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Flora and Fauna Changes During Conversion from Conventional to Organic Farming

1 Introduction (Esbjerg, P. & Petersen, B.)

1.1 Background

The present project belongs to a series of pesticides research projects derived from the governmental Pesticide Action Plan I of 1986. The goal of this plan was to reduce the pesticide use in Denmark by 50% from 1987 to 1997 both in terms of amount and in terms of intensity, expressed as treatment intensity index.

In line with this plan and its purpose of reducing environmental pressure from crop production two major project areas have been A) changes in agricultural practises aiming at reduced pesticide use, and B) effects of reductions in pesticide use. Among the latter the very recent project “Effects of reduced pesticide use on flora and fauna in agricultural fields” (Esbjerg & Petersen 2002, later in this report mostly referred to as the conventional period) has demonstrated significant improvements of plant-, insect- and birdlife by use of less than half dosages of herbicides and insecticides. Earlier Braae et al. (1988) and Hald & Reddersen (1990) found higher diversity and densities of plants, insects and birds at long established organic farms than at conventional farms but no studies had been dealing with an array of different organisms during the conversion phase. Such a possibility emerged when one of the five hosting farms, Oremandsgaard Gods, of the above mentioned project decided to convert from conventional to organic production immediately after the finalization of the project work. This possibility included the particular advantage of utilizing the former large-scale plots with three dosage levels of herbicides and insecticides in winter wheat and spring barley. Furthermore intensive studies had been carried out on wild plants, arthropods and birds during the three years prior to the conversion implying that the relevant biological history of the area was well established.

1.2 Aim and conditions

The over-all aim of the present project was to quantify changes in biodiversity of the flora and fauna in agricultural fields the first two years after the conversion from conventional to organic farming (afterwards mostly referred to as the organic period). This includes a series of underlying hypotheses:

• That the number of species of wild plants and their abundance will increase after conversion.
• That the abundance of insects and proportions between taxonomic units will change after conversion.
• That the number of birds will increase after conversion.
• That increases in plants and insects will reflect the preceding dosage.
• That increases will be correlated so that increases in bird density reflect increases in arthropod abundance, which reflect increases in density and diversity of wild plants (inter-trophic effects).

1.3 The study area

Oremandsgaard Gods, which as mentioned in the introduction was one of the host farms 1997-99 accepted the hosting of this project (2000 and 2001) also. The field area of relevance is shown on Fig. 1.1. The area has been grown conventionally for several decades in crop rotations dominated by cereals. Here it has to be underlined that dosage plots at large scale of at least 6 ha each (enabling the necessary bird counting) were established from the beginning of “the conventional period”. These plots were treated with three different dosages of herbicides and insecticides while fungicides were applied equally all over. Normal dosage was by definition the farmer’s own choice in the particular instance (based on his experience), and the two other plots were treated with 50% and 25% of normal respectively. The pesticides applied as well as application dates are listed in appendix A.

In the organic phase (2000-2001) the project used two crops, spring barley and winter wheat, obtained by using two of three fields at a time. In 2000 field 1 (Fig. 1.1) with spring barley (and undersown rye grass and clover as catch crop) and field 2 with winter wheat (sown autumn 1999) were used. As appears from appendix B (with details on field operations) field 3 was also cropped with winter wheat in 2000 but no results were taken from that field. In 2001 field 1 was not used for study, while field 2 for the second year had winter wheat (sown in September 2000), although a catch crop of rye grass and clover was sown in May. The project field with spring barley in 2001 was field 3, sown with barley mid April and further sown with a rye grass and clover catch crop late in May.

As regards the field area it may be described as generally rather uniform and flat with the exception of a slight undulation of the western part of field 3, which also has an old marl-pit (in the former half dosage plot). Such a pit can also be seen in field 2 outside the plot area. Along the western side of field 3 no tall vegetation is present, whereas the southern edge is along an old alley of Acer pseudoplatanus trees also edging field 2. Hedges of broadleaved bushes and small trees form the borders against north and east, the latter also being the edge of field 2. Between field 1 and 2 no natural vegetation is present, while field 2 has a beach forest on the eastern side and a field along the NE-corner. That field also neighbours field 1 up to the partly eastern and mostly northern forest belt dominated by deciduous trees. On the remaining north border and to the western side field 1 is aligned with a broadleaved hedge, very open and scattered along the southern part. Generally the hedges are 3-5 meters tall.

Among the field operations particularly the many mechanical weedings are noteworthy, 4 times in barley 2000 and 2 times in the already established wheat (field 2) of that year, while the wheat in the same field in 2001 was treated 3 times and barley 2001 (field 3) was treated 4 times.

View picture in full size

Fig. 1.1.:
Aerial view of the experimental fields at Oremandsgård with the dosage plots indicated. Large fields in a fairly open landscape. Single marl-pits, surrounded by trees and scrub, in Fields 2 and 3. Deciduous hedgerows, partly quite open, N and W of Field 1, between Fields 2 and 3, and N of Field 3. Alley with old, broad-leaved trees S of Fields 2 and 3. Old, deciduous wood E of Field 2 and farm buildings towards the SE. Photo by courtesy of Kampsax.

1.4 Weather

For the organic period of the present project, data on average temperatures and precipitation have been obtained from two substations of The Danish Institute of Meteorology. On this background the two years, 2000 and 2001, may briefly be characterized as follows. The late winter 2000 had a remarkably warm February (3.8ºC in contrast to a normal mean of 0). Also April 2000 was well above (8.5ºC) the normal (5.7ºC) and in addition rather dry (27 mm against 41 as normal). These April conditions may be of interest for that year’s high level of aphids in barley at the end of May, which was in any respect very normal. Except fairly dry weather in July and August the rest of 2000 was an average year. In 2001 the first four months were normal but May was rather warm (12.1ºC, normal 10.8) and dry (21 mm, normal 48 mm). While the temperature was about average June 2001 was rainy (84 mm, normal 55), whereas July was rather warm (18ºC, normal 15.6) and dry (27 mm, normal 66). The dryness was followed by excessive rain in both August (103 mm, normal 67) and September (127 mm, normal 73). Temperatures were normal and so was the weather during the last part of 2001.

1.5 Data sampling

As in the conventional period non-destructive plant studies were carried out in at least four subplots within each of the three dosage plots per field. During the organic period biomass of crop, catch crop, and weed plants was sampled. In both periods the above-ground seed biomass was sampled in September.

Arthropod sampling was as previously carried out with the special suction device constructed for the samplings in the conventional period (Navntoft & Esbjerg 2002). During the organic phase better weather conditions permitted almost weekly samplings with a total in barley of 9 in 2000 and 13 in 2001, in both cases roughly between mid May and late July. The number of samplings was the same in wheat with the exception that 10 samplings were performed in 2000 (more details appear from table 3.1).

As during the conventional period standardized bird counts (point and transect) were performed at intervals of about five days in the organic period. Counting took place between 6 May and 5 August.

In the organic period Skylark faeces were collected and arthropod fragments identified in order to identify important arthropod food items for Skylarks. The collections were carried out on the same days the bird countings were performed.

1.6 Statistical analyses

During the years of conventional farming, the experiment was run as a Latin Square design with three crops rotating between three fields in three years (Esbjerg & Petersen 2002). Each field was divided into three plots, which received different pesticide dosages and constituted the basic experimental units. Although the plots did no longer receive different treatments after conversion to organic farming, they were retained as the experimental units because carry-over effects of differences in pesticide dosages could be traced in the analyses of the vegetation. After conversion only spring barley and winter wheat were left in the rotation so analyses had to be limited to these  two crops, leading to an unbalanced and confounded experimental design (cf. section 1.3) with n=30 (5 years × 2 crops × 3 plots).

The main analytical purpose was to quantify and test the effects of the conversion to organic farming on selected response variables, representing different groups of organisms, with the null hypothesis that such effects were non-existent. This was done by comparing the conventional and the organic period (1997-1999 and 2000-2001, respectively), using the different years within each period as replications. Crop, field and dosage effects were also included in the model, together with the appropriate interactions, leading to the basic analysis of variance model shown in Table 1.1. Notice that it was not possible to include the interactions period×field and crop×field because they were confounded with period×crop and year, respectively. Starting with the full model, stepwise model reduction was carried out until the minimum adequate model was found.

Click on the picture to see the html-version of: ‘‘Tabel 1.1‘‘

The error terms in Table 1.1 are only fully valid in a balanced experimental design. Because the experiment was not balanced the F-tests had to be modified, causing the tests to be only approximate. The modifications were carried out by means of the RANDOM/TEST statement in the GLM procedure in SAS/STAT (SAS Institute 1990b), which prompts the use of Satterthwaite‘s approximation to adjust the denominator and the associated degrees of freedom.

Except for the analyses of flora composition and Skylark food preferences, all response variables were tested using this model or models derived from it. If necessary, variable transformation was performed in order to achieve an approximately normal distribution. Covariates were included if relevant, e.g. weed density was included in the model of weed species richness. Covariates were also used to model impacts of changes at one trophic level on another trophic level, e.g. possible effects of arthropod abundance on the number of  Skylarks were tested by including arthropod biomass in the model of Skylark density.

Additional statistical procedures and further details are described in the individual chapters on the botanical, entomological and ornithological studies (chapters 2-5).

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