2020, Springer Nature B.V. Plant fungal relationships should vary with abiotic and biotic factors to minimise plant stress and are likely to vary seasonally and with age. We investigated how fungal colonisation, specifically arbuscular mycorrhizal fungi and dark septate fungal endophytes, would vary with species identity and season, and how these interactions change with ontogeny. Plant roots of adults and seedlings of 9 species were collected from heathland and coastal dune habitats along the Australian east coast in New South Wales. Roots were stained and investigated for arbuscular mycorrhizal fungi and dark septate endophyte structures to determine colonisation strength. Species identity was the most important factor driving colonisation strength, while low rainfall and heatwaves were associated with declining arbuscular mycorrhizal fungi colonisation in the warmest sampling period. AMF colonisation may be supressed by plants under heat and water stress as a way of avoiding loss of limited photosynthates. Dark septate endophyte colonisation was more common in this time period and may assist with the stress of the warmer, drier conditions. Colonisation by arbuscular mycorrhizal fungi differed with age but in unpredictable ways and, along with dark septate endophytes, was evident even in plants that are considered non-mycorrhizal, although more extensive in known mycorrhizal species. The lack of arbuscular mycorrhizal fungi colonisation and the increase in dark septate endophyte colonisation during the most stressful period suggest an uncoupling mechanism in the symbiotic relationship which needs further investigation.
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