Disentangling variation patterns and partitioning strategies of net primary productivity: insights from cool-temperate forests in South Korea

Abstract

Background Understanding the processes underlying carbon storage and balance is critical for equipping the terrestrial biosphere to respond to contemporary climatic challenges. However, ecosystem-level estimates and distribution of net primary productivity (NPP), a metric for evaluating forest carbon cycling patterns and dynamics, remain constrained by uneven empirical observations between above- and belowground fractions. We herein quantified the rate and composition of NPP for four stands characteristic of the cool-temperate deciduous (Larix kaempferi, LK; Quercus mongolica, QM) and evergreen (Pinus densiflora, PD; Pinus koraiensis, PK) forests of South Korea over a complete annual cycle (2022-2023). Variations in dynamic NPP compartments, particularly (1) canopy litterfall by stand and season and (2) fine root production by stand, diameter class, and depth interval, were further characterized using litter traps and ingrowth cores, respectively. Results Total NPP varied from 1226 +/- 101 to 1796 +/- 154 g m(-2) yr(-1), with 78-84% allocated aboveground and 16-22% belowground. LK and QM exhibited total NPP up to 46% higher than PD and PK. Both litterfall and fine root production differed considerably across stands, decreasing in the order of QM > PK > PD > LK for litterfall and QM & PD > LK & PK for fine root production. Litterfall peaked in autumn, similar to the leaf phenological rhythm of many temperate deciduous species. In contrast, fine root production showed a negative vertical distribution with depth, which is consistent with decreasing nutrient availability and increasing mechanical impedance along the soil profile. Conclusions By disentangling the contribution levels and dynamic patterns of each NPP compartment, our findings demonstrate a strong inclination toward aboveground NPP investment when belowground resources are not limiting. In other words, an adequate nutrient supply enables plants to modify their priority allocation from fine root maintenance to internal resource transport, leaf production, canopy expansion, reproduction, and other critical aboveground functions. Such information underscores the necessity for forest management strategies that target soil fertility to strengthen not only canopy productivity and CO2 sequestration but also ecosystem resilience by reinforcing allocation patterns that sustain high NPP and safeguard forests against shifting climate conditions.