“An Analysis of Growth and Mortality of Planted Indigenous Tree and Shrub Species in Tiromoana Bush”

Type of content
Theses / Dissertations
Publisher's DOI/URI
Thesis discipline
Forestry
Degree name
Master of Forestry Science
Publisher
University of Canterbury
Journal Title
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Volume Title
Language
English
Date
2021
Authors
Kernahan, Alastair
Abstract

Tiromoana Bush, a 410 hectare coastal lowland forest/ecosystem restoration area (also known as Kate Valley Conservation Management Area), was created in 2005 in conjunction with establishment of the Kate Valley landfill operation, and as a legal condition implemented by the Environment Court in 2004 (Smith 2004) in exchange for granting the resource consent enabling the landfill to operate over both its intended 35 year operational term and for the 30 year period after that in which aftercare operations are required (Korhonen 2020). The purpose of creating the reserve and restoring it from existing remnants, was both to mitigate the loss of some small patches of native vegetation present on site prior to construction of the landfill, particularly a small (0.73 ha) remnant patch of Black beech (Fuscospora solandri) forest (Remnant “A”) and its associated flora. The restoration project was also intended to provide a broader social good by restoring/expanding what is a rare ecosystem type in the North Canterbury region: Coastal Lowland forest (Singers & Rogers); and to allow for the creation of a series of walking tracks and amenities within the reserve to allow the public to utilize it. The conditions of the resource consent also require Transwaste to provide long-term funding to continue track maintenance and ongoing restoration work (planting, fencing and pest control) for the reserve into the future, 30 years beyond the term of the Landfill (Korhonen 2020).

The initial 5-year restoration plan (currently a 3rd iteration is in effect) divided the reserve into seven distinct zones, with planting focussed on the 8 hectare area surrounding existing black beech remnant “B” (Zone 1), the inland faces surrounding Kate pond in a 107 hectare area (Zone 2), the 17.2 hectare Kate valley flats, which include wetland areas (Zone 3), and the 14 hectare lower portion of Kate valley adjacent to the coast (Zone 5) (Norton 2005). This study focusses solely on permanent sample plots in restoration plantings, which were established solely in Zones 2, 3 and 5, as well as some individual Black beech (Fuscospora solandri) and Kahikatea (Dacrycarpus dacrioides) plantings occurring in Zones 2 & 3.

The 2005 restoration plan envisaged a minimum of 5 hectares of new plantings being established during the first 5-year term, with a minimum survival target of 50% (Norton 2005); the subsequent restoration plan drafted in 2007 had an alternative objective, with a target of establishing an additional 3000 seedlings/annum plantings, but providing a new focus on the valley bottom aiming to create a kahikatea-dominated forest in the longer term by requiring >50% of plantings to be established in that area (Norton 2005). This change was due to some plantings comprising edge- planting and strips, which were hard to quantify in terms of area. A separate objective also existed to consolidate the beech remnant (Remnant “B”) in zone 1 and the new beech plantings which had been created in zone 2, as well as establish a new beech planting site if possible (Norton 2005).

Restoration work in the initial 8 years (first restoration Plan period) has also included construction of a deer proof fencing around the entire perimeter of the reserve (Norton 2012), as well as some internal low fencing around individual plantings to deter feral pigs (Sus scrofa) and hares (Lepus europaeus), as well as active control of pest animal species (browsing animal and also those predacious to avian life) occurring semi-annually. It was noted in the initial Plan that the perimeter deer fence would be unlikely to prevent feral pigs from accessing the reserve area and that pigs in particular would pose a threat to plantings (Norton 2012).

The planting sites range in age of establishment from 2006 to 2012, with a range of species present at each site appropriate to its relative site, aspect, and slope position. The success of the plantings was intended to be assessed through PSPs co-located at each planting site (Norton 2005) for mixed plantings. The PSPs were generally (although not always) of 100m2 size and demarcated with steel corner posts and with all trees tagged and measured for height at planting, and various times afterwards. There are also a small number of tagged individual trees clustered within the unbounded plantings of Black beech and kahikatea which were similarly remeasured in the early years following planting.

Abiotic factors impacting growth/survival of the plantings include frost at low elevation sites and drought (moisture stress), with management strategies for the plantings to address these factors having been late-winter planting to ensure the worst of the early frosts have been avoided, and that adequate root growth occurs in the first growing season to enhance survival in drought conditions (Norton 2005).

Biotic constraints for the broader area are mainly: animal browse and mechanical damage (from feral pigs, hare and deer) prior to fencing of the reserve and to a lesser degree afterwards for pigs; moisture competition and enhancement of frost damage from surrounding grass species (Norton 2005); and whether planted stock have appropriate fungal mycorrhizae to thrive on the site. Management strategies to address these issues are the fencing and ongoing active pest control, chemical release of surrounding vegetation prior to planting, and inoculation of seedlings with mycorrhizae respectively (Norton 2005).

The site elevation of all of the sampled plantings is minimal across the site as a whole (sea level to 175m approximately) and unlikely to be a primary factor affecting growth/mortality, although slope and aspect, which may influence growth, vary widely by site.

Overall the remeasurement of the PSPs and individual plantings has allowed for calculation of conservative estimates of Height, Ground line diameter (GLD) and Diameter at Breast Height (DBH) growth of the various planted species and provided an indication of which levels of stocking and species compositional mixes are most successful for the sites the PSPs and other plantings represent. Crown closure estimated from drone photography, and the presence of naturally regenerated seedlings at some sites can also give an indication of the longer-term success of the sites.

A secondary exercise in calculating above ground biomass (AGB) from various form-specific equations (Conti et al. 2019) and general averages for estimating below ground biomass (BGB) and AGB/BGB carbon ratios, has also provided a conservative estimate of carbon stored per hectare. Re- tagging of the all the species measured during the study will allow for future remeasurements to proceed with greater data precision and accuracy.

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