groves under strongly varying conditions of salinity. C. erectus is not a true mangrove, but rather a mangrove associate or ally. It does not grow as close to salt-water lagoons and estuaries or tidal plains as the mangroves sensu strictu. However, at places its distribution overlaps with that of mangroves, for instance in alluvial sand plains at the Caribbean coast of Venezuela. There are various sizes of vegetation islands on these sand plains (see Sect. 8.2.1), where C. erectus and A. marina are found in close proximity (Fig. 7.15). In the rainy season the sand plains may be flooded by fresh water to a depth of 0.5 m, but in the dry season they dry out becoming hypersaline and covered with a crust of salt (Sects. 8.2.1 and 18.104.22.168). These two species were found here growing on the same vegetation island, and Fig. 7.16 shows that in the rainy season, both species had similar CO2-uptake rates, JCO2, but that A. germinans operated at considerably lower conductance, gH2O, and internal CO2-concentration, pCO2, than C. erectus. In the dry season, JCO2 in the morning was similar to that measured in the wet season for A. germinans. There was a midday depression (see Sects. 22.214.171.124 and 10.1.2.3), which was followed, however, by considerable recovery in the afternoon; pCo was similar to that in the wet season although gH2O was somewhat reduced. Conversely, CO2-uptake in C. erectus in the dry season was greatly reduced, with only a small peak in the morning, and gH2O and pCo were low throughout the day. It is evident that with similar JCO2 for both species in the rainy season, the smaller reduction of JCO2 in the dry season allowed A. germinans to maintain productivity under salinity-stress and drought better than
the mangrove-associate C. erectus as the true mangrove maintained similar rates of JCO2 to C. erectus at lower pCo the rainy season and then JCO2 in C. erectus was greatly reduced as pCo declined in the dry season (Fig. 7.16).
Although C. erectus does not grow close to the shoreline or reach the tidally inundated mud plains, it may grow around sand dunes. It is then subject to salinity from salt spray and develops very thick succulent leaves at the windward side of the
bushes, while leaves on the sheltered side are non-succulent (Fig. 7.17). The non-succulent leaves protected from the salt spray have higher JCO2 and transpiration, Jh2o, during the second part of the day than the salt-exposed leaves (Fig. 7.18). Similar observations were described by Naidoo et al. (2002) who compared the mangrove associate Hibiscus tiliaceus with the true mangroves Avicennia marina
and Bruguiera gymnorhiza at sites with low and high salinities, respectively, and found that H. tiliaceus showed better photosynthetic performance than the mangroves at the low salinity site and vice versa at the high salinity site.
Fig. 7.18 Net CO2-uptake, Jco2, and transpiration, Jh2o, of Conocarpus erectus on coastal sand dunes. • Succulent leaves on the windward side of the bushes, o sheltered, on the leeward side. (Smith et al. 1989)
7.5.2 Water Use Efficiency
In relation to salinity and osmotic stress of mangrove trees water-use-efficiency (WUE) is of great interest. The WUEratio, as defined by JCO2 / Jh2o, decreases with salinity only slightly in A. marina and more pronouncedly at 1/1-strength sea water in A. corniculatum. WUEratio is not only affected by substrate salinity but also by leaf-to-air water vapour pressure difference (VPD) with considerable decreases as VPD increases (Fig. 7.19). However, Ball (1986) argues that notwithstanding these reductions of WUEratio the values observed remain exceptionally high. Indeed, the WUEratio values summarized for all conditions of salinity and VPD given in Fig. 7.19 are higher than in glycophytic C3-plants as well as C4-plants, and most remarkably they are in the same range as obtained for the highly water saving nocturnal CO2-uptake by CAM plants (Table 7.2).
Since stomatal control affects JCO2 and JH2O and hence WUEratio a calculation of intrinsic WUE, WUEintrinsic, is performed to include a consideration of the driving forces for JCO2, i.e. the difference between external, pCo2, and internal, pCo2, CO2-partial pressure, and for JH2O, i.e. VPD, and also the CO2-compensation point of photosynthesis, T,
Fig. 7.19 Water-use-efficiency ratios (WUEratio) of Avicennia marina and Avicennia corniculatum at approximately 1 /10-; 1/2- and 1/1-strength sea water salinity and different leaf-to-air vapour pressure differences (VPD) as indicated by the numbers (in Pa/kPa) in the graphs. (After Ball and Far-quhar 1984a; from Luttge 2002)
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