Reproductive and recruitment traits as indicators of the invasive potential of Kalanchoe daigremontiana (Crassulaceae) and Stapelia gigantea (Apocynaceae) in a Neotropical arid zone

Populations of two ornamental exotic species, Kalanchoe daigremontiana and Stapelia gigantea, have been recently detected inside a protected area containing arid ecosystems in Venezuela. We indirectly tested their invasive potential by characterizing their reproductive biology and recruitment patterns and comparing our estimates against Baker's Law and reproductive profiles reported for invasive plants. K. daigremontiana is autogamous, produces >16,000 seeds per plant and also reproduces clonally. Despite low seed viability (17.9%) and germination rates (11.9%), seeds were present in the seed bank. Plantlets of asexual origin showed high survival (75–100%) compared to seedlings of sexual origin (10%). S. gigantea is self-incompatible, xenogamous and produces close to 1500 seeds per m² of plant tissue. Seed viability (77%) and germination rates (62%) were high, but this species is not represented in the seed bank. It has a vegetative growth. A combination of reproductive and recruitment attributes, which match those considered in Baker's Law and others reported for invasive plants, confer K. daigremontiana and S. gigantea the potential to invade Neotropical arid zones; the former mainly through selfing and production of numerous asexual plantlets, and the latter through an association with a locally abundant pollinator and production of wind dispersed seeds.     

Gender-related differences of shrubs in stands of Atriplex canescens with different histories of grazing by cattle

Atriplex canescens is a relatively common dioecious shrub in western North America. It is considered a valuable forage resource for both wild and domestic herbivores. Sex ratios and shrub dimensions were recorded in stands of tetraploid A. canescens that had been either protected from cattle grazing or summer- or winter-grazed by cattle for at least 20 years. Stem diameter and crown shape were used as surrogates for shrub age which could not be estimated by counting growth rings. Shrub sex ratios in exclosures were significantly more female biased than the empirically derived ratio for tetraploid A. canescens (55 female:35 male: 10 monecious). Conversely, shrub sex ratios in grazed pastures were not significantly different from the empirical ratio. Proportion of female shrubs in exclosures was significantly higher than in grazed pastures. Proportion of male shrubs, on the other hand, was similar in exclosures and grazed pastures. Winter-grazed shrub stands were apparently younger than both summer-grazed and protected shrubs. Protected shrubs appeared to be the oldest. Grazed female shrubs were apparently younger than grazed males, however, shrub ages of protected male and female shrubs were apparently not different. Cattle-grazing may have affected female shrubs more negatively at this site, causing gender-based differential mortality, and/or sex-shifting. Such processes could account for the differences in sex ratios, and for the apparent gender-related differences in shrub age that were observed.     

A darkling beetle population in West Texas during the 1997–1998 El Niño

We examined the abundance and distribution of Eleodes extricata (Tenebrionidae) in a semi-arid region of West Texas. During the 2-year study, climatic conditions were extreme but opposite, with 1997 being unusually wet and mild, and 1998 being unusually dry and hot. These conditions were associated with striking differences in the population structure of the beetles. Under identical pitfall-sampling efforts, we captured 1562 and 671 individuals in 1997 and 1998, respectively. Capture rates were lowest in grassland, and highest in sand shinnery oak, and within the latter, they were lowest in blowouts, intermediate along the edges of blowouts, and highest under vegetative cover. Sex ratios were consistently female-biased, and the degree of bias increased from spring through fall in both years. These increases were more pronounced in 1998, with the sex ratios of early and late sampling periods differing by over four-fold, from 1·15 to 5·00. Females were heavier than males, and the dry masses of both sexes increased from spring though fall in both years. As with sex ratios, these increases were more pronounced in 1998. Only the largest beetles survived the drought, illustrating how extreme conditions can exert especially strong directional selection, and affect sex ratios in populations with sexual size dimorphism.     

Bisexual populations of the brine shrimpArtemia in Sète-Villeroy and Villeneuve Saltworks (Languedoc,France)

So far,Artemia populations in solar saltponds of southern France have been considered as belonging to the Old World parthenogenetic strain. However, two populations sampled monthly in abandoned salinas of Sète-Villeroy and Villeneuve appear to be bisexual throughout their life-cycle. Their sex ratio regularly fluctuates near equilibrium, and mating, present throughout life, is very common in early winter. Morphological characteristics, particularly the male frontal knobs structure, are discussed in relation to the diploid bisexual Mediterranean strain,Artemia tunisiana, which is distributed around the Mediterranean basin. The existence of sexual populations is discussed in regard to variation in environmental conditions in abandoned salinas.     

Influence of understory removal, thinning and P fertilization on N2fixation in a mature mesquite (Prosopis glandulosavar.glandulosa) stand

The natural abundance¹⁵N/¹⁴N method was used to estimate the influence of silvicultural and P fertilization treatments on N accretion, N₂fixation and N partitioning among tissues in a mature mesquiteProsopis glandulosavar.glandulosastand in Texas. The silvicultural treatments consisted of understory removal, herbicide treatment of brushy resprouts, thinning trees to single stems and 100 kg ha⁻¹P fertilization. The trees had a mean basal diameter of 17·8 cm with 8 to 35 cm range. The stand was slow growing with the increase in dry matter ranging from 0·465 Mg ha⁻¹year⁻¹to 0·701 Mg ha⁻¹year⁻¹for the 8 years after the treatments were applied. N accretion after 8 years ranged from 3·1 kg ha⁻¹year⁻¹to 4·4 kg ha⁻¹year⁻¹.Due to the range in δ¹⁵N of the leaves, twigs, branches and trunk, we used the weighted (by biomass) average δ¹⁵N per tree in calculations of the percent N derived from N₂fixation (%Ndfa). There was considerable variability in δ¹⁵N of the reference plants, i.e. from 3·3 to 5·9. In contrast there was low variability in the background δ¹⁵N of nearby soils (7·0±1·0). As the total above-ground biomass δ¹⁵N of a grass grown outside the influence of mesquite (7·8±0·58) had the same δ¹⁵N as the soil (7·5±1·0), we used the grass outside the influence of mesquite and the weighted tree mean δ¹⁵N to calculate % of N derived from N₂fixation.The decrease in intraspecific competition by thinning multistemed trees to single stemmed trees was the only treatment that significantly (p= 0·0001) increased growth. Interspecific competition, i.e. understory removal, did not increase growth. There were no significant differences in total N production or N fixation among treatment means. The most striking result was the highly positive correlation between tree δ¹⁵N and total N per tree and biomass per tree (R²= 0·90,F= 164·4, df. = 18, mean square error (MSE) = 0·155,p= 0·0001). This implies that the younger trees colonizing infertile soils relied more heavily on N₂fixation than larger trees which accumulated 1200 kg ha⁻¹more N under their canopies. The percentage N derived from N₂fixation ranged from 63 to 73% in the various treatments. Despite the high percentage of N derived from N₂fixation, the N₂fixation of the stand was very low, i.e. 1·98 to 2·80 kg N ha⁻¹year⁻¹, due to the low growth of the stand. We believe that comparisons of the whole tree weighted δ¹⁵N to background soil δ¹⁵N provides a more reasonable approach to estimate % N₂fixation than comparisons of leaves of fixers and reference plants.     

Effects of early season defoliation ofAnthyllis cytisoides(a Mediterranean browse species) on further herbivore attack

To determine if defoliation of a woody plant affects foraging by folivorous insects, we examined the infection rate (number of leaves damaged per total number of leaves sampled on marked stems) ofAnthyllis cytisoidesunder three experimental treatments: 10, 50 and 90% plant defoliation. Observations were made for three age classes, established by trunk base perimeter (equal to or lower than 11 cm, between 11 and 20 cm, larger than 20 cm). Plants respond positively to artificial defoliation by increasing total vegetative length of the stem and total inflorescence length. This response is most evident in young individuals.Response to herbivory was measured as overall infection rate and also as infection rate by different feeding guilds—chewing, mining, or sucking insects. We found that increased defoliation elicited increased resistance of leaves to insect attack. This was particularly evident in young plants. Different insect guilds respond in different ways. Attack by chewing insects declines with defoliation for all plant age classes; only sucking insects which feed on the oldest plants reduce feeding rate with plant defoliation. Finally, mining insects present the opposite trend in young and senescent plants.     

Salinity-induced growth and some metabolic changes in three Salsola species

Three Salsola species, Salsola dendroides Pall., S. richteri (Moq.) Karel ex Litw. and S. orientalis S.C. Gmel., were compared for their salt tolerance, inorganic ionic accumulation and their biomass production in saline conditions. Seeds were grown on sterilized quartz under five salinity levels in a factorial experimental design, with four replications, in greenhouse conditions.With salinity, Na⁺ accumulation increased while K⁺ accumulation decreased. All three species showed positive shoot growth for low levels of salinity. Root growth showed almost the same trend as shoot growth, with minor exceptions. At low levels of salinity, proline accumulated more in S. dendroides plant tissues than in the tissues of the other two species. These results suggest that the proline accumulation is a good index for salinity tolerance. Soluble sugars also increased as a result of salinity.     

Neighbor competition modifies demographic responses of desert dune annuals to gradients in sand stability

A field experiment was conducted to test the hypothesis that competitive interactions affect demographic responses of desert dune annuals to gradients in sand stability. The experiment was based on a three-way factorial design with two types of neighbor treatments (complete neighbor removalvs. control), two types of habitats (stablevs. unstable sands) and two species which characterize sandy areas of the western Negev Desert (Neurada procumbensandBromus fasciculatus). Each of the eight treatment/habitat/species combinations was characterized by three demographic variables: seedling survival to reproduction, fruit number per reproductive plant and fruit number per seedling. All of these variables exhibited significant interactions between the effects of neighbor removal and habitat type, indicating that neighbor conditions were important in determining the observed patterns of demographic variation. Moreover, while plants ofNeurada procumbensgrowing without neighbors suffered more mortality on unstable sands, conspecific plants growing at natural density conditions suffered more mortality on stable sands. In the case ofBromus fasciculatus, individuals growing in the absence of competitive effects produced more fruits on stable sands, but conspecific plants growing at natural density conditions exhibited higher fruit production on unstable sands. These findings indicate that competitive effects not only modify, but may even reverse demographic responses of desert dune annuals to gradients in sand stability. Based on these results and evidence from two other studies it is suggested that seed dispersal from abiotically favorable into less favorable habitats may increase fitness of desert annuals by reducing the average effect of competition.     

Effect of fire on growth of three perennial grasses from central semi-arid Argentina

The effect of fire on growth of Piptochaetium napostaense, Stipa tenuis, and Stipa gynerioides, three important native perennial grasses in the semi-arid region of central Argentina, was evaluated under different fire temperature regimes: 300–400°C (low temperature regime), 500–600°C (high temperature regime) and no fire (control). Fire treatments were applied with a portable propane plant burner in April and December 1994, May 1995, and January 1996. Overall results indicate that during the first months after fire occurrence, average total green length of S. tenuis, P. napostaense and S. gynerioides tillers was severely reduced (p<0.05) by fire. This effect was more pronounced in plants burned with the high temperature treatment. The observed patterns of response to fire for height of tillers were very similar to those already reported for total green length of tillers. Towards the end of each growth cycle, the number of green leaves per tiller of burned plants of P. napostaense, S. tenuis, and S. gynerioides were similar or greater (p<0.05) than the number of green leaves on tillers of control plants. Relative growth rates for total green length and for height in tillers of burned plants of S. tenuis, P. napostaense and S. gynerioides were greater than in tillers of control plants. Our results indicate that fire affected differentially the growth of the studied species; S. gynerioides was more affected by fire than P. napostaense and S. tenuis. The species most tolerant to fire was P. napostaense.     

Reproductive biology of Opuntia: A review

A review of the reproductive biology of fleshy-fruited species of Opuntia sensu stricto was conducted. Among Cactaceae, Opuntia is the most diverse and widely distributed genus in the Americas. The genus is strongly associated with bee pollination and coevolution with at least two bee genera is suggested. Fruits and vegetative parts, such as spiny cladodes, are closely linked with seed dispersal and highly efficient vegetative dissemination by animals. Vegetative multiplication appears to be more efficient than sexual reproduction for plant recruitment. Both sexual reproduction and plant multiplication seem to have contributed to the ecological and evolutionary success of the genus, but empirical evidence is lacking.     

顾及性别差异的电子地图空间认知分析

电子地图是人们进行地理空间信息应用与认知的重要载体,而性别差异影响着地图空间认知的结果。该文通过一系列地图认知实验,分析了不同性别在观察地图、认知地图、形成心象地图和表述地图信息过程中的差异。实验表明,男性对空间关系较为敏感,而女性对要素注记、形状、位置及空间面积的认知能力更为突出。     

The influence of flushing rates, terrestrial input and low salmon escapement densities on paleolimnological reconstructions of sockeye salmon (Oncorhynchus nerka) nutrient dynamics in Alaska and British Columbia

Recent advances in paleolimnology have enabled reconstructions of past sockeye salmon (Oncorhynchus nerka) dynamics using a number of proxy-indicators, including diatoms and stable isotopes. Thus far, studies have focused on nursery lakes with high escapement densities and low flushing rates, ensuring that levels of salmon-derived nutrients (SDN) are high and are incorporated into the food chain. This study examines three oligotrophic sockeye salmon nursery lakes in Alaska (Afognak and Saltery lakes) and British Columbia (Hobiton Lake) to determine if sockeye salmon populations can be tracked in nursery systems with lower salmon escapement densities, higher flushing rates and/or higher terrestrial input. We adopted a multi-proxy approach using diatoms, stable isotopes (¹⁵N), organic carbon to nitrogen (C/N) ratios and pollen to draw inferences from ²¹⁰Pb-dated sediment cores. ¹⁵N showed little response to historic variation in sockeye salmon populations, even in Saltery Lake, which has a very high escapement density, and in Afognak Lake, in which average escapement is known to have increased. Dilution effects due to high flushing rates were likely partly responsible for the low ¹⁵N and minimal variation throughout the cores, although very high terrestrial input in Hobiton Lake also dampened the salmon signal. Small changes in diatom species assemblages, however, were evident in all three lakes and may be in response to fluctuating loads of salmon-derived nutrients. Most notably, increases of mesotrophic diatom taxa, such as Asterionella formosa and Aulacoseira subarctica, corresponded to increased salmon production in Alaskan lakes as a result of enhancement (fertilization) activities and climatic changes. Changes in the relative abundance of Cyclotella pseudostelligera in Hobiton Lake may also be in response to a significant decline in sockeye salmon populations off the west coast of Vancouver Island in the 1970s. Other factors, however, such as logging and lake fertilization may also have influenced diatom species composition. These results confirm that, while salmon-derived nutrients may be of key importance in juvenile salmonid development in some lakes, this may not be the case in all systems, especially those in which flushing rates are high. Further, in these systems, diatom communities appear to respond more sensitively to fluctuations in salmon populations (and therefore nutrients) than stable isotope methods, provided that other changes in trophic status are minor.     

Effects of sand burial on dune plants: a review

Burial of different growth stages of plants （e.g., adult plants, seedlings and seeds） is frequent in dune ecosystems. The soil micro-environment, which differs from surface conditions, influences the survival and growth of dune plants. To sum up knowledge about the survival mechanisms of plants under sand burial and to promote practical rehabilitation of dune vegetation, we reviewed relevant published literature and concluded that： （1） Focus in recent years has been on impacts of sand burial on seed germination and seedling emergence. Generally, shallow burial increased seed germination and seedling emergence, but deeper burial was negative. Buried at the same depth, large seeds showed higher germination and seedling emergence rates, attributed to larger energy reserves. （2） Survival, growth and reproduction rates of dune plants show plasticity in response to sand burial. Long-term deep burial is fatal because it creates a physical barrier which overcomes the vertical growth of plants, reduces photosynthetic leaf area, and limits oxygen availability to roots. Modest burial, on the other hand, is advantageous for growth and reproduction of many dune plants, due to protection from excessive temperature and drought. （3） There are few reports concerning effects of sand burial on plant physiology, but a limited number of studies indicate that partial burial increases water use efficiency, chlorophyll content, transpiration rate and net photosynthetic rates. The antioxidant protective enzyme system and osmolyte balance were reported to be involved in the mechanisms of dune plant resistance to burial.     

Water use measurement by non-irrigated Tamarix ramosissima in arid regions of Northwest China

Tamarix ramosissima Ledeb. is a typical hardy desert plant growing in arid regions of Northwest China. Sap flow in stems ofT. ramosissima plants were measured continuously to determine the diurnal and ...     

Flower orientation in Pachycereus weberi (Cactaceae): Effects on ovule production, seed production and seed weight

Few studies have determined the importance of orientation on flower reproductive success. Flower orientation may affect reproduction of Cactaceae; the interception of photosynthetic active radiation (PAR) differs along the different sides of cacti stems, and translocation of resources between cacti ribs does not appear to occur. Therefore, PAR received by each cacti stem face may determine the resources available for reproduction. We examined the reproductive success of Pachycereus weberi flowers with contrasting orientations. Ovule production per ovary, number of seeds per fruit, seed set per fruit and seed weight were used as indicators of reproductive success for flowers facing southeast and northeast. PAR received on opposite stem sides was recorded as an estimator of resource availability for each side. Results showed that flowers facing southwards produced more ovules and seeds and produced heavier seeds. Seed set was not significantly different between flowers with contrasting orientations. The southern stem faces received more PAR than the northern faces. Reproductive success was different for flowers with contrasting orientations, suggesting that there is an association between PAR received on different P. weberi stem faces and floral reproductive success.     

Recruiting mechanisms of Cylindropuntia leptocaulis (Cactaceae) in the Southern Chihuahuan Desert

Sexual reproduction and vegetative propagation require the establishment of morphologically diverse reproductive structures (seedlings and stem segments). This study assesses both types of Cylindropuntia leptocaulis recruitment in two microenvironments protected from herbivory (in open space and under the canopy of Larrea tridentata) and with two soil types (collected below L. tridentata canopy and from open space).An observational study revealed that sexual establishment was very limited (only 8 juveniles/ha were found), while clonal establishment was more frequent (59 juveniles/ha); both types of propagule were found only under shrub canopy.Field experiments showed that, as in other Cylindropuntia species but contrasting with findings for Opuntia species, seeds germinated and seedlings survived under nurse plants as well as in open space, but they grew faster in the former environment. Faster growth under canopy shade provides better chances of successful recruitment to later life-cycle stages. Stem segments never established in open space, requiring the shade of nurse plants. This may be due to physiological constraints imposed by their small volume. In synthesis, the results indicate that clonal propagation is the most important recruitment mechanism in C. leptocaulis populations, and that it takes places under shrub canopy.     

Competition as a barrier to establishment of a native perennial grass (Elymus elymoides) in alien annual grass (Bromus tectorum) communities

The alien grass Bromus tectorum dominates stable annual-plant communities that have replaced native shrub-perennial grass communities over much of the semi-arid western United States. We conducted field competition experiments between B. tectorum and a native grass, Elymus elymoides, on two sites to determine the effects of B. tectorum competition on perennial grasses, and the role of B. tectorum competition in the stability of B. tectorum-dominated communities. B. tectorum competition acting on seedling-stage E. elymoides plants greatly reduced first-year relative growth rates and biomass which, in turn, reduced second-year survival, biomass, and flowering. However, B. tectorum competition acting on older E. elymoides plants had much less effect, which may help to explain why intact perennial-plant communities are resistant to B. tectorum invasion. At the drier site, direct effects of B. tectorum competition were less, but competition and drier habitat combined to produce greater E. elymoides mortality.     

Effects of indole-3-acetic acid and zinc on the growth, osmotic potential and soluble carbon and nitrogen components of soybean plants growing under water deficit

Three-week old soybean (Glycine max) plants were subjected to a factorial combination of four regimes of soil matric water potential (ψ_m=−0·03, −0·5, −1·0 and −1·5 MPa), two levels of supplementary Zn (O and 20 mgl⁻¹) and two levels of foliar IAA application (O and 10 mgl⁻¹). Under control conditions (no Zn, no IAA), increasing soil drying progressively retarded shoot and root growth (length and dry mass production), reduced leaf relative water content (RWC) and decreased the contents of chlorophyll (Chl) and shoot soluble sugars (SS), but increased soluble sugar content of roots and lowered osmotic water potential of shoots and roots (osmotic adjustment). Total free amino acid (TAA) content increased in shoots but decreased in roots whereas contents of soluble proteins (SP) decreased in shoots and roots. The effect of water stress was statistically significant (p<0·05) and had a major effect (as indicated by η²values) on leaf RWC, shoot and root dry masses and osmotic potential. Supplementary Zn improved root growth at all levels of stress and shoot growth under severe stress. Improvement of growth was positively correlated with the internal tissue Zn concentrations (r=0·91 and 0·86 for shoot and 0·94 and 0·82 for root length and dry mass respectively). Exogenous IAA raised (p<0·05) RWC, Chl, DM (slightly), root SS, and SP, whereas shoot TAA was lowered. Effects on root TAA and shoot SS were more complex: they were lowered at zero stress and raised under severe stress. IAA and Zn in combination had additive effects on Chl, growth and osmotic potential, but their combined effects on SP and TAA were more complex. It is concluded that the treatment of soybean plants grown under conditions of low soil water potentials and Zn deficiency with Zn and IAA solutions counteracted the deleterious effects of stress, especially at high stress levels, and helped stressed plants to grow successfully under these adverse unfavourable conditions.     

Total energy budget and prey requirements of free-ranging coyotes in the Great Basin desert of the western United States

Estimates of energetic demands for carnivore species can be valuable for estimating their annual prey requirements and thus, their potential impact on prey populations. This is the case for the coyote (Canis latrans) which has a ubiquitous distribution and preys on a wide variety of wild and domestic prey species. We took data on daily activity of coyotes and with standard energy models, estimated the daily field metabolic rate (FMR) of adult male and female coyotes in the Great Basin desert of the western United States. We then calculated the total annual energy demand and from this, extrapolated annual prey needs for lagomorph and rodent-sized prey. Daily FMR of male coyotes in the Great Basin desert averaged 1170.1 kcal±29.1, S.E. (n=11) and was significantly higher than 988.6 kcal±44.3 (n=8) for females (p=0.002). The highest reproductive cost for females was lactation (1441.1 kcal/day above FMR). Males and females need to consume 192 and 162 lagomorphs, or 3681 and 3110 rodents/year, respectively. For reproduction, females should consume 37 more lagomorphs or 700 more rodents per year. We concluded that preference for lagomorphs by coyotes reflects the most reasonable energy return on their hunting investment.     

劈裂生长的形态发生及内部解剖结构特征的研究

对绵刺劈裂生长的形态发生及内部解剖结构特征进行了观察,研究结果如下：①绵刺的劈裂生长有两种类型,一种是首先茎从基部到根部发生多次劈裂,以后地上的茎部再相应发生分裂而形成几个独立的植株;另一种是茎基部以上的部位先发生纵裂,而根部后发生分离;②劈裂生长首先在茎基部发生,当绵刺植株生长到5 a以上的时候,这个部位的形成层活动不均匀,出现不规则的木质部排列,形成花环状结构,在相邻的两环状部位进行劈裂;③由于受劈裂生长的影响,劈裂发生后的植株的根部年轮分布不规则,发生劈裂部位的形成层活动不均匀,形成的次生木质部中导管数量少,口径也小,木纤维数量增多,细胞排列紧密而形成缢缩。