Competition in mixed Scots pine and Pyrenean oak stands modifies allometry and partially affects biomass allocation during early stand development

Abstract

• Competition significantly modulates the allometric relationships of Scots pine and Pyrenean oak. • Competition modifies stem–foliage biomass allocation but does not affect stem–branch or branch–foliage allocation. • Competition indirectly affects biomass allocation through the mediation of tree height. • Scots pine allocates more biomass to branches and foliage under competition, whereas Pyrenean oak invests more in stem growth. • The results highlight the importance of early silvicultural intervention in managing mixed-species forests. Understanding how trees allocate growth and resources in response to competition is crucial for managing mixed-species forests under changing climatic conditions. Despite extensive research examining competition dynamics in mature forests, little is known about how competition influences biomass allocation strategies in early-stage mixed stands of Scots pine ( Pinus sylvestris L.) and Pyrenean oak ( Quercus pyrenaica Willd.). In this study, we examined the effects of competition on the allometric relationships between diameter at breast height (DBH) and tree height, crown base height (CBH), and crown length (CL) in both species. We also investigated how competition modifies biomass allocation among stem, branch, and foliage components and whether the two species adhere more closely to optimal allocation theory, where competition influences biomass distribution, or allometric partitioning theory, where it does not. Finally, we explored whether competition indirectly affects biomass allocation by altering its impact on structural attributes (DBH, tree height, CBH, and CL). Here, we destructively sampled 90 trees and then applied ANCOVA to quantify the direct effects of competition on tree structural attributes and biomass allocation. Additionally, we used structural equation modeling (SEM) to evaluate the indirect effects of competition. Our results demonstrated that competition significantly influenced allometric relationships (DBH–tree height, DBH–CBH, and DBH–CL). Moreover, biomass allocation between stem–branch and branch–foliage was not modified by competition and followed allometric partitioning theory, whereas stem–foliage allocation was modulated by competition and aligned with optimal allocation theory. We also found that Scots pine prioritized branch and foliage biomass under competition, whereas Pyrenean oak allocated more resources to the stem, reflecting contrasting adaptive strategies. SEM revealed that competition indirectly influences biomass allocation by modulating tree height, leading to a significant increase in stem and branch biomass but no effect on foliage biomass. These findings support early-stage silvicultural interventions, such as promoting drought-tolerant species, to guide stand development toward structurally resilient and climate-adaptive Mediterranean forests. While these findings offer informative results, they need to be interpreted with caution because of the single site and limited sample size, which may restrict broader generalizability.

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