Abstract:

Mature forest is often referred to as the old-growth forest or the late-successional forest, which plays an important role in maintaining biodiversity and long-term carbon balance. In recent ten years, the carbon sequestration in mature forests under different environmental regimes and global change scenarios has received increasing attention since the founding that the old-growth forests can accumulate carbon in soils. Understanding the carbon storage and carbon source/sink dynamics of mature forests can be helpful for evaluating and predicting the role of mature forests in regional carbon cycles. It is also essential to evaluate the full range of forest C mitigation and to develop the close-to-nature forest management practices. On the basis of introducing the thresholds of mature forest ages, this paper analyzes the dynamics of carbon storage and distribution in natural mature forest, and the mature forest productivity and their influencing factors. Three carbon source/sink hypotheses of mature forest are summarized:1) The carbon neutral hypothesis proposes that net ecosystem productivity in forests steadily increases to a maximum with the age of forests, followed by gradual decline. The total ecosystem photosynthetic assimilation is increasingly offset by higher autotrophic and heterotrophic respiration at the mature forest stage, eventually, old forests are believed to reach (that is, C neutrality). 2) The carbon sequestration hypothesis suggests that many old-growth forest ecosystems in tropical, subtropical, and northern and temperate regions can continue to accumulate carbon, and act as a net sink for atmospheric carbon dioxide for centuries. The carbon sequestration in the mature forests is mainly in the soil, which is probably related to the increasing productivity of mature forests caused by climate change, and the increasing conversion of vegetation carbon into soil carbon with the increase of forest maturity, as well as the decreasing heterotrophic respiration in mature forest soil due to the global warming and atmospheric nitrogen deposition. 3) But very few studies have shown that the old-growth forests serve as a het carbon source. Accelerated carbon cycling of litters, dead trees, and soil organic carbon pools, especially when coupled with limited growth potential, could potentially drive the mature forest ecosystems to become a net source of carbon under future regional climate change scenarios. A growing number of studies have confirmed that the mature forests can sequester carbon, supporting the mature forest sink hypothesis. The future research should be focused on:1) Ttrengthening the carbon fluxes observation of mature forests by using eddy covariance approach. 2) Exploring the process of carbon source/sink and its formation mechanism in mature forest based on ecosystems respiration and its composition. 3) Studying the effects of global change on carbon sequestration in mature forests.

Key words: mature forest, carbon storage, carbon source/sink, forest productivity, ecosystem respiration