For most of us the first thing we notice when we see a forest is the trees. Its natural, they are so much bigger than we are that their presence changes the way we feel. But the forest is much more than the trees. Forests consist of a great diversity of plants and animals that range in size from microscopic to massive. These plants and animals all interact with each other and the non-living mineral soil, water atmosphere in innumerable ways that has kept scientists busy studying them for centuries. Together the variety of plants and animals and their interactions are known as biological diversity.
Changing the mix of plants or animals alters the way the forest functions. Changes in how the forest functions is important if you are trying to manage it for an explicit outcome such habitat for a specific bird or mammal, shade and air quality for a city street or for clean water in a river. These outcomes are often called ecosystem services.
Of course people are a part of the forest also. Knowingly and unknowingly we interact with the other living members and non-living portions if the forest. They affect us and we affect them. All of this leading to changes in biological diversity and how the forest functions, and ultimately the ecosystem services that we might desire. When people dominate these interactions we called the forest an urban forest.
So managing a forest for any outcome or ecosystem service is really all about managing biological diversity.
For further information on urban forests and biological diversity check out the following:
Defining Urban Forestry