Mid Sweden University

Introduction: Rising demand for forest products, climate mitigation, and ecosystem restoration has driven international pledges to expand forests, mostly on abandoned agricultural lands and areas of low conservation value. According to a recent survey among restoration practitioners across Europe, optimal planting designs and densities are key questions for reforestation efforts. Objectives: We aimed to determine how planting density affects the performance of reforestation/afforestation plantings, how these effects vary by climate, species types, and stand age, and whether there are planting density thresholds triggering significant performance shifts. Methods: Using both descriptive statistics and meta-analyses, we systematically reviewed 120 studies from temperate and boreal forests to analyze planting density effects on the performance of tree plantings. Results: Higher planting densities increase overall yield but also mortality. Negative effects on individual stem growth occur mainly at early ages, while negative impacts on individual stem growth and survival intensify over time. Benefits on stand yield are observed at both young and old ages, and there are no clear differences in the density response of shade-tolerant and shade-intolerant species. On average, an increase in planting density of 71 and 118% is needed to cause significant impacts on performance at stand-level and individual-tree level, respectively, though effects vary across studies and variables. Conclusions: Observed patterns aligned with expectations, as higher planting densities increased mortality and lowered individual growth while promoting overall yields. However, the timing and thresholds of positive and negative effects vary, presenting opportunities to optimize management through variable densities over time. 

Forestry in boreal Sweden has significantly altered the landscape and reduced old-growth forests, promoting growth of younger, managed stands. Saproxylic polypores are in direct competition with forestry for resources and have declined significantly in diversity. These fungi play a crucial role in decomposing deadwood with their persistence reliant on habitat quality, connectivity, and continuous wood availability. We examined how log, stand, and landscape-level factors affect saproxylic polypore richness and community composition in 26 woodland key habitats (WKHs) in southern boreal Sweden. We recorded polypore fruitbodies on 1263 Picea abies deadwood units and assessed stand structure. GIS was used to quantify spruce forest amount and quality at 0.5, 5, and 15 km scales, including connections to larger high-quality habitats. Local-scale factors, especially deadwood volume and decay stage, strongly influenced fungal richness. Landscape effects were weaker, likely due to the homogenous and degraded surrounding forest matrix. However, polypore richness increased with high-quality forest cover at the 15 km scale, while medium-quality forest at 5 km had a negative effect. Red-listed species richness showed a strong eastward gradient, possibly reflecting historical forestry and extinction debt. Community composition patterns aligned with these trends, with further indirect effects from nearby forest amount and quality. We conclude that the landscape configuration matters but is context dependant. In our heavily managed study region, high-quality patches like WKHs are too isolated, leaving local conditions as the primary driver of fungal persistence. Effective conservation requires integrating landscape-scale measures, including identifying and protecting remaining forests with high natural values. 

Despite increasing evidence suggesting mixed-species plantations promote biodiversity, a comprehensive quantitative analysis of this knowledge is lacking. We systematically reviewed 71 studies to evaluate the effects of mixed versus pure tree plantations on biodiversity. Using descriptive statistics and meta-analyses, we explored: a) the effects of mixed plantations on forest-related biodiversity; b) variations in these effects with climate, stand age, and with the richness, relative abundance, and functional diversity of the planted species; and c) differences in responses across taxonomic and functional groups. Our meta-analyses revealed a significant positive effect of mixed-species plantations on taxonomic diversity. However, most observations (64%) reported no significant effects. Positive effects are more frequent in mixtures with more than two species (49%), compared to two-species mixtures (29%), and were strongest in tropical climates (78%), followed by temperate (26%) and continental climates (14%). Among taxonomic and functional groups, positive mixing effects are most frequent for birds (75%), followed by litter microbiota (47%), understory plants (40%), and above-ground arthropods (29%), while soil-dwelling micro-organisms (22%) and soil mesofauna (4%) appear less sensitive. Mixing conifers and broadleaves does not enhance biodiversity benefits, suggesting higher functional diversity may be better achieved by targeting specific species and traits. The limited effects of mixing observed in some cases may reflect the young age of plantations studied (11±9.4 years on average), which may limit the time for biodiversity to respond. The variability in biodiversity outcomes highlights the need for tailored mixing strategies and further research across broader plantation ages, settings, and underrepresented taxonomic groups to optimize biodiversity benefits in mixed-species plantations. 

In forest regions worldwide, the extent of industrial forestry footprint challenges biodiversity conservation and calls for advanced protection and ecological restoration. The conservation efficiency of protected areas needs to be improved, and forest ecosystems need to be set in a state that favors biodiversity, resilience, and provisioning of ecosystem services. Sweden hosts a large share of the European forests, with dominance of non-industrial forest ownership and extensive forestry footprint, hence with a strong need for expanded conservation, restoration, and multiple-use targets. Protection through voluntary Nature Conservation Agreements and regulated Biotope Protection Areas exists since the 1990s, supported by economic compensation to landowners. Across entire Sweden and all ecoregions, we assessed their abundance over a 30-year period, including forest types, restoration practices, rotation intervals, and selection of tree species. These nearly 14,000 patches covering over 70,000 ha are small with a median area of 3-4 ha and rarely larger than 20 ha. Their contribution is important, particularly in southern Sweden with low and fragmented forest cover distributed among many different owners. A decreasing trend in protection is alarming since these contribute to representative forest type across the forest landscape of Sweden. Active restoration dominates over passive set-asides; coniferous forest types are less represented than more rare forest types, many different tree species are favored, and different restoration practices occur, but with few dominating. While recognizing the important contribution, we find that the restoration practices are narrow and repetitive and that a greater diversification is needed to improve conservation and multiple-use targets of forests.

Forest loss, fragmentation, and transformation negatively impact forest biodiversity and ecosystem functionality worldwide. Improving landscape intactness and connectivity through restoration is critical. Determining where to restore remains, however, a challenge. As an approach for prioritizing restoration areas, we define connectivity forest (CFs) as forests outside recognized high conservation value forests (HCVFs) with capacity to support landscape-scale connectivity and green infrastructure (GI) functionality. Across a 1.3 million-ha watershed in boreal Sweden, we identified approximately 130,500 ha of CFs, equal to double the current HCVF area. By integrating CFs with consecutively lower HCVF probabilities, we demonstrate planning implementation at lower to higher ambition levels and identified specific restoration hotspots to guide local-scale restoration planning. Our CF approach has clear implications for efficient spatial targeting of restoration in forest regions where improving conservation in balance with continued forestry for wood production is required to meet national and international biodiversity and environmental goals. 

Intensifying forest management or reducing harvest levels are proposed as alternative strategies for mitigating climate change. Today, scientific disagreement over which approach is more effective impedes the development and implementation of effective climate change mitigation policies. In this paper we review studies of the climate impact of Swedish forestry to clarify the conceptual and methodological differences that underly the disagreement. To examine how assumptions concerning crucial parameters contribute to differing conclusions, we simulated various management scenarios for Gävleborg County in central Sweden. We find that support for either side in the debate can be obtained by adjusting assumptions about substitution levels and the design of management interventions. Studies favoring intensified management over reduced harvesting assume relatively high substitution levels and implement intervention levels — such as increased fertilization or expanded stump harvest — which are considerably higher (2.4–17.7 times) than the levels recommended by the Swedish Forest Agency. Conversely, when using recommended intervention levels and substitution levels based on current usage of forest biomass, reduced harvest strategies show greater climate benefits than intensified management. These findings emphasize the need to focus the scientific discussion on i) the empirical evidence for various substitution levels and ii) the relevance of alternative management scenarios for the development of effective climate change mitigation policies. 

Persistence of standing dead trees (snags) is an important determinant for their role for biodiversity and dead wood associated carbon fluxes. How fast snags fall varies widely among species and regions and is further influenced by a variety of stand- and tree-level factors. However, our understanding of this variation is fragmentary at best, partly due to lack of empirical data. Here, we took advantage of the accruing time series of snag observations in the Finnish, Norwegian and Swedish National Forest Inventories that have been followed in these programs since the mid-1990s. We first harmonized observations from slightly different inventory protocols and then, using this harmonized dataset of ca. 43,000 observations that had a consistent 5-year census interval, we modelled the probability of snags of the main boreal tree species Pinus sylvestris, Picea abies and Betula spp. falling, as a function of tree- and stand-level variables, using Bayesian logistic regression modelling. The models were moderately good at predicting snags remaining standing or falling, with a correct classification rate ranging from 68% to 75% among species. In general, snag persistence increased with tree size and climatic wetness, and decreased with temperature sum, advancing stage of decay, site productivity and disturbance intensity (mainly harvesting). Synthesis and applications: The effect of harvesting demonstrates that an efficient avenue to increase the amount of snags in managed forests is protecting them during silvicultural operations. In the warmer future, negative relationship between snag persistence and temperature suggests decreasing the time snags remain standing and hence decreasing habitat availability for associated species. As decomposition rates generally increase after fall, decreasing snag persistence also implies substantially faster release of carbon from dead wood. The effect of harvesting demonstrates that an efficient avenue to increase the amount of snags in managed forests is protecting them during silvicultural operations. In the warmer future, negative relationship between snag persistence and temperature suggests decreasing the time snags remain standing and hence decreasing habitat availability for associated species. As decomposition rates generally increase after fall, decreasing snag persistence also implies substantially faster release of carbon from dead wood.image

Monitoring of plant populations has become more and more important, especially in the current context of environmental change. In this paper, we propose methods to estimate plant density from presence/absence surveys, wherein the presence or absence of each species is recorded on sample plots. Presence/absence sampling is a useful and relatively simple method for monitoring state and change of plant communities. Moreover, it has advantages compared to traditional plant cover assessment, the latter being more prone to observer bias. We present a hybrid estimation framework, that combines model- and design-based inference features, in which a generalized linear model (for binary presence/absence data) and an inhomogeneous Poisson model (for plant locations) are used to estimate plant density in a region of interest. We look at two different cases, the first one with a known area and the second one where the area is unknown and must be estimated. Our methods are applied to real data on Vaccinium vitis-idaea from the Swedish National Forest Inventory as well as simulated data to assess the performance of our estimators of plant density and corresponding variance estimators. The results obtained are promising and indicate that this method has a potential to add considerable analytic strength to monitoring programmes that collect presence/absence data.

Fungal traits can provide a mechanistic understanding of how wood-inhabiting fungi interact with their environment and how that influences community assembly in deadwood. However, fungal trait exploration is relatively new and almost no studies measure fungal traits in their environment. In this study we tested species- and trait-environment relationships in reproducing fungal communities inhabiting 571 Norway spruce (Picea abies) logs in 55 isolated forest patches (0.1–9.9 ha) of different naturalness types, located in Northern boreal Sweden. The studied patches were (1) semi-natural set-aside patches within highly managed landscapes, or (2) old-growth natural patches located in an unmanaged landscape. We tested species and trait relationships to deadwood substrate and forest patch variables. We measured mean fruit body size and density for each of the 19 species within communities. Traits assembled in relation to log decay stage and forest patch naturalness, illustrating the important role of deterministic environmental filtering in shaping reproducing wood-inhabiting fungal communities. Early decay stage communities had larger, less dense, annual fruiting bodies of half-resupinate type and were more often white-rot fungi. Species rich mid-decay stage communities had mixed trait assemblages with more long lived perennial fruit bodies of intermediate size, and both brown- and white-rot fungi equally represented. Finally, late decay stage communities had smaller, denser and perennial fruit bodies, more often of the brown-rot type. The relationships between the studied traits and decay stages were similar in both set-aside and natural patches. However, set-aside semi-natural patches in highly managed landscapes more frequently supported species with smaller, perennial and resupinate fruit bodies compared to natural patches in an unmanaged landscape. Synthesis. We found that log decay stage was the primary driver of fungal community assembly of species and traits in isolated forest patches. Our results suggest that decay stage filters four reproduction traits (fruit body density, size, lifespan and type) and one resource-use trait (white or brown rot). Our results highlights, for the first time, that communities with diverse fungal reproductive traits are maintained foremost across all deadwood decay stages under different forest naturalness conditions. Read the free Plain Language Summary for this article on the Journal blog.

To conserve biodiversity, it is imperative to maintain and restore sufficient amounts of functional habitat networks. Therefore, the location of the remaining forests with natural structures and processes over landscapes and large regions is a key objective. Here we integrated machine learning (Random Forest) and open landscape data to scan all forest landscapes in Sweden with a 1 ha spatial resolution with respect to the relative likelihood of hosting High Conservation Value Forests. Using independent spatial stand- and plot-level validation data, we confirmed that our predictions correctly represent different levels of forest naturalness, from degraded to those with high and associated biodiversity conservation values. Given ambitious national and international conservation objectives and increasingly intensive forestry, our model and the resulting wall-to-wall mapping fill an urgent gap for assessing the achievement of evidence-based conservation targets, spatial planning, and designing forest landscape restoration.