## 1 Introduction  
Due to the wide geographical range and small habitat requirements the Scots pine, _Pinus sylvestris_ L, is one of the most important forest species in Europe and occurs in many different types of forest communities [1]. The natural Scots pine regeneration depends on many factors, starting with the abundance of seeds in the sowing year and choice of soil preparation method after clear-cutting [2]. Seed germination in the first growth phase depends strongly on the depth and variable humidity of the humus layer, until seedling radiics reach more water-stable mineral soil [3, 4]. Moreover, allelocal chemicals released from coexisting plant species affect the germination and development of young seedlings [5]. Further successful pine recruitment depends on the course of competition for resources with shrubs and trees [6] and the presence of natural enemies [7].  
The introduction of invasive tree species reduced the extent of natural Scots pine renewal [8, 9]. One of the most important invader plants in European forests is black cherry, _Prunus serotina_ [PERSON]. This alien species grows in a wide range of habitat conditions, has a high ability of generative reproduction and vegetative sprouting, its numerous seeds are dispersed by birds and mammals and it establishes a long-living shade-tolerant seedling bank [10, 11, 12].  
In forests dominated by _P. serotina_, alterations of the biotic and abiotic ecosystem properties are observed. The dense shrub of black cherry with its litter layer negatively affects the understory species through mechanical impediment and shading [13, 14, 15]. In the case of Scots pine, it is assumed that light is the factor that most limits sagling growth [16]. Previous research indicates that _P. serotonin_ could also reduce the growth of _P. sylvestris_ seedlings due to the release of phytotoxic compounds during leaf decomposition [17]. Such allebotatic interactions appear to be more intense in soils with low resource availability [18]. At the same time the fast decay of _P. serotonin_ leaf litter and humus formation accelerate nutrient circulation [19; 20; 21; 22] and soil microbial activity [23]. Moreover, self-sowing of black cherry results in increased competition with pine saplings in the regeneration layer [9].  
It cannot be ruled out that the emergence of new _P. sylvestris_ generations may be also affected by exudates or volatiles released directly from fresh _P. serotonin_ leaves, which remain on the plant. [PERSON] and [PERSON] [24] showed in ex situ experiments that water extracts from fresh black cherry leaves inhibit the germination process of Scots pine. No research studies have examined the influence of _P. serotonin_ volatile organic compounds (VOCs) on natural pine regeneration. We performed an experiment, under laboratory conditions, to evaluate the phytotoxic effects of _P. serotonin_ volatiles released from fresh leaves on _P. sylvestris_ seeds and seedlings, and the seasonal variation of these effects. We tested the following hypotheses: (1) _P. serotonin_ volatiles affect germination and seedling growth of _P. sylvestris_, (2) radicles and stems of Scots pine seedlings differ in the sensitivity to black cherry volatiles and (3) volatile compound production varies during the growing season.