Spatial scaling of ecosystem C and N in a subtropical savanna landscape

Widely occurred woody encroachment in grass‐dominated ecosystems has the potential to influence soil organic carbon (SOC) and total nitrogen (TN) pools at local, regional, and global scales. Evaluation of this potential requires assessment of both pool sizes and their spatial patterns. We quantified SOC and TN, their relationships with soil and vegetation attributes, and their spatial scaling along a catena (hill‐slope) gradient in the southern Great Plains, USA where woody cover has increased substantially over the past 100 years. Quadrat variance analysis revealed spatial variation in SOC and TN at two scales. The larger scale variation (40–45 m) was approximately the distance between centers of woody plant communities and their adjoining herbaceous patches. The smaller scale variation (10 m) appeared to reflect the local influence of shrubs on SOC and TN. Litter, root biomass, shrub, and tree basal area (a proxy for plant age) exhibited not only similar spatial scales, but also strong correlations with SOC and TN, suggesting invasive woody plants alter both the storage and spatial scaling of SOC and TN through ecological processes related primarily to root turnover and, to a lesser extent litter production, as mediated by time of occupancy. Forb and grass biomass were not significantly correlated with SOC and TN suggesting that changes in herbaceous vegetation have not been the driving force for the observed changes in SOC and TN. Because SOC and TN varied at two scales, it would be inappropriate to estimate SOC and TN pools at broad scales by extrapolating from point sampling at fine scales. Sampling designs that capture variation at multiple scales are required to estimate SOC and TN pools at broader scales. Knowledge of spatial scaling and correlations will be necessary to design field sampling protocols to quantify the biogeochemical consequences of woody plant encroachment at broad scales.     

Woody plant seed dispersal and gap formation in a North American subtropical savanna woodland: the role of domestic herbivores

The relationship between domestic cattle and vegetation change in a savanna woodland was evaluated with respect to dung deposition and the dispersal and establishment of mesquite (Prosopis glandulosa var. glandulosa, Mimosaceae), a cosmopolitan woody invader of grasslands in the southwestern USA. Dung deposited in autumn disintegrated rapidly, leaving patches of bare ground ranging from 50 to 900 cm². Herbaceous cover on gaps created by dung deposition recovered to levels comparable to neighboring vegetation by the end of the following growing season. Vegetation colonizing gaps consisted primarily of grasses not found in the surrounding vegetation. Dung deposition increased species diversity and spatial heterogeneity of the herbaceous vegetation and contributed to the development of a fine-grain mosaic of small patches of varying successional age-states.The role of cattle in facilitating the ingress and establishment of mesquite has broader implications with regard to the conversion of grasslands to woodlands. On the site with cattle, mesquite seedlings were found in 75% of dung pats surveyed in September (mean =4.2 seedlings per pat; maximum =50). Although seedling survival in dung (79%) was only 16% greater than that of mesquite emerging from seeds experimentally sown away from dung, no seedlings were found on areas without cattle. Mean (± SE) density of mesquite seedlings ranged from 12±2 to 15±2 m^-2 on the site with cattle. Seed densities away from parent plants averaged 10.7 m^-2 and 0.0 m^-2 on areas with and without cattle, respectively. Seed densities beneath adult plants were comparable between sites.The high density of seedlings on areas with cattle, in contrast to absence of seedlings on the area without cattle, suggests rates of invasion of grasslands by mesquite would have increased substantially in North America following the settlement and introduction of domestic ungulates. Prior to the introduction of livestock, poor seed dissemination and germination may have limited its Holocene spread.     

Effects of plant size on photosynthesis and water relations in the desert shrub Prosopis glandulosa (Fabaceae)

The Jornada del Muerto basin of the Chihuahuan Desert of southern New Mexico, USA, has undergone a marked transition of plant communities. Shrubs such as mesquite (Prosopis glandulosa) have greatly increased or now dominate in areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete directly with similar-sized grass plants. This is followed by a phase in which large, established shrubs sequester nutrients and water within their biomass and alter soil resources directly under their canopy, creating “islands” of fertility. We hypothesized that these two phases were associated with shrubs having different physiological response capacities related to their age or size and the resource structure of the environment. As a corollary, we hypothesized that responses of small shrubs would be more tightly coupled to variation in soil moisture availability compared to large shrubs. To test these hypotheses, we studied gas exchange and water relations of small (establishing) and large (established) shrubs growing in the Jornada del Muerto as a function of varying soil moisture during the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrubs following high summer rainfall in July, and highest seasonal soil moisture at 0.3 m. High rates of carbon assimilation and water use would be an advantage for small shrubs competing with grasses when shallow soil moisture was plentiful. Large shrubs had greater net assimilation and water-use efficiency, and lower xylem water potential than small shrubs following a dry period in September, when soil moisture at 0.3 m was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water as soil moisture is depleted. Although the study provides evidence of differences in physiological responses of different-sized shrubs, there was not support for the hypothesis that small shrubs are more closely coupled to variation in soil moisture availability than large shrubs. Small shrubs may actually be less coupled to soil moisture than large shrubs, and thus avoid conditions when continued transpiration could not be matched by equivalent water uptake.     

Genetic characterization of fast-growing rhizobia able to nodulate Prosopis alba in North Spain

Prosopis is a Mimosaceae legume tree indigenous to South America and not naturalized in Europe. In this work 18 rhizobial strains nodulating Prosopis alba roots were isolated from a soil in North Spain that belong to eight different randomly amplified polymorphic DNA groups phylogenetically related to Sinorhizobium medicae, Sinorhizobium meliloti and Rhizobium giardinii according to their intergenic spacer and 16S rRNA gene sequences. The nodC genes of isolates close to S. medicae and S. meliloti were identical to those of S. medicae USDA 1,037(T) and S. meliloti LMG 6,133(T) and accordingly all these strains were able to nodulate both alfalfa and Prosopis. These nodC genes were phylogenetically divergent from those of the isolates close to R. giardinii that were identical to that of R. giardinii H152(T) and therefore all these strains formed nodules in common beans and Prosopis. The nodC genes of the strains isolated in Spain were phylogenetically divergent from that carried by Mesorhizobium chacoense Pr-5(T) and Sinorhizobium arboris LMG 1,4919(T) nodulating Prosopis in America and Africa, respectively. Therefore, Prosopis is a promiscuous host which can establish symbiosis with strains carrying very divergent nodC genes and this promiscuity may be an important advantage for this legume tree to be used in reforestation.     

Long-term impacts of goat browsing on bush-clump dynamics in a semi-arid subtropical savanna

The effects of 16 years of continuous browsing by goats in a South African savanna at stocking rates intended for bush control were compared with plots unbrowsed for the same period of time. Differences in bush-clump density, structure and species composition were recorded. Bush-clump density did not differ between browsed and unbrowsed plots. Within individual bush-clumps, browsing was shown to impact more on structure than species composition, with smaller, shorter bush-clumps, containing fewer species but much greater stem-densities. Although species presence/absence was little affected by browsing, many species showed differences in abundance, growth and location within browsed and unbrowsed bush-clumps. Species reduced in abundance in browsed plots included Cussonia spicata, Ehretia rigida, Grewia occidentalis, Jasminum angulare and Senecio linifolius. Several species showed reduced growth in browsed plots, particularly those located at bush-clump edges. The relatively unpreferred Aloe ferox was a notable exception. Although browsing had little effect on the composition of the main clump founding species, emergents or late arrivals, there were twice as many single plants in browsed plots and emergence of several species was restricted to the middle of bush-clumps. Comparison of our findings with aerial photographic evidence and other literature suggest that browsing alone is unlikely to significantly reduce scrub cover, although it can clearly control further expansion. Combinations of fire and browsing, rather than one factor alone, are considered likely to act fastest and most effectively to significantly reduce or remove scrub cover altogether.     

Changes in aboveground primary production and carbon and nitrogen pools accompanying woody plant encroachment in a temperate savanna

When woody plant abundance increases in grasslands and savannas, a phenomenon widely observed worldwide, there is considerable uncertainty as to whether aboveground net primary productivity (ANPP) and ecosystem carbon (C) and nitrogen (N) pools increase, decrease, or remain the same. We estimated ANPP and C and N pools in aboveground vegetation and surface soils on shallow clay and clay loam soils undergoing encroachment by Prosopis glandulosa in the Southern Great Plains of the United States. Aboveground Prosopis C and N mass increased linearly, and ANPP increased logarithmically, with stand age on clay loam soils; on shallow clays, Prosopis C and N mass and ANPP all increased linearly with stand age. We found no evidence of an asymptote in trajectories of C and N accumulation or ANPP on either soil type even following 68 years of stand development. Production and accumulation rates were lower on shallow clay sites relative to clay loam sites, suggesting strong edaphic control of C and N accumulation associated with woody plant encroachment. Response of herbaceous C mass to Prosopis stand development also differed between soil types. Herbaceous C declined with increasing aboveground Prosopis C on clay loams, but increased with increasing Prosopis C on shallow clays. Total ANPP (Prosopis+herbaceous) of sites with the highest Prosopis basal area were 1.2 × and 4.0 × greater than those with the lowest Prosopis basal area on clay loam and shallow clay soils, respectively. Prosopis ANPP more than offset declines in herbaceous ANPP on clay loams and added to increased herbaceous ANPP on shallow clays. Although aboveground C and N pools increased substantially with Prosopis stand development, we found no corresponding change in surface soil C and N pools (0–10 cm). Overall, our findings indicate that Prosopis stand development significantly increases ecosystem C and N storage/cycling, and the magnitude of these impacts varied with stand age, soil type and functional plant traits     

Influence of an overstorey tree (Prosopis glandulosa) on associated shrubs in a savanna parkland: implications for patch dynamics

The arborescent legume, honey mesquite (Prosopis glandulosa), appears to play a central role in patch dynamics of southern Texas savannas by modifying soils and microclimate and by facilitating the ingress, establishment and/or growth of shrubs in its understorey. As an indirect test for the occurrence and persistence of facilitation in mature shrub clusters (patches), we examined the gas exchange, water relations and production of associated shrubs growing in patches where a Prosopis overstorey was present and in patches where Prosopis had succumbed to natural mortality. Surface (0–10 cm) soils associated with shrub patches were enriched in total [N] and [C] compared to soils of neighboring herbaceous zones. However, there were no detectable differences in soil [N] or [C] in patches with and without Prosopis. Foliar [N] and biomass of various shrub species were also statistically comparable for patches with and without Prosopis. These results are in accordance with other studies that indicate the nutrient legacy associated with Prosopis occupation of a patch may persist for decades after its demise. In comparison to plants growing in the absence of Prosopis, leaf water potentials (predawn and midday), and net photosynthesis and water vapor conductance (morning and midday) of outer-canopy sunlit leaves over an annual growth cycle were comparable for two common evergreen shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing in patches with a live Prosopis. These findings indicate that the presence of Prosopis was not enhancing the growth or activity of mature understorey shrubs; facilitation may, therefore, be important only during early stages of cluster development. In addition, we found no indication that the loss of Prosopis has initiated a downward phase in a cyclic succession of patch initiation, growth and death. Rather, the understorey shrubs appear to be able to maintain growth and productivity in the absence of a Prosopis overstorey, and may, therefore, represent persistent components of woody patches on these savanna landscapes.     

Climatic signals in growth and its relation to ENSO events of two Prosopis species following a latitudinal gradient in South America

Semiarid environments throughout the world have lost a major part of their woody vegetation and biodiversity due to the effects of wood cutting, cattle grazing and subsistence agriculture. The resulting state is typically used for cattle production, but the productivity of these systems is often very low, and erosion of the unprotected soil is a common problem. Such dry‐land degradation is of great international concern, not only because the resulting state is hardly productive but also because it paves the way to desertification. The natural distribution of the genus Prosopis includes arid and semiarid zones of the Americas, Africa and Asia, but the majority of the Prosopis species are, however, native to the Americas. In order to assess a likely gradient in the response of tree species to precipitation, temperature and their connection to El Niño southern oscillation (ENSO) events, two Prosopis species were chosen along a latitudinal gradient in Latin America, from northern Peru to central Chile: Prosopis pallida from a semi‐arid land in northern and southern Peru and P. chilensis from a semiarid land in central Chile. Growth rings of each species were crossdated at each sampling site using classical dendrochronological techniques. Chronologies were related with instrumental climatic records in each site, as well as with SOI and N34 series. Cross‐correlation, spectral and wavelet analysis techniques were used to assess the relation of growth with precipitation and temperature. Despite the long distance among sites, the two Prosopis species presented similar responses. Thus, the two species' growth is positively correlated to precipitation, while with temperature it is not. In northern Peru, precipitation and growth of P. pallida present a similar cyclic pattern, with a period of around 3 years. On the other hand, P. pallida in southern Peru, and P. chilensis also present this cyclic pattern, but also another one with lower frequency, coinciding with the pattern of precipitation. Both cycles are within the range of the ENSO band.     

Predictability of early stages of primary succession in post‐mining landscapes of Lower Lusatia,Germany

Abstract. The predictability of early primary succession in post‐mining landscapes of eastern Germany was studied at sites 5–70 yr following dumping. This chronosequence was investigated using indirect ordination methods. The position of the vegetation types in the ordination diagram was found not to infer any temporal sequence. Independent observations show that the change of vegetation type is slow and does not necessarily occur among types adjacent in the ordination diagram. Furthermore, direct ordination revealed that environmental parameters such as pH, the levels of available phosphate and organic carbon as well as the age of the study sites do not significantly account for the variance. Instead, attention needs to be paid to the influence of spatial aspects and also what recultivation measures have been carried out. A detailed account of the vegetation dynamics of individual sample plots showed ‐ depending on the respective vegetation type ‐ divergence, convergence, and fluctuation at the smallest spatial scale. While the species richness of the sample plots remained more or less constant after initial colonization, mean vegetation cover continuously increases with age, although some sites still remain free of vegetation after as long as 70 yr. No general trend in dominant life forms was indicated. A conceptual model of early succession mechanisms is outlined and five basic mechanisms are identified (i.e. site availability, site suitability, availability of diaspores, strategies of colonizing species and biotic interactions). Their respective importance in three different stages of early succession is estimated and compared. The predictability of vegetation dynamics at each stage is rated differently.     

Reactions of the fauna on the bark of trees to the frequency of fires in a North American savanna

Summary The arthropod communities living on the bark of the oak species Quercus macrocarpa and Q. ellipsoidalis were investigated in a North American oak savanna. Differences were found in the community structure of the arthropods living on the bark of these two tree species, although they have the same fissured bark type. In the North American oak savanna ecosystem the most important disturbance factor is fire, which maintains species richness. Highest numbers of species and specimens were found at moderately disturbed sites. Three main ecological groups of arthropods living on the bark of trees can be distinguished in relation to the degree of disturbance: (1) Inhabitants of bark of trees restricted to undisturbed sites: they do not occur in fire-disturbed areas; (2) Inhabitants of bark of trees adapted to a moderate degree of disturbance: many species occur in high numbers only in moderately disturbed areas; and (3) Specialist inhabitants of bark of trees in heavily disturbed areas. The number of specimens of these species increases per trunk with the frequency of disturbance.     

Acceleration of vegetation succession on eroded land by reforestation in a subtropical zone

Based on field investigations in the East River basin in Guangdong Province, south China, the processes of vegetation development and vegetation succession on bare slopes with high erosion rates are studied. Different reforestation measures have been applied to the slopes, which have resulted in very different processes of vegetation development and succession. On an experimental plot with burned forest but surviving roots, the vegetation restored naturally and quickly because there was little soil erosion. However, in the plots suffering from long-term severe soil erosion, natural vegetation recovery on these barren slopes is very slow. After 26 years, a barren slope has only been partly colonized by poorly developed vegetation composed of heliophilous herbages and scattered shrubs. On the other hand, plantation of some selected wood species on the slope land has dramatically accelerated the vegetation recovery and succession. The plots were reforested with plantation of Acacia auriculiformis in the early 1980s. Twelve years later, the vegetation cover of the artificial forests reached 90% and an understory vegetation community consisting of local species had naturally developed. Reforestation with suitable strategies may control erosion and greatly accelerate vegetation succession in the eroded slope land in the subtropical zones.     

Environmental correlates of Freckled Nightjar ( Caprimulgus tristigma ) activity in a seasonal, subtropical habitat

We examined environmental correlates of activity in the Freckled Nightjar (Caprimulgus tristigma), a nocturnal aerial insectivore that is resident year-round in the colder and drier parts of southern Africa. Specifically, we tested the prediction that air temperature (T _a), in addition to light availability, is a significant correlate of Freckled Nightjar activity. We found that aerial insect density varied by over three orders of magnitude (from <1.0 to 117.1 insects 1,000 m⁻³) during the course of the study, and was strongly and positively related to T _a. Nightjar activity was also significantly temperature-dependent, with a cessation of activity at T _a < 12°C. Consistent with previous studies, we found that activity levels were strongly related to ambient light, with markedly reduced activity levels on dark nights. The average number of Freckled Nightjars encountered per 14.6-km transect was 1.95 ± 2.44 nightjars transect⁻¹ during nights near full moon, but only 0.17 ± 0.61 nightjars transect⁻¹ around new moon. Our study reveals that caprimulgid activity can be significantly influenced by temperature as well as ambient light.     

Geographical gradients in the population dynamics of North American prairie ducks

1. Geographic gradients in population dynamics may occur because of spatial variation in resources that affect the deterministic components of the dynamics (i.e. carrying capacity, the specific growth rate at small densities or the strength of density regulation) or because of spatial variation in the effects of environmental stochasticity. To evaluate these, we used a hierarchical Bayesian approach to estimate parameters characterizing deterministic components and stochastic influences on population dynamics of eight species of ducks (mallard, northern pintail, blue-winged teal, gadwall, northern shoveler, American wigeon, canvasback and redhead (Anas platyrhynchos, A. acuta, A. discors, A. strepera, A. clypeata, A. americana, Aythya valisineria and Ay. americana, respectively) breeding in the North American prairies, and then tested whether these parameters varied latitudinally. 2. We also examined the influence of temporal variation in the availability of wetlands, spring temperature and winter precipitation on population dynamics to determine whether geographical gradients in population dynamics were related to large-scale variation in environmental effects. Population variability, as measured by the variance of the population fluctuations around the carrying capacity K, decreased with latitude for all species except canvasback. This decrease in population variability was caused by a combination of latitudinal gradients in the strength of density dependence, carrying capacity and process variance, for which details varied by species. 3. The effects of environmental covariates on population dynamics also varied latitudinally, particularly for mallard, northern pintail and northern shoveler. However, the proportion of the process variance explained by environmental covariates, with the exception of mallard, tended to be small. 4. Thus, geographical gradients in population dynamics of prairie ducks resulted from latitudinal gradients in both deterministic and stochastic components, and were likely influenced by spatial differences in the distribution of wetland types and shapes, agricultural practices and dispersal processes. 5. These results suggest that future management of these species could be improved by implementing harvest models that account explicitly for spatial variation in density effects and environmental stochasticity on population abundance.     

Nitrogen dynamics following field application of biochar in a temperate North American maize-based production system

Background and aims

Biochar additions to tropical soils have been shown to reduce N leaching and increase N use efficiency. No studies exist verifying reduced N leaching in field experiments on temperate agricultural soils or identifying the mechanism for N retention.

Methods

Biochar derived from maize stover was applied to a maize cropping system in central New York State at rates of 0, 1, 3, 12, and 30 t?ha^-1 in 2007. Secondary N fertilizer was added at 100, 90, 70, and 50 % of the recommended rate (108 kg N ha^-1). Nitrogen fertilizer enriched with ¹⁵?N was applied in 2009 to the 0 and 12 t?ha^-1 of biochar at 100 and 50 % secondary N application.

Results

Maize yield and plant N uptake did not change with biochar additions (p?>?0.05; n?=?3). Less N (by 82 %; p?<?0.05) was lost after biochar application through leaching only at 100 %?N fertilization. The reason for an observed 140 % greater retention of applied ¹⁵?N in the topsoil may have been the incorporation of added ¹⁵?N into microbial biomass which increased approximately three-fold which warrants further research. The low leaching of applied fertilizer ¹⁵?N (0.42 % of applied N; p?<?0.05) and comparatively high recovery of applied ¹⁵?N in the soil (39 %) after biochar additions after one cropping season may also indicate greater overall N retention through lower gaseous or erosion N losses with biochar.

Conclusions

Addition of biochar to fertile soil in a temperate climate did not improve crop growth or N use efficiency, but increased retention of fertilizer N in the topsoil.     

Spatial variation of the stable nitrogen isotope ratio of woody plants along a topoedaphic gradient in a subtropical savanna

Variation in the stable N isotope ratio (δ¹⁵N) of plants and soils often reflects the influence of environment on the N cycle. We measured leaf δ¹⁵N and N concentration ([N]) on all individuals of Prosopis glandulosa (deciduous tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) present within a belt transect 308 m long × 12 m wide in a subtropical savanna ecosystem in southern Texas, USA in April and August 2005. Soil texture, gravimetric water content (GWC), total N and δ¹⁵N were also measured along the transect. At the landscape scale, leaf δ¹⁵N was negatively related to elevation for all the three species along this topoedaphic sequence. Changes in soil δ¹⁵N, total N, and GWC appeared to contribute to this spatial pattern of leaf δ¹⁵N. In lower portions of the landscape, greater soil N availability and GWC are associated with relatively high rates of both N mineralization and nitrification. Both soil δ¹⁵N and leaf [N] were positively correlated with leaf δ¹⁵N of non-N₂ fixing plants. Leaf δ¹⁵N of P. glandulosa, an N₂-fixing legume, did not correlate with leaf [N]; the δ¹⁵N of P. glandulosa’s leaves were closer to atmospheric N₂ and significantly lower than those of C. hookeri and Z. fagara. Additionally, at smaller spatial scales, a proximity index (which reflected the density and distance of surrounding P. glandulosa trees) was negatively correlated with leaf δ¹⁵N of C. hookeri and Z. fagara, indicating the N₂-fixing P. glandulosa may be important to the N nutrition of nearby non-N₂-fixing species. Our results indicate plant ¹⁵N natural abundance can reflect the extent of N retention and help us better understand N dynamics and plant-soil interactions at ecosystem and landscape scales.     

Modeling of vegetation dynamics in the Mississippi River deltaic plain

An analysis was made of vegetation phenomena associated with cyclic river delta building and abandonment in the Mississippi River deltaic plain, Louisiana. Markov models of vegetation succession were completed for the new Atchafalaya delta and for the abandoned Lafourche delta. Transition matrices representing different flood conditions in the Atchafalaya delta (1979–1984) were used separately and in combinations based on long term river discharge data. Effect of grazing was introduced into the model as a special transition matrix. Succession during active delta building is heavily dependent on a particular sequence of flood conditions and, apparently, on the intensity of grazing. Natural vegetation changes and land losses in the abandoned Lafourche delta were simulated using a transition matrix based on air photographs from 1945 and 1956. A general flow diagram of long cyclic vegetation changes in the Mississippi deltaic plain was completed.We are grateful to G. W. Peterson, R. K. Abernethy, D. E. Parton, W. B. Johnson, D. A. Fuller, C. Neill, J. W. Day, R. E. Turner, F. Sklar, R. Costanza and M. G. Barbour for help and advice during this study.Nomenclature follows D. T. MacRoberts, 1984. The vascular plants of Louisiana. Bull. Mus. Life Sci., Louisiana State University in Shreveport 6: 1–165.Part of this work was supported by the Louisiana Sea Grant College Program maintained by NOAA, U.S. Department of Commerce.     

Nitrogen fertilizer and gender effects on the secondary metabolism of yaupon,a caffeine-containing North American holly

Females and males of sexually dimorphic species have distinct resource demands due to differential allocation to reproduction. Sexual allocation theory predicts that functional traits will diverge between sexes to support these demands. However, such dimorphism may be masked by the impact of current reproduction on source-sink interactions between vegetative and reproductive organs. We ask whether natural selection has led to genetic dimorphism in homologous physiological traits between sexes of the dioecious willow shrub, Salix glauca. In a common garden experiment we compared physiological responses to drought stress by male and female ramets in the absence of confounding demands from reproductive structures. Ramets experienced similar pre-dawn leaf water status (Ψ_l) as parental genets in flower within the natural population, indicating that experimental dry-down mirrored environmental conditions in nature. Male and female ramets achieved similar instantaneous water use efficiency, based on the ratio of carbon gain to water loss, under wet and dry conditions. However, female ramets experienced greater water stress (i.e., more negative Ψ_l) than males under dry conditions. Lower Ψ_l for female ramets may partly reflect the maintenance of conductance under drought; males, in contrast, maintain Ψ_l under drought by reducing conductance. Differences between sexes in terms of conductance and leaf water status of the vegetative ramets were absent in a concomitant comparison of parental flowering plants. Our results show (1) genetic divergence in physiology between sexes of S. glauca occurs in the absence of gender-specific reproductive sinks, (2) males are the more physiologically plastic sex with respect to water use, and (3) paradoxically, divergence in water relations between sexes is not detectable at sexual maturity under natural conditions.     

一类具有时滞的微生物连续培养数学模型研究

研究了具有强核函数时滞的微生物连续培养数学模型,利用泛函微分方程理论和数值解法得到系统在一定操作条件下存在Hopf分叉以及分叉值随操作参数变化的规律,并研究了Hopf分叉产生的方向及周期解的稳定性,绘制了周期解的图形和相图.该模型定性地描述了实验中的振荡和过渡现象.最后与弱核函数时滞模型、离散时滞模型进行比较,分析了它们对多态、振荡等动态行为的影响。