- Visual landscape changes after 43 years of fixed seasonal grazing in the Eastern Upper Karoo
|Last update: January 14, 2011 10:33:52 AM|
VISUAL LANDSCAPE CHANGES AFTER 43 YEARS OF FIXED SEASONAL
GRAZING IN THE EASTERN UPPER KAROO
L. van den Berg
Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg EC, 5900
E-mail: Loraine van den Berg
In December 1946, Dr. Charles Tidmarsh established the Bergkamp grazing trial at the Grootfontein Agricultural Development Institute (31° 22'S, 24° 45'E). The Bergkamp grazing trial is situated in the Eastern Eastern Upper Karoo within an extensive ecotone between the Nama Karoo Biome in the west and the Grassland Biome to the east. A complex mix of grass- and shrub-dominated vegetation types, which are subject to dynamic changes in species composition dependent on seasonal rainfall events, occurs within this vegetation type (Low & Rebelo, 1996).
The initial objective of the trial was to determine the effect of (a) continuous grazing, (b) continuous rest and (c) seasonal grazing on the species composition of mountainous vegetation in the Eastern Upper Karoo (Figure 1). The continuous grazed paddock (10.4 ha) has been continuously stocked since 1946, while the continuous rested paddock (0.5 ha) has been rested since 1946. Another portion was divided into four paddocks. The spring paddock has been grazed during spring each year (September and October) (0.9 ha). The summer paddock has been grazed during summer (November, December, January and February) (1.7 ha). A similar sized paddock was passed over and the autumn grazed paddock followed, being grazed during autumn (March and April) (0.9 ha). Following the autumn grazing, the animals return to the passed over paddock for the winter grazing (May, June, July and August) (1.7 ha).
In 1974 the Bergkamp grazing trial was terminated as a research trial and, in part, continued as a demonstration trial for training purposes. Even though the trial is currently used mainly for demonstration and training purposes, some interesting observation and monitoring surveys still continues. One of these includes photomonitoring whereby one can compile a series of photographs taken over time to determine differences in various vegetation aspects (Barker, 2001; Hurford & Schneider, 2006). While photographs reflect only limited aspects, much information can be gathered by comparing photographs taken of the same area over a number of years (McDougald et al., 2003).
Figure 1. Layout of the Bergkamp grazing trial
MATERIALS AND METHODS
Photos taken at various points in the Bergkamp grazing trial in 1967 were obtained from old research reports and repeat photographs taken in 2010. The two years (1967 and 2010) photographs were compared despite their differences and interpretations made. The photographic points included the following:
- Continuous grazed versus continuous rested paddocks
- Winter grazed versus summer grazed paddocks
- Autumn grazed versus winter grazed paddocks
- Summer grazed paddock
As a result of the fact that the 1967 photo points were not marked, the subsequent photos were taken by means of a triangulation procedure (Hall, 2001). This involved the identification of key reference points in the 1967 photographs and using the same reference points in the landscape for the 2010 photographs. A quick visual inspection was made in the field of each photograph taken in 2010 and its equivalent from 1967 to assess if they were comparable. In some instances this was difficult as vegetation density increased in some plots, making it hard to detect reference points, but an effort was made to ensure the photographs were comparable.
Photographs taken in 1967 were captured on 35 mm film and printed in black and white. These prints were scanned to obtain images in digital format. In 2010 digital photographs were taken with a CANON 450D camera (18-35 mm lens, 12 megapixels). These images were captured on a memory card and transferred to a computer. The 2010 photographs were manipulated to produce images comparable with the black and white images of 1967 (CorelDraw, 2005). Paired bitmap images from 1967 and 2010 were manipulated in CorelDraw X3 to produce vector images. Vector images store the lines, shapes and colours that make up an image by using mathematical formulas. These images are made up of solid areas of colour or gradients (Chastain, 2010), and were used to find similarities and dissimilarities between the photographs of the different years.
RESULTS AND DISCUSSION
The photographs from 1967 were taken in March (no indication as to the time at which they were taken), while the photographs from 2010 were taken in July at midday to ensure the least amount of shadows. The photographs in 2010 were however taken with some cloud cover present resulting in slight variation in colours and tones in the images. Despite the various differences in the recording of the photographs from the two years, they were compared and interpreted.
Continuous grazed versus continuous rested paddocks
Figure 2 shows bitmap and vector images comparing the continuous grazed and continuous rested paddocks from 1967 and 2010. These images showed an increase in both the density and size of woody vegetation (mostly Searsia erosa) in the continuous grazed paddock from 1967 to 2010 (to the left of the fence). The continuous rested paddock showed an increase in the size of the woody shrubs with some new shrubs appearing. The number of animals in the continuous grazed paddock was decreased from eight sheep to only four sheep in 1974 (Roux, 1974), but the continued grazing pressure in this paddock was enough to have a detrimental effect on the herbaceous layer.
Figure 2. Bitmap and vector images comparing the continuous grazed (to the left of the fence) and continuous rested (to the right of the fence) paddocks from 1967 and 2010
Figure 3 shows close-up images of the continuous grazed (to the left of the fence) and continuous rested (to the right of the fence) paddocks from 1967 and 2010. Similarly to Figure 2 it is clear that there has been an increase in woody vegetation in the continuous grazed paddock since 1967. The woody shrubs in the continuous rested paddock only showed an increase in size. The 1967 image showed a high percentage of bare soil and rock cover compared to the 2010 image that showed more grass in both paddocks. This is interesting as one would expect more grass to be present during the summer months (March) than in the winter months (July).
Figure 3. Bitmap and vector images comparing the continuous grazed (to the left of the fence) and continuous rested (to the right of the fence) paddocks from 1967 and 2010
Winter grazed versus summer grazed paddocks
When comparing images taken in 1967 and 2010 of the winter grazed versus summer grazed paddocks, an increase in woody vegetation in the summer grazed paddock (to the right of the fence) is observed (Figure 4). This could be as a result of a decrease in herbaceous cover and the resulting removal of the competition factor. Herbaceous vegetation grows mainly during summer months and is selectively grazed by sheep during this time. It, however, appears that there was an increase in herbaceous cover in the winter grazed paddock from 1967 to 2010, but this could be ascribed to the photographs not being taken in similar months or seasons.
Figure 4. Bitmap and vector images comparing the winter grazed (to the left of the fence) and summer grazed (to the right of the fence) paddocks from 1967 and 2010
Autumn grazed versus winter grazed paddock
Figure 5 shows the autumn grazed paddock (to the left of the fence) versus the winter grazed paddock (to the right of the fence) for both 1967 and 2010. It appears that more herbaceous vegetation established in both of the paddocks since 1967. In addition, the density and size of the woody vegetation did not increase very much in the autumn grazed plot since 1967. From these images it is clear that autumn grazing does not reduce grass cover but leads to a sparser grass cover as compared to that of winter grazing.
Figure 5. Bitmap and vector images comparing the autumn grazed paddock (to the left of the fence) and winter grazed paddock (to the right of the fence) from 1967 and 2010
Summer grazed paddock
When looking at the bitmap and vector images of the summer grazed paddock from 1967 and 2010 it is evident that the woody vegetation increased in density and size over a period of 43 years (Figure 6). This could be as a result of the annual removal of the herbaceous layer during the summer months. Grasses tend to compete with shrubs for water and nutrients (Ward & Esler, 2010) and this effect was therefore expected.
Figure 6. Bitmap and vector images showing the summer grazed paddock in 1967 and 2010
These images clearly show the detrimental effects of continuous grazing and continuous summer grazing on the herbaceous vegetation layer in mountainous areas of the Eastern Upper Karoo. In these paddocks there was a noticeable increase in the density and size of the woody vegetation. The continuous rested and winter grazed paddocks on the other hand showed only a slight increase in density and size of the woody vegetation.
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Grootfontein Agric 11 (1)