Agriculture Reference
In-Depth Information
temperature is not likely to affect mangroves adversely (Field
1995
). When mean
air temperature rises to 25 °C, most mangroves produce maximal shoot density
and when the mean air temperature drops below 15 °C, they stop producing leaves
(Hutchings and Saenger
1987
). At temperatures above 25 °C, leaf formation rate
declines in some species (Saenger and Moverly 1985). Temperatures above 35 °C
have led to thermal stress affecting mangrove root structures and establishment of
mangrove seedlings (UNESCO 1992). Almost no photosynthesis occurs at leaf tem-
peratures of 38-40 °C (Clough et al.
1982
; Andrews et al.
1984
). It has also been
suggested that mangroves will move polewards with increasing air temperatures
(UNEP
1994
; Field
1995
; Ellison
2005
). Migration of some species of mangroves
to higher latitudes is limited by temperature. However, extreme cold temperatures
are more likely to limit mangrove expansion into higher latitudes (Woodroof and
Grindrod
1991
; Snedaker
1995
).
4.2 CO
2
Concentration
There has been an increase in the atmospheric CO
2
by 90 parts per million by vol-
ume (ppmv) during the period from 1980 to 2000 (Houghton et al.
2001
). Most at-
mospheric CO
2
resulting from fossil fuels will be absorbed into the ocean, affecting
ocean chemistry. Increased levels of CO
2
are expected to enhance photosynthesis
and mangrove growth rates (UNEP
1994
). Ball et al. (1997) have shown that in-
creased levels of CO
2
significantly increase photosynthesis and the average growth
rates in two Australian mangrove species,
Rhizophora stylosa
and
Rhizophora apic-
ulata
, but only when grown at lower salinity levels.
4.3 Precipitation
It is predicted that the precipitation rates are likely to increase by about 25 % by
2050 in response to global warming. However, at regional scales, this increase will
be unevenly distributed with either increases or decreases projected in different
areas (Knutson and Tuleya
1999
; Walsh and Ryan
2000
; Houghton et al.
2001
).
Changes in precipitation patterns caused by climate change may have a profound
effect on both the growth of mangroves and their areal extent (Field
1995
; Sneda-
ker
1995
). Decreased precipitation results in a decrease in mangrove productivity,
growth and seedling survival, and may change species composition favouring more
salt-tolerant species (Ellison
2000
,
2004
). Reduction in precipitation may result in a
decrease in mangrove area, decrease in diversity and projected loss of the landward
zone to non-vegetated hypersaline flats (Snedaker
1995
). Increased precipitation
may increase mangrove area, diversity of mangrove zones and mangrove growth
rates in some species (Field
1995
). Increased precipitation may also allow man-
groves to migrate and outcompete salt marsh vegetation (Harty
2004
).
