THE EFFECTS OF GAP SIZE AND AGE ON THE UNDERSTOREY HERB COMMUNITY OF A TROPICAL MEXICAN RAIN-FOREST

1. The effects of gap age and size on understorey herb communities and their light environments (indirect skylight and direct sunlight estimated from hemispherical canopy photographs) were investigated in a tropical rain forest in Los Tuxtlas, Veracruz, Mexico. Thirteen gaps, ranging in size from 70 to 700 m2 and ranging in age from young (1-2 years old) to old (much greater than 5 years old), were studied. 2. The mean amounts of relative indirect skylight and relative direct sunlight were 0.083 and 0.116, respectively. Both kinds of light increased significantly with gap size and decreased significantly with gap age, but a significant interaction indicated that size was more important in new gaps than in intermediate-aged gaps. 3. Herbaceous communities were very diverse; 52 species were found in total. Most species occurred at low frequencies in most sites. The number of species per gap (mean = 15.2) was not affected significantly by gap age or size, although the Shannon - Weaver diversity index H' (mean = 2.1) was significantly highest in young gaps (mean = 2.4) followed by intermediate-aged gaps (mean = 2.1). 4. Total standing leaf area produced by herbs (mean = 0.795 m2 m-2) was significantly affected by both gap size and age; it was highest in intermediate-aged, medium-sized gaps (1.935 m2 m-2). 5. Hemiepiphytic aroids were numerically important at most sites. Terrestrial aroids and large monocots were also found to be important groups at many sites, not numerically, but by their relative contributions to total standing leaf area. 6. Species assemblages differed considerably between sites (mean proportional similarity, PS = 40.8%). Intermediate-aged gaps were more similar to each other (mean PS = 52-7%) than young gaps were to each other (mean PS = 35.3%). Mature forest sites were the least similar to all the other sites (mean PS = 28.9%). 7. Gaps of the same age and size were more similar in species assemblages than those that differed in size or age, but this trend was only of marginal significance. 8. There were reproductives at all sites; the highest relative number was in the intermediate-aged sites, not the bright sites, new gaps or young gaps. 10. The influence of gap size on the light environment is important in understanding the dynamics of the herbaceous communities. Gaps of intermediate age, the part of the forest that is in the 'building phase', are just as critical as the new-gap phase of the forest dynamics.