Fungi, forests, and fish: the role of fungi in forest biogeochemical cycling

Abstract

Forests are threatened by a multitude of stressors and assessing how forests will respond to these stressors requires a comprehensive understanding of net primary productivity (Npp). However, one parameter of significant uncertainty is belowground Npp (bNpp), or total photosynthetic carbon allocated belowground. I evaluated the effect of climatic and soil variables on bNpp using a global productivity dataset and found that soil nutrient flux rates were the main drivers of bNpp across forest biomes. Within biomes, environmental constraints on bNpp varied between forests with low versus high bNpp, indicating that environmental drivers are site-specific and the development of within-biome, site-scale classifications for forest ecosystems could be useful. Finally, I found that soil variables caused abrupt and large decreases in bNpp for boreal, but not cold temperate forests, indicating that boreal forests might have lower adaptive capacity and higher sensitivity to disturbances than cold temperate forests. I then examined nutrient cycling in an Alaskan boreal riparian forest and evaluated the role of fungi in the flow of salmon-derived nitrogen (SDN). Pacific salmon subsidize terrestrial systems with SDN, which can have ecological significance for animals, plants, and soils. However, despite the crucial role of fungi in decomposition and nutrient cycling, the importance of fungal mycelium as an SDN sink in the soil has been largely overlooked. I found that SDN was present in fungal sporocarps, soils, and foliage, but only significantly increased N concentrations in fungi, suggesting that SDN was retained in fungal hyphae. Furthermore, SDN from recent salmon carcasses increased the relative abundance and diversity of medium-distance and long-distance ectomycorrhizal fungal types. In systems such as salmon streams where nutrients are available but patchily distributed and occur in pulses, these fungi might be preferred by plant hosts due to their strong ability to maintain large network structure, mine nutrient hotspots, and mobilize organically bound N and P. These results illustrate that fungi are essential to consider to accurately assess the flow and significance of SDN in riparian systems.