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

Abstract

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.

Effect of Aspect-Slope on the Growth of Conifers in a Harsh Boreal Climate of Northwest Sweden

“…A previous study on the climatic variations of the three aspect-slopes indicated that the southwest slope was warmer than the mountaintop [13,43]. Tree growth is influenced by different factors, including neighborhood competitors [28,[44][45][46] and climate [44,47,48]. However, by using the same region, block size and shape, number of species, planting pattern (random), density and age, we tried to eliminate any possible source of variation other than those being investigated, i.e., aspectslope and species.…”

Young Silver Birch Grows Faster and Allocates Higher Portion of Biomass into Stem Than Norway Spruce, a Case Study from a Post-Disturbance Forest

“…On the other hand, birch is known to be very sensitive to snow and ice mechanical damage, especially in sparse forest stands that might be its disadvantage at some exposed sites [15]. Anyway, admixture of birch in Norway spruce plantations was pointed, especially in the newest works [44,45] as an efficient means to increase habitat and species diversity, including diversity of ground flora [46], invertebrates [47] or birds [48].…”

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

“…Birch is known to establish abundantly after disturbances like fire (Ascoli and Bovio 2010;Dzwonko et al 2015), storms (Ilisson et al 2007;Vodde et al 2010) or clearfellings (Holgén and Hånell 2000;Götmark et al 2005;Karlsson and Nilsson 2005). Birch in the boreal forest is important for several ecosystem services, in example provisioning wood for pulp, timber and bioenergy (Rytter 2004;Woxblom and Nylinder 2010;Felton et al 2021;Lidman et al 2021) or as fodder for ungulate foraging (Cederlund et al 1980;Hörnberg 2001). In Sweden, it is common to find birch growing in mixtures with other species, particularly Norway spruce (Picea abies H.…”