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August : The role of biomass in sustainable stand productivity

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19 August, 2014

The role of biomass in sustainable stand productivity

Globally many industries are pressured by consumers and incentivised by governments to reduce their carbon footprint by developing and embracing new and alternative forms of green energy. As a result the use of biomass in energy generation and biofuel production has increased significantly and has proven to be a reliable source of renewable energy in many countries. Biomass from the agricultural industry is widely used and in several countries forestry biomass is also successfully collected for energy generation.

The sheer amount of biomass generated during harvesting makes forestry biomass an attractive option for energy generation. However, these residues also play an important role in sustaining the nutrient health and  physical soil environment of the plantation. 

Wood Southern Africa and Timber Times visited the Institute for Commercial Forestry Research's    (ICFR) Dr Steven Dovey, Dr Diana Rietz and Dr Louis Titshall to learn more about the role of biomass in the sustainability of commercial forest plantations, and the ICFR's research in this area.

Dovey first explained that biomass is defined as any organic material, and in forestry relates specifically to stemwood, bark, branches, leaves, litter, roots and stumps. 

In commercial forestry, non-merchantable biomass is mostly generated during thinning and harvesting operations where trees are sometimes debarked and cut-to-length in field. The extraction of tannin from the bark of black wattle trees also results in a large amount of bark residue being removed from sites and in the sawmilling industry the by-products of wood processing such as bark, sawdust and off-cuts are also considered biomass. 

In the production of tannin and in the sawmilling industry biomass is successfully used to power some of the operations such as drying kilns. Sometimes bark from other commercial species are used to fire boilers at the processing plants. This biomass is, however, removed from the plantation site and the nutrients and organic carbon it could contribute back to the soil is thus potentially lost.

Dovey explained that forestry biomass has a massive amount of nutrients and a very high carbon content. 

"When these branches, bark and leaves decompose into the soil they can supply nutrients for around half of the next rotation of a pulpwood stand. When all the biomass is routinely removed, the plantation is at risk of nutrient depletion which can cause the stand to collapse," notes Dovey.

Further, Rietz explained that decomposing biomass replenishes soil carbon. She said this is vital for the health of the soil and prevents soil erosion. It also locks moisture into the soil and makes nutrients available when the trees need it.
"South Africa is a water scarce country and the soil's ability to lock in moisture is very important for tree survival. The nutrients and carbon also feed the organic microbial life in the soil. It gives the soil life and ensures a productive system," explained Rietz. 

Meanwhile, biomass residue also protects the surface of the soil and prevents compaction when heavy machinery is used in the plantation.

The ICFR's research goals: Linking theory and practice 
Dovey is currently researching the impact of biomass removal on the nutrient sustainability of commercial forestry plantations. 

His research aims to deplete the nutrient supply capacity of selected sites by planting a dense stand and then continuously removing all biomass. Dovey will also test the efficacy of fertiliser and biomass return on parts of the stand, and he will try to rehabilitate the site after it has lost the ability to keep supplying trees with nutrients. 

Dovey gave an example to explain the concept of this concentrated study.  "We will for instance plant a site at a very high density or up to 10 000 trees per hectare. Trees will then be harvested every year as opposed to every ten years and biomass will continuously be removed from the site."
This trial mimics 30 years of biomass removal in a time frame of three years. 

Studies done in the Northern hemisphere, where rotations are between 80 to 100 years, have detected a decline in nutrients up to 30 years after biomass removal.

In addition, Titshall noted that local and international studies and trials are looking at which biomass components can be removed from the plantation, how much of it can be removed and how often it can be removed without depriving the stand of essential nutrients and carbon.

Rietz explained that the components that are most detrimental if removed are leaves and bark which are high in nutrients and carbon.
The role of biomass in sustainable stand productivity
She added that from a site sustainability perspective the branches will be the most suitable component to harvest for biomass energy generation as they are generally the least nutrient dense component of the biomass. 

The results from Dovey's study will be of the utmost importance as it is essential in forestry to curb any potential problems right from the start. When a mistake is made in forestry the grower has to deal with the problem for the next seven to 25 years. 
Titshall stressed that the ICFR is focussed on the bigger picture of sustainability.

"We take an objective position when it comes to biomass. At the ICFR we are trying to understand the impact of biomass removal on the quality of the soil. We know that carbon is essential for healthy productive soil. Removing the carbon will have consequences and this is why we are trying to establish if and how biomass removal can be done properly and sustainably.

"There is no point in the pursuit of biomass if the plantation crashes in three rotations because then the primary and secondary products will be lost. The cost of restoring a degraded site is high and is avoidable."

Biomass residue challenges
Even though biomass residue plays an important role in the health of the plantation, the management of these residues can be challenging. 

Dovey noted that on average for every 100 tons of stemwood harvested about 10 tons of branches are left behind. He added that this amount of branches would be ideal for biomass energy generation, but is a formidable task to clear.

"It is very expensive to collect and transport biomass and can only be economically viable when moved within 50 km from the source.

Locally biomass residues are mostly burned in the plantation after harvesting. Unfortunately some of the nutrients, especially nitrogen is lost, but a fair amount is returned to the soil through ash deposits. 

Removing all the biomass will greatly impact the nutrient cycle, therefore burning is less harmful than complete biomass removal. Burning is a cost-effective way to ensure that the stand is safe and accessible for subsequent silviculture operations and it reduces the fire risk," he explained.

Other biomass residues such as roots and stumps are high in volume, but also very expensive and difficult to remove. The size of the stumps and contaminants such as dirt and rocks embedded in the roots makes this residue product practically and economically non-viable for use in biomass boilers. Extraction of these can also cause massive site disturbance potentially increasing erosion and nutrient loss.

Other challenges
Apart from challenges in the field biomass still has a lot of hurdles to cross before becoming a widely used renewable energy source in South Africa. 

Rietz explained that even though a few large corporates are considering and using alternative energy, there is currently not a large market for biofuels and biomass in the country. She added that it is also difficult to achieve a good cost balance between the energy used to remove the biomass and the energy that it actually generates. This process is only cost effective when the biomass source is near the energy plant or if the plant is mobile and can be moved to the biomass site.

Titshall noted that political and economic issues may also restrict the biomass energy industry as South Africa is not yet on par with European countries in terms of competitive energy markets, carbon tax and green policies. These aspects are being investigated by government departments at this time.

Other countries such as Ireland, where the government offers significant incentives to drive green energy, have developed whole infrastructures around biomass energy generation. This includes harvesting, transport, pelletizing production plants and energy plants. 

"At this time South African plantation forestry is geared to supply timber and pulp and is not focussed on biomass production, as this is not the core business. However as green energy opportunities arise these possibilities are being investigated both through ad-hoc investigations and more consolidated studies where the feasibility and sustainability of increasing biomass removal is being investigated. This, along with the need for South Africa to come in line with international carbon monitoring and taxation protocols, will hopefully result in a sustainable solution for both energy from biomass and site quality," explains Titshall.
Why are forestry companies interested in biomass?

Dovey explained that forestry companies are interested in biomass energy partly because of the high cost of electricity and unreliable supply and partly to contribute to the green economy.

"Markets are tough and forestry companies are trying to get more out of their plantations. Industries are also increasingly pressured to reduce their carbon footprint. Therefore many companies are looking into how they can use biomass to power their own operations," added Dovey.

He also explained that in future forestry companies may benefit from renewable energy incentives if they generate part of their energy from green sources.

"It might be more beneficial for forestry companies to produce their own energy from a certain percentage of forestry resources. These resources should only come from sites that are very resilient."
Meanwhile, Titshall added that South Africa has a massive diversity of commercial forestry plantation sites from tropical Zululand to cool temperate Mpumalanga.

He explained that the impact of natural carbon cycles varies from site to site.

"In the cooler parts of Mpumalanga a challenge is dealing with too much biomass, especially in stands planted to saw logs, which are pruned and thinned during their 25 year rotations. The resulting residues do not break down fast enough because it is too cold in the area and you get very large accumulations of forest floor biomass which poses fire and nutrient availability risks.
"In Zululand, residues break down much faster owing to the sub-tropical climate, but there foresters need to deal with a sensitive soil type that does not hold nutrients. So the carbon is very important for plantations based in Zululand and other similar sites," noted Titshall.

Proper site analysis and understanding is important in identifying which sites can be good potential biomass suppliers without being susceptible to loss of plantation productivity. 

Further, Rietz said that there is a good theoretical understanding of site sensitivity and site composition, but a lot of practical research still needs to be done to determine the long-term consequences of biomass harvesting on the various sites. 
Dovey's research could reveal essential information on the topic of site sensitivity.

Closing comments

Biomass and biofuels come at a cost, not only economically but also to the site, soil and environment. At the end of the day it is about finding the economic and sustainability balance, notes Titshall. 

He added that it is too expensive and difficult to try and fix something that is broken. "To recover soil that is depleted of nutrients and carbon is a daunting and expensive task."
Titshall and Rietz said Dovey's research on the impact of biomass removal on site health is a novel approach and will yield extremely useful, first of kind results that could pave the way for the future of biomass in South African commercial forestry. It hopes to provide the short-term answers to what are very long-term questions.

Dovey started his research twelve months ago and will collected results over the next three years. 
For more information please email Dr. Steven Dovey at
Quick Q&A on ash and fertiliser with Dr Steven Dovey Can fertiliser be used to replace soil nutrients lost as a result of the removal of biomass?Yes, fertiliser will be useful on resilient sites. However, it will require a massive amount of fertiliser and this will be very expensive. Fertiliser application is labour intensive and the production of fertiliser is carbon intensive. There may also be negative environmental impacts of excessive fertiliser use as has been found in other agricultural sectors.
Can the ash from burning biomass in a boiler be used to fertilise soil?

There has been some talk in the industry about using this ash. However, in terms of regulations, ash is still considered an industrial waste by-product that must be disposed of according to environmental regulations, especially if contaminated with hazardous compounds and metals. If it was classified as an agricultural by-product it could be used as soil ameliorant, but because it is not a dense, solid product it is difficult to transport and use. Owing to the fact that it can become airborne it makes it even more difficult to handle, so a good way to make it more manageable would be to pelletize it.

Again it is about balancing the costs. At this stage it does not seem economically viable to use ash.

Source : Wood Southern Africa & Timber Times