Forest site conditions and the threat for insect outbreaks in the Scots pine stands of Polissya

Scots pine (Pinus sylvestris L.) is the most spread forest tree species in Polissya and the most damaged by foliage browsing and stem insects. The aim of this study was to reveal the changes in the distribution of different forest site conditions for 2010–2019 in Scots pine forests of the selected forestry enterprises of Polissya and possible consequences for the spread of foliage browsing insects. Database of Production Association ‘Ukrderzhlisproekt’ (by 2010 and 2019) was analysed for five State Forest Enterprises (FE) that are located in the Central (Zhytomyr Region) and Western (Rivne and Volyn Regions) Polissya, where the large scale outbreaks of stem pests were registered last decade. The types of forest site conditions were designated in accordance with the Ukrainian typology. Distribution of the forest area by trophotops and hygrotops was evaluated for the forest-covered area, for Scots pine stands, and for pure Scots pine stands in 2010 and in 2019. For 2010–2019, the area of all Scots pine forests and its proportion in the forest-covered area has significantly decreased in the most of analysed forest enterprises. The change for 2010–2019 in the distribution both by the trophotops and by hygrotops of the entire forest-covered area, the area of all Scots pine forests and pure pine forests is not statistically significant. However, in assessment year 2019, the proportion of stands in the moist types of forest site conditions slightly increased. It may be the result of the stands’ mortality in the driest sites after an outbreak of bark beetles. A greater decrease in the proportion of Scots pine forest area in the dry poor, fresh poor, and dry relatively poor forest site conditions was found in the western direction. Therefore, the noted decrease in the foci areas of foliage browsing insects in Polissya in 2010–2012 compared to the 2000–2002 could be associated with a decrease in the most preferred stands, namely the poorest and driest types of forest site conditions.