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 degrees 26' N; 94 degrees 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.