Seed from four species of rainforest trees with widely contrasting sunlight requirements for growth and development were sown within disturbance gaps amidst mature forest on the Herberton Range in North Queensland. Observations on seedling persistence plus comparative growth of young trees of Acacia aulacocarpa, Toona australis, Flindersia brayleyana and Darlingia darlingiana (species ranked according to adaptation from full sun to deep shade) confirmed a broad tolerance of Flindersia to sunlight under all conditions, from wide to narrow gaps (minimum 0.6% full sun equivalent).
Photosynthetic attributes which underlie such broad tolerance were subsequently inferred from single leaf gas exchange, plus foliar analyses of nitrogen, phosphorus and chlorophyll on tree seedlings held for 180 days under two nutrient × three irradiance levels adjusted to represent natural irradiance incident upon the forest floor (low), mid-canopy (medium) and emergent crowns (high irradiance treatment). Medium irradiance plus high nutrients proved optimal for leaf expansion, chlorophyll content and photosynthesis in air.
Growth under low irradiance was characterised by thinner leaf palisade tissue, lower rates of dark respiration, increased leaf chlorophyll per unit nitrogen and lower light compensation point for photosynthesis. Such leaves retained a relatively high photosynthetic capacity despite these other shade-leaf attributes.
High irradiance plus low nutrients proved supraoptimal for leaf expansion and expression of photosynthetic activity. Chronic photoinhibition appeared to prevail because apparent quantum yield was reduced, while photosynthetic processes on a nitrogen basis were substantially impaired. Nitrogen use efficiency, as inferred from leaf chlorophyll content, light saturated CO2 assimilation rate, electron transport rate and carboxylation rate on a nitrogen basis declined with increasing growth irradiance.
Some ecological implications for the establishment and growth of these rainforest tree species in disturbance gaps are discussed.
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SUMMARY(1) Litterfall in a plantation of Araucaria cunninghamii and in an adjacent rain forest was compared at two sites in north-eastern Australia over 3 and 4 years respectively, for quantity, temporal distribution, and the content of nitrogen, phosphorus, sodium, potassium, calcium and magnesium.(2) The average litterfall for the four plots was 9-25 t ha-' per year, a result comparable with values from similar studies on other tropical forests. There were no consistently significant differences in the quantity of litterfall between forest types or sites, or between years for the rain forest plots.(3) Litter from the rain forest plots at both sites had significantly higher concentrations of nitrogen and potassium than litter from the Araucaria plots. Litter from both forest plots at the site where the soil was more fertile was significantly higher in concentrations of nitrogen, phosphorus, potassium and calcium than litter from the less fertile site. For nitrogen, however, the differences between plots in the two forest types were greater than the differences between sites.(4) The calcium content of Araucaria litter during the period of heavy seed production in December-January of the second study year was lower than at all other times. Sodium was the only one of the mineral elements studied to show a regular seasonal pattern of concentration, with lower concentrations in the wet season than during the dry season.(5) In spite of the significant differences in the mineral-element concentrations of litter both between forest types and between sites, there were no significant differences in annual mineral-element accessions to the soil.(6) There appears to have been little effect on the annual accession to the soil of litter and the associated mineral elements as a result of the conversion of rain forest to Araucaria plantation at each site; differences between sites in properties such as soil fertility and rainfall also had little effect.
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