The Biology of Coastal Sand Dunes M. Anwar Maun - Inecol

One of the two pits was lined with plastic
(0.13 mm thick) to prevent lateral and vertical
movement of moisture while the second
served as control. The water potential of sand
above the plastic in the pit was substantially
lower (–3.6 MPA), than control, –1.0 MPa.
The sand above the plastic in the pits dried
out very quickly. However, below the plastic
the water potential was –0.3 MPa which was
even higher than the control. The tests clearly
showed that the water moved up from below
under normal conditions, possibly by capillary
movement, or along a water potential gradient,
because water potential in upper layers
of sand was more negative than the lower layers
of sand and only a very small quantity of
water will be needed to raise the water potential
to between –1.0 MPa and –1.5 MPa.
Nevertheless, Olsson-Seffer (1909) suggested
that the rise of water by capillarity
in dune sand was extremely low and the
most probable mechanism may be internal
dew formation, whereby water moves up as
vapour and then condenses in upper cooler
layers of sand, especially at night. According
to Salisbury (1952) there was no evidence for
the hypothesis that dew formation at night
increased the moisture a few cm below the
sand surface. On a South African foredune,
Ripley and Pammenter (2004) reported that
leaf water potentials of three semi-succulent
species, Arctotheca populifolia, Ipomoea pescaprae
and Scaevola plumieri did not drop
below the turgor point, transpiration was
related to atmospheric demand, stomatal conductance
was normal and in a normal year
the water utilized by transpiration was less
than the annual rainfall. However, in a dry
year with below average rainfall, plants were
utilizing more than actual rainfall by tapping
into other water sources such as internal
dew, stored water or ground water. Among
the three species, I. pes-caprae showed stomatal
control of water loss under dry conditions.
Soil water remained above the lower limit of
about 0.5% at which plants can extract water
from sand dune soil.
Profile (cm)
160
120
80
40
0
–40
–80
THE SAND DUNE ENVIRONMENT 27
0600 hrs
8 July 1979
1400 hrs
7 July 1979
10 14 18 22 26 30 34 38 42 46
Temperature (°C)
Figure 2.5 Temperature profiles in and above the sand on
a lacustrine sand dune system at 0600 and 1400 hrs in July.
Horizontal bars represent ±1 SD (after Baldwin and Maun
1983).
2.3 Soil and air temperatures
On clear summer days the temperature of the
surface layer of sand may reach up to 60°C,
mainly because sand is a poor conductor of
heat owing to large pore spaces between particles.
At the Pinery, the soil surface temperature
reached 40 ± 6°C at 1400 hrs on 7 July 1979
at the slack site (Fig. 2.5). However, immediately
below the sand surface, temperature
declined rapidly producing very steep lapse
rates (Baldwin and Maun 1983). For example,
at 5 cm below the sand surface, the temperature
had decreased to about 30 ± 4°C, a drop
of almost 10°C. The soil temperature continued
to drop even further until it had reached
19°C at 90 cm below the sand surface. In the
evening, however, the situation was reversed.
The thin hot surface layer of sand cooled rapidly
because the low thermal conductivity of
sandy soil did not allow the replenishment
of heat from lower layers of soil. Thus, there
was a large diurnal temperature range at the
sand surface. Olsson-Seffer (1909) recorded a
diurnal temperature range of 25.6°C between
day and night at the Hangö dunes in Finland.
However, the diurnal range within the soil
decreased with an increase in soil depth. At
about 40 cm depth the soil temperature was
the same both during the day and night. In