Background: Promoting natural regeneration (including seed and sprout regeneration) of dominant woody species is essential for restoring secondary forests. However, such restoration processes have been decelerated by the enclosure under Natural Forest Protection Project of China (NFPP). It remains unclear how to implement appropriate management measures (e.g., whether to apply logging and the suitable intensity) to facilitate natural regeneration according to the responses of two regeneration modes to management measures. We monitored the early stages of seed regeneration (seed rain, soil seed bank, and 1–3-year-old seedlings) and sprout regeneration (stump sprout rate, stump survival rate, probability of sprouting, and number of sprouts per stump) over the first 3 years (2017–2019) after logging under three intensity regimes (control [0%], 25%, and 50% logging intensity) in secondary forests. Results: The seed rain density decreased markedly, seedling density increased insignificantly after logging, and logging promoted seedling survival at an increasing conversion rate of 3-year-old seedlings (37.5% under 0%, 100% under 25%, and 80.95% under 50% logging) compared to those of the control. The proportion of 3-year-old seedlings increased with logging intensity and was the highest (16.2%) at 50% logging intensity. Sprout density was not affected by logging intensity, however, under 25% and 50% logging, it decreased by 27% and 6% in 2018, and by 37% and 33% in 2019, respectively. Seedling density was 41.65- and 15.31-fold higher than that of sprouts in the 50% and 25% logging treatments, respectively. Based on the relative contributions of the two regeneration modes after logging, three groups of natural regeneration patterns were classified for dominant woody species in temperate secondary forests, i.e., seed regeneration preference (Betula dahurica, Carpinus cordata and Fraxinus mandshurica), sprout regeneration preference (Acer mono and Acer pseudosieboldianum) and no preference (Quercus mongolica, Fraxinus rhynchophylla, and Juglans mandshurica). Conclusion: In addition to enclosure, appropriate logging can be applied according to the responses of various natural regeneration patterns of dominant woody species to logging in temperate secondary forests under the NFPP.

The shift from natural mixed broadleaved forests to pure coniferous plantations results in soil degradation and the unsustainable development of plantations due to the simple stand structure and low species diversity. Thinning can practically sustain the forest structure and promote the regeneration and growth of broadleaved trees in these pure coniferous plantations. The growth of regenerated broadleaved trees is closely related to leaf ecological stoichiometry, which is strongly restricted by environmental factors such as light, soil moisture, and nutrients after thinning. However, the temporal effects of thinning on leaf C:N:P stoichiometry are still not well understood, which constrains our understanding of implementing thinning in coniferous plantations to promote the regeneration and growth of broadleaved species, and further forming the mixed larch-broadleaf forests. Here, we compared canopy openness (i.e., light availability) and the soil and leaf stoichiometry for regenerated broadleaved trees in larch (Larix keampferi) plantations in short-term (1–3 years), medium-term (4–9 years), and long-term (≥10 years) periods after thinning, taking natural mixed broadleaved forests as a control in Northeast China. The results showed that the temporal effects of thinning were not significant with respect to soil C concentrations, but significant with respect to soil C:P and N:P ratios. The regenerated broadleaved trees adjusted their leaf N concentrations and C:N ratios in response to the changed environmental conditions after thinning over time. The responses of soil and leaf stoichiometry to thinning and their significant correlation indicated a strong interaction between the soil and understory regeneration following thinning. Thus, thinning affects the soil and leaf stoichiometry of regenerated trees over time. These findings provide new insights into the conversion of pure coniferous plantations into mixed larch-broadleaf forests by controlling thinning intervals.

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 a viability rate of 82 and 10 per cent, respectively. Most seeds remained viable at 25 per cent (91 per cent 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.

Background: Granivore-mediated seed dispersal is susceptible to changes in seed availability and silvicultural management, which alters synzoochorous interactions in the antagonism-mutualism continuum and affects the seed dispersal effectiveness (SDE), and eventually, the plant recruitment. We conducted a whole-year study of seed addition to quantify the granivores-Korean pine (Pinus koraiensis) synzoochorous interactions and the SDE in the same secondary forests with two treatments. Both treatments had seed source limitations: one was caused by the disappearance of Korean pine due to the historical disturbance, the other by pinecone harvesting in Korean pine plantations adjacent to the secondary forests. Thinning with different intensities (control, 25%, and 50%) were also performed to further explore the synzoochorous interactions and SDE in response to silvicultural management in the second type of forests. Results: Source limitation increased the proportion of pre- and post-dispersal seed predation, and made the granivores-Korean pine interaction shift more towards antagonism, with the estimated SDE of 2.31 and 3.60, respectively, for the secondary forests without and with Korean pine. Thinning with different intensities did not alleviate the reactions towards antagonism but altered SDE; granivores occurrence decreased, but the proportion of pre- and post-dispersal seed predation increased, resulting in a fivefold decreased seedling recruitment in 25% thinning (the lowest SDE of 0.26). Conclusion: The source limitation coupling thinning biased the synzoochorous interactions more towards antagonism and significantly lowered granivore-mediated SDE, which limited the successful recruitment of Korean pine in secondary forests. Forest managers should control pinecone harvesting, protect the synzoochorous interaction, and take into account masting event for Korean pine regeneration in the future.