New research highlights the need for policies that encourage nurseries to produce home-grown plants and thus reduce the risk of importing tree pests and diseases that threaten the UK’s woodland. Author of Variability in commercial demand for tree saplings affects the probability of introducing exotic forest diseases, Vasthi Alonso Chavez and British Ecological Society Policy Manager, Brendan Costelloe explain more.

A Spanish version of this post is available here.

The nursery trade involves large-scale movements of trees for a forestry industry that generates considerable economic value. However,  this trade is also a pathway for the introduction of tree pests and diseases, which bring with them significant costs to the industry and the wider economy. Some examples include oak processionary moth and ash dieback. Ash dieback is native to Asia but was first discovered in the UK in 2012 in a nursery in Southern England, in plants which had been imported from the Netherlands. The disease causes high levels of mortality in all age classes of ash trees across Europe and the UK. Oak processionary moth is native to southern Europe and affects the health of oak trees, people and animals.  This pest was introduced to Britain in 2005, possibly as eggs on oak trees imported from continental Europe.

Figure 1
How does the probability of introducing an exotic pest or disease changes with demand variability when maximum profit is reached? In this scenario the probability of introducing an exotic pest or disease is increased as the demand variability grows. The probability grows faster as the production costs get closer to the import costs.

Forest nurseries grow seed, seedlings or saplings until they can be sold at a more mature stage. One of the main challenges that forest nurseries face is demand variability, which is the difference between expected and actual demand. Nurseries often receive short notice of requirements for trees for planting, even though they may require up to three years to produce a tree seedling, depending on the species, and longer if targeted seed collection is required.

In order to avoid surplus stock, businesses err on the side of caution by producing fewer trees and then importing more, as and when there is sufficient demand. Crucially, it is these imports that tend to introduce exotic diseases. Our study therefore set out to understand the extent to which demand variability contributes to the risk of introducing new tree diseases, and whether a decrease in demand variability reduces this risk.

Figure 2
Results of the dynamics of a nursery with a growing tree population. Sales take place when trees reach certain size. The blue contours show the gross margin of the nursery. The darker the blue the larger the gross margin. Black lines display contours of the probability of introducing a pest or disease. The red dotted line shows where the maximum gross margin is obtained. These contours depend on demand variability and planting rate. In this system, the expected average tree demand is μ4 = 1000 trees and the variability ranges from α=0 to α=500 trees, equivalent to α = [0, μ/2] in terms of the mean demand and the planting rate varies between R = [400,1500] trees, equivalent to R = [2μ/5, μ] in terms of the mean demand. Imports costs are 0.15 units/tree and production costs are 0.0375 (left panel) and 0.1125 units/tree (right panel).

This study by Vasthi et al. shows that when the cost of producing a tree in a UK nursery is considerably smaller than the cost of importing a tree, demand is met without importing trees and there is little increased risk of introduced pests.  Conversely, when the cost of producing a tree in the nursery is similar to the cost of importing a tree, businesses tend to import trees as this removes the risk of surplus stock. Therefore, an increase in demand variability leads to an increased risk of importing exotic diseases and pests.

However, this study demonstrates that a balanced management of demand variability and costs could reduce the risk of importing an exotic forest disease according to the management strategy adopted. Growers need a range of measures to help them manage fluctuations in demand and the difference in import and production costs.  These include developing longer-term grant schemes to allow nurseries to plan their medium and long-term tree planting to overcome demand variability while maximising their gross margin. Policies such as production subsidies or increase in import duties can incentivise growers to sell home-grown plants and can also help reduce the probability of importing tree pests and diseases. Such an approach would go a long way towards safeguarding the UK’s much-loved woodlands.

Read the full open access paper, Variability in commercial demand for tree saplings affects the probability of introducing exotic forest diseases in Journal of Applied Ecology.