The impact of fire on chemical and physical forest soil properties

November 5, 2013

One of the highly significant outcomes of research – observed by myself and others at national and international level – is that the impact of fire is positively linked to (i) fire intensity and (ii) fire residence time (i.e. the time soil is exposed to high fire temperatures). The higher the fire intensity and the longer the fire residence time, the more severe will be the impact of fire on forest soils. Light intensity, fast moving fires have little harmful effect on physical and chemical forest soil properties, and can even be beneficial to their chemical status, although this is mostly of a temporary nature.

– by Neels de Ronde

In contrast, high-intensity fires – especially when linked to long residence times – can decrease macro-element budgets, particularly in the topsoil, and serously damage physical soil properties.
There are also some basic characteristics of forest floors (above the topsoil) to consider, as these can protect the soil surface effectively against fire. In severe wildfire situations, even the thickest of forest floors will be consumed completely, exposing the underlying soil to serious damage. In medium intensity fires with a long residence time, (such as slash fires ignited after clear felling), fires can be applied in such a way that much of the forest floor layers remain intact, thus protecting the soil.

Some important observations regarding the effect of fire on soil properties are:

  • Sandy soils are often already poor in nutrients, and their loose structure and texture make them more susceptible to both physical and chemical damage.
  • Soil is a poor conductor of heat so the impact of fire is generally limited to the top few centimetres of the soil surface.
  • Most of the available macro-nutrient budgets are sitting in the topsoil layer, and can thus be susceptible to fire effects if directly exposed to high intensity fire.
  • In severe wildfire conditions, all forest floors are completely consumed, and much more of the top soil layers can be damaged seriously, requiring special management attention. The best way to identify such damaged sites is to look for discoloration of the top soil, normally in shades of orange, pink, red and/or yellow.
  • If you observe that a significant percentage of the forest floor layers has been kept intact, expect no significant damage, but rather look for 'ash-bed effects', particularly where improved seedling growth has been present for a while after the fire.

Impact of prescribed under-canopy burning
Prescribed fires beneath the canopy of even-aged plantation stands are the least of a forester's worries as, if applied correctly, such fires cannot do any chemical or physical harm to the soil properties. Instead, most benefit from the 'ash-bed effect'. Where significant forest floor layers were present before a prescribed fire was applied, most of this material will remain intact, ensuring that any fire effects will be minor and positive.

The following table illustrates the affect of prescribed fire (light intensity), slash fires (medium intensity) and wildfire (high intensity) with regard to nitrogen (N) levels:

Table 1. Illustration of Nitrogen levels above and below the soil surface after a wildfire experienced at Hogsback plantation in the Eastern Cape province


The higher the fire intensity experienced, the more above-ground N is replaced to below-ground soil levels, as the forest floor material is consumed by the fire and transported in the form of ash to below-ground levels. The above table only provides part of the nitrogen movement cycle, as some is also volatilised into the air, and the remaining levels in the forest system will be replenished over time as a result of increased nitrification.

So, nothing to worry about when applying prescribed burning correctly. However, be very careful about this type of fire application on sandy soils. Limit the application of prescribed burning to stands with a significant above-soil surface forest floor, and avoid complete fuel consumption of forest floor layers to avoid direct heat exposure to the soil.

Impact of burning slash after clear felling
Slash burning after clear felling of trees is the subject of endless debates among foresters. However, I think all agree that when slash levels are no problem with tree establishment, slash burning should not even be considered, but slash should rather be spread out within the stand. When reduction of slash is a necessity, foresters normally decide on one of the following options:
A. Spreading the slash without burning application, after clear felling and timber exploitation.
B. Spreading the slash with broadcast burning application.
C. Stacking the slash in rows without burning application.
D. Stacking the slash in heaps and then burning the heaps.

I don't want to enter into a discussion about the merits of burning or not burning here, apart from pointing out that handling the slash in some way can as much as double the establishment costs. Burning applications, on the other hand, require some precautions to avoid damage to soil and nutrient status.

Intensive work studies in the Tsitsikamma region have proven that stacking slash in heaps and then burning the heaps is an expensive treatment, with broadcasting and burning slash being significantly cheaper.
From a safety point of view, burning of slash heaps produces spots (burning stacks) of very high intensity fires, which can easily become uncontrollable, even with relatively light winds (own experience and observations).

More disadvantages of stacking and burning of slash heaps vs. broadcast burning application can be summarised in Table 2.

Table 2: Comparison of advantages and disadvantages of two common slash fire treatment methods

table 2

Experience has taught foresters that after serious wildfires, an accurate assessment of wildfire damage is needed. This should be done within weeks of the fire. Such 'assessment procedures' should preferably be carried out by experts in the fields of (i) fire impact assessments on trees and (ii) fire impact assessment on physical and chemical soil properties.

Failure to conduct a proper assessment is short-sighted, and could result in reduced future tree growth because of nutrient deficiencies and tree establishment problems. The latter is particularly common where physical soil damage has been experienced (such as water-repellency), and where serious destruction of soil texture and structure has occurred, resulting in a loss of water holding capacity and nutrient availability. Such sites require complete rehabilitation, with provision made for longer-term improvement to the sites, with recommended fertiliser application.

Forest managers should be able to identify problem areas after wildfires, as they are normally linked to where the most serious damage to trees has occurred (e.g. where crown fires were experienced).

Ash-bed effect

The 'ash-bed effect' is exactly that – ash from the fire is deposited onto the topsoil, and nutrients concentrated within it are carried into the soil with moisture and this is made available to plant roots. This effect is only temporary, but can be highly beneficial, particularly to young tree plants, as it increases growth significantly.

Prescribed burning
By far the most common application of prescribed burning inside stands is in Pinus compartments, after crown canopy closure. However, the latest results show that its application inside Eucalyptus stands is also feasible, provided that some added precautions are taken. The application of the technique is seldom required in Acacia mearnsii stands after crown canopy closure, because of its excellent decomposition rates.

Published in August 2013

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