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Biological flora of New Zealand 14: Metrosideros excelsa, pōhutukawa, New Zealand Christmas tree

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We review the biology and ecology of Metrosideros excelsa (Myrtaceae), an endemic angiosperm evergreen tree. Metrosideros excelsa belongs to a conspicuous and widely distributed Pacific Basin genus, with centres of diversity in both New Zealand and New Caledonia. Metrosideros excelsa is an iconic tree species that forms a significant component of northern New Zealand's exposed coastal headland and cliff vegetation. Where conditions are more favourable, M. excelsa forms tall coastal forest, ranging from simple young high-density stands to diverse mature forest. Inland, M. excelsa stands are confined to the margins of lakes and rivers on the Central Volcanic Plateau, where some may originate from early Māori plantings. Metrosideros excelsa is reliant on stochastic disturbance events (e.g. landslides, volcanic eruptions) to create open sites necessary for regeneration. Mass flowering (December–January), followed by abundant production of wind-dispersed seed maximises chance colonisation of such sites. Since human settlement in New Zealand, the distribution of M. excelsa forest has declined by c. 90% and the southern limit of the species has retreated north. Natural regeneration on the mainland is limited by the infrequency of large-scale disturbances and increased anthropogenic and herbivore pressures. Consequently, M. excelsa forest has become rare and localised on the mainland; monitoring and active management are fundamental to the species' long-term conservation.
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... Probably the best known of the indigenous Myrtaceae is pōhutukawa (Metrosideros excelsa Sol. ex Gaertn.), the so-called New Zealand Christmas tree, a widely acknowledged iconic species, which is popular throughout New Zealand, and indeed worldwide, as an ornamental (Simpson 2005;Bylsma et al. 2014) (Figure 1). While the tree is culturally important, it and the leaf litter it produces are also a critical food source and habitat for a diverse array of life (Anderson 2003;Bylsma et al. 2014;Cummings et al. 2014;Galbraith & Large 2017;Hosking & Hutcheson 1993;Pattemore & Wilcove 2012;Schmidt-Adam et al. 2000;Taylor et al. 2007). ...
... ex Gaertn.), the so-called New Zealand Christmas tree, a widely acknowledged iconic species, which is popular throughout New Zealand, and indeed worldwide, as an ornamental (Simpson 2005;Bylsma et al. 2014) (Figure 1). While the tree is culturally important, it and the leaf litter it produces are also a critical food source and habitat for a diverse array of life (Anderson 2003;Bylsma et al. 2014;Cummings et al. 2014;Galbraith & Large 2017;Hosking & Hutcheson 1993;Pattemore & Wilcove 2012;Schmidt-Adam et al. 2000;Taylor et al. 2007). ...
... et G.Forst.) A.Gray (Wright et al. 2000;Gardner et al. 2004). The biological flora of the species was reviewed by Bylsma et al. (2014), who considered it indigenous to the northern portion of the North Island of New Zealand, ranging from Manawatāwhi Three Kings Islands south to Urenui (38° 59´ 28.76" S) in the west and to Poverty Bay (38° 45´ 29.715" S) in the east (Bylsma et al., 2014) (Figure 2), though exact southern limits are unclear due to possible plantings by iwi and more recently European settlers. ...
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Myrtle rust (Austropuccinia psidii) poses a serious threat to the New Zealand Myrtaceae. While the threat to the host tree is reasonably well-known, the threat myrtle rust poses to the associated biota is poorly understood. As a contribution to our knowledge of this, a preliminary list of the lichenised mycobiota that utilise pōhutukawa (Metrosideros excelsa) as a phorophyte is presented, based on a survey of the specimens in two herbaria with extensive collections from the natural range of this endemic tree species. We report 187 lichen species (and lower ranks) from 76 genera and 32 families.
... At present, information on the associates of Aotearoa / New Zealand Myrtaceae is limited, and skewed toward vascular plants . For example, Bylsma et al. (2014), noted 16 fern and flowering plant taxa epiphytic on pōhutukawa. McKenzie et al. (1999) also published an annotated list of nonlichenised fungi known from Metrosideros Banks et Gaertn, and an account of the non-lichenised fungi of Kunzea Richb. ...
... The greater number of lichens reported from pōhutukawa is not surprising, especially as this phorophyte, as a much larger tree, offers a greater range of microniches (see Blanchon et al. 2020). It is also likely that the iconic status of pōhutukawa has ensured that it is better studied and collected than Lophomyrtus (Bylsma et al. 2014;Simpson 2005). ...
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The invasive rust Austropuccinia psidii, responsible for myrtle rust disease, poses a serious threat to the New Zealand Myrtaceae. Since the 2017 detection of Austropuccinia psidii in Aotearoa / New Zealand, the rust has spread rapidly, resulting in the decline and death of a range of indigenous Myrtaceae, most notably the two species of the endemic genus Lophomyrtus, ramarama (L. bullata) and rōhutu (L. obcordata). While the threat Austropuccinia psidii poses to Lophomyrtus is now widely recognised, the indirect impact the rust has on the associated biota is poorly understood. Very little has been documented about the biota found in association with Lophomyrtus. To rectify this, we undertook a survey of the specimens held in three of the key Aotearoa / New Zealand herbaria that had been collected from Lophomyrtus. This was supplemented by field work in eight sites in western Te Ika a Maui / North Island, and north-western Te Wai Pounamu / South Island of Aotearoa / New Zealand. Although the herbarium searches located few specimens, and field work was limited to a few sample points within the range of Lophomyrtus, we found 221 taxa associated with Lophomyrtus, 176 taxa on ramarama, 81 on rōhutu and one on the naturally occurring hybrid between these two species Lophomyrtus ×ralphii. Of the 176 taxa found on ramarama, 59 are bryophytes (one hornwort, 33 liverworts and 25 mosses), five pteridophytes, 16 spermatophytes and 96 are lichenised mycobiota. Rōhutu supported 81 taxa: comprising one cyanobacterium, one alga, twenty- nine bryophytes (17 liverworts and 12 mosses), four pteridophytes, two spermatophytes and 44 lichenised mycobiota. Wild populations of Lophomyrtus ×ralphii were not investigated, and herbarium searches only disclosed one plant, the mistletoe Korthalsella lindsayi, associated with it. Several lichens and liverworts collected from Lophomyrtus represent potentially new species, and Lepra erythrella is a new addition to the lichenised mycobiota of Aotearoa / New Zealand. None of the putative new species are endemic to Lophomyrtus.
... Aotearoa-New Zealand is one of the centers for Metrosideros (Myrtaceae) biodiversity with a total of 12 endemic species (Bylsma et al. 2014). In particular, pōhutukawa (Metrosideros excelsa Sol. ...
... Pōhutukawa are also valued for the production of honey and as landscape trees due to their vibrant red flowers (Anon 2021). Pōhutukawa also have an important role in ecosystem services, providing a valuable nectar source for a range of taxa including native avifauna, insects, geckos, and the shorttailed bat (Mystacina tuberculata) (Bylsma et al. 2014). ...
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Metrosideros excelsa (pōhutukawa) is an important native tree species in Aotearoa-New Zealand. Ceratocystis lukuohia is an aggressive wilt pathogen of the Hawaiian forest tree M. polymorpha (‘ōhi ‘a) and could be a threat to pōhutukawa. A host response study was conducted to evaluate the susceptibility of M. excelsa seedlings to C. lukuohia infection. Forty-eight M. excelsa seedlings from six parent trees were inoculated with C. lukuohia. Disease symptoms and mortality were observed in M. polymorpha positive controls inoculated with C. lukuohia, but not in the M. excelsa. Dissections of the inoculated M. excelsa revealed significantly lower disease severity (mean range: 3.9%-9.0%) comparable to a mean of 1.7% for the M. excelsa negative control treatment. In comparison, the mean disease severity for the M. polymorpha positive control plants was 43.3 ± 6.1%. Aleurioconidia were present in the stem tissue of 86–100% of C. lukuohia-inoculated M. excelsa selections compared to 100% of M. polymorpha and 0% of M. excelsa inoculated with sterile water. These results suggest that C. lukuohia can infect M. excelsa if introduced to a wound, but the pathogen is not aggressive enough to cause wilt and mortality. There is potential for wild M. excelsa to become infected with C. lukuohia if a pathway allowed for the accidental introduction of the pathogen into Aotearoa-New Zealand. Differential susceptibility of other M. excelsa genotypes or Metrosideros species also needs to be considered. Therefore, measures should be taken to avoid the introduction of this pathogen into the country.
... Although Mystacina tuberculata has an omnivorous diet, it consumes a substantial amount of nectar and is currently known to be an important pollinator comparable to birds on the island country (McCartney et al. 2007). The flowers visited by Mystacina tuberculata like Metrosideros excelsa, Dactylanthus taylorii and Eucalyptus sp. have some of the chiropterophilous flower characteristics in which one common trait is the abundance of nectar produced (Arkins et al. 1999;McCartney et al. 2007;Pattemore and Wilcove 2012;Bylsma et al. 2014). ...
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Durian (Durio zibethinus) brings in princely revenue for the fruit economy in Southeast Asia, ushering the current trend of clearing forests for durian plantations. Despite the thorny fruit’s popularity and increasing bat-durian papers, not many associate their vital plant-pollinator relationship. This unfamiliarity has led to the persisting negative connotations of bats as agricultural pests and worse, a disease carrier amplified by the Covid-19 pandemic. This review focuses on the bat-durian relationship comprising botanical insights and pollination ecology in relevance to the wider pteropodid-plant interactions. The majority of the studies compiled have concluded that bats are the most effective pollinator for durian than insects. Six fruit bat species (Chiroptera: Pteropodidae) have been recorded pollinating durian flowers, with several other pteropodid species speculated to pollinate durian, including in non-native countries. Lastly, we address the research gaps for the bat-durian relationship, which can also be applied to other chiropterophilous plants.
... Pohutukawa (Metrosideros excelsa) is an example; this is a native tree species in New Zealand which has been subject to fires and land clearance, and more recently possum (Didelphimorphia) browsing. 5 Pohutukawa is a multistemmed tree up to 25 m high with large, rounded crowns. Identifying and mapping this tree species have been mostly based on field data such as in Simpson,6 which is costly and time-consuming to perform. ...
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Combining data of different types and from different sources for the classification of tree species has gained popularity recently, but training models on such datasets often requires more computational demands and does not always result in higher accuracy due to feature redundancy and irrelevance. Thus preprocessing data using dimensionality reduction (DR) methods can be employed to improve the classification accuracy and reduce computations. The objective of this research is to investigate and compare tree species classification performance for different classification algorithms [naive Bayes (NB), logistic regression (LR), random forest (RF), and support vector machine (SVM)], combined with various DR methods (correlation-based feature selection filter, information gain, wrapper methods, and principal component analysis). Two primary datasets are used-QuickBird and LiDAR, as well as derived topography data. When DR is used prior to classification, the NB classifier had a significant improvement in accuracy. SVM and RF had the best classification accuracy without DR. The overall accuracies (OA) of SVM and RF are 88.2% and 87.2% (kappa 0.84 and 0.83), respectively, followed closely by LR (OA: 84.8%, kappa: 0.79) and more distantly by NB (OA: 79%, kappa: 0.72). It is recommended to use SVM and RF without DR or NB with DR for tree species classification.
... Pohutukawa (Metrosideros excelsa Sol. ex Gaertn) is such a tree in New Zealand because it has been subject to fires and land clearance, and more recently possum browsing (Bylsma et al., 2014). Pohutukawa is a multi-stemmed tree up to 25 m high with large rounded crowns growing in northern coastal regions of New Zealand. ...
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There are now a wide range of techniques that can be combined for image analysis. These include the use of object-based classifications rather than pixel-based classifiers, the use of LiDAR to determine vegetation height and vertical structure, as well terrain variables such as topographic wetness index and slope that can be calculated using GIS. This research investigates the benefits of combining these techniques to identify individual tree species. A QuickBird image and low point density LiDAR data for a coastal region in New Zealand was used to examine the possibility of mapping Pohutukawa trees which are regarded as an iconic tree in New Zealand. The study area included a mix of buildings and vegetation types. After image and LiDAR preparation, single tree objects were identified using a range of techniques including: a threshold of above ground height to eliminate ground based objects; Normalised Difference Vegetation Index and elevation difference between the first and last return of LiDAR data to distinguish vegetation from buildings; geometric information to separate clusters of trees from single trees, and treetop identification and region growing techniques to separate tree clusters into single tree crowns. Important feature variables were identified using Random Forest, and the Support Vector Machine provided the classification. The combined techniques using LiDAR and spectral data produced an overall accuracy of 85.4% (Kappa 80.6%). Classification using just the spectral data produced an overall accuracy of 75.8% (Kappa 67.8%). The research findings demonstrate how the combining of LiDAR and spectral data improves classification for Pohutukawa trees.
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Resistance to the pandemic strain of Austropuccinia psidii was identified in New Zealand provenance Leptospermum scoparium, Kunzea robusta, and K. linearis plants. Only 1 Metrosideros excelsa-resistant plant was found (of the 570 tested) and no resistant plants of either Lophomyrtus bullata or L. obcordata were found. Three types of resistance were identified in Leptospermum scoparium. The first two, a putative immune response and a hypersensitive response, are leaf resistance mechanisms found in other myrtaceous species while on the lateral and main stems a putative immune stem resistance was also observed. Both leaf and stem infection were found on K. robusta and K. linearis plants as well as branch tip dieback that developed on almost 50% of the plants. L. scoparium, K. robusta, and K. linearis are the first myrtaceous species where consistent infection of stems has been observed in artificial inoculation trials. This new finding and the first observation of significant branch tip dieback of plants of the two Kunzea spp. resulted in the development of two new myrtle rust disease severity assessment scales. Significant seed family and provenance effects were found in L. scoparium, K. robusta, and K. linearis: some families produced significantly more plants with leaf, stem, and (in Kunzea spp.) branch tip dieback resistance, and provenances provided different percentages of resistant families and plants. The distribution of the disease symptoms on plants from the same seed family, and between plants from different seed families, suggested that the leaf, stem, and branch tip dieback resistances were the result of independent disease resistance mechanisms.
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