<Li> Soil is already present </Li> <Li> Seeds, roots and underground vegetative organs of plants may still survive in the soil . </Li> <P> Imperata grasslands are caused by human activities such as logging, forest clearing for shifting cultivation, agriculture and grazing, and also by frequent fires . The latter is a frequent result of human interference . However, when not maintained by frequent fires and human disturbances, they regenerate naturally and speedily to secondary young forest . The time of succession in Imperata grassland (for example in Samboja Lestari area), Imperata cylindrica has the highest coverage but it becomes less dominant from the fourth year onwards . While Imperata decreases, the percentage of shrubs and young trees clearly increases with time . In the burned plots, Melastoma malabathricum, Eupatorium inulaefolium, Ficus sp., and Vitex pinnata . strongly increase with the age of regeneration, but these species are commonly found in the secondary forest . </P> <P> Soil properties change during secondary succession in Imperata grassland area . The effects of secondary succession on soil are strongest in the A-horizon (0--10 cm), where an increase in carbon stock, N, and C / N ratio, and a decrease in bulk density and pH are observed . Soil carbon stocks also increase upon secondary succession from Imperata grassland to secondary forest . </P>

Explain why secondary succesion takes less time than primary succession