There are both positive and negative roles that humans can play in plant succession with deforestation being one of the most influential negative factors. Deforestation is where humans will clear a mass of vegetation for reasons such as a growth in population which results in the need for the land for farming, housing e.t.c. The affect that this can have on plant succession is that it is interrupting the changes over time and is stopping the plant succession before it has had the chance to reach its dynamic equilibrium. Different species are not given the chance to develop from their pioneer species to reach a climax community. Deforestation has several serious consequences. It greatly reduces biodiversity, or the range of species. In the case of tropical rain forests, certain species cannot survive once the environmental structure has been disrupted. It also leads to dangerous changes in the carbon content of the atmosphere, in the case of rain forests; deforestation contributes to the planet's biodiversity—and to its oxygen supply. In the Amazon rainforest, one mahogany tree is cut and twenty-eight are destroyed, this amounts to 50,000 trees a year in one area being destroyed. The trees are cut for farming e.g. cocoa, rice, corn and fruit. For mining, minerals such as iron, bauxite, gold and tin, this results in high pressure so the top soil collapses. Cleared land also used for cattle ranching (approx 3,000 hectares) where the soil was acidic and infertile used for vegetation. It is difficult to grow but fertilisers are unavailable due to their lack of income.
If humans create fires in vegetation dominated areas it can have devastating affects. It can occur in grasslands, moorlands, forests and heaths. The climate plays a big part on how bad the fire can be. For example, if it is windy then the fires will move much more quickly and will burn faster. The fire is likely to be the hottest in dry weather and, in the northern hemisphere, on sunny south facing slopes where the vegetation is driest. Also the type of vegetation plays a part in the extent of the fire. Fires are strongest in forests that are dense and are very dry. Other examples are as follows: areas with Mediterranean climate suffer dense and dry vegetation following the summer drought. Coniferous forests leaf litter burns readily. Ungrazed grasslands such as the savannas which have a low biomass but a thick litter layer. Intensively grazed grasslands which have a lower biomass and a limited litter layer. Deciduous woodlands that are often slow to burn even with their presence of a thick litter layer. Fires disrupt the succession in the same way as deforestation does by wiping out lots of vegetation leaving the soil exposed, resulting in secondary succession. The blackened soil resulting from a fire then has a lower albedo and can therefore absorb heat more easily, and without its vegetation cover, the soil is more vulnerable to erosion. One positive outcome of the fire is that ash will initially increase the nutrients within the soil and so bacteria activity also increases. Any seedlings that have been left in the soil will then grow rapidly as there is plenty of light, no layer of leaf litter, plenty of nutrients, warmer soil and less competition from other species. The community may have a high proportion of species that can grow quickly after the fire, which are the plants protected by thick, insulating bark e.g. baobab in the savannas.
Desertification
One positive effect of human activity on plant succession is maintaining the diversity of habitats. At Studland Bay, the inland gorse and heather give way to birch and hazel trees. This climax community would take over the area if the trees were not deliberately cut to maintain the diversity of habitats. Dune slacks develop behind the sand dunes where the air is calmer, allowing for organic matter to be added. The soil increases in depth and the moisture content rises, encouraging different species to invade and then colonise the area. The growth of gorse and heather were also encouraged to manage the nesting of the rare Dartford Warbler bird.
The role of human activity affects plant succession both positively and negatively, although it is more so negatively. There interferences result in pioneer communities being unable to reach there climax community where the species would be in dynamic equilibrium with the environment. It may not be there direct interference, for example it may be the clearing of land resulting in infertile soils which restricts different types of species from succeeding.
