Bird breeding colonies are known to influence the habitats and phytocoenoses they occupy in different ways. Most papers concern long-lasting colonies, in which floristic composition of phytocoenoses have been already altered largely. This research was aimed to estimate the changes in the floristic composition of the forest phytocoenosis after three years of the existence of the Grey Heron (Ardea cinerea L.) breeding colony as well as to examine the influence of nutrient enrichment on the size and shape of leaves and inflorescences of three plant species Rumex acetosella L., Anthoxanthum odoratum L. and Moehringia trinervia (L.) Clairv., which have different habitat requirements. The number of vascular plant species within the colony area (0.4 ha) increased almost double and appearance of new, mostly nitrophilous, taxa like Sambucus racemosa L., Galeopsis pubescens Besser and Stellaria media (L.) Vill. were observed. The vertical structure of phytocoenosis has also changed. The undergrowth appeared and dense moss layer was mostly eliminated. In case of all investigated species, the enlargement of leaf surface was found. The greatest relative increase concerned leaf width of Moehringia trinervia and Rumex acetosella – 67 and 73%, respectively. The leaf blade of Anthoxanthum odoratum increased largely in length (99%). The enlargement of the surface of assimilative apparatus was not only due to the increase of linear dimensions, but also to rounding of the leaf blade. This was not observed in case of Rumex acetosella which is characterized by elongated, lanceolate leaf blade. Differences observed among the species are probably due to differentiated leaf blade structure. Only Moehringia trinervia can be found in well-established Grey Heron colonies, while Rumex acetosella and Anthoxanthum odoratum are known to be suppressed by heavy input of nitrogen fertilizer. Thus, it seems that after few years of benefits at least Rumex acetosella and Anthoxanthum odoratum withdraw from the area altered by Grey Heron colony, because the change in root absorption and capacity in highly fertile habitat leads to toxic accumulation of nutrients.
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