Woody plant encroachment is when bushes, shrubs and woody plants grow more and take over an area where there used to be grass and other plants. It happens mostly in grasslands, savannas and woodlands and can change the area from being open to being closed. The causes of this include things like people using the land too much, not having fires, and climate change. It can be bad for nature and for people who use the land. Some countries try to stop this by doing things like cutting down some of the bushes, using fire and managing the land. Sometimes, this type of land is good for the climate because it can take in carbon. But, it is not always good and more research is needed to understand the effects on the climate.

It is different from invasive species which are plants that are brought from other places.

Woody encroachment probably started thousands of years ago, when Earth started warming, but it has happened much faster during the last 50 years.^[1] There are different possible reasons for woody encroachment, some directly linked to the action of people, others linked to climate change. The reasons for woody encroachment are very different between dry grasslands and wet grasslands.^[2] Once the bushes and shrubs have grown and taken over an area, they make it harder for grass to grow. This makes the problem of woody encroachment continue.^[3]

When people abandon land, that means when they stop using the land for agriculture, bushes often start growing. This happens for example in the Alps in Southern Europe, because more and more people leave their farms and move to cities.^[4]

Also when people use land very intensively, bush encroachment can happen. For example when farmers bring many cows to areas where wild animals lived before, these cows eat up most of the grass quickly, but do not touch the bushes. The result is that bushes occupy more and more of the land, while grasses start to disappear.^[5] Once woody encroachment has started, it is hard to stop it, even when the number of cows is made less.^[6]

While changes in how people use the land is often seen as the main reason for woody encroachment, some studies suggest that global factors like climate change and rainfall patterns also play a role.^[7][8] Climate change can make it easier for bushes and shrubs to grow because of things like more CO₂ in the air and changes in temperature and rainfall.^{[9][10][11][12][13][14]} Once the bushes and shrubs are established, they can make it harder for grass to grow and the problem of woody encroachment can continue. According to the Intergovernmental Panel on Climate Change (IPCC) report, high latitude tundra and boreal forests are particularly at risk of woody encroachment due to climate change.^[15]

Woody encroachment is a big change for nature in areas where it happens. Often, the encroachment is seen as harmful to nature. It can make the number of different plant species less, it allows less water to go into the ground and animals have less to eat. For that reason woody encroachment is often called a form or land degradation. But this is not always the case, encroachment can also lead to good changes in nature.^[16] That is why one must study woody encroachment in each area where it happens, before making conclusions.^[17]

How woody encroachment changes nature depends on different things:^[18]

• How people use the land: encroachment can be positive in land that is not used, but is most of the times a big problem in areas where people keep animals like cows that need grass to eat.^[17][19]
• The number of bushes: a medium number of bushes is often good for nature, but a high number can be very bad.^[20][21][17]
• Climate: in areas that are very dry, woody encroachment is more negative than in wetter areas.^[22][20]

Many scientists see woody encroachment as a form of land degradation and think it can even lead to the creation of new deserts.^[23] Because bushes make nature look more green, the encroachment is sometimes called "green desertification".^[24]

Woody encroachment can have negative effects on the variety of plants and animals in that area (biodiversity).^[25] In Africa, the number of different plants and animals usually decreases when woody encroachment happens. In North America, the number of different plants and animals may increase or decrease depending on the area.^[26] In Brazil, one third of the different plant species are lost because of woody encroachment.^[27]

The following list shows what species are affected by woody encroachment:

• Grasses: In North America, the number of different types of grass types goes down to half when woody encroachment happens.^[28][29] Some types of grasses even go extinct (they disappear and do not grow anymore).^[30] The small white lady's slipper is a type of grass that is affected strongly.^[31] Large bushes allow grasses to grow, but smaller bushes do not.^[32]
• Mammals: animals that prefer to live in open areas (where there are not many large plants), have to move when woody encroachment happens^[33] Animals that hunt struggle to do that where there is a lot of bush.^[34] This is the case for cheetah,^[35][36][34] white-footed fox^[37], and antelopes like the Common tsessebe, Hirola and plains zebra.^[38] In Latin America the habitat of the almost extinct Guanaco is threatened by woody encroachment.^[39] In some places animals have 80% less food when woody encroachment happens.^[40]
• Birds: birds react differently to woody encroachment. Birds that need bushes can deal with woody encroachment better than those that need open grassland. But also the birds that live in bushes, suffer when the bushes become too dense.^[41][42] Examples of birds than do not do well under woody encroachment are the Secretarybird,^[43] Grey go-away-bird, Marico sunbird, lesser prairie chicken,^[44][45] Greater Sage-Grouse,^[46] Archer's lark,^[47][48] Northern bobwhite^[49] and the Kori bustard.^[50] In Namibia, the Cape vultures move to other places when there are more than 2,600 woody plants per hectare.^[51] In North America woody encroachment can be bad for many types of birds. They may not have enough space to live and find food, which can make it hard for them to survive. This is a big concern.^[52][53]
• Insects: there are insects that prefer to live in open areas (areas where there are not many plants). These types of insects suffer under woody encroachment^[54] Affected species include butterfly^[55] and ant.^[27]