Experimental Monocultures Research Articles

Mycotoxins in Vegetating Corn Plants from an Experimental Monoculture

Mycotoxins in Vegetating Corn Plants from an Experimental Monoculture

Microbial community attributes supersede plant and soil parameters in predicting fungal necromass decomposition rates in a 12-tree species common garden experiment

Although dead fungal mycelium (necromass) represents a key component of biogeochemical cycling in all terrestrial ecosystems, how different ecological factors interact to control necromass decomposition rates remains poorly understood. This study assessed how edaphic parameters, plant traits, and soil microbial community structure predicted the mass loss rates of different fungal necromasses within experimental monocultures of 12 tree species in Minnesota, USA. Necromass decay rates were most strongly driven by initial chemical composition, being significantly slower for fungal necromass with higher initial melanin content. Of the extrinsic ecological factors measured, variation in the amount of mass remaining for both low and high melanin necromass types was significantly predicted by soil bacterial richness and fungal community composition, but not by any soil microclimatic parameters or plant traits. Further, the microbial communities governing decay rates varied depending on the initial necromass chemical composition, suggesting that extrinsic and intrinsic factors interacted to propel decomposition. Finally, we also found significant positive relationships between the amount of remaining fungal necromass and soil carbon and nitrogen concentrations. Collectively, these results suggest that, after the initial chemical composition of dead fungal residues, soil microbial communities represent the main drivers of soil necromass degradation, with potentially large consequences for soil carbon sequestration and nutrient availability.

Influence of fertilization systems to the soil inorganic compounds. Case study: monocul-tures of cereals, leguminous and oleaginous

<p>The main objective of this article is to observe the variation of the content of inorganic compounds in the soil (cobalt, selenium, barium) under the influence of fertilization systems applied to different monocultures of cereals (wheat, corn), leguminous (peas) and oleaginous (sunflower). High concentrations of cobalt, selenium and barium with phytotoxicity effect on the soil and future crops were recorded mainly in the experimental parcels cultivated with corn, respectively with sunflower and fertilized with farmyard manure. The highest phytotoxicity effect on soil and future crop was recorded in the case of cobalt, for the experimental monoculture variant of corn fertilized with farmyard manure where the cobalt concentration in the soil exceeded the alert threshold / sensitive use (AT/SU) for inorganic compounds in the soil (51.2 mg/kg d.m.).</p>

Resistance of tropical seedlings to drought is mediated by neighbourhood diversity.

Occasional periods of drought are typical of most tropical forests, but climate change is increasing drought frequency and intensity in many areas across the globe, threatening the structure and function of these ecosystems. The effects of intermittent drought on tropical tree communities remain poorly understood and the potential impacts of intensified drought under future climatic conditions are even less well known. The response of forests to altered precipitation will be determined by the tolerances of different species to reduced water availability and the interactions among plants that alleviate or exacerbate the effects of drought. Here, we report the response of experimental monocultures and mixtures of tropical trees to simulated drought, which reveals a fundamental shift in the nature of interactions among species. Weaker competition for water in diverse communities allowed seedlings to maintain growth under drought while more intense competition among conspecifics inhibited growth under the same conditions. These results show that reduced competition for water among species in mixtures mediates community resistance to drought. The delayed onset of competition for water among species in more diverse neighbourhoods during drought has potential implications for the coexistence of species in tropical forests and the resilience of these systems to climate change.

Resource Availability Alters Biodiversity Effects in Experimental Grass-Forb Mixtures.

Numerous experiments, mostly performed in particular environments, have shown positive diversity-productivity relationships. Although the complementary use of resources is discussed as an important mechanism explaining diversity effects, less is known about how resource availability controls the strength of diversity effects and how this response depends on the functional composition of plant communities. We studied aboveground biomass production in experimental monocultures, two- and four-species mixtures assembled from two independent pools of four perennial grassland species, each representing two functional groups (grasses, forbs) and two growth statures (small, tall), and exposed to different combinations of light and nutrient availability. On average, shade led to a decrease in aboveground biomass production of 24% while fertilization increased biomass production by 36%. Mixtures were on average more productive than expected from their monocultures (relative yield total, RYT>1) and showed positive net diversity effects (NE: +34% biomass increase; mixture minus mean monoculture biomass). Both trait-independent complementarity effects (TICE: +21%) and dominance effects (DE: +12%) positively contributed to net diversity effects, while trait-dependent complementarity effects were minor (TDCE: +1%). Shading did not alter diversity effects and overyielding. Fertilization decreased RYT and the proportion of biomass gain through TICE and TDCE, while DE increased. Diversity effects did not increase with species richness and were independent of functional group or growth stature composition. Trait-based analyses showed that the dominance of species with root and leaf traits related to resource conservation increased TICE. Traits indicating the tolerance of shade showed positive relationships with TDCE. Large DE were associated with the dominance of species with tall growth and low diversity in leaf nitrogen concentrations. Our field experiment shows that positive diversity effects are possible in grass-forb mixtures irrespective of differences in light availability, but that the chance for the complementary use of resources increases when nutrients are not available at excess.

Inhibitory effects of soil biota are ameliorated by high plant diversity

The idea that plant communities with high species diversity are more stable, productive, and resistant to invasion at small spatial scales has become an important ecological paradigm. Recently, the role of soil biota has emerged as a major driver of this relationship between plant species diversity and ecosystem function. In greenhouse experiments, we found that soil collected from experimentally constructed species-rich plant assemblages (that originally contained between 10 and 16 species) promoted the growth of 4 native target plant species more than soil from species-poor communities (that originally contained between 2 and 5 species). Sterilization of soils from species-poor communities improved the growth of these target species more than sterilization of soils from species-rich plant communities, indicative that inhibitory soil biota had greater negative impacts on plant growth in low versus high diversity soils. These results suggest that strong soil biota effects in soils do not simply accrue in experimental monocultures, but can occur in low diversity assemblages that are more realistic of what occurs in nature. Our findings suggest a mechanistic explanation for the diversity-productivity relationship, and further support the importance of inhibitory soil biota as significant contributors to spatial and temporal patterns of abundance in natural plant communities through negative plant-soil feedback.

Predicting invertebrate herbivory from plant traits: evidence from 51 grassland species in experimental monocultures

Invertebrate herbivores can impact plant performance and plant communities. Conversely, plants can affect the ability of herbivores to find, choose, and consume them through their functional traits. While single plant traits have been related to rates of herbivory, most often involving single herbivore-plant pairs, much less is known about which suite of plant traits is important for determining herbivory for a pool of plant species interacting with a natural herbivore community. In this study we measured aboveground herbivore damage on 51 herbaceous species growing in monocultures of a grassland biodiversity experiment and collected 42 different plant traits representing four trait groups: physiological, morphological, phenological, and herbivore related. Using the method of random forests and multiple regression, we identified seven traits that are important predictors of herbivore damage (leaf nitrogen and lignin concentration, number of coleopteran and hemipteran herbivores potentially feeding on the plants, leaf life span, stem growth form, and root architecture); leaf nitrogen and lignin concentration were the two most important predictors. The final model accounted for 63% of the variation in herbivore damage. Traits from all four trait groups were selected, showing that a variety of plant characteristics can be statistically important when assessing folivory, including root traits. Our results emphasize that it is necessary to use a multivariate approach for identifying traits affecting complex ecological processes such as herbivory.

Tidal events and salt‐marsh structure influence black mangrove (Avicennia germinans) recruitment across an ecotone

Field experiments were conducted at a black mangrove-salt-marsh ecotone in southwest Florida (U.S.A.) to investigate retention of propagules of the black mangrove, Avicennia germinans, by salt-marsh plants as a mechanism of facilitation operating on recruitment success at landward boundaries. Buoyant A. germinans propagules are dispersed by tides, and stranding is required for establishment; therefore, processes that enable stranding should facilitate mangrove recruitment. We expected the physical structure of salt-marsh vegetation to define propagule retention capacity, and we predicted that salt-marsh plants with distinct growth forms would differentially retain propagules. Experimental monoculture plots (1 m2) of salt-marsh plants with different growth forms (Sporobolus virginicus [grass], Sesuvium portulacastrum [succulent forb], and Batis maritima [succulent scrub]) were created, and A. germinans propagules were emplaced into these plots and monitored over time. For comparison, propagules were also placed into natural polyculture plots (1 m2). Polyculture plots contained at least two of the salt-marsh plant taxa selected for monoculture treatments, and S. virginicus was always present within these polyculture plots. Natural polyculture plots retained 59.3% +/- 11.0% (mean +/- SE) of emplaced propagules. Monocultures varied in their propagule retention capacities with plots of S. virginicus retaining on average 65.7% +/- 11.5% of transplanted propagules compared to 7.2% +/- 1.8% by B. maritima and 5.0% +/- 1.9% by S. portulacastrum. Plots containing S. virginicus retained a significantly greater percentage of emplaced propagules relative to the two succulent salt-marsh taxa. Furthermore, propagule entrapment, across all treatments, was strongly correlated with salt-marsh structure (r2 = 0.6253, P = 0.00001), which was estimated using an indirect quantitative metric (lateral obstruction) calculated from digital images of plots. Overall, our findings imply that entrapment of propagules by salt-marsh plants may be facilitative if propagules are dispersed beyond the established tree line by spring or storm tides, and that facilitation may be sustained over time. We conclude that salt-marsh ecotone permeability may modulate landward encroachment by A. germinans, and that interactions among the early life history stages of black mangroves and neighboring plants may direct community responses to climate change.

Purple loosestrife suppresses plant species colonization far more than broad‐leaved cattail: experimental evidence with plant community implications

Summary 1. Dominant plant species, whether native or invasive, often change community composition and cause decreases in diversity. Still invasive species are considered more deleterious to communities than dominant natives, although evidence for this is surprisingly rare. We tested two hypotheses: (i) an exotic invasive species will have greater impacts at the community level than a dominant native and (ii) this deleterious impact will be exacerbated with eutrophication. 2. Both hypotheses were tested by evaluating colonizer success in large, well‐replicated experimental monocultures of two dominant and widespread wetland species, invasive purple loosestrife (Lythrum salicaria) and native broad‐leaved cattail (Typha latifolia). To facilitate comparisons, we standardized competitive effects by canopy biomass and by light availability beneath the canopy. The latter is a novel approach that accounts directly for resource reductions caused by community dominants. 3. Loosestrife was particularly detrimental to rare species and dramatically reduced colonizer success compared to cattail by nearly all of our metrics, including colonizer biomass (50.2% lower), species richness (34.2% lower), Shannon diversity (35.8% lower) and the proportion of mesocosms that were colonized (38.5% lower). Moreover, 15 of 16 uncommon species failed to colonize loosestrife communities. Graminoids fared poorly in loosestrife monocultures, but forb biomass (predominantly Sagittaria latifolia) was 3.5 times higher there. These results suggest that over time, plant communities under loosestrife canopies will contrast sharply with those under cattail. Contrary to our second hypothesis, fertilization did not exacerbate loosestrife’s ability to suppress colonizers, relative to that of cattail. 4. Canopy biomass and light attenuation were similar for cattail and loosestrife, yet biomass explained little variation in colonizer success. Increasing light availability in the understorey increased colonizer richness and diversity only under cattail canopies, suggesting loosestrife suppresses colonization via below‐ground competition while cattail does so via light reduction. 5. Synthesis. Ours is the first study to show that an invasive species suppresses colonizers much more than a dominant native and to identify contrasting mechanisms by which this may occur. Biomass‐based comparisons of competitive effects may have limited utility for highly productive dominant species generally, thus our approach also offers a viable new alternative that could be applied broadly.

Native shrub reestablishment in exotic annual grasslands: Do ecosystem processes recover?

The impacts of exotic plant species on ecosystem processes are well established, motivating numerous efforts to facilitate native-species recovery. Nonetheless, how the return of native species influences ecosystem processes and how these changes feed back to influence the recovery process are poorly understood. We examined these questions in exotic annual grasslands on Santa Cruz Island, California, USA, where the removal of nonnative herbivores has led to the recovery of the native shrubs Artemisia californica and Eriogonum arborescens. To examine the influence of shrub colonization on nutrient cycling, and the mechanisms by which these changes arise, we measured available nitrogen and phosphorus, and quantified nitrogen mineralization and litterfall rates under shrubs and grasses in the field and in experimental monoculture plots. Both native shrubs altered nitrogen cycling as they colonized the grassland, but they did so in opposite directions. Eriogonum depressed nitrogen pools and mineralization rates via large inputs of nitrogen-poor litter. In contrast Artemisia increased nitrogen and phosphorus pools and nitrogen mineralization rates. Last, to determine if shrub effects on soils favor shrubs or grasses, we conducted a nitrogen and phosphorus fertilization experiment in the field. Only the exotic grass was significantly limited by nitrogen. Thus the depressed nitrogen availability associated with Eriogonum colonization is more harmful to exotic grasses than to the native shrub. By contrast, the elevated nitrogen associated with recovering Artemisia favors grasses over the shrub, possibly hindering recovery of the native. Mechanistic studies of the ecosystem ,impacts of native-plant recovery are useful for managers wishing to predict which native species return ecosystem function, and whether such changes feed back to influence native recovery.

The role of plant species size in invasibility: a field experiment

Large plant species self-thin to disproportionately lower densities than smaller plant species, and therefore may leave more patches of unused space suitable for invasion. Using experimental monocultures of 11 old-field perennial plant species differing in maximum size, as well as mixtures composed of all monoculture species, we tested our primary hypothesis that monocultures of larger species will be more susceptible to natural invasion. After 3 years, monocultures of larger species were invaded by a significantly greater number of species, and more ramets, from the surrounding vegetation. Invading plant species were significantly smaller than the monoculture species being invaded, suggesting that smaller plant species may be better invaders. Thus, we quantified a trade-off between species size, which is frequently associated with increased competitive ability for light, and invasibility, suggesting one reason why large and small species coexist in virtually all plant communities. Although we expected that invasion would enhance biomass production by more fully capturing available resources, we found that the most highly invaded plots of each species produced significantly less biomass. This suggests that increased diversity resulting from invasion did not result in complementary resource use. Mixture plots containing all experimental species did not admit a significantly different number of invading ramets or species than most monocultures, indicating no obvious role for diversity in resistance to invasion, or complementary resource use. Our results suggest that relatively large species may be limited in their capacity to competitively exclude other, smaller species from communities because pure stands of the former are more susceptible to invasion by the latter.

Species and Spacing Effects of Northern Conifers on Forest Productivity and Soil Chemistry in a 50-Year-Old Common Garden Experiment

Species and Spacing Effects of Northern Conifers on Forest Productivity and Soil Chemistry in a 50-Year-Old Common Garden Experiment

Aboveground productivity and root–shoot allocation differ between native and introduced grass species

Plant species in grasslands are often separated into groups (C(4) and C(3) grasses, and forbs) with presumed links to ecosystem functioning. Each of these in turn can be separated into native and introduced (i.e., exotic) species. Although numerous studies have compared plant traits between the traditional groups of grasses and forbs, fewer have compared native versus introduced species. Introduced grass species, which were often introduced to prevent erosion or to improve grazing opportunities, have become common or even dominant species in grasslands. By virtue of their abundances, introduced species may alter ecosystems if they differ from natives in growth and allocation patterns. Introduced grasses were probably selected nonrandomly from the source population for forage (aboveground) productivity. Based on this expectation, aboveground production is predicted to be greater and root mass fraction to be smaller in introduced than native species. We compared root and shoot distribution and tissue quality between introduced and native C(4) grass species in the Blackland Prairie region of Central Texas, USA, and then compared differences to the more well-studied divergence between C(4) grasses and forbs. Comparisons were made in experimental monocultures planted with equal-sized transplants on a common soil type and at the same density. Aboveground productivity and C:N ratios were higher, on average, in native grasses than in native forbs, as expected. Native and introduced grasses had comparable amounts of shallow root biomass and tissue C:N ratios. However, aboveground productivity and total N were lower and deep root biomass and root mass fraction were greater in native than introduced grasses. These differences in average biomass distribution and N could be important to ecosystems in cases where native and introduced grasses have been exchanged. Our results indicate that native-introduced status may be important when interpreting species effects on grassland processes like productivity and plant N accumulation.

Incidence and severity of viral disease symptoms on eastern gamagrass within monoculture and polycultures

Sugarcane mosaic virus strain maize dwarf mosaic virus B (SCMV-MDMV-B) and maize dwarf mosaic virus (MDMV) are aphid-vectored potyviruses that infect eastern gamagrass ( Tripsacum dactyloides (L.) L.), a perennial grain which has potential as a human or animal foodstuff. The potential to manage these diseases through intercropping eastern gamagrass with other, non-host perennial species was examined by monitoring incidence and severity on eastern gamagrass within experimental monocultures, two-species mixtures with wildrye ( Leymus racemosus (Lam.) Tsvelev), two-species mixtures with Illinois bundleflower ( Desmanthus illinoensis (Michx.) MacM., Leguminosae), and three-species mixtures with wildrye and Illinois bundleflower at two sites in Kansas, USA, from 1991 to 1995. Wildrye and bundleflower, which are also potential grain crops, are not reported to be hosts of the viruses. At Site 1, in 1991 and 1992, disease was generally less frequent and less severe in biculture with bundleflower and in the three-species mixture than in monoculture or the biculture with wildrye. No treatment effects were observed at Site 2. Disease severity was negatively correlated with plant size and seed yield at Site 1. Seed yield per plant was generally higher in polyculture than in monoculture. The results suggest polyculture may contribute to the management of aphid-borne plant disease within plantings of perennial grains.

Density of Anomoea flavokansiensis on Desmanthus illinoensis in monoculture and polyculture

AbstractThe effect of plant species diversity on the density of the herbivore, Anomoea flavokansiensis Moldenke (Coleoptera: Chrysomelidae), on Desmanthus illinoensis (Michaux) MacMillan (Mimosaceae), a promising North American legume for exploring the principles of diverse, perennial grain agriculture was examined. From mid‐June to early August A. flavokansiensis feeds on young leaves and inflorescences of D. llinoensis. At high density, A. flavokansiensis potentially reduces seed yield and is thus an important consideration for long‐term stands that are to be grown without insecticides. The potential to manage this insect via intercropping its host species with other, non‐host perennial species by monitoring A. flavokansiensis density on D. illinoensis within experimental monocultures, two‐species mixtures with Tripsacum dactyloides, and three‐species mixtures with T. dactyloides and Leymus racemosus was examined. Insects were censused 2–3 times weekly from mid‐June to early August at two sites from 1991 to 1995. In the first three years, beetle density was generally low (<1 per plant), and did not differ among treatments. In the fourth year, however, beetle density peaked at 15 and 25 insects per plant at the two sites, and was highest within monoculture for most dates. In 1995, density was again low, but tended to remain higher in monoculture at one site. The results suggest that beetle density on D. illinoensis can be reduced in polyculture and may hold promise for the management of this insect herbivore within perennial grain polycultures.

Beetle Folivory Increases Resource Availability and Alters Plant Invasion in Monocultures of Goldenrod

One way that insect herbivores can influence plant community structure is by altering the ambient availability of resources required by plants. To examine the importance of this mechanism, I tested the following three hypotheses in a field experiment in 1990: (H1) Folivory by the leaf beetle Trirhabda canadensis (Coleoptera: Chrysomelidae) increases availability of light, water, and N in monocultures of goldenrod (Solidago missouriensis). This hypothesis was partially supported. Folivory by Trirhabda reduced leaf, root, and total biomass of goldenrod and increased light penetration, soil water content, and soil nitrate concentration. However, Trirhabda grazing did not affect overall soil N availability. (H2) The invasion of goldenrod monocultures by other plant species and the response of goldenrod to folivory reflects an increased availability of resources in monocultures grazed by Trirhabda. This hypothesis was supported. Folivory by Trirhabda raised the probability that a species would invade the experimental monocultures and increased the species richness of the invading plant assemblage. Relative growth rate of goldenrod and production of aboveground biomass by invaders were higher in monocultures grazed by Trirhabda than in ungrazed ones. Prostrate and creeping forbs increased in relative abundance following folivory by Trirhabda. (H3) Reduced N uptake by goldenrod or increased N mineralization accompanies increased N availability in grazed monocultures. This hypothesis was also supported. Net N mineralization and nutrification were higher in monocultures grazed by Trirhabda than in ungrazed monocultures. Folivory by Trirhabda larvae reduced N uptake by goldenrod, but that of adults did not. The impact of herbivores upon plant communities may be effected through increased availability of important plant resources.

Calamagrostis Canescens: Population Biology of a Clonal Grass Invading Wetlands

The preading ability of Calamagrostis canescens, a long-lived perennial grass of Central Europe, is exemplified by its behaviour in a sedge fen near Třeboň, Czechoslovakia. The process of invasion was observed within a stable community in the field. The hypothesis that the development of an invader is species-specific and does not depend on the original occupants was tested comparing natural invasion of communities by C. canescens with establishment of the species in experimental monocultures. The final stage of extensive spreading of the species was traced along a 510-m-long linear transect between 1982 and 1989. Closed stands with the maximum cover abundance (60 to 70%) and aboveground biomass (426 g m −2 ) were achieved within 5 to 6 years from a low initial occupation density of 1.5 seedlings m −2 (...)

Including competitive asymmetry in measures of local interference in plant populations.

Although considerable evidence exists that plant competition is generally asymmetric or "one-sided", with larger plants having a disproportionate competitive effect on smaller plants, currently employed measures of local interference generally assume that competition is "two-sided". We describe a simple measure of competitive asymmetry in which the effects of neighbors smaller than a focal individual are discounted by a constant factor, and include this variable in a composite measure of local interference. In this model competition varies between complete asymmetry (the effects of smaller plants are entirely discounted) and complete symmetry (the competitive effect of a neighbor is proportional to its size). The proposed method is applied to two natural populations and one experimental monoculture. In all cases an asymmetric model provides the best fit to the data. Completely two-sided models account for 26-39% of the variance in relative growth rate, while relatively one-sided models account for 44-57%. The increases in r 2 values resulting from the inclusion of asymmetry are significant in the two cases in which the data permit randomization tests. Our results suggest that interference is completely asymmetric in a population of Impatiens pallida, a species with very low root allocation and a shallow crown, and somewhat less asymmetric in an experimental monoculture of Ambrosia artemisiifolia and a natural stand of Pinus rigida, cases in which competition for water and nutrient resources is likely to be of greater importance.

The Ecology of a Montane Grassland in Sri Lanka: V. Interference in Populations of Four Major Grasses

(1) The effects of density, time of clipping and time of establishment on interference between individuals were investigated in experimental monocultures and two-species mixtures of Cymbopogon nardus, Eulalia trispicata, Pennisetum polystachyon and Themeda tremula from a montane grassland in Sri Lanka. (2) Increasing density considerably reduced vegetative size and reproductive potential of the four species. In mixtures, Pennisetum polystachyon suppressed, and Eulalia trispicata was suppressed by, every companion species; Themeda tremula depressed the growth of Cymbopogon nardus. (3) The competitive ability of the more aggressive species was accentuated by clipping, while that of the less aggressive species was reduced. (4) The time of establishment modified the competitive advantage of species; earlysown plants severely suppressed the late-sown plants regardless of the potential vigour, aggressiveness and competitive ability. (5) The results are discussed with special reference to the importance of human impact on the dynamics of these species in the grassland.