What a secondary forest is
A secondary forest is forest that has grown back naturally after most of the original trees were removed by clearing, farming, logging or fire. Untouched or only lightly disturbed forest is called primary or old-growth forest. The line between them is not always clear, since many forests have a history of past use, and definitions differ between countries and researchers. Still, the distinction is useful, because regrown forest tends to differ from the original in structure and species.
Large areas of the tropics are now covered by secondary forest. It appears wherever land is left alone: abandoned pasture, fallow fields in shifting cultivation, old logging areas, and burned sites.
The stages of regrowth
Recovery usually follows a recognizable sequence, a case of forest succession in the wet tropics. Timing varies widely depending on the site.
- Early years. Grasses, herbs, shrubs and fast-growing pioneer trees take over open ground. These pioneers need strong sunlight, grow quickly, have light wood and produce many small seeds, often spread by wind or small animals. Typical examples include trees such as cecropia in the Neotropics, though other regions have their own pioneers.
- Young forest. Within roughly a decade or two, pioneers can form a closed canopy. Shade now blocks new pioneers from germinating, and shade-tolerant species begin to appear beneath them.
- Intermediate stage. The first pioneers age and die. Longer-lived trees with denser wood grow into the canopy, and the forest gains more layers and more tree species.
- Late stage. Over many decades to centuries, the forest approaches the structure and composition of old forest, though a perfect return is never guaranteed.
Biomass and canopy height often recover faster than species composition. In some studies, regrowing forests regain a large share of the original aboveground carbon within a few decades under favorable conditions, whereas the full set of species can take much longer, if it returns at all.
What affects the speed of recovery
- Land use history. Land that was cleared briefly and not farmed intensively recovers faster. Repeated burning, long cattle grazing and heavy cultivation compact the soil and remove seeds and root systems.
- Nearby forest. Remnant forest nearby provides seeds. Animals such as birds and bats carry seeds into clearings, an effect explained in the article on seed dispersal. Isolated sites with no seed sources regrow more slowly.
- Soil. Nutrient depletion and erosion slow regrowth, while sites with some remaining fertility and organic matter respond faster.
- Rainfall. Wetter sites generally regrow faster than drier ones.
- Fire and disturbance. Repeated fires can keep a site stuck as grass or scrub.
- Surrounding landscape. Small, scattered patches are more exposed to edge effects, wind and drying than large blocks of forest, and forest fragmentation can limit recovery.
Many cases of recovery rely on natural regeneration, in which the site is simply protected and left alone. Where conditions are poor, active help such as tree planting or fire control may be needed.
How secondary forest differs from primary forest
Even when secondary forest looks lush, researchers commonly find differences:
- Fewer large old trees and less deadwood habitat, so animals that rely on cavities or fallen logs may be missing.
- Different tree species. Secondary stands often contain more pioneers and fewer rare, slow-growing or large-seeded species.
- Missing wildlife. Some forest specialists, such as certain understory birds, large mammals and amphibians, may be absent or less common, particularly if hunting continues or if there is no nearby source forest.
- Simpler structure. Fewer gaps, vertical layers and epiphytes.
At the same time, secondary forests are far from worthless. They protect soil, hold water, store carbon, support many native species, and provide wood, fruit and other products to local people. They can also act as buffers and linkages between remaining patches of primary forest.
Why it matters
Secondary forests are often cleared again after only a few years, which keeps them young and limits their benefits. When allowed to stand, they can accumulate carbon for decades. This is one of the reasons they come up in discussions of restoration carbon gains and climate policy, although the amounts vary a lot by site and are uncertain.
The key point is that regrowth is real but not a replacement for intact forest. Keeping primary forest standing preserves things that regrowth recovers only slowly, and some of which may never fully return.