Belowground communications change biomass allocation in <i>Juglans mandshurica</i> Maxim. and <i>Larix gmelinii</i> (Rupr.) Kuzen. seedlings

Biomass allocation is a critical mechanism for understanding plant responses to environmental changes. In northeast China, mixed planting with Dahurian larch (Larix gmelinii (Rupr.) Kuzen.) enhanced the productivity of Manchurian walnut (Juglans mandshurica Maxim.) plantations. However, the mechanisms underlying biomass allocation in these systems, particularly in relation to variation in root communication level, remained unclear. To address this, a 15-month pot experiment was conducted, in which Manchurian walnut and Dahurian larch seedlings were mix-planted to examine how varying degrees of root interaction influenced the trade-off between aboveground biomass (AGB) and belowground biomass (BGB) in these two species. The experiment included four treatments: complete root separation and three levels of root communication (1 μm nylon mesh segregation, 45 μm nylon mesh segregation and full root contact). Aboveground (leaf and stem) and root biomass, nutrient content (carbon, nitrogen and phosphorus), nutrient ratios and key morphological and physiological traits of leaves and roots were measured for both species. The trade-offs between AGB and BGB were also quantified. The results revealed that root communication increased the AGB and BGB of Manchurian walnut by 39.53 and 28.81%, respectively and reduced the AGB and BGB of Dahurian larch by 59.34 and 54.45%, respectively. Root communication shifted the biomass trade-off from a biased root allocation to an aboveground allocation in Manchurian walnut, while Dahurian larch exhibited the opposite trend. These trade-offs were directly associated with aboveground carbon-to-phosphorus ratios, root nitrogen-to-phosphorus ratios, root carbon-to-phosphorus ratios, specific root surface area and mycorrhizal colonization rates. In conclusion, these results indicated that mixed planting of Manchurian walnut and Dahurian larch altered the trade-off between AGB and BGB, with the magnitude varying with the level of root communication.