Responses Of Mongolian Pine Forest To Wildland Fire In The Hulun Buir Sandland, Inner Mongolia, China

Fire, a ubiquitous important disturbance, is a crucial ecological factor in most terrestrial ecosystems. Mongolian pine (Pinus sylvestris L. var. mongolica Litv.), a dominant tree species in boreal forests of China, have been widely applied in combating desertification in the northern China and is renowned for being available in windbreak building, sand dune fixation, timber productivity, and tolerance of cold, drought, and innutrition. Natural Mongolian pine forests mainly sit on the southeast margin of the boreal forest in the Hulun Buir sandland, Inner Monglia, P. R. China, where belongs to one part of high risk area of wildland fire in China. Therefore, wildland fire is a frequent disturbance and a dynamic driving force in the forests.The seedling breeding and replantation of Mongolian pine have been intensively considered since 1950s. However, natural Mongolian pine forests have been least concerned, especially, its responses to wildland fire disturbance. In this study, responses of stand structures, spatial patterns, competitive intensity, and understory vegetation species of Mongolian pine forests to surface fire are explored in the last two wildland fire events in 1994 and 2006, respectively. Stumps, fallen logs, standing dead trees and living trees old than two years in six 1 hm2 plots, which had been attacked by the last two wildland fire events or with fire-free for 38 years, were fully mapped. Diameter at basal, diameter at breast height (DBH) and bark char height of trees were also measured and each individual was identified to species. Understory vegetation species were also surveyed by nine 0.1-hm2 multi-scale modified-Whittaker plots in the fire-mediated forests with different fire intervals. One-way nonparametric analysis of variance (ANOVA) (Kruskal-Wallis) and Duncan's multiple range tests were conducted to detect the differences of surface fire intensity, stand structure, competitive intensity, and understory vegetation species richness of stands surveyed. The univariate and bivariate spatial pattern analysis of pre- or post-fire stand and the components of the fire-mediated forests were explored using the pair correlation function g(r). The linear form of Ripley's K function, L(r), was also conducted to explore univariate spatial patterns of agents concerned once in a while and provide a reference to the pair correlation function, g(r). Species-area curve and Detrended Correspondence Analysis (DCA) were also conducted to explore the composition and variations of understory plant species in the fire-mediated forests.The results show that 1) surface fire intensities had some common characteristics significantly, and positively correlated with DBHs in three burned plots. Thus, the positive regressions could also be constructed between them. 2) Surface fire mainly excluded the small size classes and recruits, and got the kinds of tree species decreased. Thus, the tree densities decreased sharply to a lower order of magnitude postfire immediately and regained again for a great number of recruits occurred as time had lapsed since the stand burned. However, the tree densities in the fire-excluded stands had little changes post-mortality. 3) Mean DBHs of the burned plots increased multiplely, while its basal areas only decreased slightly postfire. Nevertheless, the basal areas of Mongolian pine were still predominant and its proportions in the corresponding plots increased slightly. 4) Spatial patterns of Mongolian pine forests had a trend from aggregation to randomness and were strengthened as time lapsed postfire. However, variations of spatial patterns in the fire-excluded plots were relatively stable post-mortality. 5) Saplings and recruits were all significantly aggregated at small scales postfire. Survival adults were randomly and aggregatedly distributed in the burned and fire-excluded plots, respectively. Spatial patterns of survival size classes had a trend of aggregation—randomness—repulsion significantly from all to larger size classes in both burned and unburned plots. 6) Adults and saplings were repulsively distributed postfire, and positively correlated with a good deal of recruits occurred as time had lapsed postfire. However, adults and saplings exhibited repulsively or aggregatedly in the fire-excluded plots. Adults and dead trees, saplings and dead trees were all positively correlated at different scales in the stands surveyed, respectively. 7) Intraspecific competition was more intense than interspecific competion in the stands surveyed. Surface fire had the competitive intensities decreased significantly in the fire-mediated forests, and the power functions could be constructed between them significantly. 8) The understory plant speciesα-diversity increased postfire instantaneously and converged with fire-free for several dozen years, while, theβ-diversity increased along the trend from one year postfire to 12 years, then to fire-free for several dozen years. More species, especially hygrophytes species, occurred instantaneously postfire in the 2006-burned stand analysed by the Detrended Correspondence Analysis.In a word, surface fire, as a dynamic driving force of Mongolian pine forests, has the tree density of stand reduced greatly, structures the stand, changes the composition and richness of understory plant species, directs spatial patterns of the forests evolving to mature, and exhibites an intensive thinning force to the forests. Thus, we should seriously take into consideration surface fire in the conservation and resources management of the natural Mongolian pine forests and realize sustainable development in practice under the changed fire regime and the background of increasing anthropogenetic activities and climate changing.