Masting promotes individual- and population-level reproduction by increasing pollination efficiency.

Masting is a reproductive strategy defined as the intermittent and synchronized production of large seed crops by a plant population. The pollination efficiency hypothesis proposes that masting increases pollination success in plants. Despite its general appeal, no previous studies have used long-term data together with population- and individual-level analyses to assess pollination efficiency between mast and non-mast events. Here we rigorously tested the pollination efficiency hypothesis in ponderosa pine (Pinus ponderosa), a long-lived monoecious, wind-pollinated species, using a data set on 217 trees monitored annually for 20 years. Relative investment in male and female function by individual trees did not vary between mast and non-mast years. At both the population and individual level, the rate of production of mature female cones relative to male strobili production was higher in mast than non-mast years, consistent with the predicted benefit of reproductive synchrony on reproductive success. In addition, at the individual level we found a higher conversion of unfertilized female conelets into mature female cones during a mast year compared to a non-mast year. Collectively, parallel results at the population and individual tree level provide robust evidence for the ecological, and potentially also evolutionary, benefits of masting through increased pollination efficiency.

Masting in ponderosa pine: comparisons of pollen and seed over space and time.

Many plant species exhibit variable and synchronized reproduction, or masting, but less is known of the spatial scale of synchrony, effects of climate, or differences between patterns of pollen and seed production. We monitored pollen and seed cone production for seven Pinus ponderosa populations (607 trees) separated by up to 28 km and 1,350 m in elevation in Boulder County, Colorado, USA for periods of 4-31 years for a mean per site of 8.7 years for pollen and 12.1 for seed cone production. We also analyzed climate data and a published dataset on 21 years of seed production for an eighth population (Manitou) 100 km away. Individual trees showed high inter-annual variation in reproduction. Synchrony was high within populations, but quickly became asynchronous among populations with a combination of increasing distance and elevational difference. Inter-annual variation in temperature and precipitation had differing influences on seed production for Boulder County and Manitou. We speculate that geographically variable effects of climate on reproduction arise from environmental heterogeneity and population genetic differentiation, which in turn result in localized synchrony. Although individual pines produce pollen and seed, only one-third of the covariation within trees was shared. As compared to seed cones, pollen had lower inter-annual variation at the level of the individual tree and was more synchronous. However, pollen and seed production were similar with respect to inter-annual variation at the population level, spatial scales of synchrony and associations with climate. Our results show that strong masting can occur at a localized scale, and that reproductive patterns can differ between pollen and seed cone production in a hermaphroditic plant.

A chemical polymorphism in a multitrophic setting: thyme monoterpene composition and food web structure.

We investigated the effects of chemical variation in thyme (Thymus vulgaris L.) on its interactions with competitors, herbivores, and herbivore predators. Four different thyme monoterpene phenotypes (chemotypes) were grown in a 4x2x2 factorial of chemotype, caging (sham half-cages vs. full cages), and competition (control vs. the grass Bromus madritensis L.). Cages reduced numbers of arthropod predators. Thyme-feeding aphids Aphis serpylli Koch passed through full cage walls to increase more than fourfold. As a result, freed from their predators, aphids had a large negative effect on thyme size and flowering. Similarly, competition from Bromus had a negative effect on thyme size and flowering. Individual effects of aphids and competition were nonadditive, however, and their combined effect was significantly less than that predicted by a multiplicative null model. Differential thyme sizes among chemotypes were not mediated by herbivores or competitors, but differential flowering was due to the effects of chemotype on aphids. We thus document differential selection by aphids among thyme chemotypes and the influence of Bromus on the strength of these negative effects of aphids.

GENETIC VARIATION IN ALETES ACAULIS AND ITS RELATIVE, THE NARROW ENDEMIC A. HUMILIS (APIACEAE).

We compared the organization of genetic variability in Aletes humilis, a species restricted to seven populations in north-central Colorado, with A. acaulis, a closely related widespread species. Genetic variability was scored at 11 electrophoretically detectable protein loci. Nine of these loci were polymorphic (i.e., P > 0.95) in both taxa, and were used for interpopulation and interspecific comparisons. Levels of genetic variability and patterns of organization of this variability are very comparable in both species. In contrast to many other narrow endemics, A. humilis is not genetically depauperate when compared to its presumed progenitor. Both biochemical and morphological evidence suggest that A. humilis is derived from A. acaulis; the comparable levels of variability in the two species suggest that A. humilis was derived from a source containing a substantial portion of the original A. acaulis genome, because there is no evidence of a genetic bottleneck or appreciable loss of allelic variability.

Terpene-based selective herbivory by Helix aspersa (Mollusca) on Thymus vulgaris (Labiatae).

In natural populations of Thymus vulgaris in the south of France, six genetically different chemical "chemotypes" occur, each differentiated by a single dominant monoterpene that gives individual plants a characteristic smell and taste. Individual populations may contain all six chemotypes, but are often dominated by one to three chemotypes. We tested the hypothesis that this chemical polymorphism was associated with selective herbivory by the snail Helix aspersa, which feeds upon T. vulgaris in nature. The relative preference of juvenile and adult molluscs was investigated by presenting individual animals with a choice of six chemotypes in three experimental trials: (1) whole plants, (2) nutritive gels containing crushed thyme leaves, and (3) nutritive gels containing a distilled monoterpene as an additive. We found significant and consistent patterns of preference. Adult and immature molluscs preferred the linalol chemotype, and ate carvacrol and thymol-containing foods the least. This preference was more marked when using the gels than the real plants, particularly the monoterpene-based gels, demonstrating the importance of the monoterpene for the choice. Furthermore, molluscs fed exclusively on linalol-containing food gained weight whereas those fed on carvacrol-containing food lost weight. After being fed these exclusive diets both groups showed a significant preference for linalol. These results (1) demonstrate that food choice is based upon the presence of specific monoterpenes and is not learned, (2) suggest that phenolic monoterpenes such as carvacrol are repellent to the animals, and (3) indicate that selective herbivory may play a significant role in the maintenance of the chemical polymorphism of T. vulgaris.

Path analysis of natural selection on plant chemistry: the xylem resin of ponderosa pine.

We analyzed the pattern of correlations among fitness components, herbivory, and resin characteristics in a natural all-aged stand of ponderosa pine, to infer the strength and mechanism of natural selection on plant chemistry. Male and female cone production were monitored yearly for 15 years, and levels of herbivory for 9 years in 165 trees. Resin flow rate and monoterpene composition were determined for these same trees. Multiple regression of fitness components on resin characteristics showed significant associations consistent with directional selection for increased resin flow rates and increased proportions of α- and ß-pinene, myrcene and terpinolene. However, negative correlations among monoterpene fractions of the resin constrained the overall selection. Selective herbivory by aphids approached statistical significance and monoterpenes showed some (non-significant) effect as deterrents against deer browse. Resin characteristics were not correlated with attack by cone insects or porcupines. However, the association between resin characteristics and fitness is significantly different from that predicted by the path coefficients involving herbivores. Therefore the hypothesis that these herbivores mediate selection on the resin is not supported by the observed pattern of correlations among resin characteristics, herbivory, growth and fecundity. In this population, most of the association between resin characteristics and fitness appears to be mediated by some other factor independent of attack by herbivore species present.

Seed dispersal by nutcrackers causes multi-trunk growth form in pines.

We present a unique example of a seed disperser determining the basic growth architecture of two species of plants. Clark's Nutcracker (Nucifraga columbiana) is the primary disperser for Pinus flexilis and P. albicaulis, caching clusters of seeds in the process. Both pine species often occur in a multi-trunk growth form at maturity. Electrophoretic analysis based on 2 to 4 gene loci revealed that two or more trunks were genetically distinct individuals in twenty of the twenty-five multi-trunk trees sampled. This supports the hypothesis that several mature individuals can arise from single caches.

Intra-specific variation in the inhibitory effects of Thymus vulgaris (Labiatae) monoterpenes on seed germination.

The ecological role of plant secondary compounds has received a great deal of attention yet little is known regarding variation in the ecological significance of different compounds produced by a single species. This is particularly pertinent to species where variation in compound presence is under genetic control. In this study we have quantified variation in the inhibitory effects of the six dominant monoterpene oils produced by different genotypes (or chemotypes) of Thymus vulgaris on achene germination of an associated species (Brachypodium phoenicoides), and seeds of T. vulgaris itself, in controlled experimental conditions. The experiments involved the germination of seeds of the two species with crushed leaves or pure essences of the six chemotypes, i.e. four experimental trials. A significant inhibitory effect against B. phoenicoides was found for both crushed leaves and the range of concentrations of pure oils used in the trial. The two phenolic compounds had a greater effect than the non-phenolic leaves and pure oils. The significant differences between non-phenols and the controls declined faster over time for crushed leaves and low concentrations of the pure essences than did that between the phenolics and the controls. There was a significant inhibitory effect against the germination of T. vulgaris seeds for the crushed leaves and the pure oils, although the contrast between the non-phenols and the phenols was not significant. The effect of the different oils varied depending on the seed source of the thyme seeds used in the trial. This is suggested to be due to differences in the rates of outcrossing among the sampled populations. The effects of the oils on the associated species, B. phoenicoides, were not found to be greater than their effects on the germination of thyme seeds. The results are discussed in the context of a potential role for such inhibitory effects in relation to the onset of rainfall and suitable germination conditions.

Differential host utilization by two parasites in a population of ponderosa pine.

We compared phloem characteristics of individual Pinus ponderosa attacked by the dwarf-mistletoe Arceuthobium vaginatum (Viscaceae) or by the beetle Dendroctonus ponderosae (Scolytidae) or by neither species. We quantified total nonstructural carbohydrates and a broad range of chemical elements for these three categories of trees. There were significant differences between trees parasitized by Arceuthobium, trees parasitized by Dendroctonus, and non-infected trees. Discriminant function analysis of trees attacked by either Arceuthobium or Dendroctonus correctly predicted group membership for 59 of 60 trees tested. Some of the differences detected may be induced, but many probably are not. Given that the accumulation of certain elements and compounds is under genetic control, and that both parasites often cause severe reductions in fitness, including death of their host, our results suggest that the two parasites may generate diversifying selection in ponderosa pine populations.

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.

Contrasting cascades: insectivorous birds increase pine but not parasitic mistletoe growth.

1. Intraguild predation occurs when top predators feed upon both intermediate predators and herbivores. Intraguild predators may thus have little net impact on herbivore abundance. Variation among communities in the strength of trophic cascades (the indirect effects of predators on plants) may be due to differing frequencies of intraguild predation. Less is known about the influence of variation within communities in predator-predator interactions upon trophic cascade strength. 2. We compared the effects of a single predator community between two sympatric plants and two herbivore guilds. We excluded insectivorous birds with cages from ponderosa pine Pinus ponderosa trees parasitized by dwarf mistletoe Arceuthobium vaginatum. For 3 years we monitored caged and control trees for predatory arthropods that moved between the two plants, foliage-feeding caterpillars and sap-feeding hemipterans that were host-specific, and plant damage and growth. 3. Excluding birds increased the abundance of ant-tended aphids on pine and resulted in an 11% reduction in pine woody growth. Mutualist ants protected pine-feeding aphids from predatory arthropods, allowing aphid populations to burgeon in cages even though predatory arthropods also increased in cages. By protecting pine-feeding aphids from predatory arthropods but not birds, mutualist ants created a three-tiered linear food chain where bird effects cascaded to pine growth via aphids. 4. In contrast to the results for tended aphids on pine, bird exclusion had no net effects on untended pine herbivores, the proportion of pine foliage damaged by pine-feeding caterpillars, or the proportion of mistletoe plants damaged by mistletoe-feeding caterpillars. These results suggest that arthropod predators, which were more abundant in cages as compared with control trees, compensated for bird predation of untended pine and mistletoe herbivores. 5. These contrasting effects of bird exclusion support food web theory: where birds were connected to pine by a linear food chain, a trophic cascade occurred. Where birds fed as intraguild predators, the reticulate food webs linking birds to pine and mistletoe resulted in no net effects on herbivores or plant biomass. Our study shows that this variation in food web structure occurred between sympatric plants and within plants between differing herbivore guilds.

INTRAPOPULATION DIFFERENTIATION IN ANNUAL PLANTS. III. THE CONTRASTING EFFECTS OF INTRA- AND INTERSPECIFIC COMPETITION.

The roles of intraspecific and interspecific competition in producing differentiation within populations of Veronica peregrina were studied in two populations under controlled, greenhouse conditions. In nature, each population spans an environmental gradient across the center and sides of a temporary, vernal pool in California. Individuals at the center are subjected to intense intraspecific competition produced by high densities (to 30 seedlings/cm2 ) generated by quasi-simultaneous germination (90% of seeds germinate in one week). Individuals at the periphery are subjected to interspecific competition with grasses, which shade out the Veronica 4-6 weeks after the onset of winter growth. I predicted that 1) when grown under immediate intraspecific competition in the greenhouse, offspring of plants from the central subpopulation (C) would perform better (i.e., grow larger and produce more seeds) than those from the periphery (P) and that 2) when grown under delayed interspecific competition provided by Agrostis tenuis and Lollium multiflorum, offspring of plants from the periphery would perform better than those from the center. Both predictions were confirmed. The center-periphery differences were pronounced and statistically significant in an undisturbed population (V-2), while in a population disturbed by yearly plowing (V-3), the differences tended to be consistent with those in V-2 but seldom significant. Distribution of variability tended to be positively skewed and/or leptokurtic in subpopulations grown under "foreign" competition (i.e., intraspecific for P plants and interspecific for C plants) but was normally distributed following exposure to "familiar" competition. Timing of competition affected many results. There were four additional significant differences between the central and peripheral subpopulations. 1) Germination rate: the faster rate in central plants can be advantageous under immediate intraspecific competition. The slower rate in peripheral plants can be advantageous under conditions of erratic and unpredictable soil moisture. 2) Response to nutrient competition: central plants were more sensitive to N-deficiency and peripheral plants were more sensitive to P-deficiency. 3) Allocation of biomass: central plants allocated a greater proportion of biomass to seeds, while peripheral plants allocated a greater proportion of biomass to leaves under all growing conditions. 4) Root elongation: at the seedling stage, central plants have longer roots, while at the adult stage, peripheral plants have longer roots (but not more root mass). Most components of this complex pattern of differentiation are interpretable in an adaptive context. Other results defy simple explanations and underline the importance of phenotypic plasticity, which was pronounced in the competition experiments.

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, alpha-terpineol, and linalool). This is due to the presence of high amounts of precursors (gamma-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.

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.