The paper analyses the results of field investigations in the 'Yuntolovskiy' nature reserve situated in the Lakhtinskaya lowlands within the limits of St. Petersburg. The modern state of the marsh landscapes is very unstable here. In spite of drainage and other human activities over the last 70-80 years there is still a general tendency towards boggy conditions. In the next few decades a number of forest communities which have come into being largely as a result of drainage will revert to bogs; against this background the pine is also being displaced by the spruce. Spontaneous landscape dynamics can have a serious effect on the preservation of the unique biodiversity of an area.