Distribution of Calanus species off Franz Josef Land (Arctic Barents Sea)

The spatial distribution of Calanus species was examined near Franz Josef Land archipelago in August 2006 and 2007. Surface and bottom water temperatures exceeded the average multiannual values. Calanus species dominated the total mesozooplankton abundance and biomass, accounting for 818 ± 178 individuals m⁻³ (mean ± SE) and 803 ± 163 mg wet weight (WW) m⁻³, respectively, in 2006. In 2007, the values were much lower (153 ± 29 individuals m⁻³ and 192 ± 17 mg WW m⁻³, respectively), reflecting the weaker influence of warm Atlantic water that year. Calanus glacialis dominated the Calanus populations, contributing 95% and 60% of the biomass in 2006 and 2007, respectively. Older copepodite stages (CIV–CV) predominated in the C. finmarchicus (69% and 76%, respectively) and C. hyperboreus populations (80% and 77%, respectively), whereas young copepodites (CI–CIII) were predominant in C. glacialis (45% and 59%, respectively). A clear negative relationship between the average water temperature and the total Calanus biomass was observed in 2006, whereas a positive correlation between these parameters was observed in 2007. The distributions of three Calanus species off Franz Josef Land were mainly associated with hydrological conditions and circulation patterns.     

Herbaceous layer development during spring does not deplete soil nitrogen in the Portuguese montado

Nitrogen (N) content in the soil and in the herbaceous biomass were monitored during spring of 2004-2006 to determine how the herbaceous layer development influences soil N availability in the montado ecosystem of southern Portugal. Highest (246.6 ± 52.7 g m⁻²) and lowest (123.2 ± 89.5 g m⁻²) peak biomass occurred in 2006 and 2005 respectively. Total soil N within the top 20 cm soil profile ranged between 0.2 ± 0.1% in February and 0.41 ± 0.2% in May, while available soil N was lowest (5 ± 2 μg g⁻¹soil) in February but increased three-to-five fold in March and was >17.5 μg g⁻¹soil at senescence in May. Significant (p < 0.001) increase in total N in the aboveground pool occurred between February and May. There was however, no decay in soil N content. Instead, the herbaceous vegetation enhanced soil N input and N retention in the ecosystem. Most of the herbaceous plants were annuals with large reserves of organic N at senescence, which returned to the soil as detritus. The herbaceous vegetation is a critical component of the montado that contributes to N recharge and cycling within the ecosystem.     

Capacity for water conservation in invasive (Gerbillus nigeriae) and declining rodents (Taterillus pygargus and Taterillus gracilis) that exhibit climate-induced distribution changes in Senegal

Following the southward shift of rainfall isolines in the Sahel at the end of the 1960s, Gerbillus nigeriae appeared in northern Senegal in the mid-1990s, and two resident Gerbillidae (Taterillus pygargus and Taterillus gracilis) subsequently declined. We investigated the causal role of the capacity to conserve water in such climate-related shifts in the distribution of these Gerbillidae by comparing the effects of a water-poor diet on the water-efflux rate (W_−out) of freshly trapped adults pre-acclimatized to a water-rich diet. During the 12-day period of water restriction in all species, 30-50% of individuals became hyperactive and showed greater weight loss and higher W_−out than the remaining inactive individuals. Such emergence of migratory strategists within populations could accelerate the expansion of G. nigeriae. On a water-poor diet, T. gracilis showed a lower capacity to conserve water (higher W_−out) than T. pygargus and G. nigeriae, in both inactive (W_−out = 44.5 ± 1.8 vs 29.6 ± 0.8 vs 27.4 ± 0.7 ml kg^−0.82.day⁻¹, respectively) and hyperactive individuals (W_−out = 60.4 ± 1 vs 45.4 ± 0.7 vs 44 ± 0.8 ml kg^−0.82.day⁻¹, respectively). We propose that the capacity to conserve water accounted for both expansion of G. nigeriae and decline of T. gracilis, whereas competition between T. pygargus and G. nigeriae could account for the decline of T. pygargus.     

Effect of rhizobia inoculation on the seed (herbage) production of mungbean (Phaseolus aureusRoxb.) grown at Uzbekistan

In field trials conducted at Uzbekistan, inoculation of mungbean (Phaseolus aureusRoxb.) with commercial strain CIAM1901 ofBradyrhizobiumsp. (Phaseolus) increased (on average for two cultivars) the herbage mass by 46·6±6·0%, seed mass by 39·2 ± 3·6%, mass of 1000 seeds by 16·0 ± 0·8%, nitrogen content in seeds by 58·3 ± 8·9%, starch content in seeds by 30·0 ± 5·5% and number of nodules by 254%. Inoculation with this strain produced the same herbage (seed) mass as NH₄NO₃application (120 kg ha⁻¹of N), while the combined treatment of rhizobia and 60 kg ha⁻¹of N produced significantly higher yields than rhizobia inoculation alone or application of 120 kg ha⁻¹of N. The rhizobia strain M11 was isolated from Uzbekistan soils and significantly exceeded the commercial strain CIAM1901 in its influence on herbage mass by 3·9–10·6%. Two-factor analysis of variance demonstrated that the herbage mass and number of nodules are controlled mainly by the rhizobia strain genotypes, while seed yield, number of pods, mass of 1000 seeds and N and starch content in seeds are influenced by the plant cultivar genotypes.     

Dieback affects forest structure in a dry Afromontane forest in northern Ethiopia

Forests are highly susceptible to dieback under ongoing climate warming. In degraded forests, dead standing trees, or snags, have become such prominent features that they should be taken into account when setting management interventions. This study investigated (1) the extent and spatial pattern of standing dead stems of Juniperus procera and Olea europaea subsp. cuspidata along an elevational gradient, and (2) the effect of dieback on forest stand structure. We quantified abundance, size, and spatial pattern of tree dieback in 57 plots (50 m × 50 m) established at 100 m intervals along five transects. The snag density and basal area (mean ± SE) of the two species combined were 147 ± 23 stems ha⁻¹ and 5.35 ± 0.81 m² ha⁻¹, respectively. The percentages of snags were extremely high for both J. procera (57 ± 7%) and O. europaea subsp. cuspidata (60 ± 5%), but showed a decreasing trend with increasing elevation suggesting that restoration is even more urgent at the lower elevations. Snags of the two species accounted for 31 and 45% of total stand density and basal area, respectively. Living stems exhibited truncated inverse-J-shaped diameter and height class distributions, indicating serious regeneration problems of these foundation species in the study area. In addition to direct interventions to assist recruitment of climax tree species, sites with high dieback would probably benefit from snag reduction to prevent fire incidents in the remaining dry Afromontane forests in northern Ethiopia.     

Wood productivity of Prosopis flexuosa D.C. woodlands in the central Monte: Influence of population structure and tree-growth habit

A balance between forest production and protection is hard to achieve in arid zones due to their low potential for wood production. Prosopis flexuosa woodlands are the major woody formations in the Monte desert and are currently in a degraded state due to intense use. The main degradation factors in the study area are overgrazing and firewood extraction. We developed allometric models to estimate the aerial biomass of P. flexuosa, compared annual growth rates of one- and multi-stemmed individuals through dendrochronological methods, and estimated the productivity of four structurally different woodlands in the central Monte. Total dry weight was best estimated by power equations. Annual increments in basal area and dry weight were initially larger for multi- than one-stemmed individuals. However, whereas multi-stemmed individuals rapidly decreased their growth rates after 60 years of age, one-stemmed trees maintained steady growth rates during the first 100 years. Depending on woodland density and tree size, total woodland biomass varied between 4000 and 15 000 kg ha⁻¹. Wood productivity was similar in all four woodlands studied (121.6-173.7 kg ha⁻¹ year⁻¹). Our results reveal the importance of tree growth habit to productivity, and suggest that regulated extraction of firewood and poles from old multi-stemmed individuals could optimize wood productivity and contribute to the sustainable use and conservation of these woodlands.     

Biomass production, evapotranspiration and water use efficiency of arid rangelands in the Northern Cape, South Africa

Annual above-ground net primary production (ANPP), evapotranspiration (ET) and water use efficiency (WUE) of rangeland have the potential to provide an objective basis for establishing pricing for ecosystem services. To provide estimates of ANPP, we surveyed the biomass, estimated ET and prepared a water use efficiency for dwarf shrublands and arid savanna in the Riemvasmaak Rural Area, Northern Cape, South Africa. The annual production fraction was surveyed in 33 MODIS 1 km² pixels and the results regressed against the MODIS f_PAR product. This regression model was used to predict the standing green biomass (kg DM ha⁻¹) for 2009 (dry year). Using an approach which combines potential evapotranspiration (ET₀) and the MODIS f_PAR product, we estimated actual evapotranspiration (ET_a). These two models (greening standing biomass and ET_a) were used to calculate the annual WUE for 2009. WUE was 1.6 kg DM mm⁻¹ ha⁻¹ yr⁻¹. This value may be used to provide an estimate of ANPP in the absence of direct measurements of biomass and to provide a comparison of the water use efficiency of this rangeland with other rangeland types.     

Siliceous microfossil succession in the recent history of two basins in Lake Baikal, Siberia

As part of the international cooperative Baikal Drilling Project, siliceous microfossil assemblage succession was analyzed in two short ( 30-cm) sediment cores from Lake Baikal. One core was recovered from the north basin (Core 324, 55°15N, 109°30E), a second from between the central and southern basins (Core 316, 52°28N, 106°5E). The northern core had higher amounts of biogenic silica (40 g SiO₂ per 100 g dry weight sediment) compared to the southern core, and increased deposition in the more recent sediments. Weight percent biogenic silica was lower in the southern core, ranging from approximately 20–30 g SiO₂ per 100 g dry weight sediment throughout the entire core. Trends in absolute microfossil abundance mirror those of biogenic silica, with generally greater abundance in the northern core (86–275×10⁶ microfossils g^–1 dry sediment) compared to the southern core (94–163×10⁶ microfossils g^–1 dry sediment).Cluster analyses using relative abundance of the dominant diatom and chrysophyte taxa revealed four zones of microfossil succession in each core. Microfossil assemblage succession in the north basin may be reflecting shifts in nutrient supply and cycling driven by climatic changes. The most recent sediments in the northern basin (Zone 1,c. 1890's–1991 A.D.) were characterized by an increased abundance ofAulacoseira baicalensis andAulacoseira spore. Zone 3 (c. 1630's–1830's A.D.) was dominated by the endemicCyclotella spp. and reduced abundance of theAulacoseira spp. Zone 3 corresponds approximately to the Little Ice Age, a cooler climatic period. The microfossil assemblages between Zones 1 and 3 (Zone 2,c. 1830's–1890's A.D.) and below Zone 3 (Zone 4,c. 830's–1430's A.D.) are similar to one another suggesting they represent transitional intervals between warm and cold periods. Southern basin sediments record similar changes in the endemic taxa. However, the increased abundance of non-endemic planktonic taxa (e.g.Stephanodiscus binderanus, Synedra acus, Cyclostephanos dubius) during two periods in recent history (post World War II and late 1700's) suggests evidence for anthropogenic induced changes in southern Lake Baikal.     

Soil hydrological and erosional responses in areas of woody encroachment, pasture and woodland in semi-arid Australia

In arid and semi-arid areas, woody encroachment is the increase in density, cover, extent and/or biomass of woody plants. Woody encroachment is often associated with increased runoff and soil erosion. Hydrological and erosional responses of woody encroachment and of pastures established after management of encroachment in semi-arid Australia are not well understood. This study compared the hydrological and erosional responses across vegetation states comprising woody plant encroachment (>1200 stems ha⁻¹), recently established pastures (<23 years of age), long-established pasture (50-100 years of age) and open woodland (<330 stems ha⁻¹) in semi-arid eastern Australia. Responses were measured using rainfall simulation with intensity of 35 mm h⁻¹ for 30 min applied on 1 -m² plots. Runoff and sediment production did not differ significantly between vegetation states. Average runoff in woody encroachment was 9.0 mm h⁻¹, followed by recent pasture (8.2 mm h⁻¹), long-established pasture (5.9 mm h⁻¹) and open woodland (4.2 mm h⁻¹). Total sediment production in recent pasture was 11.6 g m⁻², followed by woody encroachment (9.0 g m⁻²), long-established pasture (7.3 g m⁻²) and open woodland (4.3 g m⁻²). Runoff and sediment production were significantly lower at one pasture site (0.9 mm h⁻¹ and 1.3 g m⁻²) where rotational grazing and minimum tillage had been implemented than in the adjacent paired woody encroachment site (10.3 mm h⁻¹and 6.5 g m⁻², respectively). This example of a pasture that had been managed to increase ground cover illustrated the effect of pasture management on reducing runoff and sediment production. Across all vegetation states, small scale runoff and sediment production were minimal or zero where total ground cover was 73% or higher.     

Woodland expansion’s influence on belowground carbon and nitrogen in the Great Basin U.S.

Vegetation changes associated with climate shifts and anthropogenic disturbance can have major impacts on biogeochemical cycling and soils. Much of the Great Basin, U.S. is currently dominated by sagebrush (Artemisia tridentate (Rydb.) Boivin) ecosystems. Sagebrush ecosystems are increasingly influenced by pinyon (Pinus monophylla Torr. & Frém and Pinus edulis Engelm.) and juniper (Juniperus osteosperma Torr. and Juniperus occidentalis Hook.) expansion. Some scientists and policy makers believe that increasing woodland cover in the intermountain western U.S. offers the possibility of increased organic carbon (OC) storage on the landscape; however, little is currently known about the distribution of OC on these landscapes, or the role that nitrogen (N) plays in OC retention. We quantified the relationship between tree cover, belowground OC, and total below ground N in expansion woodlands at 13 sites in Utah, Oregon, Idaho, California, and Nevada, USA. One hundred and twenty nine soil cores were taken using a mechanically driven diamond tipped core drill to a depth of 90 cm. Soil, coarse fragments, and coarse roots were analyzed for OC and total N. Woodland expansion influenced the vertical distribution of root OC by increasing 15-30 cm root OC by 2.6 Mg ha⁻¹ and root N by 0.04 Mg ha⁻¹. Root OC and N increased through the entire profile by 3.8 and 0.06 Mg ha⁻¹ respectively. Woodland expansion influenced the vertical distribution of soil OC by increasing surface soil (0-15 cm) OC by 2.2 Mg ha⁻¹. Woodland expansion also caused a 1.3 Mg ha⁻¹ decrease in coarse fragment associated OC from 75-90 cm. Our data suggests that woodland expansion into sagebrush ecosystems has limited potential to store additional belowground OC, and must be weighed against the risk of increased wildfire and exotic grass invasion.     

Historical rates of sediment and nutrient accumulation in marshes of the Upper St. Johns River Basin, Florida, U.S.A.

We used 210Pb-dated sediment cores from wetlands and Blue Cypress Lake, in the Upper St. Johns River Basin (USJRB), Florida, USA, to measure historical accumulation rates of bulk sediment, total carbon (C), total nitrogen (N), and total phosphorus (P). Marsh cores displayed similar stratigraphies with respect to physical properties and nutrient content. Wetland sediments typically contained > 900 mg organic matter (OM) g^–1 dry mass, > 500 mg C g^–1, and 30–40 mg N g^–1. OM, C, and N concentrations were slightly lower in uppermost sediments of most cores, but displayed no strong stratigraphic trends. Total P concentrations were relatively low in bottommost deposits (0.01–0.11 mg g^–1), but ranged from 0.38–2.67 mg g^–1 in surface sediments. The mean sediment accretion rate in the marsh since ~ 1900, 0.33 ± 0.05 cm yr^–1, was calculated from ten ²¹⁰Pb-dated cores. All sites displayed increases in accumulation rates of bulk sediment, C, N, and P since the early part of the 20th century. These trends are attributed to recent hydrologic modifications in the basin combined with high nutrient loading from agricultural, residential, and urban sources.     

Effect of cattle breeding on habitat use of Pampas deer Ozotoceros bezoarticus celer in semiarid grasslands of San Luis, Argentina

The largest population of the Argentinean Pampas deer Ozotoceros bezoarticus celer is found in the semiarid grasslands of the San Luis province. Despite relatively high deer numbers in the region, there has been concern that expansion of farming practices could displace the species. Since the 1990’s, cattle breeding has intensified especially as a result of the replacement of natural grassland with South African digit grass (Digitaria eriantha) and African lovegrass (Eragrostis curvula). In this study, we studied how deer in “El Centenario” ranch used available habitat types, especially to assess whether it preferentially used natural over exotic pastures. We employed 8 fixed line transects to record deer numbers and habitat use during the dry season, early rainy and late rainy seasons. We estimated a mean population size of 731 ± 121 individuals, a density of 1.95 ± 0.25 deer/km². Our results also showed that deer did not appear to select natural pastures over exotic ones, though D. eriantha grasslands with cattle were used less during the late rainy season. Grazed pastures with D. eriantha without cattle were used significantly more during all time periods. Our results therefore suggest that Pampas deer are not shifted by exotic pastures but we caution that it is important to manage these habitats sustainably (e.g. cattle load adjusted to grassland nutritional supply, rotational crops with parcel rest period), to ensure the conservation of the species within agricultural areas.     

Carbon pools in an arid shrubland in Chile under natural and afforested conditions

The pattern of carbon (C) allocation among the different pools is an important ecosystem structural feature, which can be modified as a result of changes in environmental conditions that can occur gradually (e.g., climatic change) or abruptly (e.g., management practices). This study quantified the C pools of plant biomass, litter and soil in an arid shrubland in Chile, comparing the natural condition (moderately disturbed by grazing) vs. the afforested condition (two-year-old plantation with Acacia saligna (Labill.) H.L. Wendl.), each represented by a 60 ha plot. To estimate plant biomass, allometric functions were constructed for the four dominant woody species, based on the volume according to their shape, which showed high correlation (R² > 0.73). The soil was the largest C pool in both natural and afforested conditions (89% and 94%, respectively) and was significantly lower in the afforested than natural condition at all five soil depths. The natural condition had in total 36.5 ton (t) C ha⁻¹ compared to 21.1 t C ha⁻¹ in the afforested condition, mainly due to C loss during soil preparation, prior to plantation of A. saligna. These measurements serve as an important baseline to assess long-term effects of afforestation on ecosystem C pools.     

Present status and variations in the Arctic energy balance

The total solar irradiance (TSI, or solar constant) acquired a new value: 1361 W m^?2 instead of 1365 W m^?2. However a long-term variation of TSI was not detected. The solar irradiance at the earth's surface is considerably smaller (170 W m^?2) than previously believed (e.g. 198 W m^?2 of IPCC AR4). The previous overestimation is due to the underestimation of the absorption of solar radiation in the atmosphere. The absorption of solar radiation in the atmosphere at about 90 W m^?2, or 25–28% of the primary solar radiation from space. The global mean atmospheric downward terrestrial radiation is much larger (345 W m^?2) than previously assumed (325 W m^?2 of IPCC AR4). The Arctic has regions of negative annual net radiation, a very rare phenomenon on the globe. These regions are the Central Arctic Ocean with its multi-year ice coverage and the accumulation area of the Greenland ice sheet. The energy balance of these regions is presented. Long-wave incoming radiation has been increasing in the Arctic at a rate of 4–5 W m^?2/Decade. The Greenland ice sheet exhibits a large vertical difference in net radiation from the ablation area to the dry snow zone in summer. It ranges from 80 W m^?2 in the ablation area to 20 W m^?2 at the equilibrium line and to 10 W m^?2 in the dry snow zone. This gradient determines the melt gradient on the ice sheet, and is mainly caused by the altitude variation in atmospheric long-wave radiation, seconded by the albedo variation. The effect of albedo in summer for various surfaces is discussed. Simulation capabilities of radiation for many GCMs are investigated.     

Aboveground biomass in Tibetan grasslands

This study investigated spatial patterns and environmental controls of aboveground biomass (AGB) in alpine grasslands on the Tibetan Plateau by integrating AGB data collected from 135 sites during 2001–2004 and concurrent enhanced vegetation index derived from MODIS data sets. The AGB was estimated at 68.8 g m^?2, with a larger value (90.8 g m^?2) in alpine meadow than in alpine steppe (50.1 g m^?2). It increased with growing season precipitation (GSP), but did not show a significant overall trend with growing season temperature (GST) although it was negatively correlated with GST at dry environments (<200 mm of GSP). Soil texture also influenced AGB, but the effect was coupled with precipitation; increased silt content caused a decrease of AGB at small GSP, and generated a meaningful increase under humid conditions. The correlation between AGB and sand content indicated an opposite trend with that between AGB and silt content. An analysis of general linear model depicted that precipitation, temperature, and soil texture together explained 54.2% of total variance in AGB. Our results suggest that moisture availability is a critical control of plant production, but temperature and soil texture also affect vegetation growth in high-altitude regions.     

Baptiste Lake,Alberta — A late Holocene history of changes in a lake and its catchment in the southern Boreal forest

Lead-210 assay and dating are subject to several sources of error, including natural variation, the statistical nature of measuring radioactivity, and estimation of the supported fraction. These measurable errors are considered in calculating confidence intervals for ²¹⁰Pb dates. Several sources of error, including the effect of blunders or misapplication of the mathematical model, are not included in the quantitative analysis. First-order error analysis and Monte Carlo simulation (of cores from Florida PIRLA lakes) are used as independent estimates of dating uncertainty. CRS-model dates average less than 1% older than Monte Carlo median dates, but the difference increases non-linearly with age to a maximum of 11% at 160 years. First-order errors increase exponentially with calculated CRS-model dates, with the largest 95% confidence interval in the bottommost datable section being 155±90 years, and the smallest being 128±8 years. Monte Carlo intervals also increase exponentially with age, but the largest 95% occurrence interval is 152±44 years. Confidence intervals calculated by first-order methods and ranges of Monte Carlo dates agree fairly well until the ²¹⁰Pb date is about 130 years old. Older dates are unreliable because of this divergence. Ninety-five per cent confidence intervals range from about 1–2 years at 10 years of age, 10–20 at 100 years, and 8–90 at 150 years old.This is the third of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D.F. Charles and D.R. Whitehead are guest editors for this series.     

盐角草(Salicornia europaea)对NaCl处理的生理响应

用含有NaCl的Hoagland培养液处理盐角草(Salicornia europaea)11d。检测其鲜重,干重,离子含量,电导率,溶解性总固体(TDS)含量,超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)活性及丙二醛(MDA)浓度。结果表明:随着NaCl浓度的增加,盐角草干重和鲜重呈现先上升后下降的趋势,相对电导率、TDS、SOD、POD、CAT及MDA均呈现先下降后上升的趋势,在NaCl溶液浓度为200mmol·L-1左右时,干重和鲜重的值达到最大,电导率、TDS、SOD、POD、CAT、MDA含量达到最小值;500mmol·L-1和800mmol·L-1时,SOD和CAT活性下降,电导率、TDS、POD、MDA含量则急剧上升。由此说明,一定浓度的NaCl溶液促进了盐角草的生长,200mmol·L-1左右是其生长的最适浓度,500mmol·L-1和800mmol·L-1高盐浓度会对盐角草的膜结构、酶系统等造成不同程度的损伤。盐角草主要将Na+、K+积累在地上部,且随着NaCl处理浓度的增大,Na+含量逐渐增加、K+含量逐渐降低,这可能是盐角草调节细胞内离子平衡对抗盐胁迫的一种适应策略。     

The effects of seasonal changes to in-stream vegetation cover on patterns of flow and accumulation of sediment

In-stream macrophytes are typically abundant in nutrient-rich chalk streams during the spring and summer months and modify the in-stream environment by altering river flows and trapping sediments. We present results from an inter-disciplinary study of two river reaches in the River Frome catchment, Dorset (UK). The investigation focused on how Ranunculus (water crowfoot), the dominant submerged macrophyte in the study reaches, modified patterns of flow and sediment deposition. Measurements were taken on a monthly basis throughout 2003 to determine seasonal patterns in macrophyte cover, associated changes in the distributions of flow velocities and the character and amount of accumulated fine sediment within stands of Ranunculus.Maximum in-stream cover of macrophytes exceeded 70% at both sites. Flow velocities were less than 0.1 m s^− 1 within the stands of Ranunculus and accelerated to 0.8 m s^− 1 outside the stands. During the early stages of the growth of Ranunculus, fine sediment mostly accumulated within the upstream section of the plant but the area of fine sediment accumulation extended into the downstream trailing section of the plant later in the growing season. The fine sediment accumulations were dominated by sand (63–1000 μm) with silts and clays (0.37–63 μm) comprising < 10% by volume. The content of organic matter in the accumulated sediments varied within stands, between reaches and through the growing season with values ranging between 9 and 105 mg g^− 1 dry weight. At the reach scale the two sites exhibited different growth forms of Ranunculus which created distinctive patterns of flow and fine sediment deposition.     

Late Quaternary (24–10 ka BP) environmental history of the Neotropical lowlands inferred from ostracodes in sediments of Lago Petén Itzá, Guatemala

We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Petén Itzá, Guatemala from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW–S precipitation gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6, ~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial. Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance (<940 valves g⁻¹), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 μS cm⁻¹, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6 spp.) and abundances up to 18,115 valves g⁻¹, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 μS cm⁻¹, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m. The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g⁻¹, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene.     

Effects of simulated storm sizes and nitrogen on three Chihuahuan Desert perennial herbs and a grass

Establishment and growth of three perennial herbs and a small tussock grass were studied in an experiment that provided simulated rainfall of 6 mm week⁻¹ or 25 mm once per month and nitrogen fertilization in combination with the different simulated rainfall regimes. Wild onion, Allium macropetalum, failed to establish in plots receiving 25 mm month⁻¹ simulated rainfall. The perennial composite, Bahia absinthifolia, occurred at higher densities in plots that were not irrigated but there were no differences in biomass in any of the irrigation or fertilization treatments. Desert holly, Perezia nana, failed to establish in nitrogen fertilized plots and developed higher abundance and biomass in plots receiving 25 mm month⁻¹. Nitrogen fertilization had either no effect or an adverse effect on the perennial herbs. The tussock grass, Dasychloa pulchella exhibited highest abundance and biomass with 6 mm week⁻¹ added water plus nitrogen. Since global climate change will affect both rain storm frequency and size and atmospheric nitrogen deposition, the results of this study are applicable to understanding vegetation responses climate change.