Fire

Fires are a major constraint to forest regeneration. Infrequent, low-intensity fires may slow succession and alter the composition and structure of regenerating vegetation, fire can occur naturally in all tropical forest types, even the wetter ones.

Fire spreading in agriculture plantationIt is in the seasonally dry tropics where fires are the most prevalent threat to forest regeneration. By the end of the rainy season, weedy vegetation has often grown above head height and is practically impenetrable. In the hot season, this vegetation dies back, dries out and becomes highly flammable. Each time it burns, most of the tree seedlings that may have gained a roothold amongst the weeds are killed, whereas the weeds and grasses survive, re-growing from rootstocks or seeds protected beneath the soil. Thus, the weedy vegetation creates conditions conducive to fire and in doing so prevents the establishment of trees that could shade out the weeds. Breaking this cycle is the key to restoring seasonally dry tropical forests.

Causes of fire

Fires can be started naturally by lightning strikes and volcanic eruptions. But, such natural fires are infrequent. These days, however, most fires are started by humans. The most common reason for starting fires is to clear land for cultivation. The fires spread from cultivated land into surrounding areas, where they kill young trees, effectively halting forest regeneration. 

Effects of fire on regeneration

Frequent fires reduce both the density and species richness of the tree seedling and sapling communities. Burning reduces the seed rain (by killing seed-producing trees) and the accumulation of viable seeds in the soil seed bank. Fire burns off soil organic matter, leading to a reduction in the soil’s moisture-holding capacity (the drier the soil, the less favourable it is for tree seed germination). It also reduces soil nutrients.

Prevention of fire damage

Using fire to fight fire. (A) Slash two strips of vegetation at least 8 m apart. (B) Drag the cut vegetation into the centre. (C) Allow a few days for the cut material to dry out, and then (D) burn it, taking extreme care not to allow the fire to spread outside the firebreak.The best way to prevent them is to make sure that everyone in the vicinity supports the restoration program and understands the need not to start fires. But no matter how much effort is put into raising awareness of fire prevention amongst local communities, fire remains a common cause of failure for forest restoration projects. Most local forest authorities have fire-suppression units, but they cannot be everywhere, so local, community based fire prevention initiatives are often the most effective way to tackle the problem. Preventative measures include cutting fire breaks and organising fire patrols to detect and extinguish approaching fires before they can spread to restoration sites.

What can be done if restoration sites do burn?

All is not lost. Some tree species can re-sprout (or coppice) from rootstock after having been burnt. Burnt, dead branches allow the entry of pests and pathogens, so cutting them off can speed recovery after burning. Prune dead branches right back, leaving a stump no longer than 5 mm. After fire, the blackened soil surface absorbs more heat, causing more rapid evaporation of soil moisture. This can subsequently kill young trees that have survived the initial fire. Therefore, laying a mulch of cut vegetation or corrugated cardboard around young, burnt trees can increase their chances of survival and re-growth.