Fungal and root responses to experimental warming and rainfall reduction in a temperate-boreal ecotonal forest.

Abstract

Soils––the largest terrestrial carbon (C) pool––sequester C from anthropogenic carbon dioxide (CO2) emissions linked to climate change. Fungi and tree roots contribute substantially to soil organic C (SOC) formation, particularly in boreal forests where warming is occurring rapidly, yet their responses to climate change are poorly understood. My dissertation investigated belowground effects of climate change at B4WarmED, a long-term factorial warming and rainfall reduction experiment at the temperate-boreal ecotone in northern Minnesota. My first chapter assessed responses of root-associated ectomycorrhizal (ECM) fungal symbionts of two Pinus sapling hosts with differing drought tolerances to a gradient of soil moisture induced by factorial warming (+1.7ºC, +3.2ºC) and rainfall reduction (30% reduction). Warmer, drier soils resulted in lower ECM fungal richness on roots of Pinus strobus (less drought tolerant host) and shifts in community composition of ECM fungi on P. banksiana (more drought tolerant host). My second chapter investigated seasonal patterns of soil fungal mycelium production and community composition under warming (+3.4ºC) and rainfall reduction (30%) using sequential mesh ingrowth sandbags. Mycelial abundance peaked in summer under ambient rainfall, but was lowest in summer compared to fall and spring under combined warming and rainfall reduction, indicating implications of climate change for the contribution of fungal mycelium to SOC. From spring to fall, ECM fungal relative abundance decreased, saprotroph abundance remained unchanged, and plant fungal pathogen relative abundance increased under ambient temperature but remained similar in abundance across seasons under experimental warming. My third chapter utilized root ingrowth cores to study effects of warming (+3.3ºC aboveground, +2.4ºC belowground) and rainfall reduction (40%) on sapling community fine root production and traits, SOC accumulation, and sapling root contribution to SOC. Though root production and SOC accumulation did not vary by treatment, there were warming-induced increases in root % C and root tissue density (dry root mass/root volume). Root biomass and traits were significantly related to SOC pools, indicating potential for climate-induced shifts in root production and traits to alter SOC accumulation. This research enhances our understanding of temperate-boreal fungal and root community responses to future climate change and the potential implications for C storage.