Adaptations to the Tidal Zone
Light gap in Avicennia marina near Sydney, New South Wales.
Water edge light gap in the Daintree River estuary, Queensland.Conceptual model of mangrove forest development showing processes of gap formation, regeneration and turnover.
Living Forests with Gaps and Re-establishment
Mangroves, like other forest stands, are living systems that are dynamic, evergrowing, and constantly re-establishing and renewing themselves. Mangroves differ from terrestrial forests chiefly because of their special adaptations for survival in tidal marine locations. Situated at the seawater margin, these stands are subject to both land and river runoff, as well as, the direct action of the sea itself. By using their uniquely evolved features, mangrove plants have been able to readily occupy, dominate and stabilise exposed tidal foreshore and estuarine environments. In such dynamic conditions, influenced by severe hydrological and physico-chemical conditions, and faced with pervasive and progressive changes, like sea level rise, it has been essential for these plants to have successful regenerative strategies. The mere presence today of this habitat demonstrates the success of its regenerative strategies. Mangrove stands are recognised as dynamic ecosystems that are relatively robust and stable while dominated by a small number of specialist species.
Survival or Ecosystem Collapse in a Changing World
A forest turnover model (bottom right) maps out the natural processes involved in mangrove forest dynamics by quantifying turnover in terms of small gap creation and recovery. The accumulative influence of gap replacement, as numerous small-scale disturbances on mangrove forests, seems to explain the peculiar characteristics and presence of mangrove forests. Small gaps are common in terrestrial forests also, but those in mangroves differ because they rarely involve tree falls. In mangroves, the trees usually die standing in small clusters of 10-20 trees. The particular survival advantage of this strategy for mangrove forests is most evident in exposed stands. In these locations, forest structure must be maintained because exposed sediments would rapidly erode and de-stabilise the forest. Mangrove forests are therefore believed to be at great risk of ecosystem collapse and total loss of habitat. However, their common presence in exposed locations today, is testimony to the success of current renewal strategies. But, what about the future? There is growing concern with predictions of future increases in large-scale pollution incidents, more rapid rises in sea level, and increased severity of storms. Each of these factors would seriously challenge the capacity of mangroves to regenerate. The cumulative influence of such factors is therefore expected to seriously threaten the survival of exposed mangrove habitats around Australia, and worldwide.