Aboveground nutrient conservation via resorption processes and belowground nutrient acquisition from soils are two important mechanisms for plants to maintain nutrition and ecosystem functions. However, the mechanism by which plants coordinate these two nutrient strategies, especially for ectomycorrhizal (ECM)-dominated conifers in alpine forests, remains unclear.

We investigated the relationships between aboveground nutrient conservation and belowground nutrient acquisition and their environmental drivers by measuring leaf nutrient (i.e., nitrogen [N] and phosphorous [P]) resorption efficiency, resource foraging- and uptake-related root morphological (root diameter [RD], specific root length [SRL]/area [SRA]) and physiological (root tissue density [RTD], root N and P concentration) traits, mycorrhizal colonization rate (MCR), rhizosphere effect on soil N and P cycling, and environmental factors of 40 ECM coniferous populations on the eastern Tibetan Plateau, China.

Our results show that with increasing leaf nutrient (N and P) resorption efficiency, conifers shifted from depending on the ‘outsourcing’ strategy by mycorrhizal fungi (high MCR) to relying on the ‘do-it-yourself’ strategy of root mining (high rhizosphere effect on N- and P-mining-related enzyme activities) rather than on root foraging (high SRL and SRA) and preferred more conservative roots (high RTD and low root N and P concentrations). Temperature was the main factor driving a negative relationship of ECM fungi foraging, root uptake and a positive relationship of root mining with leaf nutrient resorption, while precipitation resulted in a decoupled relationship between root foraging and leaf nutrient resorption.

Our findings demonstrate temperature-driven and diverse collaborations (e.g., tradeoff or synergy) between belowground nutrient acquisition and aboveground nutrient conservation strategies in alpine ECM conifers and highlight that the preference for belowground nutrient acquisition strategies could influence the aboveground nutrient utilization strategy. This is insightful for a holistic understanding of the adaptation and responses of alpine forests to climatic change.

This data set was collected in the alpine region of the eastern Tibetan Plateau, which spans a wide geographic range, from latitude 27.37 to 35.27° N, longitude 94.55 to 103.31° E, and with an altitude of 2562.0-4351.0 m a.s.l. Forty pure forest stands were selected for leaf, root, and soil collection in July and August 2017-2018. Specifically, we first set up three to five 30 m × 40 m experimental plots in each stand; then, in each plot, at least three mature and healthy individuals of a target conifer species were randomly selected for leaf and root collections. For each individual, three representative twigs from sun-exposed, first-order branches were collected from the mid to upper canopy using pole pruners, and then the fully expanded, current-year needles of each cohort were immediately detached from the twigs. Accordingly, freshly fallen leaf litter was collected under the canopy of the same individual in a similar position to that of the green leaves. Here, only the terminal two orders of a root branch were sampled because of their highest absorptive activity and mycorrhizal colonization in the root branch (Guo et al., 2008). Once collected, the leaf, root, and soil samples in each plot were separately combined to make one composite sample to minimize any heterogeneity caused by sampling positions and immediately placed on ice, transported to the laboratory, and kept at -20 °C prior to further analyses.