Phytosociology—A Useful Tool for the Assessment of Past and Future Human Impacts on Plants and Forest Ecosystems

The aim of this research was to answer the question of whether the floristic-phytosociological characteristics of the forest communities described in the mid-twentieth century in the Zielonka Experimental Forest (Poland) have changed, and if so, what were the reasons for these changes and what were their environmental and economic consequences. The basis of the research was a comparison of plant community descriptions drawn up in 1957-1961 to descriptions of the states of those communities in 2010-2017. The study showed a significant decrease in the number of plant species. The reasons for the disappearance of some plant communities are seen in the natural or ar-tificial changes of pine in sandy habitats, which evolved towards richer communities. The disappearance of xero-thermophile oak woods had two main causes: a natural cause, which was a consequence of regeneration towards oak-hornbeam woods after the anthropogenic phenomena responsible for their management, and anthropogenic disturbance related to the planting of Fagus sylvatica trees or Prunus serotina. The second case is described more broadly and considers both the positive and negative roles of changing the floristic compositions of forest ecosystems.

these changes can be assessed in different ways. One method that documents the current floristic composition is a phytosociological plot (relevé), a basic element of description in the field called phytosociology. Phytosociology is a botanical science as well as being part of the discipline of ecology. It describes vegetation and individual communities [1]. The essence of phytosociological relevés is to compile a list of plants in a given area, including the degree of coverage for each species expressed either as a percentage [2] or by using the scale provided by Braun-Blanquet [3]. The value of phytosociological data grows over time provided that the locations of the plants are well documented to allow repeated studies in the same place. The juxtaposition of relevés taken at different intervals allows the capture of vegetation changes that are taking place and thus aids the interpreting these changes. This is also the purpose of this article, which is based on selected relevés describing the forest ecosystem that were conducted by Nowaczyk [4]. The directions and effects of the changes were investigated by considering the natural and economic consequences.
There are many methods for studying plant cover transformation. They can be carried out at the species level and at the level of phytocoenoses [5]. However, it seems that a common feature should be observations carried out in well-defined research plots [6]. These types of observations have been carried out in different parts of the world and in different types of plant communities. In Canada, the results of a 40-year study of the alpine floor tundra were published by Danby et al. [7], pointing to an increase in the number of species in the studied communities and linking this fact to global warming. Holzinger et al. [8], Kullman [9], Vittoz et al. [10] and Odland et al. [11], obtained similar results for alpine and subarctic communities. Becker et al. [12] presented the results from 40 years of observations of vegetation changes in former coppice forests in the central part of Germany and indicated that the reasons for the changes were associated with the manner of development as well as with eutrophication with nitrogen compounds.
Despite the clear advantages of permanent experimental plots, their main disadvantages are their limited temporal and spatial extents [13]. Most field observations and experiments are short-term and yield quite limited knowledge regarding the dynamics of ecosystems operating under the influence of long-term factors [13]. The period of this type of research usually does not exceed several decades, with a few exceptions, e.g. [14]. Therefore, as an alternative to long-term observations of permanent research plots, Hedl et al. [13] reported retrospective studies. The concept underlying this type of research is not new. It was described in 1929 by Conard and Galligar [15]. In turn, the disadvantages of reproducing vegetation descriptions in a given area are mainly associated with possible shifts in the compared areas, as well as the subjectivity of the observer [16]. These two sources of undesirable variation between the first study and repeated studies cannot be completely removed, although their impacts can be estimated [17] [18].

Material and Methods
The basis of this study was a comparison of plant community descriptions made by Nowaczyk [4] in 1957-1961 with the current states of those plant communities.
The reconstruction of research plots consisted of converting a copy of the au- The descriptions of the phytosociological relevés, similar to those prepared by Nowaczyk, were prepared in accordance with the Braun-Blanquet method, which is commonly used in this respect.
All of the initial phytosociological plots described by Nowaczyk, after comparing them with the present state, were grouped into pairs, which enabled capture of the differences between the individual research areas. These differences were determined by compiling, for each relevé, a list of species that had disappeared in a given place, a list of species that had entered over the past several decades, and a list of species that had been recorded by both Nowaczyk and that are currently present.

Results
To highlight past and future human impacts on plants and forest ecosystems,

Discussion
The changes shown in Figure

Conclusion
Comparisons of phytosociological relevés conducted at different times with long intervals between them have significant scientific and practical meaning. From a scientific point of view, they provide information on the dynamics of plant cover as related to human activity, climate change and all other factors affecting plant development. From a nature protection point of view, phytosociological data provide information on the dynamics of endangered and invasive species. From an economic point of view, the analysis of phytosociological relevés allows assessments of the effects of human activities. The example presented in this paper, which shows the effects of forest management on floristic changes over 60 years, can be treated as a case study, but considering how data are used and interpreted, this example is universal. For this example, one mistake made in the past, as part of forest management, has led to the disappearance of over 80% of the plant species in a given research area. The consequences of the disappearance of each species could be considered separately both in terms of nature as well as economically. In this context, phytosociology provides the opportunity to link to a number of additional studies, e.g., entomological or mycological, as well as social aspects in the field of human usage of natural resources. This shows that phytosociology is a tool that still has great and untapped potential.