Does disturbance,competition or resource limitation underlie Hieracium lepidulum invasion in New Zealand? Mechanisms of establishment and persistence,and functional differentiation among invasive and native species

The processes underlying plant invasions have been the subject of much ecological research. Understanding mechanisms of plant invasions are difficult to elucidate from observations, yet are crucial for ecological management of invasions. Hieracium lepidulum, an asteraceous invader in New Zealand, is a species for which several explanatory mechanisms can be raised. Alternative mechanisms, including competitive dominance, disturbance of resident vegetation allowing competitive release or nutrient resource limitation reducing competition with the invader are raised to explain invasion. We tested these hypotheses in two field experiments which manipulated competitive, disturbance and nutrient environments in pre‐invasion and post‐invasion vegetation. H. lepidulum and resident responses to environmental treatments were measured to allow interpretation of underlying mechanisms of establishment and persistence. We found that H. lepidulum differed in functional response profile from native species. We also found that other exotic invaders at the sites were functionally different to H. lepidulum in their responses. These data support the hypothesis that different invaders use different invasion mechanisms from one another. These data also suggest that functional differentiation between invaders and native resident vegetation may be an important contributing factor allowing invasion. H. lepidulum appeared to have little direct competitive effect on post‐invasion vegetation, suggesting that competition was not a dominant mechanism maintaining its persistence. There was weak support for disturbance allowing initial establishment of H. lepidulum in pre‐invasion vegetation, but disturbance did not lead to invader dominance. Strong support for nutrient limitation of resident species was provided by the rapid competitive responses with added nutrients despite presence of H. lepidulum. Rapid competitive suppression of H. lepidulum once nutrient limitation was alleviated suggests that nutrient limitation may be an important process allowing the invader to dominate. Possible roles of historical site degradation and/or invader‐induced soil chemical/microbial changes in nutrient availability are discussed.     

Functional and performance comparisons of invasive Hieracium lepidulum and co‐occurring species in New Zealand

Abstract One of the key environmental factors affecting plant species abundance, including that of invasive exotics, is nutrient resource availability. Plant functional response to nutrient availability, and what this tells us about plant interactions with associated species, may therefore give us clues about underlying processes related to plant abundance and invasion. Patterns of abundance of Hieracium lepidulum, a European herbaceous invader of subalpine New Zealand, appear to be related to soil fertility/nutrient availability, however, abundance may be influenced by other factors including disturbance. In this study we compare H. lepidulum and field co‐occurring species for growth performance across artificial nutrient concentration gradients, for relative competitiveness and for response to disturbance, to construct a functional profile of the species. Hieracium lepidulum was found to be significantly different in its functional response to nutrient concentration gradients. Hieracium lepidulum had high relative growth rate, high yield and root plasticity in response to nutrient concentration dilution, relatively low absolute yield, low competitive yield and a positive response to clipping disturbance relative to other species. Based on overall functional response to nutrient concentration gradients, compared with other species found at the same field sites, we hypothesize that H. lepidulum invasion is not related to competitive domination. Relatively low tolerance of nutrient dilution leads us to predict that H. lepidulum is likely to be restricted from invading low fertility sites, including sites within alpine vegetation or where intact high biomass plant communities are found. Positive response to clipping disturbance and relatively high nutrient requirement, despite poor competitive performance, leads us to predict that H. lepidulum may respond to selective grazing disturbance of associated vegetation. These results are discussed in relation to published observations of H. lepidulum in New Zealand and possible tests for the hypotheses raised here.     

BIODIVERSITY RESEARCH: Experimental introduction of the alien plant Hieracium lepidulum reveals no significant impact on montane plant communities in New Zealand

Aim There is debate over whether alien plants necessarily alter the communities they invade or can coexist with native species without discernable impacts. We followed the fate of montane plant communities in response to the experimental sowing of the alien weed Hieracium lepidulum, looking for changes in plant community composition and structure over 6 years. Location Craigieburn Range, New Zealand. Methods We used a replicated randomised block design, with 30 × 30 cm plots (n = 756) subdivided into 5 × 5 cm cells to examine and compare the effects of H. lepidulum at 0.09 m² (plot) and 0.0025 m² (cell) scales. Plots were sown with between 0 and 15,625 H. lepidulum seeds in 2003, forming gradients of invader density and cover. Measurements comprised community richness, evenness and diversity along with H. lepidulum density and cover at both scales. The relationships between the invader and local community attributes were modelled using hierarchical mixed‐effect models. Results Plant communities differed in the extent to which they became invaded, with H. lepidulum cover in the plots ranging from 0% to 52%, with a mean of only 1.89%. Plot species richness increased from 2003 to 2009, with a component of this increase (+0.002 species per year) associated with increasing H. lepidulum density. Other relationships between the plant community and H. lepidulum were generally non‐significant. Main conclusions In these montane plant communities, it appears H. lepidulum coexists with the native community with no measurable negative effects after 6 years on species richness, evenness or diversity, even where density and cover of the invader are highest. We suggest H. lepidulum has persisted preferentially at those sites with abiotic conditions sufficient to support a species‐rich assemblage.     

Multiple successional pathways of boreal forest stands in central Canada

Predicting forest composition change through time is a key challenge in forest management. While multiple successional pathways are theorized for boreal forests, empirical evidence is lacking, largely because succession has been inferred from chronosequence and dendrochronological methods. We tested the hypotheses that stands of compositionally similar overstory may follow multiple successional pathways depending on time since last stand‐replacing fire (TSF), edaphic conditions, and presence of intermediate disturbances. We used repeated measurements from combining sequential aerial photography and ground surveys for 361 boreal stands in central Canada. Stands were measured in 8–15 yr intervals over a ~ 60 yr period, covering a wide range of initial stand conditions. Multinomial logistic regression was used to analyze stand type transitions. With increasing TSF, stands dominated by shade‐intolerant Pinus banksiana, Populus sp., and Betula papyrifera demonstrated multiple pathways to stands dominated by shade‐tolerant Picea sp., Abies balsamea, and Thuja occidentalis. Their pathways seemed largely explained by neighborhood effects. Succession of stands dominated by shade‐tolerant species, with an exception of stands dominated by Picea sp., was not related to TSF, but rather dependent on edaphic conditions and presence of intermediate disturbances. Varying edaphic conditions caused divergent pathways with resource limited sites being dominated by nutrient‐poor tolerant species, and richer sites permitting invasion of early successional species and promoting species mixtures during succession. Intermediate disturbances promoted deciduous persistence and species diversity in A. balsamea and mixed‐conifer stands, but no evidence was detected to support “disturbance accelerated succession”. Our results demonstrate that in the prolonged absence of stand‐replacing disturbance boreal forest stands undergo multiple succession pathways. These pathways are regulated by neighborhood effects, resource availability, and presence of intermediate disturbance, but the relative importance of these regulators depends on initial stand type. The observed divergence of successional pathways supports the resource‐ratio hypothesis of plant succession.     

Invasion patterns across multiple scales by Hieracium species over 25 years in tussock grasslands of New Zealand's South Island

Studying patterns of species invasions over time at multiple spatial scales may help us to elucidate important factors driving those patterns and how they change according to temporal or spatial resolution. Here we provide a large, long‐term, landscape‐scale study of the invasion of three Hieracium species using a dataset that encompasses vegetation change on 124 transects over 25 years across the lower eastern South Island of New Zealand. We investigated the relationships between key environmental and ecological factors and the invasion trajectories of H. lepidulum, H. pilosella and H. praealtum, at two spatial scales: (i) among‐transect colonization and (ii) within‐transect changes in frequency and per cent cover. Our results show that the colonization and spread of Hieracium species among and within transects reflect (i) the importance of initial environmental and biological conditions, (ii) that our sampling captured different periods of the invasion trajectories of each of the three species, and (iii) the effects of differences in life histories of the three species.     

Arbuscular mycorrhizal inoculum potential: a mechanism promoting positive diversity�Cinvasibility relationships in mountain beech forests in New Zealand?

Mycorrhizal fungi are important symbionts for the majority of plant species, but their role in determining the susceptibility of habitat to plant invasion is poorly understood. Hieracium lepidulum is an arbuscular mycorrhizal herb, currently invading the understorey of ectomycorrhizal Nothofagus solandri var. cliffortioides (mountain beech) forest in New Zealand. Mountain beech is solely ectomycorrhizal, and other plant species within the understorey occur sporadically. Hieracium has been shown to establish preferentially in microsites with higher plant species richness at a scale of less than 1 m² within mountain beech forest, and we tested the hypothesis that more diverse microsites (<1 m²) are associated with higher levels of arbuscular mycorrhizal fungal (AMF) inoculum. We found low levels of AMF inoculum across all microsites, and over a third of samples contained no inoculum at all. Higher vascular-plant species richness (but not biomass) was associated with higher AMF spore densities in field soil, and greater AMF colonization of H. lepidulum seedlings in a bioassay. Absence of AMF inoculum from much of the soil and the positive association of inoculum potential with species richness provide a potential mechanism for the establishment of a positive diversity–invasibility relationship in the mountain beech forest.     

Epilithic Diatoms as Indicators of Water Quality in the Gravataí River, Rio Grande do Sul, Brazil

The potential use of epilithic diatoms as indicators of organic pollution was evaluated in Gravataí River, RS, (latitude 29°45′–30°12′ S; longitude 50°27′–51°12′ W). The river suffers agricultural impacts in its upper course and urban and industrial organic pollution in its lower course. Epilithic diatoms were sampled eight times from September 2000 to August 2002, at six sites. Species were identified and densities and relative abundances of populations were determined. Simultaneously, physical, chemical and microbiological variables were measured (water temperature, conductivity, turbidity, pH, dissolved oxygen, biochemical oxygen demand (BOD₅), chemical oxygen demand, ammonium, organic nitrogen, total nitrogen, ortho-phosphate, total phosphate, chloride and faecal coliforms). In order to interpret the environmental and biological variables, discriminant analysis and the TWINSPAN methods (Two-Way Indicator Species Analysis) were applied. The results indicated that the concentrations of ortho-phosphate, ammonium, total organic nitrogen, BOD₅ and faecal coliforms characterized a pollution gradient along the river, where changes in the abundance or species composition were observed. Species were classified into three groups: Group A, including species more tolerant to heavy organic pollution and eutrophication, represented by Luticola goeppertiana, L. mutica, Eolimna subminuscula, Nitzschia palea and Sellaphora pupula; Group B, comprised of tolerant and widely distributed species such as Eunotia bilunaris, Frustulia crassinervia, F. saxonica, Navicula cryptocephala, N. cryptotenella, Nitzschia palea var. tenuirostris, Surirella angusta, Pinnularia microstauron and Ulnaria ulna and Group C, with less pollution tolerant species represented by Eunotia sp. and Gomphonema parvulum.     

Inter-simple sequence repeat (ISSR) diversity within Monarda fistulosa var. brevis (Lamiaceae) and divergence between var. brevis and var. fistulosa in West Virginia

Inter-simple sequence repeat (ISSR) banding patterns were used to examine genetic diversity within and among populations ofMonarda fistulosa var.brevis, a rare taxon restricted to several populations in limestone glades and barrens in eastern West Virginia and Virginia. More than 34% of the total ISSR diversity in var.brevis occurred among populations, which is high when compared to the few other rare species that have been examined for ISSR variation. Prior studies demonstrated that var.brevis is morphologically distinct from the more widespread var.fistulosa, and that the differences are maintained when the two varieties are grown together in a uniform environment. The present study utilizing ISSR markers indicated that the two varieties are distinct, though quite similar genetically, and this is concordant with prior investigations documenting their morphological and habitat differences. However, the ISSR results suggest that the two varieties have diverged relatively recently and/or there is a low level of gene flow between them.     

Differences in nitrate uptake of species from habitats rich or poor in nitrogen when grown at low nitrate concentrations,using a new growth technique

Summary A method is described for culturing plants at extremely low nutrient concentrations. Using a Braun infusion pump, a fixed amount of nitrate or ammonium was supplied continuously to plants growing in a culture vessel at a rate limiting the uptake of the plants. At a very low nitrogen concentration an equilibrium was established where uptake rate of the plants is equal to the rate of supply by the infusion pump. The nitrogen concentrations reached appeared to be in the order of 1 μM. The method compared the nitrate uptake byHypochaeris radicata L.ssp.radicata, H. radicata ssp.ericetorum Van Soest andUrtica dioica L. and ammonium uptake byH. radicata ssp.radicata andH. radicata ssp.ericetorum. Plants were cultivated in monocultures or in mixed cultures (two species per culture vessel). For the mixed cultures competition for nitrate (or ammonium) between the species was maintained for long periods. The capacities of the uptake systems of two subspecies ofH. radicata from places different in nitrogen supply and pH were adapted equally well to both low nitrate and low ammonium concentrations. Apparently factors other than nitrogen uptake play a part in the distribution of the subspecies. The capacity of the uptake system ofU. dioica, a nitrophilous species, was lower than that ofH. radicata ssp.radicata, a species from places poorer in nitrogen. This difference is related to the different distribution of the two species in the field. The present results are compared with those of previous experiments where K_m and V_max were measured and the significance of both parameters is discussed.     

Isozyme evidence for the allotriploid origin of Lycoris flavescens (Amaryllidaceae)

 Variation in isozyme patterns from ten populations of the Korean endemic Lycoris species was used to test the hypothesis that L. flavescens originated from natural hybridization between diploid L. chinensis and L. sanguinea var. koreana. Lycoris sanguinea var. koreana shows fixed heterozygosity at four of nine loci assayed, suggesting that this species is an allotetraploid instead of a diploid. Electrophoretic data suggest that Lycoris flavescens is an allotriploid species derived from the hybridization between diploid L. chinensis and tetraploid L. sanguinea var. koreana. The patterns of allelic distribution in populations of L. flavescens suggest multiple origins of the allotriploid. Within the L. flavescens complex, our isozyme data support the recognition of two taxa, L. flavescens and a recently recognized species, L. uydoensis. Received August 28, 2000 Accepted December 27, 2000     

Root traits associated with nutrient exploitation following defoliation in three coexisting perennial grasses in a semi-arid savanna

Experiments were conducted to evaluate root traits associated with nutrient exploitation following defoliation in three coexisting perennial grasses in a semi‐arid savanna. Root length density was determined within soil cores directly beneath plants, nitrogen uptake was evaluated by excised‐root assay with (¹⁵NH₄)₂SO₄, and mycorrhizal root colonization was estimated by observation of root segments. Root length density was lowest for Bouteloua curtipendula, intermediate for Eriochloa sericea, and highest for Aristida purpurea indicating that root length density was a more important trait for the mid‐seral than the late‐seral species. Rates of ¹⁵N uptake were greatest in the least grazing tolerant late‐seral species, E. sericea, intermediate in the mid‐seral species, A. purpurea, and lowest in the most grazing tolerant late‐seral species, B. curtipendula. Two successive defoliations reduced ¹⁵N uptake 60% in the late‐seral species with the greatest uptake rate (E. sericea), but not in species with lowest uptake rates (B. curtipendula). Root length colonization was consistently high (33–61%) in all three species suggesting that these C₄ perennial grasses may function as obligate mycotrophs. Contrasting responses among the two late‐seral species indicate that the least grazing tolerant species, E. sericea, appears best adapted for nutrient exploitation while the most grazing tolerant species, B. curtipendula, appears best adapted for efficient nutrient retention. Contrasting responses of nitrogen uptake to short‐term defoliation parallel the population responses of these two coexisting late‐seral species to long‐term herbivory. These data indicate that herbivory may shift interspecific competitive interactions by mediating nutrient exploitation and that a trade‐off may exist between nutrient exploitation and herbivory tolerance in these species.     

Comparisons of nutrient recovery and specific leaf area variation betweenCarex lasiocarpa var.occultans andCarex thunbergii var.appendiculata with reference to nutrient conditions and shading byPhragmites australis

The distribution of two sedge species was studied in two mires which differ in abiotic environments and in distribution ofPhragmites australis. Carex lasiocarpa var.occultans dominated in nutrient-poor valley mire, andCarex thunbergii var.appendiculata dominated in nutrient-rich flood plain subject to water fluctuations.Phragmites australis grew well in nutrient-rich conditions. The distribution ofC. lasiocarpa showed a strong negative correlation withP. australis coverage, whereasC. thunbergii coverage was not affected byP. australis. The leaf area per dry leaf mass (specific leaf area: SLA) ofC. thunbergii increased with shading byP. australis, but that ofC. lasiocarpa was stable. The SLA flexibility ofC. thunbergii to light interception might enable this species to invadeP. australis patches in nutrient-rich environments. The residual nutrient ratio of nitrogen and phosphorus (the ratio of the residual nutrient content at the end of the growing season to peak nutrient content) in the vegetative ramet ofC. thunbergii was 1.7 times higher than that ofC. lasiocarpa. This low residual ratio may indicate effective nutrient recovery to storage organs. The effective nutrient recovery inC. lasiocarpa might enable this species to grow even in nutrient-poor environments. However, it may be difficult forC. lasiocarpa to expand its habitat to nutrient-rich areas whereP. australis dominates as it is not shade tolerant.     

NITROGEN LIMITATION INCREASES BREVETOXINS IN KARENIA BREVIS (DINOPHYCEAE): IMPLICATIONS FOR BLOOM TOXICITY1

Laboratory and field measurements of the toxin content in Karenia brevis cells vary by >4‐fold. These differences have been largely attributed to genotypic variations in toxin production among strains. We hypothesized that nutrient limitation of growth rate is equally or more important in controlling the toxicity of K. brevis, as has been documented for other toxic algae. To test this hypothesis, we measured cellular growth rate, chlorophyll a, cellular carbon and nitrogen, cell volume, and brevetoxins in four strains of K. brevis grown in nutrient‐replete and nitrogen (N)‐limited semi‐continuous cultures. N‐limitation resulted in reductions of chlorophyll a, growth rate, volume per cell and nirtogen:carbon (N:C) ratios as well as a two‐fold increase (1%–4% to 5%–9%) in the percentage of cellular carbon present as brevetoxins. The increase in cellular brevetoxin concentrations was consistent among genetically distinct strains. Normalizing brevetoxins to cellular volume instead of per cell eliminated much of the commonly reported toxin variability among strains. These results suggest that genetically linked differences in cellular volume may affect the toxin content of K. brevis cells as much or more than innate genotypic differences in cellular toxin content per unit of biomass. Our data suggest at least some of the >4‐fold difference in toxicity per cell reported from field studies can be explained by limitation by nitrogen or other nutrients and by differences in cell size. The observed increase in brevetoxins in nitrogen limited cells is consistent with the carbon:nutrient balance hypothesis for increases in toxins and other plant defenses under nutrient limitation.     

How does glutamine synthetase activity determine plant tolerance to ammonium?

The wide range of plant responses to ammonium nutrition can be used to study the way ammonium interferes with plant metabolism and to assess some characteristics related with ammonium tolerance by plants. In this work we investigated the hypothesis of plant tolerance to ammonium being related with the plants’ capacity to maintain high levels of inorganic nitrogen assimilation in the roots. Plants of several species (Spinacia oleracea L., Lycopersicon esculentum L., Lactuca sativa L., Pisum sativum L. and Lupinus albus L.) were grown in the presence of distinct concentrations (0.5, 1.5, 3 and 6 mM) of nitrate and ammonium. The relative contributions of the activity of the key enzymes glutamine synthetase (GS; under light and dark conditions) and glutamate dehydrogenase (GDH) were determined. The main plant organs of nitrogen assimilation (root or shoot) to plant tolerance to ammonium were assessed. The results show that only plants that are able to maintain high levels of GS activity in the dark (either in leaves or in roots) and high root GDH activities accumulate equal amounts of biomass independently of the nitrogen source available to the root medium and thus are ammonium tolerant. Plant species with high GS activities in the dark coincide with those displaying a high capacity for nitrogen metabolism in the roots. Therefore, the main location of nitrogen metabolism (shoots or roots) and the levels of GS activity in the dark are an important strategy for plant ammonium tolerance. The relative contribution of each of these parameters to species tolerance to ammonium is assessed. The efficient sequestration of ammonium in roots, presumably in the vacuoles, is considered as an additional mechanism contributing to plant tolerance to ammonium nutrition.     

水陆生境和氮沉降对香菇草(Hydrocotyle vulgaris)入侵湿地植物群落的影响

大气氮沉降对湿地外来植物入侵的影响已成为生态学研究的热点之一。选用梭鱼草(Pontederia cordata)、水菖蒲(Acorus calamus)、黄花鸢尾(Iris wilsonii)和粉绿狐尾藻(Myriophyllum aquaticum)模拟湿地植物群落,设置有无香菇草入侵、水生和陆生生境(水位分别为15 cm和0 cm)及有无氮沉降处理(15 g N m~(-2)a(-1)和0)交叉组成的8种处理组合,进行为期70d的温室控制实验,以分析水陆生境及氮沉降对香菇草入侵湿地植物群落的影响。结果表明:(1)水生生境下,香菇草的总生物量、叶生物量、叶片数和节点数与陆生生境相比显著降低;水生生境下氮沉降处理对香菇草各指标无显著影响,陆生生境下氮沉降处理的香菇草叶片数、节点数显著多于无氮沉降处理。(2)实验周期内氮沉降和香菇草入侵没有对群落的多样性指数及群落内4种湿地植物的生物量产生显著影响;水生生境下植物群落的总生物量及梭鱼草和粉绿狐尾藻的生物量与陆生生境相比显著提高。(3)水生生境下香菇草的相对优势度相比陆生生境下显著下降;氮沉降只在陆生生境下显著提高了香菇草的相对优势度。因此,香菇草向水生生境扩散和入侵的能力不强,其入侵在短时间内对湿地植物群落影响较小。研究结果可以为入侵植物生态学研究提供案例借鉴。     

Water relations of native and introduced C4 grasses in a neotropical savanna

Introduced African grasses are invading Neotropical savannas and displacing the native herbaceous community. This work, which is part of a program to understand the success of the African grasses, specifically investigates whether introduced and native grasses differ in their water relations. The water relations of the native Trachypogon plumosus and the successful invader Hyparrhenia rufa were studied in the field during two consecutive years in the seasonal savannas of Venezuela. The two C₄ grasses differed clearly in their responses to water stress. H. rufa consistently had higher stomatal conductance, transpiration rate, leaf water and osmotic potential and osmotic adjustment than the native T. plumosus. Also, leaf senescence occurred much earlier during the dry season in H. rufa. Both grasses showed a combination of water stress evasion and tolerance mechanisms such as stomatal sensitivity to atmospheric or soil water stress, decreased transpiring area and osmotic adjustment. Evasion mechanisms are more conspicuous in H. rufa whereas T. plumosus is more drought tolerant and uses water more conservatively. The evasion mechanisms and oportunistic use of water by H. rufa, characteristic of invading species, contribute to, but only partially explain, the success of this grass in the Neotropical savannas where it displaces native plants from sites with better water and nutrient status. Conversely, the higher water stress tolerance of t. plumosus is consistent with its capacity to resist invasion by alien grasses on shallow soils and sites with poorer nutrient and water status.     

Growth and competition of several harmful dinoflagellates under different nutrient and light conditions

Three near-shore type harmful dinoflagellates, Prorocentrum minimum, Prorocentrum donghaiense and Karlodinium veneficum, and one off-shore dinoflagellate, Karenia brevis, were grown in laboratory monocultures and mixed batch cultures. The dinoflagellate cultures were grown on treatments of two ambient nitrogen (N):phosphorus (P) ratios; two N substrates (nitrate and urea) and two light intensities. The microalgae Rhodomonas and Synechococcus were also added in separate treatments to the mixed culture treatments as potential food sources. All tested species grew well on both N substrates. In mixed culture, P. minimum outgrew K. veneficum, and P. donghaiense outgrew K. brevis in most treatments reaching higher growth rates and higher biomass. However, when a third algae, Rhodomonas, was added, the growth of P. minimum was inhibited relative to that of K. veneficum. In contrast, when grown with K. brevis, the growth rate of P. donghaiense was not significantly affected by the addition of potential prey. K. brevis had a longer growth phase, and kept growing after P. donghaiense reached stationary phase, suggesting better adaptation of K. brevis to low inorganic nutrient conditions. The growth of K. brevis was also significantly limited in the low light treatment. K. veneficum overgrew P. minimum in the presence of Rhodomonas, a potential nutrient source. The growth rates of both K. brevis and P. donghaiense were reduced with the presence of Synechococcus. In addition to nutrient competition, mixotrophy and allelopathy were likely mechanisms in determining the dominant species.     

Plant N capture from pulses: effects of pulse size,growth rate,and other soil resources

In arid ecosystems, the ability to rapidly capture nitrogen (N) from brief pulses is expected to influence plant growth, survival, and competitive ability. Theory and data suggest that N capture from pulses should depend on plant growth rate and availability of other limiting resources. Theory also predicts trade-offs in plant stress tolerance and ability to capture N from different size pulses. We injected K¹⁵NO₃, to simulate small and large N pulses at three different times during the growing season into soil around the co-dominant Great Basin species Sarcobatus vermiculatus, Chrysothamnus nauseosus ssp. consimilis, and Distichlis spicata. Soils were amended with water and P in a partial factorial design. As predicted, all study species showed a comparable decline in N capture from large pulses through the season as growth rates slowed. Surprisingly, however, water and P availability differentially influenced the ability of these species to capture N from pulses. Distichlis N capture increased up to tenfold with water addition while Chrysothamnus N capture increased up to threefold with P addition. Sarcobatus N capture was not affected by water or P availability. Opposite to our prediction, Sarcobatus, the most stress tolerant species, captured less N from small pulses but more N from large pulses relative to the other species. These observations suggest that variation in N pulse timing and size can interact with variable soil water and P supply to determine how N is partitioned among co-existing Great Basin species.     

Effect of nitrogen, water and neighbor density on the growth of Hesperis matronalis and two native perennials

Disturbance is a well-recognized catalyst of exotic species invasion, depriving or releasing resources into communities and favoring the spread of some invaders. Hesperis matronalis (dame’s rocket) is widespread in North America and has potential to become a major problem in natural communities due to anthropogenic influences. We used a target-neighbor design in a greenhouse to grow H. matronalis and native seedlings (Campanula rotundifolia and Muhlenbergia montana) at various levels of neighbor density, nitrogen and water. H. matronalis clearly reduced C. rotundifolia and M. montana aboveground growth and maintained its competitive advantage across all treatments. We expected H. matronalis to maximize its growth and have the greatest negative effect on native species under high resource conditions because ruderal species are able to take advantage of excess resources. H. matronalis demonstrated a clearly negative effect on native species, but a particular ability to outcompete native species at high resource levels depended on resource and native species identity. We also expected that the native species would not exploit excess resources as well as H. matronalis, instead growing better under ambient or low resource conditions because of local adaptation. Rather, M. montana benefited from high water inputs, inconsistent with plant strategies characteristic of stress tolerators. Information on the effects of H. matronalis on native plants in a controlled setting may aid land managers to understand its potential effects in natural communities.     

Phylogenetic Relationships and Possible Hybrid Origin of Lycoris Species (Amaryllidaceae) Revealed by ITS Sequences

To examine interspecific relationships and test the hypothesis of hybrid origin within Lycoris species, this study used data from parsimony analyses with nuclear ITS sequences for 19 taxa representing 14 species of Lycoris and two outgroup taxa. The ITS sequences resolved three infrageneric clades. One clade included L. chinensis, L. longituba, L. longituba var. flava, L. anhuiensis, and L. aurea; the second one consisted of L. sprengeri, L. radiata, L. radiata var. radiata, L. radiata var. pumila, L. haywardii, L. rosea, L. sanguinea var. sanguinea, and L. sanguinea var. koreana; and the third included L. caldwellii, L. straminea, L. albiflora, L. flavescens, and two hybrids. The results strongly support the hypothesis that L. straminea originated from hybridization between L. chinensis and L. radiata var. pumila, and the allotriploid L. caldwellii and L. albiflora derived from hybridization between L. chinensis and L. sprengeri. As nucleotide additivity was observed in the artificial hybrids and several presumed hybrids, the likelihood of hybrid origin of Lycoris species is supported.These authors contributed equally to this work.