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

Abstract

Background: The frequent occurrence of storm-initiated landslides following harvesting of Pinus radiata D.Don in steep, Tertiary terrain, East Coast region, North Island, New Zealand, is of increasing concern. This paper documents the influence of tree removal and of replacement plantings on the canopy water balance and soil moisture regime when slopes are at their most vulnerable to landslide occurrence. Methods: At a previously established study site, rainfall, throughfall, and soil moisture data were collected before a mature stand of P. radiata was harvested. After harvesting, part of the study site was replanted with P. radiata at 1000 stems ha-1 and part with 500 stems ha-1. Relationships between hydrological changes and landslide occurrence are discussed in relation to planting density, site factors, root system development, silvicultural regimes, and alternative land use options for mitigating erosion in highly erodible hill country. Results: Following harvesting, soil moisture levels remained higher for longer than under a mature forest until rainfall interception and evapotranspiration returned to pre-harvest levels. This coincided with canopy closure, irrespective of planting density. After thinning, interception and evapotranspiration decreased, then regained the equivalent of a closed canopy 2 years later. Landslide occurrence was highest on slopes >25° and with a NE aspect. Sediment generation rates were highest in 2─4-year-old plantings, then decreased markedly with increasing tree age. Conclusions: Irrespective of planting density, P. radiata had little influence on the soil-water regime until canopy interception, evapotranspiration rates, soil-drying and recharge cycles returned to pre-harvest levels, coinciding with canopy closure. During this period, pore-water pressures at times of heavy or prolonged rainfall likely result in soil saturation and an increase in landslides. The progressive loss of root strength of the harvested trees had a secondary influence. The duration of the post-harvest period of heightened slope vulnerability to landslide initiation is a function of the combined influences of site factors on rates of tree growth and survival, and of the planting density regime on the canopy water balance and soil water content until the development of an effective live soil-root reinforcement system. For areas identified as high risk, the targeting of high-value timber species with longer rotation length, including consideration of coppicing species, would minimise the risk of slope failure at harvest. Very high-risk areas unsuited to rotational harvesting will ultimately require transitioning to a permanent indigenous 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

“…Although exotic forest has reduced sediment supply to the channels, it is clearly not without significant problems. Clearfelling on short-steep slopes adjacent to incised channels heightens the risk of sediment generation and delivery to the river, including any slash (Marden et al, 2023). High volumes of wood waste generated from these forests reflect the poor quality of timber established on unstable slopes (Marden and Seymour, 2022).…”

“…Selective forestry rather than clear-felling, and retention of riparian strips adjacent to channels, would help mitigate erosion risk and modulate connectivity between slopes and channels in these situations. Alternatives to exotic afforestation to mitigate slope erosion and sediment delivery to these rivers include shrubland reversion, both passive and managed, where fully stocked stands of mānuka and kānuka offer protection equivalent to closed-canopy P. radiata (Marden and Seymour, 2022), and consideration of alternative exotic species with longer rotations including Douglas-fir (Psudotsuga menziesii) and/or coppicing species including coastal redwood (Sequoia sempervirens D.Don) (Marden et al, 2023). Elsewhere, especially in Hikuwai and Waimatā catchments, wetland rehabilitation along low order streams may be helpful in trapping and storing sediment.…”

“…Delivery of slash (waste debris from forestry operations) to the channel during significant storm events has become a major concern in the Hikuwai (Figure 7D). Collapse of landings where logs are processed on steep slopes delivers large quantities of wood waste to channels (Marden et al, 2023). Slash from former forest practices adjacent to channel margins was readily mobilised during recent (2018-2023) storm events, while landslides on recently harvested (clear-felled) slopes also mobilise wood into the channel network (Figures 7B, C) .…”

“…Particular attention to the management of wood waste is required. Alongside this, native regeneration and economic retirement of steep slopes adjacent to the Hikuwai channel network is recommended as part of a whole-catchment approach that recognizes the connectivity characteristics of the Hikuwai riverscape (see Marden et al, 2023).…”

“…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.…”