The Community Ecology and Conservational Status of Shrubby Vegetation Dominated by the Endemic New Zealand Species Dracophyllum Subulatum Hook. f. (Monoao)

Abstract

Dracophyllum subulatum Hook.f. (monoao) is an epacridaceous, sclerophyllous shrub endemic to the central North lsland of New Zealand. Any qualitative and quantitative analyses of the D. subulatum communities were limited when this project was started. This primarily synecological study aims to expand understanding of these heath communities (sensu Specht 1979) focusing on aspects of the floristic, structural (sensu Atkinson 1985, Specht1979), and demographic development of D. subulatum heaths and associated communities. lncluded are autecologically-based investigations both of the response of D. subulatum populations per se and of D. subulatum-dominated communities to the environment of the region. This thesis is structured to provide the presentation of results, their statistical testing, and interpretation of their ecological significance in ten chapters. The final chapter summarizes the main findings of the study and incorporates their interpretation into an overall framework of successional theory analagous to heathland ecosystems world-wide. Suggestions are made for avenues of further research to expand on the results this study provides. Further studies are considered necessary to enable conservation management based on sound understanding of these ecosystems. The importance of disturbance regimes such as recurrent fire and drought, in the perpetuation of D. subulatum, is indicated. Strong affinities are shown between these communities and other heath communities in many parts of Europe, Australia, North and South Africa and South America. Similar global affinities are indicated with wet-heath and dry-heath variants developing in response to climate, soil compaction and moisture gradients. These affinities highlight the developmental origins of the Genus Dracophyllum under a mediterranean climate during the humid, sub-tropical conditions of the Tertiary Period. Both actual water deficit and physiological drought; and the severity and frequency of summer frosts, are all operative in the present study. The young, fertile volcanic soils on which D. subulatum grows are in contrast, however, with the old, infertile, oligotrophic soils beneath heathlands elsewhere. Dracophyllum subulatum is not restricted to soils derived from Taupo Eruption ejecta as stated in much of the literature. Similarly, Taupo tephra derived (pumice) soils are misrepresented in literature as arid or droughty. Pedological studies show these soils to retain moisture at comparatively high levels' Particularly important in the regulated seral development of D. subulatun heaths are the stratigraphic and topographic characteristics of the substrate, the proximity of communities to a suitable generative seed source; and the operative disturbance histories and the consequent age of the resident D. subulatum populations. Demographic interpretation of D. subulatun cohort structures shows both even-aged and mixed-age stands undergoing continual, gap-phase and intermittent regeneration. Growth rates, longevity and cohort establishment patterns vary according to local microenvironment, however, both synchronous and consecutive cohort establishment occurs in disjunct populations in response to widespread climatic disturbance. Successional trends within D. subulatum communities en route to reforestation are described and interpreted, along with details of the dominant exogenous and endogenous, biotic and abiotic factors involved in retention of D. subulatum dominance. A synergistic balance exists between both physical and climatic impacts on D. subulatum and its cohabitant species. Leptospermum scoparium, the natural forest precursor over the geographic range of D. subulatum, is suppressed to a temporarily prostrate form by intense frost during the summer growth season. The vegetative reproductive ability and relative frost-tolerance of D. subulatum gives this species competitive superiority over L. scoparium many areas subject to pronounced cold air inversion. Many D. subulatum communities remain relatively stable heath with. evidence of continual recruitment indicating population expansion in some areas. However, most 'are transitional or seral vegetation complexes and, as such, form part of the natural development of vegetation in the central North Island. Thisi study qualifies the level of indigenous floristic purity of D. subulatum dominated communities and documents the main allochthonous invaders. Such species pose a considerable threat to the floristic integrity of many D. subulatum stands. Many communities are becoming increasingly vulnerable to displacement and without active management intervention may undergo further degradation of displacement.