Study area

This research was conducted in 2010. The forest management unit (UMAF-2702ST) is located in the region of the southern rivers from Tabasco, at 17° 13' 22'' and 17° 28' 34'' north latitude, 90° 57' 08'' and 91° 39' 02'' west longitude: bordered in the north by the municipality of Balancán, south with the state of Chiapas and the Republic of Guatemala, east with Guatemala, west with the municipalities of Emiliano Zapata, Tabasco and Chilon, Chiapas (Figure 1). It was formed by 43 communities, Arena de Hidalgo, Redencion del Campesino, Ignacio Allende, Alvaro Obregon, Rancho Grande and Lic. Adolfo Lopez Mateos are the largest population centers, but does not exceed 2 500 inhabitants (CONAPO, 2000).

Figure 1 Geographical location of the study area. 

Weather

According to Koeppen classification system, modified by Garcia (1964), the UMAF-2702ST has a warm humid climate with rainfall throughout the year identified as Af (m) w” (i') g, with an average annual rainfall higher than 2 000 mm, with dog days during the months of Julio-August .

Soils and topography

The predominant soils in the UMAF-2702ST are Fluvisols, Cambisols, Leptozols, Luvisols, Arenosols, Vertisols and Gleysols; likewise, the altitude varies from 200 to 1 000 masl (CONAFOR, 2011). This variation in altitude creates three physiographic areas. The mountain area is characterized by a landscape of domes and conical hills and uneven landform with altitude from 60 to 1 000 masl. The altitude increases as it moves towards south from the ejidos los Rieles de San José to the ejido de Corregidora Ortiz de Domínguez.

Lowhill area is characterized by having smooth hillocks from 20 to 60 masl; it has slopes that have favored superficial erosion problems. This area begins from ejido Redencion del Campesino, Francisco Villa, Ignacio Allende, to the ejido San Francisco which borders with the Republic of Guatemala. The valley area, is characterized for having a concave relief, this area is located in the lower parts of the UMAF-2702ST Sierra and corresponds to the ejidos San Carlos, Sueños de Oro, to the ejido Carlos Pellicer Camara.

Seed collection

The seeds used in this study were obtained from mahogany fruits harvested in the UMAF-2702ST; the fruits were obtained from trees with higher phenotypic characteristics Straightness, live crown base, Crown Length and Health, two fruits of the middle part from the crown were collected, from each of the five phenotypically outstanding trees. The fruits were transported in Kraft paper bags, to the germplasm bank from CONAFOR Tabasco. For the benefit, the fruits were placed in partial shade during 3 days for 4 h and waited for fruit to open.

Seed characterization

Length and width of the seeds were determined through X-ray Faxitron (MX-20, Faxitron X-ray Corporation, Wheeling, IL, USA), calibrated to 120 s and 26 kv power ( kVp) (^{Goodman et al., 2005}) (Figure 2). Then proceeded to plant 30 seeds in germination trays of 18 x 12.5 cm; three trays per tree (90 seeds) were placed. The substrate was sand in disinfected in furnace Fischer Scientific^® series 500, at 150 °C for 2 h (ISTA, 1996). The trays were placed in a germinating Seedburo^® make, model MPG-300 programmed at 30 ± 2 °C and a photoperiod of 16 hours light and 8 hours dark, relative humidity of 75%.

Figure 2 Frequency histogram from mahogany seed size. 

The germination capacity was made for 15 days, assessed every 15 days from the twentieth day when it appeared the first radicle, and until 60 days that is the maximum time reported for seed germination in this species. The germination percentage was evaluated to determine the relationship between size and seed germination.

Statistic analysis

The data for each variable were examined through ShapiroWilk test modified to determine its normal distribution and homogeneity of variances required for analysis of variance (ANOVA). ANOVA was conducted taking the communities from UMAF-2702ST as dependent variables from collected seeds, and as an independent variable seed length and width; then a mean test was applied, Tukey with an alpha 0.05. Pearson correlation coefficient between the variables seed length, width and seed germination was calculated. Descriptive statistics were calculated and made frequency histograms for each variable, which were analyzed with SAS.

Mahogany seed size gave a normal distribution. The most common seed length was 16.5 - 18.5 mm with a record from 119 seeds (28%) of the total; seed width had a similar behavior to length, while class interval between 8.59-9.49 mm was the most frequent with 138 seeds (33%) of the total (Figure 2).

As shown in Table 1, the highest values of seed length and width were found in San Marcos (SM) and Santo Tomas 2 (ST2), respectively, while the minimum in Santo Tomas (ST). Also, it was observed a CV lower to (13%), for each of the variables evaluated.

Table 1 Descriptive statistics for mahogany seed size, collected in communities from UMAF-2702ST. 

Comunidades de la UMAF-2702ST. Longitud de semilla (L); ancho de semilla (A).

Based on square means and means comparison test, there were statistically significant differences for the variables seed length and width, being for both treatments "Francisco Villa" and "San Marcos" the ones with higher average length and width (Table 2).

Table 2 Descriptive statistics of seed size with seed germination for mahogany. 

Comunidades de la UMAF-2702ST. N= 90.

Germination

As shown in Table 1, the highest values for mahogany seed germination were collected in Boca del cerro (BC), Santo Tomás 2 (ST2) and San Marcos (SM) respectively, while the minimum germination was found in Francisco villa (BC) (Table 1). Table 2. Only shows one variable.

As for seed germination, mean squares showed statistical different between origins. Based on means comparison, the F test indicated statistical differences in germination among origins for all three sampling dates (p= 0.0008, 0.002 and 0.004, respectively). The seeds from Santo Tomás showed a good germination response to the first sampling date. For the second date, the same trend continued for the last date of evaluation. On this date, the seed from Boca del Cerro just reached 63% germination (Figure 3).

Figure 3 Mahogany germination percentage from five sources. 

Boca del Cerro (BC), Francisco Villa (FV), San Marcos (SM), Santo Tomas (ST) communities from UMAF-2702ST.

Seed size and seed germination did not correlate. Similarly, there was no correlation between seed length and width (r²= 0.9, p= 0.009).