Growth trends and site productivity in boreal forests under management and environmental change: insights from long-term surveys and experiments in Sweden

Under a changing climate, current tree and stand growth information is indispensable to the carbon sink strength of boreal forests. Important questions regarding tree growth are to what extent have management and environmental change influenced it, and how it might respond in the future. In this thesis, results from five studies (Papers I-V) covering growth trends, site productivity, heterogeneity in managed forests and potentials for carbon storage in forests and harvested wood products via differing management strategies are presented. The studies were based on observations from national forest inventories and long-term experiments in Sweden. The annual height growth of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) had increased, especially after the millennium shift, while the basal area growth remains stable during the last 40 years (Papers I-II). A positive response on height growth with increasing temperature was observed. The results generally imply a changing growing condition and stand composition. In Paper III, yield capacity of conifers was analysed and compared with existing functions. The results showed that there is a bias in site productivity estimates and the new functions give better prediction of the yield capacity in Sweden. In Paper IV, the variability in stand composition was modelled as indices of heterogeneity to calibrate the relationship between basal area and leaf area index in managed stands of Norway spruce and Scots pine. The results obtained show that the stand structural heterogeneity effects here are of such a magnitude that they cannot be neglected in the implementation of hybrid growth models, especially those based on light interception and light-use efficiency. In the long-term, the net climate benefits in Swedish forests may be maximized through active forest management with high harvest levels and efficient product utilization, compared to increasing carbon storage in standing forests through land set-asides for nature conservation (Paper V). In conclusion, this thesis offers support for the development of evidence-based policy recommendations for site-adapted and sustainable management of Swedish forests in a changing climate

Taller and slenderer trees in Swedish forests according to data from the National Forest Inventory

Changes over time in annual basal area growth and mean height for Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) over the period, 1983-2020 were studied using sample tree data from temporary plots recorded in the Swedish National Forest Inventory. The annual basal area growth was derived from the last measured full ring on increment cores. Using 20 to 60-year-old dominant trees, the mean height and annual basal area growth were examined as functions of tree, stand and site conditions, and trends were assessed mainly using residual analyses over time. A significant increase in mean height at a given age was found for both species, but the annual basal area growth level remained stable over the 38-year period. Currently, at a given age of 50 annual rings at breast height, the mean heights of pines and spruces increased on average by 10.1% (i.e. similar to 2 m), compared to 50 year-old pines and spruces in the 1980s, and the increase was similar in the different regions. The results suggest that trees have become taller and slenderer in Swedish forests. Increasing tree height over time at a given age in Northern Europe has been documented in several reports and many causes have been suggested, such as changed forest management, increasing temperatures and nitrogen deposition. We suggest that elevated CO2 in the air and improved water-use efficiency for the trees might also be strong drivers

Modelling of stand basal area from leaf and plant area indices in boreal forest systems of Sweden

Leaf or plant area index (LAI/PAI) is a useful biophysical indicator to characterize the interrelationships between forests and the atmosphere and offers greater potential to estimate productivity of forested landscapes. Recently, hemispherical photography has been used in a pilot study implemented in the Swedish National Forest Inventory (NFI) to estimate LAI. However, using this indirect approach to estimate stand basal area has been less explored in boreal forests of Sweden. This study sought to evaluate the use of LAI in estimating stand basal area for different forest structures (species composition, age, density) and site characteristics using data from the 2016 and 2017 NFI. A 10-year average of absorbed radiations and precipitation for summer months obtained from the Japanese Reanalysis-55 were used to augment a stepwise regression modeling of measured basal area for monocultures of Norway spruce, Scots pine, mixed coniferous and broad-leaved forests. Models with indirect estimates of leaf area were significant (p < 0.001) for all species. The explained variation was higher for models with LAI functions in Norway spruce (77 %) and Scots pine (71 %) compared to mixed coniferous (60 %) and broad-leaved forests (60 %) with general PAI estimates. Other predictors such as absorbed radiation, stand age and density contributed to the explained variations. It is evident that leaf area index could enhance current predictions of stand basal area and increase the sensitivity of these models to climate change. It is also acknowledged that spectral and textural variables from higher resolution satellite imagery and digital elevation models would substantially improve the model estimates of basal area in boreal forest systems

Modelling potential yield capacity in conifers using Swedish long-term experiments

Information on forest site productivity is a key component to assess the carbon sequestration potential of boreal forests. While site index (SI) is commonly used to indicate forest site productivity, expressions of SI in the form of yield capacity (potential maximum mean annual volume increment) is desirable since volume yield is central to the economic and ecological analyses of a given species and site. This paper assessed the functional relationship between SI and yield capacity on the basis of yield plot data from long-term experiments measured over several decades for Norway spruce (Picea abies), Scots pine (Pinus sylvestris), Lodgepole pine (Pinus contorta) and Larch (Larix decidua and Larix sibirica) in Sweden. Component models of total basal area and volume yield were also developed. SI was determined by existing height development functions using top height and age, whereas functions for stand-level (m2 ha- 1) basal area development were constructed based on age, SI and initial stand density using difference equations and nonlinear mixed-effects models. The relation between volume yield (m3 ha- 1) and top height was adjusted with total basal area production through nonlinear mixed-effects models. Species-specific parametric regression models were used to construct functional relationships between SI and yield capacity. The root mean square errors of the species-specific models ranged from 2 to 6% and 10-18% of the average values for the basal area and volume equations, respectively. For the yield capacity functions, the explained variations (R2) were within 80-96%. We compared our yield capacity functions to earlier functions of the species and significant differences were observed in both lower and higher SI classes, especially, for Scots pine and Norway spruce. The new functions give better prediction of yield capacity in current growing conditions; hence, they could later be used for comparing tree species' production under similar site and management regimes in Sweden

Assessment of vegetation dynamics using remote sensing and GIS: A case of Bosomtwe Range Forest Reserve, Ghana

Changing conditions owing to increasing forest fragmentation make land cover and change detection analysis an extremely important consideration for sustainable forest management. This study applied supervised classification using maximum-likelihood algorithm in Quantum GIS to detect land use land cover changes in the Bosomtwe Range Forest Reserve, Ghana from 1991, 2002 and 2017 using Landsat 4 - TM, Landsat 7 - ETM and Sentinel-2 satellite imageries respectively. Based on the results of the study, it is concluded that land use/cover of Bosomtwe Range Forest Reserve have undergone remarkable changes for over the period of 26 years. The current status of forest cover is estimated to be 2995.45 +/- 401.86 ha and 2090.03 +/- 412.78 ha of closed and opened forest canopy respectively. Conversely, built-up areas (1531.68 +/- 487.13 ha) remains virtually high (20%) though it shows a decrease in comparison to the same area in 2002. The land use land cover change map clearly identified probable areas of forest depletion especially in the north eastern and western portions of the reserve. It is recommended that potential spatial drivers of change should be identified to generate suitable image for change modelling of the reserve, coupled with earmarking of degraded areas for reforestation projects to improve upon the forest cover. (C) 2018 National Authority for Remote Sensing and Space Sciences. Production and hosting by Elsevier B.V

Volymtillväxten för träd i Sverige under 00-talet : ett faktaunderlag med anledning av den minskande nettotillväxten

I de pressmeddelanden som SLU Riksskogstaxeringen går ut med vid publicering av Skogsdata har den sjunkande tillväxten påtalats sedan 2019. I och med de preliminära uppgifterna om kolsänkans minskning som SLU levererade till Naturvårdsverket i augusti 2022 har den sjunkande tillväxten väckt en bredare uppmärksamhet (exempelvis pressmeddelanden från Naturvårdsverket, Skogsstyrelsen samt nyhet från SLU). Som underlag för nyheten från SLU har ett antal olika analyser baserade på data från Riksskogstaxeringen utförts för att kunna svara på frågan om varför tillväxten sjunkit. I denna arbetsrapport redovisas resultat från dessa analyser. • Det totala virkesförrådet fortsätter att öka, men under de senaste åren inte lika mycket som tidigare o Virkesförrådet ökade med i genomsnitt 27 miljoner m3 sk per år mellan 2005 och 2012, från totalt 3,2 miljarder m3 sk till 3,4 miljarder m3 sk. o Virkesförrådet ökade med i genomsnitt 29 miljoner m3 sk per år mellan 2012 och 2019, från totalt 3,4 miljarder m3 sk till 3,6 miljarder m3 sk. o Virkesförrådet ökade med i genomsnitt 8,0 miljoner m3 sk per år mellan 2017 och 2019, från totalt 3,55 miljarder m3 sk till 3,56 miljarder m3 sk. • Den årliga volymtillväxten i Sverige ökade med cirka 20 miljoner m3 sk (från 111 till 131 miljoner m3 sk), eller 19 procent, under perioden 2005 till 2012 o Volymtillväxten ökade i samtliga landsdelar men mest i Götaland (7 miljoner m3 sk) o Volymtillväxten ökade för samtliga trädslag men mest för gran (12 miljoner m3 sk) o Volymtillväxten ökade som en effekt av såväl gynnsamma tillväxtförutsättningar (temperatur och nederbörd) som en ur tillväxtsynpunkt positiv utveckling av åldersstruktur och ökat virkesförråd (ränta-på-ränta effekt). • Den årliga volymtillväxten i Sverige minskade med cirka 20 miljoner m3 sk, (från 131 till 112 miljoner m3 sk), eller 15 procent, under perioden 2012 till 2018 o Volymtillväxten minskade i samtliga landsdelar men mest i Götaland (10 miljoner m3 sk) o Volymtillväxten minskade för samtliga trädslag men mest för gran (14 miljoner m3 sk) o Volymtillväxten minskade beroende på ogynnsamma tillväxtförutsättningar (en kombination av temperatur och nederbörd) trots en svagt positiv utveckling av åldersstruktur och ett ökat virkesförråd. • Den totala årliga avgången ökade med cirka 11 miljoner m3 sk (från 92 till 103 miljoner m3 sk), eller 12 procent, under perioden 2005 till 2018 o Den årliga avverkningen ökade med cirka 15 miljoner m3 sk (från 74 till 89 miljoner m3 sk), eller 20 procent, under perioden 2005 till 2018. o Den årliga naturliga avgången minskade med cirka 3 miljoner m3 sk (från 18 till 15 miljoner m3 sk), eller 18 procent, under perioden 2005 till 2018. o Som en effekt av stormen Gudrun var den naturliga avgången extremt hög 2005 o Efter 2005 är den naturliga avgången betydligt högre än före 2005, främst orsakad av stormskador och barkborreangrepp. • Hur volymtillväxten kommer att utvecklas o Det finns tydliga indikationer på att den minskande tillväxten i södra Sverige efter 2012 har vänts till en ökning vilket analyser av de två senaste enskilda tillväxtåren (2019 och 2020) visar. Detta slår dock ännu inte igenom i Riksskogstaxeringens officiella statistik på grund av att den officiella statistiken grundar sig på ett genomsnitt av fem års tillväxt

Swedish forest growth decline: A consequence of climate warming?

Following an almost century-long increase, forest growth in Sweden has abruptly decreased during the last decade. Lower than expected forest biomass trajectories threaten national targets for carbon sequestration and bioeconomy. While climate-related drought is the most likely cause, the critical question is whether this recent growth decline is transient, or the beginning of a new normal where conventional management actions may risk further losses of resilience to water stress. We argue that improved mechanistic insights through better integrated research are urgently needed to avoid worsening the situation and further delaying necessary actions

Mapping site index in coniferous forests using bi-temporal airborne laser scanning data and field data from the Swedish national forest inventory

Recent advancements in remote sensing of forests have demonstrated the capabilities of three-dimensional data acquired by airborne laser scanning (ALS) and, consequently, have become an integral part of enhanced forest inventories in Northern Europe. In Sweden, the first national laser scanning revolutionised forest management planning through low-cost production of large-scale and spatially explicit maps of forest attributes such as basal area, volume, and biomass, compared to the earlier practice based on field survey data. A second scanning at the national level was launched in 2019, and it provides conditions for the estimation of height growth and site index. Accurate and up-to-date information about site productivity is relevant for planning silvicultural treatments and for the prognosis of forest status and development over time. In this study, we explored the potential of bi-temporal ALS data and other auxiliary information to predict and map site productivity by site index according to site properties (SIS) of Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris L.) in even aged stands in Sweden. We linked ground survey data of SIS from more than 11,500 plots of the Swedish National Forest Inventory (NFI) to bi-temporal ALS data to predict and map site index using an area-based method and two regression modelling strategies: (1) a multiple linear regression (MLR) model with an ordinary least-squares parameter estimation method, and (2) a non-parametric random forests (RF) model optimised for hyper parameter tuning. For model development, permanent plots were used, whereas the validation was done on the temporary plots of the Swedish NFI and an independent stand-level dataset. Species-specific models were developed, and the root mean square error (RMSE) metric was used to quantify the residual variability around model predictions. For both species, the MLR model gave precise and accurate estimates of SIS. The RMSE for SIS predictions was in the range of 1.96 - 2.11 m, and the relative RMSE was less than 10 % (7.68 - 9.49 %) of the reference mean value. Final predictors of site index include metrics of 90th percentile height and annual increment in the 95th percentile height, altitude, distance to coast, and soil moisture. Country-wide maps of SIS and the corresponding pixel-level prediction errors at a spatial resolution of 12.5 m grid cells were produced for the two species. Independent validations show the site index maps are suitable for use in operational forest management planning in Sweden

Tree growth potential and its relationship with soil moisture conditions across a heterogeneous boreal forest landscape

Forest growth varies across landscapes due to the intricate relationships between various environmental drivers and forest management. In this study, we analysed the variation of tree growth potential across a landscape scale and its relation to soil moisture. We hypothesised that soil moisture conditions drive landscape-level variation in site quality and that intermediate soil moisture conditions demonstrate the highest potential forest production. We used an age-independent difference model to estimate site quality in terms of maximum achievable tree height by measuring the relative change in Lorey's mean height for a five year period across 337 plots within a 68 km2 boreal landscape. We achieved wall-to-wall estimates of site quality by extrapolating the modelled relationship using repeated airborne laser scanning data collected in connection to the field surveys. We found a clear decrease in site quality under the highest soil moisture conditions. However, intermediate soil moisture conditions did not demonstrate clear site quality differences; this is most likely a result of the nature of the modelled soil moisture conditions and limitations connected to the site quality estimation. There was considerable unexplained variation in the modelled site quality both on the plot and landscape levels. We successfully demonstrated that there is a significant relationship between soil moisture conditions and site quality despite limitations associated with a short study period in a low productive region and the precision of airborne laser scanning measurements of mean height

On the role of forests and the forest sector for climate change mitigation in Sweden

We analyse the short- and long-term consequences for atmospheric greenhouse gas (GHG) concentrations of forest management strategies and forest product uses in Sweden by comparing the modelled consequences of forest resource use vs. increased conservation at different levels of GHG savings from carbon sequestration and product substitution with bioenergy and other forest products. Increased forest set-asides for conservation resulted in larger GHG reductions only in the short term and only when substitution effects were low. In all other cases, forest use was more beneficial. In all scenarios, annual carbon dioxide (CO2) sequestration rates declined in conservation forests as they mature, eventually approaching a steady state. Forest set-asides are thus associated with increasing opportunity costs corresponding to foregone wood production and associated mitigation losses. Substitution and sequestration rates under all other forest management strategies rise, providing support for sustained harvest and cumulative mitigation gains. The impact of increased fertilization was everywhere beneficial to the climate and surpassed the mitigation potential of the other scenarios. Climate change can have large—positive or negative—influence on outcomes. Despite uncertainties, the results indicate potentially large benefits from forest use for wood production. These benefits, however, are not clearly linked with forestry in UNFCCC reporting, and the European Union\u27s Land Use, Land-Use Change and Forestry carbon accounting, framework may even prevent their full realization. These reporting and accounting frameworks may further have the consequence of encouraging land set-asides and reduced forest use at the expense of future biomass production. Further, carbon leakage and resulting biodiversity impacts due to increased use of more GHG-intensive products, including imported products associated with deforestation and land degradation, are inadequately assessed. Considerable opportunity to better mobilize the climate change mitigation potential of Swedish forests therefore remains