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Abstract
Introduction
The impact of forest fires on pine stands health and the rate of post-fire tree mortality depends on a number of factors. They are a type of fire, its intensity and duration, a predominant type of damage, and characteristics of the damaged stand. The secondary post-fire factors include trees infestation by beetles and diseases, health condition of the stand before the fire and the level of its weakening as well as impact of droughts.
Materials and Methods
Data from 58 sample plots (SP) were used to assess post-fire tree mortality. The stands on SP varied in age (43–95 years), their mensuration characteristics (diameter: 17–51 cm, height: 18.0–25.7 m, relative density of stocking: 0.5–1.0), fire damage intensity (average bark char height ranged from 0.4 to 4.9 m), a form of fire (running and slow surface fires), a year (2005–2018) and season of fire. SP were laid out mainly in the forests of the Forest-Steppe part of Kharkiv Region, grown in the fresh infertile and fairly infertile pine sites. A one-year post-fire period was used in the study.
To assess the fire hazard level, a complex index of fire hazard by Nesterov (Nesterov 1945) was calculated. The correlation, regression and multiple regression analyses were performed according to generally accepted methods (Atramentova & Utevska 2007).
Results
Fires are one of the most dangerous destabilizing factors for forests. On determining specific features of the post-pyrogenic development of pine forests, it is possible to mitigate negative consequences caused by forest fires and assist in the management of forests damaged by surface fires. It was revealed that the health condition of the stands depended on the level of the fire hazard at the time of the fire, and worsened as fire hazard increased (r = 0.71; p = 0.05). With a high fire hazard (estimated by Nesterov methodology), the fire intensity (intensity of heat release during a fire) and amount of burnt forest fuel also increase. It has been established that with the fire danger score from 1,500 to 3,000, the categories of stand health varied from II,0 to III,9; the mortality rate reached 20%. With a further increase in the fire hazard up to more than 5,000, the health condition index of the damaged stands a year after the fire was III,5–V,0. It was found that the flame height and the bark char height on the trunks do not depend reliably on the fire hazard level by weather conditions. The approaches to predicting post-fire mortality at the stand level are improved taking into account fire hazard characteristics during a fire season, as well as the level of damage to trees in a pine stand and its morphometric characteristics.
Conclusions
A correlation between surface fire consequences (a rate of post-fire trees mortality) and the fire hazard index by weather conditions was found. It is explained by the fact that the rate of drying of surface forest fuel depends on the fire hazard index by Nesterov. With higher values of fire hazard, the intensity of heat release during a forest fire and the level of forest litter burning increase. Thus, for complex index of fire hazard from 1,500 to 3,000, a health condition index varied from II,0 to III,9. At that, the tree mortality reached 20% of total number of trees. With a further increase in the index of fire danger up to more than 5,000 points, the health condition index of pine stands a year after the fire varied from III,5 to V,0.
It was found that the height of the bark char on the tree trunks does not significantly depend on the fire danger level.
2 Figs., 2 Tables, 16 Refs.
Key words: postfire mortality models, fire resistance, fire hazard, consequences of forest fires.