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Diameter growth rates and age-size relationships are reported for 45 abundant tree species and one liana in tropical wet forest at La Selva, Costa Rica. Thirteen-year increments in each species were analysed using growth simulation, a stochastic technique which projects growth trajectories.Median growth rates ranged from 0.35 mm yr" 1 (Anaxagorea crassipetala) to 13.41 mm yr" 1 ) (Stryphnodendron excelsum). Maximum ranges ranged from 0.95 mm yr" 1 (Quararibea bracteolosa) to 14.62 mm yr' 1 (Hemandia didymanthera). Minimum rates ranged from zero growth (Capparis pittieri, Colubrina spinosa, Doliocarpus spp.) to 7.45 mm yr" 1 (Stryphnodendron excelsum).Projected lifespan (from 100 mm dbh to the maximum dbh for the species) varied from 52 years (Anaxagorea crassipetala, Guatteria inuncta) to 442 years (Carapa guianensis). The mean longevity among the 45 tree species studied is 190 years.Four main patterns of growth behaviour are recognized, based upon longevity and growth rates: (1) understorey species have slow maximum growth rates and short lifespans; (2) shadetolerant subcanopy trees live around twice as long as understorey trees and grow at approximately the same maximum rates; (3) canopy and subcanopy trees that are shade-tolerant but respond opportunistically to increased light levels have long lifespans and fast maximum growth rates; (4) shade-intolerant canopy and subcanopy species are short-lived and have fast maximum growth rates. Understorey species intergrade with shade-tolerant subcanopy species in terms of growth behaviour; shade-tolerant subcanopy species with opportunistic, shadetolerant species; and opportunistic, shade-tolerant with shade-intolerant species.Intraspecific variation in growth rates is lower in short-lived trees (understorey species with uniformly slow growth and shade-intolerant species with uniformly rapid growth) than in the two long-lived groups. These patterns are discussed in the context of tree ecophysiology and forest light environments.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology. SUMMARY(1) Mortality over a 13-year period was determined for all stems (?10-cm dbh) on 12.4 ha of primary lowland wet tropical forest at La Selva, Costa Rica. Altogether 23.2% of 5623 trees and lianas present in the initial inventory had died by the time of the subsequent inventory.(2) Mortality rates were independent of size among individuals ?10-cm dbh, and did not differ between buttressed and non-buttressed stems.(3) Of the dead individuals, 26% died standing, 31% had fallen, 7% were found buried under treefalls, and 37% had decomposed entirely, leaving no trace.(4) Mortality was nearly balanced by recruitment into the 10-cm dbh class; there was a net loss of 1 *7% of stems in 13 years.(5) La Selva appears to be among the most dynamic of tropical forests studied to date, with an annual loss of stems of 2.03% and a consequent stand half-life of 34 years.

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In Costa Rican tropical wet forest the distribution of seedlings of most tree species colonizing 51 tree-fall gaps is clumped, probably because of differences in the proximity of gaps to fruiting trees. Different gap zones (root, bole and crown) are more similar to one another, in terms of their species composition, than they are to other zones in the same gap. This nonrandom pattern is established soon after gap formation, indicating that mortality of young seedlings is both high and related to gap zones. Some tree species are strongly positively associated with one of the gap zones, whereas others are associated with the age of the gap at the time the census was taken or the species of tree whose fall caused the gap. Root zones are dominated by fewer species than are bole or crown zones. Results are consistent with the assumption that an initial random distribution of seedlings is quickly changed to a strongly non-random pattern by selective mortality of seedlings of different tree species in the different gap zones. The internal heterogeneity of gaps is probably one of the factors helping to maintain the high tree species richness characteristic of tropical wet forests.

An index of canopy closure was used to estimate closure above the crowns of all trees ≥ 10 cm dbh in 11.11 ha of undisturbed lowland tropical forest at La Selva, Costa Rica. To correct for the efFects of tree size on canopy closure, we used the residual of the regression of the canopy closure index on tree size. Analyses were carried out for the 104 species which had ≥ individuals; a total of 3224 trees were included. Nine species were found to have their crowns in significantly more open conditions than expected by chance and five species were found to have their crowns in significantly more closed conditions than expected by chance (P < 0.05). The remaining 90 species (86.5% of the assemblage) were distributed at random with respect to canopy closure, occupying the available light conditions indiscriminately. Species occurring under higher light levels did not show a narrower range of tolerance than did other species. Most species were found to occur over a substantial proportion of the canopy closure continuum present in the stand; overlap among the great majority of species in the assemblage is extensive. The results support the view that tropical forests comprise assemblages of generalist tree species, and raise questions about the classic notions of gap-phase dynamics.

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