Tree Canopy Composition in the Tropical Mountain Rainforest of Los Tuxtlas, Mexico

We studied the arboreal composition of the tropical mountain rainforest's upper canopy in the San Martín Pajapán volcano, Tatahuicapan, Veracruz, México (18°26' N; 94°17' W). Two forest stands were studied, one in an exposed position and one protected. The Shannon index of diversity and the Jaccard index of affinity were calculated to calculate affinities between plots and between stands of different environmental exposures. The average Shannon value was 3.391 +0.121 for the exposed zone and 3.511 +0.53 for the protected zone. There is a greater species number and tree density in the exposed stand. This difference might be caused be different orientation to dominant winds. The diversity index value is high is similar between the stands, despite the important difference in species composition. The Tropical Mountain Rain Forest (sensu Miranda and Hernández 1963) of Mexico has been studied from various scientific approaches analysing the floristic variations as related to environmental factors (Pérez 1967, Chavelas 1967), methodology aspects (Miranda et al. 1967) and types of management (Caballero 1978, Toledo 1978). There is significant knowledge about the diversity of trees in the hot-humid zones of Mexico and over the need There are various proposed hypotheses for explaining the high biological diversity of the tropical forest and many of these complement each other. Among the hypotheses put forward are found the effect of disturbances great stability, globally, of these environments (McArthur 1972). The objective of this study is to know the arboreal composition of the tropical mountain rainforest's upper canopy so as to contribute new data for understanding the vegetation diversity of the tropical forest. Study site: the study was undertaken in the high evergreen jungle in Mexico, in the San Martín Pajapán volcano, between 550 and 650 masl, which corresponds to the municipality of Tatahuicapan, Veracruz, México (18°26' N; 94°17' W). The soils are fine, reddish clay or laterite (Ríos Macbeth 1952) and acid in nature (pH 2.9-42). The dominant winds are the trade winds, originating in the North Atlantic and the dominant direction is north-easterly to the west 1572 or south. (García 1970). The slope exposed to the dominant winds allows for the entry of humid winds coming from the Gulf and also the cold northerlies, and prevents the passage of these winds to the protected opposite slope. The average annual temperature is greater than 20 °C and the average annual precipitation varies between 1 500 y 3 000 mm. The …

The Tropical Mountain Rain Forest (sensu Miranda and Hernández 1963) of Mexico has been studied from various scientific approaches analysing the floristic variations as related to environmental factors (Pérez 1967, Chavelas 1967), methodology aspects (Miranda et al. 1967) and types of management (Caballero 1978, Toledo 1978).There is significant knowledge about the diversity of trees in the hothumid zones of Mexico and over the need for their conservation (Carabias 1979, Wendt 1989, Dirzo and García 1992, Ochoa-Gaona et al. 2004, Mendoza et al. 2005).
There are various proposed hypotheses for explaining the high biological diversity of the tropical forest and many of these complement each other.Among the hypotheses put forward are found the effect of disturbances (Connell 1978;Whitmore and Burslem 1998;Dickinson et al. 2000), the different rate of seed predation (Janzen 1970, Connell 1971) and the great stability, globally, of these environments (McArthur 1972).
The objective of this study is to know the arboreal composition of the tropical mountain rainforest's upper canopy so as to contribute new data for understanding the vegetation diversity of the tropical forest.

MATERIALS AND METHODS
Study site: the study was undertaken in the high evergreen jungle in Mexico, in the San Martín Pajapán volcano, between 550 and 650 masl, which corresponds to the municipality of Tatahuicapan, Veracruz, México (18°26' N; 94°17' W).
The soils are fine, reddish clay or laterite (Ríos Macbeth 1952) and acid in nature (pH 2.9-42).The dominant winds are the trade winds, originating in the North Atlantic and the dominant direction is north-easterly to the west or south.(García 1970).The slope exposed to the dominant winds allows for the entry of humid winds coming from the Gulf and also the cold northerlies, and prevents the passage of these winds to the protected opposite slope.The average annual temperature is greater than 20 °C and the average annual precipitation varies between 1 500 y 3 000 mm.
The studied zone is located in the San Martín Pajapán volcano at an average altitude of 600 m.Two forest stands of 40 000 m2 (400 x 100 m) were studied, one being in an exposed situation and the other protected.They were some two kilometres apart.In each stand four plots were sampled, each of one hectare in area and arranged contiguously forming a strip.The sampling was undertaken between 1995 and 2000, following the methodology used by the majority of vegetation ecologists in Mexico, Central America and other places in the Neotropic (Miranda et al. 1967, Vazquez-Torres et al. 1995, Ramírez 1999).
Species of trees of a diameter greater than 20 cm at a height of 1.30 m above the ground were identified using characteristic flora (Rzedowski 1991, Vázquez Torres 2003) and the number of individuals of each species in each plot were counted.The approximately 2 000 botanical specimens herborized were deposited in the herbariums of the Instituto de Investigaciones Biológicas de la Universidad Veracruzana (CIB) and of the Instituto de Ecología (XAL).
The indices of diversity of Shannon (H´) (Whittaker 1972, Magurran 1988) and of affinity of Jaccard (Zar 1984) were calculated using the data obtained to calculate affinities between plots and between stands of different environmental exposures.The statistical analysis of t-Student (Abacus concepts 1996) was applied for a comparison of the averages of the studied variables between stands and plots.
In the exposed stand the total number of species was 101 whereas in the protected stand it was 83.Some 49 families were registered, 19 of which were found to be represented by a single species and 33% of the tree species belonged to five families.There were 69 species common to both zones (60%).There were 32 species exclusive to the exposed stand and 14 exclusive to the protected stand.A notable fact is that 16 of the 115 species of the upper canopy were found to be represented by a single individual, of these ten were located in the exposed stand and 6 were in the protected one (Annexe 1).
In the case of the exposed stand the species total was 101 and for each plot the results were 45, 52, 60 and 62 respectively for an average of 54.75 species per hectare.In the protected stand we found 83 species corresponding to 50, 53, 49 and 49 respectively giving an average of 50.25 species per hectare.
In terms of the diversity values, the average value of the index of Shannon was 3.391 ±0.121 for the exposed zone and 3.511 ±0.53 for the protected one.The statistical analysis doesn't show a significant difference between them.The variations between the values of the diversity indices obtained in each of the plots within each stand are very small and not statistically significant.
The index of Jaccard values between the plots within each stand varied between 0.295 and 0.469, with an average of 0.402 ±0.026.In the case of the protected stand the values varied between 0.471 and 0.571, the average being 0.516 ±0.017 (Table 1).In comparing the plots two by two within each stand, significant differences were observed between them, that being that the similarity between the plots in the protected stand was significantly greater than between those in the exposed one (Table 2).The distribution of the values between the 16 possible combinations of pairs between the plots in the exposed and protected varied between 0.346 and 0.521, the average being 0.431 ± 0.012.

DISCUSSION
There is a great concentration of vegetation richness in these forests with very different species numbers between plots in a single stand and between zones of different exposure.However, there is a greater species number and density of trees in the exposed stand compared to the protected one.One of the reasons for this difference could be caused be different orientation to dominant winds.The preponderance of any of these factors over others would demand detailed studies of the dynamics of the plots in different environmental conditions.On the other hand, the diversity index value is high and is similar between the stands, which indicates that the diversity pattern is similar in dif-  3).In comparison with some zones in Amazonia in Brazil (dry land jungles) (Black et al. 1950) they are similar in terms of species richness but the number of individuals is greater in the Mexican zones (Table 3).

ACKNOWLEDGMENTS
We thank Jacqueline Campos Jimenez and Alfredo Cruz Perez, for their valuable collaboration in integrating and using all the information; as well as Marceline Ortega Quezada, for his tremendous help during the field work.

ANNEXE 1
Tree species, indicating the families to which they belong by alphabetical order of family. The

TABLE 1
Values for the indices of similarity of Jaccard for pairs of plots within each stand Values for the indices of similarity of Jaccard by pairs of plots between stands: exposed and protected Comparison of the results obtained in this study with others undertaken in various mountain rainforests of Mexico and the New World tropics total number of individuals in the exposed and protected stands are indicated