The study was conducted in the 1000-ha forest landscape at the estates of Skabersjö and Torup (55°32′N 13°11′E) in the province of Skåne, southernmost Sweden. The mean annual temperature is about 7·5 °C and the mean annual precipitation is approximately 650 mm. The study area is situated in a hilly landscape created by glacial morainic deposits upon early Tertiary limestone. The altitude varies from 25 m to 75 m a.s.l. There are no large-scale gradients in macro-climate or geology within the study area. However, variations in topography and land-use history have created a mosaic of forest soils, including eutric and dystric cambisols and podsols on well-drained sites. Most of the ancient woodlands in the area are dominated by European beech Fagus sylvatica and, to a lesser extent, pedunculate oak Quercus robur.
Between 1920 and 1995, the estate foresters established a large number of forest plantations on arable land, pastures and ancient woodland sites. These plantations are unique in Sweden regarding the number of stands, variation in stand age and spatial isolation from colonization sources. They offer an excellent opportunity to analyse long-term colonization processes by forest plants under relatively homogeneous abiotic conditions. Whereas many stands had been planted with pedunculate oak, there were very few beech plantations. European sycamore Acer pseudoplatanus, however, had been frequently planted and was chosen to represent tree species with dense canopies.
For the Torup estate, detailed land survey maps were available for the years 1694, 1799 and 1915, and forest management plans for 1945, 1973, 1983, 1992 and 2001. For the Skabersjö estate, land survey maps were available for 1733 (part of the area), 1769–71 (part of the area) and 1915, and forest management plans for 1839, 1878, 1926–29, 1942, 1981 and 1999. Using these historical sources, the year of plantation and the distance to the nearest ancient woodland border were determined for each stand.
Areas that had been wooded continuously since the oldest survey maps were considered to be ancient woodland sites. Both former wood pastures and wood meadows were included. Today, these areas contain some semi-natural mixed broadleaved stands, considerable areas of naturally regenerated managed beech forest, and managed broadleaved and coniferous plantations. Forest stands that were planted on open pastures or on arable land (former fields) were classified as recent woodland. Oak and sycamore plantations were either established directly on fields and pastures or followed one generation (40–80 years old at felling) of Norway spruce.
The eastern part of the study area is well wooded today and all recent plantations are either directly adjacent to ancient woodland sites or linked by other recent woodland to ancient woods (further merged as contiguous plantations). Plantations in the western part are embedded in a matrix of arable land. Except for three contiguous stands, they are spatially isolated from ancient woods. Plantations on former pasture are only found in the eastern part of the study area. All contiguous plantations have been linked to ancient woodland since their establishment. Three formerly isolated plantations have become connected with ancient woodland by a new large plantation established in 1993. As this plantation was lacking typical forest species at the time of the survey, these stands were considered as isolated in the analyses.
The land survey map of 1915 (1 : 20 000) and aerial photographs from 1940 onwards show that almost no hedges or open ditches have existed in the agricultural landscape of the study area since the time of the earliest plantations. The larger landed estates in Skåne were pioneers of modern agriculture and had already introduced below-ground drainage and large field sizes in the 19th century. Most farmsteads were relocated during land redistribution in the late 18th century and their gardens are therefore of recent origin. Tree alleys and ditches are present along some roads but are not connected to recent plantations. Therefore, it was assumed that diaspore sources of forest plants were generally located in woodland.
Based on the historical data, stands of the following categories were chosen for further study (cf. Table 1):
Table 1. Mean values of stand age, stand area and species richness of typical forest plants in different types of pedunculate oak and European sycamore plantations in the Torup–Skabersjö area, southern Sweden. Separate anova analyses were carried out for oak and sycamore stands. Mean values with the same letters lack significant differences according to the Tukey test (P < 0·05)
|Stand group no.||Description||No. stands||Mean stand age (years)||Mean stand area (ha)||Mean species richness (no.)|
| ||Quercus robur planted on|
|1||ancient woodland sites||12||57·9 b||2·18 a||24·8 a|
|2||pastures||10||49·4 b||1·12 a||16·6 b|
|3||arable land, older stands contiguous with ancient woodland|| 8||77·4 a||1·51 a||21·3 a|
|4||arable land, younger stands contiguous with ancient woodland|| 8||50·5 b||1·21 a||16·0 b|
|5||arable land, isolated from ancient woodland||10||57·5 b||1·38 a|| 8·7 c|
|6||arable land, youngest plantations, contiguous and isolated||14|| 9·4 c||4·00 b|| 1·6 d|
| ||Acer pseudoplatanus planted on|
|7||ancient woodland sites||10||43·6 a||0·92 a||21·2 a|
|8||pastures|| 6||38·7 a||0·77 a||15·0 a|
|9||arable land, contiguous with ancient woodland|| 5||37·2 a||0·77 a||16·2 a|
|10||arable land, isolated from ancient woodland|| 5||34·6 a||0·71 a|| 9·2 b|
plantations on ancient woodland sites;
plantations on formerly open pastures contiguous with ancient woodland;
plantations on former arable land contiguous with ancient woodland;
plantations on former arable land isolated from ancient woodlands by open land.
A floristic survey was conducted during 2002 in 88 plantations. Vernal species were studied between 3 April and 22 May 2002; summer species were surveyed between 27 June and 1 August 2002. Each plantation was surveyed for the occurrence of typical forest-floor plants, i.e. shade-tolerant herbs, graminoids and ferns predominantly confined to woodlands (species list mainly according to group B1.1 in Schmidt et al. 2002 but excluding swamp forest species not found in well-drained plantations). Several transect walks were made through each stand until about 15 min had passed without detection of a new species.
The abundance of each forest species in a stand was estimated using an ordinal scale of 1–3: 1, cover < 1% of stand area; 2, cover 1–20% of stand area; 3, cover > 20% of the stand area.
A total of 48 typical forest species was recorded during the survey. Frequency (percentage occurrence) was calculated for each species and stand group. Differences in species frequency between ancient woodland plantations and the other stand groups were tested with the two-way contingency χ2 test for the 33 most frequent species (at least eight occurrences). Based on the results of this test, species were divided into groups with different colonization patterns. The χ2 test was also used to test differences in the frequency of cover classes 2 and 3 in colonized stands of different land-use categories. Species richness and stand characteristics of different stand groups were compared using anova and the Tukey test (Zar 1996). For plantations on arable fields, relations between species richness and quantitative environmental variables were studied with regression analysis. Stand age, stand area and distance to ancient woodland were not significantly correlated with each other in these stands.
Vegetation gradients were studied with correspondence analysis (CA) and canonical correspondence analysis (CCA; canoco version 3.12; ter Braak 1986; ter Braak 1990). In both analyses, the default options were chosen using a data set including abundance data of all stands (n = 74), except for the 14 youngest plantations, which contained very few or no forest species. Differences in CA stand scores among land-use groups were analysed with anova and the Tukey test. The following environmental variables were included in CCA. Quantitative variables: stand area (ha); stand age (years); distance to nearest ancient woodland edge (m); continuity as woodland (years, ancient woodland was given a value of 300 years); continuity as broadleaved woodland (years). Category variables (0, 1): isolated from ancient woodland by open land; pedunculate oak or European sycamore; planted on ancient woodland, open pasture or arable land; existence of Norway spruce plantation prior to oak or sycamore.