Effects of microhabitat on rodent-mediated seed dispersal in monocultures with thinning treatment

Abstract

Monoculture practices in plantation forests reduce biodiversity and soil fertility compared with the adjacent patches of primary or secondary forests. Thus, it is urgent to convert monocultures into mixed stands to resolve those problems. In Northeast China, pure larch (Larix spp., mainly including L. olgensis, L. gmelinii, and L. kaempferi) plantations (LPs) are widely converted into mixed larch-walnut stands by introducing Juglans mandshurica (a native and light-demanding broadleaved tree species in secondary forests) into the LPs. However, the success of the natural regeneration of J. mandshurica must rely on rodent-mediated seed dispersal away from parental trees, and the dispersal processes are largely influenced by microhabitat changes during the thinning management of plantations. A field trial was conducted by releasing tagged J. mandshurica seeds in LPs with three thinning intensities (0%, 25%, and 50% thinning intensities) during two continuous growing seasons. The results indicated that among the three thinning treatments, the removal rate of seeds by rodents was the fastest, and the scatter-hoard seed (6.3 m) was most plentiful, in the 25% thinning intensity of LPs. On the contrary, the seed dispersal rate was the slowest, and the seed dispersal distance (8.3 m) was the farthest, in the stand with 50% thinning intensity in the non-mast year of 2015. This finding may be because both the changes in canopy openness and variations in microhabitats (including light incidence, soil moisture, and coverage of understory shrubs and herbs) caused by different thinning intensities have a synergetic effect on rodent-mediated seed dispersal. Inter-annual variation in seed production resulted in faster removal rates with larger cache proportion in LPs (25.93%) and shorter dispersal distance (6.4 m) in the non-mast year (2015) and vice versa in the mast year (2016). Thinning is a potentially feasible practice to promote rodent-mediated seed dispersal and the potential seed germination of J. mandshurica in LPs. We could take varying measures for different silviculture objectives to make full use of the thinning advantage in rodent-mediated seed dispersal; for instance, 25% thinning intensity (437–532 trees ha−1) provided a suitable microhabitat for rodents to disperse walnut seeds into LPs faster, and 50% thinning intensity (292–355 trees ha−1) is suggested in LPs to promote rodents to cache seeds with longer distance.