Carbon Gain in Relation to Water Use: Photosynthesis in Mangroves
251
transport in relation to the expansion rate of growing tissues (Ball and
Munns 1992).
A fourth possibility concerns the increase in soil salinity that arises when
the roots extract water but exclude the salt. In tidal wetlands, where mangroves grow, the soils are typically poorly drained, indeed saturated, owing
to their fine texture, the flatness of the landscape, and their regular inundation. As a result, the soils are not well flushed by the ebb and flow of tidal
water. When roots extract water from saturated soil, the water must flow
downwards from the soil surface towards the roots. Salt is carried, by
convection, in this flow of water, but is excluded by the roots as they absorb
the water. Thus, the salinity must rise in the soil occupied by the roots. It
must keep rising until a sufficiently large concentration gradient develops to
diffuse the salt back to the soil surface (where the concentration remains at
approximately that of tidal water) as fast as it enters the soil by convection
(Fig. 12.2). Calculations based on reasonable assumptions about the distribution and activity of the roots and about the transport characteristics of the
salt in the soil suggest that the salt is likely to become so concentrated that it
will severely limit the rate of water uptake by the roots (Passioura et al.
1992). The calculated limiting rate is about 1 mm day-I, which is but a small
fraction of typical potential evaporation rates, and accords with measured
values (Miller 1972; Wolanski and Ridd 1986).
Of these possible explanations for the generally low transpiration rate
of mangroves, the second and fourth, concerning the danger of runaway
embolism and the inhibitory buildup of salt around the roots, seem the most
likely. But whatever the explanation, conservative water use has considerable
consequences for carbon gain, as we elaborate below.
12.3 Implications of Conservative Water Use for Plant Function
Interspecific differences in water-use characteristics among the Rhizophoraceae are apparent from differences in the slope of assimilation rate with
respect to stomatal conductance in response to variation in leaf temperature,
irradiance, and vpd (Table 12.1). The slope, dA/dg, increases with increasing
salinity tolerance of the species which, in turn, is correlated with a more
frugal or conservative use of water. Thus, for a wide range of environmental
factors affecting photosynthesis over the course of a day, stomatal conductance at a given assimilation rate is lower (and water use is more
conservative) the greater the salinity tolerance of the species (Ball et al.
1988).
Conservative water use has implications for leaf functioning under natural
field conditions. Salinity varies both temporally and spatially in a mangrove
swamp, but the soil salinity around the roots changes more slowly than does
251
transport in relation to the expansion rate of growing tissues (Ball and
Munns 1992).
A fourth possibility concerns the increase in soil salinity that arises when
the roots extract water but exclude the salt. In tidal wetlands, where mangroves grow, the soils are typically poorly drained, indeed saturated, owing
to their fine texture, the flatness of the landscape, and their regular inundation. As a result, the soils are not well flushed by the ebb and flow of tidal
water. When roots extract water from saturated soil, the water must flow
downwards from the soil surface towards the roots. Salt is carried, by
convection, in this flow of water, but is excluded by the roots as they absorb
the water. Thus, the salinity must rise in the soil occupied by the roots. It
must keep rising until a sufficiently large concentration gradient develops to
diffuse the salt back to the soil surface (where the concentration remains at
approximately that of tidal water) as fast as it enters the soil by convection
(Fig. 12.2). Calculations based on reasonable assumptions about the distribution and activity of the roots and about the transport characteristics of the
salt in the soil suggest that the salt is likely to become so concentrated that it
will severely limit the rate of water uptake by the roots (Passioura et al.
1992). The calculated limiting rate is about 1 mm day-I, which is but a small
fraction of typical potential evaporation rates, and accords with measured
values (Miller 1972; Wolanski and Ridd 1986).
Of these possible explanations for the generally low transpiration rate
of mangroves, the second and fourth, concerning the danger of runaway
embolism and the inhibitory buildup of salt around the roots, seem the most
likely. But whatever the explanation, conservative water use has considerable
consequences for carbon gain, as we elaborate below.
12.3 Implications of Conservative Water Use for Plant Function
Interspecific differences in water-use characteristics among the Rhizophoraceae are apparent from differences in the slope of assimilation rate with
respect to stomatal conductance in response to variation in leaf temperature,
irradiance, and vpd (Table 12.1). The slope, dA/dg, increases with increasing
salinity tolerance of the species which, in turn, is correlated with a more
frugal or conservative use of water. Thus, for a wide range of environmental
factors affecting photosynthesis over the course of a day, stomatal conductance at a given assimilation rate is lower (and water use is more
conservative) the greater the salinity tolerance of the species (Ball et al.
1988).
Conservative water use has implications for leaf functioning under natural
field conditions. Salinity varies both temporally and spatially in a mangrove
swamp, but the soil salinity around the roots changes more slowly than does