Controls of post-fire tree recruitment in great Xing'an mountains in Heilongjiang province

Fire disturbance is one of the key processes affecting forest succession in boreal forest ecosystems. Variations in disturbance characteristics have an important effect on ecosystem properties and processes. Due to the diversity of topography, fuel heterogeneity and micro-climatic conditions during a fire, the intensity of fires is quite variable, producing landscapes in different mosaic patterns. Although the resulting heterogeneity of fire characteristics (e. g., burn severity) has been widely recognized as an important driver of boreal forest regeneration, its effects on early post-fire tree recruitment has not been thoroughly studied in boreal forests of the Great Xing'an Mountains. Early post-fire tree recruitment can reflect future forest structures and succession pathways. A clear understanding of the controls and mechanisms of post-fire tree recruitment is therefore necessary for us to predict future forest dynamics, devise reasonable forest management practices, and properly maintain natural landscape structures. In addition to fire characteristics, environmental conditions and pre-fire vegetation can also have important effects on post-fire tree recruitment. Environmental conditions such as elevation, slope, aspect, soil can affect post-fire seed germination. Pre-fire vegetation can affect seed banks and the distribution of post-fire seed sources. Post-fire tree recruitment in boreal forest ecosystems, as a whole, is influenced by multiple factors, including elevation, slope, aspect, soil depth, pre-fire forest type, pre-fire stand age, burn severity, and burned patch size. The objective of this study is to discover which factors are the most important controls on post-fire conifer and broadleaf tree recruitment and to predict what succession pathways will take place. We selected Huzhong and Xinlin Forestry Bureaus as study area to carry out field survey. The forest type of the study area is typical in Great Xing'an Mountains with major tree species being white birch (Betula platyghylla) and Dahuirian larch (Larix gmelini). Post-fire forest succession in this study area can be divided into two pathways, which are self- replacement (i.e., the composition of immediately re-established post-fire tree species is the same as pre-fire canopy dominated ones, and continues as canopy dominant) and relay succession (i.e., different tree species successively assume post-fire canopy dominance, which is firstly dominated with early-successional tree species and gradually by late successional ones). This study analyzed relative influences of various controls (environmental conditions, pre-fire vegetation, and fire disturbance characteristics) on post-fire tree recruitment and their marginal effects using a boosted regression tree method and data collected from 55 recently burned sites. Our results showed that 1) elevation was the most important factor for conifer tree recruitment; 2) slope was the most important factor for broadleaf tree recruitment; 3) time- since-last-fire was the most important control of the post-fire conifer seedling proportion, and forest type was the second; 4) Post-fire conifer seedling proportion was higher in plots with conifer forest as pre-fire forest type than that with mixed or broadleaf forest; 5) conifer tree regeneration after intermediate-severity fires was better than that after low-and high-severity fires. Our study suggested that self-replacement often occurred in the sites with single-species forest as the pre-fire forest type; while relay succession often occurred in the sites with mixed forest.