Restoration Strategies for Three Dacrycarpus dacrydioides (A.Rich.) de Laub., Kahikatea Remnants in Hamilton City, New Zealand

Abstract

Identifying appropriate restoration strategies is vital for successful urban remnant restoration, but projects often lack consistent methods that distinguish them. In New Zealand, there are unique opportunities to restore depleted Dacrycarpus dacrydioides (A.Rich.) de Laub. (kahikatea, white pine) semi-swamp forest remnants in numerous urban centres. To assess potential restoration strategies for three kahikatea remnants in Hamilton City, we compared their physical features, native vascular species composition, age structures, life forms and epiphytes with a notional reference site (Te Papanui). Numerous native vascular species gaps are revealed among Te Papanui (66 species), Totara Park (40 species), Hillcrest Park (15 species) and Grove Park (nine species). Age structure analyses suggest that Hillcrest Park comprises the oldest kahikatea population, with an average age of 82 years, followed by Grove Park (70 years), Te Papanui (60 years) and Totara Park (32 years). A native floristic analysis of thirteen life forms found that Te Papanui contains the most (11), followed by Totara Park (eight), Grove Park (six) and Hillcrest Park (five). Despite the abundance of invasive plants at Totara Park, its high-water table and favourable humid, sheltered conditions support more epiphytes (nine) than Te Papanui (six), Hillcrest Park (one; Pyrrosia eleagnifolia), and Grove Park (none). Epiphytes absent from Te Papanui found at Totara Park may be due to the loss of the once abundant tree fern and host, Dicksonia squarrosa (whekī). Totara Park requires careful manipulation of troublesome weeds, whereas Hillcrest Park and Grove Park necessitate buffer extensions and native understory plantings. This study provides a simple framework that uses biophysical differences among urban remnants and a reference site to reveal suitable restoration strategies that could guide other urban restoration projects regionally and nationally.

1. Introduction

1.1. Urban Ecological Restoration

Urban ecological restoration is increasingly recognised as a pragmatic solution for reversing some of the degradation caused by urbanisation [1,2,3]. Ecological restoration provides numerous ecosystem services such as water purification, improved air quality, food and fresh water, and mitigation of the heat island effect, extreme weather events and soil erosion. In cities, restoration provides an additional body of social services due to their larger populations, including educational and recreational opportunities, social cohesion and the chance to reconnect with the environment.

In a global review, approximately 40% of urban policy plans were found to promote native biodiversity and 20% mentioned quantitative targets for specific native plant and animal taxa [4]. Urban ecological restoration generally focuses on reinstating ecosystems that resemble historic sites [5] and reintroducing and conserving diversity at the species or population level [6]. These projects vary by the type of sites selected (often due to site availability) and the adopted approaches. For example, urban restoration may focus on open parks [7], remnant restoration [8], riparian plantings [9] or rewilding gardens and infrastructure [10]. Urban ecological restoration objectives also differ with scale [2] but generally include biodiversity conservation, improving nature connectedness and wellbeing [11], and mitigating extreme weather events such as flooding [12]. In China and the United States, habitat restoration is also frequently employed to compensate for urban development [13,14]. In parts of Australia, studies have prioritised urban forest remnant restoration because they are havens for local biodiversity and are easy to monitor and access [15,16].

Ecological restoration in New Zealand has primarily focused on large national parks, offshore islands and reserves [17]. However, recognition that most of New Zealand’s threatened environments exist in urban centres [18] has shifted the focus to include urban restoration. Besides common goals like controlling invasive species, planting native flora, riparian planting, reconstructing pre-European vegetation patches and enhancing connectivity, many urban restoration projects have attempted to restore native remnants to their condition before the city’s establishment [17].

In New Zealand, Dacrycarpus dacrydioides (A.Rich.) de Laub.(kahikatea) semi-swamp forest was once widespread across most of the North Island and west and southern areas of the South Island [19]. Despite its wide natural disruption, it is predicted that more than 98 per cent of New Zealand’s pre-European kahikatea forest has been lost [20,21]. The primary cause of this clearance includes agricultural and forestry pressures resulting in artificial drainage and often the invasion of grey willow (Salix cinerea) [22]. Such lowland environments contain some of the richest species assemblages in the country despite their extensive clearance [23]. Given the lack of indigenous vegetation in New Zealand’s urban centres and the extensive clearance of kahikatea forest, it is crucial that appropriate restoration strategies are identified for these remnants. Species composition [24] or traits [25,26] are often used to guide species selection and enhance community assembly in restoration sites. Here, we investigate kahikatea age structures and indigenous plant composition, focusing on life forms to characterise each site. Life form analyses enable the rapid identification of missing functional groups to ensure restoration strategies reinstate the complete range of ecological functions. Hence, the purpose of this study is to provide a framework to identify appropriate restoration strategies that improve the ecological integrity and functioning of urban forest remnants.

1.2. Kahikatea

Dacrycarpus dacrydioides (kahikatea, white pine) is an endemic podocarp found in the lowlands of the North, South and Stewart Islands of New Zealand, reaching up to 600 m elevation [27]. Kahikatea forest typically develops in open swamp or shrublands on poorly drained gley and organic soils with a high water table. Kahikatea display several distinct features throughout their lifespan. Juvenile trees have an abundance of short shoots from their leaf axils along the trunk and branches with subdistichous, small, dark green leaves and are shade-intolerant [28]. Kahikatea grow up to 60 m as the tallest tree in New Zealand and can have a trunk as large as two metres in diameter. They shed their lower branches with maturity while the smooth bark flakes off to form thick, black, round segments. Large buttress roots support kahikatea in its preferred swamp environment with a limited supply of oxygen. Male trees have narrow cones, while females produce cones with four to five-millimetre black seeds and fruit that ripen from green to red. Kahikatea relies on birds such as tūī and kererū for dispersal [27] and provide a range of services, including habitat for vulnerable native fauna (e.g., Chalinolobus tuberculatus, New Zealand’s endemic and nationally critical long-tailed bat).

1.3. Regional and Local State of Kahikatea Forest

In the Waikato region, about one per cent of the original extent of kahikatea forest remains, and just 13 per cent are legally protected [20]. The kahikatea-dominated fragments only constitute 0.2 per cent of the dairy landscape in central Waikato [29]. These remnants demonstrate similar size and age structures, commonly having a core of 350 to 450 year-old trees surrounded by 80 to 120 year-old trees [30]. With a life span of about 450 years and relatively young fragments in the Waikato region, kahikatea are expected to dominate the canopy of these stands for at least another three centuries [29]. Kahikatea forest typically grows on flat, fertile land ideal for agriculture and face several threats, including clearance, grazing, the invasion of exotic plants, edge effects and drainage. Half of the remaining kahikatea forests in Waikato are not fenced off from stock [20]. Heavily grazed kahikatea fragments require at least 15 to 20 years of fencing for major functional groups to recover and 40 to 50 years for a site to resemble its natural state [29]. Persistent forest clearance exacerbates edge effects and the vulnerability of fragments to weed invasion and unfavourable microclimatic conditions, including prevailing winds. Kahikatea are also threatened by drainage, which encourages its replacement with other dry land species such as Podocarpus totara, Prumnopitys taxifolia and Beilschmiedia tawa [31,32].

Once a conspicuous vegetation type of Hamilton City, kahikatea forest has been reduced to just 15 ha [33]. This total area comprises 17 sites, with the largest remnants including Te Papanui (5.5 ha), Burbush Road Bush (1.8 ha), Hillcrest Park (1.5 ha) and Southwell Bush (1.2 ha). The residual area is spread across patches of less than one hectare [33]. In the city, kahikatea forest is commonly found on recent fertile alluvial soils, such as Te Kowhai silt loam [29]. They also suit waterlogged soils found on low rolling hills (footslopes), shallow depressions or swales and narrow gully floors [18]. Protecting these sites is crucial for local kahikatea forest conservation.

1.4. Characteristic Species of Kahikatea Forest

Most restoration projects strive to assist a site’s recovery or return it to its original state [34]. Therefore, determining characteristic species of a specific forest provides essential information for successful restoration. Six characteristic species lists have been developed for semi-swamp kahikatea forest [18,30,35,36,37,38]. Canopy species mentioned in most sources include kahikatea and Laurelia novae-zelandiae and, less frequently, Beilschmiedia tawa, Dacrydium cupressinum and Prumnopitys taxifolia. Species in lower forest tiers (ground cover and understory) typically included Asplenium bulbiferum, Freycinetia banksii, Streblus heterophyllus, Coprosma areolata, Ripogonum scandens, Cyathea dealbata, Dicksonia squarrosa, Melicytus micranthus, Microlaena avenacea and Myrsine australis. The species found within kahikatea forest also reflect the age, seed sources and historical disturbances, including clearance, drainage, grazing, and edge effects [39]. The natural flooding regimes responsible for the development of kahikatea forest have been irreversibly altered in many places. Maintaining kahikatea forest in such modified environments requires active management (see [40]), including planting kahikatea itself.

2. Materials and Methods

2.1. Site Selection and Descriptions

Hamilton City is located at the centre of the North Island, New Zealand. It is the country’s fourth largest city at 11,000 hectares [41] and has a population of about 170,000 [42]. Once dominated by temperate lowland rainforest and wetlands, the city’s indigenous vegetation cover has been reduced to approximately 1.8%. The city has a mild climate with an annual mean temperature of about 14.6 °C and an annual mean precipitation of about 1110 mm [43].

Three sites in Hamilton City were selected based on their existing features and restoration potential, that is, Hillcrest Park, Totara Park, and Grove Park (Figure 1). Some initial requirements of the sites were that they are publicly owned and include a kahikatea remnant because using existing vegetation as nuclei for restoration is sensible [44]. All three sites also comprise soils suitable for kahikatea semi-swamp forest [18], are significant natural areas (SNAs) [45] and adjoin areas not dominated by indigenous vegetation, thereby providing valuable restoration opportunities. Te Papanui, formally known as Jubilee Park and commonly known as Claudelands Bush (hereafter Te Papanui), was chosen as the notional reference site because it is the largest and most ecologically intact remnant of semi-swamp kahikatea forest in the city. This kahikatea remnant was previously surveyed using a plot-based methodology [36,45].

Hillcrest Park supports the third largest kahikatea remnant in Hamilton City, after Te Papanui and Burbush Road [32]. The 7.1-hectare (ha) site includes a 1.5 ha kahikatea patch surrounded by 5.6 ha of open grass on the east and west. The nearest indigenous vegetation includes the University of Waikato kahikatea stands and Mangaonua Esplanade, over a kilometre away [46]. The park comprises four soil types: Te Kowhai silt loam in the centre, Kainui silt loam and Rotokauri clay loam on the western side, and Horotiu sandy loam on the eastern side. It also contains three under-represented ecological units: shallow depressions, low mounds, and low rolling hills (footslopes). The remnant has been severely degraded, with the loss of most understory species by 1972 and drainage since 1977 [32]. Canopy species in the 1980s included Cordyline australis, Laurelia novae-zelandiae, Nestegis lanceolata, kahikatea, Elaeocarpus hookerianus and one Elaeocarpus dentatus. Since 1994, Hamilton City Council (HCC) and a local school have planted native species, fenced off the remnant and constructed a raised walkway for recreation and educational activities. A scout hall is sited on the south-western side of the forest, and a sports hall sits on the eastern edge. Although the ground cover is sparse, abundant native plants occupy the mid-tier, and the canopy is healthy with few gaps. The east and west boundaries of Hillcrest Park have the widest buffers with Kunzea ericoides, Pittosporum tenuifolium, Myrsine australis and Podocarpus totara. Kahikatea were also planted on these edges, occasionally close to the forest margin.

Grove Park was established as a reserve in 1975 [47]. It is the third-largest kahikatea forest remnant on the western side of the city in Dinsdale, supporting a rare secondary stand of about sixty trees. The site is relatively isolated, with the nearest indigenous vegetation patch about 500 m away. Additionally, the park’s flat setting, encircled by residential properties, has resulted in its drainage and the unlikely return of a high-water table. The site is only 0.3 ha, with 0.1 ha of grass on its east and west that could be restored. Its Te Kowhai silt loam soil supports kahikatea forest [29], and the shallow depressions ecological unit of the alluvial plains is severely under-represented in Hamilton City. The ground cover primarily consists of leaf litter. Additionally, the understory is mostly bare amid some native plantings that began in 2003, such as Podocarpus totara, Myrsine australis, Coprosma autumnalis, Carpodetus serratus, Blechnum parrisiae, Piper excelsum, Artistotelia serrata, Geniostoma ligustrifolium, Alectryon excelsus and Melicytus micranthus.

Totara Park is in the northwest suburb of St Andrews in Hamilton City. Surrounded by residential properties and roads, the park is more than 500 m away from any indigenous forest [46]. It is a grey willow and kahikatea forest, hosting the best-regenerating kahikatea swamp forest in the city. In 1969 the site was mined for sand, and most of its vegetation was cleared [48]. Eventually, HCC altered the drainage system and enabled a functional flooding regime with small pipes that supported the development of the wetland. The canopy is dominated by grey willow with emergent kahikatea in the central gully floor or the park’s wetter sections that retain a high water table for most of the year [49]. The 2.7 ha site comprises two distinct environments: a gully (terrace scarps) and a grassy park with peaty soils. The vegetated component of the park is dominated by Tamahana, Kaipaki peat and Kirikiriroa complex soils, whereas the open side of the park comprises Kaipaki peaty loam.

2.2. Data Collection

All 84 research-grade kahikatea observations within the Hamilton City boundaries were downloaded from iNaturalist NZ (hereafter iNaturalist) on 5 November 2021. The observations were carefully assessed to determine if the individuals established naturally or were planted. A conservative approach was taken in which observations were retained based on the descriptive notes and photographs provided for each plant. As a result, only 32 observations likely established prior to the city’s main development phase and indicative of the relict natural range of kahikatea were employed to select the case studies. Using research-grade iNaturalist data also ensures species have been identified by a minimum of two contributors and have been subject to wider scrutiny.

The diameter at breast height (DBH, 1.35 m) of the kahikatea stems at each site were measured. The size and number of naturally regenerating individuals at these remnants were somewhat limited, so an attempt was made to measure all kahikatea at each site rather than using a plot-based methodology. Species richness and composition were also recorded by observations uploaded to iNaturalist. Again, a plot-based methodology was not employed due to the size of the remnants and because many of the understory species at Hillcrest Park and Grove Park were planted and have not naturally regenerated. However, Totara Park’s well-established kahikatea saplings demonstrating recruitment were recorded by laterally traversing across the park with a sampling intensity of approximately 40 per cent.

Site assessments were conducted to identify differences in conditions and species assemblages among Te Papanui and the case studies. Research-grade observations of indigenous vascular plant species were downloaded from iNaturalist to assemble species lists for each site. A cautious approach was taken to remove any cultivated plants. The nomenclature follows the New Zealand Plant Conservation Network (NZPCN). This study focused on native vascular species, and while certain troublesome exotic species were noted, comprehensive species lists are not provided, although one has been compiled for the reference site [36].

2.3. Data Analysis

Diameter at breast height (DBH) measurements and counts of saplings were used to construct population structure histograms for each site. Further DBH data were extracted from assessments of the kahikatea at Te Papanui [50] and Totara Park [48]. Age midpoints were predicted using the DBH classes from a previous study [47] and two existing regression equations. For Te Papanui, Hillcrest Park and Grove Park, the J.R. Leathwick (unpublished) regression equation for Berkley, Hamilton was used for most of the size classes encountered. However, the D.A. McLean (unpublished) regression equation was used for the age midpoint of the youngest size class at Te Papanui, Hillcrest Park and Grove Park as it more accurately predicts the age of smaller trees. For Totara Park, the D.A. McLean (unpublished) regression equation was applied to reveal the midpoint of all size classes because it was developed from the park’s data and is better suited to its larger population size and smaller age range. Then, the age midpoints were used to compare each kahikatea population structure (Figure 2). Three kahikatea at Hillcrest Park, Grove Park and Totara Park were cored using standard methodology [51] to verify the fit of the DBH-age relationship from the equations. On average, the estimated age of the kahikatea using the coring methodology was six years older than the DBH measurements suggested, with a margin of error less than 10 per cent.

• Hillcrest and Grove Park Kahikatea Age Equation: y = 55.127Ln(x) − 132.240
• Totara Park Kahikatea Age Equation: y = 0.4262x + 24.693

3. Results

3.1. Age Structures

The DBH of Te Papanui’s 371 kahikatea ranged from two to 100 centimetres (cm). The average age was about 60 years, which is younger than Hillcrest Park and Grove Park. Most kahikatea were between 71 and 92 years. Te Papanui’s kahikatea structure exhibited a wider range of ages than Totara Park but a similar structure to Hillcrest Park and Grove Park (Figure 2a and Figure 3).

The DBH of Hillcrest Park’s 223 kahikatea ranged between four and 127 cm. The average age was 82 years, which is older than Grove Park and Totara Park. The maximum age was estimated to be 135 years. The age structure of this patch shows that most trees are between 80 and 120 years, and the most common age is 118 years (Figure 2b and Figure 3).

The DBH of Grove Park’s 64 kahikatea ranged between 14 and 83 cm, with a mean diameter of 42 cm. The average age was 70 years, approximately 12 years younger than Hillcrest Park but 38 years older than Totara Park’s kahikatea. From these measurements, the youngest kahikatea was approximately 19 years, and the maximum age was 130 years. Almost 70 per cent of the kahikatea were predicted to be 60 to 100 years, with the 60 to 70-year class containing the most trees (Figure 2c and Figure 3).

Comprehensive data collected by Don McLean and Emma Coleman at Totara Park [48] found that the DBH of these kahikatea were between 3 and 47 cm. Totara Park had the youngest kahikatea population of the three sites, with an average age of 32 years. The minimum age was approximately 26 years, and the maximum was 45 years, representing a narrower age range than the other case studies. Fieldwork also identified 25 well-established kahikatea saplings demonstrating recruitment (grey bar in Figure 2d), defined as individuals taller than 135 cm with a DBH less than four centimetres [52]. Hundreds of smaller kahikatea seedlings (<135 cm) were also encountered.

3.2. Comparison of Biophysical Features with a Reference Site

Reference sites provide a model of optimal ecological integrity that have endured minimal degradation for similar sites while planning and evaluating restoration projects [5,34]. Although no two sites are identical, information extracted from a reference site, such as historical and current species composition, threats, and solutions, can guide the restoration of similar sites towards their natural state. Te Papanui is the largest semi-swamp kahikatea remnant in Hamilton City [45]. Therefore, its species composition should indicate species gaps in similar local patches (Appendix A). The ecological integrity of Te Papanui was compared with the three case studies to identify how to improve their condition.

Most of Hamilton City’s primary forest has been lost, leaving small secondary forest patches. Primary forest includes mature stands that comprise their natural vegetation composition and have not recently been completed cleared or re-planted [53]. Te Papanui is the only kahikatea remnant in Hamilton City that partly includes primary forest. Conversely, secondary forest refers to stands regenerating through natural processes after substantial anthropogenic or natural disturbances, demonstrating a significant difference in species composition to nearby primary forest [54]. The kahikatea remnants at Grove Park, Totara Park, and Hillcrest Park are secondary forest patches.

General physical conditions that significantly influence the biotic communities of each remnant were also noted. Ecological connectivity is also poor across the sites, with the nearest healthy patch of indigenous vegetation often up to several kilometres away. All sites have been extensively drained where no kahikatea were found regenerating apart from Totara Park. Moderate buffers at Te Papanui, Hillcrest Park and Grove Park suggest their exposure to edge effects, including a less stable microclimate than Totara Park.

Site visits revealed fundamental differences in the condition of the forest tiers and the floristic composition of each remnant. Totara Park has abundant exotic plants in the canopy, shrub and ground cover tiers, while the other case studies have very few. Te Papanui supports the healthiest kahikatea patch with all three forest tiers intact, hosting rich native flora. The emergent kahikatea across all sites are healthy, with minimal yellowing or defoliating. Hillcrest Park has two intact tiers, Grove Park has a healthy canopy, and Totara Park is not dominated by native plants in any of its tiers.

Data collected from iNaturalist suggest that Hillcrest Park has only a handful of naturally established native species, including the canopy species of Laurelia novae-zelandiae, kahikatea, Cordyline australis, Beilschmiedia tawa, Nestegis lanceolata and Elaeocarpus hookerianus. Most other indigenous species in Hillcrest Park’s understory were likely planted. No kahikatea seedlings (<135 cm tall) or saplings (>135 cm tall, with a DBH < four centimetres) were found during the site assessments indicating the lack of regeneration. A few troublesome weeds exist in Hillcrest Park, including Euonymus japonicus, Hedera helix and Ligustrum lucidum. Grove Park has also faced severe disturbances, including drainage and clearance; it is now a depauperate site with sixty kahikatea, one mature Prumnopitys taxifolia and a few understory plantings. The introduced species include Laurus nobilis, Berberis glaucocarpa and Euonymus japonicus. Totara Park’s gully edge conditions, including increased light and well-drained soils, have enabled the establishment of Melicytus ramiflorus and Cyathea dealbata. Other common native species at Totara Park include kahikatea, Cordyline australis, Dicksonia squarrosa and Geniostoma ligustrifolium. The site also contains numerous problematic exotic plants, including Salix cinerea, Zantedeschia aethiopica, Lonicera japonica and Ligustrum sinense. A mature Laurelia novae-zelandiae in the patch signals that it is suitable habitat and an opportunity to reintroduce more individuals.

3.3. Life Forms and Epiphytes

The life form analyses using iNaturalist data found that Te Papanui hosts the most groups, followed by Totara Park, Grove Park and Hillcrest Park (Figure 4). The selected life forms included 13 categories of ferns and fern allies, gymnosperm trees, dicot trees, dicot shrubs, dicot lianes, dicot herbs, monocot trees, orchids, grasses, sedges, rushes, monocot herbs and monocot lianes [36]. Te Papanui supports species of all life forms except dicot herbs and rushes, also absent from the case studies. The dominant life forms at Te Papanui were ferns and fern allies and dicot trees. This trend is primarily consistent with the other sites. Totara Park’s native flora includes eight life forms, not including monocot herbs, grasses, monocot lianes, dicot herbs or rushes. Grove Park’s assessment identified six life forms but lacked dicot lianes, orchids, grasses, monocot lianes, sedges, dicot herbs and rushes. Finally, Hillcrest Park supported five life forms but lacked dicot lianes, monocot herbs, orchids, dicot shrubs, monocot lianes, sedges, dicot herbs and rushes.

Although counted separately, epiphytes play a prominent role in ecosystem functioning via biomass and nutrient partitioning [55] and are influenced by stand density and microclimate [56,57]. An epiphyte grows upon a host, is unconnected to soils and is not parasitic [58]. Life forms commonly containing epiphytes include ferns and fern allies, shrubs, herbs and orchids. Totara Park hosted the most epiphytes (nine), followed by Te Papanui (six), Hillcrest Park (one), and none were observed at Grove Park. The nine native epiphytes at Totara Park included eight ferns and fern allies (Asplenium flaccidum, Asplenium oblongifolium, Asplenium polyodon, Microsorum pustulatum, Phlegmariurus varius, Pyrrosia eleagnifolia, Tmesipteris elongata and Tmesipteris lanceolata) and one orchid (Earina mucronata). Comparably, the epiphytes at Te Papanui included four epiphytic ferns (Arthropteris tenella, Blechnum filiforme, Microsorum scandens and Pyrrosia eleagnifolia), one monocot herb (Astelia hastata) and one orchid (Earina mucronata). The fern Pyrrosia eleagnifolia was the only epiphyte found at Hillcrest Park.

Totara Park and Te Papanui support different epiphytic species apart from Earina mucronata and Pyrrosia eleagnifolia, which grow on a wide range of hosts. One explanation for the different assemblage of epiphytes at Te Papanui may be the loss of Dicksonia squarrosa (whekī), which was abundant in the mid-1900s [36]. Whekī is a ubiquitous tree fern found throughout New Zealand, which provides a prominent microsite for epiphytes [59,60]. For example, Asplenium flaccidum, Asplenium oblongifolium, Phlegmariurus varius, Tmesipteris elongata and Tmesipteris lanceolata were all found growing on whekī in Totara Park. The damp, cool microclimate of Totara Park also sets it apart from the other sites. Furthermore, epiphytes such as Arthropteris tenella, Blechnum filiforme, and Microsorum scandens that were not seen at Totara Park have typically colonised lower forest tiers in Te Papanui. The limited presence of epiphytic species at Hillcrest Park and Grove Park is likely due to their drainage and low humidity, which are not conducive to epiphytes [61].

4. Discussion

All three remnants provide significant restoration opportunities where mature kahikatea can act as nuclei for extending and connecting patches. The assortment of features at each site should guide the selection of restoration strategies and their application over time. For example, Hillcrest Park encompasses the oldest kahikatea and has the largest area available to restore, covering three different ecological units. Grove Park supports a secondary stand of kahikatea, with the smallest available restoration area of the three case studies. However, it contains the severely under-represented alluvial plains and is a key natural feature of the western side of the city. Totara Park is the best example of regenerating kahikatea in Hamilton City and offers a considerable area for restoration. The persistence of locally rare species in this remnant necessitates a sustained manipulation restoration strategy.

The kahikatea at Hillcrest Park are in reasonable condition, with plantings in the understory and a narrow buffer. The canopy layer is in the best state, with healthy kahikatea. Only one mature Laurelia novae-zelandiae, Nestegis lanceolata, and Elaeocarpus hookerianus still exist in the stand. The dense Freycinetia banksii understory tier described as a prominent feature of the stand in the 1960s [32] has been completely eradicated. The liane Ripogonum scandens has not been seen in the remnant since the 1990s [62]. Additionally, while community groups have restricted exotic plants from establishing and enabled native plants to dominate with an intact canopy tier, naturalised native plants, including Pseudopanax crassifolius × lessonii and Hoheria populnea [23], have established, and Asplenium × lucrosum has also been planted. Hillcrest Park’s ground cover has been severely degraded, with clearance exacerbating unfavourable conditions like lower humidity and greater light penetration. The shrub layer also lacks several species, with a scarce dicot shrub life form aside from a few mature Melicytus ramiflorus. For example, characteristic small-leaved coprosma species are absent, including Coprosma propinqua, Coprosma rotundifolia and Coprosma rhamnoides. Other dicot shrubs typical of kahikatea forest that could be planted at Hillcrest Park include Coprosma tenuicaulis and Melicytus micranthus. Teucridium parvifolium is an at-risk characteristic shrub species that would suit the dry kahikatea patch [30]. Hillcrest Park’s missing life forms included lianes and orchids, which would require improved semi-swamp conditions to establish. Characteristic species in Te Papanui included lianes such as Freycinetia banksii, Metrosideros perforata, Parsonsia heterophylla, Passiflora tetrandra and Ripogonum scandens and orchids such as Earina mucronata and Microtis unifolia. Only one epiphyte (Pyrrosia eleagnifolia) was found at Hillcrest Park. Extending Hillcrest Park’s buffer alongside ground cover and shrub tier plantings would reduce its exposure and enhance its ecological integrity.

Modest plantings at Grove Park since 2003 and the control of exotics have contributed significantly to its health. However, artificial drainage has also considerably modified the physical conditions of the patch from its original state. Like Te Papanui, it is unlikely that a high-water table will be re-established at Grove Park, given its proximity to residential properties. The canopy is relatively intact with a healthy kahikatea population, one Prumnopitys taxifolia, and planted Carpodetus serratus, Aristotelia serrata and Alectryon excelsus. However, there is minimal ground cover with just one native fern (Blechnum parrisiae). The shrub tier currently comprises a few planted species like Coprosma robusta, Melicytus micranthus and Geniostoma ligustrifolium. Life forms missing from this stand include grasses, sedges, herbs and rushes from the ground cover tier and lianes and orchids from the shrub and canopy tiers. Characteristic ground cover species of kahikatea forests that may be planted include sedges like Carex uncinata, Carex dissita, Carex lambertiana and Uncinia uncinata. Epiphytes typically found in kahikatea forest include Astelia hastata and Blechnum filiforme. Further planting of appropriate shrub species such as Coprosma areolata, Coprosma tenuicaulis, Melicytus micranthus and Melicytus ramiflorus should improve its ecological integrity. Teucridium parvifolium, a characteristic species that grows in dry conditions, has already been planted at Grove Park. Planting additional individuals could improve its population viability while contributing to the stand’s diversity. The restoration of Grove Park would provide numerous benefits, including strengthened ecological linkages from the southwest of the city to other native patches and gullies. Extending Grove Park’s buffer and planting its ground cover and shrub layers would reduce the edge effects, increase its diversity and improve its ecological health.

Totara Park provides an exceptional example of the effect of a functional flooding regime on the persistence of native species that are otherwise rare throughout the city. In contrast to Te Papanui, Totara Park comprises Tmesipteris lanceolata, Tmesipteris elongata, Dicksonia fibrosa and Rhopalostylis sapida, which prefer its sheltered, damp and cool microclimate. Its richer epiphytic diversity than Te Papanui and the other case studies highlight the value of restoring Totara Park to protect these species and encourage the establishment of others. Epiphytic species such as Griselina lucida, Pittosporum cornifolium and Astelia solandri could be reintroduced to the site and protected while carefully managing the profusion of troublesome weeds at Totara Park. Some of the troublesome weeds include grey willow, Lonicera japonica, Ligustrum sinense, Tradescantia fluminensis, and Euonymus japonicus. Suddenly removing all exotic species could alter the cool, damp and sheltered conditions that have enabled the locally rare native species to persist. Despite the abundance of exotic plants in Totara Park, the grey willow-kahikatea patch supports the most native species per forest tier of the three case studies. Its native dicot trees include Aristotelia serrata and Schefflera digitata. The natural establishment of mature Melicytus ramiflorus on the northern edge and one Laurelia novae-zelandiae suggest it may be practical to plant more individuals of these species. Another characteristic tree that would likely thrive in this environment is Syzygium maire. The shrub layer includes Coprosma robusta, Coprosma autumnalis and Geniostoma ligustrifolium, which could be enriched to include species such as Coprosma areolata, Coprosma tenuicaulis and Melicytus micranthus. The orchid Earina mucronata persists in the patch, and if restored, epiphytes currently at Te Papanui such as Arthropteris tenella, Astelia hastata, Blechnum filiforme, and Microsorum scandens may eventually establish (see Appendix B for planting zone maps for the case studies). Optimising the ecological potential of Totara Park requires the adoption of a manipulation strategy to progressively release native plants from the competition of abundant exotic species and encourage the persistence of uncommon native species in the other forest tiers. This strategy would be labour-intensive, requiring the careful control of exotics to gradually allow kahikatea to dominate the canopy.

Implications for Management

Kahikatea forest was once ubiquitous in the lowland basins of New Zealand. With less than two per cent of its original extent remaining nationally, one per cent in the Waikato region and only 15 ha in Hamilton City, it urgently requires protection and enhancement. Restoring kahikatea forest patches to their original state requires the careful selection of characteristic species that should be based on sites that have experienced minimal anthropogenic disturbance to achieve as full a species occupancy as practicable. Planting species from the missing life forms could fill functional gaps at each site. The leading threat facing kahikatea swamp forests is artificial drainage, which continues to modify the physical and biotic traits of Hillcrest Park and Grove Park. As seen at Totara Park, reinstating a functional flooding regime can substantially enhance the health of kahikatea forest. Plant pest control will be vital for the successful restoration of Totara Park, requiring a labour-intensive manipulation strategy. All three patches are also highly exposed to unfavourable edge conditions because of their small size. Extending the buffer at each site would likely mitigate forest edge effects and stabilise the microclimate by reducing light intensity and air temperature and increasing the humidity. Regardless of the limited size of these patches, they provide some of the last opportunities to conserve kahikatea semi-swamp forest in Hamilton City.

The restoration of the extant kahikatea remnants in Hamilton City would require substantial resources. Aside from four sites, all remaining kahikatea forest in the city is found in patches of less than one hectare. Given the wet conditions that kahikatea require for regeneration, it would be pragmatic to prioritise the few sites that provide these conditions (such as Totara Park) to conserve kahikatea semi-swamp forest. It would also be practical to focus on restoring the city’s gully floors which are predicted to have supported kahikatea-pukatea-swamp maire forest [18]. Gullies also face less threat from artificial drainage and can provide conditions conducive to kahikatea forest. For example, relic kahikatea on the narrow floors of the Mangaonua and Kirkiriroa (Onukutara) gullies signal significant restoration opportunities. Future studies may consider the restoration potential of these sites and how they could contribute to a reconnected network of kahikatea patches, like Hillcrest Park, Totara Park and Grove Park, throughout the city.

Such site assessments should consider the implications for stakeholders and include extensive consultation to obtain consensus on priorities for restoration implementation.

5. Conclusions

Restoring native forest remnants is a common goal for many urban restoration projects globally [63]. Identifying differences in species and physical features among urban remnants and a reference site can reveal species gaps and key threats to inform management decisions and enhance biodiversity in cities [64,65]. This study has presented a simple framework to improve the ecological integrity of sites, consistent with a national framework [66], by revealing differences in life forms across sites and, therefore, functional gaps that could be filled [67,68]. Similar studies could employ these methods of identifying species and life form gaps among urban remnants and reference sites to guide the selection of appropriate restoration strategies in other cities.