Various environmental factors determine the vegetation of an area, including its climate and substrate. In Iceland, the maritime climate, volcanic activity, movement of outlet glaciers, and overgrazing are among the factors that are unfavourable for vegetation growth and succession. Major changes to the vegetation have occurred since the settlement of Iceland. Various ongoing changes to the vegetation in modern times are a result of human activity.
The Icelandic wilderness is characterised by its large, bare open areas. Only a fourth of the island is vegetated. Lowland areas tend to have relatively continuous vegetation cover. At a height of 200–400 m above sea level, vegetation becomes visibly more sparse. Land above 700 m above sea level is mostly non-vegetated. Icelandic vegetation is characterised by low-growing plant species and a relatively small number of wild vascular plants. Vegetation and woodlands have been depleted over the centuries, due to natural forces and human activity. It is estimated that 60% of Iceland was vegetated at the time of its settlement, a figure that now stands at around 25%.
There are many different ways of classifying vegetation. The IINH uses a hierarchical structure with four levels (see vegetation legend, PDF). The uppermost level divides vegetation into dryland and wetland vegetation. These are then organised into seven major vegetation types (gróðursamfélög) based on the substratum and physiographical characteristics but also dominant growth-forms (e.g., moss heath). These vegetation types are sub-divided into around twenty vegetation groups (gróðurlendi), such as Racomitrium heath and Anthelia heath. Sociations (gróðurfélög) are the lowest and most specific level. Classification into sociations is based on floristic composition (e.g., mosses with Carex bigelowii and mosses with Carex bigelowii and dwarf shrubs). Natural vegetation in Iceland is classified into just over 100 sociations. This system has been used for decades in Iceland for vegetation classification and mapping, but is now being replaced by habitat type classification.