Diverging drought-tolerance strategies explain tree species distribution along a fog-dependent moisture gradient in a temperate rain forest

The study of functional traits and physiological mechanisms determining species’ drought tolerance is important for the prediction of their responses to climatic change. Fog-dependent forest patches in semiarid regions are a good study system with which to gain an understanding of species’ responses to increasing aridity and patch fragmentation. Here we measured leaf and hydraulic traits for three dominant species with contrasting distributions within patches in relict, fog-dependent forests in semiarid Chile. In addition, we assessed pressure–volume curve parameters in trees growing at a dry leeward edge and wet patch core. We predicted species would display contrasting suites of traits according to local water availability: from one end favoring water conservation and reducing cavitation risk, and from the opposite end favoring photosynthetic and hydraulic efficiency. Consistent with our hypothesis, we identified a continuum of water use strategies explaining species distribution along a small-scale moisture gradient. Drimys winteri, a tree restricted to the humid core, showed traits allowing efficient water transport and high carbon gain; in contrast, Myrceugenia correifolia, a tree that occurs in the drier patch edges, exhibited traits promoting water conservation and lower gas exchange rates, as well low water potential at turgor loss point. The most widespread species, Aextoxicon punctatum, showed intermediate trait values. Osmotic compensatory mechanism was detected in M. correifolia, but not in A. punctatum. We show that partitioning of the pronounced soil moisture gradients from patch cores to leeward edges among tree species is driven by differential drought tolerance. Such differences indicate that trees have contrasting abilities to cope with future reductions in soil moisture.     

Productive phenology of tropical montane forests: Fertilization experiments along a moisture gradient

Phenological responses of leaves and roots were studied in the tropical montane forests of Mount Kinabalu, Borneo. Soil nutrient supply, in addition to the supply of light and water, is a potential abiotic factor influencing plant phenology in the tropics. The main objective of this study was to evaluate the contribution of soil nutrient supply to plant productive phenology. Fertilization experiments, including controls, nitrogen fertilized and nitrogen and phosphorus fertilized treatments, were conducted on three vegetation types in different moisture environments. Responses of leaves and roots were compared among treatments and among vegetation types. Leaf flushing was induced by nitrogen fertilization in the upper montane forest, where extremely wet moisture conditions are associated with cloud cover. This induction of leaf flushing by fertilization was not observed in the other forests. Root growth was suppressed by fertilization when leaf flushing was not induced by fertilization. These results indicate that soil pulsed nutrient release could be a cue for leaf flushing in a tropical wet environment, and that leaf phenology could be regulated by external abiotic factors and root phenology could be regulated by internal plant demands.     

Non-random distribution of individual genetic diversity along an environmental gradient

Improving our knowledge of the links between ecology and evolution is especially critical in the actual context of global rapid environmental changes. A critical step in that direction is to quantify how variation in ecological factors linked to habitat modifications might shape observed levels of genetic variability in wild populations. Still, little is known on the factors affecting levels and distribution of genetic diversity at the individual level, despite its vital underlying role in evolutionary processes. In this study, we assessed the effects of habitat quality on population structure and individual genetic diversity of tree swallows (Tachycineta bicolor) breeding along a gradient of agricultural intensification in southern Québec, Canada. Using a landscape genetics approach, we found that individual genetic diversity was greater in poorer quality habitats. This counter-intuitive result was partly explained by the settlement patterns of tree swallows across the landscape. Individuals of higher genetic diversity arrived earlier on their breeding grounds and settled in the first available habitats, which correspond to intensive cultures. Our results highlight the importance of investigating the effects of environmental variability on individual genetic diversity, and of integrating information on landscape structure when conducting such studies.     

Spatial patterns of weeds along a gradient of landscape complexity

Processes that drive spatial patterning among plant species are of ongoing interest, mostly because these patterns have implications for the structure and function of plant communities. We investigated the spatial strategies of weeds focusing on how spatial patterns of weeds are mediated by agricultural landscape complexity and species life-history attributes. We quantified the spatial distribution of 110 weed species using data collected in ten landscapes in central western France along a gradient of landscape complexity, from structurally complex (numerous small fields) to structurally simple (few large fields). We then related differences observed in species’ distribution patterns to ecological attributes of species for resource exploitation and dispersion. Our study reveals that weeds were spatially aggregated at the landscape scale. Their spatial patterns are related to the frequency of occurrence of weeds but surprisingly not directly to the seed dispersal type, nor to the degree of habitat specialization. We show that landscape complexity had no direct effect on the spatial patterning of weeds but through interactions with species attributes. Our results point to the importance of interactions between landscape complexity and species attributes in the spatial patterning of weed species even in intensively managed fields. These patterns appear to be a consequence of the spatial arrangement of landscape elements as well as the result of landscape filtering on species attributes.     

Correlated variation of floral and leaf traits along a moisture availability gradient

Variation in flower size is an important aspect of a plant’s life history, yet few studies have shown how flower size varies with environmental conditions and to what extent foliar responses to the environment are correlated with flower size. The objectives of this study were to (1) develop a theoretical framework for linking flower size and leaf size to their costs and benefits, as assessed using foliar stable carbon isotope ratio (δ¹³C) under varying degrees of water limitation, and then (2) examine how variation in flower size within and among species growing along a naturally occurring moisture availability gradient correlates with variation in δ¹³C and leaf size. Five plant species were examined at three sites in Oregon. Intra- and inter-specific patterns of flower size in relation to moisture availability were the same: the ratios of the area of flower display to total leaf area and of individual flower area to leaf area were greater at sites with more soil moisture compared to those sites with less soil moisture. The increase in flower area per unit increase in leaf area was greater at sites with more soil moisture than at sites where water deficit can occur. Values of δ¹³C, an index of water-use efficiency, were greater for plants with larger floral size. The patterns we observed generalize across species, irrespective of overall plant morphology or pollination system. These correlations between flower size, moisture availability, and δ¹³C suggest that water loss from flowers can influence leaf responses to the environment, which in turn may indirectly mediate an effect on flower size. Electronic supplementary material The online version of this article () contains supplementary material, which is available to authorized users.     

Spatial distribution of subtidal Nematoda communities along the salinity gradient in southern European estuaries

This study investigated the spatial distribution of subtidal nematode communities along the salinity gradients of two Portuguese estuaries exposed to different degrees of anthropogenic stress: the Mira and the Mondego.The nematode communities were mainly composed of Sabatieria, Metachromadora, Daptonema, Anoplostoma, Sphaerolaimus and Terschellingia species, closely resembling the communities of Northern European estuaries. In both estuaries, nematode density and community composition followed the salinity gradient, naturally establishing three distinct estuarine sections: (i) freshwater and oligohaline – characterised by the presence of freshwater nematodes, low nematode density and diversity; (ii) mesohaline – dominated by Terschellingia, Sabatieria and Daptonema, with low total density and diversity; and (iii) polyhaline and euhaline – where nematodes reached the highest density and diversity, and Paracomesoma, Synonchiella, and Odontophora were dominant.Despite the similarities in community composition and total nematode density, the proportion of different nematode feeding types were remarkably different in the two estuaries. In Mira, selective deposit feeders were dominant in the oligohaline section, while non-selective deposit feeders were dominant in the other sections. On the contrary, in the Mondego estuary, epigrowth-feeders and omnivores/predators were dominant in the freshwater sections and in the euhaline sector of the southern arm.Differences observed along each estuarine gradient were much stronger than overall differences between the two estuaries. In the Mondego estuary, the influence of anthropogenic stressors seemed not to be relevant in determining the nematodes' spatial distribution patterns, therefore suggesting that mesoscale variability responded essentially to natural stressors, characteristic of estuarine gradients. Nevertheless, the proportion of the different feeding types was different between the two estuaries, indicating that the response of nematode feeding guilds is able to reflect anthropogenic-induced stress and can be useful in assessing biological quality in transitional waters ecosystems.     

The distribution of C3 and C4 grasses and carbon isotope discrimination along an altitudinal and moisture gradient in Kenya

Summary More than 500 species of the Poaceae are found in Kenya, East Africa. Eighteen of twenty-seven tribes are exclusively (except the Paniceae and Danthonieae) of the C₃ photosynthetic type. A floristic analysis of low altitude grasslands suggests that nearly all species at these low altitudes are of the C₄ photosynthetic type. At high altitudes, however, nearly all grasses are of the C₃ photosynthetic type. Open grassland vegetation was sampled along a transect from arid low altitude sites to the top of Mt. Kenya in an attempt to document the general distributions of the photosynthetic types.The major tribes illustrated three general patterns of distribution. The C₄ tribes Chlorideae, Eragrosteae, Sporoboleae, and Aristideae were abundant at low altitudes (or low indices of available soil moisture). The Paniceae and Andropogoneae were also exclusively C₄ but were more common at intermediate altitudes. The C₃ tribes Aveneae, Festuceae, and Agrostideae were found only at high altitudes. In these open grasslands there were no C₃ species below 2,000 m and no C₄ species above 3,000 m. The variation in ₁₃C of the live grass vegetation with altitude confirms these distributional patterns and suggests a sharp transition zone between these two photosynthetic types. The photosynthetic type accounts for broad distributions within the Poaceae but these distributions are further modified by characteristics which may be inherent in the tribal groups. Ecological and paleoecological significance of these patterns of distribution are discussed.     

An analysis of succession along an environmental gradient using data from a lawn

Differences in vegetation dynamics over a period of two years along an environmental gradient of shading on a lawn are examined. Communities (groups) recognized by numerical classification are correlated with degree of shading, season and differences between years by means of generalized linear model analysis. More of the variance is explained if environmental position is used instead of degree of shading as spatial distribution of a strongly competitive species (Trifolium repens) is confounded with shading. Transition probabilities are related to environmental position and season. Simulation with Markov matrices for each season and position demonstrate markedly different successions for different positions. These simulations have no predictive value however as accurate estimate of transition probabilities requires knowledge of the state of adjacent quadrats, i.e. individual observations of transition probabilities are not independent.Transition matrices are unlikely to be useful for predictive analysis of succession when spatial pattern is of significance in the community.We thank S. Kendall, S. Witts, A. Howard and C. Helman for their assistance with the data preparation and analysis, and R. Cunningham for advice on statistical analysis.     

Changes in plant interactions along a gradient of environmental stress

A combination of competition and facilitation effects operating simultaneously among plant species appears to be the rule in nature, where these effects change along productivity gradients often in a non-proportional manner. We investigated changes in competition and facilitation between a leguminous shrub, Retama sphaerocarpa , and its associate understorey species along an environmental gradient in semi-arid southeast Spain. Our results show a change in the net balance of the interaction between the shrub and several of its associated species, from clearly positive in the water-stressed, infertile environment to neutral or even negative in the more fertile habitat. There was a weakening of facilitation along the fertility gradient as a consequence of improved abiotic conditions. Competition was the most intense for below-ground resources in the less fertile environment while total competition tended to increase towards the more productive end of the gradient. Changes in the balance of the interaction between and among different plant species along the gradient of stress were caused by a decline in facilitation rather than by a change in competition. As both competition intensity and facilitation change along gradients of resource availability, plant interactions are best viewed as dynamic relationships, the outcome of which depends on abiotic conditions.     

Spatial heterogeneity in genetic relatedness among house sparrows along an urban-rural gradient as revealed by individual-based analysis

Understanding factors that shape patterns of kinship in sedentary species is important for evolutionary ecologists as well as conservation biologists. Yet, how patterns of relatedness are hierarchically structured in space remains poorly known, even in common species. Here, we use information from 16 polymorphic microsatellite DNA markers to study how small-scale kinship structure varies among house sparrows (Passer domesticus) along an urban-rural gradient. Average levels of relatedness were higher among urban individuals than among individuals from rural areas, suggesting lower rates of dispersal in more built-up habitats. Comparison of observed levels of relatedness with simulated distributions of known kinship values showed that central urban individuals had the highest proportion of closely related conspecifics in their immediate neighbourhood. Spatial auto-correlograms supported this small-scale genetic structure and further indicated stronger effects of genetic drift and/or limited dispersal in urban populations. Results of this study underscore the importance of individual-level analyses as a complementary approach to traditional population-level analyses when studying genetic population structure over small spatial scales.     

Spectrochromatography of photosynthetic pigments as a fingerprinting technique for microbial phototrophs

Abstract: The combination of chromatographic separation using reverse-phase high-performance liquid chromatography and continuous monitoring of the column eluate using a diode-array spectrophotometer allowed qualitative and quantitative pigment profiling of extracts of photosynthetic material in a single run. Carotenoids and the spectrally distinct types of chlorophyll and bacteriochlorophyll can be unambiguously identified even when imperfectly separated on the column. The resulting spectrochromatogram is a fingerprint useful for the rapid characterization of pure cultures or mixed populations. We have developed software to allow recording, manipulation, and presentation of the resulting spectrochromatograms and present results from photosynthetic microbes in pure and mixed cultures. We describe a number of approaches to the presentation of the resulting data in a readily comprehensible form.     

Depth distribution of photosynthetic pigments and diatoms in the sediments of a microtidal fjord

The depth distribution of photosynthetic pigments and benthic marine diatoms was investigated in late spring at three different sites on the Swedish west coast. At each site, sediment cores were taken at six depths (7–35 m) by scuba divers. It was hypothesized that (1) living benthic diatoms constitute a substantial part of the benthic microflora even at depths where the light levels are <1% of the surface irradiance, and (2) the changing light environment along the depth gradient will be reflected in (a) the composition of diatom assemblages, and (b) different pigment ratios. Sediment microalgal communities were analysed using epifluorescence microscopy (to study live cells), light microscopy and scanning electron microscopy (diatom preparations), and HPLC (photosynthetic pigments). Pigments were calculated as concentrations (mg m^–2) and as ratios relative to chlorophyll a. Hypothesis (1) was accepted. At 20 m, the irradiance was 0.2% of surface irradiance and at 7 m, 1%. Living (epifluorescent) benthic diatoms were found down to 20 m at all sites. The cell counts corroborated the diatom pigment concentrations, decreasing with depth from 7 to 25 m, levelling out between 25 and 35 m. There were significant positive correlations between chlorophyll a and living (epifluorescent) benthic diatoms and between the diatom pigment fucoxanthin and chlorophyll a. Hypothesis (2) was only partly accepted because it could not be shown that light was the main environmental factor. A principal component analysis on diatom species showed that pelagic forms characterized the deeper locations (25–35 m), and epipelic–epipsammic taxa the shallower sites (7–20 m). Redundancy analyses showed a significant relationship between diatom taxa and environmental factors – temperature, salinity, and light intensities explained 57% of diatom taxa variations.     

The detection of non-random patterns of distribution of species along a gradient

Summary A simple method for detecting non-random patterns of distribution of the boundaries of species is described. The method uses transects running across a community, where the number of upper and lower boundaries of species in each quadrat is recorded. The expected number of quadrats containing one or more boundaries can be calculated from the binomial distribution. The mean deviation of observed from expected number of such quadrats, for a set of transects, can be tested for departures from zero. Significant departures greater than zero indicate regular dispersion of boundaries. A mean deviation significantly less than zero indicates clustering of the boundaries. The method is unbiased and thus corrects previously published methods.     

Morphological indices and otolith microstructure of Atlantic croaker,Micropogonias undulatus,as indicators of habitat quality along an estuarine pollution gradient

Synopsis Atlantic croaker, Micropogonias undulatus, sampled from a transect along a pollution gradient show a trend of declining growth and physical condition. This trend is apparent in the mean size of 0-group croaker, in their recent growth rate measured by marginal otolith increment widths, in longer term growth rate as indicated by relative otolith weights, and in general physical condition as measured by an index of condition of the caudal fin. We suggest that these measures are indicators of stress associated with environmental conditions. Because croaker from different positions along the pollution gradient were distinguishable, it appears that they remain for extended periods within areas of degraded environmental quality.     

Spatial distribution of rare species: statistical approach

Many species are locally rare (have low occurrences in a given region). The method is proposed for the analysis of spatial distribution of such species. The procedure is based on the comparison of actual mean distances between records with Monte-Carlo-simulated mean distances expected for random occurrence pattern. The method was applied to two datasets: benthic macrofauna from the Barents and Kara Seas (Kucheruk, Kotov, 2002) and the Black Sea interstitial ciliates (original data). For both datasets, the number of rare species positively correlates with number of common species per site and with the total number of species. Furthermore (and irrespectively of total diversity pattern), rare species usually were locally clumped in their distribution: each of the species was mostly found at sites that were close together. This tendency, however, was significant for the macrofauna species but not for ciliates.     

Use of experimental disturbances to assess resilience along a known stress gradient

We sought to determine the effectiveness of experimental disturbances for assessing resilience and stability in a Spartina alterniflora salt marsh. To do this, we applied disturbances of different intensities along a gradient of sediment deposition that doubled as a gradient of known stress, being associated in previous studies with numerous measures of plant vigor and soil condition. Using this gradient as a standard, we postulated a priori that areas receiving sediment were less stressed than areas which received no sediment, and therefore would be more stable and recover more rapidly after experimental disturbances.For the vegetation, we found that our estimates of resilience and stability were strongly and positively affected by sediment deposition, and therefore agreed with our a priori estimates. After lethal disturbance (herbicide application), vegetation in plots not receiving sediment failed to recover, and the affected marsh turned into a mudflat and remained so during the period of observation (>2 years). In contrast, plots receiving high and moderate amounts of sediment recovered rapidly after lethal disturbance (8–11% recovery month⁻¹ to 50% of control levels [dependent variable was a composite variable describing vegetation]). After non-lethal disturbance (trimming at the soil surface) all study plots recovered, with rate of vegetative recovery being directly associated with degree of sediment deposition. For edaphic parameters, there no was effect of disturbance, and thus these parameters appeared to be resistant to vegetation removal. These parameters appeared to be more powerfully affected by other factors, such as water level fluctuations and sediment addition.We conclude that experimental disturbances accurately assessed stress along a known stress-gradient. They also provided additional information about the underlying stress in the system. In particular, it was intriguing that in stressed areas S. alterniflora grew in elevated “hummocks” which appeared unable to send rhizomes into denuded areas or to recover after root death. Because recovery after experimental disturbances depends on a variety of plant-soil processes, we suggest that this recovery can be used as a bio-indicator of ecosystem condition that provides insight into important underlying determinants of structure and function.