Physiological ecology and vegetation dynamics of North Queensland upland rainforest-open forest ecotones

Duff, Gordon (1987) Physiological ecology and vegetation dynamics of North Queensland upland rainforest-open forest ecotones. PhD thesis, James Cook University.

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Abstract

The transition between rainforest and open forest in the highlands of north Queensland is marked by an ecotonal assemblage dominated by the tall open forest species Eucalyptus grandis. This thesis examines the autoecologies of a range of species from open forest, ecotone and rainforest assemblages, in relation to the dynamics of the rainforest –open forest boundary.

Rainforest-open forest transitions were investigated at Kirrama (18' 10'S, 145' 40'E) and Paluma (19' 01' S. 146' 15'E) in the highland rainforest belt of north Queensland. Structure and floristic composition were investigated at these sites, and changes in light intensity, soil moisture and microclimate across the rainforest boundary were recorded. In the transition from rainforest to open forest, photon flux density increased dramatically. Temperature range and saturation deficit also increased, while humidity and soil moisture availability decreased under most climatic conditions.

Water relations of a range of species were studied by measuring plant water potential (y) of mature forest trees in the field, and y growth and competition of seedlings in the glasshouse. The greatest degree of drought tolerance was found in medium open forest species such as Eucalyptus intermedia, which also showed the lowest water potential values in the field. Water relations were identified as one factor limiting the movement of tall open forest and rainforest species into open forest.

Responses of a range of medium open forest, tall open forest, secondary and primary rainforest species seedlings to variations in light and mineral nutrient availability were investigated in the glasshouse. Growth experiments were conducted on these seedlings along shade gradients using three different nutrient treatments. Seedlings were grown in isolation and in species mixtures in separate experiments. Open forest species proved to be the most shade intolerant, and primary rainforest species exhibited the slowest growth rates and the greatest shade tolerance. Tall open forest and secondary rainforest species had the highest growth rates and the most substantial responses to variations in mineral nutrient concentrations. Rainforest species were more variable in their responses to light availability, and a hypothesis relating this variability to the heterogeneity of the light environment in rainforest or in the ecotone was proposed. Competition experiments conducted under shade gradients determined which species were most likely to become established at different positions on the ecotone, according to growth rate and shade tolerance. Resource allocation in seedlings was used to classify the species studied into groups, and this classification corresponded closely with the shade tolerances and growth characteristics discovered in the growth experiments.

Regrowth and regeneration after a low intensity fire at one of the study sites was investigated, demonstrating the resilience of the rainforest margin against the intrusion of fire. Open forest species recovered rapidly from the effects of fire, with coppicing as the main form of regeneration by fire damaged individuals. The effects of frost on rainforest and open forest species were examined, showing the faster growing rainforest pioneer and edge species to be the most susceptible to sub zero temperatures. As interaction between frost and subsequent fire was proposed, and evidence for this effect discussed.

Germination characteristics were investigated for a range of species. Open forest species had the most rapid germination and the greatest susceptibility to fungal attack. Dormancy was found in the seeds of rainforest pioneer species, while the secondary rainforest species investigated germinated rapidly and were successful in both rainforest and open forest substrates.

A model of vegetation dynamics on the ecotone incorporating the effects of disturbance (through fire, cyclones, frost and drought), climate, microenvironment, light, germination and dispersal was formulated. The model showed the mechanisms by which the rainforest edge advances into open forest, remains stationary or retreats as a result of severe disturbance. Fire was the critical factor controlling the position of the ecotone, while light intensity and water relations regulated the establishment and distribution of species across the ectone.

These conclusions can be coupled with the results of previous investigations by other researchers to formulate forest management strategies for forest boundary systems and small rainforest isolates in highland areas of north Queensland.

Item ID: 7878
Item Type: Thesis (PhD)
Keywords: forest ecotones, rainforest-open forest ecotones, rainforests, open forests, North Queensland, Kirrama, Paluma, highland forests, light, temperature, soil moisture, shade tolerance, forest regrowth, forest disturbances
Date Deposited: 18 Dec 2009 05:30
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060208 Terrestrial Ecology @ 34%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 33%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960806 Forest and Woodlands Flora, Fauna and Biodiversity @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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