Height and stem diameter relationships for dicotyledonous trees and arborescent palms of Costa Rican tropical wet forest. Bull. Torrey Bot. Club 113:241-246. 1986.-Allometric relationships between stem diameter and height were studied for dicotyledonous trees and arborescent palms in a tropical wet forest of Costa Rica. In a mixed population of dicotyledonous trees, stem diameter varies with the 3/2 power of height. The climax forest tree Pentaclethra macroloba (Willd.) Kuntze. appears to have a greater margin of safety against mechanical failure than the faster growing tree Pourouma aspera Trecul. This is consistent with Pourouma's shorter life span and narrower crown. As the arborescent palms Welfia georgii Wendl. ex Burret and Socratea durissima (Derst.) Wendl. grow in height, the margin of safety against mechanical failure decreases and/or the stem tissue stiffness and strength increases. Welfia shows little capacity to increase stem diameter during height growth. Socratea shows major stem diameter increase during height growth, but not enough to maintain elastic or geometric similarity. The tallest individuals of Socratea exceed McMahon's (1973) theoretical buckling limit for dicotyledonous trees. This is consistent with the observation that tall palms have stronger, stiffer stem tissue and narrower crowns than dicotyledonous trees. Differences in allometry of height and stem diameter indicate differences in stem tissue mechanical properties, the margin of safety against mechanical failure, and/or crown weight; however, we generally can not distinguish the relative importance of these three possibilities on the basis of studies of height and stem diameters alone.
The genus Bidens (Asteraceae) has undergone extensive adaptive radiation on the Hawaiian Islands. The 19 species and eight subspecies endemic to Hawaii exhibit much more morphological and ecological differentiation than the continental members of the genus. However, the Hawaiian taxa have the same chromosome number and retain the capacity to interbreed in all possible combinations. Twenty-two populations of 15 Hawaiian taxa and four populations of American taxa were compared at 21 loci controlling eight enzyme systems. Populations of Hawaiian taxa are highly polymorphic. However, little genetic differentiation has occurred among taxa in spite of the high levels of genetic variability. Genetic identities calculated for pairs of populations show that populations of the same taxon are genetically more similar than are populations belonging to different taxa, but all values are high. The level of genetic differentiation that has occurred among the species of Hawaiian Bidens is comparable to the level of genetic differences found among populations within single continental plant species. Moreover, there is no correlation between the isozyme data and morphological data. No groups of taxa are evident in the genetic data, although morphological groups exist. Genetic differentiation at isozyme loci has not occurred at the same rate as the acquisition of presumably adaptive morphological and ecological characters in Hawaiian Bidens. Adaptive radiation may be limited to a few genes controlling morphological and ecological characters.
Detailed observations and experimental studies of the reproductive biology of 12 boreal forest herbs were conducted over a 3-year period (1978–1980) in spruce–fir forests of central New Brunswick. The species examined were Aralia nudicaulis, Chimaphila umbellata, Clintonia borealis, Cornus canadensis, Cypripedium acaule, Linnaea borealis, Maianthemum canadense, Medeola virginiana, Oxalis montana, Pyrola secunda, Trientalis borealis, and Trillium undulatum. All taxa are insectpollinated perennials and most exhibit clonal growth. Floral syndromes of the understory community are relatively unspecialized with many species possessing small white or green flowers. A total of 103 taxa of insects were collected from flowers during the 1979 season. Bombus spp. are the major pollinators of 5 of the 12 species. Syrphid flies, bee flies, and halictid and andrenid bees were also commonly observed. Controlled pollinations were undertaken to determine the breeding systems of herbs. Bagged, self-, cross- and open-pollinated treatments were used to investigate the capacity for self-pollination, compatibility status, and factors influencing fecundity in each species. A diversity of reproductive systems was revealed. Six species are completely dependent on insects for pollination, four species are weakly autogamous, one is strongly autogamous, and one appears to be apomictic. Comparisons of fruit-set and seed set from controlled self- and cross-pollinations indicate that four species are strongly self-compatible, one is dioecious, and the remainder display varying degrees of self-incompatibility. Despite this variation, outbreeding appears to be the most common reproductive mode. In several species there is evidence that low pollinator service limits fruit-set.
Two experiments were designed to examine jointly the roles of maternal effect, genetic load, and other genetic variability at nuclear loci in determining offspring fitness in Lupinus texensis, an outbreeding annual herb. A diallel crossing design was used to generate seeds in the greenhouse. This design directly compares maternal and paternal contribution to offspring survival, growth, and flowering, and also assesses the role of genetic load because selfs are included in the crossing design. A nested crossing design was used to generate seeds at a field site. This design addresses the contribution of parental plants to their offspring under natural conditions. Maternal effects occur throughout the life cycle, but dominate the early growth of offspring in L. texensis. Inbreeding depression also occurs throughout the life cycle, but primarily affects size and survival of offspring. Overall inbreeding depression is substantial (8 = 0.66), exceeding the twofold threshold required for maintenance of an outcrossing mating system. However, lineages vary in the stages at which inbreeding depression occurs. Moreover, extreme heterogeneity was observed among lineages, with inbreeding depression ranging from negligible (8 = 0.02) to complete (8 = 1.00). This heterogeneity could allow the invasion and maintenance of selfing genotypes in natural populations of L. texensis.
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