Season-driven growth responses in juvenile Eucalyptus across different species and planting densities.

As a result of climate change there is an increasing need for effective forest monitoring and improved growth and yield models, to support sustainable wood production, climate resilience, and ecosystem management. The IMPACT OAL (Intensive Monitoring of Planted And Competing Trees Open Air Laboratory) located in Somerset West, South Africa, is a flagship research site that is intensively monitored and creates the opportunity to investigate how trees react to a changing environment. The current study evaluated growth increment (GI) responses of four juvenile Eucalyptus species planted across four planting densities at the IMPACT OAL from March 2025 to March 2026. To achieve this Basal Stem Diameter (BSD) of all trees were measured during every quarterly measurement campaign using different measurement technologies. BSD was used to calculate growth increment (GI) between each campaign and changes in GI was assessed using a mixed-effects model with plot and tree included as random effects to account for the experimental hierarchy, whereas fixed effects comprised diameter, species, spacing, campaign, and their interactions. Tree BSD strongly increased GI (p < 2 × 10⁻¹⁶), while overall GI declined significantly across campaigns, indicating strong seasonal effects. Although the effects of species and spacing were weak, their interactions with the campaign revealed that GI responses were season-driven across species and planting densities. Effects brought-on by planting density were also season-dependent, with certain spacings enhancing GI during specific campaigns. Therefore, tree growth was primarily season‑driven with species and planting density effects emerging through interactions with season, highlighting the role of seasonal conditions in shaping growth responses of juvenile Eucalyptus species. This finding underscores the importance of integrating continuous monitoring data into growth and yield models to improve predictions of plantation performance and support more climate-resilient and sustainable forest management.