Effects of thinning on early seed regeneration of two broadleaved tree species in larch plantations: implication for converting pure larch plantations into larch-broadleaved mixed forests

To meet the demand for timber consumption, large areas of broadleaved secondary forests have been replaced by larch (Larix spp.) plantations in Northeast China. However, larch plantations face some serious problems, including soil fertility decline, acidification of surface runoff and low levels of biodiversity. Converting pure larch plantations into mixed larch-broadleaved forests would offer a potential solution to some of these issues. Two dominant broadleaved tree species of secondary forests (i.e. Juglans mandshurica and Fraxinus mandshurica), which have been successfully mixed with larch previously, were seeded in thinned pure larch stands to confirm the feasibility of converting even-aged pure larch plantation stands into uneven-aged mixed larch-broadleaved forests. Larch stands were treated with 100, 50 and 25 per cent thinning intensities to evaluate the effects of thinning on the persistence of seeds in two burial positions (under litter and in soil) and at two burial times (6 and 12 months), and on the seed germination at five burial positions (on top of litter, on top of soil, between litter and soil, in soil and in soil with litter coverage). We found that after 1-year burial, J. mandshurica and F. mandshurica seeds had aviability rate of 82 and 10 percent, respectively. Most seeds remained viable at 25 percent (91 percent for J. mandshurica, 27 per cent for F. mandshurica) and 50 per cent (90 per cent for J. mandshurica, 23 per cent for F. mandshurica) thinning treatments. Seeds of J. mandshurica and F. mandshurica buried in soil with litter cover (21,38 per cent) and in 50 per cent thinning treatments (16, 35 per cent) of larch plantation forests had significantly higher seed germination/seedling survival rate, compared with other burial positions and thinning treatments. Regeneration results (seed persistence, seed germination and seedling survival) suggest that artificially converting larch plantations into larch-broadleaved mixed forests by thinning and adding seed sources of native broadleaved species is a potentially feasible practice. These results also provide new insights into the tending measures used in pure plantation management.