Direct and indirect effects of forest microclimate on pathogen spillover

Abstract

Disease dynamics are governed by variation of individuals, species, and environmental conditions across space and time. In some cases, an alternate reservoir host amplifies pathogen loads and drives disease transmission to less competent hosts in a process called pathogen spillover. Spillover is frequently associated with multi‐host disease systems where a single species is more tolerant of infection and more competent in pathogen transmission compared to other hosts. Pathogen spillover must be driven by biotic factors, including host and community characteristics, yet biotic factors interact with the abiotic environment (e.g., temperature) to create disease. Despite its fundamental role in disease dynamics, the influence of the abiotic environment on pathogen spillover has seldom been examined. Improving our understanding of disease processes such as pathogen spillover hinges on disentangling the effects of interrelated biotic and abiotic factors over space and time. We applied 10 yr of fine‐scale microclimate, disease, and tree community data in a path analysis to investigate the relative influence of biotic and abiotic factors on pathogen spillover for the emerging infectious forest disease sudden oak death (SOD). Disease transmission in SOD is primarily driven by the reservoir host California bay laurel, which supports high foliar pathogen loads that spillover onto neighboring oak trees and create lethal canker infections. The foliar pathogen load and susceptibility of oaks is expected to be sensitive to forest microclimate conditions. We found that biotic factors of pathogen load and tree diversity had relatively stronger effects on pathogen spillover compared to abiotic microclimate factors, with pathogen load increasing oak infection and tree diversity reducing oak infection. Abiotic factors still had significant effects, with greater heat exposure during summer months reducing pathogen loads and optimal pathogen conditions during the wet season increasing oak infection. Our results offer clues to possible disease dynamics under future climate change where hotter and drier or warmer and wetter conditions could have opposing effects on pathogen spillover in the SOD system. Disentangling direct and indirect effects of biotic and abiotic factors affecting disease processes can provide key insights into disease dynamics including potential avenues for reducing disease spread and predicting future epidemics.

Wildfire alters the disturbance impacts of an emerging forest disease via changes to host occurrence and demographic structure

“…Total stand‐level tree basal area, which may correlate with canopy cover and forest microclimate, decreased in areas that had burned since 1950 (Figure 3); however, following the 2008 fires, total surviving tree basal area had no clear effect on infestation patterns, separate from the effects of bay laurel survival and broader‐scale climatic variation. Other studies suggest that microclimate influences P. ramorum pressure but that its effect may be less than that of biotic factors, such as host density (Dillon & Meentemeyer, 2019). In our study, possible microclimatic impacts of intact host canopies cannot be separated from their effects on forest composition and structure, but the importance of dry‐season temperature in our analysis suggests that possible post‐fire microclimatic shifts warrant further study (Figure 1e).…”

Associational resistance to both insect and pathogen damage in mixed forests is modulated by tree neighbour identity and drought

“…In particular, differences in tree height between species can lead to alterations in canopy closure between plots differing in tree diversity (Muiruri & Koricheva, 2017). Alterations to the abiotic environment (such as changes to light availability, temperature and humidity) can all have direct impacts on pests and pathogens' survival or performance (Dillon & Meentemeyer, 2019;Jactel, Koricheva, & Castagneyrol, 2019;Stoepler & Lill, 2013), as well as indirect effects via changes to tree leaf traits, such as secondary metabolites Forey et al, 2016;Walter et al, 2012).…”

Vegetation ecology with anthropic drivers and consequences

“…Recent sudden oak death and white pine blister rust outbreaks illustrate processes wherein characteristics of the vegetation affect disease spread (i.e. because of alternate hosts) and the mortality of keystone species affects plant community structure and diversity (Dillon and Meentemeyer, 2019; Tomback et al, 2016). Responses to pests and weeds, especially in agriculture, can be informed by vegetation ecology (e.g.…”