Patterns of development of regeneration of tree species in a subtropical rainforest

Song, Guo-Zhang Michael (2007). Patterns of development of regeneration of tree species in a subtropical rainforest PhD Thesis, School of Integrative Biology, The University of Queensland.

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Author Song, Guo-Zhang Michael
Thesis Title Patterns of development of regeneration of tree species in a subtropical rainforest
School, Centre or Institute School of Integrative Biology
Institution The University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor David Yates
Collection year 2008
Subjects L
Abstract/Summary Factors relating to seedling establishment and the effects of selective logging on regeneration were investigated in a subtropical rainforest in south east Queensland, to determine the regeneration patterns of tree species. Along a 200m long and 0.5m wide transect, litter movement and accumulation were studied. Although litter movement was detected in every month, the total amount of litter accumulated varied slightly during the present study, due to the comparable amount of litter moved out and moved in. A larger amount of litter movement was observed with a smaller probability of occurrence. In terms of frequency, a biotic agent (the foraging behaviour of vertebrates) and abiotic agents (rain, wind and topography) respectively contributed to half of the movement of litter on the ground. Nevertheless, in terms of mean amount of litter being moved in every event, vertebrates moved significantly more litter than did abiotic agents. The understorey vegetation of this forest was dominated by a fern species ( Lastreopsis decomposita ) with a canopy height of 30-40cm. Lower light transmittance accompanied higher fern cover. The probability of being disturbed by vertebrates and covered by litter was significantly lower under the fern canopy than at the edge of fern canopy or in habitats without fern cover. The frequency of vertebrate disturbance and the amount of litter being moved in decreased as the extent of fern cover increased. In the 100 m2 transect area, even though mean growth rate of seedlings was positively correlated with light availability of habitats, over 70% of seedlings did not grow at all to their death or the end of the two-year study. Herbivory caused the largest proportion of seedling mortality (28.2%), followed by litter cover due to abiotic disturbance (27.8%), uprooting (15.4%), water stress (13.0%), vertebrate-induced litter cover (7.9%), pathogens (5.8%) and unknown agents (1.9%). Animals (herbivory, uprooting and vertebrate-induced litter cover) were responsible for more than half of seedling mortality. The effects of several factors on seedling survival were evaluated with survival analysis. The effects of herbivory, litter cover, light intensity, and litter thickness on seedling survival were not significant. Higher survival rates of seedlings were associated with greater initial height, older age and lower frequency of vertebrate disturbance. Due to a high risk of succumbing to animal disturbance, seedlings of three shrub species with woody or hard shoot structures had higher survival rates than did seedlings of canopy tree species. Understorey vegetation significantly reduced light availability and the risk of being damaged by vertebrate disturbance and litter cover. The negative effects of lower light availability were outweighed by the positive effects associated with reduced stress due to litter cover and vertebrate disturbance. As a result, seedlings in habitats with higher coverage of understorey vegetation had higher survival percentages. The 40-year vegetation records of an unlogged and a logged forest stand were compared. The selective logging operation in 1961 reduced species richness by 36.1%, the number of stems >= 6m in height by 60.1% and stem basal area in the community by 62.7%. Species richness and the number of stems recovered in ten years, and the vertical structure recovered in 40 years. Nonetheless, due to the removal of large DBH trees, it is predicted that the pre-logging basal area will not be regained within 20 to 60 years. Early successional species contributed a higher proportion of stems and basal area in the logged than in the unlogged plot. Forty years after logging, there was no significant difference in ground level light availability between the two plots, which was attributed to the dense understorey in the logged plot. For the first 30 years after logging, most individuals markedly grew faster in the logged than the unlogged plot, but in the last ten years, the situation of growth was reversed. This change was attributed to the marked reduction in growth of the smaller trees in the very dense understorey of the logged plot. The seedling study suggested there were two principal modes of regeneration establishment. Shrub species with low fecundity established seedling pools with high survivorship as a result of having woodier plant bodies. High fecundity is necessary for the establishment of seedlings of canopy tree species due to their non-durable characteristics. The study of the logging plots showed that different requirements for regeneration environments between early successional species and later successional species. Large-scale disturbances are needed for regeneration of early successional species, whilst later successional species can regenerate in smaller gaps. The prevailing natural disturbances which mainly create small canopy gaps will unilaterally facilitate the regeneration of the later successional species.

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Created: Fri, 21 Nov 2008, 16:03:18 EST