Trade-offs between environmental and economic factors in conversion from exotic pine production to natural regeneration on erosion prone land

Abstract

Background: Some of New Zealand’s exotic pine (Pinus radiata D.Don) forests were planted for erosion mitigation but cultural, legislative, environmental, and profitability limitations in some parts of the landscape have led to reassessment of their suitability. There is limited information to support landowner decisions on the viability of natural regeneration of native forest post-pine-harvest. Methods: We evaluated scenarios of post-harvest natural regeneration, compared to remaining in pine production, using erosion susceptibility determined from historical occurrence of landslides, gullies and earthflows, biophysical growth modelling of mānuka–kānuka (Leptospermum scoparium-Kunzea ericoides (A.Rich) Joy Thomps.) shrubland using the process-based CenW model, and cost-benefit analyses using NZFARM with two land use change scenarios, at two levels of erosion mitigation ± honey profits. Results: In our study area, the Gisborne Region (North Island of New Zealand), ~27% of the land has moderate–very high susceptibility to landslides, 14–22% a high probability of contributing material to waterways, and 19% moderate–very high gully erosion susceptibility. Pines grow 10 times faster than naturally regenerating mānuka–kānuka shrubland, but mānuka–kānuka is used for honey not wood production. Natural regeneration resulted in losses of $150–250 ha-1 yr-1 compared to the current profitability of pine production. Honey production offset some reduction in pine revenue, but not fully. Thus, the viability of shifting from pines to native forest is highly dependent on landowner impetus and value for non-market ecosystem services (such as cultural and biodiversity values) provided by native forest. Conclusions: A mosaic of land uses within a property may sufficiently offset income losses with other benefits, whereby highly erosion-prone land is shifted from rotational pine forest production to permanent native forest cover with honey production where possible. At the regional scale in Gisborne, the conversion of the most highly susceptible land under production forestry (315–556 ha) to natural regeneration has the potential for wider benefits for soil conservation reducing erosion by 1–2.5 t yr–1 of sediment facilitating achievement of cleaner water aspirations and habitat provision.

Effect of changes in forest water balance and inferred root reinforcement on landslide occurrence and sediment generation following Pinus radiata harvest on Tertiary terrain, eastern North Island, New Zealand

“…In the absence of stricter regulatory controls on forest establishment and removal, progress towards implementing change will be reliant to a large degree on the will of forest companies to adopt alternative management options, with or without financial incentives in the form of government subsidies, to better target erosion mitigation. Change will, however, have implications for the financial viability of many of the exotic forests located in the East Coast region (Lambie et al 2021), where issues of sustainability will, in the longer term, lead to significant areas of high-risk production forestry land transitioning to a permanent forest cover.…”

Managing at source and at scale: The use of geomorphic river stories to support rehabilitation of Anthropocene riverscapes in the East Coast Region of Aotearoa New Zealand

“…Trapping sediment at source, before it enters watercourses, requires catchment-wide management of forestry (reforestation) and pastoral practices that include the planting and management of riparian vegetation, perhaps considering prospects for re-instigation of instream wood and wetland reconstruction to improve ecological habitat. Reforestation using native timber species over longer rotation lengths than exotics is another option (Marden et al, 2023), as is abandonment of the most erosion prone areas to reversion either by passive means (i.e., walk away and allow recolonization to occur) or through planting of early colonising native species such as manuka/ kanuka with the view to the production of honey and other associated medicinal products (Marden et al, 2020;Lambie et al, 2021). Such interventions would have considerable benefits for flood management in lowland reaches, reducing delivery of fine-grained sediment and logs to the beach.…”

Knowing when native regeneration is for you, and what you should do about it. The Aotearoa New Zealand context

“…Specific examples of regeneration have been documented over tens to hundreds of hectares on disturbed land following exotic plantation clear-fell (Lambie & Marden 2020;Forbes et al 2021;Forbes 2021b) or on retired livestock pastures (Allen et al 1992;Wilson 1994;Young et al 2016). In contrast, native tree plantings have been established at comparatively small scales and often in contexts where regeneration processes are dysfunctional, such as where regeneration sites lack sufficient natural propagule sources (Overdyck & Clarkson 2012), are extremely dry (Dollery et al 2018), are weed infested (Wallace et al 2017), or where fundamental biotic impacts such as altered soil hydrology or soil compaction exist (Sullivan et al 2009).…”