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

Abstract Temperate tree-based intercropping (TBI) systems, which integrate tree rows into crop fields, offer opportunities to diversify farm income and enhance ecosystem services. Yet, adoption remains limited, given uncertainties regarding species performance, especially in mixtures of fast- and slow-growing trees intended for timber and carbon benefits. This study assessed the 10-year performance of five temperate hardwood species (Alnus glutinosa, Juglans nigra, Quercus macrocarpa, Q. rubra, Tilia americana) that are planted in monoculture or interplanted with hybrid poplar (Populus deltoides × P. nigra) within tree rows of a TBI system (50 trees ha⁻ ¹) in southern Québec, Canada. Tree growth (height and diameter), carbon accumulation, and external defects were evaluated. Most mixtures performed as expected based on monoculture productivity, with net biodiversity effects not significantly differing from zero, except in Q. rubra-poplar mixtures, which showed positive net effects. Selection effects, driven by hybrid poplar dominance, were occasionally significant, while complementary effects were weak or negative. 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. While hardwoods showed reduced diameter growth in mixtures, similar tree heights led to higher height-to-diameter ratios, suggesting improved stem form. Physical injuries and morphological defects tended to be less frequent in mixtures, although not significantly so. These findings underscore the importance of species selection in TBI design. Functional contrasts may not ensure positive interactions, especially under fertile conditions or when competition dominates. Species-specific vulnerabilities, e.g., A. glutinosa necrosis, warrant further attention. Long-term monitoring is needed to evaluate the persistence of diversity effects.