The influence of stand density on bilberry (Vaccinium myrtillus L.) cover depends on stand age, solar irradiation, and tree species composition
Introduction
The ericaceous shrub bilberry (Vaccinium myrtillus) is a keystone species of the Eurasian boreal forest. Bilberry is an essential food resource for many vertebrate and invertebrate species that feed on the flowers (nectar), berries, leaves, and twigs (e.g. Selås et al., 2011, Dahlgren et al., 2007, Hegland et al., 2010, Hertel et al., 2016, Rasmussen et al., 2016), and it is indirectly important for many animal species that prey on the bilberry consumers (Atlegrim, 1989, Jedrzejewska and Jedrzejewski, 1998). Bilberry also has an economic value through commercial berry picking, especially in Finland and Sweden (Jonsson and Uddstål, 2002, Vaara et al., 2013). Compared to the trees with which they coexist, the standing biomass of bilberry shrubs is small, but their biomass turnover is much higher; >60% of standing shrub biomass is replaced each year and the aboveground net primary productivity may be comparable to that of the trees (Nilsson and Wardle, 2005). Bilberry also has strong positive effects on litter decomposition as well as soil microbial activity, and can reduce the depletion of soil mineral nitrogen (Nilsson and Wardle, 2005). Consequently, a decreased abundance of bilberry can potentially bring about profound changes in ecosystem function and services in boreal forests.
Previous studies from Sweden and Finland have found that bilberry has declined after the introduction of industrial forestry, probably due to altered management practices and an increased proportion of young, dense forest (see Hedwall et al., 2013, Miina et al., 2009, with references). The forest density is an important determinant of the amount of light that penetrates through the canopy, and thus of the photosynthetically active irradiation available to the ground and field layer vegetation. Partial shading provides the most optimal light conditions for bilberry shrubs (Parlane et al., 2006). Increased shading, due to increased tree density or canopy cover, may thus be expected to reduce bilberry cover (Hedwall et al., 2010, Miina et al., 2009). An intensifying of forest management to increase forest production can therefore result in a decrease in bilberry cover because of increased stand density. Besides stand density, a suite of other characteristics related to site conditions and the developmental stage and structure of the forest, such as site productivity, stand age, and tree species composition, may all affect the understory vegetation (Barbier et al., 2008), including the bilberry cover (Hedwall et al., 2013). Bilberry is typically most abundant on sites of medium productivity (Ihalainen et al., 2005), less abundant on clear-cuts and in young forests than in mature stands (Atlegrim and Sjöberg, 1996, Miina et al., 2009), and is also influenced by tree species composition (Miina et al., 2009, Gamfeldt et al., 2013). Stand age and tree species composition modify the canopy and thus the transmission of light through the canopy. In rugged landscapes, the light conditions in the forest understory also vary with topography (Bonan and Shugart, 1989). Yet, surprisingly little is known about how these factors interact to determine bilberry cover.
In this study, we used a nationwide dataset, the Norwegian National Forest Inventory, to investigate the relationship between bilberry cover and stand density. Similar nationwide forest inventory (NFI) data from Sweden (Hedwall et al., 2013) and Finland (Miina et al., 2009) have been used previously to analyse relationships between bilberry cover and biotic and abiotic explanatory variables. Most of the forested area in Sweden is within the boreal zone, but a substantial part is in the boreonemoral zone (Hagen et al., 2013). In contrast, almost all forest in Finland is of southern, middle, or northern boreal types. Neither Hedwall et al., (2013) nor Miina et al., (2009) conducted a sub-selection of NFI plots, based on e.g., site or vegetation types, before analysing factors influencing bilberry cover. Both studies found that productivity was an important predictor of bilberry cover. In vegetation science, it is well known that bilberry occurs at its highest abundance in forests of low to intermediate productivity (Ritchie, 1956, Arnborg, 1990, Hill et al., 1999). We argue that including data from all forest types, including very poor and rich habitats, in which average bilberry cover is known to be (close to) zero, will overemphasise the productivity gradient and may not capture more subtle patterns of variation in the influence of forest structure. To increase our understanding of how forest structure variables may influence bilberry cover, we restricted our analyses to typical bilberry habitats.
The main aim of this study was to improve our understanding of how stand density influences bilberry abundance and how the influence of stand density is modified by other factors. Stand density is a factor that is manipulated by forest management and it is important to provide useful quantitative information for managers about the impact of stand density on bilberry cover. We hypothesized that bilberry cover would peak at intermediate stand densities. Furthermore, we expected the influence of stand density to be modified by factors such as available light, forest age, and tree species composition.
In open forests, increased amounts of light should have a negative impact on bilberry cover, whereas in denser forest, high levels of available light should have a positive influence on bilberry cover. Due to the differences in forest structure, we expected bilberry cover to be higher in old forests than in young forests with the same stand densities. Likewise, we expected the relationship between stand density and bilberry cover to differ between forests dominated by pine, spruce, or deciduous trees, due to the differences in forest structure. We expected bilberry cover to peak at higher stand densities in pine forests than in deciduous or spruce forests. We also expected the influence of stand density on bilberry cover to be less pronounced in deciduous forest. We therefore included interactions between stand density and these three factors, light availability, forest age, and tree species composition, in our analyses. Both Hedwall et al., 2013, Miina et al., 2009 included measures of forest density, stand age, and tree species composition as predictors of bilberry cover, but did not consider interactions with stand density (Hedwall et al. 2013) or concluded that they were insignificant (Miina et al., 2009). To our knowledge, our study is the first to assess whether light availability (solar irradiation) influences the relationship between stand density and bilberry cover.
Section snippets
Forest inventory data
We used data from the permanent plots of the Norwegian National Forest Inventory (NNFI) collected during 2007–2011. One fifth of the NNFI plots are measured per year, resulting in a complete coverage of Norway in five years. The plots are located on a 3 × 3 km grid on all forested land below the altitudinal coniferous forest limit, and on a 3 × 9 km grid elsewhere, except for the birch forest in the northernmost county Finnmark, where a 9 × 9 km grid is used (Viken, 2017).
At each plot,
Results
Bilberry cover was influenced by stand density, i.e., basal area per ha, but the relationship depended on stand age, solar irradiation, and the percentages of deciduous and spruce trees (Table 2). The optimal basal area with respect to bilberry cover increased with increasing stand age to approximately 30 m2 ha−1 for stands >80 years, whereas optimal basal area was ca. 20 m2 ha−1 for stands of 41–80 years, and only ca. 15 m2 ha−1 for stands ≤40 years (Fig. 2). When stand age was included as the
Discussion
As hypothesized, bilberry abundance peaked at intermediate stand densities, measured as basal area per ha, and stand density at the highest bilberry cover varied with stand age, solar irradiation, and tree species composition (i.e., the percentages of deciduous and spruce trees).
The optimal forest density for bilberry abundance differed between young and old stands; bilberry cover peaked at higher stand densities in old forest, which also concurs with our hypothesis. Changes in stand age are
Conclusions
Being able to predict the optimal forest characteristics for bilberry is of general ecological interest and important for forest managers. In this study, we focused on how bilberry cover varies with stand density, which may be manipulated by managers to facilitate optimal conditions for bilberry through e.g. choices of planting density and later thinnings. By using a nationwide dataset of sample plots in typical bilberry habitats, we demonstrate for the first time that the influence of stand
Acknowledgements
This work was supported by The Research Council of Norway, Norway (grant number 210464/E40), the National Monitoring Program for Wild Cervids in Norway; and by internal funding from the Norwegian Institute of Bioeconomy Research (NIBIO, Norway). The authors would like to thank Rune Eriksen and Stephanie Eisner (both NIBIO) for help with compiling data from the NNFI database and for updating meteorological predictions for the NNFI plots. We also thank two anonymous reviewers for constructive
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