Vegetation patterns in mature plantations of Pinus radiata and Pseudotsuga menziesii: Implications for forest transitions

Plantation forests that will not be harvested are becoming more prevalent throughout the forestry industry. The reason for not harvesting may be for carbon sequestration, facilitation of ecological restoration, or due to the establishment of plantations in sensitive environments where harvesting is no longer viable or economic. These forests are referred to as non-harvest plantation forests, a term that covers both plantations that have been established with or without the intent of harvest. The future of such forests is uncertain and there is increasing desire to transition to native forest cover without clearcut of the plantation. We used vegetation data from eight older (>40 years) abandoned exotic plantation stands, four of Pinus radiata and four of Pseudotsuga menziesii in the Hawke’s Bay region of Aotearoa New Zealand to (1) examine the structure and composition of the understorey vegetation in the plantation stands, (2) explore the influence of environmental variables and plantation characteristics on plant communities, and (3) make recommendations for the ongoing management of non-harvest plantation forests. Vegetation composition data were analysed using ordination and PERMANOVA, including the fit of environmental vectors. Hierarchical cluster analysis showed five clusters of plots, with two clusters containing plots in both P. radiata and P. menziesii stands. The communities of native understorey vegetation were different between P. radiata and P. menziesii stands, however neither plantation species contained significantly higher native species richness or importance value. Analyses of linear relationships found plantation age and stocking (stem density) had the greatest effect on native understorey vegetation importance value in P. menziesii stands. There was no relationship between importance value and stocking in P. radiata. Age could not be assessed in P. radiata due to the narrow range of stand ages. All stands sampled contained native canopy tree species: P. menziesii had dominant understorey cover of Melicytus ramiflorus and Fuscospora fusca, while P. radiata had dominant cover of Kunzea ericoides and Pterophylla racemosa. Both plantation species sampled showed potential for succession towards a native forest, however, 43 % of P. radiata plots had exotic self-seeded conifers in the understorey suggesting the need for ongoing intervention. Although emergent canopy species of the Podocarpaceae and Nothofagaceae families were found throughout the stands sampled, they were more sparse and less dominant than the native understorey and co-canopy angiosperm species indicating that disturbance of the exotic canopy may be required for further growth and establishment of Podocarpaceae and Nothofagaceae species.