Yan B. Linhart

Genetic variation in space and time in a population of ponderosa pine

SummaryThe genetic structure of a population of Pinus ponderosa was studied using seven electrophoretically-detectable protein loci. This population is composed of six groups of trees, which differ significantly from each other in their genetic constitutions. The results imply that the population is composed of groups of individuals assembled into genetically-related family units. Whenever such family clusters have been documented in other studies, marked inbreeding has also been noted as a consequence. In this population, no inbreeding is detectable. The groups also differ from one another in several characteristics which have genetic consequences within the population as a whole. These characteristics include age structures, seed output, level of infestation by woolly aphids and extent of damage by deer browsing. The population was also divided into four age classes which do not differ markedly from one another in their genetic characteristics. This suggests that genetic differentiation in time is much less marked than differentiation in space.

The evolution of bird-dispersed pines*

SummaryTwenty of approximately 100 species of pines (Pinus spp.) have wingless seeds, and 19 of these are in the subgenusStrobus. Eight of the wingless-seedStrobus pines are known to be dispersed by seed-storing corvids, particularly the nutcrackers (Nucifraga spp.), and the other 11 are presumed to be. The principal consequences of these nearly obligate bird-pine mutualisms include tree clustering and a population structure that differs from that of wind-dispersed pines. The wingless-seedStrobus pines in general are typified by ranges that include xeric conditions and/or high elevations, and large seeds, which are considered to be adaptive under either xeric or competitive conditions. The proposed evolutionary scenario for bird dependency begins with the distribution of ancestralStrobus pines into high elevation or semi-desert environments, sympatric with one or more seed-storing corvid forms, and an increase in seed size. We propose that dependency on birds for seed dispersal has occurred primarily in subgenusStrobus, becauseStrobus pines tend more towards winglessness and increased seed size in stressful environments than doPinus pines. Seed winglessness and other bird-pine traits probably arose from a combination of genetic drift in small populations and selection by corvids.

Ecological and Behavioral Determinants of Pollen Dispersal in Hummingbird- Pollinated Heliconia

Hummingbirds in Costa Rica exploit nectar resources using alternative strategies: some defend a feeding territory while others range widely for their food. In Amazilia spp., both sexes are territorial. In Thalurania furcata, Chalybura urochrysia, and Florisuga mellivora, males are territorial, but not females. In Phaethornis spp., Threnetes ruckeri, and Glaucis hirsuta, both sexes range widely. One of the primary nectar sources for humming-birds in Costa Rica are species of the genus Heliconia (Musaceae). The birds appear to be the primary pollinators of Heliconia. Heliconia species differ in their daily flower output, habitat preferences, and dispersion patterns. Heliconia tortuosa and H. acuminata usually have one flower per inflorescence per day, are forest species, and grow very dispersed in small groups of less than 20 inflorescences. Thus, the nectar they provide is very scattered. Heliconia imbricata, H. latispatha, and H. curtispatha have several flowers per inflorescence per day, are forest-edge species, and grow in large clumps or stands which can number several hundred inflorescences. Consequently, the nectar they provide is concentrated in large quantities in small areas. The dispersed forest species of Heliconia were fed upon by nonterritorial birds, whereas the clumped forest-edge species were fed upon primarily by territorial birds. This interaction may be advantageous to both organisms by lowering the competition for food among birds and the competition for pollinators among the plants. Movement of labeled pollen was extensive in the dispersed populations of forest species and much more restricted in the dense populations of forest-edge species. Possible consequences of these patterns of pollen dispersal for population structure are discussed. Interspecific pollen movement was observed. Heliconia latispatha pollen was found on H. acuminata flowers, and vice-versa. Heliconia imbricata pollen was found on H. latispatha, and putative hybrids of these two species were common in one area.

QUALITATIVE AND QUANTITATIVE VARIATION IN MONOTERPENE CO-OCCURRENCE AND COMPOSITION IN THE ESSENTIAL OIL OF THYMUS VULGARIS CHEMOTYPES

Thymus vulgaris has a chemical polymorphism with six different chemotypes that show marked spatial segregation in nature. Although some populations have a single chemotype in majority, many have two or three chemotypes. In this study we analyze the quantitative variation among T. vulgaris populations in the percentage of oil composed of the dominant monoterpene(s) for each chemotype. In general, phenolic chemotypes (thymol and carvacrol), which occur at the end of the biosynthetic chain, have a significantly lower proportion of their oil composed of their dominant monoterpene than nonphenolic chemotypes (geraniol, α-terpineol, and linalool). This is due to the presence of high amounts of precursors (γ-terpinene and paracymene) in the oil of phenolic chemotypes. The essential oil of the nonphenolic thuyanol chemotype has four characteristic monoterpenes that together make up a lower proportion of the oil than the single dominant monoterpene of the other nonphenolic chemotypes. For all chemotypes, the percentage composition of the dominant monoterpene decreased significantly at sites where the chemotype is not the majority type. This decrease is correlated with a significant increase in either the proportion of the two precursors for the thymol chemotype or the monoterpenes characteristic of the other chemotypes at the site. The latter result suggests that a plant with dominant genes is responsible for the production of different monoterpenes can produce several molecules.

Observations on the genetic structure and mating system of ponderosa pine in the Colorado front range

SummaryVariation of peroxidase enzymes is analyzed both in mature needle tissue and in open-pollinated seedling families of ponderosa pine, Pinus ponderosa, and is identified as being controlled by a single Mendelian locus. Variation at this locus, analyzed in 1, 386 individuals, is used in the analysis of population differentiation and the mating system. Significant variation of gene frequencies is detected over distances of several hundred meters, and is found to be associated with slopes of different aspects. Ponderosa pine is wind-pollinated, and an analysis of the mating system indicates that the level of outcrossing is greater than 90 %. Selection specific for different environments is evidently strong enough to overcome the homogenizing force of migration and produce population fissuring in ponderosa pine.

DIRECT AND INDIRECT ESTIMATES OF SEED VERSUS POLLEN MOVEMENT WITHIN A POPULATION OF PONDEROSA PINE

We examined the spatial distribution of maternally inherited mitochondrial DNA and paternally inherited chloroplast DNA polymorphisms in a permanently marked stand of ponderosa pine (Pinus ponderosa Laws). Movement of maternally inherited mtDNA occurs only via seed dispersal, and mtDNA haplotypes showed significant patch structure. Moreover, individuals within patches identified by mtDNA haplotypes were related approximately as half‐sibs based upon analysis of allozyme genotypes. Thus, seed dispersal is limited within the population, and creates matrilineal clusters in space. By contrast, paternally inherited cpDNA is dispersed by movement of both seed and pollen. Chloroplast DNA polymorphisms showed no evidence of patch structure, but rather a weak (and nonsignificant) trend toward hyperdispersion, suggesting nearly unlimited movement of pollen among trees within this stand. Two of the trees had unique allozyme alleles, which were used to directly measure pollen movement away from those trees. Marked pollen was as likely to disperse across the population as it was to fertilize near neighbors.

Disease, parasitism and herbivory: Multidimensional challenges in plant evolution

Plants provide feeding sites for a broad range of parasites, commensals and symbionts. In the process, they can be subjected to selection whenever feeding choices are based upon heritable traits of the plants and the feeding affects plant fitness. The outcome of such selection depends on the correlation between choices made by various taxa. Recent work suggests that this multispecies selection, although common in natural communities, now needs to be incorporated more fully into ecological and evolutionary perspectives.

Sexual Dimorphisms and Response to Low Resources in the Dioecious Plant Silene latifolia (Caryophyllaceae)

Females of dioecious species often have higher reproductive effort than males because of their production of fruits. We postulated that females of Silene latifolia would reduce growth or reproductive effort to a greater extent than males when grown under conditions of low resource availability. To test this hypothesis, plants were grown in pots divided into 11 treatments: high levels of resources (controls) and medium or low levels of light, water, nitrogen, phosphorus, and potassium. There were more females than males in all of the treatments but there was no variation in sex ratios among treatments. Females produced larger, but fewer, flowers than males. The number of flowers in both sexes was correlated with the values of many sexually dimorphic traits, including plant height, length of inflorescence branches, total biomass, reproductive biomass, and percent allocation to leaves, branches, and reproduction. Females had higher reproductive effort than males in all treatments and invested more biomass per length in their main stem. Females also invested more biomass per length in their inflorescence branches, probably as a consequence of branch architecture. Phenotypic correlations, plant morphology, and growth varied in response to the different environmental treatments, but males and females did not respond differently to low resource availability. Two hypotheses are presented to explain these results, which are contrary to the expectation that females of dioecious species are more adversely affected than males when resources are limiting.

Density-dependent seed germination strategies in colonizing versus non-colonizing plant species.

Germination is a critical stage in the life of a plant. While much is known about the physiological processes which occur at that time, the adaptive aspects of germination strategies have received much less attention (Kozlowski 1972; Heydecker 1973). However, population biologists are aware of the importance of these strategies and have emphasized their critical role in the dynamics of plant populations (Harper 1965; Cohen 1967). This study deals with an aspect of germination which has to do with both physiology and ecological strategy: how the rate of seed germination is affected by clustering of seeds into dense arrays, and the relationship between this response to density and the propensity of a species to be colonizing or weedy as opposed to being a member of an indigenous, closed plant community. A few reports on density-dependent percentages and/or rates of germination are scattered in the literature, e.g. for pollen (Brewbaker & Majumder 1961), fungus spores (Toth 1973) and seed (Ballard 1958; Palmblad 1968). The responses of germination to greater density have included both increases (Ballard 1958; Brewbaker & Majumder 1961; Linhart & Pickett 1973; Toth 1973) anct decreases (Palmblad 1968; Toth 1973). However, comprehensive reviews of seed biology (Kozlowski 1972) and seed ecology (Heydecker 1973) do not discuss this phenomenon.

Plant diversity, herbivory and resistance of a plant community to invasion in Mediterranean annual communities

Several components of the diversity of plant communities, such as species richness, species composition, number of functional groups and functional composition, have been shown to directly affect the performance of exotic species. Exotics can also be affected by herbivores of the native plant community. However, these two possible mechanisms limiting invasion have never been investigated together. The aim of this study was to investigate the relationships between plant diversity, herbivory and performance of two annual exotics, Conyza bonariensis and C. canadensis, in Mediterranean annual communities. We wanted to test whether herbivory of these exotics was influenced either by species richness, functional-group richness or functional-group composition. We also studied the relationship between herbivory on the exotic species and their performance. Herbivory increased with increasing species and functional-group richness for both Conyza species. These patterns are interpreted as reflecting a greater number of available herbivore niches in a richer, more complex, plant community. The identities of functional groups also affected Conyza herbivory, which decreased in the presence of Asteraceae or Fabaceae and increased in the presence of Poaceae. Increasing herbivory had consequences for vegetative and demographic parameters of both invasive species: survival, final biomass and net fecundity decreased with increasing herbivory, leading to a loss of reproductive capacity. We conclude that communities characterised by a high number of grass species instead of Asteraceae or Fabaceae may be more resistant to invasion by the two Conyza species, in part due to predation by native herbivores.