Agriculture Reference
In-Depth Information
sediment in the water. The process of building peat helps mangroves keep up with
sea-level rise. Mangroves can expand their range despite sea-level rise if the rate
of sediment accretion is sufficient to keep up with sea-level rise. However, their
ability to migrate landwards or seawards is also determined by local conditions
such as infrastructure (e.g. roads, agricultural fields, dikes, urbanization, seawalls
and shipping channels) and topography (e.g. steep slopes). If inland migration or
growth cannot occur fast enough to account for the rise in sea level, then mangroves
will become progressively smaller with each successive generation and may perish
(UNEP
1994
).
4.7 Environmental Factors that Affect Mangrove
Response to Sea Level
Understanding the impact of sea-level rise on mangrove ecosystems must take into
account the factors that affect the ecological balance of that ecosystem such as the
substrate type, coastal processes, local tectonics, availability of freshwater and sedi-
ment and salinity of soil and groundwater (Belperio
1993
; Semeniuk
1994
; Blasco
et al.
1996
). Climatic variability (e.g. changes in rainfall and the frequency and in-
tensity of cyclonic storms) can exacerbate the factors affecting mangrove response
to sea level because it can alter the freshwater inflow to mangroves, the sediment
and nutrient inputs and the salinity regime. In an analysis of the impacts of sea-level
rise on estuaries, Kennish (
2002
) highlights the importance of local conditions such
as the size and shape of the estuary, its orientation to fetch and local currents, the
areal distribution of wetlands, the geology of the neighbouring watersheds and land
use in upland areas. Tidal range and sediment supply are two critical indicators of
mangrove response to sea-level rise. Carbonate settings are often associated with
coral atolls and islands, where landwards migration to escape the effects of sea-lev-
el rise is not possible and sediments are often limited; thus mangrove communities
in carbonate islands are considered extremely vulnerable to sea-level rise (UNEP
1994
). Therefore, sea-level rise is expected to decrease the geographic distribution
and species diversity of mangroves on small islands with micro-tidal sediment-lim-
ited environments (IPCC
1997
). Mangroves with access to allochthonous sediments
such as riverine mangroves are more likely to survive sea-level rise than those with
low external inputs (Woodroffe
1990
; Pernetta
1993
). It is important to note that
although access to sediment is critical for mangroves to survive sea-level rise, too
much sediment (e.g. resulting from poor agricultural practices) can bury their pneu-
matophores and kill mangroves (Ellison and Stoddart
1991
).
4.8 Species Response to Sea-Level Rise
Individual mangrove species have varying tolerances of the period, frequency and
depth of inundation. Mangrove zones are related to shore profile, soils and salinity,
