Treefall Age Determination and Gap Dynamics in a Tropical Forest

(1) Most individuals of Astrocaryum mexicanum, a monopodial neotropical understorey palm, endure treefalls that form gaps in the forest, by bending under falling trees and limbs. After one year, a bent palm recovers vertical growth at its terminal meristem and forms a permanent kink in its stem. Previous detailed demographic studies have allowed us to determine with accuracy the passage of time, based on the age-constant rate of stem elongation of the palm. (2) Using this morphological feature of the palm, together with its high density (3001230 mature individuals ha-1) in a 5-ha tract of rainforest at Los Tuxtlas, Veracruz, Mx more than 50% of the quadrats suffered disturbance in the last thirty years and 28% suffered more than one disturbance in the last seventy years. These results provide evidence that canopy disturbances capable of promoting the release of suppressed seedlings and saplings of forest trees may occur at the small scale of some tens of square metres. (4) The long-term treefall patterns analysed (up to seventy years) show that canopy disturbances are a permanent ecological factor in the rainforest environment. Annual rainfall explains more than 50% of the annual variation of the proportion of forest opened to gaps per year. (5) The yearly disturbance does not indicate the gap availability for species regeneration: for example, a year of high disturbance (e.g. 6 1 % of the forest opened to gaps) may have a similar number of sizeable gaps suitable for pioneer regeneration as a year of low disturbance (e.g. 1 5% of the forest opened to gaps). (6) The results obtained in this study show that the gap-formation process operating at Los Tuxtlas forest promotes a strong temporally and spatially random variation in the physical environment of plants. This heterogeneity may be one of the factors involved in determining the high biological diversity found in most tropical rainforests.