Root-mycorrhizal foraging strategies shift with forest age more than with nitrogen manipulation

Abstract

Plant nutrient foraging depends on roots and mycorrhizal fungi, which are affected by plant carbon (C) investment and soil nutrient availability. The C supply for root metabolism and associated fungi might be diminished as the host plant size or age increases, while the quality and quantity of soil nitrogen (N) change with forest age. There is still no holistic understanding of how the organization of belowground mycorrhizal root structure and fungi in the nutrient acquisition continuum shifts with forest age and soil resources, which restrains our understanding of the functional relations among roots, fungi, and soil. Here, we examined shifts in the absorptive root, mycorrhizal strategies, and soil-associated fungal community compositions after 9 years of nitrogen manipulation (0, 20, and 50 kg N ha⁻¹ year⁻¹) in temperate larch forests across three age cohorts (11, 20, and 45 years). We found that the effect of forest age on root and fungal traits outweighs that of nitrogen treatment. Specifically, with increasing forest tree age, root respiration and specific root length decreased, while protective investments such as tissue density and phenolics increased. Meanwhile, the proportion of ectomycorrhizal fungi of the long-distance exploration type decreased, but those of the short-distance exploration type increased. Together, these patterns suggest a forest age-mediated nutrient acquisition continuum spanning from “explorative roots with long-distance exploration types” to “conservative roots with short-distance exploration types.” We propose that this nutrient acquisition continuum is functionally constrained by the “size vs. rate” trade-off between the root architecture and root segment metabolism, and the “roots vs. mycorrhizal fungi” complementarity between root architecture and mycorrhizal exploration types. Our results suggest that forest age explains shifts in systemic functional trade-offs in root architecture, root segment metabolism, and mycorrhizal exploration types.