PS 44-189 - Effects of experimental warming and reduced rainfall on fine root and ectomycorrhizal fungal mycelium production at the temperate-boreal forest ecotone

Abstract: Warming temperatures and reduced soil moisture under ongoing climate change may shift carbon allocation below-ground to fine roots and their mycorrhizal fungal partners. While fine root biomass has been shown to increase with warming, it is unclear how warming and reductions in soil moisture may interact to influence below-ground production in forests. In addition to fine root production, climate change will also affect the mycelium yield of ectomycorrhizal (ECM) fungi. This is relevant given that these ECM tree root symbionts play an important role in biogeochemical cycling, especially in boreal forests. To date, little is known about the quantity and phenology of ECM mycelium production, particularly in response to climate change stressors. To address this research gap, we leveraged the B4Warmed experiment in northern Minnesota, USA, which has a factorial combination of experimental warming (ambient, +3.3ºC) and reduced rainfall (ambient, 30% reduced rainfall). Root ingrowth cores were deployed for one growing season (May-November) in order to capture seedling fine root production. To estimate ECM fungal mycelium production, mesh ingrowth sandbags were buried and harvested after two-month intervals (May-June, July-August, September-October). Their contents will be sequenced to assess the active fungal communities throughout the growing season, and quantitative PCR will be utilized to estimate fungal biomass present in the bags.

Initially, we hypothesized that fine root production would increase with warming alone (consistent with previous studies) and decrease under combined warming and reduced rainfall due to detrimental effects on seedling growth. Instead, we observed a lower mass of fine roots produced in the experimentally warmed plots compared to the ambient plots. Further, the ambient temperature plots with reduced rainfall had the highest fine root biomass production and it was significantly higher than fine root production observed in plots with combined warming and reduced rainfall, suggesting that reduced rainfall alone stimulates seedling fine root production, but when combined with warming, leads to declines in plot-level seedling fine root production. With respect to ECM responses, we hypothesize lower fungal biomass production from May-June–– when less tree photosynthate is available–– and a greater proportion of ‘lower carbon cost’ short and contact- exploration type ECM fungi, compared to later in the growing season. This project will yield a dataset that improves our understanding of how climate change affects below-ground carbon cycling in temperate-boreal ecotonal forests.