Aboveground biomass and growth of smallholder Eucalyptus robusta under low starter fertilization and weed competition in Madagascar

Smallholder Eucalyptus plantations (EP) in Madagascar Central Highlands (MCH) address substantial fuelwood demand and reduce pressure on natural forests. However, their sustainability is challenged by low soil fertility and inadequate management. While fertilization increases tree growth, high mineral fertilizer costs limit its use by smallholder farmers. Both biomass estimation equation and impact of fertilization on smallholder EP in MCH remain poorly documented. This study aimed to evaluate the impact of low starter mineral fertilization (15 kg.ha^-1 N, 12 kg.ha^-1 P, and 17 kg.ha^-1 K) and weed competition on Eucalyptus robusta growth and aboveground biomass (AGB) while providing allometric equations for AGB estimation. Dendrometric data were collected from four stands aged 2 to 6 years, with fertilized and non-fertilized parts. AGB of 16 trees per stand (8 fertilized, 8 non-fertilized) were destructively measured. AGB of trees were estimated by regression based on tree circumference, height and their combinations. The results indicated circumference as the best single variable predictor (R² > 0.90) for all tree compartments (leaves, branches, trunk) and treatments. Fertilization significantly improved global tree survival by 7% and increased tree height by 3.1 m and circumference by 8.3 cm in 6 yo stands. AGB per hectare doubled with fertilization, reaching 55.3 Mg.ha^-1 at 6 yo compared to 29.2 Mg.ha^-1 in non-fertilized plots. Aboveground carbon stock reached 4.6 MgC.ha^-1yr^-1 in fertilized plots. Weed cover and biomass had likely negative linear relationships with tree AGB and survival rate. This study provides robust allometric equations for biomass estimation and highlights that even low fertilizer application combined with effective weed control can significantly enhance AGB production in smallholder EP.