Natural Vegetation and its Influence on Weed Populations in Neighboring Fields

Final Report for GS03-027

Project Type: Graduate Student
Funds awarded in 2003: $9,932.00
Projected End Date: 12/31/2004
Region: Southern
State: North Carolina
Graduate Student:
Major Professor:
Dr. J. Paul Mueller
North Carolina State University
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Project Information

Summary:

Natural vegetation on farms provide increased biodiversity, structural diversity,
habitat for wildlife and beneficial insects, and can act as protective buffers against
agrochemical drift. Nevertheless, farmers frequently view these areas as potential sources
of weeds, insect pests, and diseases. Objectives of this study were to examine weed
species richness and abundance in cropland bordered by managed versus unmanaged
field margins to determine if differences in weed infestation exist. Weed abundance and
richness were measured in crop fields along permanent transects that extended from the
field edge to the center of the crop fields. Presence/absence data for all plant species in
the field margin were also recorded.

Transect data from fields with margins of natural vegetation were compared to
transect data from fields with managed margins using analysis of variance. There were
no differences between log total abundance of weeds in crop edges adjacent to managed
and unmanaged field margins (P=0.44). For both margin types, more weeds were found
near the field edge than in the center of the field (1.37±0.08 to 0.52±0.07 and 1.39±0.07
to 0.41±0.06, for managed and unmanaged field margins respectively). Species richness
was slightly higher along crop edges of managed field margins (7.35±0.32) than crop
edges along unmanaged field margins (6.55±0.31). Managed field margins had lower species richness than unmanaged field margins – less than half the mean number of species (5.8±0.28 versus 14.7±0.62 species, respectively).

No association was found between plant species occurring in the field margin and
in the crop field by generating 2 x 2 contingency tables via PROC FREQ and testing the
association with Fisher’s exact two-sided test. Using logistic regression via PROC
GENMOD, margin type and weed presence in the field margin were not effective
predictors of weed occurrence in the crop field.

Introduction

Natural vegetation on farms such as field margins, fallow fields and wooded areas provide increased biodiversity, structural diversity, habitat for wildlife and beneficial insects, and can act as protective buffers against agrochemical drift (Kleijn 1997). Nevertheless, farmers frequently view these areas as potential sources of weeds, pests, and diseases. The relationship between managed and unmanaged field margins and adjacent cropland was examined relative to diversity, abundance, and distribution of weedy species.

Project Objectives:
  • to determine if crop edges adjacent to unmanaged field margins are associated with higher weed species richness and abundance than crop edges bordering managed areas

    to determine how weedy species are distributed in the field with respect to distance from the field edge

    to assess the presence of weedy species in the field margins and boundary areas and to determine if there is an association between weeds occurring in the field margin and the presence of the same species in the crop field

Research

Materials and methods:

Data Collection

In May and September of 2002 and 2003, weed diversity and abundance was measured in five fields along permanent transects that extended from field borders to the center of the crop fields. Sampling points were set at 1, 3, 7, 15, 30, and 50m into the field from the field edge. Sampling squares (90×75 cm) were laid perpendicular to the crop row.

Presence/absence data was also recorded for plant species in the field margin and boundary. Sampling points were set at the center of the managed farm tracks as well as in the natural areas at -2.5, -5, and -8m from the field edge. All species in a 75x75cm square were recorded.

Statistical Analyses

Weed species abundance was compiled by field, margin type (managed or
unmanaged), transect, distance from the field edge, and sampling date. The total counts for all species were analyzed using a stripped split plot analysis of variance with PROC GLM in SAS (SAS 1988) on log transformed data. Field and margin type were whole plot factors, distance from the field edge was the subplot factor and date was treated as stripped across other factors. The random effects were transects within field and margin type, distance by transect in field and margin type, and date by transect in field and margin type.

Species richness was determined for weed species in the crop field and for plants
found in the field margin and results were analyzed with PROC GLM. ANOVA on species richness was carried out separately for the crop field and field margin.

To determine whether an association existed between weeds appearing in the field
margin and presence in the crop field, weed abundance for each of ten dominant weed species in the crop field was converted to presence/absence data and combined with presence-absence data from the field margins. A 2 x 2 contingency table was generated using PROC FREQ in SAS by year and margin type for each species, and Fisher’s exact two-sided test was used to test for an association between the two areas.

Data from ten dominant weed species were combined across sampling dates and used to
test if margin type or presence in the field margin were predictors of presence of the weed in the crop field. Logistic regression using PROC GENMOD in SAS tested the importance of the predictors: field, margin type, presence in the field margin, and year. Reduced models were chosen for each species when appropriate.

Research results and discussion:

Weed Sampling in Crop Fields

For total weed abundance within crop field, field effect was significant (P< 0.001),
but margin type was not (P=0.44). This indicates that there were no differences, on average, between total abundance of weeds in crop edges adjacent to managed and unmanaged field margins. Distance effect was also significant (P< 0.001) indicating that total abundance of weeds differed according to distance along the transects into the crop field.

The mean of log total weed abundance for both managed and unmanaged margin types was observed to be greatest near the field edge and declined toward the center of the crop field. This pattern of weed abundance is consistent with previous research (Wilson and Aebischer 1995; Marshall 1989).

Species Richness in Crop Fields and Field Margins

Species richness for weeds in the crop field along the two field margin types differed
(P= 0.03). Crop edges along managed field margins tended to have greater mean number of weed species (7.35) than along unmanaged margins (6.55). It is not certain how biologically important this difference may be for agroecosystems.

Across all sampling dates, a total of 105 plant species were identified in the field
margins. Of these 105 species, 42 were found in the crop field representing 40% of total
species sampled (Table 8). These proportions are consistent with earlier observations of
Marshall (1989) in which 20-40% of field margin species were weedy species that commonly occurred in cultivated fields and at least 60% of plant species were found in the field margin only (i.e. not found in the cultivated field).

Species richness between the two field margin types was found to be significantly
different (P< 0.001). Managed field margins had less than half the mean number of species of unmanaged field margins (5.8 species versus 14.7, respectively). Thus, unmanaged field margins may have higher conservation and habitat potential than managed field margins.

Field Margins as Reservoirs: Association between Weeds in the Field Margin and Presence in the Crop Field and Presence in the Field Margin As Predictor

Based on the analysis of the 2 x 2 contingency tables using Fisher’s exact two-sided test, no reliable associations between presence in the field margin and presence in the crop field were found for any of the tested dominant species.

Logistic regression analysis indicated field as an appropriate predictor of the weed occurring in the crop field for most weed species. This is understandable considering that field history and knowledge of past weed populations that may have impacted the soil seedbank are very important when predicting what weeds will occur the following year and determining which management tactics to use. For most species, the factors of greatest interest to this research, presence in the field margin and margin type, were not associated with presence of the weed in the crop field.

Discussion

Previous research has indicated that some weed species invade crop edges from
adjacent field margins, however, these species are few compared to total numbers of plant species in the field margin and their impact has not been seen to be economically important. Blumenthal and Jordan (2001) suggested that because number of weeds is so limited, it may be possible to reduce management of natural habitat without negative consequences to the adjacent crop. Weed management in the field edge can be expensive and time-consuming for farmers who may not even have the proper equipment (Kleijn 1997). Eliminating this task from the list of farm chores could allow time and resources for weed management tactics or other necessary improvements to the farm.

Above all, this research supports the hypothesis that natural and semi-natural habitats
in the farm landscape are not necessarily associated with weed contamination of fields. Agroecosystems contain two dynamic systems, managed and natural, that interact in positive and negative ways. To more fully recognize the benefits of such interactions, farms should be managed on landscape as well as field levels.

References

Blumenthal, D. and N. Jordan. 2001. Weeds in field margins: a spatially explicit
simulation analysis of Canada thistle population dynamics. Weed Science. 49,
509-519.

Kleijn, D. 1997. Species richness and weed abundance in the vegetation of arable
boundaries. PhD thesis, Wageningen Agricultural University. Wageningen, 177 p.

Marshall, EJP. 1989. Distribution Patterns of plants associated with arable field edges.
Journal of Applied Ecology. 26: 247-257.

SAS Inst., Inc. 1988-2001. SAS/STAT User’s Guide: Release 8.02 Edition. SAS Inst., Inc., Cary, NC.

Wilson, P.J. and N.J. Aebischer. 1995. The distribution of dicotyledonous arable weeds
in relation to distance from the field edge. Journal of Applied Ecology. 32: 295-
310.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

The full report of this research was submitted as part of the requirements for Masters of Science in Crop Science at North Carolina State University. Manuscript is in preparation for submission to Agriculture, Ecosystems and Environment.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.