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Secondary forests (also known as second-growth forests) are forests produced by secondary succession after some form of major disturbance or perturbation. Over time, secondary forests may proceed, through succession, to a point at which they are indistinguishable from primary forests. These processes may require exceeding long time-scales. Outside of parts of the boreal and tropical zones, most remaining forests are secondary forests.

Secondary forests can arise through a variety of natural or anthropogenic disturbances including wildfire, logging, hurricanes and the abandonment of agricultural land.

The term "secondary forest" is usually only applied to forests which re-grow spontaneously. Forests that originate from planted trees (plantation forests) are usually considered distinct from secondary forests. Enrichment plantings in areas of spontaneous regeneration are often considered secondary forests.

Types of secondary forests
Secondary forests were so termed to distinguish them from primary forests, the primeval forests that existed in an area prior to modification of the landscape by humans. Inherent in this concept of primary forests was the Clementsian view of the climatic climax—the idea that some there is some stable endpoint to ecological succession. Modern ideas of plant succession see it as a dynamic process in which repeated disturbance produces a mosaic of patches which are at different successional stages. Despite the fact that modern ecological thought sees no forest stand as "undisturbed", distinctions are still made between primary and secondary forests. But given the fact that forests lie on a continuum, many different definitions of "secondary forest" exist.

Disturbance is necessary to create secondary forests, but the not all disturbance creates secondary forests. Some definitions consider any forest that has passed some threshold of disturbance to be "secondary", while others restrict the the term "secondary forest" to stands which have experienced certain types of disturbance.

Most definitions of secondary forests include forests that have experienced natural or human-caused disturbance. Some restrict the term secondary forest to stands produced by human disturbance. Francis Putz and Kent Redford, for example, restricted the term "secondary forest" for forests that have grown up following the complete removal of forest cover and used the term "degraded forest" for areas that had been subject to less intense disturbance, and Richard Corlett described secondary forests as those which had grown up after a "break in the continuity of forest occurrence". Authors who use this terminology tend to distinguish between secondary forests and degraded forests which have been modified by human actions but have not experienced complete removal of forest cover.

Others use the term for any forest that has experienced drastic or substantial disturbance, or consider all disturbed forests to be secondary. Some consider a forest to be secondary only if the species composition of the forest canopy differs from what existed prior to disturbance, while others consider forests to be secondary even if the dominant species are unchanged.

In an attempt to come up with a working definition for secondary forests, Chokkalingam and De Long defined secondary forests as:

They also recognised a number of different classes of secondary forest
 * Post-catastrophic secondary forests
 * Post-extraction secondary forests
 * Swidden fallow secondary forests
 * Secondary forest gardens
 * Post-abandonment secondary forests
 * Rehabilitated secondary forests

In their classification of secondary forests, Chokkalingam and De Long excluded forests that had been subject to low-intensity logging or other low-intensity extraction. They also excluded intensively managed forest plantations, and other planted forests. In addition, they excluded small-scale natural disturbances.

Post-catastrophic secondary forests
Forests can be significantly altered by catastrophic disturbances such as wildfire, flooding or hurricanes. These disturbances alter forest structure by killing trees or removing aboveground biomass. As a result, canopy cover and forest biomass are reduced. Forests that recover naturally after disturbances of this kind are usually significantly altered in terms of canopy structure and species composition.

Chokkalingam and De Long placed these in the first category of their classification of secondary forests.

Post-extraction secondary forests
Timber extraction alters forest biomass, and often leads to an increased dominance by early-successional species.

Chokkalingam and De Long placed these in the second category of their classification of secondary forests.

Swidden fallow secondary forests
Shifting cultivation, also known as swidden-fallow agriculture, creates secondary forests during the fallow period between cycles of cultivation.

Chokkalingam and De Long placed these in the third category of their classification of secondary forests.

Secondary forest gardens
In some cropping systems, short-term crops are planted alongside tree crops. After the end of the cropping period, succession converts these areas to secondary forest, but planted or tended tree-crops remain as part of the system.

Chokkalingam and De Long placed these in the fourth category of their classification of secondary forests.

Post-abandonment secondary forests
Secondary forests that are forest gardens or are part of swidden-fallow systems remain part of agricultural systems. In many cases, secondary forests develop after the abandonment of alternative uses.

Chokkalingam and De Long placed these in the fifth category of their classification of secondary forests.

Rehabilitated secondary forests
The development of forest on degraded land is often supplemented through restoration or rehabilitation.

Chokkalingam and De Long placed these in the sixth category of their classification of secondary forests.

Drivers and development
Secondary forests develop through a combination of deforestation and land abandonment.

Forest recovery
Although disturbance is a regular feature of forests, the end of major disturbance allows the forest to recover through secondary succession. At some point, this recovery process is expected to produce a forest which can be considered "primary" forest. Recovery can take a very long time, and can be heavily influenced by the land-use history of the site. In the Yucatan, despite the fact that almost a thousand years have passed since the collapse of the Maya civilisation, their effect on the vegetation remains, making it unclear as to whether these forests should be considered primary or secondary.