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.     

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.     

Testing the optimal defense hypothesis in Stryphnodendron adstringens (Fabaceae,Mimosoideae) leaves: the role of structure,number, position and nectar composition of extrafloral nectaries

Stryphnodendron adstringens is a common Cerrado tree that possesses extrafloral nectaries (EFNs) on its leaves, which are located at the base and apex of the rachis and along the secondary veins. The position of EFNs and their nectar production can be affected by defense strategies because plant organs possess different values and herbivory vulnerability. Here we aimed to elucidate anatomy, histochemistry, nectar composition and EFN number on leaves of S. adstringens in the light of the optimal defense hypothesis. We found a convergence on anatomy and histochemical characterization because the three studied types of EFNs have epidermis, secretory parenchyma and vascular tissue, showing phenolic compounds and polysaccharides in the secretory parenchyma cells. The nectar contained glucose, fructose and sucrose, which attract ants of the Camponotus and Cephalotes genus. We found differences in the number of EFNs along the secondary veins and in the nectar composition between EFNs located at the base and apex of the rachis of the leaf. The number of EFNs on the secondary veins increases from the base to the apex, suggesting a strategy to induce ant patrolling over the entire leaf region. EFNs at the base secreted more nectar, which should be related to the protection of the leaf base, which is the part most vulnerable to herbivore attack and the most valued organ. We concluded that EFNs of S. adstringens are anti‐herbivore defenses whose pattern matches the predictions of the optimal defense hypothesis.     

The effect of costs associated with toxin production in a competitive system

In this paper, we study a two-species competitive system where both the species produce toxin against each other at some cost to their growth rates. A much wider set of outcomes is possible for our system. These outcomes are important contrasts to competitive exclusion or bistable attractors that are often the outcomes for competitive systems. We show that toxin helps to gain an advantage in competition for toxic species whenever the cost of toxin production remains within some moderate value; otherwise it may result in the extinction of the species itself.     

Manipulation of egg production reveals costs of reproduction in the tree lizard (Urosaurus ornatus)

Life-history theory predicts that the allocation of energy to current reproduction is associated with a decrement in future fecundity, future survival, or both. I treated this notion as the cost hypothesis, and tested the assumption that current reproduction exacts a cost in future survival and fecundity. Surgical manipulations of egg production were applied to natural populations of the tree lizard, Urosaurus ornatus, in south western New Mexico by yolkectomy surgery in two different years. I reduced the number of eggs produced in the first clutch during vitellogenesis by approximately 50% in yolkectomized females relative to controls. Subsequent survival, fecundity, and growth of females were followed for two or three years, depending on the cohort. Treated females in both cohorts showed significantly higher growth and survivorship throughout the experiment than in controls. After 2 years, yolkectomized females had grown an additional 2 mm (snout-vent length) compared to controls, enough for them to add on average an additional egg to their next clutch. This demonstrated a cost in terms of future fecundity through a reduction in growth and an increase in mortality in these lizards.     

The characterisation and identification of a naturally occurring hybrid in the genusLeucaena (Leguminosae: Mimosoideae)

Phytogeographical, morphological, and molecular evidence for the widespread but sporadic occurrence of sterile hybrids betweenLeucaena leucocephala subsp.glabrata andL. esculenta subsp.esculenta in South-Central Mexico is presented. Most morphological and DNA characters studied in the putative hybrids showed states intermediate between the proposed parental taxa. The occurrence of non-additive nuclear ribosomal DNA phenotypes is discussed and the need to use a suite of nuclear taxon-specific markers to determine hybridity is emphasized. The origin of the hybrid is discussed in relation to the disruption of the distributions of both parental taxa through use by man as minor food plants, providing another example of the important influence of human interference on the evolution ofLeucaena. The successful use of dried leaf material as a source of DNA is highlighted as an efficient way to identify sterile hybrids at the molecular level.     

The effects of harvester ant (Messor ebeninus Forel) nests on vegetation and soil properties in a desert dwarf shrub community in north-eastern Arabia

In desert ecosystems, harvester ants have been shown to be important granivores and seed dispersers. Because many desert plants exist as seeds in the ground for long periods, harvester ants may greatly influence plant population dynamics. In this study, we examine the effects of harvester ant nests on vegetation and soil properties in a desert ecosystem dominated by the dwarf shrub Rhanterium epapposum in Kuwait, north-eastern Arabia. Soil properties were greatly modified in the circular refuse zone around the nests of the harvester ants, with elevated levels of nutrients (except nitrogen) and organic matter. Plant species richness and productivity were also significantly enhanced in the refuse zone. The vast majority of species inhabiting this zone were desert annuals, but there is no evidence to suggest that the species involved have any distinct association with ant nests. Harvester ants therefore contribute substantially to small-scale spatial heterogeneity in this aridland plant community. It is also suggested that the significance of ant nests in providing favourable sites for plant growth is enhanced in drier years, when many species in off-nest sites may die prematurely due to drought. Furthermore, because ants occasionally abandon their nests and create new colonies, the addition of nutrient-rich patches to the landscape over time could represent an important mechanism for maintaining fertility of desert soils, possibly with long-term implications for plant biodiversity. Harvester ants can therefore be regarded as a key ecosystem engineer in this relatively undisturbed desert ecosystem of Kuwait.     

Multi-locus DNA sequence data reveal a history of deep cryptic vicariance and habitat-driven convergence in the desert night lizard Xantusia vigilis species complex (Squamata: Xantusiidae)

The lizard genus Xantusia of southwestern North America has received recent attention in relation to delimiting species. Using more than 500 lizards from 156 localities, we further test hypothesized species boundaries and clarify phylogeographical patterns, particularly in regions of potential secondary contact. We sequenced the entire mitochondrial cytochrome b gene for every lizard in the study, plus a second mitochondrial DNA (mtDNA) region and two nuclear introns for subsets of the total sample. Phylogenetic analyses of the mtDNA recover a well-resolved, novel hypothesis for species in the Xantusia vigilis complex. The nuclear DNA (nDNA) data provide independent support for the recognition of X. arizonae, X. bezyi and X. wigginsi. Differences between the respective mtDNA and nDNA topologies result from either the effects of lineage sorting or ancient introgression. Nuclear data confirm the inference that some populations of X. vigilis in northwestern Arizona converged on rock-crevice-dwelling morphology and are not X. arizonae with an introgressed X. vigilis mtDNA genome. The historical independence of ancient cryptic lineages of Xantusia in southern California is also corroborated, though limited introgression is detected. Our proposed biogeographical scenario indicates that diversification of this group was driven by vicariance beginning in the late Miocene. Additionally, Pleistocene climatical changes influenced Xantusia distribution, and the now inhospitable Colorado Desert previously supported night lizard presence. The current taxonomy of the group likely underestimates species diversity within the group, and our results collectively show that while convergence on the rock-crevice-dwelling morphology is one hallmark of Xantusia evolution, morphological stasis is paradoxically another.     

Effects of organic matter availability on the life history and production of a top vertebrate predator (Plethodontidae: Gyrinophilus palleucus) in two cave streams

1. Surface ecosystems provide the primary source of organic matter to many cave communities. Variation in the strength of connectivity to the surface suggests that some caves may be more resource‐limited than others. To test this, we examined diet, prey availability and production of an obligate cave salamander Gyrinophilus palleucus (Plethodontidae), a top predator, in two south‐eastern U.S.A. caves with different levels of organic matter (Tony Sinks cave, 165 g AFDM m^?2; Bluff River cave, 62 g AFDM m^?2). 2. We quantified density, biomass, growth rate, production and diet of G. palleucus monthly for 21 months. Diet composition, differences in prey communities and seasonal patterns in prey consumption were also analysed. 3. Salamander density, biomass and secondary production were significantly greater in the high organic matter cave (0.10 m^?2, 0.18 g AFDM m^?2, 0.12 g AFDM m^?2 year^?1) than in the low organic matter cave (0.03 m^?2, 0.03 g AFDM m^?2, 0.01 g AFDM m^?2 year^?1). Although growth rates were not statistically different between the two cave salamander populations, low recaptures probably influenced this result. 4. Isopoda prey were the major contributor to salamander production in the high organic matter cave (69%). In the low organic matter cave, production was provided by isopods (41%) and oligochaetes (20%). The lower number of prey taxa contributing to salamander production in the high organic matter cave suggests the ability to forage more selectively. 5. The differences in foraging strategy, density, biomass and secondary production were probably related to differences in the strength of surface connectivity, which controls organic matter supply. Links between basal resource level and top predator performance show the importance of bottom‐up limitation in the food webs of caves and other detritus‐based ecosystems.     

Viral production in the Gulf of Trieste (Northern Adriatic Sea): Preliminary results using different methodological approaches

Although the temporal and spatial variability of virioplankton in the northernmost part of the Adriatic Sea has been repeatedly explored suggesting that viruses constitute an extremely dynamic component of the plankton community and hypothesizing their importance in marine food webs and mucilage events, there is still no information about viral replication rates. Hence, the contribution of viruses to bacterial mortality and the cycling of organic matter in this part of the Adriatic basin are still not fully comprehended. Assessment of the role of viral lysis requires a robust means of estimating viral production. Since, up to now, none of the available methods evolved to a state of a standard yet, in this preliminary study 3 different experimental approaches were simultaneously assayed (viral production estimated by radiotracer incorporation method [TdR], dilution technique for the estimate of viral production in already infected bacteria [DIL] and serial dilution method in manipulated phage-host assemblage [SER]). The present study provided the first evidence of viral production rates in this study area, that resulted in comprising between ∼ 3.5-15 × 10⁸ viruses L^− 1 h^− 1 and critically faced up the results obtained by different techniques with the consideration that they suffer from different biases. Based on TdR and DIL viral proliferation estimates, viral lysis was responsible for the loss of 54 to 95% of the bacterial standing stock, while the viral-induced mortality by SER (325% d^− 1) was likely consistently overestimated. These results indicate that viral lysis is a significant factor for prokaryotic mortality suggesting its implication as an important pathway for the cycling of dissolved organic matter in the Gulf of Trieste.     

Bootlegging on a desert mountain: The political ecology of Agave (Agave spp.) demographic change in the Sonora river valley,Sonora, Mexico

Recent studies suggest that wild agave (Agavespp.) plants in Sonora, Mexico, are being over-harvested by mescal makers on communal lands. Using the conceptual framework of regional political ecology (Blaikie and Brookfield, 1987), I discuss the ecological processes of agave depletion, and investigate the social, economic, and political contexts in which unsustainable harvest practices arise. Whereas all the mescal makers have knowledge of sustainable harvest methods, population growth, expansion of agriculture onto ecologically marginal lands, and increasing dependence on wild harvested products from communal lands created the socioeconomic context for increased demand for mescal income. The ideology of household autonomy, and the belief that the village has no right to internally regulate use of the commons, created the political context for rapid, unsustainable harvesting—a tragedy of the commons. However, recent cultural changes have caused a reversal of this trend, and some wild agave populations may be recovering.     

Cost of reproduction in a spring ephemeral species, Adonis ramosa (Ranunculaceae): carbon budget for seed production

BACKGROUND AND AIMS: Spring ephemerals have a specific life-history trait, i.e. shoot growth and sexual reproduction occur simultaneously during a short period from snowmelt to canopy closure in deciduous forests. The aim of this study is to clarify how spring ephemerals invest resources for seed production within a restricted period. METHODS: In order to evaluate the cost of reproduction of a typical spring ephemeral species, Adonis ramosa, an experiment was conducted comprising defoliation treatments (intact, one-third and two-thirds leaf-cutting) and fruit manipulations (control, shading and removal) over two growing seasons. In addition, measurements were made of the movements of carbon assimilated via (13)C tracing. KEY RESULTS: Survival rate was high irrespective of treatments and manipulations. The proportion of flowering plants and plant size decreased as a result of the defoliation treatments over 2 years, but the fruit manipulations did not affect flowering activity or plant size. Seed set and seed number decreased as a result of fruit shading treatment, but the defoliation treatments did not affect current seed production. Individual seed weight also decreased in the second year due to fruit shading. The (13)C tracing experiment revealed that young fruits had photosynthetic ability and current photosynthetic products from leaves were mainly transferred to the below-ground parts, while translocation to fruit was very small even when fruit photosynthesis was restricted by the shading treatment. CONCLUSIONS: Current foliage photosynthetic products are largely stored in the below-ground parts for survival and future growth, and about one-third of the resources for seed production may be attained by fruit photosynthesis. Therefore, the trade-off between current seed production and subsequent growth is weak. The cost of seed production may be buffered by sufficient storage in the below-ground organs, effective photosynthesis under high irradiation and self-assimilation ability of fruits.     

The importance of traditional uses and nutraceutical aspects of some edible wild plants in human nutrition: the case of Umbria (central Italy)

This study aimed to show how the importance of edible wild plants regards not only a question of uses linked to folk traditions but also their value in human nutrition. Data on the use of 50 species were collected through informed consent semi-structured interviews with local informants. They were eaten raw in salads (43%), boiled (35%), as ravioli filling (10%), fried without or with eggs (8%) and in vegetable soup (4%). Furthermore, the nutraceutical analysis centred on four of the commonly used wild edible plants demonstrates how these species contain many of the so-called minor nutrients, such as antioxidising vitamins and polyphenols, which were highest in Sanguisorba minor L.     

Transgenic Plant Monitor (TPM): a relational database for the management and analysis of information in plant tissue culture and transformation laboratories

Increased public concern and strict statutory regulations relating tothe generation and exploitation of genetically modified organisms, make itimperative to track accurately individual plants through DNA transformationprogrammes. The ability to rapidly retrieve information associated withspecifictransgenic events and to provide accurate reports on demand is an increasinglyimportant feature for public research laboratories. Transgenic Plant Monitor(TPM) has been developed as a database structured to allow efficient recording,monitoring and analysis of the extensive and complex data generated in planttissue culture and transformation experiments. TPM is built upon the widelyavailable Microsoft Access database engine and can be readily adoptedand/or adapted by other users. The key features and the utility of TPM as aresearch tool are discussed in this article.     

The sweet jelly of Combretum lanceolatum flowers (Combretaceae): a cornucopia resource for bird pollinators in the Pantanal, western Brazil

 The pollination biology of the neotropical scandent shrub Combretum lanceolatum was studied in the seasonally-flooded Pantanal region in western Brazil. This plant bears horizontally oriented inflorescences, whose yellowish green flowers begin to expand at dusk and are fully open at dawn. Instead of fluid nectar the flowers produce sweet gelatinous secretion in form of pellets. The glandular complex of the flower is composed of the inner wall of the receptacle and its tubular extension, being equivalent to the nectariferous disk of the nectar-producing species within the genus. The jelly is produced at night, contains mannan and is imbibed by free hexoses. It originates by swelling and disintegration of the inner wall, after contact with the nectar generated concomitantly in the mesophyll. Combretum lanceolatum is unique within the genus in its production of jelly pellets instead of liquid nectar. A new term, the jelly-flower, is proposed for flowers with this kind of reward. The pellet is not replaced once removed by a bird, and thus resembles a fruit in its availability to consumers, another unique feature that distinguishes this species within the genus. The jelly pellets offered by the many flowered branches attract a great diversity of bird visitors (28 species from eight families), which feed on this copious food resource and pollinate the flowers. The most effective pollinators probably are thrushes, tanagers, and orioles. Flocking parakeets and macaws sometimes feed on the petals, thus acting as flower plunderers. Combretum lanceolatum presents a high fruit set under natural conditions, which likely favours its spreading and becoming a weed species. Received July 11, 2000 Accepted November 18, 2000     

Population dynamics, growth and production of the neozoon Marenzelleria cf. viridis (Verrill, 1873) (Polychaeta: Spionidae) in a coastal water of the southern Baltic Sea

The propagation of an immigrant from North America, viz. the spionid Marenzelleria cf. viridis in the Darss-Zingst Bodden (DZB) (southern Baltic Sea), was studied at three stations from March 1992 to December 1995. Highest mean abundances (over 28 000 ind. m^-2) and wet weights (400 g m^-2) were recorded at station 2 in 1994. The spionid also reached its highest dominances in terms of biomass (40 to 90%) at this station, which was selected for the population dynamics, growth and secondary production studies. The spionid has a life span of about 3 years, and many individuals achieved sexual maturity after one year. Their growth curve is steepest during the first year of life, during which the animals grow to a length of about 180 segments. However, growth depression was observed during the ripening of the gametes in April, May and June. Secondary production was in the region of 55 to 85 g AFDW m^-2 y^-1. Productivity (P/B) varied considerably from generation to generation, ranging between 0 and 4.8 with an average between 1.2 and 1.6.     

The phylogeographical history of the Iberian steppe plant Ferula loscosii (Apiaceae): a test of the abundant-centre hypothesis

The geology and climate of the western Mediterranean area were strongly modified during the Late Tertiary and the Quaternary. These geological and climatic events are thought to have induced changes in the population histories of plants in the Iberian Peninsula. However, fine-scale genetic spatial architecture across western Mediterranean steppe plant refugia has rarely been investigated. A population genetic analysis of amplified fragment length polymorphism variation was conducted on present-day, relict populations of Ferula loscosii (Apiaceae). This species exhibits high individual/population numbers in the middle Ebro river valley and, according to the hypothesis of an abundant-centre distribution, these northern populations might represent a long-standing/ancestral distribution centre. However, our results suggest that the decimated southern and central Iberian populations are more variable and structured than the northeastern ones, representing the likely vestiges of an ancestral distribution centre of the species. Phylogeographical analysis suggests that F. loscosii likely originated in southern Spain and then migrated towards the central and northeastern ranges, further supporting a Late Miocene southern-bound Mediterranean migratory way for its oriental steppe ancestors. In addition, different glacial-induced conditions affected the southern and northern steppe Iberian refugia during the Quaternary. The contrasting genetic homogeneity of the Ebro valley range populations compared to the southern Iberian ones possibly reflects more severe bottlenecks and subsequent genetic drift experienced by populations of the northern Iberia refugium during the Pleistocene, followed by successful postglacial expansion from only a few founder plants.