Fate and effects of oil and dispersed oil
on mangrove ecosystems in Australia

FIELD EXPERIMENT
IMPACT AND RECOVERY OF PLANTS

In the field study, we also studied the impact of oil and dispersed oil treatments on trees and seedlings.

Treatments caused significant mortality of mangrove seedlings Oil treatments caused significant mortality of matureRhizophora stylosatrees within 2-7 months of application of treatments. Treatments caused significant mortality of mangrove seedlings, both naturally-occurringRhizophora stylosaand plantedAvicennia marina,within 1-2 months. Mortality ofRhizophora stylosatrees and seedlings was significantly lower in plots treated with dispersed oil compared with plots treated with oil only, at each of the three sites. Variations in tree mortality were not correlated with oil concentrations in sediments between treated plots. Variations in tree mortality were also correlated with sediment grain size.
Oil treatments caused significant reduction in leaf biomass in canopies of surviving trees between 2-22 months after oiling Oil treatments caused significant reduction in leaf biomass in canopies of surviving trees between 2-22 months after oiling. The one surviving oil plot had a 41% reduction in leaf standing stock at the completion of monitoring, while the two dispersed oil plots had recovered at 22 months.
Plate 8a. b, c, d. e.During field trials, plants were monitored for the impact of oil treatments.
A.Forest canopy growth and productivity was assessed from litter fall collections made approximately monthly.
B.Canopy and density was measured using both litter fall and direct observations of tagged leafy shoots. The number of leaves on these shoots was indicative of forest canopy condition.
C.Biomass of trees in plots was estimated using allometric relationships determined by cutting down other trees and systematically weighing the various parts (above-ground roots, trunk and stem, branches and twigs, and leaves) of nine trees ranging from one to 12 metres tall.
D.Trees died in some plots as seen from above the canopy.
E.Tree death in plots occurred 2-3 months after oiling, but depended on prior tree condition, presence of infauna, as well as whether oil was dispersed or not.

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April 7, 2010