Shifts in ectomycorrhizal exploration types parallel leaf and fine root area with forest age

Abstract

Abstract The abundance of fine roots and leaves in forests typically peaks during mid‐succession and then declines. If fine root area declines more rapidly than leaf area, this could contribute disproportionately to stand decline. However, trees also partner with symbiotic ectomycorrhizal (EcM) fungi that facilitate nutrient acquisition of fine roots. Ectomycorrhizal fungi can use the carbohydrates provided by their plant partners to produce emanating fungal tissues which increase soil exploration potential but may also prove costly. We suggest two competing hypotheses to frame the response of ectomycorrhizas to stand age: (a) the ‘functional offset’ hypothesis posits that the abundance of ectomycorrhizas with emanating tissues (‘Distance’ mycorrhizas) increases with stand age and (b) the ‘energy‐limited’ hypothesis posits that carbon available for root symbionts decreases with stand age resulting in fewer Distance mycorrhizas. In the first hypothesis, EcM functional traits offset root abundance, while in the second, traits parallel root abundance. To test these competing hypotheses, we sampled fine roots to a depth of 90 cm and used allometric equations to estimate changes in root and leaf area index across a chronosequence of Pinus banksiana stands ranging from 2 to 76 years average tree age. We also examined fine roots microscopically to track changes in the abundance of EcM functional types. We used DNA‐based methods to sequence EcM fungi and confirm roots were of P. banksiana. Both fine root and leaf area of pine increased for the first 30–36 years (until Mid age) and then plateaued, while the ratio of leaf to fine root area was similar across the age gradient. Also, changes to fine root area with stand age depended on soil depth. The abundance of Distance mycorrhizas was lowest at the youngest plots and increased until Mid age, where it peaked, contrary to the functional offset hypothesis. Instead, the abundance of Distance mycorrhizas paralleled changes to leaf area, aligned with the energy‐limited hypothesis. Synthesis. Ectomycorrhizas do not offset the function of roots, rather shifts in exploration type likely reflect adjustments to carbon supply from hosts.