In the plots with mature Casuarina trees, horizon A1 (0–5 cm), showed a dark
brown color (10YR2/2), with organic remains slightly decomposed and numerous
very fine roots, less than 1 mm in diameter. The lower horizon, Cg (5–60 cm) is
predominantly pale brown (60 %–10YR6/3) and brown (40 %; 10YR4/3), indicating a slight hydromorphism. This last horizon had very few roots.
In the old Casuarina plots, some of the trees have already fallen down and the
wood is dry. In this area, the phreatic level is only 30 cm deep. Horizon A1
(0–9 cm) in this site is very dark brown (10YR2/2) and medium-sized roots (with
mean diameter ranging from 2 to 5 mm) are common. The next horizon, Cg
(9–35 cm) was dark brownish grey (10YR4/2) with dark yellowish brown veins
(10YR4/6) that follow the root distribution. This soil shows a greater hydromorphism than that found at the other two sites.
The chemical characteristics of the soil showed differences between the
Casuarina stands. In general, pH values varied strongly from alkaline to extremely
alkaline in the old Casuarina plots. The carbon concentration, in the first horizon,
increased in the old plots. In this site organic matter is incorporated into the soil,
thus more root activity was observed. The concentrations of exchangeable cations
(Ca, Mg, K, Na) were, in all sites, higher in the upper horizons than in the lower,
emphasizing the high to very high calcium and sodium values. The calcium carbonate concentration showed no significant variation between sites. The chemical
composition of the saturation extract showed significant differences among sites
(Table 14.2). For example, the ion concentration measured with electrical conductivity shows that there was a tendency to decrease from the young Casuarina
stands to the old ones, a situation that can be attributable to the increment in
organic matter content (FAO 1998). In this case, the electrical conductivity values
indicate that soils are saline to moderately saline (Hillel 2000), so it is likely that
the established plant communities are semi-tolerant to salinity. Soluble salts in the
saturation extract are virtually restricted to sodium and calcium among the cations,
and chloride and sulfate between the anions. This is consistent with the marine
influence on the dune soils. The relative dominance of sodium is associated with
the high levels of chlorine, while the abundance of calcium is related to high levels
of sulfate (Richards 1954). Bicarbonate concentration tends to increase from the
young plots to the old plots, a situation explained by the greater root and microbial
activity in the older stands with a high CO 2 production, which generates
bicarbonate.
14.5 Discussion
As expected, Casuarina was the dominant tree in all our study plots (except for the
tropical dry forest, of course), creating an almost uniform canopy cover. Nevertheless, we found that natural regeneration is occurring in the hydromorphic soils,
especially in the old Casuarina stands. Here, diversity and species composition
were similar to the tropical dry forest growing on coastal dunes (tropical dry forest
228
P. Moreno-Casasola et al.
brown color (10YR2/2), with organic remains slightly decomposed and numerous
very fine roots, less than 1 mm in diameter. The lower horizon, Cg (5–60 cm) is
predominantly pale brown (60 %–10YR6/3) and brown (40 %; 10YR4/3), indicating a slight hydromorphism. This last horizon had very few roots.
In the old Casuarina plots, some of the trees have already fallen down and the
wood is dry. In this area, the phreatic level is only 30 cm deep. Horizon A1
(0–9 cm) in this site is very dark brown (10YR2/2) and medium-sized roots (with
mean diameter ranging from 2 to 5 mm) are common. The next horizon, Cg
(9–35 cm) was dark brownish grey (10YR4/2) with dark yellowish brown veins
(10YR4/6) that follow the root distribution. This soil shows a greater hydromorphism than that found at the other two sites.
The chemical characteristics of the soil showed differences between the
Casuarina stands. In general, pH values varied strongly from alkaline to extremely
alkaline in the old Casuarina plots. The carbon concentration, in the first horizon,
increased in the old plots. In this site organic matter is incorporated into the soil,
thus more root activity was observed. The concentrations of exchangeable cations
(Ca, Mg, K, Na) were, in all sites, higher in the upper horizons than in the lower,
emphasizing the high to very high calcium and sodium values. The calcium carbonate concentration showed no significant variation between sites. The chemical
composition of the saturation extract showed significant differences among sites
(Table 14.2). For example, the ion concentration measured with electrical conductivity shows that there was a tendency to decrease from the young Casuarina
stands to the old ones, a situation that can be attributable to the increment in
organic matter content (FAO 1998). In this case, the electrical conductivity values
indicate that soils are saline to moderately saline (Hillel 2000), so it is likely that
the established plant communities are semi-tolerant to salinity. Soluble salts in the
saturation extract are virtually restricted to sodium and calcium among the cations,
and chloride and sulfate between the anions. This is consistent with the marine
influence on the dune soils. The relative dominance of sodium is associated with
the high levels of chlorine, while the abundance of calcium is related to high levels
of sulfate (Richards 1954). Bicarbonate concentration tends to increase from the
young plots to the old plots, a situation explained by the greater root and microbial
activity in the older stands with a high CO 2 production, which generates
bicarbonate.
14.5 Discussion
As expected, Casuarina was the dominant tree in all our study plots (except for the
tropical dry forest, of course), creating an almost uniform canopy cover. Nevertheless, we found that natural regeneration is occurring in the hydromorphic soils,
especially in the old Casuarina stands. Here, diversity and species composition
were similar to the tropical dry forest growing on coastal dunes (tropical dry forest
228
P. Moreno-Casasola et al.