Abstract
In forest ecosystems, habitat fragmentation negatively impacts stand structure and biodiversity; the resulting fragmented patches of forest have distinct, disturbed edge habitats that experience different environmental conditions than the interiors of the fragments. In southwest Western Australia, there is a large-scale decline of the keystone tree species Corymbia calophylla following fragmentation and land use change. These changes have altered stand structure and increased their susceptibility to an endemic fungal pathogen, Quambalaria coyrecup, which causes chronic canker disease especially along disturbed forest habitats. However, the impacts of fragmentation on belowground processes in this system are not well-understood. We examined the effects of fragmentation on abiotic soil properties and ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) fungal communities, and whether these belowground changes were drivers of disease incidence. We collected soil from 17 sites across the distribution range of C. calophylla. Soils were collected across a gradient from disturbed, diseased areas to undisturbed, disease-free areas. We analysed soil nutrients and grew C. calophylla plants as a bioassay host. Plants were harvested and roots collected after 6 months of growth. DNA was extracted from the roots, amplified using fungal specific primers and sequenced using Illumina MiSeq. Concentrations of key soil nutrients such as nitrogen, phosphorus and potassium were much higher along the disturbed, diseased edges in comparison to undisturbed areas. Disturbance altered the community composition of ECM and AM fungi; however, only ECM fungal communities had lower rarefied richness and diversity along the disturbed, diseased areas compared to undisturbed areas. Accounting for effects of disturbance, ECM fungal diversity and leaf litter depth were highly correlated with increased disease incidence in C. calophylla. In the face of global change, increased virulence of an endemic pathogen has emerged in this Mediterranean-type forest.
Highlights
Forest health is threatened by multiple factors including invasive forest pests and pathogens and global change
Canker Incidence To investigate belowground drivers of canker presence, we modeled canker presence against belowground soil and fungal community factors
The most parsimonious dbRDA model for arbuscular mycorrhizal (AM) fungi suggested disturbance, soil moisture and potassium were the main drivers of AM fungal communities (F4,45 = 2.594, p = 0.001, R2 = 0.12) (Figure 2B)
Summary
Forest health is threatened by multiple factors including invasive forest pests and pathogens and global change (i.e., climate change and anthropogenic disturbance). Fragmented forests have distinct edge habitats that experience different environmental conditions than the interior of the fragments, including greater fluctuations in temperature and moisture, increased windthrow and evaporation, and increased access for people, invasive alien species including pathogens, and fire (Arroyo-Rodríguez et al, 2017; Latimer and Zuckerberg, 2017; Watson et al, 2018). Such changes in environmental conditions can cause increases in tree mortality, damage and turnover (Santo-Silva et al, 2016). The effects of exposure to multiple land uses may result in additive or interactive effects on tree health
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