Charles L. Mohler

A Model of the Effects of Tillage on Emergence of Weed Seedlings

A simple model is developed in which the density of weed seedlings emerging in a field is related to (1) the ability of seedlings to emerge from various depths in the soil, (2) the survival of seeds at different depths, and (3) the depth of seed burial in no tillage, rotary tillage, and plow tillage. Other tillage regimes are considered by analogy. Literature is reviewed to determine biologically reasonable functions describing seedling emergence, seed survival, and distribution of seeds with depth, and parameters of these equations are estimated from data in the literature. Problems related to the mathematical description of these phenomena are discussed, and it is noted that some commonly held beliefs regarding survival of seeds in the soil are mutually incompatible. Although many studies have investigated the persistence of seeds as a function of depth in the soil, few have distinguished death from the production of seedlings. The model indicates that in the first year following input of seeds to the soil, no tillage will have more seedlings than tillage, but in later years no tillage will likely have fewer seedlings unless innate or induced dormancy is high or seed survival near the soil surface is unusually good. If seed return is allowed, no tillage or minimum tillage will have more seedlings perennially. Recovery of good weed control following a year with substantial seed input may be easiest if the soil is plowed deeply to bury the seeds, and then shallow or no tillage is used in subsequent years to avoid returning seeds to the surface. Much of the literature on the effects of tillage on weed density is difficult to interpret because little indication is given of the vertical distribution of seeds in the soil at the beginning of the experiment.

Effect of grid size on runoff and soil moisture for a variable‐source‐area hydrology model

Soil chemical and biological dynamics in mixed use landscapes are dependent on the distribution and pattern of soil moisture and water transport. In this paper we examine the effect of different grid sizes on soil water content for a spatially explicit, variable-source-area hydrology model applied to a watershed in central New York. Data on topography, soil type, and land use were input at grid sizes from 10 to 600 m. Output data consisted of runoff and spatial pattern of soil moisture. To characterize the spatial variability at different grid sizes, information theory was used to calculate the information content of the input and output variables. Simulation results showed higher average soil water contents and higher evaporation rates for large grid sizes. During a wet year, runoff was not affected by grid size, whereas during a dry year runoff was greatest for the smallest grid size. While the information content (i.e., spatial variability) of soil type and land use maps was not affected by the different grid sizes, increasing grid sizes caused the information content of the slope gradient to decrease slightly and the Laplacian (or curvature of the landscape) to decrease greatly. In other words, increasing grid cell size misrepresented the curvature of the landscape. During wet periods the decrease in information content of the soil moisture data was the same as for the Laplacian as grid size increased. During dry periods, when local fluxes such as evaporation and runoff determine the moisture content, this relation did not exist. The Laplacian can be used to provide a priori estimates of the moisture content deviations by aggregation. These deviations will be much smaller for the slowly undulating landscapes than the landscape with steep valleys simulated in this study.

Measuring compositional change along gradients

A new procedure for measuring compositional change along gradients is proposed. Given a matrix of species-by-samples and an initial ordering of samples on an axis, the ‘gradient rescaling’ method calculates 1) gradient length (beta diversity), 2) rates of species turnover as a function of position on the gradient, and 3) an ecologically meaningful spacing of samples along the gradient. A new unit of beta diversity, the gleason, is proposed. Gradient rescaling is evaluated with both simulated and field data and is shown to perform well under many ecological conditions. Applications to the study of succession, phenology, and niche relations are briefly discussed.

Evaluating Seed Viability by an Unimbibed Seed Crush Test in Comparison with the Tetrazolium Test1

Abstract: The unimbibed crush test, in which seed viability is evaluated by crushing and visual inspection of dry seeds, was compared with tetrazolium staining, an established method of testing seed viability. The unimbibed crush test potentially provides an immediate and rapid method for determining seed viability. Six sets of seed lots, involving the four weed species, velvetleaf, common lambsquarters, redroot pigweed, and smooth pigweed, were tested by each method. For four of the six sets, results from the crush test were statistically indistinguishable from those of the tetrazolium test. For the other two sets of seed lots, the crush test indicated higher viability than did the tetrazolium test. The crush test may be most useful for seed bank surveys in which many samples are typically processed, and most of the variation in density of viable seeds is associated with number of seeds present rather than percentage viability. Its use in more exacting circumstances like seed survival studies requires caution. Nomenclature: Tetrazolium, 2,3,5 triphenyl tetrazolium chloride; common lambsquarters, Chenopodium album L. #3 CHEAL; redroot pigweed, Amaranthus retroflexus L. # AMARE; smooth pigweed, Amaranthus hybridus L. # AMACH; velvetleaf, Abutilon theophrasti Med. # ABUTH.

Effects of Tillage and Mulch on the Emergence and Survival of Weeds in Sweet Corn

1. The effects of tillage and non-tillage combined with no mulch and rye (Secale cereale) mulch on the emergence and survival of weeds in sweet corn (Zea mays) are reported. 2. The four most abundant weed species (Amaranthus retroflexus, Chenopodium album, Portulaca oleracea, and Digitaria sanguinalis) showed significantly lower emergence in till than in no-till treatments at one or more census times, probably because of seed burial and greater seed mortality in the till treatments. 3. Taraxacum officinale did not show lower emergence rates in till than in no-till, probably because of the short time between seed dispersal and germination. 4. Survival of Amaranthus was significantly greater in no-till than in till treatments, with the difference most pronounced late in the growing season. 5. The presence of corn or corn and rye mulch significantly reduced weed emergence for all species at one or more censuses. 6. Rye, killed with herbicide and used as a mulch, slightly decreased emergence and had no effect on survival. 7. A procedure for testing for departure of a survivorship curve from a truncated negative exponential is presented. Survival of annuals and other monocarpic plants should be considered truncated by successful reproduction. 8. In the absence of herbicides, mortality rates for Amaranthus and Chenopodium declined as the plants grew to maturity. 9. In most treatments, late-emerging plants survived to maturity better than earlier emerging plants, possibly because of shorter exposure to mortality factors. In the absence of herbicides, very dense stands of weeds led to greater mortality of later emerging cohorts.

Weed productivity and composition in sole crops and intercrops of barley and field pea

(1) The hypothesis that intercrops have fewer, smaller weeds than their component sole crops was tested by sowing pure and mixed crops of barley and field pea at commercial densities and at double these densities in two fields near Ithaca, New York, U.S.A. The effect of these cropping systems on the species composition of associated weeds was also investigated. (2) For both sole crops and intercrops weed productivity decreased and crop productivity increased with higher crop density. (3) Crop treatments had no effect on weed numbers relative to unplanted control treatments. (4) For both highand low-density treatment series, weed productivity decreased in the order unplanted controls > pea sole crops > intercrops > barley sole crops. Weed productivity in the high-density intercrop, formed by sowing together the two low-density sole crops, was not significantly different from weed productivity in the low-density barley sole crop. (The estimated difference+S.E. was 0 77+21 8 g m-2.) (5) Weed suppression by barley may have resulted from competition for soil moisture since weeds which grew in barley sole crops or barley/pea intercrops had lower predawn water potentials than weeds in unplanted controls or in pea sole crops. (6) Within both density series, above-ground crop productivity decreased in the order barley > intercrop > pea. Yield of the high-density intercrop was nearly equal to the sum of yields of the two low-density sole crops. (7) Relative abundance of the major weed species differed among crop treatments. In general, the most dominant weed species was more suppressed than other species as crop productivity increased. This response is explained in terms of a dominance hierarchy.

Effects of tillage and mulch on weed seed production and seed banks in sweet corn

Effects of tillage (till, no-till), mulch (none, dead Secale cereale), and crop competition (none, sweet corn) on weed seed production and seed banks are reported. Data are presented for Amaranthus retroflexus, Chenopodium album, Portulaca oleracea and Digitaria sanguinalis. These constituted the majority of the weeds present during the cropping season. Seed bank data for a suite of winter annuals active during the fallow season are also presented. Weed seed production was estimated by dimension analysis. Most seeds were produced by the first cohort of plants to emerge. Amaranthus and Digitaria showed significantly greater seed production in no-till than in till treatments. Seed production by Amaranthus, Chenopodium and Portulaca was significantly greater in the absence of a corn crop. The only effect of Secale cereale mulch was to decrease seed production by Portulaca in 1 year. Differences among treatments in the number of seeds produced per unit area were primarily due to effects on individual plant size and seed production. Effects on seedling density and rate of survival to maturity were of secondary importance. Seed banks of Digitaria were greater in no-till than in till treatments, whereas the seed banks of winter annual forbs were greater in till treatments. Presence of a corn crop did not affect seed banks until the final year of the study, at which time seed banks of Amaranthus, Chenopodium and Portulaca were greater in the absence of a crop. An uncropped treatment which did not receive herbicides had exceptionally large seed production and seed banks of Amaranthus and Chenopodium, but not Portulaca and Digitaria, which apparently suffered from competition with the two larger species.

Extending the resource concentration hypothesis to plant communities: Effects of litter and herbivores

We extend the resource concentration hypothesis (herbivorous insects are more likely to find and stay in more dense and less diverse patches of their host plants) to plant communities. Specifically, whenever superior plant competitors spread to form dense stands, they will be found and attacked by their specialist insect enemies. This will decrease host plant abundance, causing a reduction in standing crop biomass, which will indirectly increase subordinate competitors and plant species richness. In this study, we found that a native, specialist chrysomelid beetle (Trirhabda virgata) in an old-field community decreased total standing crop biomass, leading to an increase in plant species richness. This reduction in biomass was due solely to a reduction in the biomass of the beetles host plant, meadow goldenrod (Solidago altissima), which was the dominant plant species in this community. Our results demonstrate that when a superior competitor increases in density, the per-stem impact of herbivores increase...

Succession after elimination of buried seeds from a recently plowed field

MARKS, P. L. AND C. L. MOHLER (Ecol. & Syst., Cornell University, Ithaca, NY 14853). Succession after elimination of buried seeds from a recently plowed field. Bull. Torrey Bot. Club 112: 376-382. 1985.-The buried seed pool was eliminated from selected plots in a recently plowed goldenrod field by steam sterilization in order to determine the importance of stored seed in the early stages of old field succession. Recolonization by plants was slow despite the close proximity of abundant seed sources in the surrounding goldenrod field. Midway through the second growing season the mean per cent cover of plants in the sterilized plots was 12% (?6 SD) compared to 121% (?10) in the control plots. The results underscore the importance of both the buried seed pool and the population of perennial plants that may survive tillage and herbicide spraying in determining the dominant plants early in old field successions.

Effect of sampling pattern on estimation of species distributions along gradients

Computer simulation and statistical theory indicate that estimation of species distribution is difficult when species reach maximum abundance near one end of the sampled portion of a gradient or when they have wide ecological breadth. Relative to balanced sampling of the whole gradient, concentration of sampling effort near the ends increases accuracy in estimation of distributions of truncated species more than it decreases accuracy for other species. Hence, overall accuracy in the estimation of distributions for a collection of species with modes scattered about on a gradient is greatest when sampling is somewhat more intense near the extremes of the gradient.