Showing papers in "Oecologia in 2001"

Ecologically meaningful transformations for ordination of species data

Abstract: This paper examines how to obtain species biplots in unconstrained or constrained ordination without resorting to the Euclidean distance [used in principal-component analysis (PCA) and redundancy analysis (RDA)] or the chi-square distance [preserved in correspondence analysis (CA) and canonical correspondence analysis (CCA)] which are not always appropriate for the analysis of community composition data. To achieve this goal, transformations are proposed for species data tables. They allow ecologists to use ordination methods such as PCA and RDA, which are Euclidean-based, for the analysis of community data, while circumventing the problems associated with the Euclidean distance, and avoiding CA and CCA which present problems of their own in some cases. This allows the use of the original (transformed) species data in RDA carried out to test for relationships with explanatory variables (i.e. environmental variables, or factors of a multifactorial analysis-of-variance model); ecologists can then draw biplots displaying the relationships of the species to the explanatory variables. Another application allows the use of species data in other methods of multivariate data analysis which optimize a least-squares loss function; an example is K-means partitioning.

A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming

Abstract: Climate change due to greenhouse gas emissions is predicted to raise the mean global temperature by 1.0–3.5°C in the next 50–100 years. The direct and indirect effects of this potential increase in temperature on terrestrial ecosystems and ecosystem processes are likely to be complex and highly varied in time and space. The Global Change and Terrestrial Ecosystems core project of the International Geosphere-Biosphere Programme has recently launched a Network of Ecosystem Warming Studies, the goals of which are to integrate and foster research on ecosystem-level effects of rising temperature. In this paper, we use meta-analysis to synthesize data on the response of soil respiration, net N mineralization, and aboveground plant productivity to experimental ecosystem warming at 32 research sites representing four broadly defined biomes, including high (latitude or altitude) tundra, low tundra, grassland, and forest. Warming methods included electrical heat-resistance ground cables, greenhouses, vented and unvented field chambers, overhead infrared lamps, and passive night-time warming. Although results from individual sites showed considerable variation in response to warming, results from the meta-analysis showed that, across all sites and years, 2–9 years of experimental warming in the range 0.3–6.0°C significantly increased soil respiration rates by 20% (with a 95% confidence interval of 18–22%), net N mineralization rates by 46% (with a 95% confidence interval of 30–64%), and plant productivity by 19% (with a 95% confidence interval of 15–23%). The response of soil respiration to warming was generally larger in forested ecosystems compared to low tundra and grassland ecosystems, and the response of plant productivity was generally larger in low tundra ecosystems than in forest and grassland ecosystems. With the exception of aboveground plant productivity, which showed a greater positive response to warming in colder ecosystems, the magnitude of the response of these three processes to experimental warming was not generally significantly related to the geographic, climatic, or environmental variables evaluated in this analysis. This underscores the need to understand the relative importance of specific factors (such as temperature, moisture, site quality, vegetation type, successional status, land-use history, etc.) at different spatial and temporal scales, and suggests that we should be cautious in "scaling up" responses from the plot and site level to the landscape and biome level. Overall, ecosystem-warming experiments are shown to provide valuable insights on the response of terrestrial ecosystems to elevated temperature.

Trends in wood density and structure are linked to prevention of xylem implosion by negative pressure

Abstract: Wood density (Dt), an excellent predictor of mechanical properties, is typically viewed in relation to support against gravity, wind, snow, and other environ- mental forces. In contrast, we show the surprising extent to which variation in D t and wood structure is linked to support against implosion by negative pressure in the xy- lem pipeline. The more drought-tolerant the plant, the more negative the xylem pressure can become without cavitation, and the greater the internal load on the xylem conduit walls. Accordingly, Dt was correlated with cavi- tation resistance. This trend was consistent with the maintenance of a safety factor from implosion by nega- tive pressure: conduit wall span ( b) and thickness (t) scaled so that (t/b) 2 was proportional to cavitation resis- tance as required to avoid wall collapse. Unexpectedly, trends in Dt may be as much or more related to support of the xylem pipeline as to support of the plant.

Uncertainty in source partitioning using stable isotopes

Abstract: Stable isotope analyses are often used to quan- tify the contribution of multiple sources to a mixture, such as proportions of food sources in an animal's diet, or C3 and C4 plant inputs to soil organic carbon. Linear mixing models can be used to partition two sources with a single isotopic signature (e.g., δ13C) or three sources with a second isotopic signature (e.g., δ 15 N). Although variability of source and mixture signatures is often re- ported, confidence interval calculations for source pro- portions typically use only the mixture variability. We provide examples showing that omission of source vari- ability can lead to underestimation of the variability of source proportion estimates. For both two- and three- source mixing models, we present formulas for calculat- ing variances, standard errors (SE), and confidence inter- vals for source proportion estimates that account for the observed variability in the isotopic signatures for the sources as well as the mixture. We then performed sensi- tivity analyses to assess the relative importance of: (1) the isotopic signature difference between the sources, (2) isotopic signature standard deviations (SD) in the source and mixture populations, (3) sample size, (4) analytical SD, and (5) the evenness of the source proportions, for determining the variability (SE) of source proportion es- timates. The proportion SEs varied inversely with the signature difference between sources, so doubling the source difference from 2‰ to 4‰ reduced the SEs by half. Source and mixture signature SDs had a substantial linear effect on source proportion SEs. However, the population variability of the sources and the mixture are fixed and the sampling error component can be changed only by increasing sample size. Source proportion SEs varied inversely with the square root of sample size, so an increase from 1 to 4 samples per population cut the SE in half. Analytical SD had little effect over the range examined since it was generally substantially smaller than the population SDs. Proportion SEs were minimized when sources were evenly divided, but increased only slightly as the proportions varied. The variance formulas provided will enable quantification of the precision of source proportion estimates. Graphs are provided to al- low rapid assessment of possible combinations of source differences and source and mixture population SDs that will allow source proportion estimates with desired pre- cision. In addition, an Excel spreadsheet to perform the calculations for the source proportions and their varianc- es, SEs, and 95% confidence intervals for the two-source and three-source mixing models can be accessed at http://www.epa.gov/wed/pages/models.htm.

How well do multivariate data sets match? The advantages of a Procrustean superimposition approach over the Mantel test.

Abstract: The Mantel test provides a means to test the association between distance matrices and has been widely used in ecological and evolutionary studies. Recently, another permutation test based on a Procrustes statistic (PROTEST) was developed to compare multivariate data sets. Our study contrasts the effectiveness, in terms of power and type I error rates, of the Mantel test and PROTEST. We illustrate the application of Procrustes superimposition to visually examine the concordance of observations for each dimension separately and how to conduct hypothesis testing in which the association between two data sets is tested while controlling for the variation related to other sources of data. Our simulation results show that PROTEST is as powerful or more powerful than the Mantel test for detecting matrix association under a variety of possible scenarios. As a result of the increased power of PROTEST and the ability to assess the match for individual observations (not available with the Mantel test), biologists now have an additional and powerful analytical tool to study ecological and evolutionary relationships.

The effect of grazing on the spatial heterogeneity of vegetation

Abstract: Grazing can alter the spatial heterogeneity of vegetation, influencing ecosystem processes and biodiversity. Our objective was to identify why grazing causes increases in the spatial heterogeneity of vegetation in some cases, but decreases in others. The immediate effect of grazing on heterogeneity depends on the interaction between the spatial pattern of grazing and the pre-existing spatial pattern of vegetation. Depending on the scale of observation and on the factors that determine animal distribution, grazing patterns may be stronger or weaker than vegetation patterns, or may mirror the spatial structure of vegetation. For each possible interaction between these patterns, we make a prediction about resulting changes in the spatial heterogeneity of vegetation. Case studies from the literature support our predictions, although ecosystems characterized by strong plant-soil interactions present important exceptions. While the processes by which grazing causes increases in heterogeneity are clear, how grazing leads to decreases in heterogeneity is less so. To explore how grazing can consistently dampen the fine-scale spatial patterns of competing plant species, we built a cell-based simulation model that features two competing plant species, different grazing patterns, and different sources of vegetation pattern. Only the simulations that included neighborhood interactions as a source of vegetation pattern produced results consistent with the predictions we derived from the literature review.

Global patterns in root decomposition: comparisons of climate and litter quality effects.

Abstract: Root decomposition represents a significant C flux in terrestrial ecosystems. Roots are exposed to a different decomposition environment than aboveground tissues, and few general principles exist regarding the factors controlling rates of root decay. We use a global dataset to explore the relative importance of climate, environmental variables, and litter quality in regulating rates of root decomposition. The parameters that explained the largest amount of variability in root decay were root Ca concentrations and C:N ratios, with a smaller proportion explained by latitude, mean annual temperature, mean annual precipitation, and actual evapotranspiration (AET). Root chemistry and decay rates varied by plant life form (conifer, broadleaf, or graminoid). Conifer roots had the lowest levels of Ca and N, the highest C:N and lignin:N ratios, and decomposed at the slowest rates. In a stepwise multiple linear regression, AET, root Ca, and C:N ratio accounted for approximately 90% of the variability in root decay rates. Root chemistry appeared to be the primary controller of root decomposition, while climate and environmental factors played secondary roles, in contrast to previously established leaf litter decomposition models.

Ecological effects of the North Atlantic Oscillation

Abstract: Climatic oscillations as reflected in atmospheric modes such as the North Atlantic Oscillation (NAO) may be seen as a proxy for regulating forces in aquatic and terrestrial ecosystems. Our review highlights the variety of climate processes related to the NAO and the diversity in the type of ecological responses that different biological groups can display. Available evidence suggests that the NAO influences ecological dynamics in both marine and terrestrial systems, and its effects may be seen in variation at the individual, population and community levels. The ecological responses to the NAO encompass changes in timing of reproduction, population dynamics, abundance, spatial distribution and interspecific relationships such as competition and predator-prey relationships. This indicates that local responses to large-scale changes may be more subtle than previously suggested. We propose that the NAO effects may be classified as three types: direct, indirect and integrated. Such a classification will help the design and interpretation of analyses attempting to relate ecological changes to the NAO and, possibly, to climate in general.

Mixing models in analyses of diet using multiple stable isotopes: a critique

Abstract: Stable isotope analysis is used frequently to determine the relative contributions of different food sources to an animal’s diet (Hobson 1999). Isotopic ratios for the animal tissues and each of its potential food sources are determined. The similarity of the ratios for the animal tissues with those of individual food sources (after correcting for fractionation during digestion and assimilation) gives an idea of their relative importance in the diet; in other words “you are what you eat” (DeNiro and Epstein 1978). Two food sources can be partitioned using the isotopic ratio for a single element (e.g., δ13C), or three food sources can be partitioned using isotopic ratios for two elements (e.g., δ13C and δ15N) (Kwak and Zedler 1997). A number of recent papers have used geometric procedures to quantify the contributions of three food sources to the diet using δ13C and δ15N (Ben-David et al. 1997a, 1997b; Kline et al. 1993; Szepanski et al. 1999; Whitledge and Rabeni 1997). However, these methods do not provide correct solutions to this three-endmember mixing problem. The purpose of this paper is to point out the shortcomings of these methods and to propose an alternative procedure which avoids them. Figure 1 shows a graphical representation of the analytical situation. The dietary isotopic composition is represented by point D within the triangle bounded by the points for the adjusted food source isotopic compositions A′, B′, and C′. In the geometric procedures, Euclidean distances are calculated for line segments DA′, DB′, and DC′ and are used to compute the dietary contributions. Several variations of this calculation have been utilized. Kline et al. (1993) used the following equation:

The relative importance of allelopathy in interference: the effects of an invasive weed on a native bunchgrass.

Abstract: The relative importance of allelopathy and resource competition in plant-plant interactions has been vigorously debated but seldom tested. We used activated carbon to manipulate the effects of root exudates of Centaurea maculosa, a noxious weed in much of western North America, on root elongation rates and growth of the native bunchgrass Festuca idahoensis in order to investigate the relative importance of allelopathy in the total interference of Centaurea. In root observation chambers, Festuca root elongation rates decreased to ≈50% of the control, beginning 4 days before contacting Centaurea roots in silica sand. However, when activated carbon, which has a high affinity for adsorbing to organic compounds, was added to the sand the effects of Centaurea roots on Festuca root elongation were reduced. In other experiments, Festuca plants were 50% smaller when grown with Centaurea than with conspecifics in pure silica sand. However, Festuca grown with Centaurea in mixtures of sand and activated carbon were 85% larger than Festuca grown with Centaurea in silica sand without carbon. These results suggest that allelopathy accounts for a substantial proportion of the total interference of Centaurea on Festuca, shifting the balance of competition in favor of Centaurea. However, Centaurea outperformed Festuca even in the presence of activated carbon, demonstrating the importance of the combined roles of resource competition and allelopathy.

On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation

Abstract: The volume of shade within vegetation canopies is reduced by more than an order of magnitude on cloudy and/or very hazy days compared to clear sunny days because of an increase in the diffuse fraction of the solar radiance. Here we show that vegetation is directly sensitive to changes in the diffuse fraction and we conclude that the productivity and structure of vegetation is strongly influenced by clouds and other atmospheric particles. We also propose that the unexpected decline in atmospheric [CO2] which was observed following the Mt. Pinatubo eruption was in part caused by increased vegetation uptake following an anomalous enhancement of the diffuse fraction by volcanic aerosols that would have reduced the volume of shade within vegetation canopies. These results have important implications for both understanding and modelling the productivity and structure of terrestrial vegetation as well as the global carbon cycle and the climate system.

Elevated CO2, litter chemistry, and decomposition: a synthesis.

Abstract: The results of published and unpublished experiments investigating the impacts of elevated [CO2] on the chemistry of leaf litter and decomposition of plant tissues are summarized The data do not support the hypothesis that changes in leaf litter chemistry often associated with growing plants under elevated [CO2] have an impact on decomposition processes A meta-analysis of data from naturally senesced leaves in field experiments showed that the nitrogen (N) concentration in leaf litter was 71% lower in elevated [CO2] compared to that in ambient [CO2] This statistically significant difference was: (1) usually not significant in individual experiments, (2) much less than that often observed in green leaves, and (3) less in leaves with an N concentration indicative of complete N resorption Under ideal conditions, the efficiency with which N is resorbed during leaf senescence was found not to be altered by CO2 enrichment, but other environmental influences on resorption inevitably increase the variability in litter N concentration Nevertheless, the small but consistent decline in leaf litter N concentration in many experiments, coupled with a 65% increase in lignin concentration, would be predicted to result in a slower decomposition rate in CO2-enriched litter However, across the assembled data base, neither mass loss nor respiration rates from litter produced in elevated [CO2] showed any consistent pattern or differences from litter grown in ambient [CO2] The effects of [CO2] on litter chemistry or decomposition were usually smallest under experimental conditions similar to natural field conditions, including open-field exposure, plants free-rooted in the ground, and complete senescence It is concluded that any changes in decomposition rates resulting from exposure of plants to elevated [CO2] are small when compared to other potential impacts of elevated [CO2] on carbon and N cycling Reasons for experimental differences are considered, and recommendations for the design and execution of decomposition experiments using materials from CO2-enrichment experiments are outlined

The unexpected versatility of plants: organic nitrogen use and availability in terrestrial ecosystems.

Abstract: The recently recognized importance of organic nitrogen (ON), particularly amino acids, to plant nutrition in many types of agricultural and natural ecosystems has raised questions about plant-microbe interactions, N availability in soils, and the ecological implications of ON use by plants in the light of climate change and N pollution. In this review we synthesize the recent work on availability and plant uptake of amino acids with classic work on ON in soils. We also discuss recent work on the use of natural abundance levels of 15N to infer N sources for plants. Reliance on ON is widespread among plants from many ecosystems. Authors have reached this conclusion based on laboratory studies of amino acid uptake by plants in pure culture, amino acid concentrations in soils, plant uptake of isotopically labeled amino acids in the field and in plant-soil microcosms, and from plant natural abundance values of 15N. The supply of amino acids to plants is determined mainly by the action of soil proteolytic enzymes, interactions between amino acids and the soil matrix, and competition between plants and microbes. Plants generally compete for a minor fraction of the total amino acid flux, but in some cases this forms a significant N resource, especially in ecosystems where microbial biomass undergoes large seasonal fluctuations and contributes labile ON to the soil. A quantitative understanding of ON use by plants is confounded by incomplete data on partitioning of ON between plants, mycorrhizal fungi, and competing soil microbes. Further research is needed to predict the ecological implications of ON use by plants given the influence of climatic change and N pollution.

A meta-analysis of plant field studies simulating stratospheric ozone depletion

Abstract: The potential effects of increased ultraviolet-B radiation (UV-B, 280–320 nm) simulating stratospheric ozone depletion in field studies with vascular plants have previously been summarized only in narrative literature reviews. In this quantitative synthesis, we have assessed the significance of solar UV-B enhancement for ten commonly measured variables involving leaf pigmentation, plant growth and morphology, and photosynthesis using meta-analytic statistical methods. Of 103 papers published between 1976 and mid-1999 from field studies, more than 450 reports from 62 papers were included in the database. Effects of UV-B were most apparent for the case of UV-B-absorbing compounds with an average increase of approximately 10% across all studies when comparing the ambient solar UV-B control to the treatment (involving ambient UV-B plus a UV-B supplement from special UV lamps). Some morphological parameters such as plant height and leaf mass per area showed little or no response to enhanced UV-B. Leaf photosynthetic processes (leaf gas exchange and chlorophyll fluorescence) and the concentration of photosynthetic pigments (total chlorophylls and carotenoids) were also not affected. Shoot biomass and leaf area showed modest decreases under UV-B enhancement. The reduction in shoot biomass occurred only under very high levels of simulated ozone depletion and leaf area was affected only when studies inappropriately used the plant (i.e., the subreplicate) rather than the plot as the experimental replicate. To the best of our knowledge, this review provides the first quantitative estimates of UV-B effects in field-based studies using all suitable published studies as a database.

Natural abundance of 13C in CO2 respired from forest soils reveals speed of link between tree photosynthesis and root respiration.

Abstract: Soil respiration from a boreal mixed coniferous forest showed large seasonal variation in natural abundance of 13C, ranging from –21.6‰ to –26.5‰. We tested if weather conditions could explain this variation in δ13C of respired CO2, and found that the air relative humidity 1–4 days before the days of CO2 sampling best explained the variation. This suggested that high δ13C values were caused by effects of air humidity on isotope fractionation during photosynthesis and that it took 1–4 days for the C from canopy photosynthesis of 20–25 m trees to become available for root/rhizosphere respiration. We calculated that these new photoassimilates could account for at least 65% of total soil respiration.

Spatial and temporal niche partitioning in grassland ants

Abstract: Species coexistence can reflect niche partitioning at several spatial and temporal scales. We measured patterns of spatial and temporal niche overlap in an Oklahoma grassland ant assemblage. Ant species foraging on a 400-m2 grid of 25 tuna-fish baits were censused hourly for one 24-h period each month for 1 year. We used partial correlations to analyze pairwise associations of the four commonest species, and a null model analysis to quantify niche overlap among all seven species present. On a seasonal (monthly) time scale, niche overlap and pairwise species associations were random or aggregated, probably due to thermal constraints on ectotherms foraging in a seasonal environment. Within the warmer months of the year, there was some evidence of diurnal (24 h) niche partitioning: the variance in niche overlap was often greater than expected, and common species displayed both negative and positive associations with each other and with ambient air temperature. The strongest evidence for niche partitioning was at the spatial scale of individual baits. Species occurrences at baits were dynamic, with considerable turnover in composition and significantly less spatial niche overlap than expected by chance. These results are consistent with other studies suggesting that ant species partition resources at fine spatial and temporal scales.

Water availability and carbon isotope discrimination in conifers

Abstract: The stable C isotope composition (δ13C) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and δ13C. A significant component of the variation in δ13C was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km–1 altitude, r 2=0.49, and in foliage, 1.91‰ km–1, r 2=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO2 partial pressure into the leaf (P a–P i) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P a–P i of stemwood (r 2=0.45) and foliage (r 2=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on δ13C probably dominated the effect of water availability. We also examined the relationship between plant drought stress (Ψ) and δ13C within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha–1, Ψ and δ13C were less (more negative) than at 250 stems ha–1 indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater δ13C of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage – attributes that increase rates of photosynthesis, reduce P i and increase δ13C. δ13C was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the δ13C-Ψ relationship varied between thinning treatments. Within treatments, the relationship between δ13C and Ψ was strong (0.38

Net primary productivity and nutrient cycling across a mesic to wet precipitation gradient in Hawaiian montane forest

Abstract: Variation in rainfall in humid tropical forests has the potential to alter patterns of primary productivity andnutrient cycling. Net primary productivity (NPP) and nutrient cycling were measured at six sites similar in temperature regime, parent material, ecosystem age, vegetation and topographical relief, while mean annual precipitation (MAP) varied from 2,200 toover 5,000 mm/year. Aboveground NPP declined by a factor of 2.2 with increased MAP across the precipitation gradient. Increased water availability in excess of plant demand is likely to have decreased the other resources for plant growth. Patterns of nutrient cycling and other factors that affect plant growth suggest that increased nutrient limitation in wetter sites could be the direct cause of the decline in NPP. Foliar nitrogen (N) and soil N availability decreased with increased precipitation, corresponding with the decrease in forest growth. In contrast, patterns of foliar and soil phosphorus (P) did not correspond with the decrease in growth; P availability was highest at either end of the precipitation gradient and lowest across the middle. Natural abundance of δ15N in foliage and soils decreased with increased precipitation, further supporting the idea that N availability declined. Decreased N availability was associated with a decrease in soil reduction-oxidation potentials. Oxygen limitation in soil microsites was a factor at all sites, but became increasingly widespread at higher MAP regimes. There was no strong evidence that soil oxygen availability, expressed in foliar δ13C values, directly limited plant growth. In addition foliar micronutrients either showed no change (Ca, Mg) or declined (Al, Fe) with increased MAP while soil pH was low but constant, suggesting that toxic elements in the soil solution were also not direct factors in decreased plant growth across the gradient. Thus, the decline in NPP with associated MAP appeared to be most directly associated with decreased N availability in these humid forests. Fluctuating anaerobic conditions that increased in intensity and duration with increased rainfall could be a mechanism that slows decomposition and N mineralization while concurrently increasing P solubility from soil mineral-bound pools.

Biodiversity and invasibility in grassland microcosms

Abstract: In the years since Charles Elton proposed that more diverse communities should be less susceptible to invasion by exotic species, empirical studies have both supported and refuted Elton's hypothesis. Here, I use grassland community microcosms to test the effect of functional diversity on the success of an invasive annual weed (Centaurea solstitialis L.). I found that high functional diversity reduced the success of Centaurea by reducing resource availability. An equally important, but unstudied, question is whether diversity can buffer a community against the impacts of invasive species. In this experiment, although species diversity (independent of functional diversity) did not affect the success of the invader, the invader suppressed growth of species-poor communities more strongly. Invasion of Centaurea also increased summer evapotranspiration in species-poor communities. These results suggest that loss of species diversity alone does not affect community invasibility, but that communities with fewer species may be more likely to decline as a consequence of invasion.

Positive interactions under nurse-plants: spatial scale, stress gradients and benefactor size.

Abstract: Positive interactions often play an important role in structuring plant communities and increasing bio- logical diversity. Using three scales of resolution, we ex- amine the importance of a long-lived desert tree, iron- wood (Olneya tesota), in structuring plant communities and promoting biological diversity in the Sonoran De- sert. We examined the positive effects of Olneya cano- pies of different sizes on plant communities in mesic and xeric habitats throughout the central Gulf Coast subre- gion of Sonora, Mexico. In xeric sites, Olneya canopies had strong positive effects on plant richness and abun- dance, and small positive effects on the size of plants, underscoring the role of facilitation in extreme environ- ments. In mesic sites, Olneya canopies had very little effect on perennials and a negative effect on ephemeral richness, suggesting predominantly competitive effects in this less stressful environment. Overall, Olneya cano- pies increased biological diversity where abiotic stress was high, but did not increase diversity in more mesic areas. Thus Olneya canopies caused consistent shifts in plant-community structure among xeric and mesic sites, but not when these landscapes were combined. Benefac- tor size also mediated positive interactions, with larger Olneya canopies supporting larger perennials in both xe- ric and mesic sites. Thus stress gradients and benefactor size both influenced the balance of facilitative and com- petitive effects under nurse-plant canopies, and the spa- tial scale at which facilitative effects shape community structure.

Nitrogen and carbon isotope fractionation between mothers, neonates, and nursing offspring.

Abstract: Stable isotope signatures of lactating females and their nursing offspring were measured on 11 species, including herbivores, carnivores, hibernators, and non-hibernators. We hypothesized that: (1) nursing offspring would have stable isotope signatures that were a trophic level higher than their mothers, and (2) this pattern would be species-independent. The plasma of adult females had a δ15N enrichment over their diets of 4.1±0.7‰, but offspring plasma had a mean δ15N enrichment over maternal plasma of 0.9±0.8‰ and no C enrichment (0.0±0.6‰). The trophic level enrichment did not occur between mother and offspring because milk was depleted in both δ15N (1.0±0.5‰) and δ13C (2.1±0.9‰) relative to maternal plasma. Milk to offspring plasma enrichment was relatively small (δ15N enrichment of 1.9±0.7‰ and δ13C enrichment of 1.9±0.8‰) compared to the trophic level enrichment between the adults and their diets. While some species did have significant differences between the isotope signatures of mother and offspring, the differences were not related to whether they were hibernators or non-hibernators, carnivores or herbivores. Investigators wanting to use stable isotopes to quantify weaning or other lactation processes or diets of predators when both adults and nursing offspring are consumed must first establish the parameters that apply to a particular species/environment/diet combination.

Regulation of diversity: maintenance of species richness in changing environments.

Abstract: In order to assess how diversity changes over time at sites undergoing environmental change, we ex- amined three data sets on long-term trends in taxonomic richness and composition: (1) 22 years of rodent census- es from a site in the Chihuahuan Desert of Arizona; (2) 50 years of bird surveys from a three-county region of northern Michigan; and (3) approximately 10,000 years of pollen records from two sites in Europe. In all three cases, richness has remained remarkably constant de- spite large changes in composition. The results suggest that while species composition may be highly variable and change substantially in response to environmental change, species diversity is an emergent property of ecosystems that is often maintained within narrow lim- its. Such regulation of diversity requires maintenance of relatively constant levels of productivity and resource availability and an open system with opportunity for compensatory colonizations and extinctions. In addition to studying the effects of diversity on biogeochemical processes, it will often be useful to think of species richness as an emergent consequence of ecosystem pro- cesses.

Soil nitrogen form and plant nitrogen uptake along a boreal forest productivity gradient

Abstract: We present results from a study of soil solution concentrations of ammonium (NH4 +), nitrate (NO3 –), and amino acid N over one growing season along a local 90-m-long plant productivity gradient in a boreal forest. Three forest types are found along the gradient: an ericaceous dwarf-shrub type between 0 and 40 m, a low-herb type between 40 and 80 m, and a tall-herb type at 90 m. Soil sampling of the mor layer was performed in June, July, August and October in the three forest types. In addition, plant uptake of NH4 +, NO3 – and the amino acid glycine was investigated. A mixture of the three N forms was injected into the soil; one N form at a time was labeled with 15N, and in the case of glycine also with 13C. In the dwarf-shrub forest, where plant productivity was low, the soil N pool was strongly dominated by amino acid N. There, plants took up more NH4 + than NO3 –. Glycine uptake did not differ significantly from either NH4 + or NO3 – uptake. Along the gradient, soil concentrations of NH4 + and NO3 – increased, as did plant productivity. In the low-herb forest NH4 + comprised a major portion of the soil N pool, and plants took up more NH4 + than NO3 – or glycine. In the tall-herb forest, NO3 – was as abundant as NH4 +, and together these two N forms dominated the soil N pool. Here, plants took up nearly equal amounts of NO3 – and NH4 +, and this uptake exceeded that of glycine severalfold. Apart from the overall preference for NH4 + that plants exhibited throughout the gradient, the results show a correlation between soil concentrations of amino acids and NO3 – and plant preferences for these N forms.

Carbon cycling traits of plant species are linked with mycorrhizal strategy

Abstract: Ecosystem carbon cycling depends strongly on the productivity of plant species and the decomposition rates of the litter they produce. We tested the hypothesis that classifying plant functional types according to mycorrhizal association explains important interspecific variation in plant carbon cycling traits, particularly in those traits that feature in a hypothesized feedback between vegetation productivity and litter turnover. We compared data from standardized 'screening' tests on inherent potential seedling relative growth rate (RGR), foliar nutrient concentrations, and leaf litter decomposability among 83 British plant species of known mycorrhizal type. There was important variation in these parameters between mycorrhizal plant types. Plant species with ericoid mycorrhiza showed consistently low inherent RGR, low foliar N and P concentrations, and poor litter decomposability; plant species with ectomycorrhiza had an intermediate RGR, higher foliar N and P, and intermediate to poor litter decomposability; plant species with arbuscular-mycorrhiza showed comparatively high RGR, high foliar N and P, and fast litter decomposition. Within the woody species subset, differentiation in RGR between mycorrhizal types was mostly confounded with deciduous versus evergreen habit, but the overall differentiation in litter mass loss between mycorrhizal types remained strong within each leaf habit. These results indicate that, within a representative subset of a temperate flora, ericoid and ectomycorrhizal strategies are linked with low and arbuscular-mycorrhizal species with high ecosystem carbon turnover. The incorporation of mycorrhizal association into current functional type classifications is a valuable tool in the assessment of plant-mediated controls on carbon and nutrient cycling.

Day roost selection in female Bechstein's bats (Myotis bechsteinii): a field experiment to determine the influence of roost temperature.

Abstract: The decision where to live has far-reaching fitness consequences for animals. In contrast to most other mammals or birds that use sheltered nest sites, female Bechstein's bats frequently switch day roosts during one breeding season, and therefore must often decide where to spend the day. Selecting the right roost is important, because roost quality, e.g. microclimatic condition, influences survival and reproduction in bats. Although thermal factors are very important for the quality of roosts occupied by bats, whether bats base their day roost selection directly on roost temperature has not been tested in the field. Over one summer, we examined and tested the roost choice of 21 individually marked female Myotis bechsteinii living in one maternity colony. In a field experiment, we allowed the bats to choose between relatively warm versus cold bat boxes, while controlling for site preferences. We expected females to exhibit a preference for warm roosts during pregnancy and lactation to accelerate gestation and shorten the period of growth of their young. Roost occupancy over 160 census days reflected significant temperature differences among 89 surveyed roosts (14 tree holes and 75 bat boxes), and preferences changed with the season. Females significantly preferred cold roosts before parturition, whereas post-partum, they significantly favoured warm roosts. Temperature preferences were independent of the roost site, and thus roost selection was based directly on temperature. Boxes with significantly different daytime temperatures did not differ significantly at night. Consequently, bats would have to spend at least 1 day in a new roost to test it. Information transfer among colony members might facilitate knowledge of roost availability. Access to many roosts providing different microclimates is likely to be important for successful reproduction in the endangered Bechstein's bat.

Combining theory and experiment to understand effects of inorganic nitrogen on litter decomposition

Abstract: It has been long recognised that mineral elements, and nitrogen in particular, play an important role in determining the rate at which organic matter is decomposed. The magnitude and even the sign of the effects are, however, not universal and the underlying mechanisms are not well understood. In this paper, an explanation for the observed decreases in decomposition/CO2 evolution rates when inorganic nitrogen increases is proposed by combining a theoretical approach with the results of a 6-year litter decomposition-forest nitrogen fertilisation experiment. Our results show that the major causes of observed changes in decomposition rate after nitrogen fertilisation are increases in decomposer efficiency, more rapid formation of recalcitrant material, and, although less pronounced, decreased growth rate of decomposers. This gives a more precise description of how inorganic nitrogen modifies decomposition rates than the previously loosely used "decrease in microbial activity". The long-term consequences for soil carbon storage differ widely depending on which factor is changed; stores are much more sensitive to changes in decomposer efficiency and/or rate of formation of recalcitrant material than to changes in decomposer growth rate.

Birthdate, mass and survival in mountain goat kids: effects of maternal characteristics and forage quality

Abstract: In temperate environments, early-born ungulates may enjoy a longer growth period before winter, and so attain a higher body mass and an increased probability of survival compared to late-born ones. We assessed the effects of maternal characteristics, forage quality and population density on kid birthdate, mass and survival in a population of marked mountain goats (Oreamnos americanus) in Alberta. The duration and timing of the birth season were similar in all years. Births were highly synchronised: 80% of kids were born within 2 weeks of the first birth. Maternal age, maternal social rank and density did not affect kid birthdate or mass. Previous breeding experience was not related to kid birthdate, but kids born to pluriparous mothers were heavier during summer than kids born to primiparous mothers. Male and female kids had similar mass and accumulated mass linearly during summer. Early-born kids were heavier than late-born kids. Faecal crude protein (FCP) in late spring and maternal mass were positively related to kid mass. Survival to weaning appeared higher for males (90%) than for females (78%), but survival to 1 year was 65% for both sexes. FCP in late spring, density, birthdate and mass did not affect kid survival to weaning in either sex. Survival to 1 year increased with FCP in late spring for females, but not for males. Survival to 1 year was independent of birthdate for both sexes, but heavy females survived better than light ones. Multiple logistic regression revealed a positive effect of mass on survival to 1 year when the sexes were pooled. Our results suggest that mountain goats are constrained to give birth in a short birth season synchronised with forage productivity.

Transpiration in response to variation in microclimate and soil moisture in southeastern deciduous forests.

Abstract: Responses of forests to changes in environmental conditions reflect the integrated behavior of their constituent species. We investigated sap flux-scaled transpiration responses of two species prevalent in upland eastern hardwood forests, Quercus alba in the upper canopy and Acer rubrum in the low to mid canopy, to changes in photosynthetically active radiation above the canopy (Q o), vapor pressure deficit within the canopy (D), and soil moisture depletion during an entire growing season. Water loss before bud break (presumably through the bark) increased linearly with D, reaching 8% of daily stand transpiration (E C) as measured when leaf area index was at maximum, and accounting for 5% of annual water loss. After leaves were completely expanded and when soil moisture was high, sap flux-scaled daily E C increased linearly with the daily sum of Q o. Species differences in this response were observed. Q. alba reached a maximum transpiration at low Q o, while A. rubrum showed increasing transpiration with Q o at all light levels. Daily E C increased in response to daily average D, with an asymptotic response due to the behavior of Q. alba. Transpiration of A. rubrum showed a greater response to soil moisture depletion than did that of Q. alba. When evaluated at a half-hourly scale under high Q o, mean canopy stomatal conductance (G S) of individuals decreased with D. The sensitivity of G S to D was greater in species with higher intrinsic G S. Regardless of position in the canopy, diffuse-porous species in this and an additional, more mesic stand showed higher G S and greater stomatal sensitivity to environmental variation than do ring-porous species.

Competition for pollination: effects of pollen of an invasive plant on seed set of a native congener.

Abstract: Competition for pollination can be an important factor in plant reproduction, but little attention has been given to the effect of the growing number of invasive plant species on pollination of native species. As a first step in understanding this threat, we used hand pollination to investigate the effects of pollen from an invasive species (Lythrum salicaria) on seed set in a sympatric and co-flowering native congener (L. alatum). Dispersal of fluorescent dyes in the field confirms that pollinators (bumble bees and honey bees) transfer pollen between species. To determine the potential effect of such interspecific pollen transfer on seed set of the native, we pollinated 773 flowers on 20 plants with one of three treatments: legitimate conspecific pollen, a mixture of conspecific and foreign pollen, and foreign pollen. The mixed-pollen treatment resulted in 28.8% lower seed set relative to conspecific pollination. Foreign crosses resulted in extremely low seed set. Observations of pollen germination indicate that events at the stigmatic surface contribute to the reduction in seed set for mixed pollination. Our results indicate that the impacts of invasive species may extend beyond vegetative competition to include competition for pollination.

Differentiating aquatic mammal habitat and foraging ecology with stable isotopes in tooth enamel.

Abstract: We analyzed the carbon and oxygen isotope composition of tooth enamel from mammals inhabiting marine and terrestrial ecosystems to determine whether these stable isotopes were robust indicators of foraging and habitat preferences. Consumers were separated into six habitats (offshore, nearshore, kelp beds, estuarine, freshwater, terrestrial). Consumer δ13C values were correlated with the δ13C values of primary producers within each habitat, suggesting that δ13C values of tooth enamel are a viable proxy for foraging zones. Offshore and terrestrial consumer δ13C values were not significantly different, however, indicating that carbon isotope analysis alone is not sufficient to distinguish foraging within these two ecosystems. We propose that oxygen isotopes can be used along with δ13C values to further clarify habitat use. Oxygen isotopes were assessed as an indicator of habitat use. Consumers were grouped into four categories: aquatic-marine, aquatic-estuarine, aquatic-freshwater, and terrestrial. Populations of aquatic taxa had significantly lower standard deviations for δ18O values than those of terrestrial taxa. Mean δ18O values of aquatic taxa were significantly different among groups, but surprisingly, the mean values for freshwater taxa were higher than those for marine taxa. We conclude that variation in δ18O values of mammalian populations is a valid indicator of aquatic habits, but that mean δ18O values should be utilized with caution when trying to discriminate between marine and freshwater habitat use. Together, δ13C and δ18O values serve as valuable tools for identifying foraging and habitat preferences in modern marine and terrestrial ecosystems, and may provide similar information on ancient ecosystems.