The role of reproductive plant traits and biotic interactions in the dynamics of semi-arid plant communities

The dynamics of semi-arid plant communities are determined by the interplay between competition and facilitation among plants. The sign and strength of these biotic interactions depend on plant traits. However, the relationships between plant traits and biotic interactions, and the consequences for plant communities are still poorly understood. Our objective here was to investigate, with a modelling approach, the role of plant reproductive traits on biotic interactions, and the consequences for processes such as plant succession and invasion. The dynamics of two plant types were modelled with a spatially-explicit integrodifferential model: (1) a plant with seed dispersal (colonizer of bare soil) and (2) a plant with local vegetative propagation (local competitor). Both plant types were involved in facilitation due to a local positive feedback between vegetation biomass and soil water availability, which promoted establishment and growth. Plants in the system also competed for limited water. The efficiency in water acquisition (dependent on reproductive and growth plant traits) determined which plant type dominated the community at the steady state. Facilitative interactions between plant types also played an important role in the community dynamics, promoting establishment in the driest conditions and recovery from low biomass. Plants with vegetative propagation took advantage of the ability of seed dispersers to establish on bare soil from a low initial biomass. Seed dispersers were good invaders, maintained high biomass at intermediate and high rainfall and showed a high ability in taking profit from the positive feedback originated by plants with vegetative propagation under the driest conditions. However, seed dispersers lost competitiveness with an increasing investment in fecundity. All together, our results showed that reproductive plant traits can affect the balance between facilitative and competitive interactions. Understanding this effect of plant traits on biotic interactions provides insights in processes such as plant succession and shrub encroachment.     

The role of biotic interactions in altering tree seedling responses to an extreme climatic event

Questions: We addressed two poorly understood aspects of plant response to climate change: the impact of extreme climatic events and the mediating role of biotic interactions, through a study of heatwave effects on tree seedling survival rates and ability of the tree canopy to alter seedling responses. Location: Mountain belt of the northern French Alps (Maurienne Valley). Methods: The survival rates of two seedling cohorts from four tree species (Abies alba, Acer pseudoplatanus, Fraxinus excelsior and Picea abies) were measured during both the 2003 European heatwave and an average summer (2004) in deciduous broadleaf mountain forests. Seedlings were transplanted into two soil moisture conditions, and in experimental gaps or under the tree canopy. Results: The heatwave strongly decreased tree seedling survival rates, while there was an important species‐specific mediating role of biotic interactions. In the wettest conditions, the tree canopy strongly increased survival of Abies, buffering the negative impact of the heatwave. In contrast, in the driest conditions, the tree canopy decreased survival of Picea and Acer, amplifying the negative impact of the heatwave. We found evidence of increasing soil water stress in the understorey of the driest community, but further studies including vapour pressure deficit measurements are needed to elucidate the driving mechanism of facilitation. Conclusions: The high species specificity of the mediating role of biotic interactions and its variation along stress gradients leads to questions on our ability to predict large‐scale responses of species to climate changes.     

Interactive effects of global and regional change on a coastal ecosystem

Vertical distribution and diel vertical migration of a zooplankton community were studied at two stations off Central Peru in April 2006. Zooplankton was collected at five depth strata by vertical hauls with Hydo-Bios multinet (300-μm mesh, 0.25-m² mouth size). The zooplankton community was distributed in relation to a strong, shallow oxycline (1 ml l^?1 oxygen isopleth generally above 36 m). The highest total abundance was always in the upper, well-oxygenated layer. The most important species were: Acartia tonsa (72.86%), Centropages brachiatus (7.5%), and Paracalanus parvus (3.1%); Acartia tonsa was the dominant species at all times. Larvae of the polychaete Magelona sp. (7.5%) and larvae of the brachiopod Discinisca lamellosa (3.5%) were numerically dominant in April and small copepods e.g. Oncaea venusta (3.88%) were numerically dominant during August. Five distinct patterns of vertical distribution and migration in relation to the oxygen minimum layer were distinguished in this study: (1) Ontogenetic vertical migration through the oxycline (Acartia tonsa adults, nauplii, and copepodids), (2) permanent limitation to layers above the oxycline (e.g. Oikopleura sp., most invertebrate larvae), (3) distribution mostly below the oxycline with occasional migration into the layers just above the oxycline (Eucalanus inermis), (4) Diel Vertical Migration (Centropages brachiatus), and (5) reverse Diel Vertical Migration (larvae of the polychaete Magelona sp.).     

Effects of biotic and abiotic factors on the temporal dynamic of bat-fruit interactions

Mutualistic interactions between animals and plants vary over time and space based on the abundance of fruits or animals and seasonality. Little is known about this temporal dynamic and the influence of biotic and abiotic factors on the structure of interaction networks. We evaluated changes in the structure of network interactions between bats and fruits in relation to variations in rainfall. Our results suggest that fruit abundance is the main variable responsible for temporal changes in network attributes, such as network size, connectance, and number of interactions. In the same way, temperature positively affected the abundance of fruits and bats. An increase in temperature and alterations in rainfall patterns, due to human induced climate change, can cause changes in phenological patterns and fruit production, with negative consequences to biodiversity maintenance, ecological interactions, and ecosystem functioning.     

Experimental manipulation of biotic and abiotic parameters changes the outcome of insect-plant interactions

Plants usually respond to environmental shifts with morpho-physiological adjustments, which trickles down to biotic interactions in the insect-plant system. We evaluate how Copaifera langsdorffii, a widespread tree species adapted to multiple ecosystems, responds to shifts in nutrient and water availability through experimental supplementation and how it affects its insect communities. We also evaluate how the presence of extrafloral nectaries (EFN) exudates modifies galling insect diversity and herbivory rates. Such experimental approach is scarce, as we simultaneously evaluate biotic and abiotic factors and cover the entire reproductive cycle of a species in natural environments, bringing important contributions to better understand how bottom-up factors drive species interactions and possibly community assembly. The experiment was set in an ironstone outcrop vegetation, where we deployed supplementation treatments in 80 plant individuals divided into the following field treatments: (T1 = Fertilizer, T2 = EFN simulation (Eppendorf tubes with a 20% sugar solution), T3 = fertilizer + EFN simulation, T4 = water spray, T5 = EFN control (microtube with water), and T6 = Control. We observed lower sclerophylly and greater leaf area in plant individuals that were supplemented with nutrients and water. Herbivory rates were lower and ant abundance was higher in plants with artificial EFNs available. While we did not observe variations in richness and abundance of galling insects, the patterns of co-occurrence of galls varied with the availability of resources (nesting space) in the plant. This study illustrates how variations in nutrient availability to plants modify interactions with insect communities. Ant-plant interactions can have a negative impact on general herbivory rates, however ants seem to have a more harmonious relationship with the galling insects.     

Long-term variations of biotic and abiotic conditions in the ecosystem of the Barents Sea

Synchronism of year-class strength was noted for the majority of commercial fish in the Barents Sea. The reason for this is probably connected with a common factor, namely the intensity of water inflow which influences spawning efficiency, zooplankton food production, fish larval drift to the feeding grounds, and consequent survival of juvenile fish. Consequently, the established regular relationships of hydrological and weather processes in the ecosystem can serve as basis for long-term fishing forecasts. The overall pattern of the Barents Sea water circulation, long-term climatic changes in this region, and their effect on the fish stock reproduction are considered, using Arctic cod as an example.     

Coast of change: habitat loss and transformations in the Wadden Sea

In the southern North Sea, coastal people commenced with habitat conversions 1,000 years ago. Partly interrupted in late medieval times by large-scale inundations of marshland, progressive embankments transformed the landward half of the amphibic transition zone between a limno-terrestric and a brackish-marine ecosystem into arable land and freshwater lakes. Sea walls rigidly separated the land from the sea. Dynamic transitional habitats have vanished. Areal loss has diminished the capacity of the Wadden Sea to dissipate wave and tidal energy. A coastal ecosystem once rich in marsh plants, seagrass and diatoms on mud flats became transformed into one with less autochthonous phototroph production, dominated by sandy tidal flats, and dependent primarily on allochthonous plankton supply. The large estuaries have been dredged to serve as shipping canals, and have lost most of their former retention and filter capacity. Riverine loads are now flushed right into the North Sea. Symptoms of a syndromatic coastal habitat degradation are diagnosed, leading to a decline in natural habitat diversity. The conventional on-line coastal protection may not achieve a sustainable coastal habitat configuration. At sedimentary coasts immobilised by dikes and petrified shores, a more flexible response to sea level rise is recommended.     

Functions of the respiratory burst oxidase in biotic interactions, abiotic stress and development

The production of reactive oxygen intermediates (ROI) is among the earliest temporal events following pathogen recognition in plants. Initially, ROI were thought to be cell-death executioners. Emerging evidence, however, suggests a broader role for ROI as signals that mediate responses to infection, the abiotic environment, developmental cues, and programmed cell death in different cell types. The Respiratory burst oxidase homolog (Rboh) gene family encodes the key enzymatic subunit of the plant NADPH oxidase. Rboh proteins are the source of ROI produced following pathogen recognition and in a variety of other processes.     

Integrating spatial data and shorebird nesting locations to predict the potential future impact of global warming on coastal habitats: A case study on Farasan Islands,Saudi Arabia

One of the expected effects of the global warming is changing coastal habitats by accelerating the rate of sea level rise. Coastal habitats support large number of marine and wetland species including shorebirds (plovers, sandpipers and allies). In this study, we investigate how coastal habitats may be impacted by sea level rise in the Farasan Islands, Kingdom of Saudi Arabia. We use Kentish plover Charadrius alexandrinus – a common coastal breeding shorebird – as an ecological model species to predict the influence of sea level rise. We found that any rise of sea level is likely to inundate 11% of Kentish plover nests. In addition, 5% of the coastal areas of Farasan Islands, which support 26% of Kentish plover nests, will be flooded, if sea level rises by one metre. Our results are constrained by the availability of data on both elevation and bird populations. Therefore, we recommend follow-up studies to model the impacts of sea level rise using different elevation scenarios, and the establishment of a monitoring programme for breeding shorebirds and seabirds in Farasan Islands to assess the impact of climate change on their populations.     

Midden formation by octopuses: The role of biotic and abiotic factors

Many octopus species consume their prey in a shelter, where discarded prey items accumulate to form a midden. The shelters of Octopus bimaculatus rarely have middens. Some discarded prey items are present at 20% of the shelters of O. bimaculatus, but these do not accumulate to form middens. A field experiment using artificial middens demonstrated that currents and/or surge moved some bivalve shells, and hermit crabs rapidly removed snail shells from the middens. Snails are very important in the diet of O. bimaculatus but not most other octopus species; middens do not form around O. bimaculatus shelters because hermit crabs remove the discarded snail shells.     

The relation of biotic and abiotic interactions to eutrophication in Tjeukemeer,The Netherlands

During the summer months of 1974–1985 chlorophyll-a and total P concentration, biomass of Daphnia hyalina, smelt Osmerus eperlanus, bream Abramis brema and pikeperch Stizostedion lucioperca, water temperature and water intake from lake IJsselmeer were monitored in Tjeukemeer. During this period there were manipulations with the bream and pikeperch stocks as a consequence of the termination of a gill-net fishery in 1977, and larval smelt immigrated each year from the large lake IJsselmeer and contributed largely to the yearly smelt recruitment.The correlation matrix of the nine variables mentioned above showed a positive correlation between bream and chlorophyll-a, but surprisingly a negative one between smelt and chlorophyll-a. The latter can only be explained when smelt is the dependent variable. In a multi-linear regression there was a negative effect of temperature, chlorophyll a and pikeperch on smelt and a positive effect of water intake. Daphnia hyalina was negatively influenced by the biomass of smelt and the water intake of lake IJsselmeer. The positive relation of Daphnia hyalina and chlorophyll-a was probably related to better survival chances of D. hyalina in an Oscillatoria-rich environment when smelt is the most important predator. An increasing biomass of bream coincided with higher total-P levels and probably contributed to higher chlorophyll-a levels.     

In vivo role of nitric oxide in plant response to abiotic and biotic stress

Over the past few years, nitric oxide (NO) has emerged as an important regulator in many physiological events, especially in response to abiotic and biotic stress. However, the roles of NO were mostly derived from pharmacological studies or the mutants impaired NO synthesis unspecifically. In our recent study, we highlighted a novel strategy by expressing the rat neuronal NO synthase (nNOS) in Arabidopsis to explore the in vivo role of NO. Our results suggested that plants were able to perform well in the constitutive presence of nNOS, and provided a new class of plant experimental system with specific in vivo NO release. Furthermore, our findings also confirmed that the in vivo NO is essential for most of environmental abiotic stresses and disease resistance against pathogen infection. Proper level of NO may be necessary and beneficial, not only in plant response to the environmental abiotic stress, but also to biotic stress.     

Post‐fire shrub recruitment in a semi‐arid grassland: the role of microsites

Question: (1) Which factors regulate post‐fire recruitment and spread of the shrub Senecio bracteolatus in Patagonian grasslands? (2) What is the role of the grass Stipa speciosa on S. bracteolatus establishment in the post‐fire succession? Location: Northwest Patagonia, Argentina. Methods: We studied the effect of fire on S. bracteolatus recruitment and density by comparing these variables between burned and unburned grasslands. In burned areas, we compared abiotic characteristics and seedling establishment under the canopy of grasses (S. speciosa) and in gaps (inter‐tussock areas). Post‐fire interactions between S. bracteolatus seedlings and S. speciosa were studied using field and greenhouse experiments. Results: Density of S. bracteolatus was higher in burned than in unburned areas. In burned sites, seedlings were more abundant under tussock grasses, whereas juveniles were more abundant in gaps. Tussocks generated more attenuated micro‐environmental conditions than gaps during stressful summers. Gaps were more abundant in burned sites, while “under tussock” microsites were more frequent in unburned sites. In burned areas, tussocks allowed higher establishment of seedlings (facilitation), but gaps allowed more seedling growth and higher persistence of juveniles. Conclusions: Fire promoted S. bracteolatus recruitment in Patagonian grasslands by increasing the availability of favourable gap microsites. Grass protection for shrub seedlings became negative with time, probably due to competition with grasses. Gaps led to better performance and persistence of shrub plants. Six years after fire, higher shrub recruitment and adult density (observed as a trend) in burned grassland provides an opportunity for potential S. bracteolatus invasion.     

Metabolic regulation of leaf senescence: interactions of sugar signalling with biotic and abiotic stress responses

Sugars are important signals in the regulation of plant metabolism and development. During stress and in senescing leaves, sugars often accumulate. In addition, both sugar accumulation and stress can induce leaf senescence. Infection by bacterial and fungal pathogens and attack by herbivores and gall-forming insects may influence leaf senescence via modulation of the sugar status, either by directly affecting primary carbon metabolism or by regulating steady state levels of plant hormones. Many types of biotic interactions involve the induction of extracellular invertase as the key enzyme of an apoplasmic phloem unloading pathway, resulting in a sourcesink transition and an increased hexose/sucrose ratio. Induction of the levels of the phytohormones ethylene and jasmonate in biotic interactions results in accelerated senescence, whereas an increase in plant- or pathogen-derived cytokinins delays senescence and results in the formation of green islands within senescing leaves. Interactions between sugar and hormone signalling also play a role in response to abiotic stress. For example, interactions between sugar and abscisic acid (ABA) signalling may be responsible for the induction of senescence during drought stress. Cold treatment, on the other hand, can result in delayed senescence, despite sugar and ABA accumulation. Moreover, natural variation can be found in senescence regulation by sugars and in response to stress: in response to drought stress, both drought escape and dehydration avoidance strategies have been described in different Arabidopsis accessions. The regulation of senescence by sugars may be key to these different strategies in response to stress.     

Pathways, mechanisms and predictability of vegetation change during tropical dry forest succession

The development of forest succession theory has been based on studies in temperate and tropical wet forests. As rates and pathways of succession vary with the environment, advances in successional theory and study approaches are challenged by controversies derived from such variation and by the scarcity of studies in other ecosystems. During five years, we studied development pathways and dynamics in a chronosequence spanning from very early to late successional stages (ca. 1–60 years) in a tropical dry forest of Mexico. We (1) contrasted dynamic pathways of change in structure, diversity, and species composition with static, chronosequence-based trends, (2) examined how structure and successional dynamics of guilds of trees shape community change, and (3) assessed the predictability of succession in this system. Forest diversity and structure increased with time but tree density stabilized early in succession. Dynamic pathways matched chronosequence trends. Succession consisted of two tree-dominated phases characterized by the development and dynamics of a pioneer and a mature forest species guild, respectively. Pioneer species dominated early recruitment (until ca. 10 years after abandonment), and declined before slower growing mature-forest species became dominant or reached maximum development rates (after 40–45 years). Pioneers promoted their replacement early in succession, while mature-forest species recruited and grew constantly throughout the process, with their lowest mortality coinciding with the peak of pioneer abundance. In contrast to prevailing stochastic views, we observed an orderly, community driven series of changes in this dry forest secondary succession. Chronosequences thus represent a valuable approach for revealing system-specific successional pathways, formulating hypotheses on causes and mechanisms and, in combination with repeated sampling, evaluating the effects of vegetation dynamics in pathway variation.     

Gradient Analysis of Fire Regimes in Montane Forests of the Southern Cascade Range,Thousand Lakes Wilderness,California, USA

Species distribution and abundance patterns in the southern Cascades are influenced by both environmental gradients and fire regimes. Little is known about fire regimes and variation in fire regimes may not be independent of environmental gradients or vegetation patterns. In this study, we analyze variation in fire regime parameters (i.e., return interval, season, size, severity, and rotation period) with respect to forest composition, elevation, and potential soil moisture in a 2042 ha area of montane forest in the southern Cascades in the Thousand Lakes Wilderness (TLW). Fire regime parameters varied with forest composition, elevation, and potential soil moisture. Median composite and point fire return intervals were shorter (4-9 yr, 14-24 yr) in low elevation and more xeric white fir (Abies concolor)-sugar pine (Pinus lambertiana) and white fir-Jeffrey pine (P. jeffreyi) and longest (20-37 yr, 20-47 yr) in mesic high elevation lodgepole pine (Pinus contorta) and red fir (Abies magnifica)-mountain hemlock (Tsuga mertensiana) forests. Values for mid-elevation red fir-white fir forests were intermediate. The pattern for fire rotation lengths across gradients was the same as for fire return intervals. The percentage of fires that occurred during the growing season was inversely related to elevation and potential soil moisture. Mean fire sizes were larger in lodgepole pine forests (405 ha) than in other forest groups (103-151 ha). In contrast to other parameters, fire severity did not vary across environmental and compositional gradients and >50% of all forests burned at high severity with most of the remainder burning at moderate severity. Since 1905, fire regimes have become similar at all gradient positions because of a policy of suppressing fire and fire regime modification will lead to shifts in landscape scale vegetation patterns.     

生物质炭生物与非生物氧化特性研究进展

生物质炭是由植物生物质热解炭化产生的一类高度芳香化难熔性固态物质。生物质热解炭化还田能否成为人类应对全球气候变化的重要途径直接取决于其在土壤生态系统中的稳定性。生物质炭稳定性的研究对科学计算和评估土壤生态系统生物质炭输入的碳固持与减排作用具有重要现实意义。重点概述了土壤生态系统生物质炭生物与非生物氧化特性、影响因素及其机理研究进展,并对生物质炭在土壤环境中的稳定性预测模型研究进行了分析。在此基础上,今后需针对不同类型旱地土壤生态系统和不同类型稻田土壤生态系统生物质炭稳定性及其机理开展研究,并进一步开展土壤生态系统生物质炭稳定性预测模型研究。     

Tolerance of Pinus taeda and Pinus serotina to low salinity and flooding: Implications for equilibrium vegetation dynamics

Questions: 1. Do pine seedlings in estuarine environments display discrete or continuous ranges of physiological tolerance to flooding and salinity? 2. What is the tolerance of Pinus taeda and P. serotina to low salinity and varying hydrologic conditions? 3. Are the assumptions for ecological equilibrium met for modeling plant community migration in response to sea‐level rise? Location: Albemarle Peninsula, North Carolina, USA. Methods: In situ observations were made to quantify natural pine regeneration and grass cover along a salinity stress gradient (from marsh, dying or dead forest, to healthy forest). A full‐factorial greenhouse experiment was set up to investigate mortality and carbon allocation of Pinus taeda and P. serotina to low‐salinity conditions and two hydrology treatments over 6 months. Treatments consisted of freshwater and two salinity levels (4 ppt and 8 ppt) under either permanently flooded or periodically flushed hydrologic conditions. Results: Natural pine regeneration was common (5–12 seedlings per m²) in moderate to well‐drained soils where salinity concentrations were below ca. 3.5 ppt. Pine regeneration was generally absent in flooded soils, and cumulative mortality was 100% for 4 and 8 ppt salinity levels under flooded conditions in the greenhouse study. Under weekly flushing conditions, mortality was not significantly different between 0 and 4 ppt, confirming field observations. Biomass accumulation was higher for P. taeda, but for both pine species, the root to shoot ratio was suppressed under the 8 ppt drained treatment, reflecting increased below‐ground stress. Conclusions: While Pinus taeda and P. serotina are commonly found in estuarine ecosystems, these species display a range of physiological tolerance to low‐salinity conditions. Our results suggest that the rate of forest migration may lag relative to gradual sea‐level rise and concomitant alterations in hydrology and salinity. Current bioclimate or landscape simulation models assume discrete thresholds in the range of plant tolerance to stress, especially in coastal environments, and consequently, they may overestimate the rate, extent, and timing of plant community response to sea‐level rise.     

Zonation of spiders (Araneae) and carabid beetles (Coleoptera: Carabidae) in island salt marshes at the North Sea coast

The specific communities of spiders and carabid beetles of island salt marsh habitats of the East Frisian Island chain at the German North Sea coast were investigated. During the vegetation periods of 1997 and 1998 three pitfall trapping transects were installed on the islands of Borkum and Wangerooge. Within the salt marshes, transects extended from 0 m to 175 m. Elevation gradients varied between 10 cm and 232 cm above MHT (mean high tide). On Borkum, 35 traps were exposed in two transects, on Wangerooge 25 traps were placed in one transect. Three to five elevations above MHT were investigated per transect, each one with five traps. Highest species numbers were recorded in the higher elevated salt marshes. In contrast, highest activity values were noticed in the medium elevated salt marshes. Within both groups, spiders and carabids, four communities were distinguished by indirect gradient analysis. Indicator species were assigned to the different communities that were mainly assorted to different elevations of the salt marshes. Thus, the communities of both taxa corresponded well to the vegetational formations. The importance of sea level rise for structuring the communities of salt marsh arthropods is discussed. Overall, still great uncertainties exist on how arthropod communities and salt marshes themselves will develop.     

Habitat selection in the larvae of two species of Zygoptera (Odonata): biotic interactions and abiotic limitation

Field and laboratory experiments were set up to obtain data on the reasons for different habitat selection of Enallagma cyathigerum and Platycnemis pennipes. (1) Rearing of larvae in two different ponds showed that while P. pennipes was not able to survive conditions of low oxygen content, 50% of the E. cyathigerum larvae survived. (2) In field predation experiments with sticklebacks and dragonflies as predators, we found that E. cyathigerum suffered highest predation by the fish. In P. pennipes, mortality was highest with Anax imperator. (3) Experiments regarding larval behaviour showed that E. cyathigerum was generally more active and had higher foraging success than P. pennipes. Both species reduced activity in the presence of fish, but E. cyathigerum did so to a minor extent. In contrast to P. pennipes, E. cyathigerum showed escaping behaviour. (4) In the laboratory, the growth of E. cyathigerum was faster than that of P. pennipes.