Mixed Plant Communities Research Articles

Estimating CO2 gas exchange in mixed age vegetable plant communities grown on soil-like substrates for life support systems

If soil-like substrate (SLS) is to be used in human life support systems with a high degree of mass closure, the rate of its gas exchange as a compartment for mineralization of plant biomass should be understood. The purpose of this study was to compare variations in CO2 gas exchange of vegetable plant communities grown on the soil-like substrate using a number of plant age groups, which determined the so-called conveyor interval. Two experimental plant communities were grown as plant conveyors with different conveyor intervals. The first plant community consisted of conveyors with intervals of 7 days for carrot and beet and 14 days for chufa sedge. The conveyor intervals in the second plant community were 14 days for carrot and beet and 28 days for chufa sedge. This study showed that increasing the number of age groups in the conveyor and, thus, increasing the frequency of adding plant waste to the SLS, decreased the range of variations in CO2 concentration in the “plant–soil-like substrate” system. However, the resultant CO2 gas exchange was shifted towards CO2 release to the atmosphere of the plant community with short conveyor intervals. The duration of the conveyor interval did not significantly affect productivity and mineral composition of plants grown on the SLS.

Dynamic niche partitioning in root water uptake facilitates efficient water use in more diverse grassland plant communities

Abstract Efficient extraction of soil water is essential for the productivity of plant communities. However, research on the complementary use of resources in mixed plant communities, and especially the impact of plant species richness on root water uptake, is limited. So far, these investigations have been hindered by a lack of methods allowing for the estimation of root water uptake profiles. The overarching aim of our study was to determine whether diverse grassland plant communities in general exploit soil water more deeply and whether this shift occurs all the time or only during times of enhanced water demand. Root water uptake was derived by analysing the diurnal decrease in soil water content separately at each measurement depth, thus yielding root water uptake profiles for 12 experimental grasslands communities with two different levels of species richness (4 and 16 sown species). Additional measurements of leaf water potential, stomatal conductance, and root traits were used to identify differences in water relations between plant functional groups. Although the vertical root distribution did not differ between diversity levels, root water uptake shifted towards deeper layers (30 and 60 cm) in more diverse plots during periods of high vapour pressure deficit. Our results indicate that the more diverse communities were able to adjust their root water uptake, resulting in increased water uptake per root area compared to less diverse communities (52% at 20 cm, 118% at 30 cm, and 570% at 60 cm depth) and a more even distribution of water uptake over depth. Tall herbs, which had lower leaf water potential and higher stomatal conductance in more diverse mixtures, contributed disproportionately to dynamic niche partitioning in root water uptake. This study underpins the role of diversity in stabilizing ecosystem function and mitigating drought stress effects during future climate change scenarios. Furthermore, the results provide evidence that root water uptake is not solely controlled by root length density distribution in communities with high plant diversity but also by spatial shifts in water acquisition. A plain language summary is available for this article.

Utility of salt-marsh foraminifera, testate amoebae and bulk-sediment δ13C values as sea-level indicators in Newfoundland, Canada

Abstract We investigated the utility of foraminifera, testate amoebae and bulk-sediment δ 13 C measurements for reconstructing Holocene relative sea level from sequences of salt-marsh sediment in Newfoundland, Canada. Modern, surface sediment was collected along transects from low to supra-tidal elevations in eastern (at Placentia) and western (at Hynes Brook and Big River) Newfoundland. Consistent with previous work, low-diversity assemblages of foraminifera display an almost binary division into a higher salt-marsh assemblage dominated by Jadammina macrescens and Balticammina pseudomacrescens and a lower salt-marsh assemblage comprised of Miliammina fusca . This pattern and composition resembles those identified at other high latitude sites with cool climates and confirms that foraminifera are sea-level indicators. The lowest occurrence of testate amoebae was at approximately mean higher high water. The composition of high salt-marsh testate amoebae assemblages ( Centropyxis cassis type, Trinema spp., Tracheleuglypha dentata type, and Euglypha spp.) in Newfoundland was similar to elsewhere in the North Atlantic, but preservation bias favors removal of species with idiosomic tests over those with xenosomic tests. The mixed high salt-marsh plant community in Newfoundland results in bulk surface-sediment δ 13 C values that are typical of C 3 plants, making them indistinguishable from freshwater sediment. Therefore we propose that the utility of this proxy for reconstructing RSL in eastern North America is restricted to the coastline between Chesapeake Bay and southern Nova Scotia. Using a simple, multi-proxy approach to establish that samples in three radiocarbon-dated sediment cores formed between the lowest occurrence of testate amoebae and the highest occurrence of foraminifera, we generated three example late Holocene sea-level index points at Hynes Brook.

Above-Belowground Herbivore Interactions in Mixed Plant Communities Are Influenced by Altered Precipitation Patterns.

Root- and shoot-feeding herbivores have the capacity to influence one another by modifying the chemistry of the shared host plant. This can alter rates of nutrient mineralization and uptake by neighboring plants and influence plant–plant competition, particularly in mixtures combining grasses and legumes. Root herbivory-induced exudation of nitrogen (N) from legume roots, for example, may increase N acquisition by co-occurring grasses, with knock-on effects on grassland community composition. Little is known about how climate change may affect these interactions, but an important and timely question is how will grass–legume mixtures respond in a future with an increasing reliance on legume N mineralization in terrestrial ecosystems. Using a model grass–legume mixture, this study investigated how simultaneous attack on lucerne (Medicago sativa) by belowground weevils (Sitona discoideus) and aboveground aphids (Acyrthosiphon pisum) affected a neighboring grass (Phalaris aquatica) when subjected to drought, ambient, and elevated precipitation. Feeding on rhizobial nodules by weevil larvae enhanced soil water retention under ambient and elevated precipitation, but only when aphids were absent. While drought decreased nodulation and root N content in lucerne, grass root and shoot chemistry were unaffected by changes in precipitation. However, plant communities containing weevils but not aphids showed increased grass height and N concentrations, most likely associated with the transfer of N from weevil-attacked lucerne plants containing more nodules and higher root N concentrations compared with insect-free plants. Drought decreased aphid abundance by 54% but increased total and some specific amino acid concentrations (glycine, lysine, methionine, tyrosine, cysteine, histidine, arginine, aspartate, and glutamate), suggesting that aphid declines were being driven by other facets of drought (e.g., reduced phloem hydraulics). The presence of weevil larvae belowground decreased aphid numbers by 30%, likely associated with a significant reduction in proline in weevil-treated lucerne plants. This study demonstrates how predicted changes to precipitation patterns and indirect interactions between herbivores can alter the outcome of competition between N-fixing legumes and non-N-fixing grasses, with important implications for plant community structure and productivity.

Effects of spatial plant–soil feedback heterogeneity on plant performance in monocultures

Summary Plant–soil feedback (PSF) effects have almost exclusively been quantified on homogeneous soils, but as different plant species will influence their local soil differently, in reality PSF effects will be spatially heterogeneous. Whether plant performance in soils with spatially heterogeneous PSF can be predicted from pot experiments with homogeneous soils is unclear. In a glasshouse experiment, we tested the response of monocultures of six grassland species (two grasses, two legumes and two forbs) to three spatially explicit treatments (fine‐grained heterogeneity, coarse‐grained heterogeneity and homogeneous). Sixteen patches of conditioned soil (˜6 × 6 cm) were placed within each container. For homogeneous treatments, all patches contained the same conditioned soil within a container. The fine‐grained heterogeneous treatment contained four differently conditioned soils that were applied following a Latin square design, while for the coarse‐grained heterogeneous treatment, four contiguous square blocks of four cells each were created in each container. In general, species grew worse on soil conditioned by conspecifics. However, when the biomass production on all homogeneous soil treatments (own and foreign soils) was averaged and compared to the heterogeneous treatments, we found that biomass production was lower than expected in the heterogeneous soils. This effect of heterogeneity depended on both the conditioning and test species, but most heterogeneity effects were negative. The grain of the heterogeneity (coarse vs. fine: at the chosen spatial scale) did not affect plant performance. We hypothesize that a more diverse soil community is present in spatially heterogeneous soils. This increases (i) the chance of plants to encounter its antagonists, which may then rapidly increase in numbers; and (ii) the scope for synergistic co‐infections. Together, this may lead to non‐additive responses of plants to spatial heterogeneity in PSF. Synthesis. Plant performance was lower in spatially heterogeneous soils than predicted by spatially homogeneous soils. In natural grasslands that have mixed plant communities conditioning the soil, plant–soil feedback (PSF) effects on plant performance may therefore be more negative than what is predicted from pot experiments. Our results emphasize the need to incorporate the spatial dynamics of PSF both in empirical and modelling studies if we are to understand the role of PSF in plant–plant interactions and plant community dynamics.

Sampling method and sample size affect diversity and indigenous dominance estimates in a mixed grassland community

Estimates of vegetation attributes measured by sampling often inform scientific inference, management actions, and policy decisions. However, different sampling methods and sample sizes (i.e. number of plots) can yield significantly different estimates of vegetation attributes. This occurs because the abundance distributions and spatial distributions of species in the plant community influence their probabilities of detection and estimates of their abundances. We predicted that different sampling methods and sample sizes would produce significantly different estimates not only of vascular plant species diversity, but also of indigenous dominance (the level of indigenous influence) in mixed vegetation where indigenous and exotic floras have different abundance or spatial distributions. To test our predictions we applied three sampling methods to 24 plots in grassland and cushion vegetation in a 1058-ha scientific reserve in the Upper Waitaki (Mackenzie) Basin, New Zealand. Our methods sampled ground areas from 0.65 to 400 m 2 , and included two variants of common 'subsampling' approaches, which assessed only discrete subunits within larger plots. Indigenous plant species were both less abundant on average and more spatially-clustered (i.e. less evenly dispersed across plots) than exotic species. The two subsampling methods were less likely to detect less abundant and more spatially clustered species, leading to lower ratios of indigenous to exotic species recorded, and lower estimates of indigenous dominance of composition (% of species indigenous). Numbers of indigenous species accumulated more rapidly with increasing sample size than numbers of exotic species, so that indigenous dominance also increased with the number of plots sampled. We conclude that properly measuring species diversity and indigenous dominance in mixed indigenous-exotic plant communities requires both the searching of sizeable plots and use of rarefaction rather than plot-averaging of statistics. We suggest greater use of rarefaction and more consideration of species' detection probabilities in sampling New Zealand's mixed indigenous-exotic plant communities should improve the reliability, transparency and comparability of measures of diversity and may also provide new ecological insights.

Responses of community-level plant-insect interactions to climate warming in a meadow steppe.

Climate warming may disrupt trophic interactions, consequently influencing ecosystem functioning. Most studies have concentrated on the temperature-effects on plant-insect interactions at individual and population levels, with a particular emphasis on changes in phenology and distribution. Nevertheless, the available evidence from the community level is limited. A 3-year field manipulative experiment was performed to test potential responses of plant and insect communities, and plant-insect interactions, to elevated temperature in a meadow steppe. Warming increased the biomass of plant community and forbs, and decreased grass biomass, indicating a shift from grass-dominant to grass-forb mixed plant community. Reduced abundance of the insect community under warming, particularly the herbivorous insects, was attributed to lower abundance of Euchorthippus unicolor and a Cicadellidae species resulting from lower food availability and higher defensive herbivory. Lower herbivore abundance caused lower predator species richness because of reduced prey resources and contributed to an overall decrease in insect species richness. Interestingly, warming enhanced the positive relationship between insect and plant species richness, implying that the strength of the plant-insect interactions was altered by warming. Our results suggest that alterations to plant-insect interactions at a community level under climate warming in grasslands may be more important and complex than previously thought.

Cascading effects of belowground predators on plant communities are density-dependent.

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density-mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density-dependent population change in predators, that is, an increase in low-density treatments, but a decrease in high-density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density-dependent cascading effects on detritivore and soil microbial communities.

Plant community composition determines the strength of top-down control in a soil food web motif.

Top-down control of prey by predators are magnified in productive ecosystems due to higher sustenance of prey communities. In soil micro-arthropod food webs, plant communities regulate the availability of basal resources like soil microbial biomass. Mixed plant communities are often associated with higher microbial biomass than monocultures. Therefore, top-down control is expected to be higher in soil food webs of mixed plant communities. Moreover, higher predator densities can increase the suppression of prey, which can induce interactive effects between predator densities and plant community composition on prey populations. Here, we tested the effects of predator density (predatory mites) on prey populations (Collembola) in monoculture and mixed plant communities. We hypothesized that top-down control would increase with predator density but only in the mixed plant community. Our results revealed two contrasting patterns of top-down control: stronger top-down control of prey communities in the mixed plant community, but weaker top-down control in plant monocultures in high predator density treatments. As expected, higher microbial community biomass in the mixed plant community sustained sufficiently high prey populations to support high predator density. Our results highlight the roles of plant community composition and predator densities in regulating top-down control of prey in soil food webs.

Complementarity and selection effects in early and mid‐successional plant communities are differentially affected by plant–soil feedback

Summary Many studies that provided evidence for a positive relationship between plant diversity and productivity have proposed that this effect may be explained by complementarity among species in resources utilization, or selection of particularly productive species in high‐diversity plant communities. Recent studies have related the higher productivity in diverse plant communities to suppression of pathogenic soil biota. If soil biota plays a role in diversity–productivity relationships, the question remains about how they may influence complementarity and selection effects. Here we examine how complementarity and selection effects may depend on soil biota using a plant–soil feedback approach. We used monocultures and mixtures of early successional plant species, which are known to have mostly negative plant–soil feedback effects, and mid‐successional plant species, which generally have neutral plant–soil feedback. We found that plant–soil feedback effects differed between monocultures and mixed plant communities, as well as between early and mid‐successional plants. This resulted in a significant interaction effect between diversity and successional stage. In monocultures, plant–soil feedback tended to be negative for early and positive for mid‐successional plant species. Interestingly, the community feedback responses of the mixed communities were opposite, being positive for early and negative for mid‐successional community. Plant–soil feedback differentially affected complementarity and selection effects of early and mid‐successional plant communities: it enhanced complementarity effects of early and decreased selection effects of mid‐successional species. Synthesis. Soil biota that drive plant–soil feedback effects can influence the diversity–productivity relationship not only through decreased biomass production in monocultures compared to mixtures, but also through influencing complementarity and selection effects among species in mixed plant communities. Our results reveal that biodiversity–productivity relationships depend on plant–soil feedback interactions, which depend on the successional position of the plant. We propose that including successional position and trait‐based analyses of plant–soil feedback in diversity‐functioning studies will enhance understanding consequences of biodiversity loss for productivity and other ecosystem processes.

Đặc điểm cấu trúc quần xã thực vật rừng Khu Bảo tồn thiên nhiên Hòn Bà, tỉnh Khánh Hoà

Hon Ba Natural Reserve is considered as an unique area in Vietnam that remains relatively primitive structure of plant communities and forest landscape from a height of 150 m to 1,500 m altitude. The research results provide data on the structural characteristics of typical plant communities of Hon Ba Natural Reserve. Accordingly, the structure of plant communities has a high level of diversity, including plant communities in the lowland valleys, on slopes under 1,000 m, and on the top of mountain. The historical and stable development of the area has created several types of mixed subtropical plant communities with precious, rare coniferous species, such as plant communities of mixed broadleaf species with Ducampopinus krempfii and with Fokienia hodginsii. Canopy structure of plant communities usually has from 2 - 4 stories, but it is not follow clearly rule. In addition, soil properties, hydrological conditions of forest soil influence quite markedly on species composition, structure and morphology of plant communities of Hon Ba Natural Reserve.

Le condizioni luminose che favoriscono la rinnovazione dell'abete bianco nel Parco Nazionale del Pollino

Studies carried out on the southern Apennine mountains have evidenced that silver fir (Abies alba Mill.), a species of great ecological interest, is undergoing a sharp contraction in its spatial distribution. This contraction is mainly due to the considerable anthropic pressure exerted in the past, even if negative effects of climate change may not be ruled out. The same fate befell silver fir in the Pollino National Park, where natural regeneration of the few remaining stands is struggling to establish itself, compared to more competitive species such as oak and beech. Before planning any intervention, it is crucial to investigate which micro-climatic conditions can promote the growth and development of silver fir regeneration. In such mixed plant communities, light microclimate of the understory is undoubtedly the main factor limiting the establishment of natural regeneration. Another preeminent factor is the water resource, which in the Mediterranean environment is becoming scarce especially during the growing season, as a consequence of climate change. Dendrochronological, isotopic and photosynthetic investigations carried out on a representative regeneration survey have identified the most suitable microclimatic conditions which can help define the most appropriate management guidelines.

Evaluating population border dynamics amongArtemisia vulgarisand community constituents in an urban successional plant assemblage

Assessing border properties within vegetative assemblages is critical to the understanding of plant community dynamics and to the development of non-native species management protocols. Population border zones are the most dynamic part of a plant community and can be evaluated to determine population dynamics, management necessity, or species/species interactions. From a management perspective, changes in these borders can be used to evaluate the overall trajectory for populations in a community, the effects of disturbance, and/or non-native species on the community structure. We examined the trends of Artemisia vulgaris (L.) borders that were adjacent to other species populations in a mixed urban plant community in an effort to determine the future community identity and how interactions may vary among species of differing guilds. We analyzed trends in border properties, which were developed specifically for this study, over a four-year period. Our design included all co-dominant species bordering A. vulgaris from the following guilds: (1) Forbs, (2) Grasses, and (3) Shrubs resulting in a complete assessment of guild interactions (forb/forb, forb/grass, and forb/shrub). We found that interactions among species varied considerably and were not directly related to the guild membership. Instead, interactions and the dynamic nature of population borders varied on a species by species basis. Our results also suggest that population dynamics of differing life form guilds (i.e., forb/shrub) were not necessarily related to direct competitive interactions; rather, they may be a result of yearly fluctuations in the larger species' (shrub) population. Finally, we found that A. vulgaris did not necessarily exclude all species; in fact, A. vulgaris was thwarted by some species and co-mingled with others.

Mediterranean Shrub Diversity and its Effect on Food Intake in Goats

Mediterranean ecosystem offers a variety of shrubs that were over long periods of time involved in the evolution of complex plant-animal interactions. Biochemical components of these plants enter different metabolic pathways after digestion and absorption, resulting in development of dietary preferences in browsing animals. Herbivores in general were found to perform better when grazing in a mixed plant community composed of diverse species, and show preferential feeding behavioure for mixed vs single species diet. Our findings demonstrate an asymptotic relationship among Mediterranean shrubs species diversity and their voluntary intake by goats. Shrub biomass intake showed linear increase when number of different shrubs in diet increased from one to three. However, goats did not further increase intake when the number of shrub species increased from four to eight. As the number of shrub species offered increased, goats exhibited more preferential feeding behaviour for Quercus pubescens, Fraxinus ornus, Rubus heteromorphus and Arbutus unedo and decreased the intake of Hedera helix, Juniperus oxycedrus and Helichrysum italicum. This asymptotic relationship indicates that the maintenance of plant species richness in Mediterranean shrublands can overall benefit domestic goat farming, goat’s productive performance, and the conservation of plant biodiversity.

Changes in the understorey of mixed coniferous forest plant communities dominated by the American black cherry (Prunus serotina Ehrh.)

In the managed mixed forests of Europe, Prunus serotina increases its territory easily and gradually becomes the predominant component of the shrub layer. However, its impact on species diversity in the invaded stands remains a matter of debate. To assess the relationship between the black cherry and the understorey plant community, 120 relevés of comparable habitat properties but of variable intensity of infestation by P. serotina were made in three forest districts in south-western Poland, where the plant has been present for more than 50years. Habitat features were estimated indirectly for study plots using selected Ellenberg’s species indicator values (nitrogen availability, soil reaction, soil moisture and light conditions). The forest floor of the areas infested with black cherry showed higher proportions of certain plant species with higher requirements for nitrogen supply, soil reaction and moisture content. Furthermore, a significant negative correlation has been found between the abundance of P. serotina and the species richness of the understorey. Also in multiple regression analysis black cherry occurring in the shrub layer has appeared as the variable significantly related to the decrease of species number. Despite those established relationships, feeding species composition data into the redundancy analysis (RDA) procedure showed that the black cherry’s impact on the herb and moss layer explained only a limited proportion of the general variance. Therefore, it is likely that the effect of P. serotina on the native species composition in different forest stands is significantly modified by a set of site-specific environmental variables, interacting with a number of disturbances caused by natural factors and human activity.

Differential soil organic carbon storage at forb- and grass-dominated plant communities, 33 years after tallgrass prairie restoration

Dominance of C4 grasses has been proposed as a means of increasing soil organic carbon (SOC) sequestration in restored tallgrass prairies. However, this hypothesis has not been tested on long time scales and under realistic (e.g. N-limited) environmental conditions. We sampled a restoration in southern Illinois 33 years after establishment to determine the effects of varying plant communities on SOC sequestration in the top 50 cm of soil. SOC, total nitrogen (TN), and the stable isotopic composition of SOC (δ13C) were used to calculate SOC sequestration rates, N storage, and the relative contributions of C3 vs. C4 plant communities as a function of soil depth. While both a forb-dominated and a mixed forb-grass plant community showed positive sequestration rates (0.56 ± 0.13 and 0.27 ± 0.10 Mg C ha−1 yr−1, respectively), a C4 grass-dominated community showed SOC losses after 33 years of restoration (−0.31 ± 0.08 Mg C ha−1 yr−1). Soil δ13C values were significantly more negative for forb-dominated plant communities, increasing the confidence that plant communities were stable over time and an important contributor to differences in SOC stocks among transects. These results suggest that functional diversity may be necessary to sustain sequestration rates on the scale of decades, and that dominance of C4 grasses, favored by frequent burning, may lead to SOC losses over time.

Invertebrate herbivory increases along an experimental gradient of grassland plant diversity

Plant diversity is a key driver of ecosystem functioning best documented for its influence on plant productivity. The strength and direction of plant diversity effects on species interactions across trophic levels are less clear. For example, with respect to the interactions between herbivorous invertebrates and plants, a number of competing hypotheses have been proposed that predict either increasing or decreasing community herbivory with increasing plant species richness. We investigated foliar herbivory rates and consumed leaf biomass along an experimental grassland plant diversity gradient in year eight after establishment. The gradient ranged from one to 60 plant species and manipulated also functional group richness (from one to four functional groups-legumes, grasses, small herbs, and tall herbs) and plant community composition. Measurements in monocultures of each plant species showed that functional groups differed in the quantity and quality of herbivory damage they experienced, with legumes being more damaged than grasses or non-legume herbs. In mixed plant communities, herbivory increased with plant diversity and the presence of two key plant functional groups in mixtures had a positive (legumes) and a negative (grasses) effect on levels of herbivory. Further, plant community biomass had a strong positive impact on consumed leaf biomass, but little effect on herbivory rates. Our results contribute detailed data from a well-established biodiversity experiment to a growing body of evidence suggesting that an increase of herbivory with increasing plant diversity is the rule rather than an exception. Considering documented effects of herbivory on other ecosystem functions and the increase of herbivory with plant diversity, levels of herbivory damage might not only be a result, but also a trigger within the diversity-productivity relationship.

Evapotranspiration crop coefficients for mixed riparian plant community and transpiration crop coefficients for Common reed, Cottonwood and Peach-leaf willow in the Platte River Basin, Nebraska-USA

► Evapotranspiration crop coefficients were developed for riparian vegetation . ► Transpiration coefficients were measured for phragmites , willow, cottonwood. ► Vegetation response to flooding caused interannual variability in coefficients. ► Developed coefficients provided robust riparian ET and transpiration estimates. Application of two-step approach of evapotranspiration ( ET ) crop coefficients ( Kc ) to “approximate” a very complex process of actual evapotranspiration ( ETa ) for field crops has been practiced by water management community. However, the use of Kc , and in particular the concept of growing degree days (GDD) to estimate Kc , have not been sufficiently studied for estimation of evaporative losses from riparian vegetation. Our study is one of the first to develop evapotranspiration crop coefficient ( Kc ET ) curves for mixed riparian vegetation and transpiration ( TRP ) crop coefficients ( Kc TRP ) for individual riparian species as a function GDD through extensive field campaigns conducted in 2009 and 2010 in the Platte River Basin in central Nebraska, USA. Kc TRP values for individual riparian vegetation species [Common reed (Phragmites australis), Cottonwood (Populus deltoids) and Peach-leaf willow (Salix amygdaloides)] were quantified from the TRP rates obtained using scaled-up canopy resistance from measured leaf-level stomatal resistance and reference evapotranspiration. The Kc ET and Kc TRP curves were developed for alfalfa-reference ( Kcr ET and Kcr TRP ) surface. The seasonal average mixed riparian plant community Kcr ET was 0.89 in 2009 and 1.27 in 2010. In 2009, the seasonal average Kcr TRP values for Common reed, Cottonwood and Peach-leaf willow were 0.57, 0.51 and 0.62, respectively. In 2010, the seasonal average Kcr TRP were 0.69, 0.62 and 0.83 for the same species, respectively. In general, TRP crop coefficients had less interannual variability than the Kcr ET . Response of the vegetation to flooding in 2010 played an important role on the interannual variability of Kcr ET values. We demonstrated good performance and reliability of developed GDD-based Kcr TRP curves by using the curves developed for 2009 to predict TRP rates of individual species in 2010. Using the Kcr TRP curves developed during the 2009 season, we were able to predict the TRP rates for Common reed, Cottonwood and Peach-leaf willow in 2010 within 7%, 8% and 13% accuracy, indicating a good performance of the two-step approach proposed in this study for estimating TRP for riparian vegetation. The surface conditions of the riparian ecosystem need to be considered when using the two-step approach to estimate ETa or TRP rates of riparian plant communities. The results of this study provide important water use information and data for riparian vegetation that can be used for more robust hydrologic/water balance analyses.

Evapotranspiration and Surface Energy Balance of a Common Reed-Dominated Riparian System in the Platte River Basin, Central Nebraska

Quantifying actual evapotranspiration (ETa) of riparian zones is important for more robust water balance analyses that will enable better planning, managing, and allocating of water resources as well as developing strategies to protect delicate riparian ecosystem functions. The ETa, sensible heat flux (H), net radiation (Rn), soil heat flux (G), meteorological variables (air temperature, Ta; incoming shortwave radiation, Rs; wind speed, u3; relative humidity, RH; vapor pressure deficit, VPD; precipitation, etc.), and albedo were measured on an hourly time step, and leaf area index (LAI) and plant height were measured on a weekly basis for a common reed (Phragmites australis) dominated cottonwood (Populus deltoides) and peach-leaf willow (Salix amygdaloides) riparian plant community in 2009 and 2010 through extensive field campaigns conducted in the Platte River basin in central Nebraska. The two growing seasons were contrasted by warmer air temperatures, higher precipitation, and presence of flood water on the surface during the 2010 season. The seasonal variations of daily average ETa were mainly controlled by Rn and air temperature. In 2009, total ETa and precipitation were 679 mm and 280 mm, respectively, and the values were substantially greater in 2010 (982 mm and 508 mm, respectively). The seasonal daily ETa for the mixed plant community ranged from 0.5 to 8.5 mm d-1 with a seasonal average of 3.7 mm d-1 in 2009 and from 0.5 to 11 mm d-1 with a seasonal average of 5.5 mm d-1 in 2010. In 2010, ETa varied widely with meteorological conditions and in response to variations in phenology of the vegetation to flooding. In 2009, on a seasonal average basis, a total of 77% and 14% of the available energy was partitioned into ETa and H, respectively. In 2010, over 90% and -12% (negative due to flooding) of the available energy was partitioned into ETa and H, respectively. The research results presented here provide valuable ETa data and information for enhancing the understanding of the interactions between the surface/vegetation conditions and the surrounding microclimate and surface energy balance for mixed riparian vegetation. The results of this research should aid water managers and decision/policy makers in accounting for water use rates of phragmites-dominated cottonwood and peach-leaf willow riparian plant communities in water balance analyses to make better-informed water resources planning and management decisions.

Effects of Silver Nanoparticle Exposure on Germination and Early Growth of Eleven Wetland Plants

The increasing commercial production of engineered nanoparticles (ENPs) has led to concerns over the potential adverse impacts of these ENPs on biota in natural environments. Silver nanoparticles (AgNPs) are one of the most widely used ENPs and are expected to enter natural ecosystems. Here we examined the effects of AgNPs on germination and growth of eleven species of common wetland plants. We examined plant responses to AgNP exposure in simple pure culture experiments (direct exposure) and for seeds planted in homogenized field soils in a greenhouse experiment (soil exposure). We compared the effects of two AgNPs–20-nm polyvinylpyrrolidine-coated silver nanoparticles (PVP-AgNPs) and 6-nm gum arabic coated silver nanoparticles (GA-AgNPs)–to the effects of AgNO3 exposure added at equivalent Ag concentrations (1, 10 or 40 mg Ag L−1). In the direct exposure experiments, PVP-AgNP had no effect on germination while 40 mg Ag L−1 GA-AgNP exposure significantly reduced the germination rate of three species and enhanced the germination rate of one species. In contrast, 40 mg Ag L−1 AgNO3 enhanced the germination rate of five species. In general root growth was much more affected by Ag exposure than was leaf growth. The magnitude of inhibition was always greater for GA-AgNPs than for AgNO3 and PVP-AgNPs. In the soil exposure experiment, germination effects were less pronounced. The plant growth response differed by taxa with Lolium multiflorum growing more rapidly under both AgNO3 and GA-AgNP exposures and all other taxa having significantly reduced growth under GA-AgNP exposure. AgNO3 did not reduce the growth of any species while PVP-AgNPs significantly inhibited the growth of only one species. Our findings suggest important new avenues of research for understanding the fate and transport of NPs in natural media, the interactions between NPs and plants, and indirect and direct effects of NPs in mixed plant communities.