This description of the vegetation succession on the former arable land is for the major part based on a survey of the vegetation of the island, undertaken during the 1950s by Professor Knud Jessen (see K. Jessen, 1968: Flora og vegetation på reservatet Vorsø i Horsens Fjord. – Botanisk Tidsskrift, 63:1-201), and on my own surveys, which took place in 1991-92 and 2006, respectively. A thorough description of the vegetation succession on the island – and of its geology, history, other plant and animal life, etc. – is found in the following book (in Danish): Vorsø – et fristed for naturen, edited by Kaj Halberg and Jens Gregersen, published by Eigil Holms Forlag, 2010.
The aerial photograph (fig. 1.) shows the various localities on the island.
Near the two old forests, Østerskov (’Eastern Forest’) and Vesterskov (’Western Forest’), the grass cover was mixed with young trees, especially ash (Fraxinus excelsior) and sycamore maple (Acer pseudoplatanus), whose seeds are widely dispersed by the wind. On November 24, 1928, eastern Jutland was ravished by a hurricane-like storm from the south-west, causing seeds of predominantly ash, sycamore maple, and common elm (Ulmus glabra) to spread from the eastern wood all over the abandoned eastern field, north-east of the eastern forest. Seeds of beech (Fagus sylvatica) are too heavy to be wind-spread, so this species was not able to establish itself here. Sycamore maple quickly took over most of this area, today covering an area of c. 5 hectares (figs. 2 and 3). Ash, elm, Norway maple (Acer platanoides), and common fir (Abies alba) took part in the competition, but only a few specimens of the former three were able to reach the crown layer.
During the 1990s, Dutch elm disease (see page Dutch elm disease) killed all larger elms in the young wood on the eastern field, but new elms constantly sprout. Various shrubs were formerly growing on the forest floor, such as dog rose (Rosa canina), common hazel (Corylus avellana), guelder rose (Viburnum opulus), Midland hawthorn (Crataegus laevigata), one-seeded hawthorn (C. monogyna), and common elder (Sambucus nigra). Today, the forest floor here is very dark due to the dense foliage of sycamore maple, and no bushes grow here, apart from elder, which forms brushwood here and there, and a few hawthorns.
The grass soon disappeared from this young wood, and, since c. 1980, the forest floor is covered in places by dense growths of shadow-tolerant herbs, predominantly broad-leaved enchanter’s nightshade (Circaea lutetiana), Herb Robert (Geranium robertianum), and common avens (Geum urbanum). Near a former clay pit in the north-eastern corner of the field, sanicle (Sanicula europaea) (fig. 4) and wood false brome (Brachypodium sylvaticum) are numerous.
Other places on the island, formation of forest initially only took place near the edge of the old forests (see fig. 1). Today, these young woods are dominated by ash, with species like hazel, goat willow (Salix caprea), and Midland hawthorn in the bush layer. In a small area between the old eastern forest and the road, a dense growth of common alder (Alnus glutinosa) was established here shortly after 1928, and by 1934, the trees were already 4 metres high. Later, ash invaded this area, and today almost all alders have disappeared.
In the remaining parts of the old protected fields, the grass cover was slowly replaced by plant communities, dominated by rosebay willow-herb (Chamerion angustifolium) and raspberry, and, to a lesser extent, creeping thistle and common nettle (Urtica dioica) (see fig. 1). But after 1955, rosebay willow-herb began to spread faster. Aerial photographs show that many of the growths were circular, indicating that they were spreading by subterraneous runners. The explosive spreading was also caused by rosebay willow-herb being very competitive. It grows tall, and its withered foliage contains chemicals, which inhibit growth of other species. The rich soil on Vorsø was also beneficial to the species. In 1962, the distribution of grassy areas and rosebay willow-herb/raspberry communities on the fields was about equal, namely 45% each, while scrub and young woods covered 10%.
During the 1970s and 1980s, huge thickets of sloe (Prunus spinosa), cherry plum (Prunus cerasifera), dog rose, one-seeded hawthorn, elder, and grey willow (Salix cinerea) were established many places on the old fields, often being so dense that they were impenetrable (fig. 5). In the same period, small groups of young trees were established here and there on the fields, dominated by ash, mixed with sycamore maple, elm, common oak (Quercus robur), hazel, goat willow, and silver birch (Betula pendula).
From the plantation Vestre Remise (see more in the caption Swamp like in the 1700s below), grey poplar (Populus x canescens) spread into the surrounding field by root shoots, forming a young wood, which, however, was soon severely restricted by breeding cormorants (Phalacrocorax carbo ssp. sinensis) (fig. 6). Beneath this young wood, very dense elder shrubs were formed. Hops (Humulus lupulus) spread from Vestre Remise, and today bushes in a larger area near the plantation are completely overgrown by this vigorous vine (fig. 7).
On the old fields, by 2006, young forest had almost completely replaced the former grassy areas, as well as the rosebay willow-herb/raspberry communities, and scrubs were also slightly declining. The forest floor in these young woods was still quite open, and a large number of various herbs were growing here, especially common avens, broad-leaved enchanter’s nightshade, and dog’s mercury (Mercurialis perennis), but also many others, such as common male fern (Dryopteris filix-mas), broad buckler fern (D. dilatata), ramsons (Allium ursinum), cleavers (Galium aparine), common Solomon’s seal (Polygonatum multiflorum), red campion (Silene dioica), Herb Robert, and common bugle (Ajuga reptans). In young woods near the northern coast, species like wild chervil (Anthriscus sylvestris), sanicle, common Solomon’s seal, and garlic mustard (Alliaria petiolata) dominated.
Today (2017), only small areas with grass cover, rosebay willow-herb/raspberry communities, and meadows with tall perennials are found on the old western field. During the 1980s, in the south-eastern part of this field, several grassy areas were taken over by common male fern and a few common lady ferns (Athyrium filix-femina). Since then, the former has spread even more, today covering large areas (fig. 8). The only remaining larger area with grass cover is found south of Vestre Remise, and it is obvious that grazing roe deer (Capreolus capreolus) have delayed the succession.
Similar to the development on the old protected fields around 1930, the new protected fields, for a few years, were completely dominated by annuals and biennials, in particular scentless mayweed and couch grass (Elytrigia repens) (fig. 9). The latter, which had been a common weed on the arable land, soon took over large areas of the ploughed fields, together with e.g. species of dandelion (Taraxacum). Couch grass is able to withstand ploughing, as even a tiny part of its rhizome is able to sprout into a new plant.
In 1979, on the stubble field, seeds of rosebay willow-herb, spread from the dense growth of this species on the western field, were at once able to sprout, but on the ploughed fields, the cover of couch grass was already so dense that germination of the willow-herb seeds was inhibited. Those willow-herb plants, however, which had been able to establish themselves here, soon began to spread by subterraneous runners, and as early as 1984, this species had taken over the northern field and the western part of the southern field (fig. 10), together with e.g. creeping thistle, raspberry, and nettle. Far fewer willow-herb seeds had been spread to the eastern part of the southern field and to the central field, and the grass cover, dominated by couch grass, common cock’s-foot, and velvet grass, endured for many years.
By 2006, most of the grassy areas and the rosebay willow-herb growths on the northern and southern fields had been replaced by huge shrubs of raspberry, mixed with e.g. dewberry (Rubus caesius), nettle, creeping thistle, and cleavers. In some areas, hairy St. John’s wort (Hypericum hirsutum) and imperforate St. John’s wort (H. maculatum) were common. Grass cover was still found in small areas of the fields, mainly along the coast. In 2006, many species, which are rather rare on Vorsø, were growing here, such as common centaury (Centaurium erythraea), smooth hawksbeard (Crepis capillaris), lesser clover (Trifolium dubium), knotted clover (T. striatum), bitter fleabane (Erigeron acer), mouse-ear (Pilosella vulgatum), tansy ragwort (Senecio jacobaea), and wild angelica (Angelica sylvestris). Since then, several of these species have disappeared.
In a gravel-dominated area on the southern field, northwest of the tiny plantation Tepotten (’The Teapot’ – thus named due to a tiny waterhole in the plantation), rosebay willow-herb never managed to dominate, and in 1991 this area somewhat resembled a common, housing an abundance of species, dominated by false oat-grass (Arrhenatherum elatius) and velvet grass, but with a number of rarer species, such as orange hawkweed (Pilosella aurantiaca) (fig. 11), colt’s foot (Tussilago farfara), field garlic (Allium oleraceum), and low hop clover (Trifolium campestre). In 2006, raspberry and dewberry had taken over much of this area, and only woodland strawberry (Fragaria vesca) was of some interest.
By 2006, scrubs were not very widespread on the northern and southern fields. Grey willow and goat willow formed growths along the old western forest, on the central part of the southern field, and along a hedge row of cherry plum trees on the northern field. Small shrubs of sloe and cherry plum were established here and there. During the 1980s, along the southern and north-eastern edges of the old western forest, a young wood sprung up, dominated by elm, mixed with ash, silver birch, alder, and common aspen (Populus tremula). When the elms had reached a certain height, they were attacked by Dutch elm disease and died, but new young trees would soon sprout (see page Dutch elm disease). Today (2017), the herb layer in these young woods is dominated by nettle, mixed with e.g. common male fern, Herb Robert, and common avens.
Shortly after the protection of the southern field in 1978, thousands of seeds, originating from a few large sycamore maples in the small plantation Tepotten, were spread onto the field, and during the following years, a rather large area (except on the previously mentioned gravel strip) was covered by young trees. By 1993, a dense young wood had been established, in which the highest trees measured 8 or 9 metres. By 2006, this young forest had further spread, the tallest trees by now measuring over 15 metres (figs. 12-14). The succession here closely corresponds to the one, which took place 50 years earlier on the old protected field north-east of the old eastern forest.
By 2006, on the southern field, west of a small area of old protected field in the south-eastern corner of the island, a young wood had sprung up, dominated by ash and elm. On the forest floor in this wood, common lungwort (Pulmonaria officinalis) was very common.
Vegetation succession on the central field differed somewhat from the other two fields. This field is divided into two parts by a hedge row of cherry plum trees, and other plum hedges separate these from the northern field and from the coast. Among the plum trees grew ash and Berlin poplar (Populus x berolinensis).
Most of the central field was quickly covered by various grasses, dominated by couch grass, which, however, by 1991 had largely been replaced by velvet grass (fig. 15). A depression in the southern part of the field, along the road, is very humid, and until the mid-1980s, a dense carpet of creeping buttercup (Ranunculus repens) was found here, later largely replaced by e.g. couch grass. A young alder wood was established here at an early stage (fig. 16), today (2017) forming a dense young forest. The forest floor here is still very wet, covered by vegetation of e.g. grasses, water mint (Mentha aquatica), and Goldilocks buttercup (Ranunculus auricomus).
In 1991, the central field was one of the botanically most important areas on the island, displaying a large number of rarer herbs, e.g. common marsh orchid (Dactylorhiza majalis), oval sedge (Carex ovalis), glaucous sedge (C. flacca), field woodrush (Luzula campestris), tansy ragwort, heath cudweed (Gnaphalium sylvaticum), and heath speedwell (Veronica officinalis). In 2006, only glaucous sedge and tansy ragwort could be found again, but there were many other interesting herbs, such as common agrimony (Agrimonia eupatorium) (fig. 17), hairy St. John’s wort, imperforate St. John’s wort, pale sedge (Carex pallescens), and common bugle. Beneath the hedges, sanicle was abundant, and four small growths of broad-leaved helleborine (Epipactis helleborine) were also found here.
Thorny shrubs were quickly established on the central field, and today you see dense thickets, similar to the ones on the old fields (fig. 18). In the late 1970s, countless ash seeds were spread from the eastern edge of the old western forest onto the field, and during the 1980s, thousands of tiny ash trees sprouted here. For several years, these small trees were severely set back by roe deer, which ate the major part of the buds during winter. By 1991, however, some of the trees had been able to grow tall. The rest were only 30 to 40 centimetres high, and the growth was so dense that it was difficult to enter the area. In 2006, many of these low ash trees had managed to reach a height of 10 metres, but a remarkably large number of them had a dent on their trunk, 30 to 40 centimetres above the ground – exactly where the roe deer previously had bitten off the top shoots. Still, the growth was very dense, in some areas up to 25 per square metre. Since the 1990s, a young ash forest has also been established in the south-eastern corner of the central field (fig. 19). In the open parts of the field, dewberry is today (2017) very common, and only small areas of velvet grass remain.
The plantation Vestre Remise, which in 1929 contained two small ponds, was separated from the southern part of the old western field by a road embankment. South of this road was a depression, covering c. 0.2 hectares, which, in 1941, was still covered in grasses. Later, this depression became swampy, and, around 1960, vegetation of reed (Phragmites australis), swamp, meadow, and shrubs of grey willow had been established around the two ponds and in the depression (see fig. 20, top). In this mosaic of vegetation types, numerous species were found, e.g. sea club-rush (Bolboschoenus maritimus), branched bur-reed (Sparganium erectum), common skullcap (Scutellaria galericulata), cyperus sedge (Carex pseudocyperus), bittersweet nightshade (Solanum dulcamara), trifid bur-marigold (Bidens tripartita), celery-leaved buttercup (Ranunculus sceleratus), creeping bentgrass (Agrostis stolonifera), and field mint (Mentha arvensis).
During the following 30 years, a drastic change of the vegetation in this plantation took place. After some years with rising groundwater level in the area, the two original ponds and the depression were, for the major part of the year, transformed into one larger pond, named Vesterdam (‘Western Pond’). Those trees in the plantation, which were now standing in water most of the year, died, becoming the perfect breeding place for cormorants, which emigrated here from the western forest. When the cormorants were protected on the island in 1971, their numbers quickly increased significantly. They spread to the remaining part of the plantation, causing all trees to die. Only grey poplar survived, spreading by root shoots. In areas with many cormorants, the vegetation cover on the ground also perished. The areas, which were no longer used by the cormorants, were quickly invaded by elder, and small areas with grasses were also established.
Due to the strong influence of the guano, the swamp vegetation changed, in 1991 dominated by sea club-rush and reed, together with common bulrush (Typha latifolia), which had spread to this area around 1980 (fig. 20, centre). Other common species were trifid bur-marigold, water-pinkweed (Polygonum amphibium), spear-leaved orache (Atriplex prostrata), and red goosefoot (Chenopodium rubrum). In late summer, the latter covered large areas, when the pond started drying out (figs. 21 and 22). The grey willow shrubs had spread significantly, now also covering a former meadow with tall perennials.
By 2006, reed had spread to most of the depression, and north of the former road, sea club-rush was dominant, together with especially reed canary-grass (Phalaris arundinacea) and common bulrush (fig. 20 bottom, and fig. 23). Red goosefoot had almost disappeared, while golden dock (Rumex maritimus) had turned up. Around the pond, many of the elders had disappeared and had been replaced by a community of high perennials, dominated by great willow-herb (Epilobium hirsutum).
Until 1979, an area south-east of the farmhouse, west of the old eastern forest, was a grass field, utilized for cattle grazing. In this field, common daisy (Bellis perennis) was so numerous during the 1970s that a large part of the field appeared white (fig. 24). In the old days, before this area was drained, the field was a meadow, named Kulmade (’Coal Meadow’) – very likely because the soil here is black peat.
Following the destruction of the drainage pipes in 1980, the groundwater level rose, and the field once again turned into a meadow. In spring, during the 1980s, the vegetation was completely dominated by creeping buttercup, which is still very common here (fig. 25). In the 1990s, various grasses began to dominate, especially creeping bentgrass (fig. 26) and marsh foxtail (Alopecurus geniculatus), with large growths of reed canary-grass, water-pinkweed, water mint, bittersweet nightshade, meadow foxtail (Alopecurus pratensis), trifid bur-marigold, silverweed (Potentilla anserina), and tufted hair-grass (Deschampsia cespitosa). Cyperus sedge was also present, displaying several fine tussocks.
During the 1990s, cormorants were breeding in large numbers right out to the western edge of the eastern old forest. At this time, several guano-tolerant species were growing along the forest edge, e.g. red goosefoot, many-seeded goosefoot (Chenopodium polyspermum), pale pinkweed (Polygonum lapathifolium ssp. pallidum), and spear-leaved orache. In 2006, there were not nearly as many cormorants, and of the above-mentioned plants, only red goosefoot could be found.
As early as the 1930s, a humid depression had already been established in a small area of old protected field in the south-eastern corner of the island. Before the destruction of the drainage pipes in 1980, this depression was dominated by reed canary-grass, common nettle, and meadow sweet (Filipendula ulmaria). The rising groundwater level implied that most of the year a small pond was present here, called Østerdam (‘Eastern Pond’).
During the first half of the 1980s, this pond was almost covered by a huge growth of pond water-crowfoot (Ranunculus peltatus) (fig. 27), but this species was soon replaced by reed canary-grass, together with other species like branched bur-reed (fig. 28), common bulrush, sea club-rush, soft-stemmed bulrush (Schoenoplectus tabernaemontani), trifid bur-marigold, and water-pinkweed. In 2006, the pond had about the same distribution as in the 1980s, but the vegetation had changed. Around the edge of the pond, reed canary-grass still formed a zone, but in the pond proper, common bulrush and sea club-rush had increased, while branched bur-reed could not be found. Water mint, golden dock, and pink water-speedwell (Veronica catenata) had appeared.