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Treefall Age Determination and Gap Dynamics in a Tropical Forest

Miguel Martinez-Ramos, Elena Alvarez-Buylla, Jose Sarukhan and Daniel Pinero
Journal of Ecology
Vol. 76, No. 3 (Sep., 1988), pp. 700-716
DOI: 10.2307/2260568
Stable URL: http://www.jstor.org/stable/2260568
Page Count: 17
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Treefall Age Determination and Gap Dynamics in a Tropical Forest
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Abstract

(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, base don the age-constant rate of stem elongation of the palm. (2) Using this morphological feature of the palm, together with its high density (300-1230 mature individuals ha-1) in a 5-ha tract of rainforest at Los Tuxtlas, Veracruz, Mexico, it was possible for the first time to apply a time scale to the study of gap formation and regeneration dynamics of tropical rainforests. Surviving palms, after being knocked down or bent by treefalls, record gap=forming events for up to 100 years. By using this technique, the age structure of the forest mosaic was obtained at a scale of 25 m2. (3) A high variation in both temporal turnover rates (mean ± S.D. = 47 ± 45 years) and spatial occurrence of treefall disturbances was found; 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 spalings of forest trees may occur at the small scale of some tens of squares 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 gaps0 may have 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.

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