Reducing year-to-year variability in the demand for trees for planting could reduce the import of exotic pests and diseases that can damage local forests and woodlands.

  • 15
  • AUG
  • 2018

Oak processionary moth and ash dieback are among the most notorious tree pests and diseases introduced into the UK. And many exotic pests and diseases are suspected of having been introduced, or are known to have been introduced, through the import of commercial tree planting material.

New research, which unravels the dynamics of tree production, economics and variability in demand, shows how to reduce the risks of importing such damaging forest foes. The findings, from a team led by Rothamsted Research with the University of Cambridge, are published today in The Journal of Applied Ecology.

Tree nursery Credit: Vasthi Alonso-Chavez, Rothamsted ResearchBalancing production to demand is particularly tricky for tree nurseries where seedlings must grow for three years

“Our results suggest that a balanced management of demand variability and costs can significantly reduce the risk of importing an exotic forest pest or disease,” says Vasthi Alonso-Chavez, an epidemiological modeller at Rothamsted. She has developed a mathematical model to understand the problem.

The model assesses the impacts that three scenarios of increasing variability of demand has on the relationship between gross profit margins for nurseries and the likelihood of introducing an exotic disease: where locally producing a tree is 25%, 50% and 75% of the cost of importing a tree from Europe.

“When the cost of producing a tree in a UK nursery is considerably smaller than the cost of importing a tree, the risk of introducing an exotic disease is hardly affected by an increase in demand variability,” notes Alonso-Chavez. “But the risk increases as the cost ratio diminishes.”

Uncertainty in market demand is one of the main challenges for nursery businesses, says Alonso-Chavez. “Nursery growers often receive short notice of requirements for trees for planting…even though nurseries require up to three years to produce a tree seedling.”

Demand figure Credit: Vasthi Alonso-Chavez, Rothamsted ResearchThe figure shows how the probability of introducing an exotic pest or disease changes with demand variability (for maximum gross margin). In this scenario, the probability of introducing an exotic pest or disease is increased as the demand variability grows. The number of trees demanded in a year is a random number drawn from the interval (μ-α, μ+α), where μ is the mean demand and α is a measure of the demand variability. Here, the mean demand is 1000 trees and the variability ranges from 0 to 500 trees. The figure shows that the probability grows faster as the production costs get closer to the import costs.

As variability in demand hampers growers’ ability to gauge how many trees to produce, nurseries cut their potential losses by restricting production to reduce the number of unsold trees. If demand rises and there is a shortfall, they buy trees from foreign sources to make up the difference.

Nearly 300 plant pathogens were introduced into Britain over the past 50 years, of which 10 species affect the forestry sector. About two thirds of introduced species are native to continental Europe, chiefly the Netherlands and France, with 10% from both North America and Asia.

Introduced pathogens from across continents can have catastrophic effects, record the researchers. Ash dieback (causal agent Hymenoscyphus fraxineus), native to Asia, was first discovered in the UK in 2012 in a nursery in southern England, in plants that had been imported from the Netherlands. The disease causes high levels of mortality in all age classes of ash trees across Europe and the United Kingdom. This is a big problem as ash is among the top five most common broadleaf trees in Britain.

Ash Dieback leaves Credit: Vasthi Alonso-Chavez, Rothamsted ResearchAsh dieback, in leaves (above) or in branches (top), can have devastating effects on woodlands

Oak processionary moth (Thaumetopoea processionea), native to southern Europe, affects the health of oak trees, people and animals.  According to the Forestry Commission, “a protein in the caterpillars’ tiny hairs can cause skin and eye irritations, sore throats and breathing difficulties in people and animals who come into contact with them.”

Growers need a range of measures to help them to manage the fluctuations in demand and the differences in import and production costs, says Alonso-Chavez. She suggests longer-term grant schemes, to allow nurseries to plan their tree planting, and higher import duties, to encourage growers to sell home-grown plants.

About Rothamsted Research
Rothamsted Research is the longest-running agricultural research institute in the world. We work from gene to field with a proud history of ground-breaking discoveries, from crop treatment to crop protection, from statistical interpretation to soils management. Our founders, in 1843, were the pioneers of modern agriculture, and we are known for our imaginative science and our collaborative influence on fresh thinking and farming practices.
Through independent science and innovation, we make significant contributions to improving agri-food systems in the UK and internationally. In terms of the institute’s economic contribution, the cumulative impact of our work in the UK was calculated to exceed £3000 million a year in 20151. Our strength lies in our systems approach, which combines science and strategic research, interdisciplinary teams and partnerships.
Rothamsted is also home to three unique resources. These National Capabilities are open to researchers from all over the world: The Long-Term Experiments, Rothamsted Insect Survey and the North Wyke Farm Platform.
We are strategically funded by the Biotechnology and Biological Sciences Research Council (BBSRC), with additional support from other national and international funding streams, and from industry. We are also supported by the Lawes Agricultural Trust (LAT).
For more information, visit; Twitter @Rothamsted
1Rothamsted Research and the Value of Excellence: A synthesis of the available evidence, by Séan Rickard (Oct 2015)

The Biotechnology and Biological Sciences Research Council is part of UK Research and Innovation, a non-departmental public body funded by a grant-in-aid from the UK government.
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About LAT
The Lawes Agricultural Trust, established in 1889 by Sir John Bennet Lawes, supports Rothamsted Research’s national and international agricultural science through the provision of land, facilities and funding. LAT, a charitable trust, owns the estates at Harpenden and Broom's Barn, including many of the buildings used by Rothamsted Research. LAT provides an annual research grant to the Director, accommodation for nearly 200 people, and support for fellowships for young scientists from developing countries. LAT also makes capital grants to help modernise facilities at Rothamsted, or invests in new buildings.