Can hybrid poplar plantations accelerate the restoration of forest understory attributes on abandoned fields?

Abstract

Multi-functional plantations using fast-growing tree species can potentially be used to accelerate the restoration of forest habitat on abandoned farmland. However, few studies have directly compared the ecological effects of active afforestation using plantations to natural afforestation. The present study compares the effects of the establishment and the first 10years of growth of two different hybrid poplar clones to adjacent abandoned fields and natural second-growth forests, in terms of abiotic attributes and understory vegetation communities. Experimental plantations of a Populus maximowiczii×P. balsamifera (M×B) hybrid and a P. deltoides×P. nigra (D×N) hybrid were established in 2000 on abandoned fields at 8 sites in southern Quebec, Canada. In 2009, soil chemical properties, soil moisture, canopy openness, leaf litter biomass, and understory vegetation communities were sampled in the plantations and in neighboring unmanaged abandoned fields and second-growth woodlots. Canopy development was greatly accelerated by the plantations of both hybrid poplar clones, compared to the unplanted portions of abandoned fields, and in the more productive plantations of both clones, canopy openness and leaf litter biomass were similar to those measured in the woodlots. No difference was detected in soil chemical properties between the abandoned fields and either type of plantation. The plantation soils were drier than those of both the abandoned fields and the secondary forests, possibly due to increased transpiration by the hybrid poplars. The plantations favored the colonization of the sites by native tree seedlings compared to the unmanaged fields, producing a nurse stand effect. Among herbaceous and shrub species, native species richness was slightly higher in the plantations than in the fields. The understories of the fastest growing plantations had species in common with both the fields and the woodlots, while the understory of the slowest growing plantations was very similar to the field vegetation. The plantation understories had lower covers of open-habitat species than the fields, while the M×B plantations had slightly greater covers of native, shade-tolerant species. Plantations of the M×B clone were more distinct from the abandoned fields than were plantations of the D×N clone. In conclusion, hybrid poplar plantations appear capable of accelerating the restoration of certain understory attributes, which in turn favors their colonization by tree seedlings and suppresses competition from light-demanding herbaceous species.