Cavities are important, natural components of forest ecosystems, conditioning the high level of biodiversity. They are formed either as a result of a natural process of wood decay caused by fungi or as excavations made by woodpeckers Picidae. Distribution and density of cavity trees are regionally diversified and dependent on species composition and age of the stands as well as the management way. In the global scale, decay cavities are much more numerous than those created by woodpeckers. Natural cavities dominated in deciduous forests, whilst woodpeckers−made ones – in coniferous stands. The density of cavities increases along the age gradient due to the growing size of trees and their worse health condition. Cavities in natural forests are more numerous than in commercial, managed ones. This is caused by the removal during the thinning of trees reduced in health, attacked by insects, fungi or mechanically damaged, which are potential places for the creation of cavities. The other reason is a too low age of the final cutting. Moreover, in the managed forest, cavity trees are often removed during sanitary cuttings, although leaving them is recommended. In European forests, the density of cavities is from less than 1 to almost 100 per ha. Natural cavities dominated in most of the studied plots. In Poland, their density varied from less than 1 to 16 per ha. Cavities are habitats and breeding sites of many specialized species of animals from invertebrates to mammals, fungi, and plants. In Poland, cavities are used by about 40 bird species, about 20 mammal species, as well as several hundred species of insects. According to the Polish forestry regulations, trees with cavities should be left to natural destruction, but there are no detail recommendations how to search for such trees and what is their required density. The recommendation to leave cavity trees will not contribute to the increase in their number in managed forests, unless one provides wider availability of adequately large trees with a reduced condition. In Polish forests, actions should be taken to increase the number of potential trees, in which cavities may be formed. They should be designed at the stage of tending for young stands. The minimal density of cavities in managed forests should be in the range of 1−3 per ha in coniferous and mixed forests up to 100 years old and above 3−4 per ha in stands older than 100 years, while in deciduous forests these values should equal to 2−5 per ha and 4−6 per ha in younger and older stands respectively.
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