Written by: Bui Le Vinh, edited by: Madelline Romero

When farmers at Ma village began noticing that their acacia trees developed a rotten core, they found themselves pushed to planting eucalyptus almost exclusively. Known for its fast growth, low maintenance, and high adaptability to soil and rainfall conditions, eucalyptus timber also fetched a slightly higher price from veneer-manufacturing plants in the area.

Fifteen years on and over 90 percent of sloped land in Ma, a village 160 km northwest of Hanoi in Vietnam’s Yen Bai province, is planted with eucalyptus.

But in trying to secure a reliable livelihood, did the smallholder farmers at Ma inadvertently make themselves more vulnerable to climate change in the future?

Root rot, probably Ganoderma sp. Acacia mangium Photo grabbed from fao.org

A balancing act between providing for immediate needs and looking out for the future

“Eucalyptus is an economically important tree for many communities in Southeast Asia, but it is a tree species that is not good for soil health at all. Its strong, deep root system sucks up all water and nutrients around it, while its leaves return nearly nothing at all by way of nutrients back into the soil,” explains Dr. Didier Lesueur, senior soil microbiologist at CIAT.

A monoculture of eucalyptus for long periods degrades the quality of the soil, eventually rendering it unable to support healthy plant growth in the future. And for smallholder farmers such as those living in Ma, poor soil health would mean yet another layer of protection removed to shield them from possible impacts of a changing climate – yield and income losses due to higher temperatures, droughts and changing rainfall patterns.

While CIAT is yet to scientifically analyze the quality of soil at Ma village, farmers have noted the lowest yield in cassava among those planted on soil where eucalyptus trees had formerly grown as compared to yields from those grown on areas either exclusively planted with cassava or intercropped with acacia trees.

Crops planted in soil previously planted with eucalyptus produce very low yields. Not even grass would grow in eucalyptus plantations,” 62-year old Thuan Tran, a local farmer, has observed. 

Eucalyptus and acacia: two complementary tree species

Acacia, on the contrary, helps improve soil fertility because of its biological nitrogen-fixing capability.

“What we need to do,says Dr. Lesueur, is identify the cause of this disease, the susceptibility of acacia trees and help address this so that farmers could be encouraged to plant acacia again.The disease – Ganoderma root rot – and another called Ceratocystis canker and wilt disease have affected acacia plantations in Southeast Asia, with the latter causing up to 20 percent damage in some plantations in Vietnam.

Dr. Lesueur is convinced of the benefits of a mixed plantation consisting of both acacia and eucalyptus. While the proportion of each tree species could be determined by a number of factors including the size of the land and availability of water, he explains that the two species could complement each other – where one (eucalyptus) uses up so many soil nutrients, the other (acacia) replenishes them – while providing economic benefits to farmers in a sustainable manner. While such model has never been tried in Southeast Asia, it has shown positive outcomes in Brazil and DR Congo.

Exploring the relationship between seedling health and the tree’s vulnerability to disease

CIAT team selects samples of acacia seedlings to be studied for their nodulation and plot quality. Photo by Vinh Bui, CIAT

“We are aware of the negative effects of eucalyptus on our soils,” says Hung Tran, provincial director of the country’s Department of Agriculture and Rural Development. Two neighboring provinces – perhaps, not heavily affected by the acacia disease – have already banned the planting of eucalyptus. We would really want to plant more of the Acacia mangium but it seems more susceptible to diseases.”

In the effort to identify all potential factors contributing to the vulnerability of acacia to the disease, the CIAT team led by Dr. Vinh Bui, has approached the problem by looking first at the seedlings that farmers use to grow acacia.

In June 2016, the team collected 30 acacia seedlings from each of 26 nurseries found in and around Ma and its neighboring communes and district, to be assessed for their nodules – swellings on the roots that contain nitrogen-fixing bacteria that convert gaseous nitrogen into ammonium nitrogen – a nutrient important to plant health. The more nodules there are, the more atmospheric nitrogen will be “fixed,” and the healthier the acacia plant will become. Analysis of the plot establish? the amount of phosphorus and nitrogen present, and whether conditions encourage root symbiosis with mycorrhizal fungi, important microorganisms in the soil that help increase the capacity of the plant to absorb nutrients from the soil.

Results of the seedling nodulation analysis are expected to come out in December 2016, and even while awaiting results, the CIAT team is preparing to investigate the correlation between a robust acacia seedling and the subsequent tree’s vulnerability to the disease. With help from farmers who will be trained on optimal seedling preparation, CIAT will conduct field trials beginning in 2017.

If seedling quality is found to be an important factor in the acacia tree’s susceptibility to Ganoderma root rot, the CIAT team plans to train farmers on seedling preparation to help minimize the risk of infection and ensure this potentially important option for adapting to climate change is feasible in Ma, and beyond.

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This action is part of the Climate-Smart Agriculture (CSA) initiative addressing the vulnerability of farmers to impacts of climate change by testing, evaluating, promoting and scaling up integrated portfolios of CSA technologies and practices. As a Climate-Smart Village, Ma is a testing ground for CSA technologies and practices that would help improve agricultural productivity and the village’s climate resilience, while reducing greenhouse gas emissions from its agricultural systems. The initiative is implemented by the International Center for Tropical Agriculture through the CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS). CCAFS is supported by https://ccafs.cgiar.org/donors.

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