Causal relationships between zonal distribution patterns of coastal dune plants and their environmental conditions were studied by field surveys and by growth experiments of coastal dune plants treated with different NaCl concentrations. Seasonal changes of environmental conditions and concomitant changes of photosynthetic activities and stomatal behaviors of coastal plants were investigated at the Kado-ori coast, Ibaraki Prefecture.
Due to aridity caused by strong ceaseless wind and low water-holding capacity of sandy soil and due to low nutrient availability, the Kado-ori coast was a very stressful and unproductive habitat for survival and growth of plants living there, similar to most coastal dunes. The severity, however, was relaxed in inland sites compared with seaward sites from the viewpoint of edaphic and atmospheric conditions; that is, soil-water salinity was lowest (15 mM) at the most inland site (85 m from the seashore) and highest (90 mM) at the seaward site (30 m from the seashore). Moreover, thickness of capillary water layer increased from 13 cm at 17 m to 48 cm at 85 m from the seashore, reflecting the fine sand composition. Evaporative demand, which was evaluated by the evaporation rate of a black filter paper, decreased with increasing distance from the seashore.
Three coastal dune plants, Ischaemum anthephoroides (C4 plant), Carex kobomugi (C3 plant) and Calystegia soldanella (C3 plant) are mainly employed in this study since they are so ubiquitously distributed on Japanese coasts and also abundant on the Kado-ori coast. On the coast, Carex kobomugi and Calystegia soldanella dominated on more seaward sites where the environmental conditions were most inhospitable, and Ischaemum anthephoroides dominated on more inland sites where they were somewhat hospitable. Such a zonal distribution pattern of coastal dune plants is the same with those on most Japanese coasts.
The laboratory experiments and field measurements showed that, unless Ischaemum anthephoroides, a C4 plant, suffered from such a serious water deficiency as in summer 1990, it was superior to the two C3 plants, Carex kobomugi and Calystegia soldanella under an intense light and hot condition, ; that is, light-saturated net photosynthesis rate of Ischaemum anthephoroides was 30 micromol CO2/m2/s, whereas those of Carex kobomugi and Calystegia soldanella were 20 and 10 micromol CO2/m2/s, respectively. Optimum temperature for net photosynthesis was 35 degC in Ischaemum anthephoroides, 30 degC in Calystegia soldanella, and 25 degC in Carex kobomugi. From the viewpoint of the mean water use efficiency (WUE) and the mean nitrogen use efficiency (NUE) of the three plants growing on the Kado-ori coast, Ischaemum anthephoroides was much superior to Calystegia soldanella and Carex kobomugi; their WUE values, which were obtained by the simultaneous measurements of photosynthesis and transpiration rates by portable photosynthesis porometers, were 7.2, 2.6 and 1.8 micromol CO2 /molH2O, respectively, and their NUE values were 351, 100 and 78 micromol CO2/m2/s/mol N, respectively. Such superiority of Ischaemum anthephoroides is a well-established characteristic of a C4 plant.
From the above-mentioned physiological characteristics of the three coastal plants, we would imagine wider or seaward distribution of Ischaemum anthephoroides on the coastal dune. However, the fact is the reverse; Ischaemum anthephoroides is distributed not on seaward sites but on inland sites. This serious discrepancy between the physiological phenomena and the ecological ones was analyzed by field measurements of physiological properties and salinity experiments of the three species in a growth chamber.
The summer of 1990 was characterized by unusual aridity derived from higher summer temperature and prolonged rainless periods due to weaker development of the Baiu front. The field measurements showed that the photosynthetic activities were considerably decreased by the 1990-summer aridity. Especially those of Ischaemum anthephoroides fell to around 9 micromol CO2/m/2/s, being comparable with those of the other two C3 species, Calystegia soldanella and Carex kobomugi. Although Ischaemum anthephoroides always had the highest potential activity exceeding always 20 and sometimes 30 micromol CO2/m2/s, it showed the most pronounced simultaneous reduction of leaf water potential as low as -2.5 MPa in the summer 1990, while the reductions of Calystegia soldanella and Carex kobomugi remained over -2.0 MPa. Reflecting such a serious leaf water deficiency, Ischaemum anthephoroides showed the highest dead leaf ratio of 60 % in August 1990 among the three species. The reason why Ischaemum anthephoroides was less tolerable to a severe drought was suggested to be due to its shallow distribution of the root system, even though it had the highest potential capabilities in photosynthesis, water use efficiency and nitrogen use efficiency under a favorable or moderate environmental condition. This may be one of the reasons why the distribution of Ischaemum anthephoroides is restricted within inland sites where drought conditions are milder than in seaward sites.
Another reason to restrict the distribution of Ischaemum anthephoroides within inland sites has become apparent from a salt-treatment experiment. Seedlings of coastal sand dune plants were cultivated hydroponically with three NaCl solutions (0, 10 and 100 mM) in a growth chamber. The salinity treatment reduced relative growth rates (RGR) in all the three species in considerable extent. Independent of the salinity levels treated, however, Calystegia soldanella had the highest RGR, followed by Carex kobomugi and Ischaemum anthephoroides. For instance, the RGR values at 100 mM NaCl were 0.085, 0.066 and 0.060 g/g/d, respectively. It is worth noting that the order of RGR coincides with the order of zonal distribution of the three species in the Kado-ori coast. In addition, the overall trends of RGR against the NaCl treatment were primarily determined by those of LAR, one component of RGR related to matter allocation of a plant. The differences of RGR values among the three species and/or among the salinity levels treated were governed by those of LAR. This finding was supported by additional data obtained from the similar experiment with the salt treatments on Elymus mollis and Wedelia prostrata. On the other hand, net assimilation rate (NAR), another component of RGR related to photosynthetic activity of a single leaf, was influenced species-specifically by the salinity treatment; a higher level of salinity considerably depressed NAR in Ischaemum anthephoroides, but promoted in Calystegia soldanella.
To survive and grow in any habitat, it should be the first requisite for any plant to assume a positive value of RGR during their growth period. In sand dune plants, a high possibility has emerged from the salt-treatment experiment that keeping a higher level of LAR is major determinant to tolerate such a saline and arid habitat as seaward area of coastal dunes and succeed in establishing there, as typically demonstrated in Calystegia soldanella in the present study. In a relatively moderate habitat as inland sites of coastal dunes, on the contrary, such a plant with higher LAR would be overcome by one with lower LAR like Ischaemum anthephoroides in competition for light, due to inevitable lower stature of the higher-LAR plant. The hypothesis proposed here is very fascinating, and thus much effort should be concentrated to confirm this in future plant ecology.