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Nodulating strains of Rhizobium loti arise through chromosomal symbiotic gene transfer from ICMP3153 to nonsymbiotic rhizobia in the environment. Rhizobia were isolated from Lotus corniculatus nodules established with a single-inoculant strain, ICMP3153. Hybridization of digests generated by the enzyme Swa I revealed that the symbiotic DNA region was chromosomally integrated in the strains.

The purpose of this study was to determine the most important nonanthropogenic factors affecting understory (herbs, shrubs and low-growing vines) diversity in forested landscapes of southern Indiana. Fourteen environmental variables were measured for 46 sites. Multiple regression analysis showed significant positive correlation between understory diversity and tree seedling diversity and soil subgroup. Slope aspect, soil rooting depth and number of standing dead trees were significantly negatively correlated with understory diversity. Mesic sites were more diverse in common understory species than xeric sites but had lower total cover and different species., INTRODUCTION The understory is typically the most diverse stratum within North American Eastern Deciduous Forests. High understory diversity has been attributed to intrastand heterogeneity due to differences in nutrient, light [...]

Spatial and temporal dynamics of barrier island shrub thickets were investigated on Hog Island, a barrier island along the Eastern Shore peninsula of Virginia. The island thickets are dominated by the actinorhizal shrub, Myrica cerifera. On the accreting, northern half of the island, thickets expanded as swale area increased. Analysis of aerial photographs revealed that total thicket area increased by nearly 400% from 1949 to 1989, paralleling a substantial increase in area for the northern portion of the island. Thicket characteristics varied with the age and position of the three field sites on Hog Island. At the youngest site examined (20 yr), recruitment and shrub growth were high. Shrub growth remained high and recruitment continued in the stable central portions of the island (50 yr). However, at the oldest site near a bay side salt marsh ([approximately equal to] 130 yr), recruitment was not evident, shrub growth was reduced, and mortality was most apparent. Several 'life stages' for shrub thickets were identified relative to island accretion. After the stabilization of new land, seedlings establish and grow rapidly to form a thicket. Once established, a thicket rapidly expands in the relatively protected, mesic swale environment. Thicket decline follows, characterized by an increase in vine growth on and around the shrubs and the formation of gaps as the thicket breaks up. Spatial variation in M. cerifera thickets on Hog Island is related to length of time soils have been stabilized and, especially, to time since thicket establishment.

Females of dioecious plant species are expected to show greater trade-offs with other life history traits than males because of their high investment in reproduction. We investigated life history traits and resource allocation patterns for both males and females of the dioecious plant Silene latifolia, to document allocation both prior to and after reproductive investment. In addition, the impact of fruit production on subsequent allocation to reproduction and growth was investigated by varying the number of flowers pollinated on females. The mean date of first reproduction was the same for males and females. Early-flowering plants of both sexes invested less in leaf biomass than plants that delayed flowering. The percentage of flowers pollinated had a significant effect on how resources were allocated, and on the amount and timing of further flower production. Sexual dimorphism was observed in resource allocation patterns after, but not prior to, investment in reproduction. Overall, pollinated females were found to have more leaf biomass and invest nearly twice as much in reproduction as males, in spite of an apparent trade-off between leaf production and reproduction within both females and males. We conclude that sexual dimorphism in life history traits that affects resource acquisition causes the resource pool that is allocated to be variable, rather than static, thereby masking the underlying tradeoffs. Key words: dioecious; life history traits; sexual dimorphism; Silene latifolia; trade-offs., INTRODUCTION Traits affecting reproduction and survival will be bound together by physiological trade-offs, because allocation of limited resources to any one activity will occur at the expense of others (Cody [...]

The impermeable seed coat (ISC) trait in soybean [Glycine max (L.) Merr.] is valuable because impermeable seed retain viability longer than permeable seed under adverse conditions such as delayed harvest or lengthy storage. The objective of this study was to determine how drought stress during seedfilling affected the ISC trait. 'Forrest', a maturity group (MG) V determinate cultivar with normal seed coat, and D86-4565, an ISC MG V determinate line, were grown in the field in 1991 (hot, dry season) and 1992 (cool, wet season) at Stoneville, MS. No irrigation or furrow irrigation between beginning seedfill (R5) and full seed (R6) growth stages were used to vary soil water content during seed development. Seed was harvested by hand and machine both years. In 1991, germination of machine-harvested, unscarified D86-4565 was increased by irrigation, but the percentage of impermeable seed was not affected. In 1992, irrigation that was started at R5 and continued through R6 increased germination of machine-harvested, unscarified seed from both genotypes, and significantly decreased impermeability of unscarified seed of D86-4565. Unscarified seed from D86-4565 grown in 1992 had greater germination and higher permeability than seed from 1991. Unscarified seed from all treatments of D86-4565 germinated well below the standard of 80%. These results indicate that irrigation of soybean during the seedfilling period can influence the ISC trait and germination of unscarified seed with this trait, but the effect of diverse weather is much greater. Furthermore, adequate moisture during seedfill will not preclude preplant scarification to enhance germination of seed with the ISC trait.

Models of annual plants with a persistent seed bank have shown that temporal variation can promote coexistence if the reproductive success of species is favored in different environments determined by temporally variable conditions. This study investigates whether this mechanism may explain the coexistence of three Sonoran Desert species (Pectocarya recurvata, Plantago patagonica, Schismus barbatus). In a 2-yr experiment, factors that vary across years (water and seedling density) were manipulated. In addition, the dominant spatial feature, presence or absence of Larrea cover, was also included as a factor. Our aim was to document fitness hierarchies in different types of years. Seedlings were mapped monthly for survival and reproductive success. To compare species, we used 10 yr of data to calculate the average value that seeds of different species have for population growth. Shifts in fitness hierarchies were found for two species pairs (Pectocarya-Schismus and Pectocarya-Plantago), depending on density and either the year or the water level. Surprisingly, all species had higher survival at higher densities in one or another treatment. Habitats were not important to the fitness hierarchies; all species had higher mean survival and fitness in the open than under shrubs. Key words: annual plants; coexistence; competition; fecundity; Larrea divaricata; Pectocarya recurrata; Plantago patagonica; relative fitness; Schismus barbatus; Sonoran Desert; spatial variation; survival; temporal variation., INTRODUCTION Understanding the mechanisms allowing plant species to coexist has challenged ecologists. We often find coexisting plant species that do not appear to partition any of the fundamental axes of [...]

Comparative studies of oat phytochrome gene (phyA) overexpressing- and isogenic wild type transgenic tobacco plants to determine the function of light in the associations between neighboring plants reveal that light carries information essential for plants through the phytochrome, to reduce size variations among neighbors and to initiate structural modifications. phyA-overexpressing plants exhibit minute morphological responses to low red/far red (R/FR) ratio of incident light and to close spacing of neighboring plants and develop size variation in dense plant cultures. Wild-type plants respond largely to the R/FR ratio and size variations start increasing with the density in the experimental time frame.

Observations of Juncus and Iva, two salt-marsh plants, to assess the relation between their distribution patterns and the structure and organization of surrounding plants reveal that positive plant interactions help the plants to adjust to harsh physical conditions. Juncus and Iva share a symbiotic relationship, with Juncus decreasing the salinity of the soil and helping Iva survive. Plant interaction on marsh edges changes from positive to negative as elevation increases and physical conditions become conducive to growth.

Linum perenne L. is a nonclonal perennial herb widely distributed across a range of habitats. Variation in seed germination patterns was examined for 21 Intermountain collections and for the commercial cultivar 'Appar'. Collections from sites with long, snowy winters were largely dormant at harvest and responded positively to chilling. Collections from middle elevation sagebrush-grassland sites were generally nondormant and contained a fraction induced into secondary dormancy by chilling. Collections from palouse prairie and pinyon-juniper sites were generally nondormant and unaffected by chilling, as was 'Appar'. When seeds of contrasting populations were planted in seed retrieval experiments at low, middle, and high elevation sites, their field germination phenology was predictable from laboratory experiments. In common garden experiments, there were significant among-sibship differences in germination for each garden-grown wild accession but not for 'Appar', suggesting that differences both among and within populations may be genetically based. Garden-grown seeds were generally less dormant than wild-collected seeds, possibly because of selection during propagation. Results demonstrated the existence of ecologically relevant among-population and within-population variation in germination phenology for blue flax. Seeds of high-montane populations are spring-emerging and have little provision for between-year carryover. Populations from lower elevation habitats with less predictable weather have seeds with contrasting germination patterns, allowing for emergence in both fall and spring as well as for seed bank carryover. 'Appar' seeds lack these predictive and carryover mechanisms.

The physiological ecology of members of the Bromeliaceae is reviewed with an emphasis on photosynthesis and water relations. Terrestrial and epiphytic species are, for the most part, treated separately. Water relations, photosynthetic pathways, and photosynthetic responses to light, temperature, drought, atmospheric moisture, elemental nutrients, and pollutants are considered from an ecological perspective. In addition, appendices provide values of numerous ecophysiological parameters for all species studied thus far. Results of this review include the following: (1) the ecophysiology of terrestrial and epiphytic species is surprisingly similar; (2) approximately two-thirds of bromeliads are CAM plants and occupy arid sites or are epiphytic; (3) many species are adapted to full or partial shade, yet can grow in full sunlight; (4) photosynthesis is optimal when day temperatures are warm and night temperatures are cool; (5) species with heavy trichome indumenta on their leaf surfaces are capable of absorbing atmospheric water vapor, yet improvement of tissue water relations is unlikely; (6) heavy trichome covers also suppress CO2 exchange when leaf surfaces are wetted; (7) high levels of recycling of respiratory CO2 via CAM occur in many species, especially under stress; and (8) tissue osmotic and water potentials of nearly all bromeliads investigated are seldom more negative than -1.0 MPa. A potential explanation of the mechanisms underlying maintenance of high tissue water potentials despite large water losses during droughts is discussed. In summary, the diversity of physiological adaptations to the environment in the few bromeliads studied thus far is impressive, but likely will be surpassed with investigation of more species in the Bromeliaceae.

Spatial distribution is an important factor determining the intensity and outcome of plant competition. The commonly used measure of angular distributions of plants around a target plant (1 - r) is shown to be limited to unimodal distributions. We present a new index which is based on the variance of the differences between the azimuth of neighboring plants. The new index is an improvement as it characterizes the angular dispersion in both unimodal and multimodal distributions.

The ornithophilous and sphingophilous pollination systems in the Aquilegia formosa/Aquilegia caerulea group (Ranunculaceae), the Ipomopsis aggregata group (Polemoniaceae) and the Diplacus longiflorus group (Scrophulariaceae), and their related floral isolations are products of allopatric speciation. A possible intermediate phase in allopatric speciation is the geographical races adapted to a set of pollinations in a region where pollinations are plentiful and efficient, and to another set of pollinations in another region.

This study was conducted in a phryganic (East Mediterranean) ecosystem at Daphni, near Athens, Greece. The Labiatae, represented by ten species belonging to nine genera, dominate in this ecosystem type. They flower from February to July. Both flowering time and nectar quantity are related to the species ability to tolerate intense water stress. Labiatae are visited by 201 insect species. Of these, 43 are exclusively supported by the family and 37 are monotropous. Solitary bees (mainly Anthophoridae, Megachilidae, Halictidae) constitute 47.3% of pollinators. The family is important in hosting specialized bees (15 species) in phrygana, particularly late in the flowering season. Labiatae species form two equally represented groups in this system; namely, the late winter-early spring (early) flowering, visited by relatively few pollinator species, and the late spring-summer (late) flowering species, visited by numerous pollinators. This temporal distinction is accompanied by different pollination profiles that include duration of anthesis, reward to pollinators, floral attractiveness, and flower character differentiation. All of these attributes are maximized in the early flowering period. This strategy suggests a mechanism for resource partitioning at a time when the pollinator resource is limited and competition for the services of pollinators is expected to be intense. Contrary to the current theory concerning cornucopian species, the copiously rewarding flowers of Labiatae in phrygana are not those abundantly serviced by pollinators.

Current concepts of vegetation dynamics include that of the shifting landscape mosaic, but evidence for shifting mosaics in disturbed and undisturbed systems is primarily based on negative spatial relationships among adults and recruits, and not on measurements of actual shifts over time. We used aerial photographs to measure transition rates as evidence for mosaic shifts among grassland, coastal sage scrub, chaparral, and oak woodland communities in central coastal California between 1947 and 1989. In unburned plots without livestock, transition from grassland to coastal sage scrub was 0.69% per year, coastal sage scrub to oak woodland was 0.30% per year, and oak woodland to grassland was 0.08% per year. These transition rates, considered together, indicate that vegetation patterns may be dynamic on landscapes dominated by these communities. In burned plots without livestock, and in unburned plots where livestock were not excluded, transition rates were lower, except for the conversion of oak woodland to grassland. In burned plots, a high rate of transition of coastal sage scrub to grassland was measured. Markov chain models predicted much less directional change in community proportions in either grazed or burned conditions than in ungrazed, unburned conditions. Some transition rates varied with substrate and topographical position, indicating that fire, grazing, and the physical environment interacted to determine direction and rate of vegetation change. Variation in transition on different substrates suggests that only portions of the vegetation of these landscapes may be dynamic, with some patches in certain combinations of environment and disturbance that change rapidly, and other patches that remain static as edaphic or topographic climax communities.

Introduced grass species such as standard crested wheatgrass (Agropyron desertorum (Fischer ex Link) Schultes) are most often used to revegetate degraded rangelands in the Intermountain region of the western USA. Use of native grass species is rising because public land management policies increasingly favor plant materials representative of native flora. Our objective was to compare the population T-21076, a promising source of germplasm of the native thickspike wheatgrass (Elymus lanceolatus subsp. lanceolatus (Scribner & Smith) Gould) (Syn-Elytrigia dasystachya (Hook.) A. Love & D. Love), with 'Hycrest' crested wheatgrass for forage yield, tiller number, and tiller weight during and after establishment. The relative importance of tiller number and tiller weight for forage yield production was compared using path coefficient analysis. Seedlings were transplanted from a greenhouse to three field environments corresponding to three moisture stress levels where six harvests were evaluated through two growing seasons. Tiller number was more affected by moisture stress than tiller weight. Forage yield production of the caespitose Hycrest was more stable under moisture stress than the rhizomatous T-21076. T-21076's forage yield depended mostly on tillering through rhizomatous spread, which was more sensitive to moisture stress than the tillering of Hycrest. Tiller weight of T-21076 was similar in all environments and was equal or lower than Hycrest's at all harvests. In contrast to T-21076, Hycrest's forage yield production relied primarily on tiller weight, a more stable trait than tiller number. Increases in Hycrest's tiller weight were able to partially compensate for reductions in its tiller number with increasing moisture stress. The advantages conferred by T-21076's rhizomatous spreading and resultant tillering are probably limited by their sensitivity to moisture stress.

Plant field experiments published between 1979 and 1988 in seven journals were studied to address questions about the patterns of interspecific competition over time and space, their effects on community structure, and competitive ability among species. Competition seemed to have strong effects on community structure as shown in its significant effects on distribution patterns, relative abundances and diversity. It was also seen that interspecific competition was usually stronger than intraspecific competition with respect to competitive effect and response.

The models of Lande and Schemske predict that among species in which the selfing rate is largely under genetic control and not subject to tremendous environmental variation, the distribution of selfing rates should be bimodal. When this prediction was tested empirically using data from the literature for species of angiosperms and gymnosperms, the distribution of outcrossing rates for all species was clearly was clearly bimodal. To provide another empirical test of the prediction, we analyzed mating-system data for 20 species of Pteridophyta (ferns). Homosporous ferns and their allies are unique among vascular plants because three types of mating are possible: intragametophytic selfing (selfing of an individual gametophyte); intergametophytic selfing (analogous to selfing in seed plants); and intergametophytic crossing (analogous to outcrossing in seed plants). The distribution of intragametophytic selfing rates among species of homosporous ferns is clearly uneven. Most species of homosporous ferns would be classified ax extreme outcrossers. In contrast, a few species are nearly exclusively inbreeding. In only a few population of Dryopteris expansa and Hemionitis palmata and a single population of Blechnum spicant do we see convincing evidence of a mixed mating system. The uneven distribution of selfing rates we observed for homosporous ferns, coupled with a corresponding bimodality of the magnitude of genetic load, strongly supports the model.

(1) Three mechanisms that may promote the coexistence of the boreal mosses Tetraplodon angustatus, T. mnioides, S. ampullaceum and S. luteum (Splachnaceae) have been examined: differences between dry and wet habitats; seasonal segregation of resources, and interspecific dispersal differences. These mosses grow on the droppings of large mammals and have their spores dispersed by flies (Diptera). (2) Droppings located in dry habitats were primarily occupied by Tetraplodon spp., whereas droppings in wet habitats were primarily occupied by Splachnum spp. In dry habitats there was a seasonal segregation of dropping colonization by Tetraplodon spp. Dung placed into the field in May, when sporophytes of T. angustatus matured, were exclusively colonized by T. angustatus, whereas dung placed into the field in June, when most T. mnioides sporophytes matured, were primarily colonized by T. mnioides. (3) Each species of moss attracted between ten and seventeen species of spore-carrying flies and the fly faunas attracted to each species were 7-92% different from each other in species composition. (4) Neither species of Tetraplodon appeared to have a clear dispersal advantage: for both, spore-carrying flies were predominantly associated with dung in dry habitats; spore-carrying flies captured on T. angustatus carried significantly more spores on average than did spore-carrying flies captured on T. mnioides; however, more flies captured on T. mnioides were found to be carrying spores than were flies captured on T. angustatus. (5) Although more flies captured on Splachnum luteum than on S. ampullaceum, were found to be carrying spores, S. ampullaceum (the weaker competitor) appears to have more of its spores dispersed to dung in wet habitats than does S. luteum. Spore-carrying flies captured on S. ampullaceum and associated dung carried significantly more spores on average than did spore-carrying flies captured on S. luteum, and a much greater proportion of spore-carrying flies associated with S. ampullaceum than with S. luteum were also associated with dung in wet habitats.

In a mid-Atlantic salt marsh Spartina alterniflora overwinters with two types of aerial biomass, dead culms from the preceding growing season, and young live shoots that emerged in the fall. The importance of both shoot types for the overwintering capability and the quantity of recoverable underground reserves (RUR) available at the beginning of the following growing season was studied. No reduction in the amount of RUR in the plants' underground parts during the winter period was detected. Mobilization of RUR in March is faster than in December. Apparently, the physiological state of S. alterniflora at the onset of winter does not favor a rapid reconversion of the reserves. When both live and dead shoots were removed in December, the plants did not survive the winter. The presence of either live or dead shoots enabled the plants to survive. Though their biomass was much less, the young live shoots performed this function equally as well as the old dead shoots. The live shoots did not add significantly to the RUR through photosynthesis during the winter period, and a role of young live shoots was likely to be the conduction of oxygen to the underground plant parts to support aerobic respiration.

Ponds containing the parthenogenetic zooplankter Daphnia pulex with and without chaoborid predators were sampled over the course of a season. A significant (P Key words.--Chaoborus americanus, Daphnia pulex, evolutionary ecology, predation, reaction norm.

(1) The difference in performance of two annual haophytes, Cakile maritima and Salsola kali, was investigated in strandline habitats where plants are large and fecund and on foredunes where they are small and stunted. (2) The response of growth and photosynthetic gas exchange to nitrogen and phosphorus addition was investigated in the field. (3) Addition of nitrogen caused a substantial increase in growth on the foredune, confirming its limited availability. Phosphorus was present in the foredune sand in sufficient quantity to support the extra growth of nitrogen-fertilized plants. (4) Nitrogen addition increased the net photosynthetic rate of foredune plants to values comparable with those of unfertilized strandline plants. It is suggested that nitrogen deficiency limits growth on the foredune by restricting the rate of net photosynthesis (5) Recalculation of photosynthesis as a nitrogen use efficiency (carbon fixed per unit nitrogen content of leaf) shows that the differences in photosynthetic rate in the field could be attributed wholly to differences in leaf nitrogen status.

Few experimental studies have addressed the demographic consequences of plant-microbe symbiosis in natural communities. In this study the survival, growth, and reproduction of tall fescue grass (Festuca arundinacea), smut grass (Sporobolus poiretii), and green sedge (Cyperus virens) infected by the imperfect fungus Acremonium coenophialum, the ascomycetous fungus Balansia epichloe, and B. cyperi, respectively, were compared with uninfected conspecifics in field exeperiments conducted in natural plant communities in southern Louisiana. Equal numbers of infected and uninfected ramets of each species were propagated in the greenhouse from infected and uninfected seed lots (F. arundinacea) or from field-collected infected and uninfected plants (S. poiretii and C. virens). Ramets of the two grasses were planted into a grassland community where the two species were dominant members of the community. Ramets of the sedge were planted into a marshy area where C. virens occurred naturally. Numbers of tillers were counted for each ramet upon planting for subsequent use as a covariate, and each ramet was censused from one to three times a year for three consecutive years. All survivors were then harvested and aboveground dry biomass was determined. For F. arundinacea, infected ramets exhibited higher survival, flowering frequency, vegetative tiller production, and biomass over the 3-yr period. Relative fitness, calculated from survival and inflorescence production, was nearly twice as high for infected ramets, indicating a mutualistic relationship between F. arundinacea and its endophytic fungus. For S. poiretii, growth and survival of infected and uninfected ramets were similar, but flowering was greatly suppressed in infected ramets. Relative fitness of infected ramets was only [unkeyable] 1/10 that of uninfected ramets, indicating a pathogenic relationship. There was evidence of substantial contagious spread of the fungus in the experimental population. Infected ramets of C. virens had significantly higher survival over 3 yr and tended to be bigger than uninfected ramets. Inflorescenses were produced by both types of ramets, but the inflorescenses of infected ramets were always aborted. The results indicate that, within a closely related group of fungi, both pathogenic and mutualistic relationships with host plants exist. Maternally inherited, seed-borne endophytes similar to the one in F. arundinacea are found in many grasses, suggesting that analogous mutualisms are common. The greater survival and vegetative vigor of infected C. virens call into question the traditional nation of fitness as applied to plants capable to extensive asexual reproduction. The biological mechanisms responsible for the greater survival and growth of infected plants probably include induced alterations in the host plant's hormonal balance, greater tolerance to abiotic stresses, and increased resistance to herbivores and pathogens.

Mean pollen production and mean nearest neighbor distance were recorded for several populations of Taxus canadensis and correlated with the proportion of ovules pollinated and seed set. Distance and pollen production together explained 86% of the variation in pollination success, each variable significantly adding to the regression when adjusting for the other. Seed set was correlated significantly with pollen production and nearest neighbor distance separately, but the multiple regression including the latter two variables was not significant. Seed set was correlated most strongly with pollination success and mean ovule production ([R.sup.2] = 0.71), suggesting that variation in seed set among Taxus populations was a combination of differences in pollen and resource availability.

Author(s): Ragan M. Callaway [1]; Bruce E. Mahall [2] Knowing who are your relatives and who are not creates behavioural, ecological and evolutionary opportunities. Organisms capable of recognizing kin can [...]

Author(s): John Whitfield [1] There are about 250,000 different types of plant -- and almost as many types of leaf, from blades of grass through downy beech leaves to the [...]

Plant ecologists continue to grapple with Reid's paradox, the observation that dispersal distances of most herbs and trees are too limited to account for their recolonization of northern latitudes following glacial recession. As global climate changes and natural habitats become increasingly fragmented, understanding patterns of seed dispersal and the potential for long-distance colonization takes on new importance. We studied the dispersal and establishment of the northern pitcher plant Sarracenia purpurea, which grows commonly in isolated bogs throughout Canada and eastern North America. Median dispersal distance of S. purpurea is only 5 cm, which is insufficient to explain its occurrence throughout formerly glaciated regions of North America. Establishment probability of seeds in the field is approximately 5%, and juveniles are normally found clustered around adult plants. The large-scale population genetic structure of this species can be accounted for by rare long-distance dispersal events, but its predictable occurrence in isolated habitats requires additional explanation. Reid's paradox remains an open question, and predicting long-range colonization into fragmented habitats by species with limited dispersal ability is a novel challenge. Key words: dispersal; establishment; germination; Reid's paradox; Sarracenia purpurea; Sarraceniaceae.

Models of atmospheric dispersal of anemophilous pollen are important tools in Quaternary plant ecology for determining pollen-source areas and for applying distance-weightings to vegetation data in formal pollen-vegetation calibrations. The most widely applied model is Prentice's model, which uses a modified form of Sutton's equation for atmospheric diffusion to predict pollen-source areas from size of the depositional basin and a set of depositional parameters (deposition velocity of the pollen grains and mean wind speed) and atmospheric parameters (turbulence parameter, vertical diffusion coefficient). We review the physical theory underlying Sutton's equation and Prentice's model, explore the effects of different values of the depositional and atmospheric parameters on model predictions, and provide prescriptions for model application, parameter specification, and further research on pollen dispersal. Most applications of the models to pollen dispersal have assumed neutral atmospheric conditions. We argue that most pollen dispersal takes place in unstable atmospheric conditions, and prescribe appropriate values for the atmospheric parameters for unstable conditions. Our simulations using these parameters indicate more widespread pollen dispersal from a source than under neutral conditions. We review available data sets for sedimentation velocity of pollen grains, and compare the measured estimates with sedimentation velocities predicted from Stokes's Law to assess validity of the data. Substantial variability exists among data sets, but several are suitable for application to pollen-dispersal models. Finally, we discuss aspects of release, dispersal, and deposition of anemophilous pollen that are in need of further theoretical and empirical study. Such studies will contribute not only to Quaternary plant ecology but also to understanding of pollination biology, population genetics, and functional morphology of pollen grains and pollen-bearing organs., II. Introduction Fossil pollen assemblages are the primary source of information on past vegetation composition and pattern at timescales of 100 to 1 million years. Vegetational inferences from pollen assemblages [...]

The thinning law in plant ecology can be explained in terms of how resources are used by individuals as a function of their size. It is shown that rates of resource use in individual plants scale as about the 3/4 power of body mass, the same as metabolic rates of animals. This relationships is used to create a mechanistic model for relationships between density and mass in resource limited plants. The average plant size is predicted to scale as the -4/3 power of maximum population density, implying that constraints on metabolic rate are reflected in the scaling of population density.

Several preliminary comparative surveys are conducted to explain the impact of the ironstone outcrops on the diversity of plants in the Iron Quadrangle, a threatened landscape situated in southeast Brazil. The plant communities over these ironstone outcrops are shown to exhibit extremely high local and regional diversity.

The application of the plant-microbe-soil N transformations in increasing the range of ecosystem services is discussed. The analysis also demonstrates the need of development of new technologies that will promote the dependence of humans on a biological supply of nitrogen.

1 Facilitation between neighbouring plants can promote species survival and regulate community composition. However, the role of facilitation varies along environmental severity gradients. It is important to understand the shape of this relationship to improve our ability to predict the impact of a changing environment on biodiversity. 2 We used Scots pine saplings growing within heather to examine the shape of the relationship between facilitative interactions (protection from browsing) and the severity of the environment (deer browsing intensity). We also investigated whether protection from browsing translated into a biomass response of saplings. 3 In the first winter following planting heather had a facilitative effect on saplings by reducing the probability of browsing. This effect was strongest at intermediate deer browsing intensities, thus producing a hump-backed relationship between facilitative effects and the severity of the environment. 4 Protection from browsing did not lead to longer-term biomass gains for the saplings. The competitive effects of heather on sapling growth therefore outweighed the beneficial effects of protection from browsing. 5 These results provide much-needed information on the shape of the severityuinteractions relationship with respect to a key natural disturbance phenomenon (herbivory), and demonstrate that an observable interaction relationship does not necessarily translate into a biomass response. 6 This illustrates the complex and potentially transient nature of plantuplant interactions, and the potential difficulty that would be associated with using shelter effects of heather as a management tool to promote Scots pine regeneration.

1 Many plant species respond to herbivory with increased emission of volatile organic compounds (VOCs): these attract carnivorous arthropods and thereby function as an indirect defence mechanism. Whether neighbouring plants can 'eavesdrop' on such airborne cues and tailor their defences accordingly, remains controversial. 2 We used Lima bean plants (Phaseolus lunatus) to investigate whether herbivore-induced VOCs induce another indirect defence strategy, i.e. the secretion of extrafloral nectar (EFN) in conspecific plant neighbours, and whether this enhances the defence status of the receiving plant under natural conditions. 3 EFN secretion was induced by VOCs released from herbivore-damaged bean tendrils as well as by a synthetic VOC mixture resembling the natural one. One constituent of the herbivore-induced blend u the green leaf volatile (3Z)-hex-3-enyl acetate u was sufficient to elicit the defence reaction. 4 A long-term experiment comparing the defensive effect of EFN alone with the VOC-mediated effect (EFN induction plus attraction of plant defenders) suggested that Lima bean benefits from both indirect defences. Repeated treatment of tendrils with either an artificial blend of VOCs or with EFN led to the attraction of a higher cumulative number of predatory and parasitoid insects (i.e. ants and wasps) as well as to less herbivore damage and an increased production of inflorescences and leaves. 5 Our results demonstrate that one indirect defence mechanism can induce another one in conspecific plants, and that Lima bean plants can benefit from this VOC-induced EFN secretion under natural conditions. Both extrafloral nectaries and the capability to release VOCs upon herbivory are present in many plant taxa and airborne signalling may thus represent a common mechanism for regulating the secretion of EFN in plant parts which face an increased risk of herbivory.

The mountain birch populations suffer from regular autumnal moth defoliations that affect tree survival and growth where it is found that birch individuals show clear differences in their leaf chemistry and it may be relevant to herbivore performance. The amino acids had less genetic variation than phenolic compounds and the high genetic variation of the phenolic compounds indicate that the mountain birch is able to evolve its phenol based resistance.