A METHOD FOR THE GRAPHING OF PHENOLOGICAL CYCLES
Grootfontein Agricultural Development Institute, Middelburg CP, 5900
The phenological stage of a plant plays a very important role in the diet selected by sheep. Sheep will readily graze fresh grass (in summer) but will not readily graze dry grass (in winter). Danckwerts (1984) found that the digestibility of sweet-veld drops from about 64 % in spring to about 57 % in winter. When some species are flowering, sheep will select them in considerable quantities, as opposed to when they are not in flower. Danckwerts (1987) also found that, during certain times of the year, plants are more sensitive to grazing. He showed, by isolating the effect of time of the year, that Sporobolus fimbriatus and Themeda triandra are particularly sensitive to perturbations in spring.
The growth stage of a plant is a factor that influences the palatability of a plant. Animals prefer younger growth to older growth. Flower buds, flowers, fruits and seeds are high on the priority list of grazers because they contain much more crude protein, fats and carbohydrates than the leaves and stems (Botha, 1979). With the maturation of grass the fibre and lignin content increases and makes the grass less acceptable to sheep. Sheep and cattle also concentrate more on eating leaves and fruit than stems (Heady & Torrel, 1959).
Plants are more sensitive to grazing during their active growing period and, since there are differences in the phenology of plant components, the time of the year when a camp is grazed will favour some components more than other. It can be reasoned that by knowing the phenological cycles of the key plants in a region, certain grazing treatments or adjustments can be carried out. Tidmarsh (1951) and Roux (1964) found that the effect of repeated summer grazing on karoo vegetation resulted in the elimination of the perennial summer grass component and the palatable karoo bushes, and conversely, promoted the growth of unpalatable and invader species.
The objective of this article is to devise and test a method to determine the phenological cycles of plant species in the Karoo. The collection of data must be rapid, accurate within certain limits and easily quantified for the construction of phenographs.
For this study an exclosure paddock (2 ha) at the Grootfontein Agricultural Development Institute in the Middelburg CP district was selected. The geographic coordinates for this site are 31 0 25' 45" S; 25 0 03' 37" E. The site is situated within Veld Type 36, False Upper Karoo (Acocks, 1988) and RHFA (Reasonably Homogenous Farming Area) 4.8 of the Karoo Region (Vorster, 1985). Small communities and representative species were marked with a steel wire with a metal tag attached. The names of the species were stamped on the tags. By means of these identifications the communities and species were easy to relocate quickly. Nine predominantly grass communities and nine karoo bush communities were scored. Table 1 gives a list of the plants that were phenologically scored, and their different group classifications according to Vorster (1980) and Blom (1981).
A field score sheet was drawn up to score the different phenophases of the species (Fig. 1). The phenological data were collected every third week over a period of 500 days. The observation of 18 species started on 1 July 1989 and ended on 15 November 1990. After a thorough investigation of a community the phenophases were scored according to the scales set out in Fig. 1 (the field score sheet).
Some of the items and categories are similar to those used by Hoffman (1989) and Zietsman, Van Wyk & Botha (1989). Fig. 1 was, however, found to provide a very sound base for determining phenological cycles of karoo species.
Environmental condition could range from arid to moist (see the categories in Fig. 1) and was largely based on the soil and rainfall.
Plant condition ranges from dry to very fresh (lush) and is also directly related to growth activity. The categories were relatively easy to determine.
Stem and shoot growth ranges from no growth to visible growth to very active growth. Stems and shoots usually clearly showed degrees of growth. Stem growth was more difficult to determine.
Leaf growth ranges from no growth to active growth and could be relatively easily determined. Young, fresh leaves usually indicated active growth, whereas the absence of new and young leaves indicated little or no growth.
Flower buds range from none to many. This category was very easy to determine.
Date and number of flowers were largely based on the presence of open flowers and, as such, was an easy item to categorise. In the case of grasses this was somewhat more difficult and often required close inspection.
If necessary, phenophases (items) like leaf colour, presence of fruit and seeds, grazing utilisation, leaf abscission, etc. could be added to Fig. 1.
The simplicity and wide range of the category scales made determination quite accurate. The categories, e.g. arid, dry, moderate, moist, etc. (in Fig. 1) were quantified by making use of a scores table (Table 2).
Table 2 should be used in conjunction with the completed field score sheet (Fig. 1) to quantify the different observations in that sheet. Observe the different categories, 1 to 4 (e.g. arid, dry, moderate and moist) of the different phenophases (e.g. environmental condition, with one of the scores or score combinations (i.e. X, or X√ or √X) in Table 2 to quantify that phenophase. If a plant is scored and the dominant category of the phenophase (item) is, for example, category 3, then the score for that phase is 70 (see Table 2). If a phase is between two categories (cross-tickmark or tickmark- cross-combination) match this combination with one of the category scores under columns B to G. The particular score for the combination is indicated in brackets. Once all the data is quantified in this fashion, phenographs can be constructed to show the march of the phenology of the key plants, or groups, over a period of time.
RESULTS AND DISCUSSION
For the purpose of this article only the quantified scores of Themeda triandra are illustrated in Table 3.
From the results of all the species concerned it is clear that there is a tendency for species in the same groups to have similar phenological cycles. Fig. 2 was constructed from the data as set out in Table 3.
Certain deductions can now be made from Fig. 2. It is clear that the severe grazing of this particular grass between March and April 1990 would definitely have had an effect on seed production. The reason is that during this period the development of flower buds and flowers reaches a peak. Grazing of the grass during this period removes the buds and flowers. It is also clear that during the winter months the grass is dormant. This is a very suitable period to graze the grass extensively without the fear of causing severe damage.
This proved to be an easy and rapid method to determine the phenological cycles of plant species in the Karoo. If similar results were to be accumulated over 10 years or more it would be possible to predict certain phenological trends for the different species. This method can contribute towards effective veld management in the Karoo.
ACOCKS, J.P.H., 1988. Veld Types of South Africa. Mem. Bot. Surv. S. Afr. 57. Dept Agric., Pretoria. (Third Edition).
BLOM, C.D., 1981. Group Classification of Karoo Vegetation. Unpublished (Director Karoo Region, Middelburg, CP).
BOTHA, P., 1979. Factors influencing the palatability of herbage and species selection by the animal. Karoo Agric 1 (2): 13-18.
DANCKWERTS, J.E., 1984. Towards improved livestock production off sweet grassveld. Ph.D. Thesis, Univ. Natal.
DANCKWERTS, J.E., 1987. The influence of tiller age and time of year on the growth of Themeda triandra and Sporobolus fimbriatus in semi-arid grassveld. J. Grassld Soc. Sth Afr., 4,3:89-94.
HEADY, H.F. & TORREL, D.T., 1959. Forage preferences exhibited by sheep with oesophageal fistulus. J. Range Mgmt. 12: 2s-43.
HOFFMAN, M.T., 1989. A preliminary investigation of the phenology of subtropical thicket and karroid shrubland in the lower Sundays River Valley, S.E. Cape. S. Afr. J. Bot., 55(6): 586-597.
ROUX, P.W., 1966. The effects of seasonal rainfall and grazing on mixed Karoo veld, Proc. grassld. Soc. Sth. Afr. 1: 103-110.
TIDMARSH, C.E.M., 1951. Veld management studies at the Grootfontein College of Agriculture, 1934-1950. Pasture Research in South Africa. Prog. Report No. III, Part 1, Dept Agric., Pretoria.
VORSTER, M., 1980. Eienskappe wat 'n rol speel by veldevaluering in the Oostelike Karoo. M.Sc.(Agric.)-verhandeling. UOVS Bloemfontein.
VORSTER, M., 1985. Die ordening van landtipes in die Karoostreek in redelik homogene boerderygebiede deur middel vanplantegroei en omgewingsfaktore. D.Sc.-proefskrif, PU vir CHO, Potchefstroom.
ZIETSMAN, P.C., VAN WYK, A.E. & BOTHA, F.C., 1989. Vegetative and reproductive phenology of Ziziphus mucronata subsp. mucronata (Rhamnaceae). S. Afr. J. Bot., 55(6): 564-573.
Karoo Agric, Vol. 5, No 2, 1993 (1-4)