DEMOGRAPHY AND ALLOMETRY OF CECROPIA-OBTUSIFOLIA, A NEOTROPICAL PIONEER TREE - AN EVALUATION OF THE CLIMAX PIONEER PARADIGM FOR TROPICAL RAIN-FORESTS

被引:175
作者
ALVAREZBUYLLA, ER
MARTINEZRAMOS, M
机构
[1] Centro de Ecologia, Universidad Nacional Autonoma de Mexico, Aparatado 70-275
关键词
LIFE HISTORIES; PIONEER SPECIES; TREEFALL GAPS AND PATCH DYNAMICS; UNPREDICTABLE ENVIRONMENTS;
D O I
10.2307/2261011
中图分类号
Q94 [植物学];
学科分类号
071001 ;
摘要
1. Demography and allometry of the dioecious tree Cecropia obtusifolia were studied in a 5-ha permanent plot at Los Tuxtlas rain forest, Mexico. Treefall gaps were aged and a significant positive correlation was found between gap age and tree size. Active regeneration of the species was suggested by the high proportion (>35%) of young individuals. The population was highly clumped among young juvenile trees and became less aggregated as density within-gaps decreased among older trees. New and successful recruitment was restricted to recently formed (<3 years since opened) and large (>100-m2) gaps. Oldest individuals (35 m high) were growing in 37-year-old gaps. 2. Tree height and stem diameter increased allometrically with a slope <1.5 in trees >0.3 m tall. The margin between the actual tree diameter (d) and the theoretical minimum buckling diameter (d(min.)) declined exponentially from c. 20 in juvenile trees to almost 1 in the oldest trees (>30 years old). Whilst the number of leaves remains fairly constant before branching, it increases linearly as the number of branches increase. Changes in area per leaf during the life cycle yield an overall allometric pattern between tree height and total leaf area. 3. Seed and seedling mortality was >99%. Reduced light, falling debris, movement of rotting logs and soil dryness were important mortality factors in pre-reproductive stages. Oldest adults died standing, apparently due to senescence. Maximum life expectancy is reached when trees attain a non-suppressed canopy position (c. 20 m high). Individuals showed high growth rates (a stem-diameter increment of up to 2cm year-1). Growth-rate variation in juvenile stages seem to be related to high environmental heterogeneity. In adult stages the number of conspecifics growing in the same gap seems to affect diameter growth rate negatively. First reproduction occured at about 3 years of age (when 10cm in diameter) and increased with age. Annual fecundity rates of individuals ranged from 1.4 x 10(4) to 1.4 x 10(7) seeds and increased allometrically with stem diameter and number of branches. 4. The most important environmental filter in the life-cycle of C. obtusifolia operates from the seedling to the juvenile stage. This finding does not support the suggestion that pioneer limitation to gap regeneration is determined during germination. The allometric and demographic traits of C. obtusifolia correspond to those expected for a shade-intolerant species that colonizes ephemeral sites, and they contrast with those of purported shade-tolerant tree species reported in the literature. However, rather than finding two clearly distinguishable types of life-histories, a continuum of demographic life-cycles was found. Therefore, the dichotomous pioneer-climax framework should be viewed as a means of identifying the extremes of a continuum of tree life histories.
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页码:275 / 290
页数:16
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