Red flag over deteriorating health of forest soils

June 30, 2023
Planting successive rotations of the same tree species on the same piece of land over and over is likely to have a negative impact on soil health and long term productivity of the site will decline – especially when the residue from the previous harvest has been burnt off, leaving the soil unprotected from the elements, as is the case with this community forestry enterprise in Eastern Cape.

Planting the same tree species over successive rotations carries a high risk of deteriorating soil health, say FABI researchers …

It is widely accepted in agriculture circles that practicing crop rotation is beneficial for soil health. This is because the continuous cultivation of the same plant species on the same piece of land over and over again impacts negatively on soil health, and leads to a build-up of harmful micro-organisms. There is plenty of scientific evidence to support this theory.

To address this issue, good farmers all over the world practice crop rotation, planting different crops over successive rotations, or do inter-cropping where for instance they will plant a legume in between rotations to improve soil health and biodiversity.

Many small-scale tree farmers in Zululand practice inter-cropping, planting mielies, beans or peanuts in between their Eucalyptus seedlings when they are still small, and before the canopy closes. This could be beneficial for soil health.

So why not treat short rotation tree crops the same way – and if we don’t, and we keep on planting the same species rotation after rotation, will we eventually find that soil health deteriorates to the point where the trees will no longer grow properly?

Common sense would suggest that this will be the case, but there is no ‘conclusive’ scientific evidence to support this theory. In any event crop rotation in forestry is not so easily done as rotation lengths are long (8 – 20 years), land available for forestry is limited (in South Africa) and only three species are widely grown – eucalypts, pine and wattle – all of which are geared to serve specific markets.

Moreover improvements in silvicultural practices and advances in tree breeding have thus far masked any impacts of deteriorating soil health on successive tree crops, thus reducing growers’ appetite for experimenting with crop rotation...

This Sappi compartment in Zululand has been mulched and the residue retained to cover the soil with a nutrient-rich layer that provides protection, retains moisture and gives the young trees a growth boost.

But now a group of highly regarded researchers have raised a red flag and warned that deteriorating soil health is a real risk for short rotation plantation forestry, as it is practiced in South Africa and in many other parts of the world.

A team of FABI researchers including Dr Tanay Bose, Prof Bernard Slippers, Almuth Hammerbacher, Jolanda Roux and Mike Wingfield, have analysed existing data on soil microbiomes from short-rotation forestry environments around the world which, they say, provides evidence that a build-up of harmful micro-organisms, depletion of beneficial micro-organisms, and deterioration of the physical and chemical properties of soil can result from continuous replanting of the same tree species on the same piece of land.

“Populations of unfavourable microbes can be expected to become more abundant over successive rotations,” state the FABI researchers in an article published in the journal Current Forestry Reports. “This is strongly supported by data from recent soil microbiome studies involving commercially managed forests, which provide convincing evidence of an increase in pathogenic microbes in soils of continuously replanted forests.”

Building on these findings and considering similar approaches in agriculture, the team proposes a number of practical solutions that have the potential to mitigate the deterioration of soil health resulting from planting the same species over successive rotations.

Weza farmer Jon Mackenzie uses a tractor-powered chipping machine to chip the post-harvest residue after clear felling, which is then spread evenly over the site before the re-planting operation begins. This is his strategy to conserve the health of the soil on his farm for the log term.

• Retaining post-harvest residue on the site is likely to result in healthier soils over successive rotations. This is why many growers have stopped burning the post-harvest residues and have opted for mulching or chipping.
• Application of biochar has considerable potential to enhance soil properties, nutrients and microbes in continuously replanted forests. Biochar is a carbon-rich, stable organic product made from the pyrolysis of organic biomasses such as leaves, sawdust, animal dung and wood. During carbonization, biochar releases phosphate into the soil along with other mineral nutrients, improving its fertility. Biochar also improves the physical properties and microbial biodiversity of the soil, which could further increase soil nutrient availability and carbon storage. However more research is needed to assess the impact of biochar on plantation soils.
• Crop rotation and intercropping could alleviate the negative consequences of continuous replanting of the same species in short-rotation plantation forestry. For example, rotating between eucalypts, black wattle and pine species could prevent the accumulation of harmful soil microbes detrimental to these trees. Rotating nitrogen fixing leguminous tree species such as Acacia with eucalypts or pines has the potential to further promote both soil and tree health increasing the availability of nitrogen in the soil and improving the quality of plant litter.
• Practicing agro-forestry, where different crops are inter-planted on the same piece of land, would also have a positive impact on soil health.
• Innoculation of tree seedlings in the nursery with beneficial microbes.
“Continuous replanting practised in short-rotation plantation forests is likely to be accompanied by a high risk of ‘replanting syndrome’ in plantations. While long-term monitoring programs to document the changes in soil microbiomes are still lacking and should be urgently initiated, the available evidence suggests that short-rotation forest plantation enterprises could be restrictive when successively establishing plots with the same or nearly the same genotypes,” the FABI team concluded.

Mulching after clear felling is an expensive exercise, but many growers have opted for this approach instead of burning off post-harvest residues, in the interests of boosting soil health for the long term.

For the full Report see: Current Forestry Reports.

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