Liaodong oak (Quercus liaotungensis) is an ecologically important tree species on the Loess Plateau in China that experiences strong intraspecific competition. Therefore, here, we aimed to clarify the strategies of Liaodong oak under intraspecific competition by examining the growth, architecture, and biomass allocation, and analyzed their relationships with the competition intensity. We found that intraspecific competition severely limited the accumulation of biomass in Liaodong oak and this competition was asymmetric among individuals. Intense competition among these trees resulted in a greater allocation of biomass to the belowground parts, allowing them to compete for moisture, which is a limited resource. Aboveground, the trees tended to allocate more resources to their trunks to ensure that tree height growth was not affected, resulting in a decreased diameter at breast height (DBH)/tree height ratio and tapering of the tree, thereby improving stem quality. By contrast, the amount of biomass that was allocated to the branches was greatly reduced through a reduction in the diameter and length of the branches rather than the number of branches. ARTICLE HISTORY

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Interactions between plants are complicated: they can change resource availability, the proportion of resources acquired and the efficiency of resource use. This can eventually lead to changes in stand yield, but the driving mechanisms remain controversial. This study aimed to clarify the effects of mixing on stand yield. We established 120 mixed and pure plots of Chinese pine (Pinus tabulaeformis) and Liaodong oak (Quercus liaotungensis) in the Loess Plateau, China. Based on the inventory data in 2009 and 2016, we compared the differences in yields between mixed and pure stands. Our results indicated that the mixing of pine and oak resulted in over-yielding, but there was no transgressive over-yielding and pure pine stands consistently produced the highest yields. The over-yielding was due to facilitation between pine and oak, especially the improvement of light conditions in the understory, and an increase in the light use-efficiency of young pine and the availability of light to young oak. Mixing did not reduce competition: pine was dominant in interspecific competition and inhibited the growth of oak. The mixing effect was affected by site quality and stand density: improvements in site quality reduced over-yielding, while increases in stand density increased over-yielding.