Effects of wild boar rooting on wildflower populations

Submitted by nordicjbotany on 16 February 2016.

By Jörg Brunet (author)

Read the freely available article here: http://onlinelibrary.wiley.com/doi/10.1111/njb.01010/epdf

Dalby Söderskog, a temperate broadleaf wood in southernmost Sweden, is a classic site of ecological research. Being a forest reserve since 1918, ecological studies have been carried out in the area since 1925. Swedish ecologist Bertil Lindquist published a monograph on the history, dynamics and ecology of the wood and, in 1935, also established a grid of 74 survey plots which has been used in later studies. Lindquist´s studies were partly repeated during 1969-1983 by a team of plant ecologists from Lund university. These studies showed a picture characteristic for many European temperate forest reserves which developed from semi-open oak-dominated pasture woodlands to closed forest with old-growth characteristics during the 20th century.

However, during the late 1980s, the first in a row of unforeseen disturbance events struck the area, when the Dutch elm disease started to kill the elm (Ulmus glabra) that at the time had become the most common tree species in the forest. When I took the baton from my former colleagues in Lund and did the first of my resurveys of the forest floor vegetation in 2002, most of the large elm trees had succumbed to the disease. I soon realized that not only the dominant tree species had almost been wiped out, but that also the population of the former dominant forest herb, the dog´s mercury (Mercurialis perennis), had crashed and just survived in patches here and there. During the field work that summer I could observe how enormous numbers of the invasive Spanish slug (Arion vulgaris) selectively browsed this particular herb and consumed the shoots down to the ground.

During these years, young ash trees started to fill the gaps after the elms and, instead of dog´s mercury, ground-living mosses kept the forest floor green during summer after the springflower carpets had wilted. In 2010 we started a new taxation of the tree population, which had not been repeated since 1970. At that time the first signs of a new tree disease, the ash dieback, had appeared. This new invasive fungal disease had probably been brought to Europe with nursery ash trees from East Asia. While the fungus of the Dutch elm disease does not kill tree saplings because the transmitting bark beetle does not attack small trees, the fungus causing the ash dieback is airborne and infects ash (Fraxinus excelsior) trees of all sizes.

When we continued with our taxation in 2011 we observed traces of yet another uninvited guest in Dalby Söderskog. At many places, the soil was rooted, and the moss carpets that had been expanding suddenly had disappeared at many places. As you already may suspect, the wild boar (Sus scrofa) had made also this forest their home in the course of their conquest of southern Sweden! As we happened to have repeated the survey of the ground vegetation in 2010 just before the boar started to plough the forest soil, we had a reference situation to compare with the emerging effects of wild boar rooting.

In 2013 we repeated the vegetation survey in the same 74 one square meter sample plots that we used in 2010. The difference between 2010 and 2013 was quite dramatic! Rooting frequency in sample plots increased from 0% in 2010 to 61% in 2013. In heavily rooted plots, the mean cover of springflowers, mainly wood anemone (Anemone nemorosa), yellow anemone (A. ranunculoides) and lesser celandine (Ranunculus ficaria) decreased from 75% to less than 40% during only three years. This decrease is probably caused by consumption of rhizomes and bulbs but also by dessication of uprooted plants. Species richness of summer green herbs, however, generally increased between 2010 and 2013, as a result of increased plant recruitment and seed dispersal. Rooting also made the herbaceous layer composition more heterogeneous and amplified ongoing compositional changes induced by changing light conditions due to tree diseases.

Dalby Söderskog is a popular recreation area, and in spring many people visit the forest to enjoy the extensive carpets of springflowers. Since 2013, rooting has continued and probably further reduced the springflower populations. The effects of the wild boar, the Spanish slug, and the two tree diseases have now severely compromised the ecological and recreational value of Dalby Söderskog. Among these four human-induced disturbances, only the effects of the wild boar can be reduced by active measures in a short-term perspective. To avoid long-term losses of characteristic spring flora elements, local population control of wild boar is necessary to reduce abundance and frequency of soil rooting.

During the past 25 years, Dalby Söderskog has experienced severe disturbance by invasive herbivores and pathogens. However, favorable growing conditions have also initiated new successions, for example a wave of natural regeneration of light-demanding tree species such as oak (Quercus robur) and wild cherry (Prunus avium), and shrubs such as hazel (Corylus avellana). The unique long-term data that are available make the area a much valued research site to study the long-term effects of disturbance and succession in broadleaf forests.


References on the vegetation of Dalby Söderskog

Brunet, J., Bukina, Y., Hedwall, P.-O., Holmström, E. & von Oheimb, G. 2014. Pathogen induced disturbance and succession in temperate forests: evidence from a 100-year data set in southern Sweden. - Basic Appl. Ecol. 15: 114-121.

Brunet, J., Hedwall, P.-O., Holmström, E. & Wahlgren, E. 2016. Disturbance of the herbaceous layer after invasion of a eutrophic temperate forest by wild boar. – Nord. J. Bot. (early view).

Brunet, J. & von Oheimb, G. 2008. Almsjukan och mördarsniglar – dramatik i Dalby Söderskog. - Svensk Bot. Tidskr. 102: 27-38.

Lindquist, B. 1938. Dalby Söderskog, en skånsk lövskog i forntid och nutid. - Acta Phytogeographica Suecica 10: 1-273.

Malmer, N., Lindgren, L. & Persson, S. 1978. Vegetational succession in a south Swedish deciduous wood. - Vegetatio 36: 17-29.

Persson, S. 1980. Succession in a south Swedish deciduous wood: a numerical approach. - Vegetatio 43: 103-122.

Persson, S. et al. 1987. Leaf litter fall and soil acidity during half a century of secondary succession in a temperate deciduous forest. - Vegetatio 73: 31-45.

von Oheimb, G. and Brunet, J. 2007. Dalby Söderskog revisited: long-term vegetation changes in a south Swedish deciduous forest. - Acta Oecol. 31: 229-242.