Evaluating the Progress of Restoration Planting on Motutapu Island, New Zealand
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University of Auckland, 2015 Evaluating the progress of restoration planting on Motutapu Island, New Zealand Robert Vennell1* 1 University of Auckland, Private Bag 92019, Auckland 1142, New Zealand * Email for correspondence: [email protected] Introduction Ecological restoration is an intentional human activity that attempts to accelerate the recovery of an ecosystem that has become degraded, damaged or destroyed (SER, 2004). Often the impacts to ecosystems are so great that they cannot recover to their ecological state prior to the disturbance (SER, 2004; Suding et al. 2004). Therefore the key focus of most restoration efforts is to return ecosystems back to their historic trajectories (SER, 2004; Forbes & Craig, 2013) in order to enhance their overall health, integrity and sustainability. In order to achieve this goal, a historic reference state is generally used as a framework and to provide a starting point for designing the restoration (Balauger et al. 2012). There is concern however that we currently lack sufficient knowledge of historic ecosystems to replicate their composition and function faithfully (Davis, 2000; Choi et al. 2007). In order to mitigate this problem, a modern analogue is often used – the reference ecosystem (SER, 2004). This consists of a contemporary habitat that is assumed to representative of the desired historic reference state, and can serve as a model that restoration activities attempt to emulate (White & Walker, 1997). As a result it also provides an ideal opportunity for evaluating the success of restoration activities, allowing comparisons to be made against restored ecosystems and outcomes effectively measured (e.g. Stephenson, 1999; Palmer et al. 2005; Klein et al. 2007; Matthews et al. 2009). Motutapu Island in the Hauraki Gulf of New Zealand represents an ideal opportunity for restoration; as a complex physical and geological landscape, protected from invasion by exotic pests by an ocean barrier but still close to New Zealand’s major metropolitan centre (Miller, 1994). Palynological evidence suggests the island was once covered in a mixed podocarp/broadleaf forest (Murray, unpublished data, cited in Miller et al. 1994). However extensive human cultivation has severely reduced native vegetation cover (Esler, 1980) and today the island consists primarily of introduced European pasture (Miller, 1994). The Motutapu Restoration Working Plan (Hawley, 1993) seeks to restore ‘the ecological community that was present on Motuapu after the Rangitoto eruption’ by 1 University of Auckland, 2015 revegetating up to one third of the island in native forest. This study seeks to evaluate the success of the restoration activities on Motutapu Island, ten years after the first planting programme begun. In order to effectively evaluate whether or not the programme has been successful, a reference ecosystem - the oldest mature remnant forest stand on the island – was compared against three age classes of restoration plantings. Density, basal area and species composition of each of the forest stands were analysed to determine whether the restoration plantings are converging on the reference state in terms of plant community structure and composition. Methods Study Site & Data Collection Motutapu Island is situated in the Hauraki Gulf, approximately 16km away from the Auckland Isthmus – New Zealand’s most densely populated urban centre. The island has an extensive cultural history for both European and Māori (Hawley, 1993; Miller et al. 1994). Since the 19th century the island has been cleared and grazed and is largely covered in pasture, save for a few remnant forest patches that remained in the gullies and around the coastal fringe (Esler, 1980) The point-centred-quarter (PCQ) method (Mitchell, 2015) was used to quantify density, basal area and species composition of the largest remnant of old-growth forest on the island against three age classes of restored forest (Planting A – c. 1995; Planting B – c. 2001, Planting C – c. 2010). At each forest stand, a 75 m transect was run along the edge of the forest fragment. PCQ plots were located at 15m intervals along the transect and sited 20 m apart working perpendicular from the length of the transect (25 plots total for each stand). Within each quarter of the plot, the closest woody tree with a DBH > 5cm was identified and its distance to the centre measured. Planting C contained very few individuals > 5cm DBH, so if no sufficiently large trees were identified within 20 m of the quarter, the distance and identity of the nearest smaller tree was used instead (DBH was not recorded). For Multi-stemmed individuals the diameter of all stems at breast height were recorded. Data Analyses Density was calculated as the mean density of stems per ha, with confidence intervals calculated as per Mitchell (2015). Basal area was calculated in m2/ha by calculating mean the basal area of each stem for each site, and then multiplying by the density of stems per ha for that site. Similarity in species composition was assessed between sites using Sorenson’s index. Similarity was assessed further using multivariate analysis in PAST software (Hammer et al. 2001). First the data was 2 University of Auckland, 2015 weighted to give greater emphasis to larger individuals – by allocating the two closest individuals within a plot a value of two, with all other species recording a one. Then non-metric multi-dimension scaling (nMDs) Ordination was calculated using the Bray-Curtis dissimilarity measure to evaluate whether the forest sites could be distinguished from one another in terms of species composition. PERMANOVA was used to explore whether the sites were significantly different and SIMPER analysis was used to identify the species most responsible for driving the relationship. Results The reference forest contained a significantly lower density of stems than was found in the restored forest (Fig. 1). The restored plantings on the other hand were not shown to be statistically different from one another in terms of density. The reference forest also contained significantly higher mean basal area/m2/ha than the planted forest stands (Fig. 2). The planted forest stands showed a small but significant difference from one another in terms of basal area, with the oldest stand having the highest measure of basal area and progressively less in each of the younger stands. Fig. 1. Mean density of stems per hectare for the reference forest and Fig. 2. Mean basal area per m2 per hectare for the reference three planted stands of different ages at Motutapu Island Reserve. forest and three planted stands of different ages at Motutapu Error bars = 95% CI as per Mitchell (2015). Island Reserve. Error bars = standard error. None of the planted forests scored highly on the Sorenson’s index for measuring the similarity of species composition between the forest sites (0.25 – 0.12; Table 1). The older Plantings A and B showed a greater amount of similarity to the forest remnant than the more recent Planting C. Planting A and Planting B were the most similar in terms of species composition (0.33). Table 1. Sorenson’s Index values comparing reference forest and three PERMANOVA provided strong evidence planted stands of different ages at Motutapu Island Reserve. Values are presented as a figure from 0 – 1, with 1 representing an identical that there were significant differences composition. between all of the forest sites examined Reference Planting A Planting B Planting C Reference . 0.25 0.25 0.12 (Bonferroni-corrected p-values <0.001) Planting A 0.25 . 0.33 0.25 however this a fairly conservative measure Planting B 0.25 0.33 . 0.17 Planting C 0.12 0.25 0.17 . and does not consider the extent of the 3 University of Auckland, 2015 difference. The nMDS ordination recorded a high stress value (0.488) suggesting that only broad generalisations can be made from the ordination. The ordination plot PA suggests that the reference forest is the most dissimilar PB from the other sites, with only a small degree of overlap RF between planting A and planting B and very little PC similarity with Planting C (Fig. 3). The planted forest sites are difficult hard to distinguish, and appear very similar in terms of composition with a large degree of overlap. SIMPER analysis identified the most influential species Fig. 3: nMDs Ordination of PCQ data at Motutapu Island reserve using Bray-Curtis Dissimilarity driving the above relationship. In particular, mānuka Index. Sites were colour coded according to their forest state (Reference – RF – Blue; Planting A – (Leptospermum scoparium), cabbage tree (Cordyline PA, Green; Planting B – PB, Yellow, Planting C – PC, Red). australis), karo (Pittosporum crassifolium) and karamu (Coprosma robusta) were highly abundant in the planted forest stands but absent in the reference forest. In contrast, kohekohe (Dysoxylum spectabile) and tōtara (Podocarpus totara) were predominantly found in reference forest and rare or absent in the restored plantings. Discussion To return to the central focus of the study; we aimed to evaluate whether the restoration plantings were converging on the reference state in terms of plant community structure and composition. Our results indicate that the restored forest plantings on Motutapu Island differ significantly from the reference ecosystem in terms of all the variables measured - density, basal area and species composition. However, they also suggest that there is a slight trend towards convergence in older plantings, with a small increase in average basal area and increasing similarity in species composition. Therefore in answer to the research question we suggest that currently, the restoration plantings do not resemble the reference state in any of the variables measured, however given a much longer length of time they may come to do so. Reference sites are typically selected because they represent a well-developed, mature forest with high levels of biodiversity. However sites in the process of restoration generally represent a much earlier ecological stage (SER, 2004).