Relationships between understory vegetation and hybrid poplar growth and size in an operational plantation

Abstract

The effect of varying composition and abundance of neighbouring herbaceous vegetation on tree growth in commercial hybrid poplar plantations remains unclear. We evaluated relationships between tree stem volume, volume increment, and the abundance of various growth forms (perennial grasses, legumes, and perennial forbs excluding legumes) adjacent to the tree base in four-year-old plantation trees under conditions protected from ungulate herbivory. Associations between tree metrics and the understory revealed that perennial grasses consistently reduced tree size and current annual volume growth, and the negative effects of grasses were made worse by increasing legume cover. Under little to no perennial grass cover however, the presence of legumes had a positive impact on tree size and growth, suggesting net facilitative versus competitive effects. The pesence of forbs other than legumes led to no apparent competitive effects after four years, and may instead signify the absence of more competitive understory vegetation, particularly grasses. While final modelled relationships showed that the understory accounted for a limited amount of variance in tree volume (R2=0.10) and volume increment (R2=0.09) in four-year-old trees, these results nevertheless have implications for the development of silvicultural prescriptions targeting understory vegetation in order to maximize commercial hybrid poplar plantation yields.