How to live with trees and keep them healthy
During autumn, many trees in the urban environment drop their leaves in large amounts. People sweep or rake them together to collect them in bags so that the refuse removal services can take them away. Meanwhile, people buy bags of fertiliser at great costs to keep their garden plants healthy. Removing the leaf litter is like preparing a good meal for someone and just before you put the plate on the table, you dump it into the waste bin.
by Dr Coert Geldenhuys, Forest Ecologist
Left: Dry forest maintained on shallow soils on rock substrate steep slopes through closed nutrient cycle, with shallow roots system developed over many years. Victoria Bay, Southern Cape. Centre: This road fill covered the root systems of the trees on the forest edge and killed all the trees during construction of Tsitsikamma Toll Road, Bloukrans. Right: Typical shallow root system of the evergreen forest.
The pattern of monthly leaf fall over four years in Groenkop evergreen forest near George, Southern Cape.
Left: This road through the Plaatbos forest, Tsitsikamma, caused the accumulation of standing water inside the forest with much tree mortality. Centre: This retainer wall across a stream and next to a large tree of Podocarpus falcatus ensured a healthy forest because of no soil fill onto the tree roots. Right: A boardwalk along a foot path through the forest to reduce soil compaction and damage to the feeder roots of the trees, Tsitsikamma forests.
Somebody once said that the forest thrives on its own decay. We need to understand how and why the cycling of nutrients is critical for the health and vitality of the forest and trees in general, to help us to minimise the disruption of this critical, everyday ecological process. We should be taking into consideration the health of existing trees as well as trees we may wish to plant in the future, when designing buildings, roads, footpaths and parking spaces.
Many people associate the falling of leaves with deciduous trees and with the cold time of the year, but for most African trees, leaf fall is associated with the dry season. The dry season in woodland areas is also the cold season, but this is not necessarily true for the evergreen forests. But do evergreen trees drop their leaves, and if they do, when do they do it because we never see them without leaves?
Every year, all trees of all species produce new leaves and at some stage also drop all the old leaves, and mostly all during a specific time of the year. In the Southern Cape evergreen forests, we studied litter fall over four years, collecting litter once a month in litter bags. Total litter fall was 3 000 kg/ha/yr in the drier forest and 4 500 kg/ha/yr in the moister forest, and about 10 000 kg/ha/yr in another study in Woodbush forest, Limpopo Province. Total litter mass increases with both increasing total mass of living material (biomass) of the forest and rainfall. Leaves form about 80% of the falling litter, with the rest made up of twigs, bark, flowers, fruit, and animals (mainly insects).
Contrary to general perception, leaf fall in evergreen forests occurs throughout the year but with a definite peak in mid-summer (December to January). Why this time of the year? This is a nutrient-conservation mechanism to cope with the characteristic nutrient-poor soils of the leached evergreen forest environment. New leaves flush in spring with the onset of new growth from the increasing temperatures and rainfall. More soluble nutrients are translocated within the tree from the old to new leaves. During mid-summer, rainfall is high, but the trees experience a physiological drought due to the high temperatures and associated high loss of water due to transpiration through the leaves (potential evapotranspiration or PET), and drop the old leaves.
The falling litter accumulates on the forest floor as standing litter, i.e. the litter on the ground in various stages of decomposition. In the Southern Cape forests this amounted to about 8 700 kg/ha in the moist forest, and about 12 000 kg/ha in the medium-moist to dry forest. The litter decomposed within two years in moist forest and four years in drier forest, which causes a gradual build-up of litter on the forest floor.
The nutrient content of leaf litter varies between different sites and different seasons. Most nutrients showed higher levels during spring (September) and/or mid-summer (December; nitrogen, phosphorous, potassium and boron). The levels of more soluble nutrients decreased from the falling litter, through the standing litter to the upper part of the soil profile, caused by the leaching of released nutrients through the high rainfall in forest environments. However, the dense mat of feeder roots concentrated at the soil surface rapidly absorbs the released nutrients before they get leached. This is known as the closed nutrient cycle, and includes large amounts of nutrients stored in the biomass of forest plants above and below the soil surface, the plant litter on the ground, and the standing litter and the feeder roots at the soil surface.
Shallow root systems
Forests in high rainfall areas grow on leached, nutrient-poor, often water-logged soils, but the closed nutrient cycle enables forests to grow and persist in a healthy condition, even on very shallow and nutrient-poor soils. In the South African mixed evergreen forests with generally higher rainfall, the trees show the typical, very shallow root systems associated with this closed nutrient cycling process. The maximum depth of the root system is immediately below the main stem, and this depth decreases towards the edge of the root plate. Most of the root system is almost confined to the top 300 mm of the soil profile. Even large standing trees of, for example, the Outeniqua yellowwood Podocarpus falcatus, have roots of >200 mm diameter that extend horizontally above the soil surface for over 40 m away from the tree. A dense mat of living and dead fine (feeder) roots is concentrated in the upper 100 mm of the soil profile; they are exposed when the upper layer of the standing litter is removed. These feeder roots absorb nutrients released from decomposing leaf and other debris on the forest floor. They are also the breathing organs of the tree at ground level. This rooting profile is further associated with the drainage of water flowing through the soil profile above the heavy clays or the firm ferri-humic layers which inhibit the penetration of feeder roots deeper down the soil profile.
Management of nutrient cycling
The mat of dense feeder roots on the forest floor is an important mechanism for direct nutrient cycling and nutrient conservation. Disturbance of the root and litter layers disrupts the closed nutrient cycle, affects forest nutrition negatively, as well as regeneration and recovery of degraded forest. This closed nutrient cycle requires careful management:
- Minimise forest destruction, but where this has happened, do not burn the debris on site but use it to facilitate rapid forest recovery. Forest clearing accelerates decomposition of the litter and burning destroys the litter and feeder roots. Rapid vegetation cover, for example, on mined sites, helps to minimise and control soil erosion, provide for litter input to build up the organic material and restore the nutrient cycling, in addition to creating a suitable micro-climate for the establishment of tree regeneration.
- Maintain the natural drainage lines during road and infrastructure construction. Most trees are particularly sensitive to poor aeration owing to flooding, siltation and soil compaction. It reduces absorption of water and nutrients, reduces aerobic root respiration and root growth, causes decay of the root system, and reduces overall plant physiology and response.
- Maintain good stands of trees with spreading roots in and near streams and rivers to buffer the violence of floods during extreme rainfall events. The dense root systems can limit the uprooting of trees and filtered the soil washed down from agricultural lands or eroding areas to limit the siltation of rivers.
- Prevent cutting through the root systems of existing large trees. Use concrete pillars with above-ground cross-beam foundations in building construction, or place pipes for water, telephone and electrical cables underneath the laterally spreading feeder roots, without cutting the roots.
- Use boardwalks and paving bricks with no cement fillings to prevent soil compaction and damage to the shallow feeder roots at picnic sites, forest walks, around big trees, and parking sites under beautiful trees.
- Use appropriate timber and use harvesting equipment designed to minimise soil compaction and damage to the tree roots.
- Accept that falling leaves are part of the health of our environment. If it is considered necessary to remove the leaves, then at least pile them in a corner of the garden to turn into compost to be used instead of fertilisers.
Published in June 2011