Regular pyrogenic impact influence on forest phytocenosis structure in the Middle Priamurie. coniferous-deciduous ecotone

Ecotone is characterized by a peculiar biodiversity that forms the ecosystem safety margin before external destructive factors. The pyrogenicfactor influences the habitat, specific composition and structure of phytocenoses. We divide all biocenosis and phytocenosis structuralelements into two categorical units: endocenogenetic and exocenogenetic ones. With relation to functional participation in thephytocenosis dynamics and stability maintenance, its every structural block may be considered to be a biocenotic island, in other words,a relatively independent cenosis with its own food chains, specific compositions, element cycles. The main characteristic of the island isweakening or even absence of biocenotic connections with associations of the same sort. The island concept reflects the function of thegiven cenosis in relation to bigger, enclosing and similar ones. Islands, just like structural elements, are divided into the same cenogeneticcategories. The examples of endocenogenetic islands are stubs and fallen trunks. In any forest biocenosis, its inner environmentdiffers from the external one. But the trunks biota is under the pressure of the host biocenosis, and it adapts easily to its types. An exampleof an exocenogenetic island is cenoses of mountain tops, or, otherwise, relief depressions we as they have a different compositionor proportion than the enclosing biocenosis. The influence of the pyrogenic factor on the phytocenosis structure depends on the prepyrogenicphytocenosis state, various nature factors combination and pyrogenic pressure magnitude. To estimate the pyrogenic factorinfluence on the coniferous-deciduous structure we mark out the following fire periodicity: annual - 1-2 times a year; very frequent -once per 2-5 years; frequent - once per 6-10 years; infrequent - once per 11-20 years; very infrequent - once per 21-30 years; irregular- once per 31-100 years; episodical - more infrequent than once per 100 years. Such periodisation is based on the ecosystems potentialityto rehabilitate itself up to its initial state. Originally, ground fires increase the degree of horizontal and vertical ecosystem structurednessdue to the deciduous species activity increase. Then the fire impact leads to a decrease and, later, elimination of the endocenogeneticislands and to an increase of the number of the exocenogenetic ones. Very frequent fires may form forest paraclimax with awell-defined vertical structure and an almost complete absence of the horizontal structure. Under the impact of annual fires rapiddisafforestation occurs because of trees pyrotraumas. At the same time the degree of the horizontal patchiness increases due to the rhizomegrass overgrowth. The increase of fire frequency and the pyrogenic factor duration weakens and, finally, eliminates the endocenogeneticislands. Firstly, the number of exocenogenetic islands increases, then it drops to zero. The reason of it is the consecutive pyrogenicreduction of the phytocenosis biodiversity and ontogenesis conditions unification.

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