Natural regeneration and management of birch

This thesis offers guidance for those who want to naturally establish, maintain and manage birch in monocultures and mixed stands. Silver and downy birch are the most common broadleaf tree species in northern Europe. In Sweden, the two species together make up approximately 12% of the standing forest volume. This thesis presents results from four studies (papers I - IV), with the aim to increase the level of knowledge about establishment and regeneration of birch, management of naturally regenerated birch in pure and mixed stands, and the distribution of birch over Sweden. The studies were based on experimental data from field trials, survey data from practical forestry, Swedish national forest inventory data and predictive modelling. On dry soil, mechanical site preparation is necessary in order to get a successful regeneration of birch; in wet soil moisture conditions, natural regeneration of birch will appear without effort. It is possible to manage the birch regeneration success if the soil scarification is adapted to the soil moisture conditions (paper I). The proportion of silver and downy birch varied in Sweden’s young forests, and the temperature sum explained most of the variation (paper II). In dense, naturally regenerated stands of birch and Norway spruce, pre-commercial thinning (PCT) had a significant impact on the development of the future stand, and there are several profitable management strategies for the owner of this type of stand (paper III). The proportion of birch tends to decrease after canopy closure in mixtures of Norway spruce with stand age in southern Sweden, regardless of thinning (paper IV). Active forest management is key, in order to maintain the proportion of birch over the full rotation period. In conclusion, this thesis offers knowledge that can contribute to a more varied forestry, and forestry with a greater element of broadleaf trees

Kvävefixering av cyanobakterier associerade till hus- och väggmossa : en jämförelse mellan tall och granbestånd

The limitations of available nutrients are strong in the boreal forest, especially of nitrogen (N). Due to slow mineralization rate of organic soil nitrogen, caused by low temperatures and acidic soils. Few organisms can fixate atmospheric nitrogen. Among those who have the ability there are some that are common in the boreal forest. Cyanobacteria associated to feather mosses are one example. This study is focused on finding if there is a difference in N2 fixation rate by cyanobacteria associated to feather mosses between forest stands of different tree species. The two tree species that are compared are Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) H. Karst). In a study by Nilsson et al. (2012) the Scots pine stands produced much more volume compared to the Norway spruce. The hypothesis of this study is that N2 fixation by cyanobacteria associated to feather mosses, would enhance N input in particularly Scots pine stands. Thereby contribute to the larger volume production of these stands compared to the Norway spruce stands. The study is carried out on some of the plots used by Nilsson et al. (2012), along with some other similar ones. Samples of the feather moss species Hylocomium splendens (Hedwig) and Pleurozium schreberi (Bridal) was collected from 14 sites on 6 locations, half of which was Scots pine stands and half Norway spruce stands. An acetylene reduction assay was used in combination with a moss biomass inventory, to estimate N2 fixation rates. Results showed that the moss biomass of the two species Hylocomium splendens and Pleurozium schreberi was significantly larger in the Norway spruce stands than in the Scots pine stands. There was also a tendency that estimated N2 fixation rate per unit moss mass was higher in Norway spruce than Scots pine stands. The difference between the stands in estimated N2 fixation rate per hectare and year was not significant. In conclusion the study indicates that the N2 fixation in the moss layer does not have notable effect on the volume growth of the trees in this study.I den boreala skogen är tillgången på näringsämnen, speciellt av kväve, begränsande. Detta beror på den långsamma nedbrytningen av organiskt material orsakad av låga temperaturer och jordar med lågt pH. Få organismer kan fixera kväve från luften. Ett exempel på en av de organismer som har denna förmåga är cyanobakterier associerade till boreala bladmossor, vilka är vanligt förekommande i den boreala skogen. Denna studie är fokuserad på att jämföra kvävefixeringen för cyanobakterier associerade till boreala mossor mellan tall- (Pinus sylvestris L.) och gran- (Picea abies (L.) H. Karst) bestånd. Bakgrunden till denna studie kommer från en studie gjord av Nilsson et al. (2012) där trädtillväxten i jämförbara tall och granbestånd var betydligt högre i tallbestånden. Hypotes för denna studie är att kvävefixeringen associerad till boreala mossor är större i tallbestånden. Detta skulle kunna leda till större tillgång på växttillgängligt kväve i tallbestånden som i sin tur skulle leda till högre tillväxt. Prover av mossorna husmossa (Hylocomium splendens (Hedwig)) och väggmossa (Pleurozium schreberi (Bridal)) samlades in från 14 bestånd på 6 olika lokaler. Hälften av bestånden var tallbestånd och hälften granbestånd. Acetylen-reduktions analys användes i kombination med en mossbiomassainventering i de olika bestånden, för att skatta kvävefixeringen per hektar. Resultaten visade att moss-biomassan för de två arterna H. splendens och P. schreberi var signifikant större i gran- än i tallbestånden. Det fanns även en tendens till att kvävefixeringshastigheten per gram mossa var högre i gran- än i tallbestånden, men skillnaden var inte signifikant. Kvävefixering associerad till mossor verkade alltså inte ha någon betydelse för den observerade högre trädtillväxten i tall- än i granbestånd

The effect of fertilization on production with varying stems per hectares on Scots pine

Studien är uppbyggd på röjningsförsök som lagts ut av Sven-Olof Andersson i början av 1970-talet. Studien handlar om samspelseffekten mellan röjningsförband och gödsling. Försöket är uppdelat på tre olika lokaler i Sverige, Virsbo, Hedeviken och Siljansfors. Alla lokaler har ett antal avdelningar som efter röjning fått olika stamantal (600, 1400, resp. 2500 stammar/ha) och man har sedan försökets början mätt och behandlat avdelningarna med gödsling på olika sätt. Data från de tre lokalerna har sedan använts som underlag till analyser i Excel och Minitab. Målet med arbetet var att se om det fanns någon samspelseffekt mellan stamantal/ha och gödslingens effekt på total volymproduktion, diametertillväxt samt höjdtillväxt. Resultatet visade inte på någon statistiskt signifikant samspelseffekt. En del brister uppmärksammades i datamaterialet rörande antalet replikat och med tanke på detta skulle man efter ytterligare analyser med bättre material och fler replikat kunna komma fram till att det kanske kan finnas ett svagt samspel.The study is built on trials about pre-commercial thinning plots founded by Sven-Olof Andersson in the beginning of the 1970’s. The study is about the interaction effect between pre-commercial thinning and nitrogen fertilization. The trials are dispersed over three different locations in Sweden, in Virsbo, Hedeviken and Siljansfors. Every site has a number of sections, where each section have received a specific number of stems (600-,1400- & 2500- stems per hectare) after pre-commercial thinning. Since the beginning, the sections have been measured and treated with fertilization in different ways. Data from the three sites were used as material for analyses in Excel and Minitab. The purpose of this study was to find an interaction effect between a specific number of stems per hectare and fertilization on the production of tree volume growth, height and diameter. The result of the study showed that there were no significant effect of the interaction effect between the number of stems and fertilization. There were some flaws detected in the material considering replications. Because of this lack in the dataset, further studies and analyses with more replications could possibly show a small interaction effect between the two factors

Keeping mixtures of Norway spruce and birch in production forests: insights from survey data

Admixtures of birch in Norway spruce plantations are being promoted as a means to increase habitat and species diversity. The implications of this mixture were analysed with regional survey data from southern Sweden. Permanent sample plots from the Swedish National Forest Inventory (NFI), with Norway spruce and admixture of birch, were used to describe the temporal trends in the admixture, regarding species composition and competitive strength. Observations from thinned plots show a higher harvest removal in birch (35%) than for Norway spruce (19%). Observations without thinnings in the period before measurement showed that individual birch tree growth was lower compared to Norway spruce and it decreased even more with increasing stand age and competition. In addition, a complementary field survey, with multiple distributed sample plots in each stand, was used to detect within-stand variation of species composition and density. Although within-stand heterogeneity was larger in mixed stands in terms of species composition, it was not different from Norway spruce monocultures in terms of stand density. These two surveys show that the admixture of birch, for several reasons, decreases over stand age and although birch increases tree species diversity, it does not necessary imply a change in density

Birch establishes anywhere! So, what is there to know about natural regeneration and direct seeding of birch?

We tested three soil scarification approaches of varying intensity (intensive preparation exposing bare mineral soil, medium intensity with a mixture of organic material and mineral soil, and control without site preparation) on six clear-felled sites in two localities in northern and central Sweden between 2018 and 2021. The effect of soil scarification intensity and soil moisture on the occurrence of naturally regenerated birch seedlings was tested one, two and three years after soil scarification, and the density of direct-seeded birch seedlings one year after seeding. In addition, we tested the effect of the annual seed rain, and differences between the two birch species, on the density of direct-seeded seedlings. Soil scarification and its interaction with soil moisture had a significant positive effect on both the occurrence of naturally regenerated birch seedlings and the density of direct-seeded birch seedlings. There was no significant effect of neither annual natural seed rain nor species choice on direct-seeded seedling density. Time since soil scarification had a significant effect on the occurrence of naturally regenerated birch seedlings. In moist soils with high volumetric water content (>= 28%), birch seeds germinate at high rates and seedlings survive without soil scarification. In mesic soils, birch seeds germinate with higher rates after soil scarification. In dry soils, birch seeds rarely germinate regardless of any disturbance of the humus layer

Management of spontaneously regenerated mixed stands of birch and Norway spruce in Sweden

Timber production and profitability were evaluated for spontaneously-regenerated mixtures on two formerly clearcut areas. The abandoned areas developed into birch-dominated (Betula pendula Roth and Betula pubescens Ehrh.) stands with successional ingrowth of Norway spruce (Picea abies (L.) H. Karst.). An experiment with randomized treatments within blocks was established, using three management strategies and one unthinned control, resulting in variation in optimal rotation age, merchantable volume and species composition. The management strategies were evaluated based on total production (volume) by using measured growth data 42 years after clearcutting and the modelled future stand development. The long-term effects of spontaneous regeneration and management strategies were evaluated based on land expectation value (LEV) and compared with a fifth management strategy using artificial regeneration and intense thinnings. 12 years after treatment, at a stand age of 42 years, the unthinned control had produced the highest total stem volume. At interest rates of 2% or higher, the unmanaged forest was an economically viable strategy, even compared to an intensive management strategy with a preferred merchant-able timber species. Interest rates clearly impacted the profitability of the different management strategies. This study shows that when spontaneous regeneration is successful and dense, the first competition release can have a high impact on the development of future crop trees and on the species mixture