Adjacent Quadrats Research Articles

Establishment of Fitted Models for Topographical Factors and Coexisting Plants Influencing Distribution of Natural Wild Jujube

Fitting mathematical models to describe the influence of topographic factors and coexisting plants on wild jujube distribution was performed to provide a scientific basis for wild jujube forestation. Investigation quadrats, with straight-line distances between adjacent quadrats of longer than 100 m, were set up in areas of wilderness or low human disturbance, which were rich in wild plant species. Data concerning altitude, slope aspect, slope position and slope degree of each investigation quadrat, as well as the type and number of coexisting wild plants, were collected. Based on this, correlations with the average number, occurrence probability and density of wild jujube and these variables were analyzed, and data models were established. Results of analyses show that topographic factors such as altitude, aspect, gradient, slope and position, play an important role in the distribution of wild jujube; and that Vitex negundo var. heterophylla (Franch.) Rehd. coexistence is related to wild jujube distribution. Both average number and occurrence probability of wild jujube conform to a GaussAmp model with altitude. The highest average number was recorded at 581.24 ± 13.78 m above sea level, and the highest occurrence probability at 462.53 ± 36.67 m above sea level. Average number and occurrence probability of wild jujube were fit to a linear model with slope aspect—with mathematical slope 0.49 ± 0.16—indicating that wild jujube is a light-loving and drought-tolerant plant. Average number and density of wild jujubes were fit to GaussAmp models with slope position. The highest average number and the highest density of wild jujube appears on the upper part of the middle slope. Wild jujube occurrence probability was correlated to slope degree in a quadratic equation model. With an increase in slope degree, the distribution number of wild jujube increased sharply. The survey data of slope position and slope degree further reinforced the observed drought-resistance qualities of wild jujube. Average number and density of wild jujubes were correlated to the number of Vitex negundo var. heterophylla by quadratic equation models. No other plants investigated conformed to a statistically significant relationship with wild jujube distribution. Our results suggest altitude, slope aspect, slope position and slope degree play an important role in wild jujube distribution, and that Vitex negundo var. heterophylla is an important coexistent plant species for wild jujube.

Small-scale switch in cover–perimeter relationships of patches indicates shift of dominant species during grassland degradation

Abstract Aims Grasslands are globally threatened by climate changes and unsustainable land-use, which often cause transitions among alternative stable states, and even catastrophic transition to desertification. Spatial vegetation patch configurations have been shown to signify such transitions at large spatial scale. Here, we demonstrate how small-scale patch configurations can also indicate state transitions. Methods The whole spatial series of degradation successions were chosen in alpine grasslands characterized as seven typical communities. Patch numbers, and perimeter and cover of each patch were recorded using adjacent quadrats along transects in each type of the communities. Species abundance within each patch was measured. Important Findings Across seven grazing-induced degradation stages in the world’s largest expanse of grassland, from dense ungrazed turf to bare black-soil crust, patch numbers and perimeters first increased as patch cover decreased. Numbers and perimeters then decreased rapidly beyond an intersection point at 68% of initial continuous vegetation cover. Around this point, the vegetation fluctuated back and forth between the sedge-dominated grassland breaking-up phase and the forb-dominated phase, suggesting impending shift of grassland state. This study thus demonstrates how ground-based small-scale vegetation surveys can provide a quantitative, easy-to-use signals for vegetation degradation, with promise for detecting the catastrophic transition to desertification.

Vegetation, recent pollen deposition, and distribution of some non-pollen palynomorphs in a degrading ice-wedge polygon mire complex near Pokhodsk (NE Siberia), including size-frequency analyses of pollen attributable to Betula

Vegetation distribution and pollen deposition were studied in a complex of degraded ice-wedge polygon mires near Pokhodsk (NE Siberia) in order to obtain insight in the relation between actual vegetation and pollen deposition in microtopographic landscape elements. Pollen surface samples with 1m spacing were collected along a transect and compared with the vegetation at the sample quadrats, with the vegetation in the adjacent quadrats, and with the vegetation at larger distances. A clear regional pollen signal of plants growing outside the study plot was found in the large central depression for pollen types attributable to Comarum palustre, Betula, and Ericales. Local pollen values (i.e. exceeding the regional background deposition values) are restricted to quadrats where the associated taxa are present within 1m distance (e.g. Comarum palustre, Betula and Rubus chamaemorus). Several plant taxa (e.g. Cyperaceae, Ericales, Pedicularis and Sphagnum) show both high and low pollen deposition values in quadrats where the taxa actually occur, indicating differential pollen production/deposition among specimens of the same taxon. A local signal of grasses and Salix is not noticeable and the pollen signal of these taxa is mainly attributable to plant specimens growing in a larger area. The non-pollen palynomorphs include principally testate amoebae, which dominate on the polygon ridges. Size-frequency analyses show that pollen attributable to dwarf Betula has in the NE Siberian Arctic a large morphological variation.

Accounting for tree spatial distribution in a comparison of plot sizes and shapes in dense forest and woodland in Benin (West Africa)

AbstractThe study examined simultaneously, the effect of tree spatial distribution, inventory plot size and shape on the estimation error of basal area in two contrasting environments. Twenty and fifteen square plots of 1 ha each (divided into 100 quadrats of 0.01 ha) were randomly set in dense forest and woodland, respectively. Thirteen subplots of various shapes and sizes were obtained from the association of adjacent quadrats. Estimation error was calculated using residual mean square of one‐way ANOVA, based on replications of subplot within 1 ha plots. Tree spatial distribution was measured using Green index. Weighted linear regression and mixed effect models were applied to Box & Cox transformed data. In general, the estimation error of basal area decreased with increase in subplot size. However, the effects of tree spatial distribution and plot shape varied with the vegetation type. Where trees tended to be aggregated, estimation error increased with degree of aggregation, and rectangular plots of 0.24 ha produced an acceptable precision. It was concluded that 0.24 ha rectangular plots can be used in tropical environments where the target parameters vary constantly according to one direction, while square plots of the same size are optimal for reliable analysis in case of randomness.

Structural analysis of regeneration in tropical dense forest: combined effect of plot and spatial distribution patterns

This study assessed the combined effect of the size and shape of inventory plots on the analysis of the density and diameter structure of saplings and juvenile trees. Thirty square plots of 400 m² were randomly laid out and each one was divided into 400 quadrats of 1 m². Stem number of recruits was recorded for all tree species and their diameter at breast height was measured in the quadrats. Seven types of subplots with different shapes and sizes were considered by grouping adjacent quadrats of 1 m². The Green index was used to assess the spatial distribution of regeneration and Weibull distribution was applied to model stem diameter distributions. Box–Cox transformations and weighted regression were used to model the relationship between the Green index, the plot patterns and the standard error of the estimation of the regeneration density. The same analyses were applied to model the relationship between the Green index, the plot patterns and the mean absolute error of the shape parameter of the Weibull distribution. The Green index and the plot size had a significant impact on the error of the regeneration density estimation. For clumped distribution of individuals, the plot shape had a significant impact on the error of their density estimation. As for stem diameter distribution, only the plot size had a significant effect on the mean absolute error of the stem diameter structure. The rectangular plots and square plots of 100 m² (14 m × 7 m and 10 m × 10 m) appeared more suitable for an effective structural analysis of woody species regeneration in the tropical dense forest for an aggregated distribution and a random distribution, respectively.

Soil moisture variability in a temperate deciduous forest: insights from electrical resistivity and throughfall data

In deciduous forests, soil moisture is an important driver of numerous physical, microbial, and biogeochemical processes. Therefore, characterizing the interactions between vegetation and soil moisture is critical to forecast long-term water resources and ecosystem health. However, these interactions are difficult to measure, both in time and space. Recent studies have shown the ability of electrical resistivity imaging (ERI) to characterize the spatial and temporal dynamics of soil moisture at a range of scales. However, no study has yet attempted to use ERI to describe spatiotemporal variability of soil water in relation to vegetation structure and throughfall. In this study, at a mature forest site in Michigan, USA, we captured spatial and temporal dynamics of soil moisture using weekly ERI measurements augmented with throughfall and soil temperature measurements, and a detailed vegetation survey for five adjacent quadrats. Our results show that throughout the growing season, the soil moisture gradually declined despite strong variations in cumulative monthly rainfall. This decline was occasionally halted, but not reversed, during weeks with high precipitation. Spatial variability of electrical resistivity and soil moisture was not related to soil temperature differences but showed a strong correlation with canopy variables.

Efficiency of inventory plot patterns in quantitative analysis of vegetation: a case study of tropical woodland and dense forest in Benin

The main issue in forest inventory is the reliability of data collected, which depends on the shape and size of inventoried plots. There is also a need for harmonisation of inventoried plot patterns in West Africa. This study focused on the impact of plot patterns on the quantitative analysis of two vegetation types of West Africa based on case studies from Benin. Twenty and fifteen plots of 1 ha each were demarcated in dense forest and woodland, respectively. Each 1 ha plot was divided into 100 quadrats of 100 m2 each and diameter at breast height (dbh) of trees was recorded in each quadrat. The required time to measuring trees diameter in each 1 ha plot was also recorded to compute the mean inventory effort. From the 100 quadrats in each 1 ha plot, 14 subplots of different shapes and sizes were considered by grouping together adjacent quadrats. The basal area of each subplot was computed and the relationship between estimation bias of the basal area and the size of subplots was modeled using Smith's Law (Smith 1938). The mean absolute error of the shape parameter c of Weibull distribution was computed for each of the subplot shape, size and direction. The direction and shape of subplots did not influence significantly (P > 0.05) the precision of the quantitative analysis of vegetation. However, square subplots were suitable in practice. On the contrary, plot size was significantly (P < 0.05) and inversely correlated to estimation efficiency. The optimal plot size for quantitative analysis of vegetation was 1 800 and 2 000 m2 with an inventory effort of 0.51 and 0.85 man-days per subplot in woodland and dense forest, respectively. It is concluded that use of standard sample sizes will help to harmonise a forestry database and to carry out comparisons at regional level.

Effect of inventory plot patterns in the floristic analysis of tropical woodland and dense forest

This study was set up to examine the effect of plot patterns on the accuracy of phytosociological characterization of tropical vegetation. Fifteen and twenty square plots of 1 ha were demarcated, respectively, in woodland and dense forest in BEach 1 ha plot was divided into 100 quadrats of one 100 m 2 . Species of trees in each quadrat were identified and recorded. The cost in terms of time required to record tree species in each 1 ha plot and five random quadrats in a 1 ha plot were also recorded to compute the mean inventory effort for a team of three foresters. From the 100 quadrats in a 1 ha plot, fourteen independent subplots of square and rectangular plots with different sizes were considered by grouping together adjacent quadrats of 100 m 2 . Eigenanalysis was carried out to compare the subplots. Moreover, the relationship between the relative loss of accuracy (RLA) and the size of subplots was modelled. Plot size highly influenced the RLA (P < 0.05). Findings indicated that the square plots of 1500 and 1000 m 2 with an inventory effort of 0.35 and 0.20 man-days per subplot, respectively in tropical dense forests and woodlands appeared to be the most efficient in the phytosociological characterization of woody

Nesting habitat use by river chubs in a hydrologically variable Appalachian tailwater

AbstractAs hydrologic alteration continues to affect aquatic biodiversity, knowledge of the spawning requirements of fishes, especially ‘keystone’ or ‘foundation’ species, is critical for conservation and management. The objectives of this study were to quantify the spawning micro‐ and mesohabitat use of river chub Nocomis micropogon, a gravel mound nesting minnow, in a hydrologically regulated river in North Carolina, USA. At the microhabitat scale, substrate sizes on nests were compared with pebble counts in 1‐m2 adjacent quadrats. Average depths and current velocities at nests were compared with measurements from paired transects. At the mesohabitat scale, generalised linear mixed models (GLMMs) were used to identify the importance of average bed slope, average depth and percentages of rock outcrops (a measure of flow heterogeneity and velocity shelters) for predicting nest presence and abundance. To relate nesting activities to hydrologic alteration from dam operation, nest dimensions were measured before and after a scheduled discharge event approximately six times that of base flow. Additionally, linear regression was used to predict changes in the use of flow refugia and overhead cover with increased fluvial distance from the dam. Microhabitats in which nests were placed had, on average, slower current velocities and shallower depths. Gravel diameters of nests were significantly smaller than substrate particles adjacent to nests. GLMMs revealed that mesohabitats with nests were shallower, had more moderate slopes and greater proportions of rock outcrops than mesohabitats without nests. The scheduled discharge event significantly flattened nests. Near the dam, nests were built in close proximity (≤2 m) to velocity shelters; this relationship diminished with distance from the dam. River chubs are spawning habitat specialists. Because multiple species rely on river chub nests for reproduction and food, the needs of this species should be considered when managing instream flows.

Study of the distribution of microvessels in normal and pathologic prostate using an information-based similarity analysis

Categorizing biologic signals by analysis of symbolic sequences was employed in the study of prostate microvessels. The estimates of the volume fraction of the vessels immunostained to Factor-VIII was mapped to binary sequences. The distance between sequences was estimated by comparing the rank and frequency of repetitive elements. These measurements were applied to detect whether there are unique microvascular patterns for each individual, and to search for patterns describing prostate microvessels of different conditions. Normal prostate, benign prostate hyperplasia and prostate carcinoma groups were studied. All the specimens were immunostained to F-VIII and strips formed by adjacent quadrats were explored. At each point of the long axis of the strip, the V(V) F-VIII was calculated. These values were processed with the information-based similarity software to estimate the dissimilarity between two space series. The following comparisons were carried out: intrasubject versus intragroup distances; intragroup distances among the groups studied and intergroup distances. The distance defined between a vessels immunostained to Factor-VIII space series and its randomized surrogate was considered as an index of the nonrandomness of the space series. These indices were compared for all the groups. We conclude that (a) The information-based similarity analysis can be adapted to vessels immunostained to Factor-VIII space series from prostate microvessels. (b) There are no unique microvascular patterns associated with each individual. (c) There are characteristic patterns describing the microvessels from normal prostate, benign prostate hyperplasia and carcinoma. (d) This method is able to account for the differences between prostate cancer and both normal and benign prostate conditions, with respect to the microvessel patterns.

Size of sampling unit strongly influences detection of seedling limitation in a wet tropical forest

Seedling limitation could structure communities, but often is evaluated with sampling units that are orders of magnitude smaller than mature plants. We censused seedlings for 5.5 years in five 1 x 200-m transects in a wet Neotropical forest. For 106 common species (> or = 10 seedlings in a transect), we calculated prevalence (occurrence of > or = 1 newly emerged seedlings per sampling unit) at 1 m(2) and at 1 m x mature crown diameter units by aggregating adjacent quadrats. For most species, prevalence was 2-25% at 1 m(2), but 20-92% at mature crown scales. Increased prevalence arose from broadly distributed seedlings within transects, with unoccupied segments generally shorter than crown diameters. At the landscape scale, 69% of 301 species were locally rare (< 10 seedlings) and only 16% were represented in all transects (maximally separated by 2.4 km). Nonetheless, for more common species, much lower estimates of seedling limitation at mature crown scales suggest weaker influence of seedling limitation on community dynamics than previously assumed.

Weed Seedling Emergence and Survival as Affected by Crop Canopy

This study measured impact of cool-season crops on seedling emergence, survival, and seed production of weeds common in corn and soybean. Weed dynamics were monitored in permanently marked quadrats in winter wheat, spring wheat, and canola. Three species, green foxtail, yellow foxtail, and common lambsquarters, comprised more than 80% of the weeds observed in the study. Seedling emergence was reduced by winter wheat, but not by spring wheat or canola, when compared with adjacent quadrats without a crop canopy. Approximately 10% of seedlings in spring wheat and canola developed into seed-bearing plants, but no seed-bearing plants were present in winter wheat at harvest. Common lambsquarters produced more than 1,100 seeds/plant, whereas a foxtail plant produced 85 seeds, averaged across spring wheat and canola. At harvest, new seedlings were present in all crops; thus, control after harvest will be required to prevent seed production in the fall. Winter wheat may provide an opportunity to disrupt population dynamics of weeds common in corn and soybean without requiring herbicides.

Aggregated seed dispersal by wreathed hornbills at a roost site in a moist evergreen forest of Thailand

AbstractHornbills (Bucerotidae) are widely regarded as important seed dispersers in tropical forests in Africa and Asia. We investigated how the roosting behavior of wreathed hornbills (Aceros undulatus) influences seed deposition and seedling survival at a roost site in a moist evergreen forest of Khao Yai National Park, Thailand. Fallen fruits and seeds were collected in traps that were placed around a roosting site for 14 months, and seedlings were monitored in adjacent quadrats for 3 years. Seedfall and seedlings of species represented in the hornbill diet occurred at significantly higher densities in the traps and quadrats located beneath the crown of the roosting tree than in those located beyond the crown. With the exception of Cinnamomum subavenium, the seeds and seedlings of most diet species rarely survived beyond the first year. The quality of hornbill dispersal to this roosting site may be poor due to the highly concentrated seedfall, which results in high seed and seedling mortality. However, the number of seeds deposited by each hornbill each day at roosting sites is relatively low. Wreathed hornbills are primarily scatter dispersers during the day and probably serve as agents of seed dispersal in the moist evergreen forest of Khao Yai.

Experimental design for spatial sampling applied to the study of tropical forest regeneration

For practical reasons, estimating seed production in tropical forest is only possible by sampling. Classical sampling designs (random or systematic) give poor estimations of seed abundance. The spatial disposition of the trees, combined with nonuniform seed dispersal, leads to a highly heterogeneous spatial distribution of the seeds. We propose a random stratified sampling design based on a model that takes account of seed dispersal processes and the location of the trees. We assume a gamma distribution for dispersal distances. The overall seed dispersal area is divided into adjacent quadrats. In each quadrat, the number of seeds follows a Poisson distribution with the mean derived from the model. We estimate model parameters from the results of a previous study and give the variance of the Horvitz–Thompson estimator of population total for stratified and random sampling designs. A simulation study is used to find the optimal number of strata, and the performance of the sampling design is evaluated. For each model, we compared the variance of the estimator of population total obtained with the stratified sampling design with that obtained with the random sampling design with the same sample size. The stratified sampling design is, on average, 25 times as precise as the random sampling design.

ADAPTIVE CLUSTER SAMPLING FOR ESTIMATION OF ABUNDANCES WITHIN LOCAL POPULATIONS OF LOW-ABUNDANCE PLANTS

Quantitative estimates of abundance for rare plants can be difficult, as the most widely used sampling techniques are ill-suited for rarity. Adaptive cluster sampling (ACS) can take advantage of the spatial clustering common in rare plant populations to provide more efficient unbiased estimates of population sizes than simple random sampling. When plants are found in a quadrat, all adjacent quadrats are adaptively added to the sample. Despite this biased sampling, the Horvitz-Thompson estimator for adaptive cluster sampling provides unbiased estimates of population means or totals, and variances of those estimates. Because ACS disproportionately samples quadrats with plants, it can provide additional efficiency whenever further attributes of rare individuals need to be assessed, such as demographic parameters or genotypes. Adaptive cluster sampling was performed on a population of Aletris bracteata, a moderately visible herbaceous species, in a savanna near Chekika, Everglades National Park, Florida. Both 1-m2 and 4-m2 quadrats provided reasonable estimates of the population size. The 1-m2 sampling included 30–36% of the estimated total plants while sampling only 5% of the total area. The 4-m2 sampling captured 78% of the estimated total population while sampling only 21% of the area.

A GENERALIZED, VARIOGRAM-BASED FRAMEWORK FOR MULTI-SCALE ORDINATION

Multi-scale ordination (MSO) deals with potential scale dependence in spe- cies assemblages by studying how results from multivariate ordination may be different at different spatial scales. MSO methods were initially based on two-term local covariances between species and, therefore, required sampling designs composed of adjacent quadrats. A variogram-based MSO, recently introduced by H. H. Wagner, is applicable to very diverse sampling designs and for use with principal-components analysis, correspondence analysis, and derived ''two-table'' (also called ''direct'') ordination methods, i.e., redundancy anal- ysis and canonical correspondence analysis. In this paper we put forward an enlarged framework for variogram-based MSO that relies on a generalized definition of inter-species covariance and on matrix expression of spatial contiguity between sampling units. This enables us to provide distance-explicit decompositions of variances and covariances (in their generalized meaning) that are con- sistent with many ordination methods in both their single- and two-table versions. A spatially explicit apportioning of diversity indices is proposed for some particular definitions of variance. Referring to two-table ordination methods allowed the multi-scale study of re- sidual spatial patterns after factoring out available environmental variables. Some aspects of the approach are briefly illustrated with vegetation data from a Neotropical rain forest in French Guiana.

Pattern and impact of hornbill seed dispersal at nest trees in a moist evergreen forest in Thailand

Following the entire process of frugivore seed dispersal, from intake of seeds to seed deposition, is a difficult task. One alternative is to monitor areas of heavy seed rain deposited by animals. We quantified the number of seeds deposited by hornbills and followed the fates of these seedlings for 3 y to evaluate the effectiveness of hornbill seed dispersal at nest trees, on the basis of seedling survival. For 14 mo, fallen fruits and seeds were collected in traps established around four nest trees of each of two hornbill species (Aceros undulatus and Anthracoceros albirostris) and the seedlings were monitored in adjacent quadrats. Seedfall and seedlings of species represented in hornbill diets occurred at significantly higher densities in the traps/quadrats in front of nest cavities than in other traps/quadrats. Fewer seedling species and individuals germinated under nest trees than expected from the composition of the seedfall. Our results suggest that the quality of hornbill seed dispersal might be poor at nest trees due to the highly concentrated seedfall, which results in high seed and seedling mortality. Although seed deposition at nest trees is a useful guide to hornbill diet during the breeding season, it is clearly not of benefit to the plants involved. However, the pattern and consequences of hornbill seed dispersal at nest sites is likely very different from that during the non-breeding season.

Disease spread patterns of Banana streak virus in farmers, fields in Uganda

SummaryThree surveys were conducted to establish the disease spread patterns of Banana streak virus (BSV) in farmers, fields in Uganda. Transects were traced both across the fields and from infection foci within a field. BSV incidence in adjacent quadrats was also determined to quantify statistically the spatial relationships of infected plants in the fields. Severity assessment along transects across fields revealed clusters of plants with moderate to high severity and clusters of plants with no BSV or low severity. Symptom severity decreased away from foci of infection (b=−0.014; P=0.0081). Observed frequency of infected quadrat counts differed from corresponding expected frequency of infected quadrat counts (Poisson, s distribution, x2; P&lt;0.01). BSV– infected plants, therefore, were aggregated in well‐established fields. Aggregation of infected plants in farmers, fields and the decrease of severity away from infection foci suggest the likely involvement of a slow moving vector in BSV transmission.

Foci of Stagonospora Nodorum Blotch in Winter Wheat Before Canopy Development

ABSTRACT Stagonospora nodorum blotch (SNB) often develops explosively on upper leaves and glumes of wheat. Inoculum for late season infections may arise from early disease foci in the lower canopy or from recent immigration of wind-dispersed ascospores. Research was conducted to determine if foci of SNB are present and secondary spread has occurred in fields before tiller elongation. We determined the incidence of infection by Stagonospora nodorum for plants sampled at the mid-tillering stage in 96 1-m(2) quadrats in each of two fields. Isolates of S. nodorum were recovered from 32 quadrats, one per infected plant where possible. Multilocus restriction fragment length polymorphism haplotypes were determined for each isolate. Of 55 isolates collected from one field, there were 22 distinct haplotypes. Diseased plants were aggregated in both fields; aggregates sometimes extended to adjacent quadrats. Plants within aggregates were often infected by the same haplotype, suggesting that secondary spread had occurred. Foci overlapped because some aggregates were infected by more than one haplotype. Our results show that genetically diverse populations of S. nodorum were already established in fields before canopy development and were comprised of sometimes overlapping foci undergoing clonal expansion.

Identifying associations among site properties and weed species abundance. II. Hypothesis generation

Identification of associations between site properties and weed species abundance led to the generation of hypotheses as to why weed populations occur where they do, or do not, in agricultural fields. The objective of this research was to use a multivariate statistical technique, canonical correlation analysis, to identify the associations. Two continuous Zea mays production fields under center-pivot irrigation in the central Platte River Valley of Nebraska were grid-sampled between 1994 and 1997 for nine site properties and six to seven weed species. Weed species were identified and counted just prior to postemergence weed control in two adjacent quadrats (1 by 0.38 m) at each grid sampling point. These quadrats represented untreated weed populations emerging between crop rows and treated populations that survived preemergence herbicide banded within the crop row. Canonical correlation analysis identified one to five significant correlations between linear combinations of site properties and weed species abundance depending on field site, years, and between- vs. on-crop row weed populations. The first pair of linear combinations consistently described an association that separated weed species across a gradient of topography and soil type. The second pair of linear combinations described associations between weed species and soil fertility. In all cases, it was hypothesized that management practices strongly interacted with site properties to create the observed associations with weed populations. Other hypothesized mechanisms for weed patchiness include patchiness in available soil moisture that would influence weed seed germination, emergence, and seedling growth. Additional variation in plant-available preemergence herbicide concentration across the field site would vary weed control efficacy. Another mechanism would be variation in soil fertility that affects the growth, reproduction, and competitive ability of both the crop and the weed.