Environmental Consequences of Landscape Fires in Trans-Baikal Forests

Landscape fires play the leading role in the modern anthropogenic dynamics of forest ecosystems in south-eastern Siberia. Forest pyrologists consider them the most destructive manifestation of fires - an environmental factor that acts across wide areas to change the state of landscapes. Due to the insufficient knowledge of the nature of pyrogenic forest ecosystems, especially its dynamic aspects in the forestforming process, obtaining data on the pyrogenic dynamics of forest ecosystems belonging to various ranks is of broad scientific interest. A new aspect of this study is assessing ecosystem transformations under various pyrological regimes that determine the pyrogenic successions of plant communities, the hydrothermal regime of soils, a post-fire runoff, etc. The history of catastrophic landscape fires in the Baikal and Trans-Baikal regions began after the construction of the Trans-Siberian Railway. At that time, a long strip of coastal hemiboreal small-leaved deciduous forests formed along the southern shore of Lake Baikal, in the area of indigenous dark coniferous taiga, as a result of the “selective” impact of a lingering series of fires that followed intense forest harvesting. It happened regardless of the average productivity of the pyrogenic stands being 30-40% lower vs. the potential level. The landscape fires of the 1930-1950s that arose during industrial development, especially in the areas of intense forest harvesting, caused heavy damage to forest ecosystems throughout Transbaikalia. Now, due to a regress in the Russian forest management, landscape fires have turned into an archaic alternative to the ecological progress as of the 1970s and 1980s. Therefore, the relevant goal of this article is to consider the transformation of forest ecosystems after landscape fires. The emergence and spread of landscape fires were studied during two fireintensive seasons through the air monitoring of the Trans-Baikal territory. Geographic coordinates: 49-57° N, 99-122° E. Regular pyrological studies of the seasonal dynamics of the forest fire danger implied the use of the method suggested by N.P. Kurbatsky (1970) [9]. The permanent trial plots were located on the Malkhansky and Khamar-Daban Mountain Ranges. The route studies covered the Selenga Highlands, as well as the basins of the Rivers Barguzin, Turka, Upper Angara, Muya, and Chara. Fig. 1 shows the schematic map of the sites. The pyrological regimes of vegetation complexes were established based on research data (see the table). The environmental consequences of landscape fires were studied in the forests of South-Eastern Baikal and Central Transbaikalia. The closest observations of the dynamics of burned (including through prescribed burning) and control stands were performed on permanent sample plots. There the researchers also ensured regular accounting of litterfall using special 1x1m collectors. The grain composition and physical and mechanical properties of the soils were determined via the common methods [21]. The pyrogenic variations of the forest-forming process after landscape fires were observed in different natural districts (Fig. 2). Pyrological regimes differ in the altitude-belt complexes (ABC) of vegetation and correspond to a wide range of atmospheric moistening. Forest-steppe vegetation prevails in the valleys of large rivers and low mountains in the conditions of extremely insufficient moistening. This vegetation is exposed to an extreme fire regime in the spring and in the early summer. The Scots pine (Pinus sylvestris L.) prevails in subtaiga-forest-steppe forests. There, an intense regime takes place even in a standard weather situation. The fire maturation of forests in the light-coniferous taiga belt with predominant larches (Larix sibirica Ledeb., Larix gmelini Rupr.) is approximately two times longer vs. forest-steppe and hemiboreal forests. Accordingly, the pyrological regimes vary from intense to moderate. Dark coniferous forests comprising the Siberian pine (Pinus sibirica Du Tour.), fir (Abies sibirica Ledeb.) and spruce (Picea obovata Ledeb.) are confined to wet belts with a moderate pyrological regime. A safe regime prevails in the subalpine ABC. The general fire danger is determined by ground fires, after which most burned forests retain their vitality at the level of their fire resistance, as well as depending on the actual intensity of fire in this area. Fig. 3 illustrates the typical dynamics of Scots pine stands established based on forest management data. In fact, diverse structural transformations of plant communities are observed after landscape fires - from a moderate fire damage to heavy thinning caused by an intense fire, and even complete death after head fires with probable local deforestation. Fig. 4 shows the comparative dynamics of the population of trees in normal and pyrogenic larch forests. Larch trees have an advantageous position vs. pines, as they are better protected from thermal damage by their thick bark. Besides, the crowns of larch trees are capable of regenerating needles and shoots damaged by fire. The duration of recovery to the pre-fire basal area increases exponentially with age, since forest stands inevitably face a growth decline as they become old. The pyrogenic digression of larch forests in permafrost areas with a burned peat horizon was followed by a tree fall, which was observed during the air monitoring of fires. Later thickets of birch and willow trees formed there. The fire damage to trees (trunks and crown), the ground cover, as well as the hydrothermal regime of soils are most comprehensively studied in the Scots pine forests of Central Transbaikalia where the ground cover and lower forest layers were explored not only at natural fire sites but also through prescribed burning. The integral picture of forest disturbance with landscape fires is complex and diverse. On mountain slopes, the strongest fire was observed when it was moving upwards. The speed and direction of the wind, as well as the daily temperature and humidity rhythm played a significant role in the spread of the fire line. Therefore, extensive landscape fires left behind a chaotic structure of damaged and dead stands. Later, a complex mosaic of uneven-aged stands modified by repeated fires formed during post-fire reforestation. After high-intensity fires, the forest plant communities were locally replaced by steppe ones, while reforestation took entire decades. Successions after fires were different. Extensive fires in the upper taiga ABC used to lower the upper boundary of the forest belt. Meanwhile, situations being destructive for the forest ecosystems of Transbaikalia regularly recurred, which was clearly seen based on the 200-year dynamics of the tree-ring width of the Scots pine in the River Turka valley. The tree diagram in Fig. 5 unambiguously shows a sharp decline in the width of tree rings at the turn of the 19^th-20^th centuries when the TransSiberian Railway was built. One may also identify drought periods in the 1930-1970s. The pyrogenic anomaly of 2015 that covered over 10% of the Baikal region turned out to be extremely dangerous. The article contains 5 Figures, 1 Table and 58 References. The Authors declare no conflict of interest.

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