Forests 2022, 13, 1575. https://doi.org/10.3390/f13101575 www.mdpi.com/journal/forests Article The Impact of Long-Term Fertilisation of Potato Starch Wastewater on the Growth of Scots Pines: A Retrospective Analysis Longina Chojnacka-Ożga 1, *, Jerzy Lendzion 2 and Wojciech Ożga 1 1 Department of Silviculture, Institute of Forest Sciences, Warsaw University of Life Sciences, 02-776 Warsaw, Poland 2 Iława Forest District, 14-200 Smolniki, Poland * Correspondence: longina_chojnacka_ozga@sggw.edu.pl; Tel.: +48-698-561-411 Abstract: The article discusses the impact of the application of potato starch wastewater as a ferti- liser on the growth responses of Scots pines at the Forest Wastewater Treatment Plant (FWTP) in Iława. More specifically, our study sought to determine the direction, extent, and duration of changes in the trees’ growth responses caused by the application of fertiliser and the influence of climatic conditions on secondary growth in the trees to which the fertiliser had been applied. As part of the study, the extent of and changes in the growth responses were determined with refer- ence to annual ring widths and earlywood and latewood widths using dendrochronological methods. The research was carried out in four pine stands: two stands of different ages (80 and 110 years) located within the FWTP site and two control stands of corresponding ages located outside that area. Core samples were collected from 12 trees in each stand. We found a two-way impact of potato starch wastewater on secondary growth in the trees under study, 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). The trends of these changes could be seen in both the overall annual ring widths and the widths of earlywood and latewood. The direction of the changes was the same for trees of different ages, although age was found to have affected the extent and duration of the stimulatory or inhibitory effect. Over the entire period during which the fertiliser was applied, changes occurred in the structure of the wood as mani- fested in the increased share of earlywood. The sprinkler application of potato starch wastewater and the accompanying irrigation caused a shift in dendroclimatic relationships in comparison to the control plots. Surface irrigation and the resulting changes in water balance reduced the drought susceptibility of the pines under study. At the same time, however, trees weakened by the exces- sive concentration of toxic nitrates became more sensitive to temperature conditions in winter. The results confirm that the implementation of substances containing significant amounts of organic nitrogen and potassium into forest ecosystems may impair the vigour of trees, reduce stand productivity, cause an imbalance in the ecosystem and may consequently lead to forest degrada- tion. Keywords: tree ring widths; organic sewage; forest experiment; Pinus sylvestris; dendrochronology; Poland 1. Introduction Biogenic disposal of wastewater sludge is a relatively common solution used in ag- riculture, and in some countries also in forestry [1–8]. The potential benefits of utilising sludge as a forest fertiliser have been the subject of many studies conducted in the con- text of both increasing forest productivity and identifying potential environmental threats [2,5,9–14]. Citation: Chojnacka-Ożga, L.; Lendzion, J.; Ożga, W. The Impact of Long-Term Fertilisation of Potato Starch Wastewater on the Growth of Scots Pines: A Retrospective Analysis. Forests 2022, 13, 1575. https://doi.org/10.3390/f13101575 Academic Editors: Angela Lo Monaco, Rodolfo Picchio and Mark Vanderwel Received: 28 August 2022 Accepted: 21 September 2022 Published: 26 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/).