This paper presents the results of studies carried out on Ptelea trifoliata populations in the Wyszków Forest District in 1998 and 2013. P. trifoliata is a native species of North America (United States of America, northern part of Canada) and has a wide ecological range. However, it prefers fertile, wet soils and moderate light. In Europe, it is planted for its decorative value and is mainly found in synanthropic habitats (parks, graveyards, roadsides, fortifications) in Poland. The station of P. trifoliata is situated in the oak-hornbeam forest, Tilio-Carpinetum typicum, with a significant fraction of the stand consisting of Pinus sylvestris. Hop trees occur mainly along forest section lines and are rarely found inside the sections. In the last 15 years, an increase in the number and size of P. trifoliata clusters has been observed. The species spreads along forest section lines, which form a convenient migration route by creating favourable conditions for the germination and growth of seedlings (good access to light, fragments of bare soil). The presence of new individuals far from the pre-existing clusters indicates that the generative way of propagation dominates. Biometric measures indicate significant differences in length and width of whole leaves as well as leaflets, with leaves and leaflets of vegetative specimens significantly larger than generative ones.
As a consequence of the high rate of P. trifoliata expansion along forest section lines and occurrence of single specimens inside the forest sections, we assume this species to be potentially invasive.
Linnaea borealis, the twinflower, is considered a critically endangered species in the Południowopodlaska Lowland. The disappearance of the twinflower is mainly caused by habitat changes resulting from forest management, but also light deficiency due to the increase in canopy cover and growth of the shrub layer (processes of succession).
The aim of the paper is to present the actual distribution and phytosociological characteristics of L. borealis in the northern part of the Południowopodlaska Lowland. In this region, only three out of ten known areas of occurrence (Werchliś, Serpelice, Grala-Dąbrowizna) have endured to the present day. The twinflower occurs in pine forest, Peucedano-Pinetum, and mixed forest, Querco roboris-Pinetum, communities.
A decrease in the ground cover of twinflower populations was observed in all existing twinflower locations. Although the area covered by the Linnaea borealis population in Werchliś increased tenfold during the last 20 years (from about 200 m2 in 1993 to 1970 m2 in 2013), its cover-abundance according to the Braun-Blanquet scale decreased from 5 to 3. The increase in the area covered by the twinflower population together with the accompanying decrease in cover-abundance is probably not a manifestation of species dynamics but rather a response to disadvantageous environmental changes (growth of canopy cover and shrub layer). Progressive light reduction can initiate the process of fragmentation of a population.
A decrease in the cover-abundance of the L. borealis population (from 4 to 2) was also observed in Serpelice. Its area was reduced to half of its original size due to anthropogenic destruction of this location. In the third region, Grala-Dąbrowizna, also the negative effects of competitive species (e.g. Vaccinium myrtillus, V. vitis-idaea) are considered.