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Characterizing regeneration of woody species in areas with different land-history tenure in the Tamaulipan Thornscrub, Mexico.

Tamaulipan thornscrub is distributed in northeastern Mexico (states of Coahuila, Nuevo Leon, and Tamaulipas) covering a total area of 32,188 [km.sup.2] (Palacios-Prieto, 2000; Jimenez et al., 2009). This ecosystem is composed of ca. 60 woody species, many of which are important for forestry and grazing activities (wood, posts, firewood, charcoal, forage, etc.; Foroughbakhch et al., 2009; Jimenez and Alanis, in press). The woody vegetation of Tamaulipan thornscrub has been subjected to different activities of agroforestry in the last decades, which have caused changes in structure and composition (Alanis et al., 2008; Jimenez et al., 2009; Foroughbakhch et al., 2009). Extensive large-scale production of cattle has been practiced during the last 350 years in this ecosystem (Foroughbakhch et al., 2009), resulting in the loss of quality and quantity of species for forage followed by a reduction in the layer of plants that covers and protects the soil (Eviner, 2003; Gonzalez et al., 2009). In the last five decades, the main uses have been agriculture and establishment of grazing lands for intensive production of livestock (Manzano and Naivar, 2000; Manzano et al., 2000).

For the period 1980-1996, Tamaulipan thornscrub experienced an annual deforestation rate of 2.3% indicating that ca. 600 [km.sup.2] of this community of plants were lost every year and that 60% of the original Tamaulipan thornscrub in Mexico has been lost since the 1950s (Navar-Chaidez, 2008). Knowledge of the regeneration of the woody species is basic for understanding the nature of the ecosystems resulting from activities of agroforestry (Jimenez et al., 2012). The objective of our study was to evaluate the regeneration of woody species ([d.sub.0. 10] [greater than or equal to] 1 cm) in four areas with different histories of land-tenure (extensive production of livestock, intensive production of livestock, agriculture, and clearcutting) in Tamaulipan thornscrub.

MATERIALS AND METHODS--The present study occurred in native thornscrub on the campus of the Facultad de Ciencias Forestales, Universidad Autonoma de Neuvo Leon, in northeastern Mexico (25[degrees] 09' and 24[degrees] 33'N, and 99[degrees] 07'W; 350 m in elevation). Woody species with high abundance and cover in the study area were Acacia rigidula, Acacia farnesiana, Havardia pallens, Cordia boissieri, Karwinskia humboldtiana, and Prosopis glandulosa (Gonzalez et al., 1997; Espinoza and Navar, 2005; Alanis et al., 2008).

In August 1984, four abandoned areas of Tamaulipan thornscrub with different uses were selected: extensive production of livestock; intensive production of livestock; agriculture; clearcutting. The area of extensive production of livestock was managed under the selective extraction system of its woody components and used simultaneously for grazing by livestock. The area of intensive production of livestock was cleared with agricultural machinery, removing the vegetation and cultivating pastures of nonnative plants, such as Pennisetum ciliare for grazing by livestock. This area was used for grazing by cattle during 6 years. The agricultural area was cleared with agricultural machinery, removing the vegetation for cultivating purpose (corn, Zea mays, and sorghum, Sorghum bicolor) without irrigation systems during a 5-year period. The clearcut area was cleared with agricultural machinery (bulldozer) and did not present any subsequent activity. In September 1984, use of the four areas ended, and the areas were excluded by the school of forest sciences for its conservation by means of a perimeter fence.

In April 2005 (21 years later), four sampling sites were established in each area (16 total) to evaluate the regeneration of the woody vegetation. The sampling plots were 250-[m.sup.2] rectangles (10-x-25-m; Alanis et al., 2008; Jimenez et al., 2009). The rectangular shape of the sites was used due to the ease of delimitation and measurement in dense vegetation (Alanis et al., 2008). The distribution of plots was systematic, with a distance of 50 m between them. The four areas had similar physiographic characteristics (350-380 m in elevation, slope of <3%, and vertisol soil).

In the sampling sites, an inventory of all woody species was made including measurements of their total height (ht) and diameter at 10 cm ([d.sub.0,10] [greater than or equal to] 1 cm). The measurement of diameter was made at 0.10 m above ground, given that it is a standard measurement employed for the regeneration of the woody species of Tamaulipan thornscrub (Alanis et al., 2008; Jimenez et al., 2009). Only individuals with d0,10 > 1 cm were included.

We analyzed relative abundance, dominance, importance value index and mean height of woody individuals. The absolute abundance is [A.sub.i] = [N.sub.i]/S, where Ai is the absolute abundance of species i, [N.sub.i] is the number of individuals of species i, and S the sample surface (in hectares). The relative abundance of species was calculated using the equation: [Ar.sub.i] = ([A.sub.i]/[summation][A.sub.i])100, where [Ar.sub.i] is the relative abundance and [summation][A.sub.i] is the total abundance of species I (Jimenez et al., 2009). To estimate dominance the basal area for each individual was calculated. The absolute dominance is [D.sub.i] = [Ab.sub.i]/S, where [D.sub.i] is the absolute dominance of species i, [Ab.sub.i] is the basal area (in square meters) of species i, and S the sample surface (in hectares). The relative dominance was calculated using the equations [Dr.sub.i] = ([D.sub.i]/[summation][D.sub.i])100, where [Dr.sub.i] is the relative dominance of species i total dominance over and [summation][D.sub.i] is the total dominance of species. The relative frequency is [Fr.sub.i] = ([F.sub.i]/[summation][F.sub.i) 100, where Fri is the relative frequency of species i over the total frequency, [F.sub.i] is the number of sites that present the species i and [summation][F.sub.i] is the total frequency of all species (Harrison, 1994; B. Mostacedo and T. S. Fredericksen, in litt.).

The importance value index (IVI) was calculated as follows: IVI = [Ar.sub.i] + [Dr.sub.i] + Fri, where [Ar.sub.i] is relative abundance, [Dr.sub.i] is relative dominance and Fri is relative frequency (MuellerDombois and Ellenberg, 1974). Species richness was determined as the total number of species for each site. Diversity was calculated for each site (Shannon, 1948):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where H' is the Shannon index of diversity, ln is the natural logarithm, [p.sub.i] is the proportion of the species in the site, ni is the number of individuals of species i and N is the total number of individuals.

To characterize the mean height of the woody vegetation, only the 20% of the higher individuals (upper strata) found on the sampling plots were selected (Kramer and Akca, 1995). To evaluate if there were significant differences between the variables among the evaluated sites, we calculated the average values of the sampled sites and performed an analysis of variance (P > 0.05). If differences were found, a multiple comparisons with the Tukey test was conducted. All analyzes were conducted using the SPSS program version 15.0 (SPSS Inc., Chicago, Illinois).

RESULTS--From the four evaluated areas, we recorded 14 families consisting of 24 genera and 29 species (Table 1). The most common family was Fabaceae with 10 species.

The area of extensive production of livestock had the greatest abundance of woody species (16,810 [+ or -] 3,429 N/ ha), followed by clearcut area (6,310 [+ or -] 523 N/ha). The areas of intensive production of livestock (1,760 [+ or -] 704 N/ ha) and agriculture (2,370 [+ or -] 823 N/ha) were similar with the lowest values of abundance. Abundance in these latter two areas did not differ significantly (Fig. 1a).

Dominance (basal area) presented significant differences among the four areas (P < 0.000). The area of extensive production of livestock had the highest dominance (26.1 [+ or -] 4.9 [m.sub.2]/ha), a result of the high abundance of individuals in the area. The areas of agriculture and clearcutting had medium values that were similar (14.9 [+ or -] 1.7 an d 17.4 [+ or -] 3.4 [m.sup.2]/ha, respectively) were not significantly different (Fig. 1b).

Twenty percent of the tallest individuals were selected from the sampling sites, and their average height was estimated following Kramer and Akca (1995). Height did not differ significantly among the evaluated areas (F = 1.97, df = 3, P = 0.172) and ranged from 3.4 [+ or -] 0.2 (extensive production of livestock) to 4.0 [+ or -] 0.4 m (intensive production of livestock).

Species richness differed significantly (P < 0.000) among the evaluated areas. The areas of extensive production of livestock (18.7 [+ or -] 4.6) and clearcutting (15.0 [+ or -] 0.8) had the highest species richness, followed by the areas of agriculture (11.0 [+ or -] 3.3) and intensive production of livestock (4.7 [+ or -] 1.2; Fig. 1c).

Shannon diversity indices (H') differed (P < 0.001) among the evaluated areas (Fig. 1d). The clearcut area had the highest value (2.12 [+ or -] 0.1) differing from that of the area of intensive production of livestock (1.11 [+ or -] 0.4). The areas of agriculture (1.90 [+ or -] 0.1) and extensive production of livestonck (1.91 [+ or -] 0.0) were similar to the other two areas.

DISCUSSION--The high representation of the family Fabaceae agrees with the studies of Gonzalez et al. (1997), Estrada and Jurado (2005), Espinoza and Navar (2005), Jimenez et al. (2009) and Gonzalez et al. (2010), which documented that most species present in Tamaulipan thornscrub are from this family. The large number of individuals from this family in areas with relatively young secondary vegetation can be related with diverse factors, such as scarce availability of nutrients in the soil, intolerance to shade, and mechanisms related to reproduction (Estrada et al., 2004; Estrada and Jurado, 2005).

Gonzalez et al. (1997) and Jimenez et al. (2012) suggest that the areas that have been stripped of their natural plant cover and later subjected to agricultural and farming use, are likely to show low availability of nitrogen when they are abandoned. Therefore, species capable of fixing atmospheric nitrogen (a trait common in Fabaceae) are likely to be abundant. Fabaceae also have other adaptations that would likely make them abundant, such as small leaflets in compound leaves and the ability to be drought deciduous. They also might be abundant because some have seeds that are eaten by cattle and spread in droppings.

The area that presented the highest relative abundance of woody species was the area of extensive production of livestock, perhaps because this was the only land in which the existing plant cover was not completely removed and goats might have dispersed some species, such as Bernardia myricifolia, H. pallens, and Diospyros texana (Romero and Ramirez, 2003; Ramirez et al., 2006; Cooper et al., 2008). The lower abundance of woody species in the areas of intensive production of livestock and agriculture might be a consequence of removal of all plant cover and possible changes in soil characteristics. The soil of the livestock area possibly became compacted and had changes in chemical composition

The area with lowest dominance (6.7 [+ or -] 1.3 [m.sup.2]/ha) was the area of intensive production of livestock; perhaps, this was due to the least favorable conditions in the area for germination and establishment of seedlings, with few individuals occupying the location. This could be due to compaction of soil from trampling by livestock. In relation to height, parameters are not significantly different between the areas assessed. It is remarkable that this area has a high coverage of exotic species introduced such as Pennisetum ciliare, which increases the competition for resources with the native species (Fowler et al., 2011).

According to the Shannon diversity analysis, the clearcut area had the highest diversity, perhaps because the area had a history of less severe human activities, given that agricultural activities had not occurred in this area. Maybe, this meant less pressure on the soil and less impact on diversityofplants, and, therefore, there maybe a direct effect on the composition of species. This area presented a Shannon diversity index of 2.1, similar to that reported by Gonzalez et al. (2010) in three areas of Tamaulipan thornscrub in Nuevo Leon (2.4, 2.6, and 2.8). Their values, however, are higher than those found in our study, perhaps because their areas did not have a history of use for agroforestry. The area of intensive production of livestock presented the lowest diversity as well as the lowest value of species richness, which indicates that the activities in this area had a higher impact on the area, presenting a greater change in its composition.

The area that was not cleared (extensive production of livestock) presented the highest values of abundance, dominance, and species richness. The area that was cleared and later trampled by livestock (intensive production of livestock) had the lowest values of abundance, dominance, and species richness.

The results provide valuable information on the state of the natural regeneration of woody species locally. Nevertheless, it is important to continue with further research to evaluate the issue at the regional level to increase the knowledge of this ecosystem. The use of this type of information has increased among scientists, technicians, and managers of natural resources, and the information serves as a starting point for correct decision making within programs of rehabilitation and ecological restoration in the state of Nuevo Leon (Gonzalez Tagle et al., 2007; Alanis et al., 2011).

We would like to thank the Consejo Nacional de Ciencia y Tecnologia (CONACYT) for the grant awarded to the second author. R. Dominguez helped with identification of the species. This study was partially supported by Programa de Mejoramiento al Profesorado (PROMEP/103.5/12/3585) and Programa de Investigation Cientifica y Tecnologica (PAICYT).

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Submitted 28 December 2011. Accepted 16 October 2013. Associate Editor was Mara L. Alexander.

Javier Jimenez Perez, Eduardo Alanis Rodriguez, Marco Aurelio Gonzalez Tagle,* Oscar A. Aguirre Calderon, and Eduardo J. Trevino Garza

Universidad Autonoma de Nuevo Leon, Facultad de Ciencias Forestales, Linares, Nuevo Leon, Mexico, C.P. 67700

* Correspondent: marco.gonzaleztg@uanl.edu.mx

Table 1--Absolute abundance (number per hectare), absolute dominance
(square meters per hectare), and importance value index (IVI,
percent) for four areas of Tamaulipan thornscrub in Nuevo Leon,
Mexico.

                                Intensive production of livestock

                                Absolute      Absolute
     Family and species         abundance     dominance      IVI

Agavaceae
  Yucca filifera                    0            0.0         0.0
Boraginaceae
  Cordia boissieri                 10            0.0         0.8
  Ehretia anacua                   10            0.1         1.0
Ebenaceae
  Diospyros palmeri                 0            0.0         0.0
  Diospyros texana                  0            0.0         0.0
Euphorbiaceae
  Bernardia myricifolia             0            0.0         0.0
  Croton torreyanus                 0            0.0         0.0
Fabaceae
  Acacia berlandieri                0            0.0         0.0
  Acacia farnesiana                720           3.8        44.4
  Acacia rigidula                   0            0.0         0.0
  Acacia schaffneri                30            0.4         3.2
  Acacia wrightii                   0            0.0         0.0
  Eysenhardtia texana               0            0.0         0.0
  Havardia pallens                  0            0.0         0.0
  Mimosa monancistra               610           1.0        24.1
  Parkinsonia texana               10            0.9         5.0
  Prosopis laevigata               280           0.5        16.2
Malpighiaceae
  Malpighia glabra                  0            0.0         0.0
Oleaceae
  Forestiera angustifolia          10            0.0         0.9
Rubiaceae
  Randia obcordata                  0            0.0         0.0
Rhamnaceae
  Condalia hookeri                  0            0.0         0.0
  Karwinskia humboldtiana           0            0.0         0.0
  Ziziphus obtusifolia             20            0.0         1.1
Rutaceae
  Amyris texana                     0            0.0         0.0
  Zanthoxylum fagara               30            0.1         1.8
Sapotaceae
  Sideroxylon celastrinum          20            0.0         1.2
Scrophulariaceae
  Leucophyllum frutescens           0            0.0         0.0
Ulmaceae
  Celtis pallida                    0            0.0         0.0
Zygophyllaceae
  Guaiacum angustifolium            0            0.0         0.0
Total                             1,750          6.8        100.0

                                             Agriculture

                                Absolute      Absolute
     Family and species         abundance     dominance      IVI

Agavaceae
  Yucca filifera                    0            0.0         0.0
Boraginaceae
  Cordia boissieri                 70            0.6         2.8
  Ehretia anacua                    0            0.0         0.0
Ebenaceae
  Diospyros palmeri                 0            0.0         0.0
  Diospyros texana                 660           2.3        21.3
Euphorbiaceae
  Bernardia myricifolia             0            0.0         0.0
  Croton torreyanus                300           0.1         5.7
Fabaceae
  Acacia berlandieri               110           2.6         9.7
  Acacia farnesiana                350           2.6        16.0
  Acacia rigidula                  270           3.3        15.8
  Acacia schaffneri                 0            0.0         0.0
  Acacia wrightii                  10            0.0         0.6
  Eysenhardtia texana              270           1.1         8.7
  Havardia pallens                 40            0.4         3.0
  Mimosa monancistra                0            0.0         0.0
  Parkinsonia texana                0            0.0         0.0
  Prosopis laevigata               20            0.6         2.0
Malpighiaceae
  Malpighia glabra                 20            0.1         1.2
Oleaceae
  Forestiera angustifolia          60            0.9         4.0
Rubiaceae
  Randia obcordata                  0            0.0         0.0
Rhamnaceae
  Condalia hookeri                 10            0.0         0.6
  Karwinskia humboldtiana          50            0.0         1.6
  Ziziphus obtusifolia              0            0.0         0.0
Rutaceae
  Amyris texana                    10            0.1         0.7
  Zanthoxylum fagara               70            0.5         3.3
Sapotaceae
  Sideroxylon celastrinum          20            0.1         1.0
Scrophulariaceae
  Leucophyllum frutescens           0            0.0         0.0
Ulmaceae
  Celtis pallida                   20            0.1         1.4
Zygophyllaceae
  Guaiacum angustifolium           10            0.1         0.8
Total                             2,370         15.6        100.0

                                             Clearcutting

                                Absolute      Absolute
     Family and species         abundance     dominance      IVI

Agavaceae
  Yucca filifera                    0            0.0         0.0
Boraginaceae
  Cordia boissieri                 210           0.4         3.7
  Ehretia anacua                    0            0.0         0.0
Ebenaceae
  Diospyros palmeri                30            0.0         4.7
  Diospyros texana                 330           0.3         0.5
Euphorbiaceae
  Bernardia myricifolia             0            0.0         0.0
  Croton torreyanus                220           0.2         2.2
Fabaceae
  Acacia berlandieri                0            0.0         0.0
  Acacia farnesiana                 0            0.0         0.0
  Acacia rigidula                 1,060          5.4        22.2
  Acacia schaffneri                 0            0.0         0.0
  Acacia wrightii                  290           1.6         7.1
  Eysenhardtia texana              90            0.4         2.0
  Havardia pallens                 160           1.1         5.1
  Mimosa monancistra              1,790          1.9        19.5
  Parkinsonia texana               30            0.6         2.0
  Prosopis laevigata               50            0.1         1.2
Malpighiaceae
  Malpighia glabra                 140           0.1         2.0
Oleaceae
  Forestiera angustifolia          20            0.2         0.9
Rubiaceae
  Randia obcordata                 30            0.0         0.5
Rhamnaceae
  Condalia hookeri                 110           0.2         1.6
  Karwinskia humboldtiana          20            0.0         0.5
  Ziziphus obtusifolia             140           0.1         1.7
Rutaceae
  Amyris texana                     0            0.0         0.0
  Zanthoxylum fagara              1,050          3.5        16.0
Sapotaceae
  Sideroxylon celastrinum          150           0.3         2.4
Scrophulariaceae
  Leucophyllum frutescens           0            0.0         0.0
Ulmaceae
  Celtis pallida                   310           0.6         4.2
Zygophyllaceae
  Guaiacum angustifolium            0            0.0         0.0
Total                             6,230         17.0        100.0

                                 Extensive production of livestock

                                Absolute      Absolute
     Family and species         abundance     dominance      IVI

Agavaceae
  Yucca filifera                   34            2.8         3.4
Boraginaceae
  Cordia boissieri                 251           1.8         3.6
  Ehretia anacua                    0            0.0         0.0
Ebenaceae
  Diospyros palmeri                 0            0.0         0.0
  Diospyros texana                 731           4.0         7.5
Euphorbiaceae
  Bernardia myricifolia           7,349          4.1        26.7
  Croton torreyanus                183           0.1         1.3
Fabaceae
  Acacia berlandieri               286           0.7         1.5
  Acacia farnesiana                11            0.1         0.3
  Acacia rigidula                 1,909          3.1        11.5
  Acacia schaffneri                 0            0.0         0.0
  Acacia wrightii                   0            0.0         0.0
  Eysenhardtia texana             1,520          1.2        11.5
  Havardia pallens                1,634          2.6         8.4
  Mimosa monancistra               23            0.0         0.2
  Parkinsonia texana               46            0.1         0.3
  Prosopis laevigata               11            0.0         0.2
Malpighiaceae
  Malpighia glabra                  0            0.0         0.0
Oleaceae
  Forestiera angustifolia          823           1.6         4.3
Rubiaceae
  Randia obcordata                 46            0.0         0.3
Rhamnaceae
  Condalia hookeri                 331           3.0         4.8
  Karwinskia humboldtiana          91            0.0         0.6
  Ziziphus obtusifolia             57            0.1         0.5
Rutaceae
  Amyris texana                    229           0.1         1.3
  Zanthoxylum fagara               423           1.0         2.6
Sapotaceae
  Sideroxylon celastrinum          217           2.4         3.3
Scrophulariaceae
  Leucophyllum frutescens          171           0.1         1.7
Ulmaceae
  Celtis pallida                   80            0.1         0.4
Zygophyllaceae
  Guaiacum angustifolium           526           1.0         3.6
Total                            16,983         29.8        100.0
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Author:Perez, Javier Jimenez; Rodriguez, Eduardo Alanis; Tagle, Marco Aurelio Gonzalez; Calderon, Oscar A.
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1USA
Date:Sep 1, 2013
Words:4173
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