Trees rely on both root‐based ‘do‐it‐yourself’ and mycorrhizal‐based ‘outsourcing’ adaptations to shifting soil nutrient availability during forest stand development

Abstract Soil nitrogen (N) and phosphorus (P) availability generally decrease with forest stand development, yet how tree species acclimate to this decline remains largely unclear. The functional coordination hypothesis proposes that trees concurrently improve leaf nutrient recycling and root nutrient acquisition to cope with nutrient deficiency. Conversely, the resource limitation hypothesis posits that trees prioritize either strategy, as simultaneous employment of both strategies is resource costly. To test these conflicting hypotheses, we leveraged a chronosequence of Pinus sylvestris var. mongolica stands with six age classes ranging from 16‐ to 62‐year‐old. We measured a comprehensive set of 70 plant and soil variables including needle nutrient retranslocation and resorption, and root architectural, morphological, physiological and mycorrhizal traits to assess the response of needle nutrient recycling and root nutrient acquisition to changes in stand nutrient status. Stand development reduced soil N and P availability and caused a shift from N to P limitation as evidenced by increasing needle N:P ratios and decreasing N:P‐acquisition enzyme ratios. There was no evidence of N or P retranslocation from old to young needles, and nutrient resorption efficiency was independent of soil nutrient availability, indicating that adjustments of needle nutrient recycling contribute little to mitigating nutrient deficiency. Root length density decreased exponentially with stand development, whereas cord‐forming ectomycorrhizal fungi (EMF), root phosphatase activity and rhizosphere effects on plant‐available soil P logarithmically increased, implying a shift from root nutrient scavenging to combined EMF nutrient scavenging and root P mining. Synthesis . Our findings suggest that tree species do not necessarily rely on both improved recycling and acquisition to acclimate to soil nutrient scarcity with stand development. However, trees can simultaneously depend on physiological adaptations of absorptive fine roots themselves, as well as on a constrained selection of EMF. This reveals that complex sets of adaptations, both root‐based ‘do‐it‐yourself’ and mycorrhizal‐based ‘outsourcing’, can be effectively used by trees to cope with changes in soil nutrient availability.