Pulp refining is a mechanical treatment of fibres using special equipment to initiate changes in structural and electro-kinetic characteristics, which lead to the improvement of the fibre quality. The nature and intensity of the fibre changes depend on several factors, including wood species. The objective of this study was to assess the impact of pine wood species to their processability during pulp refining. Two pine wood species lodgepole (Pinus contorta) and Scots (Pinus sylvestris) pines were mechanically treated and differences in fibre properties between pines were compared and estimated. A strong and significant correlation between the time of mechanical treatment (PFI mill beating for 5, 10 and 15 min) of pulp (Shopper-Riegler number and water retention value), and mechanical, optical, and surface paper properties were found. Higher refining grade and better water retention value and mechanical properties of paper were recognized in the case of lodgepole pine. Also, paper of beaten lodgepole pine fibres had better air permeability and surface smoothness. Light scattering and ISO brightness were higher for Scots pine; there was no difference between species in light absorption.

Keywords: fibre refining; paper properties; paper optics; water retention; air permeability.

   The importance of wood biomass as a source of bioenergy at wider scales, i.e. for a centralized production of heat and electricity, is increasing. Hence, precise estimates of biomass are necessary for the calculation of available resources and carbon budget. Mathematical models based on easily measurable variables can notably facilitate the estimation of biomass of trees. However, such models are usually locally (regionally) applicable. The biomass of the aboveground parts (stems and branches and total) and the dimensions were measured for 29 Norway spruce (Picea abies (L.) Karst.) trees at the age of 40 years, representing the eastern part of Latvia. Different linear models were fitted to the biomass data; tree height (H), stem diameter at 1.3 m height (D) and their combinations were used as the independent variables. The estimates of stem biomass and their mean relative errors were compared between the best of the developed models, traditionally used derivatives of wood volume tables and models developed for Finland by J. Repola (2009, Silva Fennica 43: 625–647).

   A linear model with D and H as two independent variables showed the best fit to the biomass of stem; the power model with D²H as the independent variable performed best for the total aboveground biomass; the branch biomass was best predicted by the power model based on D. The mean relative errors of the stem and total aboveground biomass was ~ 8 %, while the model of branch biomass had higher relative error, ~ 20 %. The estimates of stem biomass by the traditionally used volume tables and the developed models did not differ significantly, but the relative errors were lower for the developed model. However, the relative errors of the stem biomass estimated by the volume tables were correlated with D, suggesting systematic bias. The developed models performed better than those established in Finland, which underestimated stem and branch biomass, but overestimated the total aboveground biomass and had significantly higher relative errors of the estimates.

Keywords: Picea abies, aboveground biomass, biomass estimation, allometric equations, tree dimensions, hemiboreal forest zone.

   The dynamics of un-managed forest ecosystems is led by the natural disturbances, among which storms are one of the most important, especially due to occurring and predicted increase in their frequency. Mimicking of the natural disturbances is considered a part of the forest management strategy in the particular areas. However, the diversity of the possible initial stand and landscape composition leads to the various corresponding pathways of recovery after the disturbance. So far, a limited number of short term (up to 25 years after the windthrow) empirical studies have assessed them in the hemiboreal forests. In this study, the stand development 45–46 years after the stand replacing windthrow was assessed in the north-western part of Latvia, Slitere National Park. In the areas formerly dominated by Norway spruce, the post-storm tree species composition was significantly altered by the site type. Birch dominated in the overstorey; however, the dominance of spruce was found in 12 and 33 % of the areas in the mesic and wet sites, respectively; the understorey and the advanced regeneration were mostly (61 and 46 %, respectively) formed by spruce. In most cases, the diameter at breast height (DBH) of the overstorey spruce was similar or larger than for pioneer species (except aspen), regardless of the proportion of spruce in the stand composition. The dimensions of spruce were significantly larger in mesic than in wet sites: DBH and height was 23.8 ± 1.0 vs. 19.4 ± 1.5 cm and 22.5 ± 0.5 vs. 17.7 ± 1.0 m, respectively. Regardless of the distance from the undamaged stand, the number of the overstorey spruce was similar, indicating the sufficient seed dispersal from the windthrown trees. However, on the wet sites, the proportion of spruce from the total number of trees was significantly higher in the areas up to 50 m from spruce stand that remained after the storm. Also the abundance of the understorey spruce had significant positive relation to the distance to the undamaged stand. The results suggest that 45 years after the stand-replacing windthrow spruce is abundant within all stand layers and might dominate the overstorey besides the pioneer species.

Keywords: semi-natural dynamics, windthrow, stand replacing disturbance, post-storm regeneration.

   The increasing use of bioenergy and therefore the expanding market for biomass have boosted interest in the establishment of short rotation plantations, where biomass could be obtained either as a production goal or as a by-product. Biomass equations are specific to species and growing conditions; however, their development takes resources and is time consuming. In the Baltic States, several Populus hybrids differing by a number of traits are established in small areas; therefore, interest in generalised biomass equations is increasing. The aim of our study is to develop above-ground biomass equations for the hybrid aspen (Populus tremula x P. tremuloides) and the hybrid poplar (Populus balsamifera x P. laurifolia) in Latvia and to test their robustness regarding tree age and stand density.

   Sample trees were collected during the winter period in four stands located on mineral soil with normal moisture regime and similar fertility (corresponding to the Oxalidosa forest type) in the central part of Latvia. In total, 82 hybrid aspen trees from 12 to 19 years of age and 16 hybrid poplar trees from 62 to 64 years of age were sampled.

   Differences in the above-ground biomass in both Populus hybrids for trees with similar dimensions were non-significant; however, a noticeable difference in biomass allocation was found. Stem biomass formed 69% and 90% of the above-ground biomass (in leaf-less state) for hybrid aspens and hybrid poplars, respectively. Present biomass equations for the hybrid aspen significantly underestimate the real above-ground biomass by 8% to 24%. All of the developed biomass estimation models were statistically significant (p < 0.01), and the R-squared values ranged from 0.85 to 0.96 for hybrid poplars and from 0.96 to 0.98 for hybrid aspens, suggesting good explanatory power. The developed equation based on the hybrid aspen might be applied to the hybrid poplar without significant error.

Keywords: hybrid aspen, hybrid poplar, biomass allocation.