The ecoregions of Denmark consist of two chief units: Atlantic mixed forests and Baltic mixed forests. Most of these temperate forests have been substantially destroyed or fragmented, chiefly through conversion to agricultural uses over the last several millennnia. Although the Faroe Islands and Greenland are both self-governing entities within the Kingdom of Denmark, the ecoregions of those geographic units are treated in separate articles.
In addition to the two forest ecoregions, there is a recognizable ecosystem along the coastal zone of southern Jutland which consists of expansive tidelands, mudflats and marshes; this area includes not only the continental coast but the area comprised by numerous low-lying islands along the very shallow Wadden Sea.
Atlantic mixed forests
See main article: Atlantic mixed forests
The Atlantic mixed forests ecoregion includes coastal vegetation formations of dune fields and heathlands with vegetation that thrives in salty soil. Several mixed oak forests are also found in the ecoregion, dominated by Quercus robur and Betula pendula, or Q. robur and Fagus sylvatica. On the coastline, heathlands with Ulex gallii occur, adapted to local ecological conditions of wind and sea spray. In general, heathlands with Ericaceae (Calluna, Erica, and Ulex spp.) have replaced natural forests. This ecoregion includes the riparian forests along the rivers discharging to the Wadden Sea and North Atlantic. There are a number of endemic species in the Atlantic mixed forests. Mean annual temperatures in this ecoregion are approximately nine to eleven degrees Celsius.
Portions of these low lying forests have significant prehistoric and extant bogland. Notably there have been discoveries dating to the Iron Age of well preserved human remains as well as other biological materials. The acidic and anaerobic conditions of these bogs provide ideal conditions for preservation of such ancient biota that became enmired in these bogs. Perhaps the most outstanding example of such preservation is the Haraldskaer Woman, preserved in toto near Vielje; the state of preservation was so complete that not only was the corpse accurately dated to 500 BC, but the exact stomach contents were determinable.
Agricultural expansion and intensification are the most serious threats to this ecoregion in Denmark. Additionally, urbanization accompanied by the pollution of air, water, and soil brings increased problems. Recreation and tourism, unsustainable exploitation, development and habitat fragmentation and disturbance of wildlife are other major threats.
Baltic mixed forests
Occurrence of Baltic mixed forests (in yellow). Source: WWF
See main article: Baltic mixed forests
the Baltic mixed forests are present in the northern portion of Denmark rimmed by the Baltic Sea and the Kattegat. These forests occupy most of the island portions of Denmark, and also the eastern part of the Jutland peninsula. This ecoregion is within a temperate zone much farther north than comparable areas in the western part of the Eurasian continent or compared eastern North America. Comprised of both forests and wetlands, the Baltic mixed forests ecoregion displays many different habitats and, accordingly, a broad diversity of species. Migrating and breeding water-birds, shorebirds, and wading birds visit this habitat each year.
Baltic mixed forests have been exploited by man throughout the Holocene period; moreover, there are significant finds of sunken forests along the Baltic coastline, that reveal earlier forest composition, but that also suggest an early beginning of global warming and sea level rise, perhaps earlier than generally reported. Presently, the ecoregion is being threatened by ongoing habitat destruction and water pollution discharge, especially from agricultural runoff to rivers and wetlands.
North Frisian coastal tidelands and islands
Mando tidelands, Wadden Sea coastal zone. @ C.Michael Hogan Inside the North Frisian Wadden Sea, a number of Halligen islands (small salt marsh low lying landforms) and polder islands occur, of which Fohr has a Pleistocene core. Other islands that manifest a glacial Pleistocene core are Sylt, Texel and Amrum.
The chief Danish Wadden islands are Mando, Fano and Romo; in addition, two prominent sandbanks are Jordsand and Koresand. Some of these islands such as Mando have diked areas to allow water defended areas for sheep grazing. They have perimeters of expansive mudflats and marshes. Much of this mudflat and tidelands are within designated Important Bird Areas of western Europe.
Structural definition of ecoregions
Ecoregions are areas that:  share a large majority of their species and ecological dynamics;  share similar environmental conditions; and,  interact ecologically in ways that are critical for their long-term persistence. Scientists at the World Wildlife Fund (WWF), have established a classification system that divides the world in 867 terrestrial ecoregions, 426 freshwater ecoregions and 229 marine ecoregions that reflect the distribution of a broad range of fauna and flora across the Earth.
- Udo Bohn, Gisela Gollub, and Christoph Hettwer. 2000. Reduced general map of the natural vegetation of Europe. 1:10 million. Bonn-Bad Godesberg
- S.D.Davis, V.H. Heywood, and A.C. Hamilton. 1994. Centres of plant diversity. Vol. 1: Europe, Africa, Southwest Asia and Middle East. WWF and IUCN, Washington DC. ISBN: 283170197X
- M.F.Heath and M.I. Evans, editors. 2000. Important bird areas in Europe: Priority sites for conservation. 2 vols. BirdLife International, Cambridge, UK. ISBN: 0946888361
- World Wildlife Fund. 2002. Atlantic mixed forests.