Resilience of forests to pathogens: an evolutionary ecology perspective

R A Ennos

doi:10.1093/forestry/cpu048

Abstract

Both natural and managed forests are currently suffering from increases in damage by pathogens. Here, an evolutionary ecology approach is adopted to analyse the factors that influence the levels of pathogen damage experienced by forest tree populations and consider the conditions under which stable co-existence of trees and pathogens occurs in natural populations. The demographic and genetic responses of tree–pathogen systems to anthropogenic perturbations are explored to identify where the greatest threats to resilience lie. Problems caused by native pathogens are likely to arise as a consequence both of rapid climate change and of forest management practices that lead to increases in species density, drastic reductions in genetic diversity and planting outside the native range. The most serious threats to forest trees are posed by introduction of exotic pathogens derived from related exotic tree species. Recovery following spread of exotic pathogens is likely to be both slow and uncertain and may not be possible without intensive programmes involving rapid selection and widespread dissemination of genotypes resistant to the exotic pathogen.

Highlights

In natural and managed forest ecosystems, trees coexist with their pathogens
Given the power of pathogens to alter forest tree abundance and distribution, an understanding is needed of how pathogens interact with trees at the individual, population and community level in order both to identify the factors that have caused an upsurge in forest pathogen damage and to manage their impacts (Desprez-Loustau et al, 2007)
In terms of practical forestry, what can this analysis tell us about the disease threats to our forests and the resilience of forests to pathogens? A strong conclusion from the review is that rapid anthropogenic climate change is likely to lead to increased disease problems in forests caused both by greater pathogen pressure and by elevated stress resulting from loss of environmental adaptation

Summary

Introduction

In natural and managed forest ecosystems, trees coexist with their pathogens. Under most circumstances, the damage inflicted by pathogens on tree populations is small and acceptable, giving rise to little comment. Effective selection for higher quantitative resistance has occurred and is associated with better containment of the disease by the seedlings Another possible consequence of rapid anthropogenic climate change will be loss of local environmental adaptation in tree populations and a corresponding permanent increase in stress. Apart from transposing tree populations into higher hazard areas, range extension may lead to the establishment of plantations that are poorly adapted to local environmental conditions Under stress, their quantitative resistance to local pathogens will be compromised and severe pathogen damage may ensue. The planting of tree species on non-native sites either within their natural range or as exotics may lead to significant disease problems involving indigenous pathogens either as a consequence of greater pathogen pressure in the new planting site or due to poor adaptation to the site and increased stress. Current damage by Dothistroma on P. contorta in British Columbia may be due to climate change and to planting of P. contorta on sites that would naturally have carried western hemlock, Tsuga heterophylla, and subalpine fir, Abies lasiocarpa (Dale et al, 2011)

Introduction of exotic pathogens

Findings

Conclusions