Leptochloa Dubia Research Articles

Emergence and development of Chihuahuan Desert grasses under simulated wet-dry sequences

We evaluated the response (emergence, speed of germination, survival) of nine grasses (six native, three introduced) commonly occurring in the State of Chihuahua, Mexico to four irrigation “wet-dry” sequences that simulate sequences of days with and without rainfall, under greenhouse conditions. The sequences provide a total of 120, 100, 80 and 60 mm (high, medium, low, and very low) of water for 40 days. The native species were Bouteloua curtipendula, B. dactyloides, B. eriopoda, B. gracilis, Digitaria californica, and Leptochloa dubia. The introduced grasses were Cynodon dactylon, Eragrostis curvula, and E. superba. A decrease in water, resulting from the drier sequences, delayed the emergence from 4 to 7 days in most species. The native B. dactyloides displayed the best potential for growth with high emergence (18%) and high survival (44%) under low or very low water sequences. In general, survival for native species was higher than for introduced species under water restrictions. The native species showed high survival and high root:shoot ratio (RSR) under drought, which was not observed in introduced species. As expected, we found that plant species responded differently to modeled rainfall sequences, providing insight into plant adaptations to drought and potential for survival under environmental variation.

Fungal endophytes of the invasive grass Eragrostis lehmanniana shift metabolic expression in response to native and invasive grasses

Plant-fungal interactions shape ecosystem dynamics and are increasingly recognized as important in the success of invasive plants. Although diverse fungal endophytes are known to inhabit plants, including grasses, the precise chemical mechanisms through which they influence their hosts remain inadequately understood. We used untargeted metabolomics to characterize substrate use and compound production of three fungal endophytes isolated from an invasive grass, Eragrostis lehmanniana, characterizing the metabolome of these fungal isolates grown alone (axenically) and in the presence of seeds from invasive E. lehmanniana and co-occurring native grasses (E. intermedia, Bouteloua curtipendula, and Leptochloa dubia). We found that each fungal isolate expressed a different metabolic profile in response to Eragrostis seeds, relative to seeds of non-Eragrostis native grasses. Coupled with results of germination trials, these findings suggest that plant-fungal interactions mediated by the fungal metabolome may play a key role in determining the success of a major invasive species.

Potential Distribution of Five Native Grass Species in Northern Mexico and their Dynamics due to Climate Variability

Grasslands are recognized as the most threatened communities on the planet due to environmental and anthropogenic disturbances. In the state of Chihuahua, Mexico, native grasslands have a great ecological importance due to their wide distribution; however, studies on the current and future potential distributions of grasslands, as well as their temporal dynamics based on climatic variability, are lacking. Thus, we analysed the potential distribution of five native grasses (Bouteloua curtipendula, Bouteloua gracilis, Leptochloa dubia, Digitaria californica and Setaria macrostachya) based on climatic factors. The present data of these species were consulted at the Global Biodiversity Information Facility (GBIF) and recorded with location information: latitude and longitude. This database was complemented with presence data of the five species from previous studies. To characterize the climate niche, the values of the climatic factors at the locations of the species presence, were obtained from the WorldClim. Then, current and future potential distributions of these native species, were generated based on three climate change scenarios, based on Representative Concentration Pathways - RCP2.6, RCP4.5 and RCP6.0). Under the RCP2.6 scenario, the estimated area of S. macrostachya was 12,525.5 ha, representing 7.4% of the state. For B. gracilis, the potential areas under the RCP4.5 and RCP6.0 scenarios were 20,391.49 and 23,079.08 ha, respectively. B. curtipendula represented the smallest area, with 4.6% under the scenario RCP2.6. The distribution of grassland species in northern Mexico is mainly determined by climatic factors; as it may apply for the rest of the grassland species within this area, so those results could increase our knowledge about their potential distribution.

ReducingEragrostis lehmannianapopulations by preparing seedbeds with unconventional tillage implements and seeding in a semiarid grassland

AbstractThe invasion of Lehmann lovegrass (Eragrostis lehmannianaNees) in rangelands of Chihuahua, Mexico, has resulted in a need for revegetation to recover lost forage productivity. Thus, new knowledge on generating alternatives to improve these invaded grasslands is of great importance. This study evaluated seedbeds prepared with unconventional tillage implements and seeded with a grass mixture to reduce the plant density ofE. lehmannianawhile increasing the productivity of an invaded semiarid grassland of Chihuahua. The unconventional tillage implements were: a Rangeland Harrow, which was used to prepare the Striped Harrowing and Full Harrowing seedbeds; Rangeland Rehabilitator, which was used to prepare the Deep-Stingray Subsoiler seedbed; and a Tandem-type Aerator Roller, which was used to prepare the Double-Digging Aeration seedbed. An area without tillage was left as a control. The seed mixture was composed of blue grama [Bouteloua gracilis(Willd. ex Kunth) Lag. ex Griffiths var. Hachita] (25%); sideoats grama [Bouteloua curtipendula(Michx.) Torr. ‘6107 Kansas’] (25%); green sprangletop [Leptochloa dubia(Kunth) Nees var. Van Horn] (5%); weeping lovegrass [Eragrostis curvula(Schrad.) Nees var. Ermelo] (40%), and Columbus grass [Sorghum almumParodi] (5%). The experiment was conducted across 4 yr, and the evaluation started at the second year. Plant density and dry matter (DM) production were evaluated per species. In the control plot, the plant density ofE. lehmannianaincreased approximately 180% from the 2nd to the 4th year (18 to 50 plants m−2). The use of unconventional tillage implements for seedbed preparation and the inclusion ofE. curvulain the seed mixture decreasedE. lehmannianadensity in more than 50% of plots and increased DM production in around 100% of plots. Considering the whole experimental period, in all the prepared seedbed treatments,E. curvulahad the highest establishment and DM production of all the seeded species. The native speciesB. gracilis,B. curtipendula, andL. dubiahad poor establishment in all the prepared seedbeds.

Trampling and Cover Effects on Soil Compaction and Seedling Establishment in Reseeded Pasturelands Over Time

A field study in Randall County, Texas, was conducted to determine how soil bulk density and plant cover change over time in response to deferment following a high-density, high-intensity, short-term grazing/trampling event. Green Sprangletop (Leptocloa dubia Kunth.) and Kleingrass (Panicum coloratum L.) were broadcasted at 4.5 kg ha−1 pure live seed (PLS) on former cropland that had a partial stand of WW-Spar Bluestem (Bothriochloa ischaemum L.). A high-density, high-intensity trampling event was achieved with twenty-four 408-kg Bos taurus heifers occupying four 0.10-ha plots (97 920 kg live weight ha−1) for 10 h, with four adjacent 0.10-ha control plots left untrampled. Canopy and basal cover were determined by plant functional group using the Daubenmire method after rainfall events of > 0.254 cm, and a 5.08 × 7.62 cm core was collected to determine soil bulk density. Strips of supplemental plant material were applied in March to test the effects of 100% soil cover on seedling recruitment. Trampled treatments had 30% less vegetative cover (P < 0.01) and average soil bulk densities that were 0.20 g cm−³ higher (P < 0.01) than untrampled plots post trampling. Bulk density decreased with deferral until there were no significant differences between treatments (240 d). However, WW-Spar basal cover increased in both treatments, with no differences between treatments. Trampling did not affect seedling recruitment, but supplemental cover increased seedling density on three of five subsequent sampling dates (P < 0.05). Canopy cover of warm season perennial grasses in trampled treatments surpassed that of the untrampled treatments during the early growing season of 2016 (P < 0.01) but were no different after mid-June. Hydrologic function can be maintained with high stock densities by providing adequate deferment to reestablish sufficient cover and allow natural processes to restore porosity.

Seed size, bill morphology, and handling time influence preferences for native vs. nonnative grass seeds in three declining sparrows

ABSTRACT The invasion of exotic grasses is a potential threat to the winter habitat of migratory grassland birds by reducing native sources of seed food. We compared seed preferences among 3 native (blue grama [Bouteloua gracilis], sideoats grama [Bouteloua curtipendula], and green sprangletop [Leptochloa dubia]), and 3 exotic (Lehmann lovegrass [Eragrostis lehmanniana], buffelgrass [Pennisetum ciliare], and natal grass [Melinis repens]) grass seeds in captive Baird's (Ammodramus bairdii), Grasshopper (A. savannarum), and Savannah (Passerculus sandwichensis) sparrows to investigate factors that determine vulnerability of grassland passerines to exotic grasses in the Chihuahuan Desert. We hypothesized that seed handling time would determine the ability of sparrows to exploit exotic grass seeds, and that larger-billed birds would be able to better exploit a larger variety of seeds, including exotic seeds. We offered seeds in choice and nonchoice trials and determined handling times for the different seed an...

Germination of six native perennial grasses that can be used as potential soil cover crops in drip-irrigated vineyards in semiarid environs of Argentina

Native desert plants have structural adaptations that maximize photosynthesis rates and minimize water loss. They can be successfully utilized as soil cover crops in drip-irrigated vineyards where water availability is low. The objective of this paper is to study seed germination conditions and to recommend which best species is most apt as soil cover crop in drip-irrigated vineyards. Seed weight (the higher the seed weight, the greater the success of mechanical seeding) and germination tests (at 15, 20, 25, 30, and 35 °C in light and dark conditions) were carried out for six species native to Mendoza, Argentina. Germination percentage (GP) and mean time germination (MTG) were calculated. Digitaria californica (C4) had the highest GP (97% in light condition), which is recommended as a cover crop because of its seed germination potential. Pappophorum phillippianum (C4) had a 70% GP in light conditions (regardless of temperature) and a high seed weight. Leptochloa dubia (C4) and Sporobolus cryptandrus (C4) reached the highest GP at the highest temperature, although S. cryptandrus had the lowest seed weight. Nassella tenuis (C3) averaged 54% GP at 25 °C in light conditions. The GP of Setaria leucopila (C4) was not affected by temperature (26% in light, and 16% in darkness). Based on the GP results, S. leucopila was the worst choice of the six species. Hence, during seeding, soil temperature should be high (>20 °C) to ensure a rapid plant establishment of all species.

Chemical composition, antioxidant activity and bioactive compounds of vegetation species ingested by goats on semiarid rangelands

The first goal of this study was to evaluate the chemical com- position of vegetation consumed by goats on semiarid rangelands. Secondly, antioxidant activity and identified bioactive compounds of this vegetation were estimated. Twenty-five samples were analysed, including leaves, stems, fruits, or a combination of them. Chemical composition demonstrated large differenc- es among species and smaller differences within species. Some species had high protein contents, e.g. Acacia schaffneri, Celtis pallida and Prosopis lae- vigata. Methanol:water was the best solution to extract bioactive compounds, which we found in larger amounts in complete plants and stems than in fruits and leaves. A. farnesiana pods had the largest polyphenol contents, whereas A. farnesiana, A. schaffneri, Leptochloa dubia and others showed the best an- tioxidant activity. A positive correlation was observed between antioxidant ac- tivity and polyphenol-flavonoid concentration. Based on these results, it seems necessary to study in more detail some species, e.g. the fruits of Acacia far- nesiana and A. schaffneri, to better understand their implications for ruminant feeding and nutrition.

Establishment of Native Grasses with Biosolids on Abandoned Croplands in Chihuahua, Mexico

The objective of the work was to evaluate establishment and forage production of native grasses with application of biosolids, a byproduct of waste-water treatment, at an abandoned field, in Ejido Nuevo Delicias, Chihuahua, Mexico. Four biosolids rates from 0 (control) to 30 dry Mg ha−1and two methods of application, surface applied (BioSur) and soil incorporated (BioInc), were evaluated. Seedbed preparation included plowing and harrowing before rainfall. Field plots of 5 × 5 m were manually sown with a mix of blue grama (Bouteloua gracilis) (50%) and green sprangletop (Leptochloa dubia) (50%) in early August 2005. Experimental design was a randomized block with a split plot arrangement. Grass density, height, and forage production were estimated for three years. Data were analyzed with mixed linear models and repeated measures. Green sprangletop density increased under all biosolids rates regardless of method of application, while blue grama density slightly decreased. Biosolids were more beneficial for green sprangletop height than for blue grama height. Blue grama forage production slightly increased, while green sprangletop forage production increased the most at 10 Mg ha−1biosolids rate under BioSur method. It was concluded that BioSur application at 10 and 20 Mg ha−1rates had positive effects on the establishment and forage production of native grasses, especially green sprangletop.

Constituents of Calamintha ashei: Effects on Florida Sandhill Species

Chemical constituents of the perennial shrub Calamintha ashei have been characterized as part of our investigation of the allelopathic properties of this plant. Besides the known monoterpenes, (+)-evodone, (-)-calaminthone and (+)-desacetylcalaminthone, fresh aerial parts of C. ashei provided six new menthofurans, two new germacrane sesquiterpenes, and the six, known flavonoids: 5-desmethoxynobiletin, 5-hydroxy-6,7,8,4'-tetramethoxyflavone, 5,4'-dihydroxy-6,7,8,3'-tetramethoxyflavone, thymonin, 5,4'-dihydroxy-6,7-dimethoxyflavone and 6-hydroxy-7,3'-dimethoxyluteolin. The structures of the new compounds were elucidated by spectroscopic methods and comparison of their 1H NMR spectra with those of structurally related compounds. The molecular structures of (+)-evodone, 5-desmethoxynobiletin (5-hydroxy-6,7,8,3',4'-pentamethoxyflavone) and the triacetate of thymonin (7,8,3'-trimethoxy-5,6,4'-triacetoxyflavone), were determined by single crystal X-ray diffraction. Saturated aqueous solutions of menthofuran, (+)-evodone, (-)-calaminthone, (+)-desacetylcalaminthone, 4alpha,5beta-diacetoxymenthofuran, as well as mixtures of (+)-evodone and (+)-desacetylcalaminthone inhibited the germination and root growth of Schizachyrium scoparium and Leptochloa dubia, two native Florida sandhill grasses, as well as Lactuca sativa. (+)-Evodone and (+)-desacetylcalaminthone were the most active. 5-Hydroxy-6,7,8,3',4'-pentamethoxyflavone and 5,4'-dihydroxy-6,7,8,3'-tetramethoxyflavone and mixtures of the two flavonoids in aqueous as well as in saturated aqueous solutions ofursolic acid were tested on the same species, but showed no significant activity.

Evaluation of Herbicides for Restoring Native Grasses in Buffelgrass‐Dominated Grasslands

Abstract Buffelgrass (Pennisetum ciliare) is an exotic grass that threatens arid and semiarid ecosystems. The objective of this study was to determine effectiveness of several herbicides at reducing competition from buffelgrass to enhance establishment of planted native grasses. In Duval County, Texas, plots were delineated in two experiments in a buffelgrass‐dominated pasture and mowed on 2 September 2002. On 18 September 2002 and 7 October 2002, a 41% glyphosate (N‐(phosphonomethyl) glycine) herbicide was applied to all plots. A mixture of three native grasses—green sprangletop (Leptochloa dubia), plains bristlegrass (Setaria leucopila), and four‐flower trichloris (Chloris pluriflora)—was planted on 8 October 2002. On 9 October 2002, 1.12 and 2.24 kg/ha of a 80% tebuthiuron (N‐[5‐(1,1‐dimethyethyl)‐1,3,4‐thiadiazol‐2‐yl]‐N,N’‐dimethylurea) herbicide was applied preemergence to the first experiment, and all other herbicides were applied postemergence on 27 July 2003 to the second experiment. Percent canopy cover of vegetation was estimated with a 20 × 50–cm sampling frame during April, June, and October 2003 and August 2004. Postemergent herbicides had no significant effect on canopy cover of buffelgrass or planted species (p≥ 0.05). Canopy cover of native grasses did not exceed 8% on any treatment or sampling date, and buffelgrass cover returned to pre‐treatment conditions in less than 1 year; however, the 2.24 kg/ha rate of tebuthiuron suppressed (p &lt; 0.05) canopy cover of buffelgrass compared with controls and increased (p &lt; 0.05) native grasses almost 2 years past application. Tebuthiuron may have potential value in reducing buffelgrass canopy cover and increasing cover of native grasses, particularly Chloris spp.

Photosynthetic oxygen exchange in C4 grasses: the role of oxygen as electron acceptor

ABSTRACTC4 grasses of the NAD‐ME type (Astrebla lappacea, Eleusine coracana, Eragrostis superba, Leptochloa dubia, Panicum coloratum, Panicum decompositum) and the NADP‐ME type (Bothriochloa bladhii, Cenchrus ciliaris, Dichanthium sericeum, Panicum antidotale, Paspalum notatum, Pennisetum alopecuroides, Sorghum bicolor) were used to investigate the role of O2 as an electron acceptor during C4 photosynthesis. Mass spectrometric measurements of gross O2 evolution and uptake were made concurrently with measurements of net CO2 uptake and chlorophyll fluorescence at different irradiances and leaf temperatures of 30 and 40 °C. In all C4 grasses gross O2 uptake increased with increasing irradiance at very high CO2 partial pressures (pCO2) and was on average 18% of gross O2 evolution. Gross O2 uptake at high irradiance and high pCO2 was on average 3.8 times greater than gross O2 uptake in the dark. Furthermore, gross O2 uptake in the light increased with O2 concentration at both high CO2 and the compensation point, whereas gross O2 uptake in the dark was insensitive to O2 concentration. This suggests that a significant amount of O2 uptake may be associated with the Mehler reaction, and that the Mehler reaction varies with irradiance and O2 concentration. O2 exchange characteristics at high pCO2 were similar for NAD‐ME and NADP‐ME species. NAD‐ME species had significantly greater O2 uptake and evolution at the compensation point particularly at low irradiance compared to NADP‐ME species, which could be related to different rates of photorespiratory O2 uptake. There was a good correlation between electron transport rates estimated from chlorophyll fluorescence and gross O2 evolution at high light and high pCO2.

Effects of surface-applied biosolids on grass seedling emergence in the Chihuahuan desert

Plant establishment in semi-arid rangelands is difficult because of low, unpredictable soil water and extreme soil temperatures. In these rangelands, biosolids disposal is limited to topical application; resulting soil coverage ameliorates microenvironmental conditions and may affect plant establishment. We investigated biosolids effects on soil water, soil temperature, and seedling emergence and growth of blue grama ( Bouteloua gracilis) and green sprangletop ( Leptochloa dubia) in a Chihuahuan desert grassland. Greenhouse and field experiments were conducted for 2 years. Biosolids did not affect mean soil temperature (at seed depth) but usually increased minimum and reduced maximum temperatures. Biosolids generally reduced soil water loss. These benefits may be insufficient under harsh conditions to promote seedling establishment, and unnecessary under favorable conditions. Under intermediate conditions, seedling establishment may be enhanced by biosolids application.

Establishment of Native Semidesert Grasses into Existing Stands of Eragrostis lehmanniana in Southeastern Arizona

AbstractThe semidesert grassland in southern Arizona has changed from a native grassland to a scattered Prosopis juliflora var. velutina (mesquite) woodland with an understory of African Eragrostis lehmanniana (Lehmann lovegrass) on many sites. To determine native grass restoration potential, seven species were direct seeded into E. lehmanniana stands that were left alive, burned, sprayed with an herbicide and then either left standing, or mowed. Initial native grass establishment was limited in the live standing treatment but was successful for all other treatments when either June or August sowing was followed by consistent summer precipitation and soil water availability. Four species, Bothriochloa barbinodis (cane beardgrass), Bouteloua curtipendula (sideoats grama), Digitaria californica (Arizona cottontop), and Leptochloa dubia (green spangletop) initially established most successfully, while only Muhlenbergia porteri (bush muhly) had consistently limited or no establishment. E. lehmanniana establishment from the seed bank was increased by canopy removal associated with burning. Densities of native grasses one year after successful initial establishment were much lower than that of E. lehmanniana. A possible revegetation strategy would be to spray emergent E. lehmanniana seedlings and surviving plants with an herbicide during the summer rainy season after spring burning. Native grasses could then be established by sowing in early August of that year or June and August of subsequent years until consistent precipitation produces a native grass stand.

Sesquiterpenes fromChrysoma Pauciflosculosa

Abstract The aerial parts of Chrysoma pauciflosculosa (syn. Solidago pauciflosculosa) gave two new sesquiterpenes, (+)-β-turmerone and a bisabolane endoperoxide, together with the known (-)-α-trans-bergamotene and (+)-β-sesquiphellandrene. When exposed to air and light, (+)-β-turmerone and (+)-β-sesquiphellandrene gave oxidative degradation products, involving hydroperoxide intermediates. Photosensitized singlet oxygen reactions of (+)-β-turmerone provided a series of bisabolane-type endoperoxides. The structures of the natural compounds as well as those of the degradation products and derivatives were elucidated by chemical and spectroscopic methods, mainly NMR and MS. Aqueous solutions of (+)-β-turmerone,(+)-β-sesquiphellandrene and (-)-α-trans-bergamotene were tested for their effects on the germination and radicle growth of three Florida sandhill species, Rudbeckia hirta, Schizachyrium scoparium, Leptochloa dubia, as well as Lactuca sativa. (+)-β-Turmerone significantly inhibited germination of L. sat...

In search of allelopathy in the Florida scrub: The role of terpenoids

The hypothesis was tested that allelopathic agents released from fire-sensitive plants of the Florida scrub community deter the invasion of fireprone sandhill grasses. The structures of the constituents of four endemic scrub species,Conradina canescens, Calamintha ashei, Chrysoma pauciflosculosa, andCeratiola ericiodes, were established and their phytotoxic activity against two grasses of the sandhill was examined. Effects of the secondary metabolites from the above scrub species and their degradation products upon the germination and radicle growth of little bluestem (Schizachyrium scoparium) and green sprangletop (Leptochloa dubia), two native grasses of the Florida sandhill community, were determined. The studies included determination of the water solubility and release mechanism of terpenes and other allelopathic agents from the source plants and their aqueous transport to the target species. Some of the natural products were nontoxic until activated by light and/or oxidation after release from the source plant into the environment.

Diterpenes from Chrysoma pauciflosculosa: Effects on Florida sandhill species

The aerial parts of Chrysoma pauciflosculosa (syn. Solidago pauciflosculosa) gave the known diterpenes, 17-oxygrindelic acid, 17-oxogrindelic acid and 17-carboxygrindelic acid, as well as the new diterpene, 17-hydroxy-7α,8α-epoxygrindelic acid. 17-Oxogrindelic acid decomposes in the presence of air giving three products. The structural data of the diterpenes as well as those of degradation products and derivatives were elucidated by chemical and spectroscopic methods. 17-Oxygrindelic acid and 17-oxogrindelic acid were tested for their effect on the germination and radicle growth of three Florida sandhill species and Lactuca sativa. At concentrations of 12 to 48 ppm, 17-oxogrindelic acid reduced the germination and radicle growth of Schizachyrium scoparium and Leptochloa dubia, two native sandhill grasses, but had no significant effects on germination and only a slight stimulatory effect on radicle growth of Rudbeckia hirta and Lactuca sativa. Mixtures of 17-oxogrindelic acid with an equimolar mixture of three C. pauciflosculosa sesquiterpenes, (+)-curlone, (+)-sesquiphellandrene and (−)-α- trans-bergamontene, did not enhance activity. 17-Oxygrindelic acid was in general less active in reducing germination and growth than 17-oxogrindelic acid.

Allelopathic actions in the Florida scrub community

The hypothesis that allelochemicals released from members of the Florida scrub community deter the invasion of fire-prone sandhill grasses was investigated. Constituents of the endemic scrub members, Ceratiola ericoides, Conradina canescens and Calamintha ashei, were examined for their phytotoxic activity. Effects of the plant natural products on the germination and radicle growth of lettuce (Lactuca sativa), as well as little bluestem (Schizachyrium scoparium) and green sprangletop (Leptochloa dubia), two native grasses of the Florida sandhill community, were tested.The data suggest that ursolic acid and other natural detergents which are released from a source plant enhance the solubilization of allelopathic lipids via micellization. In general, natural surfactants seem to play a significant role in the water transport of lipophilic phytotoxins to target seeds or seedlings.

Allelopathic Effects of Shrubs of the Sand Pine Scrub on Pines and Grasses of the Sandhills

Abstract Aqueous leachates prepared monthly from foliage of eight species and from litter of two of them from Florida's sand pine scrub community were tested for potential inhibitory activity on four receiver species: three grasses native to Florida's sandhill community (Andropogon gyrans, Schizachyrium scoparium, and Leptochloa dubia) and commercial lettuce (Lactuca sativa). Seed germination of the grasses was inhibited significantly by all ten leachates. Inhibition varied with month of preparation of the samples and was highly correlated with monthly precipitation. However, only two of ten leachates significantly inhibited radicle growth of the grasses, with a seasonal peak in late spring before the rainy season. Leachates from species dominant on open scrub sites were much more inhibitory than those from mature scrub. Lettuce seed germination was inhibited significantly, whereas radicle length was stimulated; neither effect exhibited significant seasonal variation. Andropogon gyrans watered with runoff from leaf misting of potted Ceratiola ericoides and Conradina canescens had significantly lower dry weights than control seedlings receiving distilled water. In a field transplant experiment designed to control resource competition, seedling pines (P. palustris, P. elliottii, P. clausa) and plugs of wiregrass (Aristida stricta) grew more slowly at scrub sites than at sandhill sites. Chemicals released from the shrubs may deter pines and grasses that otherwise provide fuel for surface fires which cause shrub mortality. For. Sci. 34(3):592-605.

Effects of Seed Predation by a Tropical Ant on Competition Among Weeds

The native fire ant, Solenopsis geminata, harvests small—sized seeds, especially grasses, in disturbed habitats in wet tropical areas of Mexico and Central America. When common, this ant can lower overall abundance of many weedy species, grasses in particular. We studied experimentally the effect of the ant on pairwise competition between weed plants, using a replacement series design; the pairs were (1 Paspalum distichum and Daucus carota, (2) Leptochloa dubia and Capsella bursapastoris, (3) Panicum clandestinum and Pennisetum ciliare, and (4) Cynodon dactylon and Festuca arundinacea. When the ant strongly preferred seeds from one of the plant species, total plant biomass was significantly lower (by up to 50%) in the plots with ants for about the first 50 after plants. Subsequently, the nonpreferred species increased, and 83 d after planting, total plant biomass was about the same in the presence and absence of ants. A crop growing with ants could thus benefit from reduced competition early in the season. For one of the plant pairs, ants reversed the course of plant competition and changed which of the species was eventually excluded from the plots. For two of the weed pairs, when the ant preferentially ate the seeds of the inferior competitor, the inferior competitor disappeared more rapidly. For the fourth pair, ants created a stable equilibrium between two species, whereas otherwise one of the plant species would have disappeared. Feeding preference studied conducted in the field demonstrated that the amount of each seed type removed by the ants was strongly influenced by the amount and kinds of other seeds in the immediate area. A strongly preferred seed was removed less frequently if it occurred in the midst of nonpreferred seeds. Nonpreferred seeds were taken much more readily if they occurred with preferred seeds. The importance of this "seed environment" effect was less significant in determining final composition of a weed mixture than: (1) inherent preference of the ants for certain weed species, or (2) relative competitive ability of the weeds in the mixture. Although S. geminata can increase the proportion of dicots in a weed assemblage, several factors limit this process: (1) as grasses become rare, a large fraction of their seeds will fall into the protective cover of dicots, (2) as grasses become rare, S. geminata may move their nests to areas of higher food abundance, and (3) the vegetative reproduction of some grasses occurs independently of seed removal by ants, or may even be accelerated by the reduced competition with grass seedlings.