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By PTA News Bureau

A simple allometric model, which is a study of the relationship of body size to shape, has been developed by Sri Lankan rubber researchers to quantify the amounts of carbon fixed in the rubber tree.

Developed by researchers at the Rubber Research Institute (RRISL), it offers mLakshman300uch convenience in field applications. As the rubber tree is capable of fixing ca. 1 MT of CO2 during its 30-year economic life cycle, a hectare of rubber having over 300 trees can produce a minimum of 300 MT of CO2 for trade. This is under an average management conditions, but more carbon could be fixed with good prices, says Dr Rodrigo, an award-winning researcher and Additional Director of the Rubber Research Institute of Sri Lanka (RRISL).

The simple allometric model offers much convenience in field applications. Using this, scientists have assessed the amount of carbon sequestered through the process of photosynthesis. In particular, growth curves of rubber under wet and dry conditions in Sri Lanka have been established for average management conditions and so, the amount of carbon fixed in rubber trees at varying age levels is known.

Therefore, sufficient information is available to build up CDM projects on rubber cultivation, said Dr Rodrigo, explaining his research. The aim is at building allometric models using simple growth indicators such as tree diameter and total height to assess the timber, biomass and carbon in rubber trees. These are commonly available aboveground.

“Although the difficulty in assessing the biomass in belowground components has been the cause for such limitation, the importance of root system in fixing carbon cannot always be ignored,” he said.

Instead of cut and felling for biomass harvesting in most of other forestry plants, rubber trees are uprooted at the end of their economic lifespan in most occasions and so, significant component of the root system is also harvested. It means the root system of rubber can be assessed for biomass and carbon to a greater extent.

With various objectives in mind, one may require quantifying biomass and carbon in different components of the rubber tree. “Our objective was to cater to such diverse requirements of rubber-based project developers; allometric models were used to estimate biomass, carbon, timber and also some other empirical equations to quantify the time cause variation in growth parameters.”

Tree diameter and total height of rubber tree reflect the growth condition and the age, and so allometric models based on these parameters are not site-specific. Also, different genotypes were used in building up these allometric models to make them universally accepted.

More importantly, allometric models developed initially were validated in completely different locations with a different combination of genotypes. This has not been the case in previous studies. No significant changes in parameters of original functions were observed in the validation; therefore these models could be used anywhere with high level of accuracy.

“Obviously, the accuracy level in biomass and carbon estimation was as high as over 95 per cent. Even the lowest level of accuracy   recorded for timber was over 85 per cent. Root biomass assessments were limited to 1 cm diameter and so, some root biomass was left behind in model development.”

This means chances for overestimation which is of most concern in carbon projects, are very remote. “Only limitation we see in our allometric models is that those cannot be used in small size plants, particularly below six years old. However, it cannot be an issue as rubber based projects are always long-term.”

(Full Text in PTA Aug/Sep issue)

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