Growth-driven heartwood formation in oak: evidence across monocultures and mixed-species plantations

Heartwood formation represents a major physiological transition in tree development, converting hydraulically active sapwood into structurally and chemically resistant tissue. This process has important implications for tree longevity, hydraulic regulation, and long-term carbon storage, yet the drivers of variation in heartwood formation among conspecific trees remain poorly quantified. In particular, it is unclear how accelerated growth, management interventions, and short-term stress interact to influence heartwood development in temperate hardwood species within forestry systems designed to enhance carbon sequestration and resilience. Here, we assess the combined effects of tree size, age, growth rates, and growth suppression on heartwood formation in Quercus robur across a network of planted stands of contrasting ages and species compositions in central England. These include young (14-year-old) intimate mixture plantations comprising up to 27 tree species, established with the explicit aim of improving carbon storage and forest resilience, alongside older planted stands and managed trees subjected to canopy pollarding. We measured heartwood and sapwood areas in 183 trees spanning ages from 11 to 120 years, using full stem cross-sections and increment cores. Heartwood boundaries were validated using ferrous sulphate staining and tylosis detection. Growth histories were reconstructed using tree-ring analysis, allowing estimation of lifetime mean growth rates, size-independent instantaneous growth rates, and post-disturbance growth resilience following the 2018 drought. Statistical analyses combined nonlinear allometric models, generalized additive models, and mixed-effects approaches to disentangle the roles of size, age, growth, management, and stress. Heartwood area increased strongly with stem diameter, explaining most of the variation among individual trees (R² ≈ 0.98), while age exerted an additional but secondary influence. For trees of similar diameter, older individuals consistently contained more heartwood, indicating that heartwood formation is not solely a function of size. Heartwood onset occurred early, with a 50% probability at a diameter of 8.5 ± 0.8 cm. Following onset, heartwood expansion accounted for an increasing fraction of total basal area increment, rising from approximately 40% in small trees to over 80% in large trees. Despite declining sapwood proportion with size, absolute sapwood area continued to increase, indicating sustained canopy development even in large trees. Both lifetime mean and size-independent instantaneous growth rates were positively associated with heartwood expansion, demonstrating that faster-growing trees consistently allocate more biomass to heartwood formation. In contrast, short-term growth suppression following drought or canopy pollarding did not reduce heartwood development. Trees with lower post-drought growth resilience and pollarded trees that had already initiated heartwood formation exhibited equal or greater heartwood proportions, suggesting a shift in allocation towards durable tissues under stress. Our results support a sapwood homeostasis mechanism linking growth, canopy function, and heartwood formation in Q. robur. Importantly, accelerated growth in mixed-species plantations does not compromise heartwood development and may enhance long-term carbon residence times through earlier and greater heartwood accumulation. These findings provide mechanistic evidence that climate- and biodiversity-smart forestry strategies based on species mixtures and productivity gains can simultaneously support resilience and long-term carbon storage in temperate hardwood systems.