreviations used: GSn, growing season number ‘n’; leaf water potential, LWP; water super-absorbent polymers, SAP; soil conditioner with water super-absorbent polymers, SCwSAP; Soil Plant Analysis Development, SPAD

Ecosystem services provision by Mediterranean forests will be compromised above 2℃ warming

Forests provide a wide range of provisioning, regulating and cultural services of greatvalue to societies across the Mediterranean basin. In this study, we reviewed the sci -entific literature of the last 30 years to quantify the magnitude of projected changesin ecosystem services provision by Mediterranean forests under IPCC climate changescenarios. We classified the scenarios according to the temperature threshold of 2℃set by the Paris Agreement (below or above). The review of 78 studies shows thatclimate change will lead to a general reduction in the provision of regulating services(e.g. carbon storage, regulation of freshwater quantity and quality) and a general in-crease in the number of fires, burnt areas and generally, an increase in climate-relatedforest hazards (median + 62% by 2100). Studies using scenarios above the 2℃ thresh-old projected significantly more negative changes in regulating services than stud-ies using scenarios below this threshold. Main projected trend changes on materialservices (e.g. wood products), were less clear and depended on (i) whether or notthe studies considered the interaction between the rise in temperatures and otherdrivers (e.g. forest management, CO2 fertilization) and (ii) differences in productivityresponses across the tree species evaluated. Overall, the reviewed studies projectedsignificant reductions in range extent and habitat suitability for the most drought-sensitive forest species (e.g. −88% Fagus sylvatica), while the amount of habitatavailable for more drought-tolerant species will remain stable or increase; however,the magnitude of projected change for these more xeric species was limited whenhigh-end extreme climatic scenarios were considered (above Paris Agreement). Ourreview highlights the benefits that climate change mitigation (to keep global meantemperature increase <2℃) can bring in terms of service provision and conservationof Mediterranean forests

The relationship between fire severity and burning efficiency for estimating wildfire emissions in Mediterranean forests

Forests are exposed to changing climatic conditions reflected by increasing drought and heat waves that increase the risk of wildfire ignition and spread. Climatic variables such as rain and wind as well as vegetation structure, land configuration and forest management practices are all factors that determine the burning potential of wildfires. The assessment of emissions released by vegetation combustion is essential for determining greenhouse gases and air pollutants. The estimation of wildfire-related emissions depends on factors such as the type and fraction of fuel (i.e., live biomass, ground litter, dead wood) consumed by the fire in a given area, termed the burning efficiency. Most approaches estimate live burning efficiency from optical remote sensing data. This study used a data-driven method to estimate live burning efficiency in a Mediterranean area. Burning severity estimations from Landsat imagery (dNBR), which relate to fuel consumption, and quantitative field data from three national forest inventory data were combined to establish the relationship between burning severity and live burning efficiency. Several proxies explored these relationships based on dNBR interval classes, as well as regression models. The correlation results between live burning efficiency and dNBR for conifers (R = 0.63) and broad-leaved vegetation (R = 0.95) indicated ways for improving emissions estimations. Median estimations by severity class (low, moderate-low, moderate-high, and high) are provided for conifers (0 .44 − 0.81) and broad-leaves (0.64 − 0.86), and regression models for the live fraction of the tree canopy susceptible to burning (&lt; 2 cm, 2 − 7 cm, &gt; 7 branches, and leaves). The live burning efficiency values by severity class were higher than previous studies

Author&apos;s personal copy Understory light predictions in mixed conifer mountain forests: Role of aspect-induced variation in crown geometry and openness

a b s t r a c t 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 &lt; 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

Biogeographic context mediates multifaceted diversity-productivity relationships in island and mainland forests

Biotic and abiotic conditions have been found to strongly influence how biodiversity affects ecosystem functioning in forests. This context dependency of biodiversity-productivity relationships in real-world ecosystems may be shaped by the biogeographic context via deep-time processes acting on the size and composition of the species pool such as dispersal limitation, environmental filtering, speciation and invasibility. However, the role of the biogeographic context in shaping multifaceted biodiversity and forest productivity relationships remains uncertain. Using data from the Spanish National Forest Inventory in climatically similar forests on the Canary Islands (637 plots) and mainland Spain (1434 plots), we investigate the extent to which above-ground productivity is determined by ecological and evolutionary processes associated with the biogeographic context. We used structural equation models to test the drivers of above-ground productivity in both contexts, that operate via multifaceted tree diversity (taxonomic, phylogenetic, and functional diversity) or alternative mechanisms, that is environmental conditions, non-native species, and the number of trees. Our results show that mainland and island forests exhibit similar levels of productivity, yet island forests display overall lower multifaceted diversity. We found that the number of trees increased multifaceted diversity and also increased productivity directly and via their effects on phylogenetic diversity in both mainland and island forests. Further, non-native species increased productivity in island forests. Synthesis: Our results suggest that multifaceted diversity, by capturing the diversity of evolutionary history, contributes to elucidating diversity-productivity relationships in mainland and island forests that could not be detected by taxonomic diversity alone. By filling empty niches in island forests, we find that non-native species are fundamentally altering ecosystem functioning on islands and thus, reveal how biogeographic context can shape biodiversity-productivity relationships

Bimodal and unimodal radial growth of Mediterranean oaks along a coast-inland gradient

Wood formation during the growing season is shaped by the intra-annual variability of climatic conditions. In the Mediterranean, the cambial activity is seasonally constrained by winter low temperature and summer drought, resulting in bimodal growth patterns. Although bimodal growth is an ecologically important adaptation of woody species to seasonally dry environments, its variability across space and among species remains unclear. We combined direct monitoring of intra-annual radial growth using dendrometers and xylogenesis micro-sampling with indirect wood formation modeling to assess growth bimodality for two coexisting oak species across six sites in North-Eastern Spain. We focused on species with two contrasting life strategies, evergreen with diffuse-porous wood (Quercus ilex) and winter deciduous with ring-porous wood (Q. faginea/Q. pubescens). Study sites spanned from oceanic areas with high autumn precipitation to dry continental inland regions. We hypothesized that growth bimodality reflects the interaction between local precipitation patterns and species-specific traits. Our results revealed cambial activity in autumn after summer quiescence across the entire region and for both oak species. However, growth bimodality and the relative contribution of the autumn growth peak to the total annual growth was geographically structured and species-specific. Growth was unimodal under continental and Atlantic conditions. By contrast, growth bimodality was significant along the Mediterranean coast, where precipitation showed a bimodal pattern. Moreover, evergreen Q. ilex showed more growth bimodality compared with deciduous Q. faginea/Q. pubescens at Mediterranean sites. Different intra-annual distribution of radial growth could facilitate coexistence of both oak species through temporal niche partitioning. The growth bimodality helps to compensate for reduced growth in summer by autumn cambial resumption and, therefore, the differences in its intensity between sites and species may determine divergent responses of forest ecosystems to ongoing climate change

Knowledge gaps about mixed forests : What do European forest managers want to know and what answers can science provide?

Research into mixed-forests has increased substantially in the last decades but the extent to which the new knowledge generated meets practitioners' concerns and is adequately transmitted to them is unknown. Here we provide the current state of knowledge and future research directions with regards to 10 questions about mixed forest functioning and management identified and selected by a range of European forest managers during an extensive participatory process. The set of 10 questions were the highest ranked questions from an online prioritization exercise involving 168 managers from 22 different European countries. In general, the topics of major concern for forest managers coincided with the ones that are at the heart of most research projects. They covered important issues related to the management of mixed forests and the role of mixtures for the stability of forests faced with environmental changes and the provision of ecosystem services to society. Our analysis showed that the current scientific knowledge about these questions was rather variable and particularly low for those related to the management of mixed forests over time and the associated costs. We also found that whereas most research projects have sought to evaluate whether mixed forests are more stable or provide more goods and services than monocultures, there is still little information on the underlying mechanisms and trade-offs behind these effects. Similarly, we identified a lack of knowledge on the spatio-temporal scales at which the effects of mixtures on the resistance and adaptability to environmental changes are operating. Our analysis may help researchers to identify what knowledge needs to be better transferred and to better design future research initiatives meeting practitioner's concerns.Peer reviewe

Bimodal and unimodal radial growth of Mediterranean oaks along a coast-inland gradient

Wood formation during the growing season is shaped by the intra-annual variability of climatic conditions. In the Mediterranean, the cambial activity is seasonally constrained by winter low temperature and summer drought, resulting in bimodal growth patterns. Although bimodal growth is an ecologically important adaptation of woody species to seasonally dry environments, its variability across space and among species remains unclear. We combined direct monitoring of intra-annual radial growth using dendrometers and xylogenesis micro-sampling with indirect wood formation modeling to assess growth bimodality for two coexisting oak species across six sites in North-Eastern Spain. We focused on species with two contrasting life strategies, evergreen with diffuse-porous wood (Quercus ilex) and winter deciduous with ring-porous wood (Q. faginea/Q. pubescens). Study sites spanned from oceanic areas with high autumn precipitation to dry continental inland regions. We hypothesized that growth bimodality reflects the interaction between local precipitation patterns and species-specific traits. Our results revealed cambial activity in autumn after summer quiescence across the entire region and for both oak species. However, growth bimodality and the relative contribution of the autumn growth peak to the total annual growth was geographically structured and species-specific. Growth was unimodal under continental and Atlantic conditions. By contrast, growth bimodality was significant along the Mediterranean coast, where precipitation showed a bimodal pattern. Moreover, evergreen Q. ilex showed more growth bimodality compared with deciduous Q. faginea/Q. pubescens at Mediterranean sites. Different intra-annual distribution of radial growth could facilitate coexistence of both oak species through temporal niche partitioning. The growth bimodality helps to compensate for reduced growth in summer by autumn cambial resumption and, therefore, the differences in its intensity between sites and species may determine divergent responses of forest ecosystems to ongoing climate change.JT received institutional financial support from the Charles University (UNCE/HUM 018). JJC thanks the support of project "Factores climáticos y de crecimiento que determinan el secuestro de carbono en la carrasca (Quercus ilex subsp. ballota) y el quejigo (Quercus faginea) en condiciones mediterráneas" financed by INIA (2007–2010). GS-B was supported by a Spanish Ministry of Economy, Industry and Competitiveness Postdoctoral grant (IJC2019–040571-I; FEDER funds), and PROWARM project (PID2020–118444GA-I00, Spanish Ministry of Economy and Competitiveness). AG was supported by the “Ramón y Cajal” Program of the Spanish MICINN under Grant RyC2020- 030647-I and by CSIC project PIE-20223AT003. FC was supported by the R&D unit CFE (FCT/UIDB/04004/2020).Peer reviewe

The use of scenarios and models to evaluate the future of nature values and ecosystem services in Mediterranean forests

Science and society are increasingly interested in predicting the effects of global change and socio-economic development on natural systems, to ensure maintenance of both ecosystems and human well-being. The Intergovernmental Platform on Biodiversity and Ecosystem Services has identified the combination of ecological modelling and scenario forecasting as key to improving our understanding of those effects, by evaluating the relationships and feedbacks between direct and indirect drivers of change, biodiversity, and ecosystem services. Using as case study the forests of the Mediterranean basin (complex socio-ecological systems of high social and conservation value), we reviewed the literature to assess (1) what are the modelling approaches most commonly used to predict the condition and trends of biodiversity and ecosystem services under future scenarios of global change, (2) what are the drivers of change considered in future scenarios and at what scales, and (3) what are the nature and ecosystem service indicators most commonly evaluated. Our review shows that forecasting studies make relatively little use of modelling approaches accounting for actual ecological processes and feedbacks between different socio-ecological sectors; predictions are generally made on the basis of a single (mainly climate) or a few drivers of change. In general, there is a bias in the set of nature and ecosystem service indicators assessed. In particular, cultural services and human well-being are greatly underrepresented in the literature. We argue that these shortfalls hamper our capacity to make the best use of predictive tools to inform decision-making in the context of global change.This work was supported by the Spanish Government through the INMODES project (grant number CGL2017-89999-C2-2-R), the ERA-NET FORESTERRA project INFORMED (grant number 29183), and the project Boscos Sans per a una Societat Saludable funded by Obra Social la Caixa (https://obrasociallacaixa.org/). AMO and AA were supported by Spanish Government through the “Juan de la Cierva” fellowship program (IJCI-2016-30349 and IJCI-2016-30049, respectively). JVRD was supported by the Government of Asturias and the FP7-Marie Curie-COFUND program of the European Commission (Grant “Clarín” ACA17-02)