What happens to the leaves
Every year a forest drops a large amount of plant material to the ground: leaves, needles, twigs, bark, flower parts and fruit. Together this layer is called leaf litter, and it covers the floor of most forests. In a deciduous forest much of the fall happens in fall. In evergreen and tropical forests it is spread through the year, though often with seasonal peaks.
The litter does not pile up forever. Something is always breaking it down, and the speed at which it vanishes tells you a lot about a forest. In warm, moist tropical conditions a leaf may disappear within months. In cold or dry places, or when the leaves are tough, the litter can stay for years and build up thick layers.
Who does the work
Decomposition is a team effort, and no single organism does it alone.
Soil animals as shredders. Earthworms in some regions, along with millipedes, springtails, mites, woodlice and the larvae of many insects, chew litter into small pieces. They mostly do not digest the leaf completely; instead they increase its surface area, mix it with soil and leave droppings that microbes can colonize easily.
Fungi. Fungi are the main decomposers of tough plant material. Their thread-like bodies grow through the litter, releasing enzymes that break large molecules into smaller ones they can absorb. Different fungi specialize in different materials, and they often take over in sequence as the litter ages.
Bacteria. Bacteria are abundant and fast-reproducing. They tend to work on the easier-to-digest compounds and on the products released as fungi and animals break the leaf apart.
Larger animals such as beetles and slugs play a part, as does the slow rotting of logs and snags. The overall process, though, depends heavily on microbes.
A leaf in stages
The breakdown of a single leaf often follows a rough pattern.
- Leaching. Within days to weeks of landing, rain washes out soluble compounds such as sugars and some minerals. These are quickly used by microbes or taken up by roots.
- Colonization and early decay. Fungi and bacteria move in and begin consuming the easily digested parts, such as simple sugars and proteins.
- Fragmentation. Animals shred the leaf, which exposes more surface and mixes the fragments into the upper soil.
- Slow decay. What remains is richer in tougher compounds such as lignin, which decay slowly. This stage can last months or years.
- Humus. Some of the material is transformed into dark, stable organic matter that blends into the soil and may persist for a long time.
Along the way, much of the carbon in the leaf returns to the air as carbon dioxide through the respiration of the decomposers. A smaller portion is built into soil organic matter, which contributes to soil carbon storage over the longer term.
Nutrients on the move
Leaves contain nitrogen, phosphorus, potassium, calcium and other nutrients that the tree took up from the soil. Before dropping leaves, many trees pull back a portion of their nitrogen and phosphorus into the branches for reuse, so fallen leaves are somewhat depleted compared with green ones. The rest returns to the ground.
Decomposers need these nutrients too. When litter is rich in nitrogen, microbes can release the excess in forms that plants can take up, a process often called mineralization. When litter is very low in nitrogen relative to its carbon, microbes may temporarily hold onto available nitrogen, which can slow the release of nutrients to plants. The ratio of carbon to nitrogen in litter is therefore one of the factors that influences how quickly it decomposes.
Once released, nutrients are absorbed by fine roots, often with the help of partner fungi, a partnership known as mycorrhizal fungi. The nutrients are then built into new leaves, and the cycle repeats. In many forests, this loop is tight: a large proportion of the nutrients the trees use each year comes from recycled litter rather than from rock weathering or rain.
This is especially important in places where the mineral soil holds few nutrients. In tropical rainforests with nutrient poor soils, much of the available nutrient supply is held in living plants and the fast-recycling litter layer, which helps explain why clearing such forests can quickly reduce soil fertility.
What affects the speed
Several things influence how fast litter decomposes:
- Climate. Warmth and moisture generally speed it up. Cold, waterlogged or very dry conditions slow it down.
- Leaf quality. Soft, nutrient-rich leaves from some broadleaf species often break down faster than tough needles or leathery leaves. Species differ a lot, so it is best to treat this as a general pattern.
- Soil life. The presence of active soil animals and fungi makes a large difference.
- Acidity and chemistry. Very acidic soils or litter containing defensive chemicals, such as tannins or resins, can slow microbial activity. Some of these compounds are the same ones that serve as tree defenses while the leaf is still alive.
- Disturbance. Fire consumes litter and releases some nutrients quickly; this is part of fire ecology. Compaction and removal of litter can also change decomposition.
Other jobs the litter layer does
Besides feeding the soil, leaf litter acts as a blanket. It shades the ground, reduces evaporation, moderates temperature swings and protects the soil surface from the force of raindrops, which helps limit soil erosion and keeps water moving into the ground rather than across it. This links to the broader picture of forest soil health.
Litter is also a habitat. Many small animals, from salamanders to beetles to ground-nesting insects, spend their lives in it, and it feeds the chains of predators that eat them. Some seeds germinate in it; others are blocked by a thick layer.
When the cycle is interrupted
Removing litter by raking, burning or heavy harvesting can reduce the return of nutrients to the soil. How much this matters depends on the site: on fertile soils it may make little difference over the short term, while on poor soils the effect can be significant. Invasive earthworms in some northern forests that had no native earthworms have been reported to consume the litter layer quickly, changing the soil and the plants that grow in it, though the effects vary by location.
Even this simple layer, which most people walk over without noticing, is one of the main ways the forest feeds itself.