Ecological Questions 4/2004: 105-114 Early medieval fortified settlements at Kaldus and Piutowo (Chehnno land, northern Poland) — places of plant invasion and refuges Dariusz Kamiriski Department of Plant Taxonomy and Geography, Institute of Ecology and Environment Protection, Nicolaus Copernicus University, Gagarina 9, 87-100 Torun, Poland, e-mail: daro@biol.uni.t0run.pl Abstract. This study presents the results of research conducted at two early-medieval fortified settlements situated on the border of Wysoczyzna Chehninska (Cheiminska Upland) and neighbouring areas. Phytosociological surveys were performed on site and compared with literature data concerning analogous plant communities from less synanthropised areas. The influence of ancient colonisation on expansion and survival of steppe relicts was underscored. The Kaldus settlement was presented as a place of refuge for plants connected with ancient colonisation, and a place to which some of the neighbouring steppe relics arrived. The Piutowo settlement was included to demonstrate the natural process of forest regeneration in anthropogenic habitats. Keywords: medieval settlements, relicts, steppe species, anthropopressure. 1. Introduction The settlements at Kaldus and Piutowo are situ- ated on the main route of plant migration in Po- land and within the largest concentration of xerothermic vegetation in the area of the lower Wisia River. This community has long been influ- enced by Man, who contributed to the endurance and secondary expansion of steppe species. First steppe plants arrived in the area of Central and Northem Poland already in the first periods after the glacier recession. They survived subsequent changes of weather and climate as well as expan- sion of forests, receding to relict stands - under- mined river valley-sides - which guaranteed good lighting and thermal conditions. Intensified settle- ment and clearing of forests growing on the val- ley-sides, which were then tumed into pastures, contributed to improvement of the conditions for development of steppe plants and their renewed migration. In the area of Kaidus they did not only enter the natural habitats, but also settled in the enviromnent exposed to strong pressure of Man. It is justified by a small patch of Potentillo- Stipetum capillatae Libb. 1933 em. Krausch 1960 persisting on the rampart of the settlement. Its sta- bility is due to buming and tending of the greens, a practice so frequent in the past (Ceynowa 1971). Extensive use is a prerequisite for the sustenance of grass communities. When tending discontinues forests may be regenerated. As an example, the area of the settlement at Piutowo is a woodland. Long-lasting utilisation of medieval cities resulted in formation of specific soils, characterised by a higher content of phosphorus and nitrogen (Cook, Heizer 1965; Ceynowa 1968). Such settlements are