In Chapter 3 we introduced some cornerstones in ecosystem science. Chapters 4–9 have dealt in detail with structures and functions of terrestrial ecosystems. Here we take a broader view and derive from these chapters 17 principles, which can serve as a point of departure when analysing questions about ecosystem behaviour (Figure 10.1). The principles are divided into five classes, depending upon which component of the ecosystem they are dealing with:

1. Boundary conditions (B1–B5). These principles pertain to the interaction between the ecosystem and its environment.

2. Energy and water processes (A1). This principle relates to abiotic (energy and water) constraints on ecosystem processes.

3. Plant processes (P1–P5). These principles deal with constraints on plant growth.

4. Soil processes (S1–S3). These principles deal with the turnover of soil organic matter.

5. Element cycling processes (E1–E3). These principles couple plants and soils through the element cycles.

Principle B1. On boundaries and storage

The total storage of an element in an ecosystem depends on the balance between fluxes over the ecosystem boundary. As long as these fluxes do not change, the total storage will remain the same, although the internal distribution can change. For many elements an ecosystem can be described as a series of n pools (Xi) interconnected by fluxes (Fij). Some of these pools are also connected to the surrounding environment. In the simplest of cases there is an inflow (I) to only one of the pools, say 1, and an outflow from one of the pools, say n. Let the inflow be constant and independent of the state of the ecosystem, as would be the case with nitrogen in wet deposition, and let the outflow be proportional to the size of pool n (kXn), which might be the case if this flow represents nitrate leaching. A model of such an ecosystem could be