The study presents the growth response of Scots pine to long-term the application of potato industry sewage. The research was carried out in a fresh pine forest in northern Poland, where organic wastewater from the Potato Industry Plant in Iława (FWTP) was sprinkled for over 25 years. Our research sought to determine (i) the direction, extent, and duration of changes in the trees’ growth caused by the application of the fertiliser and (ii) the influence of climatic conditions on radial growth in those trees.

The extent of and changes in the growth were estimated by changes in annual ring widths and earlywood and latewood widths using dendrochronological methods. The research was carried out in different-aged pine stands located within the FWTP site and control stands located outside that area. Core samples were collected from 15 trees in each stand. The last sewage application took place ten years before core sampling.

We found a two-way impact of potato sewage on radial growth of Scots pine, with a stimulatory effect (27–30%) in the first decade of fertiliser application followed in the subsequent years by a strong reduction in growth (30–45%, depending on the age of the trees). Over the entire period during which the fertiliser was applied, changes occurred in the structure of the wood as manifested in the increased share of earlywood. The sprinkler application of potato starch wastewater and the accompanying irrigation caused a shift in dendroclimatic relationships of Scots pine in comparison to the pine from control plots. Surface irrigation and the resulting changes in water balance reduced the drought susceptibility of the pines under study. At the same time, trees weakened by the excessive concentration of toxic nitrates became more sensitive to thermal conditions in winter.