Total Plant Density Research Articles

Effects of Applications of Fungicide, Phosphorus and Nitrogen on the Structure and Productivity of an Annual Serpentine Plant Community

Benomyl (a fungicide) and phosphate (as NaH2PO4-H20) were applied to patches of two annual serpentine communities dominated by forbs. After one year neither Benomyl nor phosphate had affected plant productivity (except for legume productivity which was increased by phosphorus application), the abundance of any species or mycorrhizal infection. However, at both research sites Benomyl decreased phosphorus concentration in the shoots whereas added phosphorus increased its concentration. At one site, added phosphorus also increased nitrogen concentration in the shoot. In the following year, nitrogen (as NH4NO3) was added to the experimental design. In that second year, both nitrogen and phosphorus applications decreased the abundance and total above-ground dry weight per unit area of Plantago erecta Morris, a dominant annual forb, and decreased the density of all plants taken together. Additionally, Benomyl increased total plant density. Phosphorus and nitrogen increased productivity of the whole community. The data presented herein support the hypothesis that communities occupying inherently infertile sites are not very responsive to nutrient amendment and that mycorrhizal infection may serve as a controller of community structure. Key-words: Annual grassland, Benomyl, nitrogen, phosphorus, serpentine soils, nutrient limitation, forbs, mycorrhiza, community structure * Address for correspondence: Dr R.T. Koide, Department of Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. t Present address: Dr L.F. Huenneke, Department of Biology, New Mexico State University, Box 3AF, Las Cruces, New Mexico 88003, USA t Present address: Dr S.P. Hamburg, Department of Systematics and Ecology, University of Kansas, Lawrence, Kansas 66045, USA. Introduction Plant productivity in serpentine soils can be limited by a number of factors. These include low levels of macronutrients, high concentrations of nickel or chromium, calcium deficiency or magnesium toxicity (Proctor & Woodell, 1975). It has become clear that no single factor is sufficient to explain growth limitations in all serpentine communities. Indeed, serpentine soils are quite variable chemically and physically (Walker, 1954) and the plant species occupying them appear to reflect this variability (Kruckeberg, 1954). Differential tolerance or preference of species to the particular soil chemical regime may thus influence the structure of the community (Tilman, 1986). Indeed, nutrient availability has been shown to affect species composition of both serpentine (Ferriera & Wormell, 1971) and nonserpentine substrates (le Roux & Mentis, 1986; Tilman, 1986). Turitzin (1982) identified both nitrogen and phosphorus as limiting nutrients in an annual serpentine grassland community in coastal central California (the Jasper Ridge Biological Preserve of Stanford University). He showed that total community productivity increased when either nitrogen or phosphorus were added, that nitrogen had a greater effect than phosphorus, and that annual grasses were more responsive than annual forbs. However, it was not reported how fertilizer application quantitatively altered species composition or other community properties. This same serpentine grassland supports welldeveloped vesicular-arbuscular mycorrhizal infections. As is well documented, infection may enhance plant phosphorus uptake (Smith, 1980), particularly under conditions of soil drought (Fitter, 1985). Because the benefit of infection may differ for each species in the community and because a mycorrhizal fungal mycelial network may mediate the transfer of materials from plant to plant within the community (Grime et al., 1987), mycorrhizal infection may serve as another controller of community structure. This content downloaded from 157.55.39.104 on Mon, 20 Jun 2016 07:32:35 UTC All use subject to http://about.jstor.org/terms 336 In order to document the effects of nutrient R. T. Koide supply and mycorrhizal infection on annual et al. serpentine community characteristics, a study was conducted in two serpentine sites in coastal central California. In the Jasper Ridge Biological Preserve, applications of Benomyl, a fungicide reported to suppress mycorrhizal infection [Menge, 1982]) and of phosphate were made during a single growing season. At another site (referred to as the Kirby Canyon site), applications of Benomyl and of phosphate were made over two growing seasons and nitrogen application was made during a single growing season. Effects on the abundance and shoot productivity of various species, mycorrhizal infection and nitrogen and phosphorus concentrations in the shoots of Plantago erecta Morris, a dominant annual forb, are reported. Materials and methods

Wild Oat (Avena fatua) Competition with Spring Wheat: Effects of Nitrogen Fertilization

Field experiments were conducted to determine the effect of nitrogen fertilization on competition between wild oat (Avena fatuaL. # AVEFA) and spring wheat (Triticum aestivum‘Anza’). Nitrogen fertilizer treatments were applied over several wild oat-wheat density combinations. Wheat grain yield in wild oat-infested plots generally declined with fertilization while the density of wild oat panicles increased. Apparently, in competition with wheat, wild oat was better able to utilize the added nitrogen and thus gained a competitive advantage over the wheat. The increased competitiveness of wild oat resulted in reduced crop yields. Under the conditions of these experiments, nitrogen fertilization resulted in positive wheat yield response only when the wild oat plant density was below 1.6 percent of the total plant density.

Herbicides for Sod-Seeding Establishment of Alfalfa (Medicago sativa) in Quackgrass (Agropyron repens)-Infested Alfalfa Swards

The herbicides 2,4-D [(2,4-dichlorophenoxy) acetic acid], dalapon (2,2-dichloropropionic acid), glyphosate [(N-phosphonomethyl)glycine], paraquat (1,1’-dimethyl-4,4’-bypiridinium ion), and pronamide [3,5-dichloro-(N-1,1-dimethyl-2-propynyl)benzamide] were applied at selected rates and dates to quackgrass [Agropyron repens(L.) Beauv. ♯ AGRRE]-infested alfalfa (Medicago sativaL.) sod in 1979 and 1980 to suppress vegetation for subsequent establishment of alfalfa without tillage. The influence of herbicides on weed control, alfalfa seedling density and visual stand, and forage yield and quality was assessed for 3 yr. During establishment, all herbicides except 2,4-D effectively controlled quackgrass. Glyphosate applied broadcast increasingly suppressed the existing sod as the rate was increased from 0.6 to 1.6 kg ai/ha. The effectiveness of herbicides as measured by total (old and new) plant density was closely related to the suppression of the sward. While the substitution of new alfalfa plants for old ones was not necessary under dense (40 plants/m2) alfalfa stands, completely controlling the vegetation with glyphosate resulted in sustained productivity throughout the experimental period. After 3 yr, alfalfa percentage and yield were highest when glyphosate was broadcast at 1.6 kg/ha. Improvement in forage quality, as estimated by the concentrations of crude protein and neutral-detergent fiber, was closely related to grass control.

Insect Herbivore Abundance in Tropical Monocultures and Polycultures: An Experimental Test of Two Hypotheses

The population dynamics of six beetle pests (Chrysomelidae: Galerucinae) were studied for three seasons in monocultures and polycultures of the corn—bean—squash agroecosystem in Costa Rica. The study was designed to determine if beetle abundance differed among these treatments, and if so, to determine whether the differences were caused primarily by predators and parasites (Enemies Hypothesis) or by movement patterns of adult beetles (Resource Concentration Hypothesis). In polycultures containing at least one nonhost plant, the numbers of beetles per unit host plant (squash or bean) were significantly lower relative to the numbers of beetles on host plants in monocultures. This pattern was generally observed for each of the beetle species studied between 40 and 60 d after planting and continued until the end of the season. When a given beetle species fed on both crop species in a diculture (i.e., the squash—bean diculture), the general polyculture effect was reversed: beetle numbers were significantly greater on one or both of the host plants in the diculture relative to the respective monocultures. These effects were apparently caused by changes in species richness of the plots per se rather than changes in host density or total plant density in the plot. There were no differences in rates of parasitism or predation of beetles between monocultures and polycultures. Neither were there significant differences in beetle emergence between treatments that could account for differences in adult beetle numbers between monocultures and polycultures. Instead, several lines of evidence show that different movement patterns of adult beetles were primarily responsible for the observed pattern of beetle abundance and distribution. Direct measurements of beetle movements in the field showed that beetles tended to emigrate more from polycultures that included a nonhost plant than from host monocultures. Field observations and experiments showed that this was due to several factors: (1) beetles avoided host plants shaded by corn; (2) cornstalks interfered in some way with flight movements of beetles; (3) as beetles moved through polycultures they remained on nonhost plants significantly less time than on host plants and their overall rate of movement in polycultures was higher. These data suggest that the Resource Concentration Hypothesis rather than the Enemies Hypothesis can account for differences in beetle abundance between monocultures and polycultures. The implication of this finding for predicting herbivore abundance in yet untried agreocosystems are discussed. It is suggested that it is not sufficient merely to recognize that herbivores are less common in particular polycultures. Practical issues of agroecosystem design and more theoretical questions of plant—herbivore coevolution depend on knowing exactly what ecological processes create reduced herbivore loads in polycultures.

Neighborhood competition in several violet populations.

Research into local (neighborhood) competition has heretofore focused on either natural populations of long-lived woody plants or artifical populations of herbaceous annuals. To explore the usefulness of these techniques for herbaceous perennials, 1 applied them to 11 natural populations of 4 species of violet (Viola blanda, V. pallens, and V. incognita, alone and with V. adunca). Ramet density ranged from 132 to 508 per square meter. The analysis tried to account for the size of each ramet (expressed as the number of leaves) using information on the number, size, and angular aggregation of neighboring plants in three concentric annuli (0-3, 3-6, and 6-9 cm). Simple rank correlations between leaf number and these measures of local competition were usually of the expected sign, but were consistently significant in only two of the populations (1 pallens and 1 incognita). No significant correlations were of the "wrong" sign. When the spatial independent variables were combined into multiple regression equations they accounted for between 5 and 59% of the total variance in leaf number. The equations were best in predicting individual size in the Newfoundland populations of V. incognita which had the greatest range in individual size. Total plant density appeared unrelated to predictive success.The neighborhood competition approach can be applied to natural populations of herbaceous perennials, but low density, inaccurate measures of plant performance and microvariation in site quality limit its usefulness. In particular, it seems impossible to determine the mechanism by which neighbors exert their competitive effects with this approach.

Methods of analysing yield from trials in which the produce is graded according to diameter

SUMMARYA study was made of the relationship between yields in particular size grades of carrots and onions and the number of plants per unit area with a view to providing adjustments to yields for differences in plant densities. It is concluded that the relationship for individual small grades cannot be fitted consistently by a single mathematical equation but that estimates of yields in small grades are best obtained by fitting a common equation to the accumulated yield at the limits of the grade and obtaining the yield by difference. Eleven previously published equations which have been shown to fit the relationship between total yield and plant density for a number of crops are compared with one newly developed for graded produce. It was found that the latter,where y = yield/ha, ρ = number of plants/m2 and A, B and C are constants, generally leads to the best fits when a large range of densities is present, but it is argued that, for adjustment of yields for small differences in densities such as are obtained in variety trials, a simpler equation such as a second degree polynomial is equally effective.

Effects of Plant Density and Diversity on the Population Dynamics of a Specialist Herbivore, the Striped Cucumber Beetle, Acalymma Vittata (Fab)

To determine the effects of plant density and diversity on the population dynamics of a specialist herbivore, the striped cucumber beetle (Acalymma vittata [Fab.]), cucumbers (Cucumis sativus L.) were planted in monocultures and in polycultures with corn (Zea mays L.) and broccoli (Brassica oleracea L.).Population densities of A. vittata and its insect predators, as well as cucumber growth, survivorship, and reproduction were monitored in these experimental plots. By controlling total plant density, host plant density, and plant diversity, it was possible to distinguish the effects of these three confounding variables. Densities of A. vittata were 10–30 times greater in monocultures than in polycultures, both per plot and per plant, even when total plant density and host plant density were held constant. The lack of any effect of plant density on per—plot beetle abundances emphasized the importance of diversity per se in influencing beetle populations. These differences in beetle abundance caused by diversity resulted from differences in tenure time and movement patterns among plots, rather than from differences in colonization, reproduction, or predation. In mark—recapture studies, a greater proportion of beetles marked in monocultures than in polycultures was later found, and of those marked in monocultures, a greater number were later found in monocultures than in polycultures. All measures of growth (leaf area, growth rate, and vine length) and reproduction (fruit production and number of flowers) of Cucumis sativus were most strongly affected by diversity, but also were affected by plant density. Both per—plot and per—plant values were greater in monocultures than in polycultures. The number of beetles was strongly correlated with the total amount of plant growth and reproduction in monocultures but not in polycultures. For plots with equal amounts of leaf area, monocultures had an order of magnitude greater number of beetles; thus, differences in host plant quantity did not explain the differences in beetle abundances between monocultures and polycultures. Numbers of beetles were much more strongly related to total plot characteristics than individual plant characteristics. Although defoliation was negligible, beetles significantly decreased cucumber survivorship and longevity by disseminating bacterial wilt disease (Erwinia tracheiphila [E. F. Sm.]).

The Population Dynamics of Several Herbivorous Beetles in a Tropical Agroecosystem: The Effect of Intercropping Corn, Beans and Squash in Costa Rica

SUMMARY The population dynamics of six species of chrysomelid beetle pests (Diabrotica, Acalymma and Cerotoma) were studied in monocrops, dicrops and tricrops of maize, beans and squash for three seasons in Costa Rica. The beetles include both generalist and specialist species and they damage the plants by consuming the flowers and leaves, by transmitting viral and fungal diseases, and by eating the roots of the plants. The numbers of beetles per unit plant biomass were determined at approximately six sampling times each season (for each crop type in each treatment). Whenever an intercrop contained at least one non-host plant for a given beetle species, the numbers of that beetle species per host plant in the intercrop were significantly reduced relative to the numbers in the monocrop. This pattern was generally observed for each of the species 40 and 60 days after planting and continued until the end of the season when the differences in the numbers of beetles per host plant between monocrops and intercrops were sometimes tenfold. In the cases when a given beetle species fed on both crop types in a dicrop and when there was no non-host plant present, the general intercrop effect was usually reversed: the numbers of beetles were significantly greater on one or both of the host plants in the dicrop relative to the numbers on the respective monocrops. The differences in beetle abundance appear to have been caused by differences in plant species richness of the plots per se rather than by differences in host plant or total plant density between the different treatments. Laboratory studies showed that beetles preferred to eat the more diseased squash leaves growing in the intercrops than the healthier leaves in the monocrops, so the decreased beetle abundance in intercrops does not conform with apparent differences in leaf palatability. Within squash monocrops the number of adult beetles was only weakly correlated with the size of individual plants. There were significantly more of several beetle species per unit squash biomass on smaller than larger plants so factors besides size of plant are important in determining beetle abundance within one cropping system. The effect of beetle damage could not be separated from the effects of root and shoot interactions of the crops. Yet considering the mechanisms and time at which the beetles typically cause economic damage to the crops, it is possible that decreased beetle abundance contributed to greater yields of the intercrops. The traditional practice of intercropping may therefore be a cultural method of pest control.

Competition between two annual herbs, Atriplex gmelini C.A. Mey and Chenopodium album L., in mixed cultures irrigated with seawater of various concentrations.

Atriplex gmelini and Chenopodium album were grown in mixed stands with various combinations of plant density and mixing ratio, and irrigated with seawater of different concentrations (f) to formulate the effect of changing concentration on the competitive relationship between the species.In single-species stands, the mean plant weight (w)∼plant density (ρ) relation for each level of seawater concentration could well be described by Shinozaki-Kira's reciprocal equation of crowding effect. On the other hand, the response of w to f followed Hozumi-Shinozaki's formulation for an optimum growth factor at respective levels of ρ.By introducing the density conversion factor (q) that enabled the conversion of the density of one species to that of the other species on the basis of their effects on growth of respective species, the results of mixed culture experiments could be successfully formulated by similar reciprocal equations. The dependence of q and coefficient values of the equations on seawater concentration was also formulated in a way similar to the case of pure stands.Based on all these quantitative relations, a comprehensive formulation was developed to describe the effects of plant density and seawater concentration on the growth of two species in mixed stands. The behavior of species biomass in mixed stands was then examined by means of the formulation.It was thereby demonstrated that the relative dominance of two species in a mixed stand was strongly affected not only by total plant density and density ratio between the two species but also by concentration of irrigated seawater. Even the optimum seawater concentration that resulted in the maximum species biomass differed between pure and mixed cultures.

Structural Analysis and Dynamics of the Plant Communities of Wizard Island, Crater Lake National Park

Wizard Island, a ½—square—mile volcanic cinder cone located in Crater Lake, Oregon, was sampled by continuous belt transects. Separation of the one herbaceous and four forested community types, which were mapped as major vegetation units, was remarkably distinct, as 60% of the herbaceous species were found in only one or two communities. Physical factors such as the distribution and stability of substrate materials, temperature extremes, and soil—moisture availability exert the greatest control over plant community structure and development. Biotic factors have reduced effects on the generally sparsely vegetated island. The unstable cinder slope community was dominated by Polygonum newberryi Small., Eriogonum pyrolaefolium Hook. var. coryphaeum T. & G., and Arenaria pumicola Cov. & Leib. Pinus albicaulis Engelm. constituted 55% of the importance value of the crater rim community, but appears to be declining in importance. Pinus contorta is invading the crater area and may slowly replace P. albicaulis. Other characteristic crater rim species were Holodiscus microphyllus Rydb. var. glabrescens (Greenm.) Ley. and Castilleja applegatei Fern. The lower cone and north slope forests were dominated by Abies magnifica Murr. var. shastensis Lemm. and Tsuga mertensian (Bong.) Carr., with Pinus monticola Dougl. the only other important tree species. An importance value of 51% for T. mertensiana in the north slope community as compared with 25% in the lower cone reflects the cooler, moist microclimate and greater community maturity on the north slope. The more tolerant T. mertensiana increases in importance as the stands mature. Characteristic shrub and herb species were Vaccinium membranaceum Dougl. and Luzula glabrata (Hoppe) Desv. in the north slope, and Arctostaphylos nevadensis Gray and Pyrola secunda L. in the lower cone communities, respectively. The lava flow community was largely restricted to recesses between lava ridges where moisture and soil conditions are improved. Tsuga mertensiana was the dominant tree species on the lava flow at 51% importance value, with Sambucus microbotrys Rydb. and Cheilanthes gracillima D. C. Eaton as indicators of the shrub community development was and herb strata. Diversity, total plant density, and herbaceous and shrub community development was greatest in the favorable forest microclimates of the north slope. The north slope, crater rim, lower cone, cinder slope, and lava flow communities had 28, 23, 21, 12, and 10 nonarborescent species, respectively. The reported vascular flora of the island comprises 105 species and varieties. The best forest development occurred in an encircling belt at about 6,400—6,500 ft (1,950—1,980 m) elevation, but tree species characteristic of an elevational range of 2,500 ft (820 m) are telescoped onto the 760—ft—(250—m) high island. Substrate and local climatic diversity and climatic moderation due to lake effects and shielding by the caldera wall permit survival of trees above their usual elevation range. Tree invasion on the upper cone (particularly of Tsuga mertensiana) is slowed more by unstable slopes than by climatic severity. Mat—forming plants, such as Arctostaphylos nevadensis and Penstemon davidsonii Greene, anchor the substrate and serve as nurseries for tree seedlings. Ecesis of tree seedlings on the lava flow occurs only in crevices which exhibit some soil development and microclimatic amelioration. Log density—size class plots gave a straight line relationship for north slope tree species, but all other community types had marked density reductions in some size classes. As such, the north slope community is considered to be in equilibrium at the present level of substrate and microclimatic alteration.