Compensatory growth responses to clipping defoliation in Leymus chinensis (Poaceae) under nutrient addition and water deficiency conditions

Compensatory growth responses of Leymus chinensis, a dominant species in Inner Mongolia steppe, to clipping defoliation were evaluated in a pot-cultivated experiment under different nutrient (N and P) and water availability conditions. Leymus chinensis exhibited over-compensatory growth at the light and moderate clipping intensities (20% and 40% aerial mass removed) with a greater accumulated aboveground biomass, higher relative growth rate (RGR), more rhizomatic tillers and a stimulation of compensatory photosynthesis to the remnant leaves as compared with those of the unclipped plants. Intense clipping (80% aerial mass removed), which removed most of the aboveground tissues, greatly reduced the growth of aboveground biomass in comparison with that of the unclipped plants. Nitrogen addition only slightly improved the biomass production and RGR in light and moderately clipped plants, and it did not allow plants in the intense clipping condition to over-compensate. Phosphorus addition had no obvious influences on the growth and physiological responses to clipping defoliation. These results indicated that nutrient addition could not compensate for the negative effects of severe clipping on the defoliated grass. On the other hand, there were no distinct positive responses under water deficiency condition for L. chinensis at all clipping intensities with a significant reduction of aboveground and belowground biomass, lower RGR, fewer rhizomatic tillers, and a lower net photosynthetic rate than other wet treatments. Additionally, the chlorophyll contents of remnant leaves gradually increased with the increase of clipping intensities in each treatment. In conclusion, although L. chinensis could compensate for tissues removal by some morphological and physiological responses, intense clipping and drought can result in a significant decrease of biomass and growth rate, even under enriched nutrition conditions.     

Interspecific variations in responses of <Emphasis Type="Italic">Festuca rubra</Emphasis> and <Emphasis Type="Italic">Trifolium pratense</Emphasis> to a severe clipping under environmental changes

Understanding and predicting possible responses of grassland species to global change is of important meaning for adapting grassland management to a changed and changing environment. A laboratory clipping experiment was conducted to examine the interspecific responses in an ecological context of competition and environmental changes. Festuca rubra and Trifolium pratense, either in monoculture or two- and three-species mixtures, were grown in three environmental combinations (ambient and increased temperature, repetitive N supply, and simulated acid rain), respectively. After a growth time of three months, plants were clipped at the height of 1.0 cm above soil surface. Plant height and aboveground biomass prior clipping, and survival rate and regrowth (height and biomass) after clipping were analyzed. F. rubra and T. pratense responded differently in compensatory growth and competition intensity to environmental change and co-existing species. The differences in their physiological and ecological traits may account for species-dependent responses. The present study emphasizes that predicting the plant assemblage response in the face of global change requires in understanding the integrating effects of abiotic and biotic factors.     

Intensity and Importance of Competition for a Grass (Festuca rubra) and a Legume (Trifolium pratense) Vary with Environmental Changes

How plant competition varies across environmental gradients has been a long debate among ecologists. We conducted a growth chamber experiment to determine the intensity and importance of competition for plants grown in changed environmental conditions. Festuca rubra and Trifolium pratense were grown in monoculturs and in two- and/or three-species mixtures under three environmental treatments. The measured competitive variations in terms of growth (height and biomass) were species-dependent. Competition intensity for Festuca increased with decreased productivity, whilst competition importance displayed a humpback response. However, significant response was detected in neither competition intensity nor importance for Trifolium. Intensity and importance of competition followed different response patterns, suggesting that they may not be correlated along an environmental gradient. The biological and physiological variables of plants play an important role to determine the interspecific competition associated with competition intensity and importance. However, the competitive feature can be modified by multiple environmental changes which may increase or hinder how competitive a plant is.     

Environmental and physiological controls on the carbon isotope composition of CO2 respired by leaves and roots of a C3 woody legume (Prosopis velutina) and a C4 perennial grass (Sporobolus wrightii)

Accurate estimates of the δ¹³C value of CO₂ respired from roots (δ¹³C_{R_root}) and leaves (δ¹³C_{R_leaf}) are important for tracing and understanding changes in C fluxes at the ecosystem scale. Yet the mechanisms underlying temporal variation in these isotopic signals are not fully resolved. We measured δ¹³C_{R_leaf}, δ¹³C_{R_root}, and the δ¹³C values and concentrations of glucose and sucrose in leaves and roots in the C₄ grass Sporobolus wrightii and the C₃ tree Prosopis velutina in a savanna ecosystem in southeastern Arizona, USA. Night‐time variation in δ¹³C_{R_leaf} of up to 4.6 ± 0.6‰ in S. wrightii and 3.0 ± 0.6‰ in P. velutina were correlated with shifts in leaf sucrose concentration, but not with changes in δ¹³C values of these respiratory substrates. Strong positive correlations between δ¹³C_{R_root} and root glucose δ¹³C values in P. velutina suggest large diel changes in δ¹³C_{R_root} (were up to 3.9‰) influenced by short‐term changes in δ¹³C of leaf‐derived phloem C. No diel variation in δ¹³C_{R_root} was observed in S. wrightii. Our findings show that short‐term changes in δ¹³C_{R_leaf} and δ¹³C_{R_root} were both related to substrate isotope composition and concentration. Changes in substrate limitation or demand for biosynthesis may largely control short‐term variation in the δ¹³C of respired CO₂ in these species.     

Different growth responses of C3 and C4 grasses to seasonal water and nitrogen regimes and competition in a pot experiment

Understanding temporal niche separation between C₃ and C₄ species(e.g. C₃ species flourishing in a cool spring and autumn whileC₄ species being more active in a hot summer) is essential forexploring the mechanism for their co-existence. Two parallelpot experiments were conducted, with one focusing on water andthe other on nitrogen (N), to examine growth responses to wateror nitrogen (N) seasonality and competition of two co-existingspecies Leymus chinensis (C₃ grass) and Chloris virgata (C₄grass) in a grassland. The two species were planted in eithermonoculture (two individuals of one species per pot) or a mixture(two individuals including one L. chinensis and one C. virgataper pot) under three different water or N seasonality regimes,i.e. the average model (AM) with water or N evenly distributedover the growing season, the one-peak model (OPM) with morewater or N in the summer than in the spring and autumn, andthe two-peak model (TPM) with more water or N in the springand autumn than in the summer. Seasonal water regimes significantlyaffected biomass in L. chinensis but not in C. virgata, whileN seasonality impacted biomass and relative growth rate of bothspecies over the growing season. L. chinensis accumulated morebiomass under the AM and TPM than OPM water or N treatments.Final biomass of C. virgata was less impacted by water and Nseasonality than that of L. chinensis. Interspecific competitionsignificantly decreased final biomass in L. chinensis but notin C. virgata, suggesting an asymmetric competition betweenthe two species. The magnitude of interspecific competitionvaried with water and N seasonality. Changes in productivityand competition balance of L. chinensis and C. virgata undershifting seasonal water and N availabilities suggest a contributionof seasonal variability in precipitation and N to the temporalniche separation between C₃ and C₄ species. Key words: Chloris virgata, competition, growth, Leymus chinensis, nitrogen seasonality, water seasonality Received 19 November 2007; Revised 29 January 2008 Accepted 4 February 2008     

Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions

C4 plants contribute ≈ 20% of global gross primary productivity, and uncertainties regarding their responses to rising atmospheric CO₂ concentrations may limit predictions of future global change impacts on C4-dominated ecosystems. These uncertainties have not yet been considered rigorously due to expectations of C4 low responsiveness based on photosynthetic theory and early experiments. We carried out a literature review (1980–97) and meta-analysis in order to identify emerging patterns of C4 grass responses to elevated CO₂, as compared with those of C3 grasses. The focus was on nondomesticated Poaceae alone, to the exclusion of C4 dicotyledonous and C4 crop species. This provides a clear test, controlled for genotypic variability at family level, of differences between the CO₂-responsiveness of these functional types. Eleven responses were considered, ranging from physiological behaviour at the leaf level to carbon allocation patterns at the whole plant level. Results were also assessed in the context of environmental stress conditions (light, temperature, water and nutrient stress), and experimental growing conditions (pot size, experimental duration and fumigation method). Both C4 and C3 species increased total biomass significantly in elevated CO₂, by 33% and 44%, respectively. Differing tendencies between types in shoot structural response were revealed: C3 species showed a greater increase in tillering, whereas C4 species showed a greater increase in leaf area in elevated CO₂. At the leaf level, significant stomatal closure and increased leaf water use efficiency were confirmed in both types, and higher carbon assimilation rates were found in both C3 and C4 species (33% and 25%, respectively). Environmental stress did not alter the C4 CO₂-response, except for the loss of a significant positive CO₂-response for above-ground biomass and leaf area under water stress. In C3 species, stimulation of carbon assimilation rate was reduced by stress (overall), and nutrient stress tended to reduce the mean biomass response to elevated CO₂. Leaf carbohydrate status increased and leaf nitrogen concentration decreased significantly in elevated CO₂ only in C3 species. We conclude that the relative responses of the C4 and C3 photosynthetic types to elevated CO₂ concur only to some extent with expectations based on photosynthetic theory. The significant positive responses of C4 grass species at both the leaf and the whole plant level demand a re-evaluation of the assumption of low responsiveness in C4 plants at both levels, and not only with regard to water relations. The combined shoot structural and water use efficiency responses of these functional types will have consequential implications for the water balance of important catchments and range-lands throughout the world, especially in semiarid subtropical and temperate regions. It may be premature to predict that C4 grass species will lose their competitive advantage over C3 grass species in elevated CO₂.     

Mechanistic understanding of interspecific interaction between a C4 grass and a C3 legume via arbuscular mycorrhizal fungi,as influenced by soil phosphorus availability using a 13C and 15N dual-labelled organic patch

Arbuscular mycorrhizal fungi (AMF) can improve plant nutrient acquisition, either by directly supplying nutrients to plants or by promoting soil organic matter mineralization, thereby affecting interspecific plant relationships in natural communities. We examined the mechanism by which the addition of P affects interspecific interactions between a C4 grass (Bothriochloa ischaemum, a dominant species in natural grasslands) and a C3 legume (Lespedeza davurica, a subordinate species in natural grasslands) via AMF and plant growth, by continuous ¹³C and ¹⁵N labelling, combined with soil enzyme analyses. The results of ¹⁵N labelling revealed that P addition affected the shoot uptake of N via AMF by B. ischaemum and L. davurica differently. Specifically, the addition of P significantly increased the shoot uptake of N via AMF by B. ischaemum but significantly decreased that by L. davurica. Interspecific plant interactions via AMF significantly facilitated the plant N uptake via AMF by B. ischaemum but significantly inhibited that by L. davurica under P-limited soil conditions, whereas the opposite effect was observed in the case of excess P. This was consistent with the impact of interspecific plant interaction via AMF on arbuscular mycorrhizal (AM) benefit for plant growth. Our data indicate that the capability of plant N uptake via AMF is an important mechanism that influences interspecific relationships between C4 grasses and C3 legumes. Moreover, the effect of AMF on the activities of the soil enzymes responsible for N and P mineralization substantially contributed to the consequence of interspecific plant interaction via AMF for plant growth.     

A trade-off between nitrogen uptake and use increases responsiveness to elevated CO2 in infrequently cut mixed C3 grasses

The aim of this study was to evaluate whether the responsiveness of mixed C3 grass species to elevated CO2 is related more to nitrogen uptake or to N-use efficiency. Nitrogen uptake and whole-plant N-use efficiency were investigated with two binary mixtures: Lolium perenne was mixed either with Festuca arundinacea or with Holcus lanatus. The swards were grown on sand with or without CO2 doubling, and subjected to two cutting frequencies. A C20 alcohol was used as a marker to determine species proportion in the total root mass of the mixtures. The mean residence time of N was calculated from that of 15N-labelled fertilizer. Lolium perenne took up significantly more N per unit root mass than its grass competitors, but its N-use efficiency was lower. Elevated CO2 significantly reduced the N uptake of the three grass species. A trade-off between N capture and use was found, as N-use efficiency and N-uptake rate were negatively correlated. A high N-use efficiency, and conversely low N uptake appeared to favour the responsiveness to elevated CO2 of the infrequently cut grasses.     

Drought limitation of photosynthesis differs between C3 and C4 grass species in a comparative experiment

Phylogenetic analyses show that C₄ grasses typically occupy drier habitats than their C₃ relatives, but recent experiments comparing the physiology of closely related C₃ and C₄ species have shown that advantages of C₄ photosynthesis can be lost under drought. We tested the generality of these paradoxical findings in grass species representing the known evolutionary diversity of C₄ NADP‐me and C₃ photosynthetic types. Our experiment investigated the effects of drought on leaf photosynthesis, water potential, nitrogen, chlorophyll content and mortality. C₄ grasses in control treatments were characterized by higher CO₂ assimilation rates and water potential, but lower stomatal conductance and nitrogen content. Under drought, stomatal conductance declined more dramatically in C₃ than C₄ species, and photosynthetic water‐use and nitrogen‐use efficiency advantages held by C₄ species under control conditions were each diminished by 40%. Leaf mortality was slightly higher in C₄ than C₃ grasses, but leaf condition under drought otherwise showed no dependence on photosynthetic‐type. This phylogenetically controlled experiment suggested that a drought‐induced reduction in the photosynthetic performance advantages of C₄ NADP‐me relative to C₃ grasses is a general phenomenon.     

开放式空气CO2浓度增高条件下C3作物(水稻)与C4杂草(稗草)的竞争关系

通过田间试验,研究了FACE（开放式空气CO2浓度升高）条件下C3作物水稻（Oryza sativa）和C4杂草稗草（Echinochloa crusgalli）的生长和竞争关系,结果表明,FACE条件下C3植物水稻生物量和产量增加,吉片数增加,分蘖数增加,叶面积系数（LAI）增大;而C4植物稗草相反,FACE条件下水稻和稗草中面积均减少,而净同化率（NAR）均增加;FACE条件下水稻-稗草比例为1：1时,水稻与稗草的生物量比率、产量比率、LAI比率、茎蘖比率和NAR比率均增加,水稻-稗草的竞争关系发生变化,水稻（C3植物）竞争能力增加,稗草（C4植物）竞争能力下降。     

Patterns and determinants of potential carbon gain in the C3 evergreen Yucca glauca (Liliaceae) in a C4 grassland

Yucca glauca is a C(3) evergreen rosette species locally common in the C(4)-dominated grasslands of the central Great Plains. Most congeners of Y. glauca are found in deserts, and Y. glauca's morphological similarities to desert species (steeply angled leaves, evergreen habit) may be critical to its success in grasslands. We hypothesized that the evergreen habit of Y. glauca, coupled with its ability to remain physiologically active at cool temperatures, would allow this species to gain a substantial portion of its annual carbon budget when the C(4) grasses are dormant. Leaf-level gas exchange was measured over an 18-mo period at Konza Prairie in northeast Kansas to assess the annual pattern of potential C gain. Two short-term experiments also were conducted in which nighttime temperatures were manipulated to assess the cold tolerance of this species. The annual pattern of C gain in Y. glauca was bimodal, with a spring productive period (maximum monthly photosynthetic rate = 21.1 ± 1.97 μmol·m·s) in March through June, a period of midseason photosynthetic depression, and a fall productive period in October (15.6 ± 1.25 μmol·m·s). The steeply angled leaves resulted in interception of photon flux density at levels above photosynthetic saturation throughout the year. Reduced photosynthetic rates in the summer may have been caused by low soil moisture, but temperature was strongly related (r = 0.37) to annual variations in photosynthesis, with nocturnal air temperatures below -5°C in the late fall and early spring, and high air temperatures (>32°C) in the summer, limiting gas exchange. Overall, 31% of the potential annual carbon gain in Y. glauca occurred outside the "frost-free" period (April-October) at Konza Prairie and 43% occurred when the dominant C(4) grasses were dormant. Future climates that include warmer minimum temperatures in the spring and fall may enhance the success of Y. glauca relative to the C(4) dominants in these grasslands.     

Leaf vascular systems in C(3) and C(4) grasses: a two-dimensional analysis

BACKGROUND AND AIMS: It is well documented that C(4) grasses have a shorter distance between longitudinal veins in the leaves than C(3) grasses. In grass leaves, however, veins with different structures and functions are differentiated: large longitudinal veins, small longitudinal veins and transverse veins. Thus, the densities of the three types of vein in leaves of C(3) and C(4) grasses were investigated from a two-dimensional perspective. METHODS: Vein densities in cleared leaves of 15 C(3) and 26 C(4) grasses representing different taxonomic groups and photosynthetic subtypes were analysed. KEY RESULTS: The C(4) grasses had denser transverse veins and denser small longitudinal veins than the C(3) grasses (1.9 and 2.1 times in interveinal distance), but there was no significant difference in large longitudinal veins. The total length of the three vein types per unit area in the C(4) grasses was 2.1 times that in the C(3) grasses. The ratio of transverse vein length to total vein length was 14.3 % in C(3) grasses and 9.9 % in C(4) grasses. The C(3) grasses generally had greater species variation in the vascular distances than the C(4) grasses. The bambusoid and panicoid C(3) grasses tended to have a denser vascular system than the festucoid C(3) grasses. There were no significant differences in the interveinal distances of the three vein types between C(4) subtypes, although the NADP-malic enzyme grasses tended to have a shorter distance between small longitudinal veins than the NAD-malic enzyme and phosphoenolpyruvate carboxykinase grasses. CONCLUSIONS: It seems that C(4) grasses have structurally a superior photosynthate translocation and water distribution system by developing denser networks of small longitudinal and transverse veins, while keeping a constant density of large longitudinal veins. The bambusoid and panicoid C(3) grasses have a vascular system that is more similar to that in C(4) grasses than to that in the festucoid C(3) grasses.     

Contrasting physiological responsiveness of establishing trees and a C4 grass to rainfall events,intensified summer drought,and warming in oak savanna

Climate warming and drought may alter tree establishment in savannas through differential responses of tree seedlings and grass to intermittent rainfall events. We investigated leaf gas exchange responses of dominant post oak savanna tree (Quercus stellata and Juniperus virginiana) and grass (Schizachyrium scoparium, C₄ grass) species to summer rainfall events under an ambient and intensified summer drought scenario in factorial combination with warming (ambient, +1.5 °C) in both monoculture and tree‐grass mixtures. The three species differed in drought resistance and response of leaf gas exchange to rainfall events throughout the summer. S. scoparium experienced the greatest decrease in A_area (?56% and ?66% under normal and intensified drought, respectively) over the summer, followed by Q. stellata (?44%, ?64%), while J. virginiana showed increased A_area under normal drought (+13%) and a small decrease in A_area when exposed to intensified summer drought (?10%). Following individual rainfall events, mean increases in A_area were 90% for S. scoparium, 26% for J. virginiana and 22% for Q. stellata. The responsiveness of A_area of S. scoparium to rainfall events initially increased with the onset of drought, but decreased dramatically as summer drought progressed. For Q. stellata, A_area recovery decreased as drought progressed and with warming. In contrast, J. virginiana showed minimal fluctuations in A_area following rainfall events, in spite of declining water potential, and warming enhanced recovery. J. virginiana will likely gain an advantage over Q. stellata during establishment under future climatic scenarios. Additionally, the competitive advantage of C₄ grasses may be reduced relative to trees, because grasses will likely exist below a critical water stress threshold more often in a warmer, drier climate. Recognition of unique species responses to critical global change drivers in the presence of competition will improve predictions of grass–tree interactions and tree establishment in savannas in response to climate change.     

Identification of a α‐helical molten globule intermediate and structural characterization of β‐cardiotoxin,an all β‐sheet protein isolated from the venom of Ophiophagus hannah (king cobra)

β‐Cardiotoxin is a novel member of the snake venom three‐finger toxin (3FTX) family. This is the first exogenous protein to antagonize β‐adrenergic receptors and thereby causing reduction in heart rates (bradycardia) when administered into animals, unlike the conventional cardiotoxins as reported earlier. 3FTXs are stable all β‐sheet peptides with 60–80 amino acid residues. Here, we describe the three‐dimensional crystal structure of β‐cardiotoxin together with the identification of a molten globule intermediate in the unfolding pathway of this protein. In spite of the overall structural similarity of this protein with conventional cardiotoxins, there are notable differences observed at the loop region and in the charge distribution on the surface, which are known to be critical for cytolytic activity of cardiotoxins. The molten globule intermediate state present in the thermal unfolding pathway of β‐cardiotoxin was however not observed during the chemical denaturation of the protein. Interestingly, circular dichroism (CD) and NMR studies revealed the presence of α‐helical secondary structure in the molten globule intermediate. These results point to substantial conformational plasticity of β‐cardiotoxin, which might aid the protein in responding to the sometimes conflicting demands of structure, stability, and function during its biological lifetime.     

Molecular phylogenetics of Alternanthera (Gomphrenoideae,Amaranthaceae): resolving a complex taxonomic history caused by different interpretations of morphological characters in a lineage with C4 and C3–C4 intermediate species

Alternanthera (Amaranthaceae) is a diverse genus (80–200 species) largely restricted to the American Tropics. With Pedersenia and Tidestromia, it makes up the ‘Alternantheroid clade’ in Gomphrenoideae. Parsimony and Bayesian analyses of nucleotide sequences of nuclear (ITS) and plastid (rpl16, trnL‐F) and morphological characters identify that the capitate stigma of Alternanthera is a synapomorpy within the Alternantheroids. Within Alternanthera, two major clades were resolved, both of which were marked by otherwise homoplasious characters of the gynoecium: Clade A [99% jackknife (JK); 1.0 posterior probability (PP)] with nine species and Clade B (60% JK; 0.98 PP) with 22 species. Four subclades (B1–B4), strongly supported statistically, were identified in Clade B. Previous subgeneric classifications of Alternanthera appear artificial in light of our new molecular phylogenetic analyses. Most major lineages are congruently resolved by nuclear and plastid data but some incongruence between the nrITS and plastid phylogenetic trees suggests hybridization may have played a role in the rampant speciation in Alternanthera. Whereas C₄ photosynthesis appears to have evolved in a single clade, the position of A. littoralis var. maritima (C₃) in this clade may be explained by hybrid speciation rather than a reversal from C₄ to C₃. All C₃–C₄ intermediates belong to a different clade that also contains C₃ species, but species limits, including the widely studied A. tenella, are unclear. The clade including A. tenella and A. halimifolia contains most of the species endemic to the Galápagos whereas A. nesiotes, also endemic to the islands, is nested among widespread American taxa. This suggests that the Galápagos radiation of Alternanthera may have arisen from at least two independent colonization events followed by a subsequent radiation in the former lineage. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 169 , 493–517.     

Growth decline and divergent tree ring isotopic composition (δ13C and δ18O) contradict predictions of CO2 stimulation in high altitudinal forests

Human‐induced changes in atmospheric composition are expected to affect primary productivity across terrestrial biomes. Recent changes in productivity have been observed in many forest ecosystems, but low‐latitude upper tree line forests remain to be investigated. Here, we use dendrochronological methods and isotopic analysis to examine changes in productivity, and their physiological basis, in Abies religiosa (Ar) and Pinus hartwegii (Ph) trees growing in high‐elevation forests of central Mexico. Six sites were selected across a longitudinal transect (Transverse Volcanic Axis), from the Pacific Ocean toward the Gulf of Mexico, where mature dominant trees were sampled at altitudes ranging from 3200 to 4000 m asl. A total of 60 Ar and 84 Ph trees were analyzed to describe changes in growth (annual‐resolution) and isotopic composition (decadal‐resolution) since the early 1900s. Our results show an initial widespread increase in basal area increment (BAI) during the first half of the past century. However, BAI has decreased significantly since the 1950s with accentuated decline after the 1980s in both species and across sites. We found a consistent reduction in atmosphere to wood ¹³C discrimination, resulting from increasing water use efficiency (20–60%), coinciding with rising atmospheric CO₂. Changes in ¹³C discrimination were not followed, however, by shifts in tree ring δ¹⁸O, indicating site‐ and species‐specific differences in water source or uptake strategy. Our results indicate that CO₂ stimulation has not been enough to counteract warming‐induced drought stress, but other stressors, such as progressive nutrient limitation, could also have contributed to growth decline. Future studies should explore the distinct role of resource limitation (water vs. nutrients) in modulating the response of high‐elevation ecosystems to atmospheric change.     

Changing gull diet in a changing world: A 150‐year stable isotope (δ13C, δ15N) record from feathers collected in the Pacific Northwest of North America

The world's oceans have undergone significant ecological changes following European colonial expansion and associated industrialization. Seabirds are useful indicators of marine food web structure and can be used to track multidecadal environmental change, potentially reflecting long‐term human impacts. We used stable isotope (δ¹³C, δ¹⁵N) analysis of feathers from glaucous‐winged gulls (Larus glaucescens) in a heavily disturbed region of the northeast Pacific to ask whether diets of this generalist forager changed in response to shifts in food availability over 150 years, and whether any detected change might explain long‐term trends in gull abundance. Sampled feathers came from birds collected between 1860 and 2009 at nesting colonies in the Salish Sea, a transboundary marine system adjacent to Washington, USA and British Columbia, Canada. To determine whether temporal trends in stable isotope ratios might simply reflect changes to baseline environmental values, we also analysed muscle tissue from forage fishes collected in the same region over a multidecadal timeframe. Values of δ¹³C and δ¹⁵N declined since 1860 in both subadult and adult gulls (δ¹³C, ~ 2–6‰; δ¹⁵N, ~4–5‰), indicating that their diet has become less marine over time, and that birds now feed at a lower trophic level than previously. Conversely, forage fish δ¹³C and δ¹⁵N values showed no trends, supporting our conclusion that gull feather values were indicative of declines in marine food availability rather than of baseline environmental change. Gradual declines in feather isotope values are consistent with trends predicted had gulls consumed less fish over time, but were equivocal with respect to whether gulls had switched to a more garbage‐based diet, or one comprising marine invertebrates. Nevertheless, our results suggest a long‐term decrease in diet quality linked to declining fish abundance or other anthropogenic influences, and may help to explain regional population declines in this species and other piscivores.     

Holocene boundary dynamics of a northern Australian monsoon rainforest patch inferred from isotopic analysis of carbon, (14C and δ13C) and nitrogen (δ15N) in soil organic matter

Abstract Soil organic matter (SOM) was sampled from lateritic soil profiles across an abrupt eucalypt savanna–monsoon rainforest boundary on the north coast of Croker Island, northern Australia. Accelerator mass spectrometry dating revealed that SOM that had accumulated at the base of these 1.5 m profiles had a radiocarbon age of about 5000 years. The mean carbon and nitrogen stable isotope composition of SOM from 10 cm deep layers from the surface, middle and base of three monsoon rainforest soil profiles was significantly different from the means for these layers in three adjacent savanna soil profiles, suggesting the isotopic ‘footprint’ of the vegetation boundary has been stable since the mid Holocene. Although there were no obvious environmental discontinuities associated with the boundary, the monsoon rainforest was found to occur on significantly more clay rich soils than the surrounding savanna. Tiny fragments of monsoon rainforest and abandoned ‘nests’ (large earthen mounds) of the orange‐footed scrubfowl, an obligate monsoon rainforest species, occurred in the savanna, signalling that the rainforest was once more extensive. Despite episodic disturbances, such as tropical storm damage and fires, the stability of the boundary is probably maintained because clay rich soils enable monsoon rainforest tree species to grow rapidly and achieve canopy closure, thereby excluding grass and reducing the risk of fire. Conversely, slower tree growth rates, grass competition and fire on the savanna soils would impede the expansion of the rainforest although high rainfall periods with shorter dry seasons may enable rainforest trees to grow sufficiently quickly to colonize the savanna successfully.     

Carbon isotope compositions (δ13C) of leaf,wood and holocellulose differ among genotypes of poplar and between previous land uses in a short‐rotation biomass plantation

The efficiency of water use to produce biomass is a key trait in designing sustainable bioenergy‐devoted systems. We characterized variations in the carbon isotope composition (δ¹³C) of leaves, current year wood and holocellulose (as proxies for water use efficiency, WUE) among six poplar genotypes in a short‐rotation plantation. Values of δ¹³C_wood and δ¹³C_{holocellulose} were tightly and positively correlated, but the offset varied significantly among genotypes (0.79–1.01‰). Leaf phenology was strongly correlated with δ¹³C, and genotypes with a longer growing season showed a higher WUE. In contrast, traits related to growth and carbon uptake were poorly linked to δ¹³C. Trees growing on former pasture with higher N‐availability displayed higher δ¹³C as compared with trees growing on former cropland. The positive relationships between δ¹³C_leaf and leaf N suggested that spatial variations in WUE over the plantation were mainly driven by an N‐related effect on photosynthetic capacities. The very coherent genotype ranking obtained with δ¹³C in the different tree compartments has some practical outreach. Because WUE remains largely uncoupled from growth in poplar plantations, there is potential to identify genotypes with satisfactory growth and higher WUE.     

In Vitro and In Vivo Evaluation of N‐{2‐[4‐(3‐Cyanopyridin‐2‐yl)piperazin‐1‐yl]ethyl}‐3‐[11C]methoxybenz­amide,a Positron Emission Tomography (PET) Radioligand for Dopamine D4 Receptors,in Rodents

The D₄ dopamine receptor belongs to the D₂‐like family of dopamine receptors, and its exact regional distribution in the central nervous system is still a matter of considerable debate. The availability of a selective radioligand for the D₄ receptor with suitable properties for positron emission tomography (PET) would help resolve issues of D₄ receptor localization in the brain, and the presumed diurnal change of expressed protein in the eye and pineal gland. We report here on in vitro and in vivo characteristics of the high‐affinity D₄ receptor‐selective ligand N‐{2‐[4‐(3‐cyanopyridin‐2‐yl)piperazin‐1‐yl]ethyl}‐3‐[¹¹C]methoxybenzamide ([¹¹C] 2 ) in rat. The results provide new insights on the in vitro properties that a brain PET dopamine D₄ radioligand should possess in order to have improved in vivo utility in rodents.