Dalby Söderskog Revisited: Long-Term Vegetation Changes in a South Swedish Deciduous Forest. von Oheimb, G. & Brunet, J. 31(2):229–242.
Dalby Söderskog Revisited: Long-Term Vegetation Changes in a South Swedish Deciduous Forest [link]Paper  doi  abstract   bibtex   
In this study, we report on 67 years of secondary succession in the temperate deciduous forest Dalby Söderskog, southern Sweden. In 2002 vegetation analyses were conducted in 74 systematically distributed plots and the results compared with similar investigations during the years 1935, 1969 and 1976. Additionally, a floristic survey of the forest was made and compared with inventories of the years 1925, 1935 and 1970. From 1970 to 2002, overall species richness of the forest continued to decline, but at a lower rate than before. In 2002, for the first time, shade-tolerant forest species constituted the largest group. In the upper tree layer Fraxinus excelsior became the most important species, at the expense of Quercus robur and Ulmus glabra. In the herb layer, no changes in total or mean species richness were exhibited at plot scale between 1976 and 2002. In contrast to the lack of quantitative changes in species richness, we observed major changes in species composition at the plot level. In particular, the former dominant species Mercurialis perennis had declined markedly. After release from a historical regime of livestock grazing and irregular cuttings, the predicted steady-state community dominated by Ulmus and Mercurialis only prevailed for a few decades before unexpected external events, in this particular case Dutch elm disease and invasion by the slug Arion lusitanicus, initiated a successional change. We conclude that unexpected diseases or pests or rare climatic extremes can play a decisive role in forest dynamics and strongly interact with more gradual autogenic changes of forest structure and composition. [Excerpt: Changes in the tree and shrub layer] A similar long-term decline in regeneration of Quercus spp., such as observed in Dalby Söderskog, has been reported from a wide range of forest reserves in Europe and North America ( Glitzenstein et al., 1986, Emborg et al., 1996, Vera, 2000, Cowell and Jackson, 2002, Harcombe et al., 2002, Runkle et al., 2005 and von Oheimb et al., 2005). The present Quercus canopy trees are relicts of the more open stand structure of former times and do not regenerate under a closed canopy ( Table 2). In the absence of management, Lindquist, 1938 and Malmer et al., 1978 and Diekmann (1994) suggest that Ulmus would be the dominating tree species on moist eutric cambisols due to its characteristics as a shade-bearing, late-successional species, with the advanced regeneration exhibiting a higher shade-tolerance and the canopy trees having a higher competitive vigour than the early- to mid-successional species Fraxinus. Leemans (1992) used the forest succession model FORSKA to simulate species composition and stand structure of Dalby Söderskog. Starting the simulation run in 1920, the author found that species composition stabilized after 100 years as a forest of Ulmus, with scattered individuals of Fraxinus and Quercus. Fagus remained limited to the patches which initially had a high Fagus abundance. [\n] However, Diekmann (1994) suggested the Dutch elm disease (DED) had the potential to reverse these future trends. DED is a response of Ulmus spp. to infection by the fungus Ophiostoma novo-ulmi, which is spread by Scolytus beetles ( Röhrig, 1996). A particularly aggressive strain of DED was first recorded in Sk˚ane in 1979. In 1986, Dalby Söderskog was not yet visibly damaged ( Persson, 1987), but DED started to spread in the reserve during the following years. In order to reduce damage, 10-70 Ulmus trees were cut per year in Dalby Söderskog between 1988 and 1997 (pers. comm., 2002, County Board of Sk˚ane). By 2002, a considerable proportion of the Ulmus canopy trees had been killed or infected by DED (personal observation). [\n] In an unmanaged Ulmus glabra population in Lady Park Wood (western Britain), which has been recorded since 1945 and which has been subjected to DED since 1972, Peterken and Mountford (1998) found that about 65\,% of the individuals present 40 years ago have been killed by DED. However, the population increased 1.4 times during this period. In particular on moist sites of the forest, seedling regeneration was the main survival mechanism and a few individuals survived by producing basal sprouts. While Ulmus recruitment from seedlings was infrequent before the onset of DED in Lady Park Wood, new saplings established soon after the canopy had been opened up. Peterken and Mountford (1998) suggest that, unless DED abates, Ulmus seems likely to persist as a fast turnover population of poles that never reaches the forest canopy. In a population of Ulmus americana in Michigan most of the larger trees were killed by DED, but the species remained dominant in the understorey ( Richardson and Cares, 1976). Based on the vegetation data of the plots, the impact of DED seems to result in similar changes of the Ulmus population in Dalby Söderskog. [\n] [...] [Conclusions] Considering seven decades of vegetation changes in Dalby Söderskog, our results highlight the role of unpredictable historical events that strongly influence the process of autogenic succession. In most places, disturbance after windstorms, fire, pathogen outbreaks or other unpredictable large-scale disturbance will prevent the persistence of stable late-successional forest communities (Pontailler et al., 1997 and McLachlan et al., 2000). Long-term studies are essential in separating the effects of autogenic processes, extreme events, and more gradual changes in the environment upon community structure (Mountford et al., 1999, Runkle, 2000, Woods, 2000 and Harcombe et al., 2002). [\n] After release from a historical regime of livestock grazing and irregular cuttings, the predicted steady-state community in Dalby Söderskog dominated by Ulmus and Mercurialis perennis only prevailed for a few decades before unexpected external events, in this particular case Dutch elm disease and invasion by the slug Arion lusitanicus, initiated a successional change which will be interesting to follow. When walking through the forest in 2006, large numbers of juvenile Fraxinus and Fagus can be seen in gaps, but young Ulmus trees are also common, and in several places Quercus saplings can be observed. The last tree inventory was in 1970 and a new survey which includes regeneration will be of particular importance in an analysis of the changing structure of the forest.
@article{vonoheimbDalbySoderskogRevisited2007,
  title = {Dalby {{Söderskog}} Revisited: Long-Term Vegetation Changes in a South {{Swedish}} Deciduous Forest},
  author = {von Oheimb, Goddert and Brunet, Jörg},
  date = {2007-03},
  journaltitle = {Acta Oecologica},
  volume = {31},
  pages = {229--242},
  issn = {1146-609X},
  doi = {10.1016/j.actao.2006.12.001},
  url = {https://doi.org/10.1016/j.actao.2006.12.001},
  abstract = {In this study, we report on 67 years of secondary succession in the temperate deciduous forest Dalby Söderskog, southern Sweden. In 2002 vegetation analyses were conducted in 74 systematically distributed plots and the results compared with similar investigations during the years 1935, 1969 and 1976. Additionally, a floristic survey of the forest was made and compared with inventories of the years 1925, 1935 and 1970. From 1970 to 2002, overall species richness of the forest continued to decline, but at a lower rate than before. In 2002, for the first time, shade-tolerant forest species constituted the largest group. In the upper tree layer Fraxinus excelsior became the most important species, at the expense of Quercus robur and Ulmus glabra. In the herb layer, no changes in total or mean species richness were exhibited at plot scale between 1976 and 2002. In contrast to the lack of quantitative changes in species richness, we observed major changes in species composition at the plot level. In particular, the former dominant species Mercurialis perennis had declined markedly. After release from a historical regime of livestock grazing and irregular cuttings, the predicted steady-state community dominated by Ulmus and Mercurialis only prevailed for a few decades before unexpected external events, in this particular case Dutch elm disease and invasion by the slug Arion lusitanicus, initiated a successional change. We conclude that unexpected diseases or pests or rare climatic extremes can play a decisive role in forest dynamics and strongly interact with more gradual autogenic changes of forest structure and composition.

[Excerpt: Changes in the tree and shrub layer]

A similar long-term decline in regeneration of Quercus spp., such as observed in Dalby Söderskog, has been reported from a wide range of forest reserves in Europe and North America ( Glitzenstein et al., 1986, Emborg et al., 1996, Vera, 2000, Cowell and Jackson, 2002, Harcombe et al., 2002, Runkle et al., 2005 and von Oheimb et al., 2005). The present Quercus canopy trees are relicts of the more open stand structure of former times and do not regenerate under a closed canopy ( Table 2). In the absence of management, Lindquist, 1938 and Malmer et al., 1978 and Diekmann (1994) suggest that Ulmus would be the dominating tree species on moist eutric cambisols due to its characteristics as a shade-bearing, late-successional species, with the advanced regeneration exhibiting a higher shade-tolerance and the canopy trees having a higher competitive vigour than the early- to mid-successional species Fraxinus. Leemans (1992) used the forest succession model FORSKA to simulate species composition and stand structure of Dalby Söderskog. Starting the simulation run in 1920, the author found that species composition stabilized after 100 years as a forest of Ulmus, with scattered individuals of Fraxinus and Quercus. Fagus remained limited to the patches which initially had a high Fagus abundance.

[\textbackslash n] However, Diekmann (1994) suggested the Dutch elm disease (DED) had the potential to reverse these future trends. DED is a response of Ulmus spp. to infection by the fungus Ophiostoma novo-ulmi, which is spread by Scolytus beetles ( Röhrig, 1996). A particularly aggressive strain of DED was first recorded in Sk˚ane in 1979. In 1986, Dalby Söderskog was not yet visibly damaged ( Persson, 1987), but DED started to spread in the reserve during the following years. In order to reduce damage, 10-70 Ulmus trees were cut per year in Dalby Söderskog between 1988 and 1997 (pers. comm., 2002, County Board of Sk˚ane). By 2002, a considerable proportion of the Ulmus canopy trees had been killed or infected by DED (personal observation).

[\textbackslash n] In an unmanaged Ulmus glabra population in Lady Park Wood (western Britain), which has been recorded since 1945 and which has been subjected to DED since 1972, Peterken and Mountford (1998) found that about 65\,\% of the individuals present 40 years ago have been killed by DED. However, the population increased 1.4 times during this period. In particular on moist sites of the forest, seedling regeneration was the main survival mechanism and a few individuals survived by producing basal sprouts. While Ulmus recruitment from seedlings was infrequent before the onset of DED in Lady Park Wood, new saplings established soon after the canopy had been opened up. Peterken and Mountford (1998) suggest that, unless DED abates, Ulmus seems likely to persist as a fast turnover population of poles that never reaches the forest canopy. In a population of Ulmus americana in Michigan most of the larger trees were killed by DED, but the species remained dominant in the understorey ( Richardson and Cares, 1976). Based on the vegetation data of the plots, the impact of DED seems to result in similar changes of the Ulmus population in Dalby Söderskog. [\textbackslash n] [...]

[Conclusions]

Considering seven decades of vegetation changes in Dalby Söderskog, our results highlight the role of unpredictable historical events that strongly influence the process of autogenic succession. In most places, disturbance after windstorms, fire, pathogen outbreaks or other unpredictable large-scale disturbance will prevent the persistence of stable late-successional forest communities (Pontailler et al., 1997 and McLachlan et al., 2000). Long-term studies are essential in separating the effects of autogenic processes, extreme events, and more gradual changes in the environment upon community structure (Mountford et al., 1999, Runkle, 2000, Woods, 2000 and Harcombe et al., 2002).

[\textbackslash n] After release from a historical regime of livestock grazing and irregular cuttings, the predicted steady-state community in Dalby Söderskog dominated by Ulmus and Mercurialis perennis only prevailed for a few decades before unexpected external events, in this particular case Dutch elm disease and invasion by the slug Arion lusitanicus, initiated a successional change which will be interesting to follow. When walking through the forest in 2006, large numbers of juvenile Fraxinus and Fagus can be seen in gaps, but young Ulmus trees are also common, and in several places Quercus saplings can be observed. The last tree inventory was in 1970 and a new survey which includes regeneration will be of particular importance in an analysis of the changing structure of the forest.},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-13713915,~to-add-doi-URL,arion-lusitanicus,corylus-avellana,crataegus-spp,disturbances,dutch-elm-disease,forest-pests,forest-resources,forest-succession,fraxinus-excelsior,ground-vegetation,mercurialis-perennis,plant-pests,quercus-robur,sweden,ulmus-glabra},
  number = {2},
  options = {useprefix=true}
}

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