**2. Forest degradation around the world**

The growth in human population has forced mankind to explore existing resources, including forest for multiple purposes [9], for instance, logging of timber for wood-based industries, agricultural activities, urban areas, road, and mining. All these practices are done at different stages of forest growth which give significant impacts on the stand structure, composition, and other components of the forest and will consequently reduce forest productivity and their ecosystem functions [10]. During early civilizations, forest occupied about half of the earth's surface. However, it has, at the moment, reduced into less than one-third of the total earth's surface. The significant reduction of the forest areas requires appropriate planning and management for the conservation [8], especially forest in the tropical and subtropical areas in the developing nations. Today, the degradation of the forest due to anthropogenic activities and natural disturbances is significantly reducing total forest areas in the world. Originally, tropical forest covered about 1.5–1.8 billion ha

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

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 deterioration [14].
