Mechanistic-statistical model for the expansion of ash dieback

TL;DR

This paper presents a mechanistic-statistical model for the spread of ash dieback disease in France, incorporating climate, pathogen dynamics, and host density, successfully fitting 20 years of observed data and highlighting summer temperature as a key factor.

Contribution

The study introduces a novel mechanistic-statistical model that accurately predicts ash dieback spread, emphasizing the role of summer temperature and simplifying factors like the Allee effect.

Findings

Model accurately reproduces 20-year disease spread in France.

Summer temperature, especially days above 28°C, is the main driver.

Allee effect and spring rainfall heterogeneity are less influential.

Abstract

Hymenoscyphus fraxineus is an invasive forest fungal pathogen that induces severe dieback in European ash populations. The spread of the disease has been closely monitored in France by the forest health survey system. We have developed a mechanisticstatistical model that describes the spread of the disease. It takes into account climate (summer temperature and spring rainfall), pathogen population dynamics (foliar infection, Allee effect induced by limited sexual partner encounters) and host density. We fitted this model using available disease reports. We estimated the parameters of our model, first identifying the appropriate ranges for the parameters, which led to a model reduction, and then using an adaptive multiple importance sampling algorithm for fitting. The model reproduces well the propagation observed in France over the last 20 years. In particular, it predicts the absence of disease impact in the south-east of the country and its weak development in the Garonne valley in south-west France. Summer temperature is the factor with the highest overall effect on disease spread, and explains the limited impact in southern France. Among the different temperature indices tested, the number of summer days with temperatures above 28{\textdegree}C gave the best qualitative behavior and the best fit. In contrast, the Allee effect and the heterogeneity of spring precipitation did not strongly affect the overall expansion of H. fraxineus in France and could be neglected in the modeling process. The model can be used to infer the average annual dispersal of H. fraxineus in France.