Individual Plant Size Research Articles

El alga Digenea Simplex (Ceramiales: Rhodomelaceae) en México: variación biogeográfica

The alga Digenea simplex (Wulfen) C. Agardh has been reported in forty seven Mexican localities. A detailed description of reproductive and morphological features is given hete for the first time. Its presence in Bahia Banderas, Nayarit (Pacific of Mexico) is a first record. The lack of sexual structures suggests that Mexican populations are monophasic. The only morphological difference observed in distant populations (Nayarit, Tamaulipas and Quintana Roo) is lhe size of individual plants (not statistically significant). There is not enought evidence to support taxonomic seggregates.

Pathways of Leymus chinensis Individual Aboveground Biomass Decline in Natural Semiarid Grassland Induced by Overgrazing: A Study at the Plant Functional Trait Scale.

Natural grassland productivity, which is based on an individual plant’s aboveground biomass (AB) and its interaction with herbivores, can obviously affect terrestrial ecosystem services and the grassland’s agricultural production. As plant traits have been linked to both AB and ecosystem success, they may provide a useful approach to understand the changes in individual plants and grassland productivity in response to grazing on a generic level. Unfortunately, the current lack of studies on how plant traits affect AB affected by herbivores leaves a major gap in our understanding of the mechanism of grassland productivity decline. This study, therefore, aims to analyze the paths of overgrazing-induced decline in the individual AB of Leymus chinensis (the dominant species of meadow-steppe grassland in northern China) on a plant functional trait scale. Using a paired-sampling approach, we compared the differences in the functional traits of L. chinensis in long-term grazing-excluded and experimental grazing grassland plots over a continuous period of approximately 20 years (located in meadow steppe lands in Hailar, Inner Mongolia, China). We found a highly significant decline in the individual height and biomass (leaf, stem, and the whole plant) of L. chinensis as a result of overgrazing. Biomass allocation and leaf mass per unit area were significantly affected by the variation in individual size. Grazing clearly enhanced the sensitivity of the leaf-to-stem biomass ratio in response to variation in individual size. Moreover, using a method of standardized major axis estimation, we found that the biomass in the leaves, stems, and the plant as a whole had highly significant allometric scaling with various functional traits. Also, the slopes of the allometric equations of these relationships were significantly altered by grazing. Therefore, a clear implication of this is that grazing promotes an asymmetrical response of different plant functional traits to variation in individual plant size, which influences biomass indirectly. Furthermore, we detected paths of individual AB decline in L. chinensis induced by grazing by fitting to a structural equation model. These results indicate that grazing causes AB decline primarily through a ‘bottom-up’ effect on plant height and stem traits. However, leaf traits, via the process of allometric scaling, affect plant AB indirectly.

Seasonal and within-plant variation in fatty acid content and composition in the brown seaweed Spatoglossum macrodontum (Dictyotales, Phaeophyceae)

We investigated seasonal and within-plant variation in total fatty acids (TFAs) and biomass increase in the tropical brown seaweed Spatoglossum macrodontum which was sampled from Magnetic Island (Queensland, Australia) at monthly intervals over 1 year. In this habitat, S. macrodontum is an annual species with a growth period from June to September where mean biomass changed from 8- to 136-g fresh weight. Although TFA content and fatty acid (FA) composition were not directly correlated to individual plant size, there was clear seasonal variation in TFA content with a peak in July (82.7 mg g−1 dry weight (dw)) followed by a 30 % decline in August and little subsequent variation from September to November (65.9–55.5 mg g−1 dw). The FA profile was rich in polyunsaturated FAs (PUFAs) (39 %); however, there was a change to a higher percentage of saturated FAs (SFAs) (42 %) and reduced PUFA (31 %) as plants reached the end of the growth period. Averaged across sampling periods, TFA content ranged from 77 mg g−1 dw in the tips to 30 mg g−1 dw in the base section. While PUFA content (37–38 %) was similar across sections, the base had less SFAs and a higher content of monounsaturated FAs (MUFAs) (29 %). These results are the first data on the seasonal biomass increase and the temporal and internal variations in FAs for this species with important implications when targeting large brown seaweeds as a source of FAs for nutraceuticals (PUFA(n-3), 21.8 % of TFA) or chemicals (C18:1 (n-9), 17.6 % of TFA).

The limited role of snow water in the growth and development of ephemeral plants in a cold desert

AbstractQuestionThe ephemeral layer is an important component of the plant community in cold deserts. Conventional wisdom has it that the ephemeral layer primarily depends on early spring snowmelt water for germination and development. However, this hypothesis has never been subjected to rigorous testing.LocationSouthern edge of the Gurbantunggut Desert, a cold desert and a natural habitat of ephemeral plants (the inter‐sand dune lowland), in the far northwest of China and around the geographical centre of the Eurasian continent.MethodsAt the end of the snow cover season between 2009 and 2011, four different snow treatments (0, 50, 100 and 200% snow cover, with 100% representing the natural amount of snow) were applied, using five replicates. Seedling establishment, species diversity and plant growth, population, cover and productivity were monitored throughout or sampled at the end of the growing season.ResultsThe ephemeral plant germination and seedling density were significantly influenced by changes in snow cover. Germination, density of mature ephemeral plants and plant coverage rose as snow cover depth increased. However, the height, rooting depth, numbers of lateral roots and above‐ground biomass of individual plants decreased as snow cover increased, and plant size was negatively correlated with plant density. At the ephemeral layer level, above‐ground biomass showed no significant variation between snow treatments. There was also no significant difference in species richness among the snow treatments.ConclusionsIncreased snow resulted in higher plant density of ephemerals. However, this high density induced strong competition, reducing the body size of individual plants. By the end of the growing season, snow treatments did not induce significant changes in primary productivity of the ephemeral layer. Primary productivity or growth after seedling establishment depended mainly on rainfall that occurred after the snow had melted, not on snowmelt water itself. Thus conventional wisdom was shown to be only partially true. Snow was important to these ephemeral plants, but its role was limited to seedling establishment.

GENETICALLY MODIFIED BANANAS: PAST, PRESENT AND FUTURE

Edible bananas comprise several characteristics that make them an ideal target for improvement through genetic engineering: (i) they constitute the N° 1 fresh fruit crop in the world, (ii) they are highly sterile which makes classical breeding extremely difficult but at the same time prevents transgene drift via pollen into the environment, (iii) their cultivation is monoclonal, and (iv) they are susceptible to a wide range of pests and diseases. The latest outbreak of Fusarium wilt disease caused by Fusarium oxysporum f. sp. cubense Race 4 or TR4, which is seriously threatening the international banana trade, strengthens the need for developing resistant bananas. Despite these facts, the number of publications and research groups dealing with genetic engineering of banana remains low compared to other crops. Possible reasons are among others the difficulties in obtaining transformation competent tissues (in case of banana embryogenic cell suspensions or ECS), the duration to obtain fully transformed plants, the cost and effort of maintaining transgenic lines, the lack of a proper and functional legal biosafety system for testing and culturing GMOs in many banana producing countries and the size of individual banana plants rendering large scale testing of transgenes extremely costly. In this review, we discuss the 19 years history of banana genetic modification. An overview of the different transformation methodologies will be given, from the first reports on protoplast electroporation to Agrobacterium-mediated transformation of ECS. Also the applications of this technology for banana improvement (agronomic treats, quality treats, etc.) will be discussed along with examples of confined field trials of genetically modified bananas. Finally, a brief overview of the research topics on banana genetic engineering at the Laboratory of Tropical Crop Improvement (LTCI, KU Leuven, Belgium) is discussed. Topics include selectable marker genes, promoter and gene characterization as well as molecular breeding for fungus and drought resistance.

Hybridization, cryptic diversity, and invasiveness in introduced variable‐leaf watermilfoil

Hybridization may be important in the evolution of invasiveness, but few empirical studies compare introduced hybrid and parental lineages. Invasive ‘variable-leaf watermilfoil’ (Myriophyllum heterophyllum) in the northeastern United States consists of at least three distinct lineages: an interspecific hybrid (M. heterophyllum × Myriophyllum laxum) and two historically allopatric lineages of pure M. heterophyllum. Previous observations suggested that hybrid populations of variable-leaf watermilfoil may be comparatively more ‘invasive’ than pure lineages. However, no quantitative data comparing hybrid and parental lineages have been collected, nor has invasiveness been compared between parental lineages. Here, we demonstrate that these distinct lineages are also ecologically distinct. We find some support for the hypothesis that hybridization has played a role in the evolution of invasiveness: hybrids exhibited higher biomass, individual plant size, and greater branching than at least one parental lineage of M. heterophyllum. However, parental lineages did not differ from the hybrid for some traits, demonstrating that pure parental lineages can also be invasive. In addition, we found no evidence for a role of intraspecific hybridization in the evolution of invasiveness in these lineages of variable-leaf watermilfoil, even where they co-occurred locally. Our study suggests that distinguishing among cryptic lineages will help prioritize rapid response control efforts.

Effects of wave exposure on population structure, demography, biomass and productivity of the kelp Laminaria hyperborea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 451:45-60 (2012) - DOI: https://doi.org/10.3354/meps09594 Effects of wave exposure on population structure, demography, biomass and productivity of the kelp Laminaria hyperborea Morten F. Pedersen1,*, Lars B. Nejrup1, Stein Fredriksen2, Hartvig Christie3, Kjell Magnus Norderhaug3 1Centre for Integrated Population Ecology (CIPE), Department of Environmental, Social & Spatial Change (ENSPAC), Roskilde University, Box 260, Roskilde 4000, Denmark 2Department of Biology, Oslo University, Box 1072, Blinderen, Oslo 0316, Norway 3Norwegian Institute for Water Research (NIVA), Gaustadaleen 21, Oslo 0349, Norway *Email: mfp@ruc.dk ABSTRACT: Patterns of potential recruitment, survival, age-structure, density, biomass and primary production were studied in the kelp Laminaria hyperborea along a wave-exposure gradient in western Norway. The distribution of L. hyperborea is positively related to wave exposure, and the present work aimed to study how demography and population dynamics are affected by wave exposure. Populations at 9 sites representing 3 different levels of wave exposure were investigated. The biomass and production of L. hyperborea doubled along the gradient from low- to high-exposure sites. This increase was caused by an increase in plant density and individual plant size. Recruits and sub-canopy plants made up the majority of all individuals at all sites, but potential recruitment increased more than mortality rate among recruits and young sub-canopy plants as wave exposure increased, leading to a higher density of these small individuals at high-exposure sites. Despite their large numbers, recruits and sub-canopy plants were not important for total biomass and production, and variations in those parameters with wave exposure were largely driven by variations in the density and size of adult canopy plants. Canopy plants suffered higher rates of mortality at low wave exposure, leading to shorter longevity and lower density than at high-exposure sites. The density and individual size of canopy plants both increased by ca. 50% with increasing wave exposure, explaining the higher biomass and productivity at high-exposure sites. We suggest that the high mortality rate, low density and small size of canopy plants at low-exposure sites is caused by a combination of high epiphytic load and self-shading, which may impair light conditions, affect nutrient uptake and increase drag on the blades during extreme wind events. The reason why plants at high-exposure sites reach a larger individual size remains unknown. KEY WORDS: Macroalgae · Physical disturbance · Population dynamics · Demography · Biomass · Productivity Full text in pdf format PreviousNextCite this article as: Pedersen MF, Nejrup LB, Fredriksen S, Christie H, Norderhaug KM (2012) Effects of wave exposure on population structure, demography, biomass and productivity of the kelp Laminaria hyperborea. Mar Ecol Prog Ser 451:45-60. https://doi.org/10.3354/meps09594 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 451. Online publication date: April 11, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Effects of bark damage by feral herbivores on columnar cactus Echinopsis (= Trichocereus) terscheckii reproductive output

Columnar cacti are an important feature of the arid “cardonal” landscape in South America. The predominant columnar cactus in the southern zone is Echinopsis (= Trichocereus) terscheckii. At some sites, the species suffers considerable trunk damage, attributed in previous studies to feral livestock grazing (donkey and cattle). In order to understand the potential impact of this damage on columnar cactus populations, we investigated whether reproductive output (flower, fruit and seed production) of Echinopsis terscheckii individuals was related to damage, controlling for the effect of individual plant size and habitat characteristics measured at meso and micro-scale. Cactus trunk damage negatively correlated with the number of flowers and fruits, but not with fruit size. We also found significant effects of meso and micro-habitat conditions on all the analyzed reproductive parameters while plant size was positively related to flower and fruit production. These results show that bark damage reduces the reproductive potential of cacti, and therefore that large feral herbivores could have negative effects on reproduction of columnar cacti. Such findings highlight the need for management programs to monitor population trends of these plants and also exotic herbivores.

Size-related auto-fragment production and carbohydrate storage in auto-fragment of Myriophyllum spicatum L. in response to sediment nutrient and plant density

Size-related asexual reproduction of submersed macrophytes is still poorly understood. Here, we investigate how size-related auto-fragmentation in Myriophyllum spicatum L. responds to sediment nutrients and plant density. An experiment was carried out with sediments containing two different nutrient levels and with two levels of plant density. The results show that sediment nutrients and plant density brought about a strong dependency of auto-fragment production and the amount of total non-structural carbohydrate (TNC) storage in auto-fragments on individual plant size (total plant biomass). However, these two factors acted differently on size dependency. Sediment nutrients positively affected auto-fragment production and the amount of TNC in auto-fragments of M. spicatum. High concentrations of sediment nutrients significantly increased these two traits in absolute value and the value relative to plant size. Although the auto-fragment biomass and the amount of TNC in auto-fragments did not differ between density treatments when plant size was considered, the absolute values of these two traits were much larger in the low plant density treatment than in the high plant density treatment, which suggested an indirect negative effect of plant density on the auto-fragmentation of M. spicatum. In addition, higher percentages of large auto-fragments (>100 mg) were produced by plants that grew in nutrient poor sediment and low plant density environment than plants in nutrient rich sediment and high plant density environment. These results do not solely highlight a size-dependent effect, but also a size-independent effect of auto-fragment production and the amount of TNC in auto-fragments of M. spicatum. Furthermore, such size-independent effects can be explained by the significant biomass partitioning differences and the similar TNC-concentrations in auto-fragments under different environmental conditions.

Environmental heterogeneity, species diversity and co-existence at different spatial scales

The positive relationship between spatial environmental heterogeneity and species diversity is a widely accepted concept, generally associated with niche limitation. However, niche limitation cannot account for negative heterogeneity–diversity relationships (HDR) revealed in several case studies. Here we explore how HDR varies at different spatial scales and provide novel theories for small-scale species co-existence that explain both positive and negative HDR. At large spatial scales of heterogeneity (e.g. landscape level), different communities co-exist, promoting large regional species pool size and resulting in positive HDR. At smaller scales within communities, species co-existence can be enhanced by increasing the number of different patches, as predicted by the niche limitation theory, or alternatively, restrained by heterogeneity. We conducted meta-regressions for experimental and observational HDR studies, and found that negative HDRs are significantly more common at smaller spatial scales. We propose three theories to account for niche limitation at small spatial scales. (1) Microfragmentation theory: with increasing spatial heterogeneity, large homogeneous patches lose area and become isolated, which in turn restrains the establishment of new plant individuals and populations, thus reducing species richness. (2) Heterogeneity confounded by mean: when heterogeneity occurs at spatial scales smaller than the size of individual plants, which forage through the patches, species diversity can be either positively or negatively affected by a change in the mean of an environmental factor. (3) Heterogeneity as a separate niche axis: the ability of species to tolerate heterogeneity at spatial scales smaller than plant size varies, affecting HDR. We conclude that processes other than niche limitation can affect the relationship between heterogeneity and diversity.

Community‐level consequences of species interactions in an annual plant community

AbstractQuestion: How does the intensity of species interactions affect species and functional group composition of an annual plant community?Location: Sede Boqer, Negev Desert, Israel.Methods: The potential for competitive interactions in two annual plant communities (desert and coastal) from semi‐stabilized sand dunes was manipulated by varying seed bank density and therefore the number of potentially interacting individuals. Communities were exposed to three different irrigation regimes, mimicking precipitation at the desert site, the coastal site, and an intermediate precipitation level. Plots were maintained for 3 years, and percentage cover of each species in the plots was recorded at the end of each growing season. We used redundancy analysis to test for effects of initial density, irrigation, and year on the species and functional group composition of the communities.Results: Initial density had significant effects on species composition, and these effects remained significant over 3 years, even as total community percentage cover became more similar among treatments over time. Density effects did not depend on resource availability (irrigation level). Functional group identity or individual plant size did not predict which species would be good competitors, and a species' competitive ability did not predict its abundance in the field.Conclusions: Species interactions strongly affect community composition, and those effects carry over into subsequent years such that competition does not lead to convergence in community structure over time. However, the particular changes in composition observed were not predictable by some of the traits that have been found important in individual‐level experiments. We speculate that the outcome of competition in diverse communities will depend on multiple traits, in contrast to the outcome of individual‐level pairwise experiments. We also speculate that the shift in composition with density could mean that local variation in density may contribute to maintenance of diversity in this system.

Positive biodiversity–productivity relationship due to increased plant density

1. Positive effects of biodiversity on plant productivity may result from diversity-induced changes in the size or density of individual plants, yet these two possibilities have never been tested at the same time in a biodiversity experiment with a large species pool. Here, we distinguish between size effects and density effects on plant productivity, using data from 198 experimental grassland communities that contained 1–16 species. Plant modules such as tillers or rosettes were defined as relevant units, being equivalent to plant individuals in the majority of species. 2. In agreement with previous studies, we found positive effects of species richness on above-ground productivity. We show that this positive biodiversity effect resulted from diversity-induced increases in module density rather than from increases in module size. In contrast, variation in productivity within diversity levels was related to module size rather than module density. 3. The size–density relationships varied among plant functional groups and among species but their average response to increasing species richness paralleled the pattern observed at the level of the entire plant communities: species richness had a positive effect on above-ground species biomass and species module density but not on species module size. Twenty-four out of 26 overyielding species had denser populations and 25 out of 28 underyielding species had smaller modules in mixtures than in monocultures. 4. Synthesis: In grasslands, an increase in community productivity must involve an increase in plant size or density. We found that diversity-induced increases in productivity were related to diversity-induced increases in density, whereas diversity-independent increases in productivity were related to increases in plant size. Our results suggest that increased density of overyielding species in mixtures was the main driver of the positive biodiversity–productivity relationship in our experiment. We conclude that the mechanisms leading to enhanced productivity of species-rich as compared with species-poor communities cannot be derived from mechanisms explaining high productivity within communities that contain a particular number of species.

Density-Dependent Phytotoxicity of Impatiens pallida Plants Exposed to Extracts of Alliaria petiolata

Invasive plants are by definition excellent competitors, either indirectly through competition for resources or directly through allelopathic inhibition of neighboring plants. Although both forms of competition are commonly studied, attempts to explore the interactions between direct and indirect competition are rare. We monitored the effects of several doses of extracts of Alliaria petiolata, a Eurasian invader in North America, on the growth of Impatiens pallida, a North American native, at several planting densities. The density-dependent phytotoxicity model predicts that as plant density increases, individual plant size will decrease, unless a toxin is present in the soil. In this case, individual plant size is predicted to increase as plant density increases, as plants share a limited toxin dose. We tested this model using fractions of an A. petiolata extract enriched in flavonoids or glucosinolates, as well as a combined fraction. The flavonoid-enriched fraction and the combined fraction suppressed I. pallida growth but only when applied at a dose eight times higher than that expected in the field. When treated with a dose equivalent to estimated field exposure levels, I. pallida growth was not distinguishable from that of control plants that received no extract, showing that indirect competition for resources was more important for determining the growth of I. pallida than direct allelopathic inhibition by A. petiolata. This is an important reminder that, even though many plants have the demonstrated potential to exert strong allelopathic effects, those effects may not always be apparent when other forms of competition are considered as well.

Size‐dependent sex allocation in Aconitum gymnandrum (Ranunculaceae): physiological basis and effects of maternal family and environment

Theory predicts size-dependent sex allocation (SDS): flowers on plants with a high-resource status should have larger investment in females than plants with a low-resource status. Through a pot experiment with Aconitum gymnandrum (Ranunculaceae) in the field, we examined the relationship between sex allocation of individual flowers and plant size for different maternal families under different environmental conditions. We also determined the physiological base of variations in plant size. Our results support the prediction of SDS, and show that female-biased allocation with plant size is consistent under different environmental conditions. Negative correlations within families showed a plastic response of sex allocation to plant size. Negative genetic correlations between sex allocation and plant size at the family level indicate a genetic cause of the SDS pattern, although genetic correlation was influenced by environmental factors. Hence, the size-dependency of sex allocation in this species had both plastic and genetic causes. Furthermore, genotypes that grew large also had higher assimilation ability, thus showing a physiological basis for SDS.

Individualism in plant populations: Using stochastic differential equations to model individual neighbourhood-dependent plant growth

We study individual plant growth and size hierarchy formation in an experimental population of Arabidopsis thaliana, within an integrated analysis that explicitly accounts for size-dependent growth, size- and space-dependent competition, and environmental stochasticity. It is shown that a Gompertz-type stochastic differential equation (SDE) model, involving asymmetric competition kernels and a stochastic term which decreases with the logarithm of plant weight, efficiently describes individual plant growth, competition, and variability in the studied population. The model is evaluated within a Bayesian framework and compared to its deterministic counterpart, and to several simplified stochastic models, using distributional validation. We show that stochasticity is an important determinant of size hierarchy and that SDE models outperform the deterministic model if and only if structural components of competition (asymmetry; size- and space-dependence) are accounted for. Implications of these results are discussed in the context of plant ecology and in more general modelling situations.

Genetic identity affects performance of species in grasslands of different plant diversity: an experiment with Lolium perenne cultivars.

Recent biodiversity research has focused on ecosystem processes, but less is known about responses of populations of individual plant species to changing community diversity and implications of genetic variation within species. To address these issues, effects of plant community diversity on the performance of different cultivars of Lolium perenne were analysed. Populations of 15 genetic cultivars of Lolium perenne were established in experimental grasslands varying in richness of species (from 1 to 60) and functional groups (from 1 to 4). Population sizes, mean size of individual plants, biomass of individual shoots and seed production were measured in the first and second growing season after establishment. Population sizes of all cultivars decreased with increasing community species richness. Plant individuals formed fewer shoots with a lower shoot mass in more species-rich plant communities. A large proportion of variation in plant size and relative population growth was attributable to effects of community species and functional group richness, but the inclusion of cultivar identity explained additional 4-7 % of variation. Cultivar identity explained most variation (28-51 %) at the shoot level (biomass of individual tillers and reproductive shoots, seed production, heading stage). Coefficients of variation of the measured variables across plant communities were larger in cultivars with a lower average performance, indicating that this variation was predominantly due to passive growth reductions and not a consequence of larger adaptive plastic responses. No single cultivar performed best in all communities. The decreasing performance of Lolium perenne in plant communities of increasing species richness suggests a regulation of competitive interactions by species diversity. Genetic variation within species provides a base for larger phenotypic variation and may affect competitive ability. However, heterogeneous biotic environments (= plant communities of different species composition) are important for the maintenance of intra-specific genetic variation.

Mating patterns and contemporary gene flow by pollen in a large continuous and a small isolated population of the scattered forest tree Sorbus torminalis

The influence of population size and spatial isolation on contemporary gene flow by pollen and mating patterns in temperate forest trees are not well documented, although they are crucial factors in the life history of plant species. We analysed a small, isolated population and a large, continuous population of the insect-pollinated tree species Sorbus torminalis in two consecutive years. The species recently experienced increased habitat fragmentation due to altered forest management leading to forests with closed canopies. We estimated individual plant size, percentage of flowering trees, intensity of flowering, degree of fruiting and seed set per fruit, and we determined mating patterns, pollen flow distances and external gene flow in a genetic paternity analysis based on microsatellite markers. We found clear effects of small population size and spatial isolation in S. torminalis. Compared with the large, continuous population, the small and isolated population harboured a lower percentage of flowering trees, showed less intense flowering, lower fruiting, less developed seeds per fruit, increased selfing and received less immigrant pollen. However, the negative inbreeding coefficients (F(IS)) of offspring indicated that this did not result in inbred seed at the population level. We also show that flowering, fruiting and pollen flow patterns varied among years, the latter being affected by the size of individuals. Though our study was unreplicated at the factor level (i.e. isolated vs non-isolated populations), it shows that small and spatially isolated populations of S. torminalis may also be genetically isolated, but that their progeny is not necessarily more inbred.

Population structure and population dynamic of Pulsatilla patens (L.) Mill. in relation to vegetation characteristics

In 2003, the last existing population of Pulsatilla patens in Germany was studied in the nature reserve “Garchinger Heide”, a calcareous grassland north of Munich. The aim of our study was to evaluate the endangerment of the species in ancient and young grasslands of the nature reserve. Therefore, the size, structure and generative reproduction of the population in relation to vegetation characteristics were investigated. Total population size and mean density of individual plants in different zones of the nature reserve were determined by a GPS-supported counting and compared to a mapping of 1991. The population structure was analyzed by counting the number of individuals of different age–state classes on 4 m 2 plots in ancient and young grasslands of the nature reserve. The cover of phanerogams, cryptogams, litter and bare soil was studied in order to characterize habitat quality. In 2003, about 9700 individuals of P. patens were counted which means that population size strongly decreased in comparison to 1991 (about 27 000). In the ancient grasslands, the number of individuals of all age–state classes of P. patens correlated negatively with the cover of phanerogams and litter. In contrast, phanerogam and cryptogam cover had a positive influence on the number of individuals of all age–state classes on a topsoil-removal site due to protection from desiccation. This site was the only one in the nature reserve, where population size increased between 1991 and 2003. The large population size indicates that short-term endangerment or extinction of the species in the nature reserve is not very likely. Nevertheless, the management of the nature reserve has to be optimized in order to prevent a further long-term decrease of the population size of P. patens.

Nutrient Availability and Atmospheric CO2 Partial Pressure Modulate the Effects of Nutrient Heterogeneity on the Size Structure of Populations in Grassland Species

Size-asymmetric competition occurs when larger plants have a disproportionate advantage in competition with smaller plants. It has been hypothesized that nutrient heterogeneity may promote it. Experiments testing this hypothesis are inconclusive, and in most cases have evaluated the effects of nutrient heterogeneity separately from other environmental factors. The aim of this study was to test, using populations of Lolium perenne, Plantago lanceolata and Holcus lanatus, two hypotheses: (a) nutrient heterogeneity promotes size-asymmetric competition; and (b) nutrient heterogeneity interacts with both atmospheric CO2 partial pressure (P(CO2)) and nutrient availability to determine the magnitude of this response. Microcosms consisting of monocultures of the three species were grown for 90 d in a factorial experiment with the following treatments: P(CO2) (37.5 and 70 Pa) and nutrient availability (NA; 40 and 120 mg of N added as organic material) combined with different spatial distribution of the organic material (NH; homogeneous and heterogeneous). Differences in the size of individual plants within populations (size inequality) were quantified using the coefficient of variation of individual above-ground biomass and the combined biomass of the two largest individuals in each microcosm. Increases in size inequality were associated with size-asymmetric competition. Size inequality increased when the nutrients were heterogeneously supplied in the three species. The effects of NH on this response were more pronounced under high nutrient supply in both Plantago and Holcus (significant NA x NH interactions) and under elevated P(CO2) in Plantago (significant P(CO2) x NA x NH interaction). No significant two- and three-way interactions were found for Lolium. Our first hypothesis was supported by our results, as nutrient heterogeneity promoted size-asymmetric competition in the three species evaluated. Nutrient supply and P(CO2) modified the magnitude of this effect in Plantago and Holcus, but not in Lolium. Thus, our second hypothesis was partially supported.

(372) Water Requirements of High bush Blueberry Cultivars Planted at Normal and High Density Spacings

Many growers in the Pacific Northwest are planting blueberry fields at higher densities to improve yields and increase the number of berries harvested per acre in the first few years after planting. The objective of this study was to determine the effect of high-density planting on blueberry water requirements. Although close spacing reduces individual plant size, we expected that plants spaced closer together would require more irrigation per unit land area than those spaced further apart due to increased canopy coverage within rows. The study utilized a 5-year-old planting of highbush blueberry, consisting of three cultivars, `Duke', `Bluecrop', and `Elliott', planted at 0.5- and 1.2-m in-row spacings. Plant water use was calculated from changes in soil water content measured using TDR probes for shallow depths and a neutron probe and access tubes for deeper depths. Stem water potentials were also measured periodically using a pressure chamber to determine how well irrigation was meeting crop water demands throughout the season. Surprisingly, plants spaced 0.5 m apart required only slightly more water than those spaced 1.2 m apart. They did, however, require more frequent irrigations due to their smaller root systems, especially during fruit filling. Water use by each cultivar increased during fruit filling and then rapidly decreased after harvest. `Duke' required the most water among cultivars, using 5–10 mm/day from mid-May to mid-August, while `Elliott' required the least, using 3–5 mm/day.