Scots pine radial growth response to surface fire in young stands in Zhytomyr Polissia

Abstract

Introduction

Forest fires pose a critical environmental threat, particularly to Scots pine (Pinus sylvestris L.) stands in the Polissia region of Ukraine. Since 2022, full-scale military aggression has drastically increased both the frequency of forest fires and the total burned area. Surface fires cause cambium damage, reduce radial growth, and may lead to tree mortality. Dendrochronological analysis provides an effective tool for assessing the impact of fire on tree vitality. This study aimed to evaluate the response of radial growth in young Scots pine stands to a spring surface fire in Zhytomyr Polissia under current climatic conditions and military-induced pressures.

Materials and Methods

The study was conducted in a 30-year-old Scots pine stand under fresh relatively poor site conditions (B?) in Korostyshiv forestry, Zhytomyr region. A surface fire occurred in April 2022. Two temporary sample plots were established: an unburned control plot (TPP-1) and a burned plot (TPP-2). Mean tree height was 15.8 m on the burned plot and 14.7 m on the control plot, while mean diameter was 18.3 cm and 17.9 cm, respectively. Relative density of stocking was 0.3 on the burned plot following fire-induced mortality and sanitary felling, and 0.7 on the control plot. Fifteen increment cores per plot were collected at breast height (1.3 m) in September 2024 using a Pressler increment borer. Earlywood, latewood, and annual ring widths were measured with a precision of 0.001 mm. Cross-dating and chronology development followed standard dendrochronological procedures. Meteorological data from the Zhytomyr weather station for the period 1995–2024 were analysed using the hydrological year approach. Statistical analysis included descriptive statistics and polynomial regression analysis.

Results

In the fire year (2022), total radial growth on the burned plot decreased by 34% compared to the control plot (0.96 mm vs. 1.46 mm). Latewood width decreased by 28% (0.25 mm versus 0.35 mm), while earlywood width decreased by 27% (0.83 mm versus 1.13 mm). Hydrothermal conditions in 2022 were close to optimal, with precipitation exceeding the long-term average by 7% and air temperature by 0.6°C, which partially mitigated immediate fire stress. However, the severe drought of 2023 (precipitation deficit of 32% and air temperature 14% above the long-term average) prolonged the recovery period. By 2024, earlywood width on the burned plot exceeded that of the control plot by 23% (1.02 mm versus 0.78 mm). This increase was associated with a 57% reduction in stand density following sanitary fellings (from 0.7 to 0.3). Post-fire variability in radial growth increased substantially: earlywood variance on the burned plot reached 0.88 in 2024 compared to 0.22 on the control plot. In 1% of sampled trees, the annual ring for 2022 was absent. Regression analysis revealed significant relationships between radial increment and health condition (R² = 0.44, ? = 0.66), as well as stem diameter (R² = 0.69, ? = 0.83). Trees with larger diameters demonstrated greater fire resistance due to thicker bark and improved cambial protection.

Conclusions

The surface fire of April 2022 caused substantial physiological stress in the 30-year-old Scots pine stand, resulting in a 34% reduction in total radial growth during the fire year. Favourable moisture conditions in 2022 reduced the immediate impact of fire stress; however, the severe drought of 2023 prolonged the recovery process. By the second year after the fire, surviving trees exhibited increased earlywood growth, exceeding control values by 23%, due to reduced competition following a 57% reduction in stand density. Post-fire variability in annual ring width increased markedly, indicating considerable individual variation in fire response. Stem diameter and health condition were identified as statistically significant predictors of post-fire radial growth. Trees with larger diameters demonstrated greater resilience to fire. Overall, surviving trees on the burned plot successfully restored radial growth by the second year following the fire.

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