Floristic changes in the understory vegetation of a managed forest in Denmark over a period of 23 years – Possible drivers of change and implications for nature and biodiversity conservation

Abstract

We studied changes in the understory vegetation in two Danish forests (212 permanent plots in a 330 ha area) over a period of 23 years from 1993 to 2016. Changes were analyzed in relation to a shift in forest management towards close-to-nature silviculture, local natural disturbance (caused by storms), nitrogen deposition, and climatic change.The composition of the forests has gradually changed over time and have more deciduous forest stands, fewer coniferous stands and fewer open areas in 2016 compared to 1993.A total of 260 plant species was recorded in 212 plots surveyed four times between 1993 and 2016. Each plot consisted of two nested quadrats of 1 and 25 m2, respectively. The number of species found per survey was steady in the first three surveys, but decreased from 2005 to 2016: 1993: 172; 1998: 169; 2005: 175; and 2016: 147.The abundance of individual species changed over the years: 32 species became significantly more abundant (winner species) and 24 species became significantly less abundant (loser species). Among the winners were ancient-forest species, forest species in general, broad-leaved tree species and stress-tolerant species. The loser species were mainly ruderals and light-demanding species.The mean number of species per plot increased during the study and, at the same time, plots became more homogenous indicating a narrower ecological space in terms of species composition. An analysis of Ellenberg Indicator values showed that mean L (light) and T (temperature) values per plot decreased during the study period while Ellenberg M (moisture) showed a small increase and Ellenberg R (reaction of soil) and N (nitrogen) showed no significant change during the study period.We conclude that changes in the understory vegetation were mainly caused by a general darkening of the forests. This is partly driven by a switch in forestry management practice from high forestry to close-to-nature silviculture, and partly by recovery after a severe storm in 1981. On average, the tree crowns became denser, and the cover of overstory increased. Consequently, the light availability for understory vegetation is significantly lower. No change in Ellenberg Indicator Values related to effects of climate warming or nitrogen deposition could be detected in this study.The benefits of close-to-nature forest management are discussed in relation to biodiversity protection, where it seems to promote at least plant species characteristic of old-growth forest stands. However, close-to-nature forest management does not necessarily create suitable conditions for other organism groups and is not a general quick-fix for biodiversity protection as such.