Impacts of Downed Dead Wood Poplar Trees on Forest Regeneration in the Semi-Arid Region of Northern China

In the past few decades, due to rising temperatures and changes in precipitation, the degree of drought in semi-arid areas has increased, leading to a large number of tree deaths and threatening the natural forests distributed in the semi-arid areas of North China. This article takes the forest ecosystem of Saihanwula Nature Reserve in the southern section of Greater Khingan Mountains in China’s semi-arid region as a research area and studies the distribution of downed dead wood and its impact on forest renewal in the area. We used the sample plot survey method, investigated the number of downed dead wood, decay class, dumping direction, existence form, and the number of regenerated seedlings in the sample plot, and calculated the density of regenerated seedlings in different plots. The renewal density is 4050 ± 824, 2950 ± 265, plants/ha, and 2625 ± 237 plants/ha, respectively, in the sample plots for Later-death plot, Mid-death plot, and Early-death plot. The average storage of downed dead wood in Saihanwula Nature Reserve is 58.51 ± 16.56 m3/ha. The distribution densities of downed dead wood are 50 ± 21, 806 ± 198, 189 ± 76, and 22 ± 5 plants/ha for decay classes II, III, IV, and V respectively. The main form of downed dead wood in the research area is “trunk base fracture”, accounting for 68.78% of the total number of downed dead wood. A large number of downed dead wood had serious negative effects, such as crushing and injuring the regeneration seedlings and other plants under the forest at the moment of dumping and for a long time after dumping. The crushed and injured rate is 5.3~7.8%, with downed dead wood accumulated in the forest from the early stage of downed dead wood. It had negative effects on the regeneration of seeds, seedlings, and young trees, such as obstructing and hiding the light from the soil surface and inhibiting the regeneration and growth of seedlings. However, after the trees were dumped, large gaps appeared in the forest, increasing the sunlight area on the soil surface. In the later stage of tree death, moderately high decayed downed dead wood changed the soil structure in terms of soil softness, water holding capacity, and nutrient content, thus promoting the growth of seedlings and young trees. Reasonably utilizing the relationship between downed dead wood and forest renewal can effectively promote the healthy development of forests.