Competition for pulsed resources: an experimental study of establishment and coexistence for an arid-land grass

In arid environments, episodically-pulsed resources are important components of annual water and nutrient supply for plants. This study set out to test whether seedlings have an increased capacity for using pulsed resources, which might then improve establishment when in competition with older individuals. A second aim was to determine whether there is a trade-off in competitive strategies when resources are supplied continuously at low concentrations, or as pulses with pronounced inter-pulse periods. A glasshouse experiment used a target-neighbour design of size-asymmetric competition, with juveniles of Panicum antidotale (blue panicgrass) introduced into contrasting densities of adult plants. Stable isotopes of nitrogen were used for measuring plant resource uptake from pulses, and tolerance to inter-pulse conditions was assessed as the mean residence time (MRT) of nitrogen. A higher root/shoot ratio and finer root system enhanced the capacity of juveniles to use resources when pulsed, rather than when continuously supplied. Higher resource uptake during pulses improved the establishment of juvenile Panicum in mixed cultures with older individuals. However, a trade-off was observed in plant strategies, with juveniles showing a lower MRT for nitrogen, which suggested reduced tolerance to resource deficit during inter-pulse periods. Under field conditions, higher utilization of pulsed resources would lead to the improved seedling establishment of Panicum adjacent to “nurse” plants, whereas mature plants with well-developed roots, exploiting a greater soil volume, maintain more constant resource uptake and retention during inter-pulse periods.     

养分资源脉冲供给对几种微藻种间竞争的影响

以淡水生态系统常见的5种微藻为研究对象,通过稳态条件下单一培养的方式获取各微藻在氮素或磷素缺乏条件下对应的生长特征参数和R*值,同时将5种微藻在养分资源脉冲供给的方式下进行混合培养,进而检测养分资源脉冲供给对几种微藻种间竞争的影响作用,并与基于稳态条件下所预测的微藻种间竞争结果进行比较。研究结果显示,在稳态条件下具有最小R*值的纤细角星鼓藻在与其它微藻的种间竞争过程中始终保持优势地位,而其余4种微藻在混合培养状态下的相对比重亦与其稳态条件下的R*值紧密相关。然而,资源脉冲供给条件下的竞争优势种纤细角星鼓藻能与其它种群数量较小的微藻共存。此外,在氮素和磷素的不同供给比例情况下,对应微藻群落的结构也会发生相应的变化。实验养分资源脉冲作用下多种微藻的共存现象与自然水体中的观测现象相一致,显示了资源脉冲可能是维持生物多样性水平的一个重要机制。     

Competitive impacts and responses of an invasive weed: dependencies on nitrogen and phosphorus availability

Changes in competitive interactions under conditions of enhanced resource availability could explain the invasion success of some problematic plant species. For one invader of North American grasslands, Centaurea diffusa (diffuse knapweed), we test three hypotheses: (1) under ambient (high resource) conditions, C. diffusa is better able to tolerate competition from the resident community (competitive response), (2) under ambient conditions, C. diffusa strong impacts the competitive environment (competitive effect), and (3) reductions in nitrogen and/or phosphorus availability diminish these advantages. In support of our first hypothesis, C. diffusa was the most tolerant to neighbor competition of the four focal species under current resource conditions. In opposition to our second hypothesis, however, neighborhoods that contained C. diffusa and those where C. diffusa had been selectively removed did not differ in their impact on the performance of target transplant individuals or on resource conditions. Reduction in resource availability influenced competitive tolerance but not competitive impact, in partial support of our last hypothesis. Reduction in soil nitrogen (via sucrose carbon addition) enhanced the degree of neighbor competition experienced by all species but did not change their relative rankings; C. diffusa remained the best competitor under low nitrogen conditions. Reduction of soil phosphorus (via gypsum addition) weakened the ability of C. diffusa to tolerate neighbor competition proportionately more than the other focal species. Consequently, under low phosphorus conditions, C. diffusa lost its competitive advantage and tolerated neighbor competition similarly to the other focal species. We conclude that C. diffusa invasion may be double-edged: C. diffusa is less limited by nitrogen than the other focal species and is better able to utilize phosphorus to its competitive advantage.     

Mineral allocation to reproduction in Sorhum bicolor and Sorghum halepense in relation to parental nutrient supply

Summary This experiment investigated the effect of parental nutrient shortage on the allocation of five nutrients to seeds and rhizomes in Sorghum halepense, a perennial, noxious weed, and to seeds in Sorghum bicolor, an annual, cultivated species. Plants from both species were grown from seeds and supplied with fertilizer at three concentrations. The allocation of biomass and nutrients (N, P, K, Ca and Mg) to reproductive and vegetative parts was determined. Relative biomass allocation to reproduction (either sexual or vegetative) remained constant in S. halepense in spite of large differences in total plant weight. In S. bicolor, however, biomass allocation to sexual reproductive structures decreased significantly with decreasing nutrient supply. Individual seed weight was not modified by parental nutrient supply in S. halepense, but it increased with decreasing nutrient availability in S. bicolor. Important differences in mineral allocation to seeds were found between the two species. While S. bicolor seeds were largely buffered from the differences in parental nutrient status, concentration of nutrients in S. halepense seeds decreased significantly with decreasing supply for all the nutrients analyzed except Ca. However, mineral nutrient concentration in S. halepense rhizomes remained remarkably constant despite differences in the external supply, evincing the priority given to vegetative reproduction at the expense of sexual reproduction. Overall, the pattern of nutrient allocation in S. bicolor seeds under different nutrient supply resembled the pattern observed in S. halepense rhizomes, but it had little resemblance to the pattern of nutrient allocation in S. halepense seeds. The results are discussed in terms of differences and similarities in the reproductive strategy of these two species.     

Species effects on resource supply rates: do they influence competitive interactions?

We examined two ways in which species effects on nitrogen supply rates could influence species competitive interactions and lead to coexistence between species with very different patterns of resource use. First, through differential effects on the rates of resource cycling, species may modify resources in divergent directions to better tolerate neighbors under self-modified conditions (tolerance modification). Second, plant-induced shifts in resource supply could enhance the degree to which species can suppress individuals (suppression modification). We addressed these hypothesized mechanisms in the non-successional moist meadow alpine tundra, which is codominated by Acomastylis rossii, a slow-growing roseaceous forb with high nutrient retention that is associated with slow rates of nitrogen supply, and Deschampsia caespitosa, a graminoid with rapid growth potential that is associated with fast rates of nitrogen (N) supply. Neighbors inhibited Acomastylis transplants (the species with high N retention) less than Deschampsia transplants (the species with rapid potential growth) in both neighborhood types, and hence neighborhood type did not influence species tolerance to neighbors. Likewise, Acomastylis neighborhoods inhibited transplant growth more than Deschampsia neighborhoods regardless of transplant species identity. When N supply rates were enhanced through repeated N additions in the two neighborhood types or in the presence of each species’ litter, the inhibitory effects associated with Acomastylis were offset to a greater degree than those associated with Deschampsia, as predicted by the suppression modification. These effects appeared to be density or size-dependent, with few effects observed at low density. Our results suggest that species effects on supply rates may influence competitive interactions, particularly if these effects are complemented by other sources of temporal or spatial variation such as pulses in resource availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of the invasive Centaurea maculosa and two native grasses to N-pulses

Nitrogen is often a limiting resource on semi-arid grasslands. During the growing season, N is often only available during short-term pulses associated with wetting events. The Eurasian forb Centaurea maculosa Lam. has invaded millions of hectares of semi-arid grasslands in western North America. C. maculosa's success could be attributed to greater use of N-pulses, or more efficient use of N supplied in those pulses compared with native grasses. In a glasshouse, C. maculosa and two native grasses, the caespitose Pseudoroegneria spicata [Scribn. and Smith] A. Love and the rhizomatous Pascopyrum smithii [Rybd.] A. Love, were established in mixed- and monoculture combinations, and then conditioned to weekly N-pulses of 8, 24, or 72 h for 8 weeks. These pulse durations are typical on semi-arid grasslands. At the end of the 8 weeks, plants were exposed to ¹⁵N-labeled nitrate (¹⁵NO₃ ^–) for 8 h and harvested 16 h later to compare short-term root uptake of ¹⁵NO₃ ^–. C. maculosa did not have greater enrichment (atom % ¹⁵N), rate of ¹⁵N-uptake (mol g^–1 h^–1), or ¹⁵N acquired (relative to ¹⁵N applied) than the grasses. C. maculosa's ¹⁵N-uptake per unit mass was relatively consistent across pulse durations, whereas ¹⁵N-uptake was lower at the longer pulse durations for the grasses. In general, C. maculosa acquired more of the applied ¹⁵N than P. spicata but less than P. smithii. ¹⁵N acquired was often influenced by the neighbour's identity. Regarding growth responses, C. maculosa produced more total biomass than the grasses, except for P. smithii plants growing with C. maculosa conditioned to 72 h pulses of N. Root mass ratios varied depending on the neighbor. Overall, C. maculosa used nitrogen less efficiently than the grasses. C. maculosa's success as an invasive species cannot be explained wholly by a greater response to N-pulses or more efficient use of N-pulses compared with native grasses with which it competes.     

Intraspecific heritable variation in life-history traits can alter the outcome of interspecific competition among insect herbivores

Competition is one of the most important biotic factors determining the structure of ecological communities. In this study, we show that there is variation in competitive ability between two clones of the pea aphid, Acyrthosiphon pisum, both of which out-compete a clone of the vetch aphid, Megoura viciae, in the laboratory. We tested whether this variation in competitive ability would alter the outcome of interspecific competition in the field. While one pea aphid clone followed the pattern set in the laboratory, out-competing the Megoura viciae clone, another showed the reverse effect with Megoura viciae dominating. These differences appear to be the result of variation in early population growth rate between the pea aphid clones, rather than predation, although predation did lead to the eventual extinction of colonies. We also questioned whether intra- and interspecific differences in predator escape behaviour could affect the outcome of competition in the field. All three clones responded similarly to the presence of foraging hoverfly larvae (Episyrphus balteatus), but the Megoura viciae clone dropped from the plant significantly less often in response to the presence of a foraging two-spot ladybird (Adalia bipunctata). This work provides evidence that intraspecific variation in competitive ability can alter the outcome of interspecific competitive interactions in nature and suggests that species–specific behavioural traits may have the potential to modify the outcome of these interactions.     

油茶低产林养分需求和时间配置动态变化研究

以常宁油茶低产林为研究对象,分析了油茶低产林树体各器官及土壤养分时间动态变化。结果表明:油茶低产林树体在不同生长期需求的大量元素均为全N、Ca和全K最多,而需求的Mg和全P最少;需求的微量元素均为Mn和Fe最多,Cu和Cd最少;春梢期供应的土壤养分元素是速效N和Mg,夏梢期主要供应的土壤元素是速效K、全N、全P、Fe和有机质,果实成熟期主要供应的土壤养分元素是全K,开花期主要供应的土壤元素是速效P和Ca;不同时间油茶低产林养分需求为春梢期(28.36%)夏梢期(26.17%)果实成熟期(22.75%)开花期(22.73%);土壤养分供应为夏梢期(2999.83±87.04 mg/kg)果实成熟期(2703.93±292.26 mg/kg)开花期(2554.60±508.84 mg/kg)春梢期(2385.88±199.62 mg/kg);油茶低产林在不同生长期需要的养分和土壤供应的养分并不一一对应。研究结果可为油茶低产林的施肥时间配置和养分时间变化提供科学依据,在春梢期多施肥。     

The resurrection of Erigeron parryi (Asteraceae)

Erigeron parryi was described in 1890 from a single collection from southwest Montana. No additional collections were made during the following 90 years. Recent floristic treatments have synonomized E. parryi under E. ochroleucus. I used principal components and discriminant analyses of morphological characters to examine the relationship of E. parryi to its presumed closest relatives, E. ochroleucus, E. radicatus, and E. scribneri, in light of several populations discovered in the past 20 years. Erigeron parryi should be considered a distinct species. It can be readily distinguished from closely related species based on indument and caudex morphology. It is allopatric with E. ochroleucus and E. scribneri and ecologically separated from E. radicatus. Erigeron scribneri is intermediate between E. ochroleucus and E. radicatus and is probably best treated as a variety of one or the other of these species.     

Differences in life history strategies between an invasive and a competing resident seed predator

Competitive interactions may arise from biological invasion if a successful invasive species requires the ecological niche of a resident one. Life-history traits that make a species a successful invader are of particular interest in elucidating both invasion success and how interspecific competition may emerge. In southeastern France, the invasion of cedar forests by the seed chalcid Megastigmus schimitscheki generated competitive relationships with the resident M. pinsapinis for the exploitation of the seed resource. Among the numerous ecological traits allowing these seed predators to exploit their niche successfully, the timing of adult emergence, initial egg load and age-specific realized fecundities of females were investigated to help understanding the issue of such interspecific relationships. Spring adult emergence of M. schimitscheki under natural conditions was significantly earlier than that of M. pinsapinis, suggesting that an advantage for the access to the seed resource for the invasive species may be associated with this trait. Initial egg load was significantly higher in M. schimitscheki than in M. pinsapinis and the analysis of age-specific realized fecundity in semi-natural conditions indicated that both M. schimitscheki and M. pinsapinis females lay a large proportion of their eggs during the early days of their lives. In the light of these findings, both earlier phenology and higher reproduction abilities of M. schimitscheki may have the potential to confer an advantage within a competitive context with M. pinsapinis through enhanced seed resource preemption. This may potentially explain the invasiveness of M. schimitscheki in southern France despite the presence of its closely related competitor M. pinsapinis.     

Nitrogen addition increases intraspecific competition in the invasive wetland plant Alternanthera philoxeroides,but not in its native congener Alternanthera sessilis

Nitrogen is often released in pulses with different frequencies, and N supply pulses may affect growth, reproduction, and biomass allocation of plants. However, few studies have examined how N supply pulses affect intraspecific competition of clonal plants and whether such an effect depends on the N supply amount. We grew one (no competition) or 12 ramets (with intraspecific competition) of both an invasive clonal plant Alternanthera philoxeroides and its native congener Alternanthera sessilis in five different N treatments: control (no N addition), low/high amount with low/high frequencies (pulses). Nitrogen addition significantly increased the growth of both species, while intraspecific competition decreased it. Nitrogen addition significantly increased intraspecific competitive intensity of A. philoxeroides as measured by the log response ratio of growth traits, but did not affect that of A. sessilis. Despite the N supply amount, N pulses had little effect on the growth and thus intraspecific competition of the two species. Therefore, increasing N deposition may change population structure and dynamics and the invasion succession of A. philoxeroides, but changes in N pulses may not.     

Uptake and transport of organic and inorganic nitrogen by arbuscular mycorrhizal fungi

New information on N uptake and transport of inorganic and organic N in arbuscular mycorrhizal fungi is reviewed here. Hyphae of the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (BEG 107) were shown to transport N supplied as ¹⁵N-Gly to wheat plants after a 48 h labelling period in semi-hydroponic (Perlite), non-sterile, compartmentalised pot cultures. Of the ¹⁵N supplied to hyphae in pot cultures over 48 h, 0.2 and 6% was transported to plants supplied with insufficient N or sufficient N, respectively. The increased ¹⁵N uptake at the higher N supply was related to the higher hyphal length density at the higher N supply. These findings were supported by results from in vitro and monoxenic studies. Excised hyphae from four Glomus isolates (BEG 84, 107, 108 and 110) acquired N from both inorganic (¹⁵NH₄ ¹⁵NO₃, ¹⁵NO₃ ⁻ or ¹⁵NH₄ ⁺) and organic (¹⁵N-Gly and ¹⁵N-Glu, except in BEG 84 where amino acid uptake was not tested) sources in vitro during short-term experiments. Confirming these studies under sterile conditions where no bacterial mineralisation of organic N occurred, monoxenic cultures of Glomus intraradices Schenk and Smith were shown to transport N from organic sources (¹⁵N-Gly and ¹⁵N-Glu) to Ri T-DNA transformed, AM-colonised carrot roots in a long-term experiment. The higher N uptake (also from organic N) by isolates from nutrient poor sites (BEG 108 and 110) compared to that from a conventional agricultural field implied that ecotypic differences occur. Although the arbuscular mycorrhizal isolates used contributed to the acquisition of N from both inorganic and organic sources by the host plants/roots used, this was not enough to increase the N nutritional status of the mycorrhizal compared to non-mycorrhizal hosts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Extensive summer water pulses do not necessarily lead to canopy growth of Great Basin and northern Mojave Desert shrubs

Plant species and functionally related species groups from arid and semi-arid habitats vary in their capacity to take up summer precipitation, acquire nitrogen quickly after summer precipitation, and subsequently respond with ecophysiological changes (e.g. water and nitrogen relations, gas exchange). For species that respond ecophysiologically, the use of summer precipitation is generally assumed to affect long-term plant growth and thus alter competitive interactions that structure plant communities and determine potential responses to climate change. We assessed ecophysiological and growth responses to large short-term irrigation pulses over one to three growing seasons for several widespread Great Basin and northern Mojave Desert shrub species: Chrysothamnus nauseosus, Sarcobatus vermiculatus, Atriplex confertifolia, and A. parryi. We compared control and watered plants in nine case studies that encompassed adults of all four species, juveniles for three of the species, and two sites for two of the species. In every comparison, plants used summer water pulses to improve plant water status or increase rates of functioning as indicated by other ecophysiological characters. Species and life history stage responses of ecophysiological parameters (leaf N, ¹⁵N, ¹³C, gas exchange, sap flow) were consistent with several previous short-term studies. However, use of summer water pulses did not affect canopy growth in eight out of nine comparisons, despite the range of species, growth stages, and site conditions. Summer water pulses affected canopy growth only for C. nauseosus adults. The general lack of growth effects for these species might be due to close proximity of groundwater at these sites, co-limitation by nutrients, or inability to respond due to phenological canalization. An understanding of the connections between short-term ecophysiological responses and growth, for different habitats and species, is critical for determining the significance of summer precipitation for desert community dynamics.     

Competition and coexistence of rhizomatous perennial plants along a nutrient gradient

I studied competition and coexistence of three tall clonal perennial plant species, Calamagrostis epigejos (L.) Roth, Solidago canadensis L., and Tanacetum vulgare L. along a gradient of soil productivity over five years. A replacement series field experiment was conducted with high, moderate and low fertility levels in 1m×1m plots. There were significant effects of soil type on ramet density (P<0.001), mean height (P<0.01), and total biomass (P<0.01). Ramet density, mean height, and total biomass increased with increasing soil fertility. There were also significant effects of mixture on ramet density (P<0.01), but not on mean height and total biomass for all species. Significant neighbor effects on ramet density and total biomass (P<0.01) were found for Solidago, showing that it is important whether Tanacetum or Calamagrostis is its neighbor within mixtures. During the five years there was only one case of competitive exclusion: Calamagrostis excluded Solidago on the most fertile substrate in the fifth growing season. In most cases species coexisted over the five years. Each of the three species was able to dominate in at least one combination of substrate type and mixture. The experiment showed that asymmetric competition for light on substrates of high fertility, symmetric competition for nutrients on nutrient-poor soil and positive interactions especially on substrates of intermediate fertility played a role. A founder effect was evident in aggregated mixtures of Calamagrostis and Solidago on the nutrient-rich substrate. A conceptual model of the relative importance of root competition for soil nutrients, shoot competition for light, and positive interactions along the fertility gradient is presented. The model emphasizes that positive interactions play an important role over a broad range of the productivity scale with a peak at intermediate levels of fertility. On the substrate of high productivity shoot competition for light is more important than positive interactions and root competition for soil nutrients as well. The competitive superiority of Calamagrostis on the most productive substrate was evident only in the long run. Rare events like extreme summer drought or selective herbivore pressure caused a switch in dominance in mixtures with Solidago, respectively Tanacetum. The guerrilla growth strategy of Calamagrostis and interference competition through a dense cover of aboveground biomass and litter could further cause competitive exclusion.     

Fluctuating resource availability increases invasibility in microbial microcosms

The fluctuating resource hypothesis (FRH) proposes that fluctuations in resource supply can temporally reduce competitive pressure from resident species, thereby providing ephemeral opportunities for invading species. Although FRH has the potential to integrate many existing hypotheses regarding mechanisms of community invasibility, previous tests and evaluations of FRH were based on single trophic level, did not take the timing effect into account, and had difficulties in distinguishing the effects of resource pulses from other simultaneous processes. Here we test FRH in multi‐trophic aquatic microcosms by creating resource pulses, by controlling resource quantity, propagule supply and pulse recurrence frequency, and by manipulating the timing of pulses relative to the timing of the arrival of new species (i.e. invaders) to local communities. The novelty of our work lies in that we directly manipulate resource pulse timing relative to invader introduction events and thus demonstrate the importance of this timing effect for community invasibility. Our study supports FRH in general: invasion success was positively related to resource pulses, and invaders had strong performance in treatments receiving coincident pulses, although not all invaders gained more benefit when resources were supplied at large‐magnitude than supplied at continuous rates. Since many ecosystems worldwide are experiencing high rates of anthropogenic nutrient input and increasing rates of precipitation, these ecosystems are potentially more fragile and susceptible to invasion. More experiments across multiple ecosystem types are needed to help formulate a general theory of community invasibility.     

Effects of soil organic matter and nitrogen supply on competition between Festuca ovina and Deschampsia flexuosa during inland dune succession

We tested the prediction that the successional replacement of plant species during succession on inland sand dunes results from the effects of an increase in nitrogen mineralization on competitive interactions. The growth and competitive strength of Festuca ovina and Deschampsia flexuosa on soil substrates with different amounts of soil organic matter or nitrogen supply were measured. Small tillers of Festuca ovina and Deschampsia flexuosa were grown in monocultures and 1:1-mixtures on soil columns with undisturbed layers of soil organic matter from different successional age. There was (a) no visible soil organic matter, (b) a thin soil organic layer (0.5 cm) and (c) a thicker soil organic layer (6.0 cm) present on the soil columns. The species were also grown on columns with no visible soil organic matter (bare sand) with two different levels of N fertilization to mimic the increased N mineralization in the older successional stages.In monoculture, Festuca produced more biomass on the substrates with a soil organic layer compared to the unfertilized sand substrate. It also produced more biomass on sand substrates with N fertilization. Deschampsia produced more biomass in treatments with a soil organic layer compared to the bare sand treatments, but did not respond to the ammonium-nitrate addition. In competition, Festuca seemed to be the stronger competitor on the unfertilized sand substrate. Festuca was also the better competitor on the N fertilized sand treatments, while on the treatments with a soil organic layer Deschampsia was the winning species. Our results do not support the hypothesis that an increase in N supply is responsible for the replacement of Festuca by Deschampsia that concur with the accumulation of soil organic matter during succession in inland dunes.     

Interactions between a detrital resource pulse and a detritivore community

Detritivore communities influence the decomposition of detrital resources in virtually all natural systems. Conversely, detrital resources can also have considerable bottom-up effects on detritivore communities. While many investigations have examined detritivory and decomposition processes, few have considered interactions between detritivores and detritus as concurrent processes in the same system, or in the context of natural detrital pulses. In many systems, resource pulses contribute substantial detrital inputs to belowground systems. These detrital pulses may influence interactions between the detritivore community and detrital decomposition. I conducted field experiments to investigate interactions between detrital resource pulses of periodical cicada (Magicicada spp.) carcasses and scavenging detritivorous macroarthropods. Cicada litterfall pulses influenced several broad groups in the macroarthropod community, including relatively specialized necrophilous taxa and relatively generalized detritivores, omnivores and predators. Conversely, detritivore activity increased the rate of cicada carcass decomposition by 4,082% compared to caged control carcasses. These results suggest that interactions between pulses of cicada detritus and the detritivore community influence both the persistence of ephemeral detrital resources, and the distribution, abundance and behavior of detritivore populations.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Competition between the green alga Scenedesmus and the cyanobacterium Synechococcus under different modes of inorganic nitrogen supply

In this study, we evaluated growth responses of the green alga Scenedesmus and the cyanobacterium Synechococcus supplied with inorganic nitrogen in different ways. A competitive situation in which nitrogen was limiting was created in mixed cultures as well as in cultures growing in the same vessel but separated by a permeable dialysis membrane. Supplying inorganic nitrogen in small pulses at a high frequency favoured the cyanobacterium Synechococcus, whereas batch additions favoured the green alga Scenedesmus. When using a large-pulse/low-frequency supply mode, the yield of the green alga was higher when ammonium was added as nitrogen source compared to when nitrate was added. By contrast, the yield of the cyanobacterium was higher in the nitrate regime. However, uptake experiments using unialgal cultures showed that both organisms depleted the medium of ammonium more rapidly than they depleted the medium of nitrate; i.e. the higher yield of the cyanobacterium in the nitrate regime than in the ammonium regime can be attributed to the effects of competition with the green alga. Since nitrate assimilation involves the consumption of reductive power, we suggest that the outcome of competition was governed by the fact that green alga was light limited and therefore better able to compete for ammonium than for nitrate. The results from the laboratory studies are discussed in relation to results from an enclosure experiment performed in Lake Erken, Sweden. In that field experiment, in which additions of both phosphate and ammonium were applied every second day to 350-l enclosures, the green algal biomass increased exponentially during an incubation period of 22 days.     

The timing and degree of root proliferation in fertile-soil microsites for three cold-desert perennials

Summary Root proliferation in nutrient-rich soil patches is an important mechanism facilitating nutrient capture by plants. Although the phenomenon of root proliferation is well documented, the specific timing of this proliferation has not been investigated. We studied the timing and degree of root proliferation for three perennial species common to the Great Basin region of North America: a shrub, Artemisia tridentata, a native tussock grass, Agropyron spicatum, and an introduced tussock grass, Agropyron desertorum. One day after we applied nutrient solution to small soil patches, the mean relative growth rate of Agropyron desertorum roots in these soil patches was two to four times greater than for roots of the same plants in soil patches reated with distilled water. Most of the increased root growth came from thin, laterally branching roots within the patches. This rapid and striking root proliferation by Agropyron desertorum occurred in response to N-P-K enrichment as well as to P or N enrichment alone. A less competitive bunchgrass, Agrophyron spicatum, showed no tendency to proliferate roots in enriched soil patches during these two-week experiments. The shrub Artemisia tridentata proliferated roots within one day of initial solution injection in the N-enrichment experiment, but root proliferation of this species was more gradual and less consistent in the N-P-K and P-enrichment experiments, respectively. The ability of Agropyron desertorum to proliferate roots rapidly may partly explain both its general competitive success and its superior ability to exploit soil nutrients compared to Agropyron spicatum in Great Basin rangelands of North America.     

Nitrogen preferences and plant-soil feedbacks as influenced by neighbors in the alpine tundra

Plant resource partitioning of chemical forms of nitrogen (N) may be an important factor promoting species coexistence in N-limited ecosystems. Since the microbial community regulates N-form transformations, plant partitioning of N may be related to plant–soil feedbacks. We conducted a ¹⁵N tracer addition experiment to study the ability of two alpine plant species, Acomastylis rossii and Deschampsia caespitosa, to partition organic and inorganic forms of N. The species are codominant and associated with strong plant–soil feedbacks that affect N cycling. We manipulated interspecific interactions by removing Acomastylis or Deschampsia from areas where the species were codominant to test if N uptake patterns varied in the presence of the other species. We found that Deschampsia acquired organic and inorganic N more rapidly than Acomastylis, regardless of neighbor treatment. Plant N uptake—specifically ammonium uptake—increased with plant density and the presence of an interspecific neighbor. Interestingly, this change in N uptake was not in the expected direction to reduce niche overlap and instead suggested facilitation of ammonium use. To test if N acquisition patterns were consistent with plant–soil feedbacks, we also compared microbial rhizosphere extracellular enzyme activity in patches dominated by one or the other species and in areas where they grew together. The presence of both species was generally associated with increased rhizosphere extracellular enzyme activity (five of ten enzymes) and a trend towards increased foliar N concentrations. Taken together, these results suggest that feedbacks through the microbial community, either in response to increased plant density or specific plant neighbors, could facilitate coexistence. However, coexistence is promoted via enhanced resource uptake rather than reduced niche overlap. The importance of resource partitioning to reduce the intensity of competitive interactions might vary across systems, particularly as a function of plant-soil feedbacks.