Landscape History of Moawhango Ecological District

Abstract

Landscapes of the Moawhango Ecological District are studied by evaluating historical processes rather than contemporary environments in an attempt to elucidate the essential ecological character of the district. The important physical criteria of geology, geomorphology, climate and soils are analysed and data indicates that two distinct physiographic landscapes exist A northern region - Moawhango River headwaters, northern Ngamatea and Owhoako Plateaux - is dominated by resurrected greywacke landforms following subduction and cover by marine sediments in the late Cenozoic. Marine sediments are now largely removed and soils are of andesitic ash and Taupo Pumice. A rainshadow climate produces unequable humidity. The remaining region is of marine sediment block plateaux separated by fault activity and deeply incised rivers. Holocene andesitic ash mantles the region, smoothing microrelief. Surrounding axial ranges also dominate the climate with rainfall and humidity diminishing along centripetal gradients away from the ranges. A survey of subfossil forest evidence is undertaken and integrated within the fragmented array of relict pre-fire vegetation in order to reconstruct pre-Polynesian deforestation vegetation patterns. Subfossil charcoal is abundant beneath the dominant tussockland cover. The only habitats originally in open vegetation were plateau high watertable sites and valley floors and basins of the Moawhango River headwaters. Plateau forests were dominated by Libocedrus bidwillii with associated Podocarpus cunninghamii in the north and west and Halocarpus biformis in the south and east. On plateau margins a bordering narrow peripheral strip of Nothofagus forest intergraded with conifer associations. Northern greywacke landforms supported Nothofagus forest the cover of which was reduced in basins and valleys by edaphic dryness of Taupo Pumice Ignimbrite. The history, pattern and ecological impact of fire is assessed. Stand size and age-classes of cohort woody vegetation that resulted from fire, are investigated. The cohorts form sporadic borders to steady-state forest and constitute a transition zone between forest and fire induced tussockland. Radiocarbon dates and maximum ages of Libocedrus cohort trees indicate that the western sector was deforested c. 570 years ago and deforestation on the Mangaohane Plateau occurred c, 43O years ago. Narrow, < 80 m wide bands of cohort Libocedrus bordering unaffected parent stands owe their origins to the initial conflagrations. Two later fire phases produced additional cohorts in which Phyllocladus aspleniifolius is important. In the west Waiokotore Stream basin successional forests resulting from Polynesian fires are dominated by Phyllocladus that demonstrates a superior seed dispersal capacity to that of Libocedrus. Rates of successional advancement of forest through open vegetation are fastest on steep slopes but slow on flat tussockland topography. The invasion of Chionochloa rubra tussockland by Dracophyllum shrubs is a prerequisite to advancing forest species overcoming the successional inertia of tussockland on flat topography. An examination of radial increment growth rates of woody conifer species shows that cohort trees initially grow at faster rates than steady-state community trees. Cohort growth rates after 100 - 200 years decline to levels comparable to steady-state trees when the 'ash-bed' fertility boost disappears and intrastand competition increases. It is concluded from an analysis of pre-historic Maori land-use practices that deforestation fires and subsequent fires were lit in the north to foster ease of high altitude travel. The district was comprehensively deforested as undulating relief and plateaux offered no topographical barriers to the passage of fires that burnt from the northwest. Studies of the post-glacial history of the distribution of the two major forest classes, conifer and Nothofagus are undertaken. Input is used from tephro-stratigraphy, palynology and the dynamics of tree populations constituting transition zones between the two forest classes. Plateaux were largely unvegetated at the close of the Otiran glaciation. The primary late-glacial successions c. 12,000 yr. B.P. were grasses and halocarpus spp. followed by Phyllocladus and then Libocedrus. A superior freezing resistance and mobility through open vegetation enabled Libocedrus to dominate plateaux ahead of Nothofagus. However, through the Holocene, Nothofagus has universally been the aggressive species, expanding into bordering conifer associations on plateaux margins. The primary limitation to expansion is lack of seedling establishment opportunities. Nothofagus range expansion occurs by two processes - the formation of outlier stands and expansion at the margin of the main body of beech. Expansion at a mean rate of between 5.5 and 8 m/century occurs by taller stature Nothofagus shading adjacent conifer community vegetation. Canopy gaps are created which Nothofagus juveniles exploit generally at tie margins of the outstretched limbs of parent trees. The position of the transition zones suggests that such a process has operated throughout the Holocene but steeper slopes may have encouraged faster Nothofagus expansion rates than flatter topography. From an analysis of the ecologies and distributions of plants in the lower North Island, several groups of mainly non-forest species show peculiar endemic and disjunct distributions, many involving Moawhango E.D. Apart from small groups of endemic species in Moawhango E.D and in central North Island mountains other endemic species only occur in these regions in North Island then are disjunct to South Island. Another 80 species show wide North Island disjunctions between central North Island and south Wellington-Wairarapa. Aspects of these patterns are compatible with the tectonic history of lower North Island as shown by palaeographic and tectonic reconstruction models for the late Cenozoic. Combinations of these processes have preserved a fragmented record of an early Miocene flora in lower North Island. Moawhango E.D. landscapes represent uplifted and intact portions of lowland Miocene surfaces that escaped complete marine inundation at the northern limit of the Wanganui sedimentary basin in Pliocene times.