Do hardwood species benefit from mixing with hybrid poplar? Evidence from a 10-year temperate tree-based intercropping system

Abstract

Knowledge of how tree species with contrasting growth strategies establish in temperate tree-based intercropping (TBI) systems is still limited. We assessed the 10-year performance of five hardwood species (Alnus glutinosa, Juglans nigra, Quercus macrocarpa, Quercus rubra, Tilia americana), planted either in monoculture or with hybrid poplar (Populus deltoides × P. nigra), in a TBI system (50 trees ha⁻¹) in southern Québec, Canada. Mixtures generally performed as expected from component monocultures, with no significant net biodiversity effects, except in Quercus rubra-poplar mixtures that showed positive interactions. Productivity in mixtures was mainly driven by poplar dominance, reflecting a selection effect rather than complementarity, with early biomass outcomes shaped more by fast-growing poplar than niche differentiation. Tree carbon sequestration rates averaged 1.2 Mg C ha⁻¹ yr⁻¹ in hardwood monocultures, 4.4 Mg C ha⁻¹ yr⁻¹ in mixtures, and 7.1 Mg C ha⁻¹ yr⁻¹ in poplar monocultures. In mixtures, hardwoods had reduced diameter growth but maintained similar heights, resulting in higher height-to-diameter ratios that indicate straighter, more desirable stem form for timber. Juglans nigra and Quercus macrocarpa showed the greatest diameter growth reductions, likely due to shade intolerance. Physical injuries and morphological defects tended to be less frequent in mixtures, although not significantly. Species-specific vulnerabilities, such as necrosis in Alnus glutinosa and frequent suckering in poplar, warrant further attention. Despite some reduced growth in individual hardwoods, plots with hardwoods mixed with poplar sequestered substantially more biomass C than pure hardwood plots, providing ecosystem services more rapidly and supporting long-term functions in productive TBI systems.