Showing papers in "Oecologia in 1994"

Relative importance of photosynthetic traits and allocation patterns as correlates of seedling shade tolerance of 13 tropical trees.

Abstract: Among 13 tropical tree species on Barro Colorado Island, species with high seedling mortality rates during the first year in shade had higher reltive growth rates (RGR) from germination to 2 months in both sun (23% full sun) and shade [2%, with and without lowered red: far red (R:FR) ratio] than shade tolerant species. Species with higher RGR in sun also had higher RGR in shade. These interspecific trends could be explained by differences in morphological traits and allocation paterns among species. Within each light regime, seedlings of shade-intolerant species had lower root: shoot ratios, higher leaf mass per unit area, and higher leaf area ratios (LAR) than shade tolerant species. In contrast, leaf gas exchange characteristics, or acclimation potential in these traits, had no relationship with seedling mortality rates in shade. In both shade tolerant and intolerant species, light saturated photosynthesis rates, dark respiration, and light compensation points were higher for sungrown seedlings than for shade-grown seedlings. Differences in R:FR ratio in shade did not affect gas exchange, allocation patterns, or growth rates of any species. Survival of young tree seedlings in shade did not depend on higher net photosynthesis or biomass accumulation rates in shade. Rather, species with higher RGR died faster in shade than species with lower RGR. This trend could be explained if survival depends on morphological characteristics likely to enhance defense against herbivores and pathogens, such as dense and tough leaves, a well-established root system, and high wood density. High construction costs for these traits, and low LAR as a consequence of these traits, should result in lower rates of whole-plant carbon gain and RGR for shade tolerant species than shade-intolerant species in shade as well as in sun.

13C discrimination during CO2 assimilation by the terrestrial biosphere

Abstract: Estimates of the extent of the discrimination against13CO2 during photosynthesis (ΔA) on a global basis were made using gridded data sets of temperature, precipitation, elevation, humidity and vegetation type. Stomatal responses to leaf-to-air vapour mole fraction difference (D, leaf-to-air vapour pressure difference divided by atmospheric pressure) were first determined by a literature review and by assuming that stomatal behaviour results in the optimisation of plant water use in relation to carbon gain. Using monthly time steps, modelled stomatal responses toD were used to calculate the ratio of stomatal cavity to ambient CO2 mole fractions and then, in association with leaf internal conductances, to calculate ΔA. Weighted according to gross primary productivity (GPP, annual net CO2 asimilation per unit ground area), estimated ΔA for C3 biomes ranged from 12.9‰ for xerophytic woods and shrub to 19.6‰ for cool/cold deciduous forest, with an average value from C3 plants of 17.8‰. This is slightly less than the commonly used values of 18–20‰. For C4 plants the average modelled discrimination was 3.6‰, again slightly less than would be calculated from C4 plant dry matter carbon isotopic composition (yielding around 5‰). From our model we estimate that, on a global basis, 21% of GPP is by C4 plants and for the terrestrial biosphere as a whole we calculate an average isotope discrimination during photosynthesis of 14.8‰. There are large variations in ΔA across the globe, the largest of which are associated with the precence or absence of C4 plants. Due to longitudinal variations in ΔA, there are problems in using latitudinally averaged terrestrial carbon isotope discriminations to calculate the ratio of net oceanic to net terrestrial carbon fluxes.

Adaptive variation in growth rate: life history costs and consequences in the speckled wood butterfly,Pararge aegeria.

Abstract: An important assumption made in most lifehistory theory is that there is a trade-off between age and size at reproduction. This trade-off may, however, disappear if growth rate varies adaptively. The fact that individuals do not always grow at the maximum rate can only be understood if high growth rates carry a cost. This study investigates the presence and nature of such costs inPararge aegeria. Five females from two populations with known differences in life history (south Sweden and Maderia) were allowed to oviposit in the laboratory and their offspring were reared in environmental chambers under conditions leading to direct development. We measured several aspects of life history, including development times, pupal and adult weights, growth rate, female fecundity, longevity and larval starvation endurance. In both populations there seemed to be genetic variation in growth rate. There was no evidence for a trade-off between age and size at pupation. As predicted, larvae with high growth rates also lost weight at a relatively higher rate during starvation. High weight-loss rates were furthermore associated with a lower probability of surviving when food became available again. This is apparently the first physiological trade-off with growth rate that has been experimentally demonstrated. In both populations there were significant differences in growth rate between the sexes, but the populations differed in which sex was growing at the highest rate. In Sweden males had higher growth rates than females, whereas the reverse was true for Madeira. These patterns most likely reflect differences in selection for protandry, in turn caused by differences in seasonality between Sweden and Madeira. Together with the finding that males had shorter average longevity than females in the Swedish, but not in the Maderiran, population, this indicates that a lower adult quality also may be a cost of high growth rate. We argue that for the understanding of life history variation it is necessary to consider not only the two dimensions of age and size, but also to take into full account the triangular nature of the relationship between size, time and growth rate.

Photosynthesis-nitrogen relations in Amazonian tree species : II. Variation in nitrogen vis-a-vis specific leaf area influences mass- and area-based expressions.

Abstract: The relationships between leaf nitrogen (N), specific leaf area (SLA) (an inverse index of leaf “thickness” or “density”), and photosynthetic capacity (Amax) were studied in 23 Amazonian tree species to characterize scaling in these properties among natural populations of leaves of different ages and light microenvironments, and to examine how variation within species in N and SLA can influence the expression of the Amax-to-N relationship on mass versus area bases. The slope of the Amax-N relationship, change in A per change in N (μmol CO2 gN-1 s-1), was consistently greater, by as much as 300%, when both measures were expressed on mass rather than area bases. The x-intercept of this relationship (N-compensation point) was generally positive on a mass but not an area basis. In this paper we address the causes and implications of such differences. Significant linear relationships (p<0.05) between mass-based leaf N (Nmass) and SLA were observed in 12 species and all 23 regressions had positive slopes. In 13 species, mass-based Amax (Amass) was positively related (p<0.05) with SLA. These patterns reflect the concurrent decline in Nmass and SLA with increasing leaf age. Significant (p<0.05) relationships between area-based leaf N (Narea) and SLA were observed in 18 species. In this case, all relationships had negative slopes. Taken collectively, and consistent in all species, as SLA decreased (leaves become “thicker”) across increasing leaf age and light gradients, Nmass also decreased, but proportionally more slowly, such that Narea increased. Due to the linear dependence of Amass on Nmass and a negative 4-intercept, “thicker” leaves (low SLA) therefore tend, on average, to have lower Nmass and Amass but higher Narea than “thinner” leaves. This tendency towards decreasing Amass with increasing Narea, resulting in a lower slope of the Amax-N relationship on an area than mass basis in 16 of 17 species where both were significant. For the sole species exception (higher area than mass-based slope) variation in Narea was related to variation in Nmass and not in SLA, and thus, these data are also consistent with this explanation. The relations between N, SLA and Amax explain how the rate of change in Amax per change in N can vary three-fold depending on whether a mass or area mode of expression is used.

Vascular plants as bioindicators of regional warming in Antarctica

Abstract: Monitoring selected populations of the only two native Antarctic vascular plant species (Colobanthus quitensis andDeschampsia antarctica) over a 27-year period has revealed a significant and relatively rapid increase in numbers of individuals and populations at two widely separated localities in the maritime Antarctic. There is strong evidence that this increase is a response to a warming trend in summer air temperatures, which has been evident throughout the region since the late 1940s, enhancing seed maturation, germination and seedling survival. This study provides the only known long-term monitoring data for any terrestrial organisms in Antarc-tica. Because their response to ameliorating conditions is more rapid than that of the dominant cryptogamic groups, Antarctic phanerogams may be useful bioindicators of climate change in West Antarctica.

Photosynthesis-nitrogen relations in Amazonian tree species : I. Patterns among species and communities.

Abstract: Among species, photosynthetic capacity (Amax) is usually related to leaf nitrogen content (N), but variation in the species-specific relationship is not well understood. To address this issue, we studied Amax-N relationships in 23 species in adjacent Amazonian com- munities differentially limited by nitrogen (N), phospho- rus (P), and/or other mineral nutrients. Five species were studied in each of three late successional forest types (Tierra Firme, Caatinga and Bana) and eight species were studied on disturbed sites (cultivated and early sec- ondary successional Tierra Firme plots). Area x expressed on a mass basis (Amass) was correlated (p < 0.05) with N.... in 17 of 23 species, and Area x on an area basis (Aarea) was correlated (p<0.05) with N.... in 21 of 23 species. The slopes of Amax-N relationships were greater and intercepts lower for disturbance adapted early suc- cessional species than for late successional species. On a mass basis, the Amax-N slope averaged g 15 gmol CO2 (g N) -~ s -1 for 7 early secondary successional species and ~4 pmol CO2 (g N) -1 s -1 for 15 late successional species, respectively. Species from disturbed sites had shorter leaf life-span and greater specific leaf area (SLA) than late successional species. Across all 23 species, the slope of the Amass-N.... relationship was related (p < 0.001) positively to SLA (r 2 = 0.70) and negatively to leaf life-span (r 2= 0.78) and temporal niche during sec- ondary succession (years since cutting-and-burning, r2= 0.90). Thus, disturbance adapted early successional species display a set of traits (short leaf life-span, high SLA and Amax and a steep slope of Amax-N) conducive to resource acquisition and rapid growth in their high resource regeneration niches. The significance and form of the Amax-N relationship were associated with the rela- tive nutrient limitations in the three late successional communities. At species and community levels, Amax was more highly dependent on N in the N-limited Caatinga than in the P- and N-limited Bana and least in the P- and Ca-limited Tierra Firme on oxisol- and dif- ferences among these three communities in their mass- based Ama slope reflects this pattern (6.0, 2.4, and 0.7 pmol CO2 (g N)- 1 s-1, respectively). Among all 23 spe- cies, the estimated leaf N.... needed to reach compensa- tion (net photosynthesis ~ zero) was positively related to the A.... -N.... slope and to dark respiration rates and negatively related to leaf life-span. Variation among species in the Amax-N slope was well correlated with potential photosynthetic N use efficiency, Ama x per unit leaf N. The dependence of Ama x on N and the form of the relationship vary among Amazonian species and com- munities, consistent with both relative availabilities of N, P, and other mineral nutrients, and with intrinsic ecophysiological characteristics of species adapted to habitats of varying resource availability.

Separation of allelopathy and resource competition by the boreal dwarf shrub Empetrum hermaphroditum Hagerup

Abstract: An experimental technique was used to separate and evaluate the magnitude of allelopathic interference relative to resource competition by the boreal dwarf shrub Empetrum hermaphroditum Hagerup (Empetraceae). To test for resource competition and allelopathy, respectively, Scots pine (Pinus sylvestris L.) seedlings were grown in both the greenhouse and in the field over a 3 year period, in four different treatments within E. hermaphroditum vegetation: (1) PVC tubes were used to reduce effects of interspecific below-ground competition; (2) activated carbon was spread on the soil to adsorb toxins leached from E. hermaphroditum litter and green leaves, thus reducing effects of allelopathic interference; (3) E. hermaphroditum vegetation was left untreated to evaluate inhibiting effects when both allelopathy and resource competition were present; (4) PVC tubes, placed in E. hermaphroditum vegetation spread with activated carbon were used to determine growth of seedlings when both allelopathy and resource competition were reduced. Scots pine seedlings grown in untreated vegetation (with both root competition and allelopathy present) had the lowest shoot length and dry weight; seedlings with both allelopathy and root competition reduced (activated carbon in tube) were the largest. Reducing either root competition alone (tube treatment) or allelopathy alone (carbon treatment) produced seedlings of intermediate size, but reduced competition had a greater effect than reduced allelopathy (although, in the greenhouse, significantly so only for root biomass). In the greenhouse experiment, biomass production of seedlings grown free of both interactions (carbon in tube) was greater than the simple sum of the growth response to the individual interactions (tube treatment and carbon treatment, respectively). Larger shoot:root ratios were also found when pine seedlings were grown without tubes (i.e. when resource competition was occurring). In the field, the removal of allelopathy (carbon treatments) increased shoot:root ratio when compared to the removal of resource competition. The study showed that two different interference mechanisms of E. hermaphroditum can be separated and quantified, and that below-ground competition and allelopathy by E. hermaphroditum are both important factors retarding growth of Scots pine.

Environmental and developmental controls over the seasonal pattern of isoprene emission from aspen leaves.

Abstract: Isoprene emission from plants represents one of the principal biospheric controls over the oxidative capacity of the continental troposphere. In the study reported here, the seasonal pattern of isoprene emission, and its underlying determinants, were studied for aspen trees growing in the Rocky Mountains of Colorado. The springtime onset of isoprene emission was delayed for up to 4 weeks following leaf emergence, despite the presence of positive net photosynthesis rates. Maximum isoprene emission rates were reached approximately 6 weeks following leaf emergence. During this initial developmental phase, isoprene emission rates were negatively correlated with leaf nitrogen concentrations. During the autumnal decline in isoprene emission, rates were positively correlated with leaf nitrogen concentration. Given past studies that demonstrate a correlation between leaf nitrogen concentration and isoprene emission rate, we conclude that factors other than the amount of leaf nitrogen determine the early-season initiation of isoprene emission. The late-season decline in isoprene emission rate is interpreted as due to the autumnal breakdown of metabolic machinery and loss of leaf nitrogen. In potted aspen trees, leaves that emerged in February and developed under cool, springtime temperatures did not emit isoprene until 23 days after leaf emergence. Leaves that emrged in July and developed in hot, midsummer temperatures emitted isoprene within 6 days. Leaves that had emerged during the cool spring, and had grown for several weeks without emitting isoprene, could be induced to emit isoprene within 2 h of exposure to 32°C. Continued exposure to warm temperatures resulted in a progressive increase in the isoprene emission rate. Thus, temperature appears to be an important determinant of the early season induction of isoprene emission. The seasonal pattern of isoprene emission was examined in trees growing along an elevational gradient in the Colorado Front Range (1829-2896 m). Trees at different elevations exhibited staggered patterns of bud-break and initiation of photosynthesis and isoprene emission in concert with the staggered onset of warm, springtime temperatures. The springtime induction of isoprene emission could be predicted at each of the three sites as the time after bud break required for cumulative temperatures above 0°C to reach approximately 400 degree days. Seasonal temperature acclimation of isoprene emission rate and photosynthesis rate was not observed. The temperature dependence of isoprene emission rate between 20 and 35°C could be accurately predicted during spring and summer using a single algorithm that describes the Arrhenius relationship of enzyme activity. From these results, it is concluded that the early season pattern of isoprene emission is controlled by prevailing temperature and its interaction with developmental processes. The late-season pattern is determined by controls over leaf nitrogen concentration, especially the depletion of leaf nitrogen during senescence. Following early-season induction, isoprene emission rates correlate with photosynthesis rates. During the season there is little acclimation to temperature, so that seasonal modeling simplifies to a single temperature-response algorithm.

Nitrogen nutrition and isotope differences among life forms at the northern treeline of Alaska.

Abstract: Natural abundances of nitrogen isotopes, δ15N, indicate that, in the same habitat, Alaskan Picea glauca and P. mariana use a different soil nitrogen compartment from the evergreen shrub Vaccinium vitis-idaea or the deciduous grass Calamagrostis canadensis. The very low δ15N values (-7.7 ‰) suggest that (1) Picea mainly uses inorganic nitrogen (probably mainly ammonium) or organic N in fresh litter, (2) Vaccinium (-4.3 ‰) with its ericoid mycorrhizae uses more stable organic matter, and (3) Calamagrostis (+0.9 ‰) exploits deeper soil horizons with higher δ15N values of soil N. We conclude that species limited by the same nutrient may coexist by drawing on different pools of soil N in a nutrient-deficient environment. The differences among life-forms decrease with increasing N availability. The different levels of δ15N are associated with different nitrogen concentrations in leaves, Picea having a lower N concentration (0.62 mmol g-1) than Vaccinium (0.98 mmol g-1) or Calamagrostis (1.33 mmol g-1). An extended vector analysis by Timmer and Armstrong (1987) suggests that N is the most limiting element for Picea in this habitat, causing needle yellowing at N concentrations below 0.5 mmol g-1 or N contents below 2 mmol needle-1. Increasing N supply had an exponential effect on twig and needle growth. Phosphorus, potassium and magnesium are at marginal supply, but no interaction between ammonium supply and needle Mg concentration could be detected. Calcium is in adequate supply on both calcareous and acidic soils. The results are compared with European conditions of excessive N supply from anthropogenic N depositions.

Long term effects of naturally elevated CO2 on mediterranean grassland and forest trees.

Abstract: We investigated the carbon supply status in species-rich mediterranean plant communities growing in a bowl-shaped 1-ha “CO2 spring” area near Sienna, Italy. A geothermic “lime-kiln” has provided these communities, for as long as historical records are available, with pure CO2 that mixes with ambient air at canopy level to daytime means of 500–1000 ppm CO2. Immediately outside the spring area similar plant communities are growing on similar substrate, and in the same climate, but under ca. 355 ppm CO2. We found no evidence that plants in the CO2 spring area grow faster, flower earlier or become larger. However, we found very large differences in tissue quality among the 40 species studied inside and outside the spring area. Depending on weather conditions, the mean concentration of total non-structural carbohydrates (TNC, sugars and starch) in leaves of herbaceous plants was 38–47% higher in the spring area. Fast growing ruderals growing on garden soil inside and outside the spring area show the same response. Among trees, leaves of the deciduousQuercus pubscens contain twice as much TNC inside as outside the vent area, whereas evergreenQ. ilex leaves show no significant difference. TNC levels in branch wood paralleled leaf values. TNC in shade leaves was also higher. Elevated CO2 had no effect on the sugar fraction, therefore differences in TNC are due to starch accumulation. Leaf nitrogen concentration decreases under elevated CO2. These observations suggest that the commonly reported TNC accumulation and N depletion in leaves growing under elevated CO2 are not restricted to the artificial conditions of short-term CO2 enrichment experiments but persist over very long periods. Such an alteration of tissue composition can be expected to occur in other plant communities also if atmospheric CO2 levels continue to rise. Effects on food webs and nutrient cycling are likely.

Effects of leaf age, nitrogen nutrition and photon flux density on the distribution of nitrogen among leaves of a vine (Ipomoea tricolor Cav.) grown horizontally to avoid mutual shading of leaves.

Abstract: Effects of leaf age, nitrogen nutrition and photon flux density (PFD) on the distribution of nitrogen among leaves were investigated in a vine, Ipomoea tricolor Cav., which had been grown horizontally so as to avoid mutual shading of leaves. The nitrogen content was highest in newly developed young leaves and decreased with age of leaves in plants grown at low nitrate concentrations and with all leaves exposed to full sunlight. Thus, a distinct gradient of leaf nitrogen content was formed along the gradient of leaf age. However, no gradient of leaf nitrogen content was formed in plants grown at a high nitrate concentration. Effects of PFD on the distribution of nitrogen were examined by shading leaves in a manner that simulated changes in the light gradient of an erect herbaceous canopy (i.e., where old leaves were placed under increasingly darker conditions with growth of the canopy). This canopy-type shading steepened the gradient of leaf nitrogen content in plants grown at a low nitrogen supply, and created a gradient in plants grown at high concentrations of nitrate. The steeper the gradient of PFD, the larger the gradient of leaf nitrogen that was formed. When the gradient of shading was inverted, that is, younger leaves were subjected to increasingly heavier shade, while keeping the oldest leaves exposed to full sunlight, an inverted gradient of leaf nitrogen content was formed at high nitrate concentrations. The gradient of leaf nitrogen content generated either by advance of leaf age at low nitrogen availability, or by canopy-type shading, was comparable to those reported for the canopies of erect herbaceous plants. It is concluded that both leaf age and PFD have potential to cause the non-uniform distribution of leaf nitrogen. It is also shown that the contribution of leaf age increases with the decrease in nitrogen nutrition level.

Offspring fitness in relation to population size and genetic variation in the rare perennial plant species Gentiana pneumonanthe (Gentianaceae)

Abstract: Seeds were sampled from 19 populations of the rare Gentiana pneumonanthe, ranging in size from 5 to more than 50,000 flowering plants. An analysis was made of variation in a number of life-history characters in relation to population size and offspring heterozygosity (based on seven polymorphic isozyme loci). Life-his-tory characters included seed weight, germination rate, proportion of seeds germinating, seedling mortality, seedling weight, adult weight, flower production per plant and proportion of plants flowering per family. Principal component analysis (PCA) reduced the dataset to three main fitness components. The first component was highly correlated with adult weight and flowering performance, the second with germination performance and the third component with seed and seedling weight and seedling mortality. The latter two components were considered as being maternally influenced, since these comprised life-history traits that were significantly correlated with seed weight. Multiple regression analysis showed that variation in the first fitness component was mainly associated with heterozygosity and not with population size, while the third fitness component was only correlated with population size and not with heterozygosity. The latter relationship appeared to be non-linear, which suggests a stronger loss of fitness in the smallest populations. The second (germination) component was neither correlated with population size nor with genetic variation. There was only a weak association between population size, heterozygosity and the population coefficients of variation for each life history character. Most correlation coefficients were negative, however, which suggests that there is more variation among progeny from smaller populations. We conclude that progeny from small populations of Gentiana pneumonanthe show reduced fitness and may be phenotypically more variable. One of the possible causes of the loss of fitness is a combination of unfavourable environmental circumstances for maternal plants in small populations and increased inbreeding. The higher phenotypic variation in small populations may also be a result of inbreeding, which can lead to deviation of individuals from the average phenotype through a loss of developmental stability.

Sources of water used by riparian Eucalyptus camaldulensis overlying highly saline groundwater.

Abstract: Water sources of Eucalyptus camaldulensis Dehn. trees were investigated on a semiarid floodplain in south-eastern Australia. The trees investigated ranged in distance from 0.5 to 40 m from a stream, with electrical conductivity 0.8 dSm−1, and grew over groundwater with electrical conductivity ranging from 30 to 50 dSm−1. The sources of water being used by the trees were investigated using the naturally occurring stable isotopes of water and measurements of soil water potential. Xylem water potential and leaf conductance were also examined to identify the trees' response to using these sources of water. Trees at distances greater than about 15 m from the stream used no stream water. The trees used groundwater in summer and a combination of groundwater and rain-derived surface-soil water (0.05–0.15 m depth) in winter. In doing so they suffered water stress at electrical conductivities higher than approximately 40 dSm−1 (equivalent to approximately −1.4 MPa). Trees adjacent to the stream used stream water directly in summer, but may have used stream water from the soil profile in winter, after the stream had risen and recharged the soil water. E. camaldulensis appeared to be partially opportunistic in the sources of water they used.

CO2 alters water use, carbon gain, and yield for the dominant species in a natural grassland.

Abstract: Global atmospheric CO2 is increasing at a rate of 1.5-2 ppm per year and is predicted to double by the end of the next century. Understanding how terrestrial ecosystems will respond in this changing environment is an important goal of current research. Here we present results from a field study of elevated CO2 in a California annual grassland. Elevated CO2 led to lower leaf-level stomatal conductance and transpiration (approximately 50%) and higher mid-day leaf water potentials (30-35%) in the most abundant species of the grassland, Avena barbata Brot. Higher CO2 concentrations also resulted in greater midday photosynthetic rates (70% on average). The effects of CO2 on stomatal conductance and leaf water potential decreased towards the end of the growing season, when Avena began to show signs of senescence. Water-use efficiency was approximately doubled in elevated CO2, as estimated by instantaneous gas-exchange measurements and seasonal carbon isotope discrimination. Increases in CO2 and photosynthesis resulted in more seeds per plant (30%) and taller and heavier plants (27% and 41%, respectively). Elevated CO2 also reduced seed N concentrations (9%).

Interspecific hybridization of plants and resistance to herbivores: hypotheses, genetics, and variable responses in a diverse herbivore community.

Abstract: We studied the morphology, molecular genetics, and hebivory of two species of willows (Salix sericea and S. eriocephala) and their interspecific hybrids to test four alternative hypotheses concerning the effects of hybridization on plant resistance. Individually marked plants were identified using morphological traits in the field and measurements of stipule and leaf pubescence were made and compared using Canonical Discriminant Function Analysis. DNA was extracted from the leaves of a sample of the marked plants and RAPD-PCR analysis was performed to establish the genetic status of parental and hybrid plants. RAPD band analysis generally verified the genetic status of parental plants. Hybrid plants were usually correctly identified in the field with a few exceptions. However, the hybrid plants were a heterogeneous group of plants made up of most plants that appear to be F1s and a few plants that appear to be backcrosses to S. sericea. Morphological variables were useful for distinguishing S. sericea from S. eriocephala and hybrids, but were not as dependable in distinguishing between S. eriocephala and hybrids. We compared the densities of 11 herbivore species and the infection by a leaf rust pathogen (Melampsora sp.) on the leaves and stems of two parents and the hybrids in the field. We found support for the Additive hypothesis (3 species), the Dominance hypothesis (2 species) and the Hybrid Susceptibility hypothesis (7 species, 6 herbivores and the Melampsora rust). We found no evidence for the Hybrid Resistance hypothesis. Guild membership was not a good predictor of similar responses of species to hybrid versus parental plants. A Canonical Discriminant Function Analysis showed discrete separation of the taxa based on herbivore densities, illustrating different community structures on hybrid and parental plants. This study demonstrates the diversity of responses of phytophages in response to interspecific hybridization.

Trap-nesting bees and wasps colonizing set-aside fields: succession and body size, management by cutting and sowing

Abstract: Trap-nesting bees and wasps (Hymenoptera Aculeata) colonizing crop and fallow fields in an agricultural landscape were studied using 20 sown fields (pea, barley, rye, clover-grass mixtures, Phacelia tanacetifolia) and 20 fields with naturally developed vegetation (1- and 2-year old fields, both mown and unmown, and old meadows) Fourteen species of Apoidea, 4 of Sphecidae, 1 of Eumenidae and 4 of parasitoids were reared from reed nests exposed in these 40 fields of 10 field-types Fields with naturally developed vegetation had twice as many species as sown fields, due to the distribution pattern of the 14 bee species, whereas the 9 predatory species (wasps and parasitoids) showed a rather uniform distribution None of the trap-nesting bees were found in Phacelia fields, despite contrasting expectations of beekeepers Old meadows showed a particularly high abundance and species richness, since only 10% of all traps were exposed, but 32% of all bee nests were sampled in old meadows, including 4 bee species that were not found elsewhere Accordingly, species richness of fields with naturally developed vegetation showed a significant increase with age Variability in Hymenoptera species numbers could be explained by corresponding differences in plant species numbers The alternative hypothesis that field size or field connectivity influenced species richness was not supported Habitats with great floral diversity appeared to offer better and richer food resources for the flower-visiting bees, whereas food availability apparently did not influence predatory wasps The bees Osmia caerulescens and Megachile versicolor that had colonized early-successional fields took twice as long to provision cells as those that colonized late-successional meadows characterized by a greater plant species richness In contrast, the eumenid wasp Ancistrocerus gazella took a similar period of time to provision cells in both field types In addition, bee and wasp species of plant-species-poor fields were on average significantly larger than those of plant-species-rich fields Thus, body size appeared to be a good predictor of colonization ability Management by cutting greatly increased plant species richness in early-successional set-aside fields and thus doubled species richness of bees Cutting of early-successional habitats can be expected to benefit insects and plants in general, whereas older grassland should show the greatest insect diversity when both mown and unmown parts are present

Density and distance-to-adult effects of a canker disease of trees in a moist tropical forest.

Abstract: We compared the spatial distribution of stem cankers on the canopy tree Ocotea whitei (Lauraceae) in a 20-ha plot on Barro Colorado Island, Panama, with spatial and temporal patterns of mortality in this host over the previous decade. The cankers occur both on adult and juvenile individuals, aothough juveniles are much more likely the adults to show symptoms. Disease incidence is host-density dependent, and both the presence of the disease and host mortality are more likely close to than far from a conspecific adult, which resulted in a net spatial shift of the juvenile population away from conspecific adults through time. Disease incidence is lower than expected among juveniles of O. whitei growing near to adults of the non-susceptible canopy tree Beilschmiedia pendula. The coincidence of spatial patterns of canker incidence and host mortality suggest a role for the disease in regulating host spatial distribution, in agreement with predictions of the Janzen-Connell hypothesis.

The ecological significance of nickel hyperaccumulation: a plant chemical defense

Abstract: Nickel hyperaccumulating plants have more than 1000 mg Ni kg−1 dry weight when grown on nickel-bearing soils. We hypothesized that Ni hyperaccumulation could serve as a chemical defense against herbivores In feeding experiments with potential insect herbivores and Ni hyperaccumulating plants, only those inseets fed leaves from plants grown on non-nickel-bearing soil survived or showed a weight gain. Among chemical parameters measured, only Ni content of plants was sufficient to explain this result. When subjected to herbivory by lepidopteran larvae, plants grown on Ni-amended soil showed greater survival and yield than plants on unamended soil. Ni hyperaccumulation may be an effective plant chemical defense against herbivores because of its high lethality, apparent low cost, and broad spectrum of toxicity.

Large- and small-scale effects of habitat structure on rates of predation: how percent coverage of seagrass affects rates of predation and siphon nipping on an infaunal bivalve

Abstract: Landscape ecology, predominantly a terrestrial discipline, considers the effect of large-scale (tens of meters to kilometers) spatial patterns of habitats on ecological processes such as competition, predation, and flow of energy. In this study, a landscape-ecology approach was applied to a marine soft-sediment environment to examine rates of predation and transfer of secondary production in and around vegetated habitats. Seagrass beds naturally occur in a variety of spatial configurations from patches 1–10s of meters across with interspersed unvegetated sediments (i.e., patchy coverage) to more continuous coverage with little or no bare sediment. I designed experiments to address how percent coverage of seagrass in a 100-m2 area of seafloor, and the spatial arrangement (degree of patchiness or fragmentation) of an equal area (100 m2) of vegetation affected predation (lethal) and siphon nipping (sublethal) intensity on an infaunal bivalve, Mercenaria mercenaria (hard clam). Measures of seagrass density and biomass with different percent coverage of seagrass were also made. When clams were placed in both the vegetated and unvegetated portions of the seafloor nearly twice as many clams were recovered live with 99% seagrass cover than with 23% seagrass cover, while survivorship was intermediate with 70% cover. Cropping of clam siphons from both the vegetated and unvegetated sediments was also affected by the amount of seagrass cover in a 100-m2 area of seafloor: mean adjusted siphon weights were approximately 76% heavier from the 99% seagrass cover treatment than from the 70% or 23% cover treatments. Survivorship of clams placed within an equal area of seagrass in very patchy, patchy, and continuous spatial configurations was 40% higher in the continuous seagrass treatment than in either of the two patchy treatments. This study demonstrates that transfer of secondary production in the form of predation and cropping on an infaunal organism is altered as the percent cover of seagrass changes. While large-scale changes in the amount and spatial patterning of vegetation may affect habitat utilization patterns and foraging HGLoopbehavior, increased seagrass density and biomass with increased percent coverage of seagrass limit any conclusions concerning predator foraging behavior and feeding success in response to patch shapes and sizes. Instead, local changes in seagrass characteristics provide the most compelling explanation for the observed results.

The functional significance of the browser-grazer dichotomy in African ruminants.

Abstract: The allometric relationships for the fermentation rate of dry matter, the total energy concentration of volatile fatty acids (VFAs), the energy supplied from VFA production and the mass of the digesta contents within the rumen or caecum and proximal colon (hindgut) were used to test whether the digestive strategies of grazing and browsing African ruminants differ. The wet and dry mass of the contents of the rumen and hindgut were allometrically related to body mass (BM). These relationships did not differ between browsing and grazing ruminants. The fermentation rates in the rumen were strongly allometric and the intercepts of the relationships did not differ between browsers and grazers. The fermentation rates in the hindgut were not allometrically related to BM and did not differ between ruminants with different feeding habits. Likewise, the total energy concentration of the VFAs in the rumen and hindgut showed no allometric scaling and did not differ between browsing and grazing ruminants. The energy supplied by VFA production in both the rumen and hindgut of African ruminants scaled at around 0.8 with BM. Only in the case of the energy supplied by VFAs in the rumen were there significantly different intercepts for browsing and grazing ruminants. The energy supplied by VFA production in the rumen was inadequate to meet the energy requirements for maintenance of browsers and small grazers. The retention time of digesta in the alimentary tract was positively related to BM although there was no difference in the allometric relationships for grazers and browsers. The results of these analyses suggest that, after controlling for the effects of body mass, there is little difference in digestive strategy between African ruminants with different morphological adaptations of the gut.

Meat ants as dominant members of Australian ant communities: an experimental test of their influence on the foraging success and forager abundance of other species.

Abstract: Meat ants (Iridomyrmex purpureus and allies) are perceived to be dominant members of Australian ant communities because of their great abundance, high rates of activity, and extreme aggressiveness. Here we describe the first experimental test of their influence on other ant species, and one of the first experimental studies of the influence of a dominant species on any diverse ant community. The study was conducted at a 0.4 ha savanna woodland site in the seasonal tropics of northern Australia, where the northern meat ant (I. sanguineus) represented 41% of pitfall catches and 73% of all ants at tuna baits, despite a total of 74 species being recorded. Meat ants were fenced out of experimental plots in order to test their influence on the foraging success of other species, as measured by access to tuna baits. The numbers of all other ants and ant species at baits in exclusion plots were approximately double those in controls (controlling for both the fences and for meat ant abundance), and returned rapidly to control levels when fences were removed after 7 weeks. Individual species differend markedly in their response to the fencing treatment, with species of Camponotus and Monomorium showing the strongest responses. Fencing had no effect on pitfall catches of species other than the meat ant, indicating that the effect of meat ants at baits was directly due to interference with foraging workers, and not regulation of general forager abundance. Such interference by meat ants has important implications for the sizes and densities of colonies of other ant species, and ultimately on overall ant community structure.

Seasonal changes in environmental variables, biomass, production and nutrient contents in two contrasting tropical intertidal seagrass beds in South Sulawesi, Indonesia.

Abstract: Seasonal dynamics were studied by monthly monitoring of biological and environmental variables in permanent quadrats in two contrasting intertidal seagrass beds in South Sulawesi, Indonesia, from February 1991 to January 1992. Datasets were analysed with canonical correlation analysis for correlations between environmental and biological variables. Considerable variation in biomass, production and plant tissue nutrient contents in a monospecific seagrass bed of Enhalus acoroides, growing on a coastal terrigenous mudbank (Gusung Tallang), was assumed to be related to riverine influences of the nearby Tallo River. The variation in seagrass variables at this site could, however, not be significantly correlated to seasonal patterns in rainfall, salinity, tides, nutrient availability, water motion or turbidity. A seasonal cycle in biomass, production and nutrient contents in a mixed seagrass bed of Thalassia hemprichii and E. acoroides, growing on carbonate sand on the reef flat of an offshore coral island (Barang Lompo), was found to be largely determined by tidal exposure and water motion. Exposure of the intertidal seagrass bed during hours of low water during spring tides showed a gradual shift from exposure during the night (January-June) to exposure during daylight (July-December). Daylight exposure resulted in a significant loss of above-ground plant biomass through desiccation and 'burning' of leaves. The observed seasonal dynamics of the seagrass bed on reef sediment contrast with reports from the Caribbean, where the effect of tidal exposure on comparable shallow-water seagrass communities is relatively insignificant due to a small tidal amplitude.

Plant hybrid zones as centers of biodiversity: the herbivore community of two endemic Tasmanian eucalypts

Abstract: We found the hybrid zone between Eucalyptus amygdalina and Eucalyptus risdonii to be a center of insect and fungal species richness and abundance. Of 40 taxa examined, 73% were significantly more abundant in the hybrid zone than in pure zones, 25% showed on significant differences, and 2% were most abundant on a pure host species. The average hybrid tree supported 53% more insect and fungal species, and relative abundances were, on average, 4 times greater on hybrids than on either eucalypt species growing in pure stands. Hybrids may act as refugia for rare species: 5 of 40 species were largely restricted to the hybrid zone. Also, 50% of the species coexisted only in the hybrid zone, making for mique species assemblages. Although hybrids support more species and greater abundances, all hybrids are not equal: 68% of the 40 taxa examined were significantly more abundant on one hybrid phenotype than another. While herbivore concentrations on F1 type intermediates were rare, concentrations were common on phenotypes resembling backcrosses either to E. amygdalina or E. risdonii. For specialist herbivores, the hybrid phenotype most heavily utilized appears to be determined by its phenotypic affinity to its host species. Generalists exhibit an overall greater abundance on hybrids, but are less likely to utilize one hybrid phenotype over another. Mechanistic explanations for these distributions are numerous and probably species specific, but are likely to include: increased genetic susceptibility of hybrids due to hybrid breakdown; increased stress in the hybrid zone resulting in greater plant susceptibility; and a greater diversity of resources in the hybrid zone which could support more species. Seed capsule production by hybrids and their parental species is negatively correlated with herbivory. However, it is difficult to determine whether herbivores cause this pattern as hybrids may have inherently lower sexual reproduction. Laws enacted to protect rare and endangered species do not include hybrids. We argue that a re-examination of our current "hybrid policy" is warranted. Plant hybrid zones are centers of plant evolution and speciation, sources of economically important plants and potential biocontrol agents, and, as our study suggests, also provide essential habitats for phytophagous communities.

Piscivore efficiency and refuging prey: the importance of predator search mode

Abstract: In predator-prey interactions, the efficiency of the predator is dependent on characteristics of both the predator and the prey, as well as the structure of the environment. In a field enclosure experiment, we tested the effects of a prey refuge on predator search mode, predator efficiency and prey behaviour. Replicated enclosures containing young of the year (0+) and 1-year-old (1+) perch were stocked with 3 differentially sized individuals of either of 2 piscivorous species, perch (Perca fluviatilis), pike (Esox lucius) or no piscivorous predators. Each enclosure contained an open predator area with three small vegetation patches, and a vegetated absolute refuge for the prey. We quantified the behaviour of the predators and the prey simultaneously, and at the end of the experiment the growth of the predators and the mortality and habitat use of the prey were estimated. The activity mode of both predator species was stationary. Perch stayed in pairs in the vegetation patches whereas pike remained solitary and occupied the corners of the enclosure. The largest pike individuals stayed closest to the prey refuge whereas the smallest individuals stayed farthest away from the prey refuge, indicating size-dependent interference among pike. Both size classes of prey showed stronger behavioural responses to pike than to perch with respect to refuge use, distance from refuge and distance to the nearest predator. Prey mortality was higher in the presence of pike than in the presence of perch. Predators decreased in body mass in all treatments, and perch showed a relatively stronger decrease in body mass than pike during the experiment. Growth differences of perch and pike, and mortality differences of prey caused by predation, can be explained by predator morphology, predator attack efficiency and social versus interference behaviour of the predators. These considerations suggest that pike are more efficient piscivores around prey refuges such as the littoral zones of lakes, whereas perch have previously been observed to be more efficient in open areas, such as in the pelagic zones of lakes.

Effects of inflorescence size on visits from pollinators and seed set of Corydalis ambigua (Papaveraceae).

Abstract: Female reproductive success (seed set) of a spring ephemeral plant, Corydalis ambigua Cham. et schlecht (Papaveraceae) was investigated in relation to inflorescence size and foraging behavior (frequency and duration of visitations) by pollinators (namely, overwintered queens of Bombus hypocrita sapporensis) by detailed daily observations of a natural population. Pollination experiments indicated that C. ambigua is self-incompatible and that seed set was significantly affected by the behavior of the pollinating queens. Plants with larger inflorescences were visited more often than those with fewer flowers. Fecundity also increased with increasing size of inflorescences. Visitation time (duration of foraging) rather than the frequency of visitations (number of visits) was critical for higher fecundity. Seed production was strongly enhanced by a few long visits (of more than 60 s), and seemed to be independent of large numbers of short visits (of less than 60 s). Hence, plants with larger inflorescences, which provide a conspicuous signal to pollinators and offer greater rewards in terms of nectar, received longer visits by B. hypocrita sapporensis queens and those plants exhibited higher fecundity.

Distribution of two congeneric charrs in streams of Hokkaido Island, Japan : considering multiple factors across scales

Abstract: Salvelinus leucomaenis (white-spotted charr) and S. malma (Dolly Varden) are distributed throughout Hokkaido Island, Japan, but sites where they occur in sympatry are rare. In general, S. malma inhabit upstream reaches and S. leucomaenis extend downstream to the ocean. Factors influencing their distribution were analyzed at four spatial scales ranging from the whole island to individual stream pools. At the island scale, S. leucomaenis were found in the warmer south-west region and at lower altitudes elsewhere, whereas S. malma were found in the colder north-east and at higher altitudes. At a regional scale, the downstream limit of S. malma and upstream limit of S. leucomaenis shifted to lower altitude from south-west to north-east across the island, coincident with the decrease in temperature. Further analysis showed that transition points from S. leucomaenis or sympatry to S. malma in individual watersheds were closely related to an index of cumulative mean monthly temperatures exceeding 5°C. However, at the scale of a single watershed, the transition occurred at different altitudes, gradients, and temperatures in two tributaries, apparently because stream discharge, habitat, and disturbances from floods interacted with these abiotic factors to limit distribution. The two charr species developed interspecific dominance hierarchies in individual pools, and there was strong complementary density compensation among stream pools that could be explained by interspecific competition but not by differences in habitat. However, patterns at watershed and regional scales suggested that interspecific competition interacts with temperature in complex ways. We conclude that the importance of various abiotic and biotic factors in shaping Hokkaido charr distributions depends on the scale at which they are viewed.

Diversity of ant-plant interactions: protective efficacy in Macaranga species with different degrees of ant association.

Abstract: The pioneer tree Macaranga in SE Asia has developed manyfold associations with ants The genus comprises all stages of interaction with ants, from facultative relationships to obligate myrmecophytes Only myrmecophytic Macaranga offer nesting space for ants and are associated with a specific ant partner The nonmyrmecophytic species are visited by a variety of different ant species which are attracted by extrafloral nectaries (EFN) and food bodies Transitional Macaranga species like M hosei are colonized later in their development due to their stem structure Before the colonization by their specific Crematogaster partner the young plants are visited by different ant species attracted by EFN These nectaries are reduced and food body production starts as soon as colonization becomes possible We demonstrated earlier that obligate ant partners can protect their Macaranga plants against herbivore damage and vine cover In this study we focused on nonspecific interactions and studied M tanarius and M hosei, representing a non-myrmecophyte and a transitional species respectively In ant exclusion experiments both M tanarius and M hosei suffered significantly higher mean leaf damage than controls, 37% versus 6% in M hosei, 16% versus 7% in M tanarius M tanarius offers both EFN and food bodies so that tests for different effects of these two food rewards could be conducted Plants with food bodies removed but with EFN remaining had the lowest mean increase of herbivore damage of all experimental groups Main herbivores on M hosei were mites and caterpillars Many M tanarius plants were infested by a shootborer Both Macaranga species were visited by various ant species, Crematogaster spp being the most abundant We found no evidence for any specific relationships The results of this study strongly support the hypothesis that non-specific, facultative associations with ants can be advantageous for Macaranga plants Food bodies appear to have lower attractive value for opportunistic ants than EFN and may require a specific dietary adaptation This is also indicated by the fact that food body production in the transitional M hosei does not start before stem structure allows a colonization by the obligate Crematogaster species M hosei thus benefits from facultative association with a variety of ants until it produces its first domatia and can be colonized by its obligate mutualist

Effect of periphyton biomass on hydraulic characteristics and nutrient cycling in streams.

Abstract: The effect of periphyton biomass on hydraulic characteristics and nutrient cycling was studied in laboratory streams with and without snail herbivores. Hydraulic characteristics, such as average water velocity, dispersion coefficients, and relative volume of transient storage zones (zones of stationary water), were quantified by performing short-term injections of a conservative tracer and fitting an advection-dispersion model to the conservative tracer concentration profile downstream from the injection site. Nutrient cycling was quantified by measuring two indices: (1) uptake rate of phosphorus from stream water normalized to gross primary production (GPP), a surrogate measure of total P demand, and (2) turnover rate of phosphorus in the periphyton matrix. These measures indicate the importance of internal cycling (within the periphyton matrix) in meeting the P demands of periphyton. Dense growths of filamentous diatoms and blue-green algae accumulated in the streams with no snails (high-biomass streams), whereas the periphyton communities in streams with snails consisted almost entirely of a thin layer of basal cells of Stigeoclonium sp. (low-biomass streams). Dispersion coefficients were significantly greater and transient storage zones were significantly larger in the high-biomass streams compared to the low-biomass streams. Rates of GPP-normalized P uptake from water and rates of P turnover in periphyton were significantly lower in high biomass than in low biomass periphyton communities, suggesting that a greater fraction of the P demand was met by recycling in the high biomass communities. Increases in streamwater P concentration significantly increased GPP-normalized P uptake in high biomass communities, suggesting diffusion limitation of nutrient transfer from stream water to algal cells in these communities. Our results demonstrate that accumulations of periphyton biomass can alter the hydraulic characteristics of streams, particularly by increasing transient storage zones, and can increase internal nutrient cycling. They suggest a close coupling of hydraulic characteristics and nutrient cycling processes in stream ecosystems.

Water and nitrogen dynamics in an arid woodland

Abstract: Arid environments are characterized by spatial and temporal variation in water and nitrogen availability. differences in δ15N and δD of four co-occurring species reveal contrasting patterns of plant resource acquisition in response to this variation. Mineralization potential and nitrogen concentration of surface soils associated with plant canopies were greater than inter-canopy locations, and values decreased with increasing depth in both locations. Mineralization potential and nitrogen concentration were both negatively correlated with soil δ15N. The spatial variation in soil δ15N caused corresponding changes in plant δ15N such that plant δ15N values were negatively correlated with nitrogen concentration of surface soils. Plants occurring on soils with relatively high nitrogen concentrations had lower δ15N, and higher leaf nitrogen concentrations, than plants occurring on soils with relatively low nitrogen concentrations. Two general temporal patterns of water and nitrogen use were apparent. Three species (Juniperus, Pinus andArtemisia) relied on the episodic availability of water and nitrogen at the soil surface. δ15N values did not vary through the year, while xylem pressure potentials and stem-water δD values fluctuated with changes in soil moisture at the soil surface. In contrast,Chrysothamnus switched to a more stable water and nitrogen source during drought. δ15N values ofChrysothamnus increased throughout the year, while xylem pressure potentials and stem-water δD values remained constant. The contrasting patterns of resource acquisition have important implications for community stability following disturbance. Disturbance can cause a decrease in nitrogen concentration at the soil surface, and so plants that rely on surface water and nitrogen may be more susceptible than those that switch to more stable water and nitrogen sources at depth during drougnt.

The effect of elevated carbon dioxide and fertilization on primary and secondary metabolites in birch,Betula pendula (Roth).

Abstract: Seedlings of European white birch (Betula pendula Roth) were grown in growth chambers for one growth season under four carbon dioxide regimes (350, 700, 1050 and 1400 ppm) and at three fertilization levels (0, 100 and 500 kg ha−1 monthly). The soluble carbohydrates and secondary phenolics in the leaves and stems were analysed. It was found that fertilizer addition reduced the amounts of glucose and fructose while sucrose remained almost unaffected. The sugar content of leaves increased at 700 ppm and 1050 ppm of CO2 and decreased at the highest CO2 concentration (1400 ppm). The amounts of proanthocyanidins and flavonoids in leaves decreased with fertilization addition and increased with CO2 enrichment. The production of simple phenolic glucosides varied according to the fertilization and CO2 treatments. The triterpenoid content of stems seemed to increase with fertilization and CO2-addition. Our results indicate that the production of phytochemicals in the birch seedlings is very sensitive to both fertilization and CO2 addition, which is in agreement with earlier studies, and thus provide some support for the hypothesis of carbon allocation to plant defence when there is an excess of carbon and nutrient. The considerable variation in the production of secondary components may indicate that the synthesis of these defensive metabolites can be regulated by a plant to certain extent, depending on the ability of the plant to acclimate to changes in the physical environment.