Which of the following is NOT classified as a wetland?

Wetlands are defined as areas where water covers the soil, either permanently or seasonally, and this presence of water leads to unique ecosystems that support a specific set of plant and animal species adapted to wet conditions. Marshes and swamps are both types of wetlands: marshes are dominated by herbaceous plants, while swamps are characterized by woody plants such as trees and shrubs.

The correct choice is not classified as a wetland because it does not describe a habitat that is defined by the presence of standing water. Forest, although it can include wetland areas, mainly refers to an ecosystem with trees that might not necessarily require wet conditions. On the other hand, 'substrate' refers to the underlying layer of material beneath a habitat, such as soil or rock, and does not imply any specific ecological characteristics of wetlands. Thus, 'substrate' serves as a foundational element but does not itself constitute a wetland.