Changes in understorey vegetation of Scots pine stands under the decreased impact of acidifying and eutrophying pollutants

Abstract

Understorey vegetation of Scots pine (Pinus sylvetris L.) forests subjected to intense air pollution stress in the impact zone of one of the largest air pollution sources in central Lithuania – the nitrogen fertilizer plant Achema – were studied in 31 sample plots first in 1988 and again in 2004. A recovery of the vegetation was observed since the first sample time and interpreted as due to a distinct reduction of emission of nitrogen and sulphur oxides. Study stands were 85-100 years old with vegetation assigned to the Vaccinio-myrtillosa site type. Species which frequency changed little (Vaccinium myrtillus, Pteridium aquilinum) and increased in frequency (Trientalis europaea, Luzula pilosa, Vaccinium vitis-idaea, Melampyrum pratense) were typical of site type species. During the study period nitrophilous species with indicator values for nitrogen >= 6 decreased in frequency. In 1988 nitrophilous plants comprised 58% of all herbaceous cover, while in 2004 only 21%. According to the unweighted Ellenberg indicator values the stands had become darker and less acidic. A significant decrease in nitrogen indicators was observed in 2004. The process of ground vegetation recovery was documented in terms of an increase in keystone species (Vaccinium myrtillus, V.vitis-idaea) and a reduction of nitrophilous species (e.c. Rubus idaeus).