Study area
This study was carried out in a Tamaulipan thornscrub vegetation regenerated after livestock activities in Northeastern Mexico in the municipality of Pesquería, Nuevo León (Fig. 1). This area is located at an altitude of 310 m. The climate in the area is considered as dry BSOhw according to Köppen as modified by García [16] for the Mexican Republic. The mean annual temperature is 20° a 22 °C and the hottest months are July and August whereas the lower temperatures occur in December and January with 13 °C a 14 °C.
Some of the species with the highest importance value index in the region are Leucophyllum frutescens (Cenizo), Cordia boissieri (Anacahuita) y Acacia amentácea (Gavia), Prosopis glandulosa (Mezquite), Havardia pallens (Tenaza), Acacia farnesiana (Huizache), Parkinsonia texana (Palo verde) and Celtis pallida (Granjeno) [6].
Floristic inventory
In order to accomplish the main objective of this study, two vegetation communities with different historical livestock uses were selected. The extensive livestock area (25°43′25.46″, 99°58′7.15″ and 18.58 ha), was influenced by decades of use with selective extraction of woody species as well as for tramping and feeding activities of livestock.
In 1998 productive activities in this area were abandoned. In the area with a historical intensive livestock activity (25°43′25.63″, 99°58′19.43″ and 28.66 ha), the secondary vegetation was eliminated and in 1977 an exotic grassland (Cenchrus ciliaris) was established and livestock were grassing there for 10 years (i.e. 1978–1988). In both areas, productive activities were abandoned and regeneration occurred naturally.
After twenty five years livestock activities (i.e. in 2013), the vegetation communities were evaluated in both areas. In order to evaluate regeneration 12 sampling sites (i.e. 24 in total) in each area were established. Rectangular sampling sites of 50 m2 (5 × 10 m) were used for facilitating the limits and measurements in dense vegetation when compared with circular shaped sites [17].
Distribution of sampling sites was randomly selected and information of all trees, shrubs and herbaceous species was registered. Trees and shrubs with a basal diameter of d0.10 > 1 cm, were evaluated as well as all herbaceous species.
Data analysis
Abundance was determined for each species, considering the number of individuals, their dominance regarding the function of the canopy coverage and its frequency based on its existence in sampling sites. Results were used in order to obtain a balance value at the taxon level named Importance Value Index (IVI), which has percentage values in a scale of 0–100 [18]. For relative abundance estimate the following equation was used:
$$ {AR}_i=\left(\raisebox{1ex}{${A}_i$}\!\left/ \!\raisebox{-1ex}{$\sum \limits_{i=1\dots n}{A}_i$}\right.\right)\times 100\kern2em {A}_i=\raisebox{1ex}{${N}_i$}\!\left/ \!\raisebox{-1ex}{$S$}\right. $$
Where Ai = absolute abundance, ARi = relative abundance of the species i respect to the total abundance, Ni = number of individuals of the species i, S = sampling surface (ha). Relative dominance was evaluated by means of:
$$ {DR}_i=\left(\raisebox{1ex}{${D}_i$}\!\left/ \!\raisebox{-1ex}{$\sum \limits_{i=1\dots n}{D}_i$}\right.\right)\times 100\kern2em {D}_i=\raisebox{1ex}{${Ab}_i$}\!\left/ \!\raisebox{-1ex}{$S(ha)$}\right. $$
Where Di = absolute dominance, DRi = relative dominance of the species i respect to the total dominance, Abi = canopy area for the species i and S = surface (ha). Relative frequency was obtained with the following equation:
$$ {FR}_i=\left(\raisebox{1ex}{${F}_i$}\!\left/ \!\raisebox{-1ex}{$\sum \limits_{i=1\dots n}{F}_i$}\right.\right)\times 100\kern2em {F}_i=\raisebox{1ex}{${P}_i$}\!\left/ \!\raisebox{-1ex}{$ NS$}\right. $$
Where Fi = absolute frequency, FRi = relative frequency of the species i with respect to the total frequency, Pi = number of site per species occurrence i, NS the total sampling sites number. The importance value index (IVI) has percentage values from 0 to 100% and is taken in accordance to [19, 20]:
$$ IVI=\frac{AR_i+{DR}_i+{FR}_i}{3} $$
Where ARi = Relative abundance of the species i with respect to total abundance, DRi = relative abundance of the species i respect ot total dominance, FRi = relative frequency of the species i respect to total frequency.
In order to determine alfa diversity two indexes were used, Margalef (DMg) [21] corresponding to the number of present species (richness of species) and that by Shannon (H´) [22] corresponding to the community structure, i.e. the proportional distribution of the value given to each species. The following formulas were used for its calculation:
$$ {\displaystyle \begin{array}{l}{D}_{Mg}=\frac{\left(S-1\right)}{1\mathrm{n}(N)}\\ {}{H}^{\hbox{'}}=-\sum \limits_{i=1}^S{p}_i\times 1\mathrm{n}\left({p}_i\right)\\ {}{p}_i=\raisebox{1ex}{${n}_i$}\!\left/ \!\raisebox{-1ex}{$N$}\right.\end{array}} $$
Where S = number of present species, N = number of total individuals, ni = number of individuals per species i, pi = proportion of individuals of species i with respect to the total number of individuals.
In order to determine variability of species composition between the sampling units, the beta diversity was used. Similarity of vegetation communities was calculated using the Sorensen similarity coefficient used for quantitative data (IScuant) using the formula by Magurran [23]:
$$ {I}_{Scuant}=\frac{2 pN}{aN+ bN} $$
Where aN = total number of individuals in the site A, bN = total number of individuals in the site B, pN = Summary of the lower abundance of the shared species between both sites.
In order to determine differences between density and canopy, as well as between the Margalef and Shannon indexes of both thornscrub areas, the mean values from the sampling sites were added. When data was arranged with the normality and homogenicity criteria a T test with a (α = 0.05) was carried out.