5. Suggestions for forest sustainable management
Under the pressure of climate change, China government commit to reduce carbon emission, achieve carbon peak by 2030 and carbon neutrality by 2060. Although the forest carbon stocks increased in China in past several decades, our results showed that AGB stability in planted and natural forests decreased significantly over time, productivity also decreased over time in natural forest but not in planted forest. In combination with the increasing scarcity of land available for new forest plantations (Lambin and Meyfroidt, 2011), our results draw a much more complex and worrying picture.
The existing plantation forests are commonly stocked with fast-growing tree species that dominate early pioneer stages in natural forests. In turn, these trees are generally more short-lived than species typically dominating later successional phases (Norden et al., 2012), while the low diversity, often not only in view of species richness, but also genetic diversity (Liu et al., 2018) of plantation forests also makes them less resilient to pest outbreaks (Jactel and Brockerhoff, 2007) and other environmental stressors (Drever et al., 2006). One way to sustain the C sequestration specifically of the plantation forests into the future could therefore be enrichment planting of plantation forest plots with a diverse set of local, shade tolerant late-successional species under the existing plantation forest canopy. These trees could then take over once the initially planted fast-growing pioneer species start to die off. Human interventions, especially the targeted extraction of individual stems of fast-growing, not locally native trees like Eucalyptus spp. or Poplarspp., could further accelerate this transformative process, while also creating a more heterogeneous forest environment that potentially allows a greater mix of trees differing in shade tolerance to colonize or be planted under the existing trees. If extracted timber was used for example in buildings or other ways that store the carbon contained in the timber over long periods of time, this approach could actually further optimize the carbon sequestration of these forests over long time-spans, eventually creating a forest structure that is species-rich and heterogeneous also in its age and size class distribution that could be sustained almost indefinitely under a targeted, selection cutting regime, hence providing sustainable C sequestration benefits while increasingly mimicking the structure and diversity of the natural forest environment.
In contrast, we strongly suggest to strictly protect all – mature and secondary – natural forests from any human interventions - apart from a potential enrichment planting with tree species well adapted to the predicted, dryer and warmer future climatic conditions. The target here should be to allow the eventual natural domination of well-adapted long-lived late-succession tree species that commonly store significant amounts of carbon in their trunks (Smith and Knapp, 2003), with natural disturbances allowing for an overall heterogeneous forest environment that not only sustainably stores carbon, but also supports the extremely high levels of biodiversity encountered across our study region (Sun et al., 2021).