The impact of fire and density-dependent mortality on the spatial patterns of a pine forest in the Hulun Buir sandland, Inner Mongolia, China

Wildland fire, especially surface fire, is one of the major disturbances in forest ecosystems such as the Mongolian pine forest of the Hulun Buir sandland. However, little is known about the impact of fire on the spatial patterns at the stand level and its consequences for successional dynamics. To fill this gap we mapped three plots of Mongolian pine forest comprising sites of the same age that were affected by fire in 2006 and 1994, and a site without fire. We explored the stand structure using diameter at breast height (DBH) distributions and basal area, and used point pattern analysis to quantify the observed spatial patterns. Null models included homogeneous Poisson and Thomas cluster processes for univariate analyses, toroidal shift to test for independence, and random labelling for analyzing mortality. Large stems showed at all three stands a tendency to regularity, whereas small stems were mostly clustered. However, small stems did not become more regular at plots with earlier fire, and only the “overstocked” 2006 pre-fire stand showed negative association between large and small trees. Mortality was non-random, showing clustering of dead (or surviving) trees and clear density dependence where stems with more neighbors had a higher risk to die. Fire killed mostly smaller trees and after prolonged fire-free periods the stand approached the pre-fire patterns structurally and spatially. Our study showed that surface fire causes strong thinning in later succession stages and pushes the forest into maturity. It may also enhance resistance of our forest to more severe fire under the relatively harsh environmental conditions. A novel test statistic for random labeling allowed directly testing for density-dependent mortality and could be widely applied in point pattern analysis.