Welfia Research Articles

The Occurrence of Trichomanes godmanii (Hymenophyllaceae) on Welfia georgii (Arecaceae) at the La Selva Biological Station, Costa Rica

Field observations suggested that the epiphytic fern Trichomanes godmanii occurred more frequently and abundantly on the trunks of the palm Welfia georgii than on the trunks of dicotyledonous trees. We tested this observation statistically by randomly selecting 25 individuals of W. georgii and the nearest dicotyledonous tree of similar dbh, for a total of 50 trunks. For each trunk up to a height of three meters, we recorded the presence or absence of T. godmanii and, if present, we visually estimated percent cover using a ranked scale. We found that the fern occurred more frequently and abundantly on the palm than on dicotylendonous trees. No relationship was found between the diameter of the trunks and vegetative cover. This is one of the few host-specific preferences recorded among epiphytic ferns. We cannot fully explain why the fern occurs more frequently and abundantly on the trunks of Welfia georgii instead of dicot trees, but the fern's adhesive hairs on its rhizomes and petioles probably help attachment to the smooth trunk of the palm.

Edaphic and Human Effects on Landscape‐Scale Distributions of Tropical Rain Forest Palms

We studied the landscape—level spatial variation in distribution and abundance of seven species of canopy and subcanopy palms in a neotropical rain forest. Within 568 ha of nonswamp old—growth lowland forest at the La Selva Biological Station, Costa Rica, we sampled at 516 intersections of a reserve—wide grid to systematically assess the species' distributions across the major edaphic gradients. We found mosaics of community structure related to marked within—forest variability in both soil and topography. Total stem density of this guild varied with both edaphic factors. Steep sites had twice as many large palms (>10 m tall) per hectare than those on gentler topography or at lower slope positions. The combined density of subcanopy and canopy palms also varied significantly among soil types in the smallest size class (1—5 m tall), but not for larger individuals. Local (point) species richness of the subcanopy and canopy palms varied among soil types. Of the seven species, two (Astrocaryum alatum and A. confertum) were rare. All others showed significant edaphic variation in their distribution and/or estimated density. Two closely related species had strong and opposite edaphic associations. Euterpe macrospadix was biased toward steep topography and less fertile sites. Prestoea decurrens was nearly ommipresent on soil types with gentle topography while absent from half the points on soils with steep slopes. Two other closely related species, Iriartea deltoidea and Socratea exorrhiza, while virtually omnipresent across soil types and topographic positions, showed marked reciprocal variation in density between related soils. Iriartea's spatial distribution further indicates local removal of this species from one sector of the old growth by human harvesting (with subsequent apparent release of Socratea in this site). The most abundant species, Welfia georgii, while present at all sample points, showed significant among—soil variation in density. This substructuring of the arborescent palm guild results from the interplay of edaphic variation and past human activity. These findings and evidence from other sites suggest that marked spatial heterogeneity in community structure, at small to large scales (0.5—103 ha), may be general among tropical wet forests. Study of the spatial scales and causes of this variability will produce a more robust understanding of these complex ecosystems.

Differential Predation of Welfia georgii Seeds in Treefall Gaps and the Forest Understory

We report the results of an experimental study of predation by vertebrates on the seeds of the palm Welfia georgii in gaps and the forest understory of La Selva, Costa Rica. Seed survival in gaps and beneath fruiting adults was very low, and significantly less than survival in the understory 10 m from an adult. Thus, the increase in survival with dispersal of seeds away from the parent is habitat-dependent. This result is consistent with other studies in neotropical lowland wet forests, but contrasts with results from other geographical regions. The difference is likely caused by the differential responses of various seed predator species to treefall gaps.

Age-Size Relationships and Growth Behavior of the Palm Welfia georgii

Height growth rates and age-height relationships are reported for the abundant arborescent geonomoid palm Welfia georgii growing in tropical wet forest at La Selva, Costa Rica. Height increments over the 10-yr period range from 0.3 to 9.9 m, with a modal value of 2-3 m/10 yr. The data were analyzed using growth simulation, a stochastic bootstrapping technique that projects growth trajectories from periodic increment data. The number of years needed to attain the maximum height of 20 m from an initial height of 2.3 m ranges from 31 at the fastest growth rate to 80 at the slowest. The median value is 68 yr. Welfia palms appear to grow more or less continuously once they have reached 2.3 m. The wide variation in canopy conditions under which the present sample of surviving Welfia is growing indicates that the species tolerates a considerable range of light regimes. PALMS (LIKE OTHER TREES) in nonseasonal tropical environments seldom leave interpretable physical evidence of their growth history and age. Repeated measurements of tagged individuals over long periods are therefore necessary. Because palms lack a vascular cambium and do not show ordinary secondary stem growth, other variables must be measured to assess growth rates and infer age (see Piniero et al. 1977, Sarukhan et al. 1984). We report height growth behavior over a 10-yr period of the arborescent geonomoid palm Welfia georgii Wendl. (referred to hereafter as Welfia) and apply a stochastic growth simulation procedure (Lieberman & Lieberman 1985) that uses height increment data to project age-height relationships and longevity values for the species.

DEVELOPMENTAL ANATOMY OF THE STEM OF WELFIA GEORGII, IRIARTEA GIGANTEA, AND OTHER ARBORESCENT PALMS: IMPLICATIONS FOR MECHANICAL SUPPORT

Changes in stem anatomy with radial position and height were studied for the arborescent palms Welfia georgii, Iriartea gigantea, Socratea durissima, Euterpe macrospadix, Prestoea decurrens, and Cryosophila albida. Vascular bundles are concentrated toward the stem periphery and peripheral bundles contain more fibers than central bundles. Expansion and cell wall thickening of fibers within vascular bundles continues throughout the life of a palm, even in the oldest tissue. Within individual vascular bundles, the fibers nearest the phloem expand first and fiber cell walls become heavily thickened. A front of expanding fibers moves outward from the phloem until all fibers within a vascular bundle are fully expanded and have thick cell walls. Peripheral vascular bundles differentiate first and inner bundles later. In the stem beneath the crown, vascular bundles and ground tissue cells show little or no size increase, but marked cell wall thickening during development for Welfia georgii. Beneath the crown, diameters of peripheral vascular bundles increase more than twofold for Iriartea gigantea, while diameters of central bundles do not increase. In Iriartea stems, ground tissue cells at the periphery elongate to accommodate expanding vascular bundles and cell walls become thickened to a lesser degree than in fibers; central ground tissue cells elongate markedly, but cell walls do not become thickened; and large lacunae form between central parenchyma cells. For Iriartea, Socratea, and Euterpe, sustained cell expansion results in limited, but significant increases in stem diameter. For all species, sustained cell wall thickening results in dramatic increases in stem stiffness and strength.