The dynamics of Gymnostoma maquis on ultramafic soils in New Caledonia

Abstract

Ultramafics cover extensive areas of New Caledonia and present a variety of unique nutritional and metal toxicity problems for plant growth. The flora of these substrates is highly diverse and occurs as vegetation types ranging from a low sclerophyllous maquis (heath) to rainforest. These substrates have been the focus of intensive nickel mining over the last century and are often devoid of vegetation for several decades. This lack of plant establishment is a major environmental concern in New Caledonia and has been the focus of revegetation research. The genus Gymnostoma has received particular attention because of its nitrogen fixing association with Frankia and dominance in vegetation types which might be interpreted as a reflecting successional progression after fire. An understanding of the mechanisms surrounding these successional patterns might assist in solving practical issues of mine site revegetation with native species. This thesis set out to firstly determine whether vegetation types containing Gymnostoma are successional communities, secondly to examine certain key processes which appear to be driving vegetation patterns, and thirdly, to identify the role of Gymnostoma in ultramafic vegetation. Key processes associated with vegetation were examined experimentally to determine their influence on plant growth on a range of ultramafic substrates. Field surveys exammmg vegetation patterns and processes were carried out on three ultramafic soil types supporting Gymnostoma vegetation: iron crust oxisol, eroded oxisol and hypermagnesian soil. These three soil types have distinct chemical and physical properties which may affect plant growth. Floristic records from each soil type were firstly examined using MDS to determine floristic associations and subsequently with a successional index constructed from a PCA ordination of vegetation structure. The successional indices explained much of the variation on MDS axes which suggested that ultramafic vegetation patterns primarily reflects a post-fire succession. Fire records, annual growth rings of fire sensitive Dacrydium araucarioides and demographic trends in species composition also support a post-fire succession pattern. The succession pattern was found to be composed of an early successional group of maquis species which either (i) decline or (ii) persist, and (iii) a late successional group of forest species. Another important pattern revealed in surveys is that early ultramafic successional stages progress more slowly than tropical vegetation on other soil types. More importantly, certain processes appear to be critical at early successional stages and predetermine later successional development. Surveys of abandoned mine sites set out to determine what processes appear to be influencing plant establishment at early successional stages. The evidence implies that most species are dispersal limited and show an abrupt decline in abundance away from adjacent vegetation. Primary colonist establishment is more abundant on crevice sites which trap seed, and seedlings show higher rates of survival in such areas. The primary colonists eventually generate a micro-environment providing shade and litter which supports all of the subsequent colonisation of bare ground. Shade and litter levels were regarded as important factors driving succession on ultramafics and their effects at both the community and plant level were examined. Measures of light regimes in successional vegetation indicated that light (PAR) declined with the development of vegetation cover over time. These changes in incident solar radiation effect the community composition depending on species light requirements for photosynthesis. Chlorophyll fluorescence measurements of plants in natural vegetation and in field experiments indicated that seedlings on bare ultramafic soil experienced chronic photoinhibitive stress from a combination of high light and substrate conditions. Seedlings under shade cloth and underneath tree canopies potentially received less light for photosynthesis but exhibited less stress indicative of photoinhibition. Therefore, shade may be crucial for seedling survival at early successional stages. Furthermore, plant species responded to high light conditions depending on their successional status. Maquis species generally overcame photoinhibitive effects of bare ground environments once they attained a certain age. In contrast, forest species continued to exhibit photoinhibitive stress on bare ground environments. Measures of soil and litter nutrient content indicate that N, P, Kand Ca content increased with successional progression. Gymnostoma plays a key role in the development of later successional phases by providing a major source of nitrogen through its Frankia association. This nitrogen is released along with other nutrients as an abundant litterfall that gradually increases soil nutrient pools over time through slow decay. Glasshouse experiments indicated that leachates arising from slow cladode decay had no allelopathic effect on plant growth. However, litter build up in late maquis phases physically excluded large seeded species from establishing. A fire susceptibility index was constructed from litter and vegetation properties of successional vegetation. It indicates that Gymnostoma may raise the susceptibility of maquis because its open canopy allows litter beds to remain dry and flammable. However, Gymnostoma deplancheanum and G. chamaecyparis are able to persist on their respective soil types even at high fire frequencies because rocky terrain reduces fire spread and protects patches of vegetation. In contrast, G. intermedium appears to have been excluded from early successional vegetation on eroded oxisol by a continuous cover of woody sedge maquis which is highly flammable. The eventual dominance of broad leafed vegetation at later succession forest stages decreases the fire susceptibility of these communities. Results of field experiments indicate that slow maquis species growth is primarily due to the low nutrient status of ultramafic soils. Plants which received low doses of fertiliser and/ or litter showed a significant positive response in terms of height and relative shoot growth. Fertiliser applications also raised the total Frankia nodule weight per plant, indicating that Frankia responds positively to slight increases in phosphorus. Contrary to expectation, lime applications had no effect on maquis species shoot growth. Lime additions resulted in fewer Frankia nodules and more extensive root systems in Gymnostoma seedlings by reducing the availability of other nutrients. In summary, it is concluded that Gymnostoma dominated maquis represents a post-fire succession. Colonisation at early successional stages is dispersal limited and highly dependent upon the availability of crevices. Once pioneer plants have established, litter and shade levels become crucial factors in facilitating further succession. The relative abundance of Gymnostoma determines the rate and direction of this successional change.