Tree species impact on understory vegetation: Vascular plant communities of Scots pine and Norway spruce managed stands in northern Europe

Abstract

The tree species used in production forests can have direct implications for the understory vegetation, due to species-specific differences in the environmental conditions provided, and silviculture applied. In southern Sweden, there is an ongoing shift from even-aged managed stands of Scots pine (Pinus sylvestris) to even-aged Norway spruce (Picea abies). Here we evaluate the potential ecological consequences of these conversions by surveying the understory vascular plants, stand structural characteristics, and associated understory light levels in three age categories of Scots pine and Norway spruce stands. We quantify the extent of difference in light transmittance between the two stand types, and find large differences in understory abundance, especially among the dwarf shrubs species. Whereas bilberry (Vaccinium myrtillus) generally dominated Scots pine stand understory, the understory of Norway spruce stands was comparably sparse. Differences in vascular plant community composition indicate associated differences between the two stand types in the conditions prior to stand establishment, and subsequent understory development. In Scots pine stands the high abundance of dwarf shrubs may be outcompeting other species of vascular plants, causing a decline in species richness. In Norway spruce stands, species richness and abundance was instead apparently inhibited by limited light transmittance. In these stands generalist ruderal species and remnants from previous land use constituted a considerable part of plant species composition. Our findings suggest that the regional replacement of Scots pine with Norway spruce will lead to a noticeable loss of understory diversity, for which the decline in Vaccinium species will be the most prominent. We discuss the associated implications of our results for forest ecosystem services, and how shortened rotation lengths in Norway spruce stands would compound these impacts.