14.3 Methods
14.3.1 Vegetation
The Casuarina artificial forest was subdivided into three stands with different
mean trunk width. Diameter at breast height (DBH; approximately 1.50 m) was
measured in five 10-m 9 10-m plots in each of these three forest stands, allowing
us to define them as young (smallest DBH values), mature, and old Casuarina
plots (largest DBH values). These Casuarina stands were planted on different
dates and in rows parallel to the coast. In each of these different-aged stands two
transects of five 10 9 10 m adjacent quadrats were marked, parallel to the
shoreline, so that we could observe changes in the community composition in each
of these stands. In each quadrat, species were collected for identification, and plant
cover per species was estimated with the Westhoff and van der Maarel (1978)
scale. For comparison, the same type of sampling took place in a coastal tropical
dry forest growing on sand dunes further inland, on a slope of the same dune
system, but where Casuarina trees had not been planted. Finally, another set of ten
10 9 10 m quadrats was sampled on a foredune area stabilized with Casuarinas,
5 km north of the study site, with a DBH structure similar to the mature stand
(referred to here as Casuarina). For each stand Shannon’s diversity index was
calculated. In all cases, once species were identified, the type of fruit was determined, either from herbarium species or from the literature and then a dispersal
syndrome was assigned to each species. A matrix was built with the species cover
values and analyzed with a principle component analysis (PCA) ordination using
the program PCOrd (McCune et al. 2002).
14.3.2 Soil Attributes
Soil profiles 1 m deep were excavated at two sites in each stand, close to the
quadrats where floristic composition was sampled. In each horizon soil samples
were collected and transported to the laboratory. Samples were air dried and sieved
(up to 2 mm) before chemical analyses were performed. The following analyses
were carried out following standard procedures as described by Sparks (1996): pH
in water was measured using a 1:2 relation, total organic carbon (Walkley and
Black 1934), total nitrogen using the Kjeldahl method, exchangeable cations (Ca,
Mg, Na, K) through soil lixiviation with ammonium acetate 1 N pH 7, CaCO 3
percentage was calculated using the neutralization method of HCl with a known
concentration and a posteriori titulation of the excess acid with NaOH. In a saturation extract the following chemical properties were determined: electrical
conductivity (dS m
-1 ), calcium (mg L
-1 ), magnesium (mg L
-1 ), potassium (mg
L
-1 ), sodium (mg L
-1 ), chloride (mg L
-1 ), bicarbonates (mg L
-1 ), and sulfates
(mg L
-1 ). The saturation extract is an important aqueous solution because many
14 The Impacts on Natural Vegetation
221
14.3.1 Vegetation
The Casuarina artificial forest was subdivided into three stands with different
mean trunk width. Diameter at breast height (DBH; approximately 1.50 m) was
measured in five 10-m 9 10-m plots in each of these three forest stands, allowing
us to define them as young (smallest DBH values), mature, and old Casuarina
plots (largest DBH values). These Casuarina stands were planted on different
dates and in rows parallel to the coast. In each of these different-aged stands two
transects of five 10 9 10 m adjacent quadrats were marked, parallel to the
shoreline, so that we could observe changes in the community composition in each
of these stands. In each quadrat, species were collected for identification, and plant
cover per species was estimated with the Westhoff and van der Maarel (1978)
scale. For comparison, the same type of sampling took place in a coastal tropical
dry forest growing on sand dunes further inland, on a slope of the same dune
system, but where Casuarina trees had not been planted. Finally, another set of ten
10 9 10 m quadrats was sampled on a foredune area stabilized with Casuarinas,
5 km north of the study site, with a DBH structure similar to the mature stand
(referred to here as Casuarina). For each stand Shannon’s diversity index was
calculated. In all cases, once species were identified, the type of fruit was determined, either from herbarium species or from the literature and then a dispersal
syndrome was assigned to each species. A matrix was built with the species cover
values and analyzed with a principle component analysis (PCA) ordination using
the program PCOrd (McCune et al. 2002).
14.3.2 Soil Attributes
Soil profiles 1 m deep were excavated at two sites in each stand, close to the
quadrats where floristic composition was sampled. In each horizon soil samples
were collected and transported to the laboratory. Samples were air dried and sieved
(up to 2 mm) before chemical analyses were performed. The following analyses
were carried out following standard procedures as described by Sparks (1996): pH
in water was measured using a 1:2 relation, total organic carbon (Walkley and
Black 1934), total nitrogen using the Kjeldahl method, exchangeable cations (Ca,
Mg, Na, K) through soil lixiviation with ammonium acetate 1 N pH 7, CaCO 3
percentage was calculated using the neutralization method of HCl with a known
concentration and a posteriori titulation of the excess acid with NaOH. In a saturation extract the following chemical properties were determined: electrical
conductivity (dS m
-1 ), calcium (mg L
-1 ), magnesium (mg L
-1 ), potassium (mg
L
-1 ), sodium (mg L
-1 ), chloride (mg L
-1 ), bicarbonates (mg L
-1 ), and sulfates
(mg L
-1 ). The saturation extract is an important aqueous solution because many
14 The Impacts on Natural Vegetation
221