Study area

This research was carried out in the area known as Rancho Palmar de Abajo, located in La Matanza, La Paz municipality, Baja California Sur, between 23°38 ́02 ́ ́ N and 110°17 ́05 ́ ́O (Figure 1). The climate of this area is desert-dry with average temperatures of 21.2 °C and temperatures of 9 °C in winter; the rainy season displays from July to September, but rains can occur in winter (^{Ramírez et al., 2011}). The dry season is from February to June (^{Troyo et al., 2014}).

Figure 1 Location of the Rancho Palmar property, La Paz municipality, Baja California Sur (The study area is marked in red in the polygon). 

In the study area, the soil type is Regosol, and, for the mostly covered by the sarcocaule scrub (^{INEGI, 2009}). In general, the land is used in vegetable agriculture and the sowing of fodder for livestock.

Sampling method

The quadrant method was selected (^{Ferro-Díaz, 2015}; ^{Pequeño et al., 2017}) to describe the arboreal, shrub and herbaceous vegetation at the sampling site. Twenty 5 m × 5 m quadrants were delimited with metal stakes at an approximate distance of 100 m between them; its geoposition was taken at the vertex of each one of them with a Garmin Oregon 650 GPS. Inside each quadrant, the following data were taken: number of species and of individuals per species, height (cm) crown width (cm) and crown length (cm) of each plant with a 10 m Truper tape.

The results thus measured were used to determine the ecological indicators of Relative Abundance (AR _i ), Relative Dominance (DR _i ) and Relative Frequency (FR _i ) of each species in the 20 quadrants. These data were used in the calculation of the Importance Value Index (IVI), which obtains values on a scale from 0 to 100 expressed in per cent (^{García and Zavala, 2018}). The formulas for each indicator are shown below (^{Marroquín et al., 2017}):

Where:

AR _i = Relative abundance of the i species, in respect to total abundance

A _i = Number of individuals of the i species

S = Sampling area (ha)

Where:

DR _i =Relative dominance of the i species, in respect to total dominance

D _i = Absolute dominance of individuals

∑Di = Sum of the dominance of individuals of all species

S = Area (ha)

Where:

FR _i = Relative frequency of the i species, in respect total frequence

F _i = Number of quadrants in which the species is present

∑Fi = Sum of the number of quadrants in which all the species are present

P _i = Number of quadrants in which the i species is present

NS = Total number of sampling sites

Where:

IVI= Importance Value Index

AR _i = Relative abundance of the i species in respect to total abundance

DR _i = Relative dominance of the i species in respect to total dominance

FR _i = Relative frequency of the i species in respect to total frequency

In the same way, the Margalef index (S´) was determined to know the diversity of species and the Shannon-Wienner index (H’) for species richness (^{Marroquín et al., 2017}) and finally the Jaccard similarity index between the quadrants sampled in the two periods (I _j ) (^{Reyes and Torres-Florez, 2009}), using the following formulas:

Where:

S´= Margalef index

S = Number of species

ln = Natural logarithm

N = Total number of individuals

Where:

H’ = Shannon-Wienner diversity índex

S = Number of present species

ln = Natural logarithm

p _i = Rate of the i species

n _i = Number of individuals of the i species

N= Total number of individuals

Where:

I _j = Jaccard similarity index

a = Number of species present at the A quadrant

b = Number of species present at the B quadrant

c = Number of species present at both A and B quadrants

Samplings were carried out in the same quadrants, in the wet season (September 28 to November 2, 2018) and dry (May 27 to August 5, 2019); It should be noted that rain events were delayed during these activities.

Data analysis

The information was analyzed using a multivariate test to detect the differences between each season (wet and dry), coverage and species similarity in each quadrant, using the Past 3.2 program (^{Medina et al., 2017}). For the statistical analysis of the data, the SPSS version 22.0 package was used (^{IBM, 2013}), using the non-parametric test for Wilcoxon-dependent samples, for the variables of Abundance (N ha^-1), Dominance (m² ha^-1 ), Species diversity (H´) and Species richness (S´) in order to detect differences between the wet and dry seasons (^{Ramírez et al., 2013}). Each one of the species and families sampled throughout the investigation, were corroborated with what was published in the work of ^{Rebman and Roberts (2012)} referring to the botany of the Baja California peninsula.

Floristic composition

A total of 1 466 individuals of 60 species and 27 families were gathered during the wet season. The largest number of species was recorded for the families Asteraceae and Fabaceae (8), followed by Cactaceae (7), Euphorbiaceae (6), Solanaceae and Rhamnaceae (3 each), Burseraceae, Amaranthaceae, Acanthaceae and Malvaceae (2 each), the other families present a single species, for the wet season. In the dry season, a total of 168 individuals from 27 species and 14 families were recorded, which were distributed as follows: from highest to lowest diversity: Cactaceae (7), Euphorbiaceae (4), Fabaceae (4) and Malvaceae (2), the other families showed only one species per family.

Ecological indicators

Table 1 orders the results obtained in the two seasons of the year, and Table 2 lists the families and life forms of each species recorded in the research.

The IVI results show among the main species Jatropha cinerea with 10.7 %, Ruellia californica I.M. Johnst, with 6.7 %, Turnera diffusa Willd. ex Schult. with 5.46 %, Lippia palmeri S. Watson with 4.48 % and Fouquieria diguetti with 4.33 %, as species with the highest IVI for the rainy season (wet). In contrast, for the dry season, an IVI was obtained in Ruellia californica of 12.48 %, Prosopis articulata S. Watson with 11.76 %, Stenocereus gummosus with 9.87 %, Cylindropuntia cholla Engelm. & Bigelow) F.M. Knuth with 8.61 % and Mammillaria armillata K. Brandegee with 7.7 %, with the highest values in the dry season. The richness (S´) of species is greater for the rainy season, with an S´ = 8.09 and for the dry season an S´ = 4.88 is shown.

For the diversity of species (H´), in the rainy season a value of H´ = 3.36 was observed and in the dry season a value of H´ = 2.88, which indicates high diversity during the wet season, as expected, which coincides with the results of ^{Alanís et al. (2015a)} in this type of scrub in Nuevo León. For abundance (N ha^-1) and dominance (m² ha^-1), significant differences were detected between the two seasons (p = 0.000087) and (p = 0.000085), respectively, which is extensive for diversity (H´) and species richness (S´) (p = 0.000088) between the two seasons (Figure 2).

Estación = Season; Abundancia = Abundance; Dominancia = Dominance; Índice de Shannon = Shannon Index; Húmeda = Wet; Seca = Dry.

Figure 2 Abundance (N ha^-1), Dominance (m² ha-¹), Shannon-Wienner Index and Margalef Index during the wet and dry seasons of the study site. The bars indicate the values found for the different variables, with differences (p = ≤0.05) between the two seasons. 

Floristic similarity

The Jaccard index shows values of I _j = 38.71, in species similarity between the two seasons. The results show similarities between sites 16, 17, 18 and 19 (I _j > 0.05) through the dendogram (Figure 3) with coverage (m² * quadrant) and species in common, in each of the quadrants (Cilindropuntia cholla, Pachycereus pringlei and Stenocereus gummosus). Those with the greatest similarity between seasons are 16 and 19 (I _j = 0.06) and coincide with species such as Acacia farnesiana, Bourreria sonorae, Cylindropuntia cholla, Cnidoscolus angustidens, Krameria parvifolia, Mammillaria armillata, Prosopis articulata, Ruellia californica, Solanum hindsian Stenocereus gummosus and Turnera diffusa. The other group of quadrants that indicates homogeneity in coverage and species are sites 8 and 13 (I _j <0.05) with Cylindropuntia cholla, Cnidoscolus angustidens, Ferocactus chrysacanthus and Mammillaria armillata.

Figure 3 Dendrogram of floristic similarity between sites sampled for the two seasons.