**4. Carbon sequestration in forests**

*Forest Degradation Around the World*

deterioration [14].

in the later succession.

grow [18].

**3. The role of natural regeneration**

of the land surface [6, 11]. Unfortunately, one-third of these original forests have been lost because of deforestation, resulting into large degraded areas. To date, the degraded areas are common in many countries, most of which are converted into other land uses, although some of them will regenerate into secondary forests [12]. Currently, the secondary forest can be found in all areas in the world and it is

The decrease of world's forest is caused by many activities such as clearing to open land for agriculture, roads, settlement, logging for timber, and cutting for fuel. Indirectly, forest is succumbing to the effect of environmental pollution and further threat by climatic change. Together, these causes have been responsible for decimating many of the world's forest, and they threaten to significantly degrade those that remain. Moreover, the burning of trees, shrubs, and other vegetation during land clearing and after logging further contributes to environmental

Sapling regeneration is important to restore forest formation after disturbance, balancing the loss of primary forest and as a main sequester of CO2. Their radial growth characteristics are the main performer of canopy closure, and it can be used to identify the canopy formation in the secondary forest after disturbances. Moreover, the canopy closure can be used to predict the vertical structure in the future; therefore, it will explain the pressure that will be given by dominant species

The continuing loss of primary tropical forest has made the secondary forest to be increasingly important for maintaining the ecology across the large forested landscapes [15]. The regeneration in the tropical secondary forest is strongly influenced by disturbances that resulted from natural tree fall in primary forest [15, 16]. Regeneration is widely recognized as being a key process in determining the structure and dynamism of tropical secondary forest [17]. For instance, the logging of mature trees can reduce seed resource in the forest [9]. Nevertheless, it may increase the light intensity, which reaches the forest floor and will stimulate the seeds' germination and growth of saplings. The ability of regeneration in the degraded areas is determined by the success of the seedlings and saplings to

The success of forest regeneration in terms of its distribution and diversity can be used as parameters to understand the level of forest succession and to figure out the population dynamic condition in the areas that have been controlled [18]. A lot of efforts have been made to study forest regeneration to provide information on ecological condition [18–20]; however, the data which have been collected cannot solve the same problem in different areas in the world. This is because every area in the world has different conditions which make it possible to create the different microclimate after fragmentation, and then influence the recovery process. The change of microclimate because of forest fragmentation will influence the plant species survival. Therefore, only species that can adapt to the environmental change will survive as different species have different capabilities to response to the ecological pressure. Furthermore, there are crucial chemical elements in the soil that have been recognized for growth of all plants [21], and the loss one of these elements will influence the plant growth, especially macronutrient. Besides the chemical elements, forest regeneration is influenced by light intensity which reaches under the canopy. The increasing of light intensity which reaches under the canopy will

estimated to reach about 64% of the total world forest [13].

**4**

Rapid development of industrial technologies in the world has contributed to the increment of greenhouse gases. The increase of greenhouse gases, such as CO2 in the atmospheres, has led to global warming phenomenon. Nowadays, rise of global temperatures has constituted a challenge to the mankind. The increment of CO2 today is estimated to be 1.3–1.9 ppm year<sup>−</sup><sup>1</sup> [22]. According to [23], from the analysis recorded in Malaysia, the average of temperatures has increased by 1.1°C within 50 years, which is consistent with the trend on global warming.

In reality, trees have important function in the carbon sequestration process [24–26]. In this case, the conservation of plant resources in a sustainable manner is the best solution. Forests play important roles in the CO2 sequestration process. Trees in the forests are important in order to perform the canopy closure in the forest and then influence the other species. Several studies have shown that the light intensity in the understory affects the growth and crown morphology of several tree species [17, 21, 27] and soil properties at allocation of plant biomass [28–31]. Additionally, the effect of soil properties and light intensity has been known to correlate with growth rate, and distribution pattern as well as species composition. In reality, each species has different capability to respond to their environmental factors.

In the tropical rainforests, nutrient cycling is probably one of the most significant ecological processes [32]. Similarly, according to Hamzah et al. [33], tropical rainforests are recognized as the richest ecosystems in the world in terms of structures, species diversity, and ability to maintain the global climate change by reducing the accumulation of greenhouse gases. According to Sands [34], tropical rainforests contain 70% of the world's plants and animals, 70% of the world's vascular plants, 30% of all birds, and more than 90% of all the invertebrates. There can be more than 200 tree species per hectare in tropical rainforest.
