Production Phenology of Fine Roots and Ectomycorrhizal Hyphae Are Closely Linked but Vary Across Monospecific Stands of Temperate Tree Species

Authors

Luke McCormack¹* (lmccormack@mortonarb.org), Newton Tran¹, Marvin Lo¹, Colleen M. Iversen²

Institutions

¹Center for Tree Science, The Morton Arboretum, Lisle, IL; ²Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN

Abstract

The production of fine roots and mycorrhizal fungi together account for between 15 and 45% of annual productivity in forest ecosystems though these estimates are highly uncertain. The root-fungal synergy is responsible for acquisition of water and nutrients heterogeneously distributed throughout the soil profile and sporadically available in pulses of availability throughout the year. Given the high cost to construct and maintain roots and mycorrhizal fungi, and the variability in resource availability, the timing of root and fungal production plays an important role in determining the effectiveness and efficiency of belowground resource acquisition.

However, little is known about the timing of belowground production, especially fungal production, and how fungal phenology varies with tree hosts. In this study, the team used in situ minirhizotron imaging to characterize the timing of fine root production and the production of fungal hyphae and rhizomorphs in 10 monospecific forestry plots, each dominated by either an angiosperm or gymnosperm tree host that associates with ectomycorrhizal fungi. Images were collected at The Morton Arboretum in Northern Illinois for 3 years (2019 to 2021) that encompassed both relatively wet years (2019 and 2020) and a relatively dry year (2021).

Both roots and mycorrhizal fungi showed strong seasonality in their production. Across years, root production typically peaked around July to August (most angiosperms) to September (most gymnosperms). Production of fungal hyphae peaked shortly after peaks in root production, typically in late September to October. The similar timing and strongly positive correlation between root and fungal production echoes the close relationship between the two partners.

However, it is important to note that compared to the variation in root production observed across different tree species, and between angiosperm and gymnosperm hosts in general, the timing of peak fungal production was more consistent across all plots indicating that environmental conditions were also important. Water in particular, known to be important for fungal growth in general, appeared to be an important factor controlling fungal production across years. Peak fungal production in 2019 was roughly a month earlier than peak production in 2020. This is consistent with strong fall wetting events that occurred roughly 1 month earlier in 2019 compared to 2020. Fungal production overall was strongly reduced in 2021 compared to 2019 and 2020, with 2021 being the notably drier year. Future work will link patterns of fine root and fungal phenology to leaf phenology and environmental conditions and enable accurate parameterization of belowground phenology in Earth system models.