Dry Plots Research Articles

Changes in the yield components of durum wheat (Triticum durum Desf.) during irrigation controlled by soil sensors

The effect of irrigation water on the yield and on individual yield components was examined for 19 durum wheat varieties by continually recording weather data and carrying out measurements on the moisture content, temperature, electrical conductivity and tension of the soil. Dry (rain-fed) and irrigated treatments were included in the experiment, which was carried out in the framework of the EU FP7-244374 DROPS project.During the rainless spring of 2011 the soil moisture content of the non-irrigated area dropped to 21–22 vol% and the effect of drought stress was still felt at harvest. The quantity of irrigation water applied during the growing season ensured normal conditions for generative development and a significant difference could be detected between the yield components in the two treatments. The thousand-kernel weight of the varieties was identical in the dry and irrigated plots, but in response to irrigation there was an increase in the number of grains per ear and the grain weight, and an improvement in fertilisation, resulting in higher yields.

Long-term (13-year) effects of repeated prescribed fires on stand structure and tree regeneration in mixed-oak forests

The survival and growth of oak advance regeneration is often limited by shade-tolerant species that are abundant in the understory of oak stands. Evidence of historic burning has prompted the use of prescribed fire as a tool to improve the competitive status of oak regeneration in mature stands. A primary shortfall of fire effects research in oak forests has been a lack of long-term studies on the effects of multiple fires. Here we describe the effects of repeated fires on stand structure and tree regeneration over a 13-year period in mature mixed-oak forests located in southern Ohio, USA. Three stands were burned 3–5 times from 1996 to 2005 with low-intensity dormant-season fires, and two stands remained unburned. Woody vegetation was sampled periodically on nine 0.125ha plots per stand. Plots were located across the upland landscape and were characterized by an Integrated Moisture Index. Fire altered stand structure by reducing the density of large saplings (3.0–9.9cm DBH) and midstory trees (10–25cm DBH) by 76% and 34%, respectively. Fire had little impact on trees >25cm DBH. Small saplings (1.4m tall to 2.9cm DBH) were dynamic over time on dry plots that were burned. After being repeatedly topkilled from year 1–8, the small sapling layer had redeveloped on dry burned plots by year 13 and species composition had shifted from dominance by shade-tolerant species to a more equal distribution of shade-tolerants, oaks+hickories, and sassafras. The density of oak+hickory and sassafras advance regeneration (stems 30cm tall to 2.9cm DBH) was significantly greater on burned plots than on unburned plots in year 13, though variability among plots was high. Advance regeneration of shade-tolerant species was equally abundant on burned and unburned plots. Density of oak+hickory advance regeneration in year 13 was positively related to its weighted frequency (a surrogate for size and abundance) in year 0 (r2=0.67, p<0.0001) and inversely related to stand density (r2=0.33, p<0.0001) and canopy cover (r2=0.31, p<0.0001), both of which were reduced by fire. Although oak+hickory advance regeneration was more abundant on burned plots, we conclude that other methods (e.g., herbicide, partial cutting) are necessary to further reduce stand density and promote the development of larger oak+hickory regeneration, particularly on mesic sites.

The influence of spatiotemporal variability and adaptations to hypoxia on empirical relationships between soil acidity and vegetation

ABSTRACTSoil acidity is well known to affect the species composition of natural vegetation. The physiological adaptations of plants to soil acidity and related toxicity effects and nutrient deficiencies are, however, complex, manifold and hard to measure. Therefore, generally applicable quantifications of mechanistic plant responses to soil acidity are still not available. An alternative is the semi‐quantitative and integrated response variable ‘indicator value for soil acidity’ (Rm). Although relationships between measured soil pH and Rm from various studies are usually strong, they often show systematic bias and still contain high residual variances. On the basis of a well‐documented national dataset consisting of 91 vegetation plots and a dataset with detailed, within‐plot, pH measurements taken at three periods during the growing season, it is shown that strong spatiotemporal variation of soil pH can be a critical source of systematic errors and statistical noise. The larger part of variation, however, could be explained by the moisture status of plots. For instance, Spearman's rho decreased from 93% for dry plots and 87% for moist plots to 59% for wet plots. The loss of relation between soil pH and Rm in the moderately acid to alkaline range at increasingly wetter plots is probably due to the establishment of aerenchyma‐containing species, which are able to control their rhizosphere acidity. Adaptation to one site factor (oxygen deficit) apparently may induce indifference for other environmental factors (Fe2+, soil pH). For predictions of vegetation response to soil acidity, it is thus important to take the wetness of plots into account. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Heritable Effect of Plant Water Availability Conditions on Restoration of Male Fertility in the “9E” CMS-Inducing Cytoplasm of Sorghum

Heritable changes of phenotype arising in plant ontogenesis by the influence of environmental factors belong to the most intriguing genetic phenomena. An unusual inheritance pattern was detected during examination of male fertility restoration in the CMS-inducing “9E” type cytoplasm of sorghum: Rf-genes were functional in self-pollinated progeny of F1 hybrids yet were either not expressed or poorly expressed in backcrosses of these hybrids to CMS-lines with the same cytoplasm type. In experiments on parallel growing of the same F1 hybrid combinations in the “dry plot” and in the “irrigated plot,” it was found that high level of plant water availability during panicle and pollen developmental stages significantly increased male fertility of F1 and test-cross hybrid populations, in which fertility-restoring genes were in heterozygote state, whereas in F2 populations the influences of water availability conditions cause less pronounce effects. Similarly, male-sterile F1 plants, being transferred from the “dry plot” to greenhouse, produced male-fertile panicles. In addition, male-sterile plants from F2 families, which segregated-out as recessives, being transferred to greenhouse also produced male-fertile panicles. In the progenies of these revertants that were grown in field conditions and in the “dry plot,” stable inheritance of male fertility for three cycles of self-pollination was observed, and a number of stable fertile lines in the “9E” cytoplasm were obtained. However, in test-crosses of these fertile lines to CMS-lines with the “9E” cytoplasm restoration of male fertility was not observed, except the progeny of one revertant that behaved as fertility-restorer line. These data suggest that the functional state of fertility-restoring genes for the “9E” sorghum cytoplasm is epigenetically regulated trait established by the influence of environmental factors and is transmitted to sexual generations.

The role of microhabitat and food availability in determining riparian invertebrate distributions on gravel bars: a habitat manipulation experiment

AbstractExposed riverine sediments (ERS) are an important ecotone, where aquatic and terrestrial habitats and species interact. Within an area of ERS, invertebrate species richness tends to be highest along the water's edge ecotone. In this article, we discuss a habitat manipulation experiment which was conducted to test the hypotheses that ERS invertebrate distribution is linked to: (1) the availability of food items (emerging and stranded aquatic invertebrates) and (2) favourable physical microhabitat and microclimate (temperature and moisture). Four experimental plots were created (wet, wet‐fed, dry‐fed and dry control plot), replicated three times. The plots were wetted using a capillary pump system, and fed with dried blood worms (Chironomids) for 28 days. Sediment temperature was measured at 15‐min intervals. Hand searches were undertaken on 25% of each plot after 7, 14, 21 and 30 days. Significant temperature differences were observed between the wet and dry sediment and the air temperature. Wet plots were 1·9 °C cooler on average than the dry plots and had a smaller temperature range; all plots remained significantly warmer than the air temperature. The wet and wet‐fed plots yielded significantly greater numbers of beetles than the dry and dry‐fed plots; however, no significant difference was found between the wet and wet‐fed plots. Spiders were significantly associated with dry plots and showed no response to food variability. These results suggest that micro‐environment and not food availability is the main driving factor underlying beetle distribution and movement, although it is possible that they are using lower temperatures and increased moisture as a cue for aquatic food availability. Copyright © 2011 John Wiley &amp; Sons, Ltd.

The use of diachronic spatial approaches and predictive modelling to study the vegetation dynamics of a managed heathland

According to the EU Habitats Directive, heathlands are a semi-natural habitat type of community interest. This status aims at conserving these habitats, especially where and when they are threatened by various changes, including natural vegetation succession. We present results of a study of the dynamics of a typical dry heathland plot located in the Fontainebleau massif (France). An exhaustive observation of vegetation changes were made on this area of four hectares between 2000 and 2008, employing a spatial approach. We recorded the expansion of Molinia caerulea (L.) Moench at the expense of Ericaceae. The potential future vegetation of the site was modelled using Markov chains coupled to a GIS programme. This model predicted a gradual change in the floristic composition of heathland in favour of M. caerulea at the expense of Calluna vulgaris (L.) Hull and Erica tetralix L., and the expansion of Pinus sylvestris L. The study demonstrates how spatial methods can contribute to the design of reliable management methods of habitats such as the heathlands.

Do photosynthetic limitations of evergreen Quercus ilex leaves change with long‐term increased drought severity?

Seasonal drought can severely impact leaf photosynthetic capacity. This is particularly important for Mediterranean forests, where precipitation is expected to decrease as a consequence of climate change. Impacts of increased drought on the photosynthetic capacity of the evergreen Quercus ilex were studied for two years in a mature forest submitted to long-term throughfall exclusion. Gas exchange and chlorophyll fluorescence were measured on two successive leaf cohorts in a control and a dry plot. Exclusion significantly reduced leaf water potential in the dry treatment. In both treatments, light-saturated net assimilation rate (A(max)), stomatal conductance (g(s)), maximum carboxylation rate (V(cmax)), maximum rate of electron transport (J(max)), mesophyll conductance to CO2 (g(m)) and nitrogen investment in photosynthesis decreased markedly with soil water limitation during summer. The relationships between leaf photosynthetic parameters and leaf water potential remained identical in the two treatments. Leaf and canopy acclimation to progressive, long-term drought occurred through changes in leaf area index, leaf mass per area and leaf chemical composition, but not through modifications of physiological parameters.

Análise Fenotípica e Genotípica de Bactérias Isoladas do Solo da Floresta Nacional dos Tapajós, pará, Brasil, sob Efeito de Estresse Hídrico

Os solos da Floresta Nacional dos Tapajós são classificados como argilosos e podzólicos, possuem acidez elevada, altos teores de alumínio e são pobres em nutrientes. Visando avaliar o impacto das secas na floresta, foi desenvolvido o projeto "Dry Forest". Este projeto inclui as parcelas Floresta Primária (FP, controle) e Floresta Seca (FS, com exclusão de chuvas). O presente estudo avaliou o efeito do estresse hídrico sobre a microbiota do solo nessas parcelas. Foram analisadas 12 amostras de solo por parcela quanto às populações microbianas, incluindo organismos celulolíticos, proteolíticos e solubilizadores de fosfato. Foi observada uma redução nas contagens microbianas no solo sob estresse hídrico, podendo atingir duas ordens de grandeza, sendo mais expressiva para os microrganismos celulolíticos. Das 286 culturas bacterianas isoladas, os bastonetes Gram positivos representaram 47% (FP) e 62% (FS) destes isolados. Os gêneros prevalentes foram Bacillus, Corynebacterium, Listeria e Micrococcus. Foi encontrada significativa diversidade genotípica entre as estirpes isoladas (FP e FS) nos perfis de BOX-PCR. Os resultados permitem concluir que a exclusão de chuvas na parcela seca pode provocar redução nas populações microbianas. No entanto, a diversidade encontrada, em nível de gênero, foi pouco influenciada. A elevada diversidade bacteriana detectada ao nível de espécie/subespécie sugere uma resiliência dos solos de ambas as parcelas ao estresse ambiental.

Human and Climatic Influences on Fire Occurrence in California’s North Coast Range, USA

Outside of the immediate coastal environments, little is known of fire history in the North Coast Range of California. Fire scar specimens were collected from ponderosa pine (Pinus ponderosa C. Lawson), sugar pine (Pinus lambertiana Douglas), incense cedar (Calocedrus decurrens [Torr] Florin), and Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) for seven plots in mixed-conifer forests from the Mendocino National Forest, California, USA. Five plots were on high ridges immediately adjacent to the Sacramento Valley (DRY plots). The other two plots were on mesic north facing slopes interior in the range (MESIC plots), and were separated from the Sacramento Valley by at least one to several ridge systems. These two plots were selected because they supported populations of rare lady’s slipper orchids (Cypripedium fasciculatum [Kellogg ex S. Watson] and C. montanum [Douglas ex Lindl.]). We found that DRY plots had unusually short fire return intervals (FRI) compared to other areas in northwestern California. The median FRI for these plots ranged from 4.5 yr to 6 yr in comparison with a tenth percentile of 11 yr, grand median of 24 yr, and ninetieth percentile of 66 yr for FRIs from other mixed conifer plots (n = 109) in the region. In northwestern California, most fire scars have been found primarily at ring boundary (68%) and secondarily in latewood (23%) with few in earlywood (9%). In contrast, in the DRY plots 35% (88) of the fire scars were in earlywood with only 15% (39) at the ring boundary. Fire occurrence was associated with drought conditions in the year of fire, and with wet conditions three years before the fire year. Before ∼1850, fires that scarred at least two trees on a site were quite frequent for the DRY plots while being less frequent and more variable on the MESIC plots. However, the MESIC orchid habitats burned with frequency and seasonality similar to mixed conifer forests in the Klamath Mountains of northern California. Fires were less frequent after ∼1850, with fires ceasing on most plots shortly after 1900. We suggest that these unusually low FRIs and high incidence of fire scars in earlywood were due to the adjacency of the DRY plots to the hot, relatively dry Sacramento Valley grasslands that were likely influenced by the burning practices of Native Americans.

Remote sensing of tundra gross ecosystem productivity and light use efficiency under varying temperature and moisture conditions

Satellite observations have shown greening trends in tundra in response to climate change, suggesting increases in productivity. To better understand the ability of remote sensing to detect climate impacts on tundra vegetation productivity, we applied a photosynthetic light use efficiency model to simulated climate change treatments of tundra vegetation. We examined changes in the Normalized Difference Vegetation Index (NDVI) and photosynthetic light use efficiency ( ε) in experimental warming and moisture treatments designed to simulate climate change in northern Alaska. Plots were warmed either passively, using Open Top Chambers, or actively using electric heaters in the soil. In one set of plots water table depth was actively altered, while other plots were established in locations that were naturally wet or dry. Over two growing seasons, plot-level carbon flux and spectral reflectance measurements were collected, and the results were used to derive a light use efficiency model that could explore the effects of moisture and temperature treatments using remote sensing. Warming increased values of canopy greenness (NDVI) relative to control plots, this effect being more pronounced in wet plots than in dry plots. Light use efficiency (LUE), the relationship between absorbed photosynthetically active radiation (PAR) and gross ecosystem production (GEP), was consistent across warming treatments, growing season, subsequent years, and sites. However, LUE was affected by vegetation type, which varied with moisture; plots in naturally dry locations showed reduced light use efficiency relative to moist plots. Additionally moss exhibited reduced LUE relative to vascular plants. Understory moss production, not accounted for by the usual definition of the fraction of absorbed PAR ( f APAR), was found to be a significant part of total GEP, particularly in areas with low vascular plant cover. These results support the use of light use efficiency models driven by spectral reflectance for estimating GEP in tundra vegetation, provided effects of vegetation functional type (e.g. mosses versus vascular plants) and microtopography are considered.

Accurate environmental bioindication in floodplains in spite of an extreme flood event

AbstractThe ecological reaction of species on environmental factors can differ both, temporally and geographically. Therefore, bioindicator systems must be shown to remain accurate at different places and points of time. Extreme events pose challenges for testing this robustness, which has not yet been explored systematically. The unique opportunity of the extreme flooding of the river Elbe in August 2002 was used for such a test. The bioindicator system for mean groundwater depth during the vegetation period and annual duration of inundation in floodplains studied here was developed with data collected in 1999. Data from 2003 were used to analyse the dependence of the bioindication results on the humidity of the study plots and taxonomic group (plants, molluscs and carabid beetles). For the duration of inundation, the accuracy of bioindication in 2003 was similar to that in 1999, except for the medium‐humid and dry plots using molluscs as indicators. For groundwater depth, the accuracy of bioindication in 2003 was similar to that of the reference year except in the dry plots with significantly lower accuracy for all species groups. With a few exceptions, the bioindicator system is shown to be robust and to remain accurate in spite of an extreme flood event. In conclusion, the relevance of extreme events for the robustness of bioindicator systems differs between the indicated parameters and taxonomic groups. The results stress the need to take into account the limits of the working range and the occurrence of extreme events, when testing and applying bioindicator systems. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Ozone fluxes and foliar injury development in the ozone-sensitive poplar clone Oxford (Populus maximowiczii x Populus berolinensis): a dose-response analysis

Between 2004 and 2005 a combined open plot and open-top chamber (OTC) experiment was carried out at Curno (Northern Italy) with cuttings of the poplar clone Oxford (Populus maximowiczii Henry x Populus berolinensis Dippel) grown in open plots (OPs, ambient air), charcoal-filtered OTCs (CF, ozone concentration reduced to 50% of ambient) or non-filtered OTCs (NF, ozone concentration reduced to 95% of ambient). Plants in half of the chambers were kept well-watered (WET), and plants in the remaining chambers were not watered (DRY). The onset and development of visible foliar injury and the stomatal conductance to water vapor (g(w)) were assessed during each growing season. A stomatal conductance model was parameterized by the Jarvis approach, allowing the calculation of ozone stomatal fluxes of plants in each treatment. The pattern of visible symptoms was analyzed in relation to ozone exposure (AOT40, accumulated ozone over a threshold of 40 ppb) and accumulated ozone stomatal fluxes (AF(ST)). Symptoms became visible at an AOT40 between 9584 and 13,110 ppb h and an AF(ST) between 27.85 and 30.40 mmol O(3) m(-2). The development of symptoms was more widespread and faster in plants in WET plots than in DRY plots. A slightly higher dose of ozone was required to cause visible symptoms in plants in DRY plots than in WET plots. By the end of each growing season, plants in the CF OTCs had absorbed a high dose of ozone (31.60 mmol O(3) m(-2) in 2004 and 32.83 mmol O(3) m(-2) in 2005, for WET plots), without developing any visible symptoms. A reliable dose-response relationship was defined by a sigmoidal curve model. The shape of this curve expresses the change in leaf sensitivity and physiologic state over a prolonged ozone exposure. After the appearance of the first symptoms, foliar injury increased more rapidly than the increases in ozone exposure and ozone absorbed dose; however, when the injury incidence reached 75%, the plant response declined.

Patterns of frequency in species-rich vegetation in pine savannas: Effects of soil moisture and scale

Our principal objective was to document dominant plant composition and the species frequency pattern in a plant community type, longleaf pine savanna, known for its extraordinary number of vascular plant species. We also tested whether an important habitat factor, soil moisture, affected the resulting patterns, and whether the patterns were scale-dependent. We began with a collection of 120 sample plots (1- × 1-m) in a wet coastal plain savanna. These plots contained 126 plant species. The 3 dominant species by cover were Rhynchospora gracilenta (15.37%), Schizachyrium tenerum (13.36%), and Scleria pauciflora (10.07%). The species frequency distribution was a skewed unimodal pattern with most species occurring infrequently, in less than 10% of the plots. There was no evidence for bimodality. To test whether soil moisture affected species frequency patterns, we sorted the plots into 2 groups: 60 representing wetter conditions and 60 representing drier conditions. Measured by percent cover, the dominants in the wetter plots were R. gracilenta, Dichanthelium scabriusculum, and S. pauciflora, whereas in the drier area they were S. tenerum, Ilex glabra, and D. dichotomum. The species frequency pattern was similar for both wet and dry plots (χ2 = 8.97, P > 0.05). To explore possible effects of sample area on this pattern, we sampled a further 75 plots in a larger tract of similar habitat in De Soto National Forest, using 4 sample areas 0.1, 1, 10, and 100 m2 Again, the species frequency distributions all had a skewed unimodal pattern. These patterns are consistent with other studies of savannas but do not appear consistent with the bimodal patterns reported from some grasslands. Further studies of frequency will determine the degree of generality of such patterns and their relationship to mechanistic processes in plant communities.

Aldicarb and carbofuran transport in a Hapludalf influenced by differential antecedent soil water content and irrigation delay

Pesticide use in agroecosystems can adversely impact groundwater quality via chemical leaching through soils. Few studies have investigated the effects of antecedent soil water content (SWC) and timing of initial irrigation (TII) after chemical application on pesticide transport and degradation. The objectives of this study were to investigate the effects of antecedent soil water content (wet vs dry) and timing of initial irrigation (0 h Delay vs 24 h Delay) on aldicarb [( EZ)-2-methyl-2-(methylthio)propionaldehyde O-methylcarbamoyloxime] and carbofuran [2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate] transport and degradation parameters at a field site with Menfro silt loam (fine-silty, mixed, superactive, mesic Typic Hapludalf) soils. Aldicarb and carbofuran were applied to plots near field capacity (wet) or near permanent wilting point (dry). Half of the dry and wet plots received irrigation water immediately after chemical application and the remaining plots were irrigated after a 24 h Delay. The transport and degradation parameters were estimated using the method of moments. Statistical significance determined for SWC included averages across TII levels, and significance determined for TII included averages across SWC levels. For the dry treatment, aldicarb was detected 0.10 m deeper ( P < 0.01) on two of the four sampling dates and carbofuran was detected at least 0.10 m deeper ( P < 0.05) on all of the sampling dates compared to the wet treatment. Pore water velocity was found to be higher ( P < 0.10) in the dry vs wet treatments on three of four dates for aldicarb and two of four dates for carbofuran. Retardation coefficients for both pesticides showed similar evidence of reduced values for the dry vs wet treatments. These results indicate deeper pesticide movement in the initially dry treatment. For aldicarb and carbofuran, estimated values of the degradation rate were approximately 40–49% lower in the initially dry plots compared to the initially wet plots, respectively. When the initial irrigation was delayed for 24 h, irrespective of antecedent moisture conditions, a 30% reduction in aldicarb degradation occurred. This study illustrates the deeper transport of pesticides and their increased persistence when applied to initially dry soils.

Atrazine and Alachlor Transport in Claypan Soils as Influenced by Differential Antecedent Soil Water Content

Increased attention to ground water contamination has encouraged an interest in mechanisms of solute transport through soils. Few studies have investigated the effect of the initial soil water content on the transport and degradation of herbicides for claypan soils. We investigated the effect of claypan soils at initial field capacity vs. permanent wilting level on atrazine and alachlor transport. The soil studied was Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualf) with a subsoil clay content, primarily montmorillonite, of >40%. Strontium bromide, atrazine, and alachlor were applied to plots; half were at field capacity (Wet treatment), and half were near the permanent wilting point (Dry treatment). Soil cores were removed at selected depths and times, and cores were analyzed for bromide and herbicide concentrations. Bromide, atrazine, and alachlor were detected at the 0.90-m depth in dry plots within 15 d after experiment initiation. Bromide was detected 0.15 m deeper (P < 0.05) in the Dry compared with the Wet treatment at 1, 7, and 60 d after application and >0.30 m deeper (P < 0.01) in the Dry treatment at 15 and 30 d after application; similar treatment results were found for atrazine and alachlor, although on fewer dates with significant differences. The mobility order of the applied chemicals was bromide > atrazine > alachlor. The atrazine apparent half-life was significantly longer in the Dry plots compared with the Wet plots. The retardation factor determined from the relative velocity of each herbicide to that of bromide was higher for alachlor than for atrazine. This study identifies the impact that shrinkage cracks have for different moisture conditions on preferential transport of herbicides in claypan soils.

Net ecosystem CO2 exchange of a cutover peatland rehabilitated with a transplanted acrotelm

Natural peatlands are an important component of the global carbon cycle, representing a net long-term sink of atmospheric carbon dioxide (CO2). The natural carbon storage function of these ecosystems can be severely impacted due to peatland drainage and peat extraction, leading to large and persistent sources of atmospheric CO2 following peat extraction abandonment. Standard restoration techniques aim to restore the peatland to a carbon-accumulating system through various water management techniques to improve hydrological conditions and through reintroducing Sphagnum at the surface. However, restoring the hydrology of peatlands can be expensive due to the cost of implementing the various restoration techniques. This study examines the net ecosystem CO2 exchange of a new extraction—restoration technique where the acrotelm is preserved and replaced directly on the cutover surface. Peat respiration at the experimental restored acrotelm (110.5 g C·m−2over the study period: May 20–August 18) was significantly lower than at the natural peatland (144.8 and 203.7 g C·m−2, wet and dry plots respectively). However, gross ecosystem production (GEP) at the experimental site was significantly reduced (-54.0 and -34.4 g C·m−2) compared to the natural site (-179.2 and -162.0 g C·m−2). Consequently, this was a growing season net emitter of CO2 to the atmosphere over the season at the experimental site (78.5 and 56.5 g C·m−2 and a growing season sink of CO2 at the natural site (-17.6 and -22.8 g C·m−2). Light response curves indicated that maximum GEP was significantly lower at the experimental site; however, it is likely that the percentage of living and dead vegetation at the plots post restoration had a large control on this lower productivity as plots with more living vegetation had higher overall productivity (GEP). Nevertheless, the experimental site did maintain limited productivity post-extraction, indicating that the carbon dynamics of the system was maintained due to this acrotelm restoration process, thereby likely returning the ecosystem toward a natural sink of atmospheric CO2 faster than natural rehabilitation and current restoration techniques. As such, this approach to peat extraction may be an environmentally preferred approach to use.

Floristic and functional affiliations of woody plants with climate in western Amazonia

AbstractAim To test whether a direct relationship exists between the relative abundance of woody plant genera and precipitation regime along the north–south climate gradient of the western Amazon.Location Lowland rain forests in the western Amazon.Methods Floristic data on 91 woody plant genera, from 39 0.1‐ha plots across the western Amazon, and precipitation data from a 0.5° global data set were used to test for correlations between plant relative abundance (defined as percentage number of stems ≥ 2.5 cm diameter at breast height for each woody plant genus per plot) and derived dry‐season variables. Moisture preference was then assessed in terms of pioneer and shade‐tolerant life‐history strategy.Results There were significant associations between the distribution of plant relative abundances and seasonal precipitation variables in 34% of genera analysed. Significant differences were identified in size‐class distribution between dry affiliates and generalists. Dry affiliates were not dominant in any size class in any plot type, whereas climate generalists dominated most of the size classes in the dry plots and the mid‐range size classes in the wet plots. Dry‐affiliate genera were a minority, even in dry forests. Wet‐affiliate genera were correlated with shade tolerance, whereas genera with no rainfall affiliation were often pioneers.Main conclusions The results suggest that moisture variable seasonality influences community composition in a manner that can be related to the life‐history trade‐off between shade tolerance and pioneer ranking. One possible reason for higher diversity in wetter forests is that high rainfall amplifies the niche space available to shade‐tolerant plants. Determining which plant groups are constrained by which environmental variables can contribute to our understanding of how forest composition may be changing now, and how it may change under future climate: if shade‐tolerant trees are also drought‐intolerant, community structure in wet forests may be more vulnerable to future increases in moisture stress.

BIOTIC AND ABIOTIC CONTROLS OF ARGENTINE ANT INVASION SUCCESS AT LOCAL AND LANDSCAPE SCALES

Although the ecological success of introduced species hinges on biotic interactions and physical conditions, few experimental studies--especially on animals--have simultaneously investigated the relative importance of both types of factors. The lack of such research may stem from the common assumption that native and introduced species exhibit similar environmental tolerances. Here we combine experimental and spatial modeling approaches (1) to determine the relative importance of biotic and abiotic controls of Argentine ant (Linepithema humile) invasion success, (2) to examine how the importance of these factors changes with spatial scale in southern California (USA), and (3) to assess how Argentine ants differ from native ants in their environmental tolerances. A factorial field experiment that combined native ant removal with irrigation revealed that Argentine ants failed to invade any dry plots (even those lacking native ants) but readily invaded all moist plots. Native ants slowed the spread of Argentine ants into irrigated plots but did not prevent invasion. In areas without Argentine ants, native ant species showed variable responses to irrigation. At the landscape scale, Argentine ant occurrence was positively correlated with minimum winter temperature (but not precipitation), whereas native ant diversity increased with precipitation and was negatively correlated with minimum winter temperature. These results are of interest for several reasons. First, they demonstrate that fine-scale differences in the physical environment can eclipse biotic resistance from native competitors in determining community susceptibility to invasion. Second, our results illustrate surprising complexities with respect to how the abiotic factors limiting invasion can change with spatial scale, and third, how native and invasive species can differ in their responses to the physical environment. Idiosyncratic and scale-dependent processes complicate attempts to forecast where introduced species will occur and how their range limits may shift as a result of climate change.

Effects of partial throughfall exclusion on the phenology of Coussarea racemosa (Rubiaceae) in an east-central Amazon rainforest

Severe droughts may alter the reproductive phenology of tropical tree species, but our understanding of these effects has been hampered by confounded variation in drought, light and other factors during natural drought events. We used a large-scale experimental reduction of throughfall in an eastern-central Amazon forest to study the phenological response to drought of an abundant subcanopy tree, Coussarea racemosa. We hypothesized that drought would alter the production and the timing of reproduction, as well as the number of viable fruits. The study system comprised two 1-ha plots in the Tapajos National Forest, Para, Brazil: a dry plot where 50% of incoming precipitation (80% throughfall) was diverted from the soil during the six-month wet season beginning in January 2000, and a wet plot that received natural rainfall inputs. Fruit production of C. racemosa was quantified every 15 days using 100 litter traps (0.5 m(2)) in each plot. The production of new leaves and flowers was recorded monthly for C. racemosa individuals. Soil water, pre-dawn leaf water potential and solar radiation were measured to help interpret phenological patterns. Over the approximately 3.5-year period (April 2000 through December 2003), total fruit production remained similar between plots, declining by 12%. In 2003, production was four times higher in both plots than in previous years. In the dry plot, fruit fall shifted 40 and 60 days later into the dry season in 2002 and 2003, respectively. Total fruit fall dry mass production was variable across the study period. Foliage and flower production coincided with peak irradiance early in the dry season until delays in flowering appeared in the dry plot in 2002 and 2003. Plant water stress, through its influence on leaf developmental processes and, perhaps, inhibition of photosynthesis, appears to have altered both the timing of fruit fall and the quality and number of seeds produced.

Effects of water additions, chemical amendments, and plants on in situ measures of nutrient bioavailability in calcareous soils of southeastern Utah, USA

We used ion-exchange resin bags to investigate effects of water additions, chemical amendments, and plant presence on in situ measures of nutrient bioavailability in conjunction with a study examining soil controls of ecosystem invasion by the exotic annual grass Bromus tectorum L. At five dryland sites in southeastern Utah, USA, resin bags were buried in experimental plots randomly assigned to combinations of two watering treatments (wet and dry), four chemical-amendment treatments (KCl, MgO, CaO, and no amendment), and four plant treatments (B. tectorum alone, the perennial bunchgrass Stipa hymenoides R. & S. alone, B. tectorum and S. hymenoides together, and no plants). Resin bags were initially buried in September 1997; replaced in January, April, and June 1998; and removed at the end of the study in October 1998. When averaged across watering treatments, plots receiving KCl applications had lower resin-bag NO3− than plots receiving no chemical amendments during three of four measurement periods—probably due to NO3− displacement from resin bags by Cl− ions. During the January–April period, KCl application in wet plots (but not dry plots) decreased resin-bag NH4+ and increased resin-bag NO3−. This interaction effect likely resulted from displacement of NH4+ from resins by K+ ions, followed by nitrification and enhanced NO3− capture by resin bags. In plots not receiving KCl applications, resin-bag NH4+ was higher in wet plots than in dry plots during the same period. During the January–April period, resin-bag measures for carbonate-related ions HPO42−, Ca2+, and Mn2+ tended to be greater in the presence of B. tectorum than in the absence of B. tectorum. This trend was evident only in wet plots where B. tectorum densities were much higher than in dry plots. We attribute this pattern to the mobilization of carbonate-associated ions by root exudates of B. tectorum. These findings indicate the importance of considering potential indirect effects of soil amendments performed in␣conjunction with resource-limitation studies, and they suggest the need for further research concerning nutrient acquisition mechanisms of B.␣tectorum.