Comparison of spatial patterns of soil seed banks between larch plantations and adjacent secondary forests in Northeast China: implication for spatial distribution of larch plantations

In response to increasing timber demands, the natural secondary forest (NSF) is largely converted to larch plantation (LP) in Northeast China. However, it causes a series of problems such as mono-species composition, declined soil fertility and unsustainable productivity. On the other hand, it brings a great challenge for the forest managers to answer how to keep the LP with higher natural regeneration capability and how to change the pure plantations into mixed forests with more complex stand structures. Two typical spatial distribution patterns of LP within the NSF ecosystem, i.e., the Contour Type (CT, NSF and LP locating at the same slope position and aspect) and the Up-and-Down Type (UDT, LP locating at the down slope of adjacent NSF in the same aspect) were selected. The objectives of this study were to find out the optimal spatial distribution type of the LP to improve the regeneration potential (i.e., seeds in the soil stock) and the possibility inducing them into larch-broadleaved mixed forests. The spatial patterns of seed bank along the transects with the NSF-boundary-LP gradient were compared between CT and UDT. We found that in the CT stands, seed density of all species varied as a quadratic curve from NSF to LP and the largest seed density occurred at the boundary; for the dominant tree species in NSF, seed density of Fraxinus rhynchophylla tended to decrease logarithmically from NSF to LP and the seeds could only appear at 4 m away from the boundary in LP; litter depth was closely correlated with seed density and species richness in seed bank. In the UDT stands, seed density of all species tended to increase linearly from NSF to LP, and seed density in LP was 1.95 times higher than that in NSF significantly; seed densities of F. rhynchophylla and Acer mono increased as a quadratic curve from NSF to LP; the canopy openness was linearly correlated with seed density. Our results indicated that the establishment of the Up (NSF)-and-Down (LP) Type of stands could be more feasible for seed invasion. Comparative studies on the patterns of seed banks across the two spatial distribution types of larch plantations within the secondary forest ecosystem may provide some new insights into the tending measures on the LP management.