Windsor Great Park and forest has the largest collection of mature, over mature and ancient oak and beech trees that remain in Europe north of the Mediterranean and the Pyrenees. It is important not only because of the trees themselves but also for the a vast array of organisms which require and depend on them for their survival.

Such a large number of trees covering the whole spectrum of age classes in one area provides a very rare, but declining, habitat. Together with the New Forest, Windsor is unequalled elsewhere in Europe.

Scattered throughout the forest and park are approximately 3,000 oak trees over 300 years old, 500 between 400 and 600 years old and up to 100 ranging between 600 and 1,000 years of age. This large number of trees provides continuity of dead and dying wood which is considered to be one of the key resources for animal, fungal and invertebrate species in a natural forest.

As a habitat, Windsor is exceptional.

• Over 2,000 species of beetle have been recorded, including at least 200 rare beetles dependent on oak trees. 15 of these are found at only a few sites in Europe and nowhere else in the UK.
• Over 60 rare species of fly have been recorded. Of these, 4 have not been found elsewhere in the UK and a further 30 are limited to fewer than 10 sites in the country.
• Over 1,000 species of macro fungi have been recorded, many for the first time in the UK, with 4 species discovered for the first time ever; they are new to science. A further 250 species are regarded as rare in Europe and over 40 species are entirely, or almost entirely, confined to Windsor.

Ornithologically, the area is important for tree hole nesting birds. Nationally, it is important for two species of owl and, internationally, for the mandarin duck, an introduced species threatened in its home range in Asia. The rich diversity produced by the forest and park and the continuity of biomass supports very large numbers of breeding and winter visitors. This diverse array of species, some in substantial populations, makes this an important site too for bird predators, which is reflected in the fact that Windsor holds the highest breeding density of sparrow hawks studied in its breeding range throughout Britain and Europe.

The Crown Estate recognises the international importance of Windsor as a nature reserve and the collection of natural organisms, such as flowers, insects, mushrooms and toadstools is not permitted.

A large part of the Windsor estate is designated as a Site of Special Scientific Interest and since 1990 the park has been managed under a Statement of Intent agreed with Natural England to help promote the important nature conservation interests of this unique estate.