The paper presents a rather speculative overview of sacred groves and forests; there is no comprehensive data available at the moment because the survey of sacred natural sites has not been completed yet. Only true sacred forests (those known as hiis) will be analysed below although many single sacred objects (trees, tiny groves, boulders, springs, etc.) might be biologically interesting, and some are under nature protection.
Sacred forests can be compared to the special management zones of the Estonian contemporary nature reserves and national parks. Human inf luence on the plant communities incl. tree and shrub layers of the sacred areas was kept to a minimum. However, certain human activities were accepted, such as erecting fences, religious buildings and sacred figures, or leaving various ceremonial items in the sacred area (e.g. bones and ropes of sacrificial animals). Thus, sacred forests differ slightly from strict nature reserves (without any direct human impact), but they are close to them.
All northern European forests undergo ecological succession from one site type to another over a long period. Therefore, it can be presumed that sacred forests unaffected by logging and similar human impact are climax communities. Boreonemoral sprucedominated forest (Aegopodium or Hepatica site type) is the climax community in most parts of Estonia, whereas it is the broad leaved nemoral forest (e.g. Ulmo-Tilio-Acerion) in many places in western Estonia. Sacred forests are often associated with pure oak groves, which was probably not the case. Oaks dominate in Estonian conditions only if a forest is grazed or mowed, but that was forbidden in sacred areas. Sacred forests could have included also poorer site types, especially in eastern Estonia, and esker forests. Some sacred areas are (in) bog or swamp forests now. They were originally probably located on mineral land which is presently covered by a peat layer (the average rate of paludification in Estonia is ca 1 m of peat per millennium).
The secondary use of sacred groves and forests started in the 1200s (cutting, erection of Christian churches) and intensified in the 19th century. Lack of agricultural land during the rapid growth of rural population coincided with the gradual disappearance of traditional religion, and thus a number of sacred forests were turned into wooded meadows or fields.
There is no uniform solution for the future of the Estonian sacred forests. Many have definitely undergone natural succession or will do it again, others might be more valuable as e.g. wooded meadows from the viewpoint of biodiversity and some have been built up or turned into gravel pits. Consequently, some sacred forests should be protected as natural objects, whereas others might be saved only by heritage protection or spatial planning measures. Unfortunately some destroyed sacred areas have totally lost their holiness.