Fire and stand dynamics in different forest types of the West Khentey Mountains, Mongolia

Abstract

This research focused on the reconstruction of historical fire regimes, structure and dynamic patterns of different types of forests, specifically to examine present tree species composition, regeneration patterns, size distribution, radial growth patterns of trees, composition, and structure of downed logs within different types of forests in the north-west Khentey Mountains, Mongolia. The Pinus sibirica-Abies sibirica (PiSi-AbSi) forests and Picea obovata-Abies sibirica (PcOb-AbSi) dark taiga forests consisted of more diverse tree species and had different size structures leading to a higher stand complexity than the Larix sibirica-Betula platyphylla (LaSi-BePl) and Pinus sylvestris-Larix sibirica (PiSy-LaSi) light taiga forests. Overall, the main tree species which occurred in the research area were present in all forest types, but they differed by the proportion of their Importance Value (IV).The PiSi-AbSi forest had a high number of P. sibirica and A. sibirica saplings, indicating that a future canopy could remain dominated by these species. Other tree species, such as P. obovata and B. platyphylla, which constituted a rather small part of the canopy, were also underrepresented in the sapling composition. In PcOb-AbSi forest, P. obovata made up only 5.8% of the saplings, while A. sibirica saplings made up the highest percentage (53.3%) of the total saplings, followed by P. sibirica saplings (31.7%). Also, the percentage of B. platyphylla saplings was higher (9.2%) than P. obovata. In LaSi-BePl forests, L. sibirica made up 54.3% of the canopy trees, however in the sapling tally B. platyphylla was dominant (77.6%); also it was well distributed in the different height classes of saplings. L. sibirica saplings made up only 22.4% of the total saplings. PiSy-LaSi forests consist of a high number of P. sylvestris saplings, indicating that a future canopy could remain dominated by the species. However, B. platyphylla sapling numbers were slightly higher (15.4%) than L. sibirica (11.5%).The continuous and stable tree recruitment of the different tree species in the P. sibirica dominated forest suggested that this forest did not experience any large fires for more than 200 years. Due to the openness of the canopy, light penetrates and warms the forest floor, enhancing the conditions for species diversity in the area. As a response to this fact, the understory growth is stimulated and species with the ability to sprout start to colonize the area. The tree recruitment pattern in PcOb-AbSi forest confirms the explained reaction of vegetation. In LaSi-BePl forests, B. platyphylla is abundant only during the first seven to twelve decades. After its decline P. sylvestris and L. sibirica became the main species. The tree recruitment wave indicates that this forest experienced major disturbances (e.g., fire) every 40-60 years. In PiSy-LaSi forest, the tree recruitment was more or less restricted to P. sylvestris, which may be explained by dry site conditions. The tree recruitment pulse occurred every 20-40 years in this forest.The tree-ring chronologies were developed for each trees species in order to examine growth patterns. A strong radial growth decrease of P. sylvestris trees in single years greatly fits to the fire event years. Such years are; 1790, 1822, 1866, 1905, 1929, 1948, 1954, 1972, 1981, 1996 and 2009.The climate response of each population (P. sibirica, A. sibirica, P. obovata, L. sibirica and P. sylvestris trees) was determined by correlating the residual chronology against a 17-month climate window from prior April to August of the current growth year. Correlation analysis for the five tree species indicated very different responses to climatic variables likely related to differences in elevation, topography, and microclimate at the study sites. On the other hand a few climatic variables were found to be correlated with the growth of tree species (P. sibirica, A. sibirica, and P. obovata), which are found at the wetter sites perhaps due to a stronger relative influence of non-climatic factors, leading to more individualistic tree growth patterns. P. sylvestris showed high responses to precipitation, snow-melt water and drought indices. With regard to the analysis of tree growth in response to climate factors, L. sibirica and P. sylvestris could be suitable species for climate studies in this area.Mean fire intervals were clearly different between the dark and light taiga forest types. The mean fire interval of 46 years (range 17.4-62 years) in PiSi-AbSi forests, and 39 years (range 15.8-68.5 years) in PcOb-AbSi dark taiga forests indicate that fire-return intervals are longer than those in LaSi-BePl and PiSy-LaSi light taiga forests. The seasonality of fires was, for most periods, constant and did not change within each forest type.