Journal Article

Assessing native woody composition, structure, and carbon stocks along elevation-climate gradients in mature Pinus radiata plantations as a baseline for transitional forestry: a regional pilot study

Forbes, Adam Sean

2025

Overview

Background: The ecological science associated with transitioning exotic forest to native dominance (hereafter transitional forestry, transition forests) is currently limited, yet this form of forest has expanded rapidly. In part, this is related to forest-based carbon credit schemes which have driven large-scale afforestation in exotic trees. In other circumstances (e.g., for soil conservation or values-based reasons such as biodiversity conservation), forest owners wish to transition their exotic forest to native forest without harvest. Knowledge is required to inform management of realistic expectations for regeneration and succession both spatially and temporally. Methods: Mature (>20-year-old) Pinus radiata plantations were surveyed along three elevation-climate gradients in the Waikato region of New Zealand to explore the composition and structure (including native carbon stocks) of plantation understories and whether these parameters vary spatially over several decades. Mammalian browsing was also recorded. Results: Native woody stem densities were variable. Factors indicated as driving variability were stand age, elevation, topographic shelter, soil hydrology, solar radiation, and air temperature. On average, understories comprised five native woody species in the seedling tier, a single native species in the sapling tier, and a single native species in the tree tier. The most common species were the sub-canopy tree species Melicytus ramiflorus, Geniostoma ligustrifolium, and Aristotelia serrata. Tall old-growth species, such as Beilschmiedia tawa, Podocarpus totara, Pectinopitys ferruginea, and Prumnopitys taxifolia, occurred in only particular circumstances and on average at densities too low to form a meaningful part of a future forest canopy. Average species richness was low, although some diversity hotspots occurred. Carbon stocks in native trees and tree ferns in the understories were on average 1.55±0.38 tCha–1. Heavy browsing by mammalian herbivores was recorded at 60% of plots. Conclusions: These data indicate typical understorey conditions in mature P. radiata plantations for this area of New Zealand in the absence of management to promote a native transition. These data also highlight the importance of browser control, enrichment planting of tall old-growth species, and canopy manipulations to accelerate regeneration and succession in non-harvest P. radiata plantations. The data suggest transitional forestry should only be attempted at scales that can be reasonably managed, and there is a need for caution against large-scale establishment of P. radiata for transitional forestry as at large scales, achieving adequate levels of management are uncertain.