Tree biomass allocation in temperate mixed forests

Abstract

Forests are critical for global climate regulation, biodiversity conservation, and the provision of ecosystem services. However, estimating aboveground biomass (AGB) in mixed-species forests remains challenging because of the complex interactions among species, structural variability, and competitive dynamics. This study addresses this gap by investigating biomass allocation, allometric relationships, and competition effects in mixed and monospecific stands of Scots pine (Pinus sylvestris L.), sessile oak (Quercus petraea), and Pyrenean oak (Quercus pyrenaica) across different forest developmental stages in northern Spain.

The research combined empirical data from three complementary studies: mature triplet-based plots of pine and sessile oak (Study I), young mixed stands of Scots pine and Pyrenean oak subjected to thinning treatments (Study II), and an analysis of competition effects on biomass allocation and tree structure using structural equation modeling (Study III). Tree attributes such as DBH, height, crown dimensions, and biomass components (stem, branches, foliage) were collected using destructive sampling and analyzed using log-transformed regression, nonlinear mixed-effects models, Dirichlet regression, and ANCOVA.

The results show that biomass allocation patterns and model performance are species- and stand specific. While Scots pine displayed stable allometric relationships across stand types, sessile oak required distinct models for mixed and monospecific stands. Nonlinear mixed-effects models incorporating both DBH and height outperformed traditional log-linear approaches. Dirichlet regression effectively predicted biomass component proportions, preserving additivity. Neighborhood competition significantly improved AGB model performance for Scots pine but had negligible effects on Pyrenean oak, revealing differing sensitivities to interspecific and intraspecific competition. Furthermore, competition altered allometric scaling relationships and tree crown development, with Scots pine investing more in foliage and branches, whereas Pyrenean oak prioritized stem growth.

Structural equation modeling confirmed the indirect effects of competition on stem and branch biomass through modifications in tree height, supporting a mediation framework in tree allometry. These findings advance the understanding of biomass partitioning under mixed-species conditions and highlight the importance of incorporating species-specific traits and competitive interactions into biomass estimation models. Ultimately, this research provides refined modeling tools and insights to support adaptive forest management and carbon accounting under changing climatic and ecological conditions.