Seasonal Cambial Activity for Pentaclethra, Goelthalsia, and Carapa Trees in a Costa Rican Lowland Forest

Abstract

At the La Selva Biological Station, Costa Rica, dendrometer bands were read monthly for a year to monitor the seasonality and rate of girth increment for three common tree species: Pentaclethra macroloba (Mimosaceae), Goethalsia meiantha (Tiliaceae), and Carapa guianensis (Meliaceae). Annual diameter increment varied with tree size but was greatest for Goelthalsia (1.26 cm/yr), least for Carapa (0.04-0.34 cm/yr), and intermediate for Pentaclethra (0.20-1.06 cm/yr). Cambial activity for Pentaclethra was greatest during drier months (December to April), the only months when Pentaclethra was not in flower or fruit. In contrast, Goelthalsia and Carapa had less cambial activity during drier months (April and May), the time of leaf flushing for these species. Greater growth during the drier months, a characteristic not shared by relatively common Goelthalsia and Carapa trees, may provide a competitive advantage for Pentaclethra and help explain its abundance at La Selva. PENTACLETHRA MACROLOBA (Willd.) Kuntze (Mimosaceae) is the dominant tree species in all three of the major primary forest types of the La Selva Biological Station, Costa Rica. The importance values for Pentaclethra in these forests range from 18 to 23 percent (Hartshorn 1983a). For Pentaclethra to exert this degree of dominance in a speciesrich, Neotropical lowland suggests that this species competes well with over 450 tree species also native to La Selva (30-135 m elevation). Hartshorn (1983b) attributed the success of Pentaclethra to: (1) excellent seedling establishment in the absence of serious seed predation (toxic seed); (2) tolerance of relatively infertile soils that are probably restrictive to potentially competing species; (3) well-distributed and abundant rainfall that enables a swamp species, such as Pentaclethra, to extend to the ridgetops; and (4) shade tolerance that enables Pentaclethra to survive and even grow in the dense primary forest understory. In addition to these characteristics, Grime and Hunt (1975) suggested that dominance by one species is sometimes explained by differences in timing and rate of growth between the community's dominant and competing species. Also, Pentaclethra's water utilization patterns, evidenced by times of tree growth, may be different from those of competing species, as suggested for different tree species in a North American forest (Bunce et al. 1977). To examine this possibility at La Selva, the seasonal girth increment of the dominant Pentaclethra was compared with the girth increments of Goelthalsia meiantha (D. Sm.) Burret (Tiliaceae), a common gap-phase and secondgrowth species, and Carapa guianensis Aubl. (Meliaceae), a codominant, shade-tolerant, swamp species. MATERIALS AND METHODS Study areas were the 4-ha permanent inventory plots I and II at La Selva (Hartshorn 1983a). From 88 tree species in plot I, Pentaclethra is the dominant (importance value of 23%) and Goelthalsia is sixth in importance value at 2.55 percent. From 115 tree species in Plot II, Pentaclethra and Carapa are codominants (importance values of 18.1 and 11.3%, respectively). Plot I soil is fairly well-drained, alluvial material of the La Selva soil series, a dominant soil type in the area. Plot II is also on old alluvium, but has more poorly drained soils of the Swampo series (Bourgeois et al. 1972). Mean annual rainfall at La Selva is 3991 ? 748 mm (N = 22). Although nearly all canopy trees are in leaf throughout the year, the driest months of March and April are associated with maximum leaf drop and leaf flushing. A total of 14 Pentaclethra, 6 Carapa, and 3 Goelthalsia trees were selected for study. In Plot I, 3 large Pentaclethra trees (40-70 cm DBH), 8 small Pentaclethra trees (5-40 cm DBH), and 3 small Goelthalsia trees (1015 cm DBH) were banded. In Plot II, 3 Pentaclethra trees (30-40 cm DBH), 4 small Carapa trees (5-20 cm DBH), and 2 larger Carapa trees (30-45 cm DBH) were banded. All smaller trees (<25 cm DBH) were in recent treefall gaps, but their crowns were shaded most of the day by canopy trees. At least a portion of the crown of each larger individual reached to the main canopy. In September 1976 aluminum dendrometer bands, similar to those described by Woodman (1980), were installed on each of the 23 study trees. Bands were secured on tree boles at a height of 1.3 m by stainless steel expansion springs. Increments in girth were recorded for 13 months (near the beginning of each month) by reading vernier scales on each band. The initial interval reading I Received 14 October 1985, revision accepted 8 May 1986. BIOTROPICA 19(4): 357-36