Competition-based mortality and tree losses. An essential component of net primary productivity

Summary There is a rising interest in the role of species diversity in ecosystem functioning and services, including productivity. Yet, how the diversity–productivity relationship depends on species identity and abiotic conditions remains a challenging issue. We analysed mixture effects on species productivity along site productivity gradients, calculated from a set of abiotic factors, in two biogeographic contexts (highlands and lowlands). We compared the productivity of 5 two‐species mixtures (i.e. 10 cases of mixed species) with that of monocultures of the same species. Five main European tree species were considered: sessile oak (Quercus petraea Liebl.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) H. Karst). Our data set was compiled from the 2006 to 2010 French National Forest Inventory data base and covers 2361 plots including pure and mixed stands. Overall productivity of mixtures in highlands, that is European beech–Norway spruce, European beech–silver fir and to a lesser extent, silver fir–Norway spruce, was found to be higher than expected from the productivity of corresponding monospecific stands. Overyielding was mainly due to European beech for the first two mixtures and to silver fir for the third one. No effect of mixture was found for sessile oak–Scots pine and sessile oak–European beech stands in lowlands. Overyielding of sessile oak mixed with Scots pine was not strong enough to significantly increase overall stand productivity. Overyielding of European beech was balanced by an underyielding of sessile oak. The mixture effect changed along site productivity gradients for six cases out of the 10 studied, with a stronger and positive effect on sites with low productivity. The magnitude of this change along site productivity gradients varied up to 89% depending on the tree species. Synthesis. The nature of species interaction in mixtures with regard to productivity changes with species assemblage and abiotic conditions. Overyielding is strongest when species grow in highlands on less productive sites. A negative link between mixture effect and site productivity was found, in line with the stress‐gradient hypothesis.

Sequestering carbon in forest ecosystems is an important nature-based solution for mitigating climate change. One way to assess the effectiveness of carbon uptakes in forestry is to determine the effect that the inclusion of carbon price has on the length of economic rotation period of forest stands. Whether and until when forests should be left unharvested and store carbon in living biomass and deadwood, or harvested and store carbon in harvested wood products is the issue, which is investigated in this study by computing rotation ages that consider both commercial timber and carbon prices. The optimization is carried out with an economic-ecological model that includes size-structured matrix growth model and accounts for climate effects on forest dynamics. The study concerns with uneven-aged mixed-species forests consisting of Norway spruce (Picea abies L.), European beech (Fagus sylvatica L.) and silver fir (Abies alba Mill.). The data on forest initial distributions comes from the Czech National Forest Inventory. The results showed that inclusion of carbon pricing to economic criteria for determining optimal rotation ages prolongs the rotation period. An increase in the carbon prices postpones optimal harvest age and leads to higher total net carbon sinks. However, the optimal solutions are sensitive not only to the discount rate, but also to the growth conditions of forest site and future climate evolution.

Consistent growth responses of silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.) to drought in mixed and monospecific forests: Insights from Central European forests

Long-term research plots in multi-aged stands managed with the plenter system were assessed to evaluate sustainability of the plenter system in Central Europe. Plots primarily consisted of Norway spruce (Picea abies (L.) Karst., silver fir (Abies alba Mill.) or European beech (Fagus sylvatica L.) and were measured for seven to 16 measurement intervals over 60–91 years. Sustainability was assessed with four types of criteria: stand density, tree species diversity, basal area increment, and stand structure. Comparable even-aged stands were also analysed to compare and evaluate the performance of the measures of sustainability. Measures of species diversity, increment and stand structural diversity generally experienced increasing trends over time in these even-aged stands. Basal area generally increased and trees ha-1 decreased in multi-aged stands following similar patterns as in even-aged stands. These results suggest that the plenter system is still evolving and is not the model of sustainability often assumed. Many of the measures used have potential as indicators of sustainability in multi-aged stands.

We compared long-term C sequestration in the pools of aboveground portions of live trees, dead wood, and harvested wood products among highly contrasting forest management scenarios on a rotation (30-100 years) and 100-year basis. Average annual net change in C (AAC) and the cumulative sum of net changes in C were calculated using 65 years of data from permanent plots representing contrasting approaches to managing mixed-species stands dominated by shade-tolerant coniferous species on the Penobscot Experimental Forest in Maine, USA. Simulations of tree growth and mortality were used to estimate C pools to 100 years. On a rotation basis and for all pools combined, scenarios with selection cutting had greater AAC than those with shelterwood cutting followed by thinning or with diameter-limit cutting (p < 0.05). For combined pools, the cumulative sum of net changes in C for the unmanaged, selection, and guiding diameter-limit stands was positive for most of the study period. Our results suggest that strategies that maintain overstory stocking levels necessary to regenerate desired species and promote the development of sawlog-sized trees can enhance long-term C sequestration in mixed-species, naturally regenerated northern temperate forests.

Dead wood and saproxylic beetles in set-aside and non set-aside forests in a boreal region

Old-growth forests are often looked at as reference for close to nature silviculture, which aims to manage forests in a natural way. An important component of these forests is the large amount of deadwood they possess; the role of dead wood in the forest ecosystem has been well recognised. A detailed investigation of dead wood characteristics (the amount per stand, species, dead wood type, decay class, size and spatial distribution) was performed in two Romanian old-growth European mixed beech forests: Runcu Grosi Reserve (sessile oak-beech) and Sinca (silver fir-beech). Dead wood pieces were classified as belonging to one of seven dead wood types and one of five decay classes. The total amount of the dead wood was greater in Runcu Grosi (240.8 m3 ha-1) than in Sinca Forest (135.5 m3 ha-1). The majority of the dead wood in Runcu Grosi was composed of sessile oak (91.7%), whereas in Sinca Forest, the main dead wood species was silver fir (67.0%); both species exhibited higher values of probability density than beech, the second most important species. The dead wood exhibited much variation in tree size and in dead wood type, and covered the entire spectrum of decomposition classes. The main spatial pattern of all standing dead trees was random in both forests (over 55%), with a reduced participation of regularity and aggregation. The results of this investigation could be used as key values derived from natural conditions to enhance the biodiversity related to dead wood in managed temperate beech mixed forests.

Three softwood species, spruce (Picea abies Karst.), fir (Abies alba Mill.), and larch (Larix decidua Mill.), and five hardwood species, oak (Quercus petraea Liebl.), ash (Fraxinus excelsior L.), beech (Fagus sylvatica L.), cherry (Prunus avium L.), and black locust (Robinia pseudoacacia L.) were treated at high temperatures under vacuum conditions with Termovuoto® technology. All of the wood species were treated at different temperatures (from 160 to 220°C), different times (from 45 minutes to 5 hours), and under different pressure conditions (160, 210, and 330 mbar). The treated material was characterized in terms of mass loss, color changes, and equilibrium moisture content. Results showed dissimilar behavior of various wood species and their different sensitivities to treatment schedules. Consequently, the series of tests performed allowed a detailed characterization of the Termovuoto® process and its effect on product quality.

Optimising raw material use in the sawing process requires understanding variations in quality indicators. Cracks are defects that significantly reduce product value and can lead to the downgrading of the product. This study aimed to characterise heart crack variability in softwood stems before sawing and improve wood inspection. 1026 Norway spruce (Picea abies (L.) H. Karst.) and silver fir (Abies alba Mill.) stems were scanned using an industrial CT Log X-ray scanner. Among these, 250 stems exhibited heart cracks longer than 50 cm: 37% in silver fir and 18% in Norway spruce. Radial and longitudinal variability of crack metrics were analysed. Results confirm that internal cracks are present in approximately 25% of stems before sawing, with heart cracks length averaging 32% of total stem length. A significant positive linear relationship (R2 ≈ 0.28) was identified between heart crack length, stem diameter and taper. This suggests that factors associated with larger stem dimensions may contribute to increased crack length. These findings are crucial for the wood industry, enabling better crack management, minimising defects in end products and optimising yield and value. Further research on the relationship between silvicultural treatments and the formation of internal cracks is recommended.

Currently, a change in the species composition of the Białowieża Forest, eastern Poland, is occurring. Parallel to the dying of spruce (Picea abies L. Karst.), caused by Ips typographus (L.), there is a spread of deciduous tree species, among which hornbeam (Carpinus betulus L.) is the most active. Against the background of climate change, it is of interest to consider the possibility of reintroducing relict species in areas occupied until recently by spruce. One of these is silver fir (Abies alba Mill.), which is biologically and ecologically very similar to spruce. The Tisovik tract (the Belarusian part of the Białowieża Forest) is the most northeastern and the only preserved refuge of autochthonous silver fir in the region. Therefore, it is the most suitable source of propagation of this species outside the mountainous part of its range. The target area of our study was a 26-year old artificial stand included in the information system of forest genetic resources conserved in Europe. It was created in 1996 in the Polish part of the Białowieża Forest in the Hajnówka Forest District (Forestry Wilczy Jar) from 10 half-sib families originating from seeds collected in the Tisovik tract. The goal of the study was to assess the influence of provenance factors and spatial structure on productivity and phenotypic variation as measured by diameter (DBH). The study’s initial results showed that the provenance factor’s influence at the pole wood stage manifests itself more clearly than spatial structure. Even within the framework of individual half-sibs, there was a clear differentiation of diameters between investigated trees. Interpretation of the obtained results allowed us to conclude that there is a “threshold value” of the distance between trees in a stand of a given age, which determines the point when its increase no longer contributes to an increase in DBH. The most promising phenotypes were selected for further research and practical actions to reproduce silver fir further.

Heavy crop tree thinning can enhance net overyielding in Scots pine-sessile oak mixed stands by reducing pine tree mortality

The stochastic backward shifts model better corresponds to the fine-scale structural heterogeneity of old-growth Abies-Fagus-Picea forests than the ontogenic life cycle model

Species-rich mixed silver fir (Abies alba Mill.) forests dominated in the northern Apennines, but climate and, mainly, anthropogenic land use provoked a sharp silver fir decline approximately 5000 years bp. The conservation of the silver fir in this region was mainly due to the establishment of monastic orders that preserved and even planted silver fir for its spiritual and economic value. In 1993, the best silver fir stands were included in the Parco Nazionale delle Foreste Casentinesi (FCNP), Monte Falterona e Campigna, and have been submitted to low-intensive management or strict protection regardless of past land use and cultural history. In this study, we have (1) analyzed the current structure of three silver fir forests that have had different ownership histories and (2) compared the structure of the three Italian forests among them and with two old-growth forests from the Dinaric Alps as a reference of naturalness. The results show that the current structures of the three Italian forests are very different among them and are strictly related to past land use and, mainly, to monastic legacies. Even if the Italian forests have experienced decades of low-intensity management or strict protection, they are currently structurally very different from Dinaric old-growth forests. Developing an old-growth structure in these forests can be very slow and, in some ways, unpredictable. The results also highlight the importance of recognizing protected areas as cultural landscapes that host an important biocultural diversity. The current risk is that by applying almost exclusive biodiversity-centered management and setting difficult or impossible-to-achieve biodiversity goals, total diversity will decrease, and biocultural diversity, the greatest richness of most European parks, will be lost.

The paper presents gross and net calorific values, ash content, conversion factors, and bulk density for different-sized spent cones of Scots pine Pinus sylvestris L., Norway spruce Picea abies L., European larch Larix decidua Mill., and Silver fir Abies alba Mill. harvested from various sites. Gross and net calorific value and bulk density were measured in accordance with the relevant EN and ISO standards. The density conversion factors were determined based on free space measurement by means of water immersion. Gross calorific value for Scots pine, Norway spruce, European larch, and Silver fir was 19.04 ± 0.70 MJ·kg−1, 20.08 ± 0.87 MJ·kg−1, 20.37 ± 0.48 MJ·kg−1, and 20.79 ± 0.61 MJ·kg−1, respectively. The bulk density of larch cones was the highest at 223 kg·m−3, which corresponds to 9–18% of their specific density. The ANOVA test showed that the bulk density depends on the origin of the cones and is different for individual species. The conversion factors for the cones of Scots pine, Norway spruce, and Silver fir were similar and ranged from 0.18 to 0.26, while those for the European larch were much greater with a maximum of 0.55. All of the studied cones have shown a good potential as energy source, based on their physical characteristic and can be considered as a supplementary fuel. In the future, the study of chemical properties, such as the elemental composition and the ash melting temperature, will allow for a comprehensive characterization of the energy potential of the tested raw material.

Understanding the biological mechanisms underlying tree responses to drought is critical for preserving forest biodiversity, as current global climate change is challenging the ability of drought-sensitive trees to cope with water shortage. In this study, we investigate how silver fir (Abies alba Mill.) responds to experimental drought stress, more specifically, atmospheric drought caused by high vapor pressure deficit (VPD), by analyzing the gene expression and DNA methylation profiles of different organs alongside physiological variables under well-watered, drought and recovery conditions. Roots exhibited a stronger transcriptomic response than leaves, with 50 times more altered transcripts, revealing their value for assessing water stress in this species through the expression of genes involved in water transport. In addition, brassinosteroid-related genes can serve as stress markers both in roots and leaves. VPD-induced drought also affected DNA methylation, which, like transcriptomic and physiological variables, begins to normalize once the stress is over, suggesting some resilience to drought. However, A. alba struggles to improve intrinsic water use efficiency, which raises its vulnerability to VPD-induced drought. Our results suggest that silver fir forests might be able to cope with short drought events, but prolonged periods of water shortage, which are likely to increase with climate change, may surpass their resilience thresholds, increasing the likelihood of hydraulic failure and carbon starvation.