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Structure, composition and dynamics of Metrosideros excelsa (pōhutukawa) forest, Bay of Plenty, New Zealand

The structure, composition and dynamics of a rare forest type, Metrosideros excelsa (henceforward referred to as Metrosideros) forest, were assessed in the Bay of Plenty Region, North Island, New Zealand. Metrosideros and associated species’ shade tolerances were also investigated. Study sites extended from Waihi Beach in the west, to Opape headlands in the east of the region. Thirty five quadrats ranging from 50–400 m² in size were measured resulting in a total survey area of 14,200 m². Metrosideros forests were floristically poor in comparison to other indigenous forest types; though semi-coastal Metrosideros forest hosted a more diverse understory than coastal forest. A total of 122 indigenous species were recorded, however only a minority of these were considered common and widespread, such species included Coprosma macrocarpa, Coprosma robusta, Cyathea dealbata, Melicytus ramiflorus, Myrsine australis, Pseudopanax lessonii, Pseudopanax arboreus, Beilschmiedia tawa, Dysoxylum spectabile and Litsea calicaris. In general, forest understories comprised of a sparse shrub layer, overtopped by a sub-canopy dominated by either tree ferns in steep inland localities, Pseudopanax spp., Coprosma spp. and Melicytus ramiflorus on coastal headlands, or Coprosma spp., Myrsine, Beilschmiedia, Dysoxylum and Litsea in mature and semi-coastal forest. Metrosideros growth rates were determined in two ways; first from the diameter growth of stems in four permanent quadrats, and second from ring counts on cut stem disks. Metrosideros stems initially had high growth rates, which could exceed 4 mm year⁻¹ in the first 80 years, however diameter growth subsequently slowed down as trees developed. All Metrosideros populations had regenerated in cohorts. The derived diameter age relationship suggested that stands ranged from 20–>300 years old, with mature forest being formed within 250 years. The oldest trees measured may be >1000 years old, and represent relics of the regions original vegetation. The sequence of forest development quantified for Bay of Plenty forests shows a decline in Metrosideros stems from >2000–<400 stems ha⁻¹ over a period of c. 300 years; a result of self-thinning. This was coupled with an increase in Metrosideros basal area from <20 m² ha⁻¹ to an average of 50 m² ha⁻¹; plateauing after 70 years. Total stem density and basal area were greatest in stands between 60–300 years old. The diameter frequency distributions quantified for key species suggested that appreciable shifts in species composition were occurring. The replacement strategies among key canopy species involved establishment at different phases of forest development, and this directly reflected species’ differing shade tolerances. Shade intolerant species Metrosideros and Kunzea ericoides (which could form mixed stands) established solely following disturbance. Metrosideros forest is likely to replace itself indefinitely on the unstable coastal headlands cliffs in the region, where frequent disturbances provide suitable regeneration sites. However, in semi-coastal localities, where disturbance is less frequent, Metrosideros is succeeded by shade tolerant species. Litsea, Dysoxylum and Corynocarpus laevigatus establish in mid-successional forest, however may be somewhat reliant on canopy gaps to stimulate height growth. Conversely Beilschmiedia, the most shade tolerant species, commonly establishes in mature forest and is capable of continued regeneration; thus is likely to be a dominant component of the forest community that replaces Metrosideros forest, in the Bay of Plenty. This research contributes to the national reporting of quantitative data describing the structure and composition of New Zealand’s indigenous forests, provides the first quantitative model of forest succession in Metrosideros forest in the Bay of Plenty, and contributes to the understanding of linkages between species’ shade tolerance, regenerative strategies and successional status.
Type of thesis
Bylsma, R. J. (2012). Structure, composition and dynamics of Metrosideros excelsa (pōhutukawa) forest, Bay of Plenty, New Zealand (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/6615
University of Waikato
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