Root morphology and mycorrhizal symbioses together shape nutrient foraging strategies of temperate trees
Significance Plant growth requires acquisition of soil nutrients in a patchy environment. Nutrient patches may be actively foraged by symbioses comprising roots and mycorrhizal fungi. Here, we show that thicker root tree species (e.g. tulip poplar, pine) respond weakly or not at all to nutrient heterogeneity. In contrast, thinner root tree species readily respond by selectively growing roots [arbuscular mycorrhizal trees (e.g. maple) or mycorrhizal fungal hyphae [ectomycorrhizal trees (e.g. oak) in nutrient rich "hotspots." Our results thus indicate predictable patterns of nutrient foraging among tree species with contrasting mycorrhiza types and root morphologies. These findings can pave the way for a more holistic understanding of root microbial function, which is critical to plant growth and biogeochemical cycles in forested ecosystems. Photosynthesis by leaves and acquisition of water and minerals by roots are required for plant growth, which is a key component of many ecosystem functions. Although the role of leaf functional traits in photosynthesis is generally well understood, the relationship of root functional traits to nutrient uptake is not. In particular, predictions of nutrient acquisition strategies from specific root traits are often vague. Roots of nearly all plants cooperate with mycorrhizal fungi in nutrient acquisition. Most tree species form symbioses with either arbuscular mycorrhizal (AM) or ectomycorrhizal (EM) fungi. Nutrients are distributed heterogeneously in the soil, and nutrient rich "hotspots" can be a key source for plants. Thus, predicting the foraging strategies that enable mycorrhizal root systems to exploit these hotspots can be critical to the understanding of plant nutrition and ecosystem carbon and nutrient cycling. Here, we show that in 13 sympatric temperate tree species, when nutrient availability is patchy, thinner root species alter their foraging to exploit patches, whereas thicker root species do not. Moreover, there appear to be two distinct pathways by which thinner root tree species enhance foraging in nutrient rich patches: AM trees produce more roots, whereas EM trees produce more mycorrhizal fungal hyphae. Our results indicate that strategies of nutrient foraging are complementary among tree species with contrasting mycorrhiza types and root morphologies, and that predictable relationships between below ground traits and nutrient acquisition emerge only when both roots and mycorrhizal fungi are considered together.
Proceedings of the National Academy of Sciences