Research Results - (PDF, 101 mb) - USAID

15 main wooden branches, the larger class
contained shrubs with more than 15 main
branching axes. On average there were 834
shrubs per plot that fell into these two size class
categories, with a little more than half (58%) of
the sample group in the larger class. Every
month, 12 plants in each size class in each plot
were harvested, and a corresponding number of
permanently marked shrubs was measured.
Measurements on marked plants were taken on
permanently marked axes, three axes in the case
of plants in the small size class and five for
plants in the large size class. On each marked
main axis, the number of branches and the
number and length of shoots on each branch
were measured. For the destructive sampling,
axes comparable to the marked plants were
chosen and number of branches on each axis
was recorded, the length of each shoot on a
branch was measured, and the dry weight of
each shoot (current growth) determined. Total
plant weight and dry weights of woody and
consumable fractions were obtained. Data were
also taken on plant and soil water potentials
each month.
The above experimental design and
methodology are presented at some length to
show the detail and magnitude of the data set
and to justify its use in the current synthesis
effort as a data base for a model which simulates
plant growth and shoot demography under a
variable climatic pattern. General features are
emerging as these data are being massaged,
such as thL recruitment of new shoots that
occurs if there are late spring rains (after May
10), the summer mortality (20-27%) of all young
shoots irrespective of rainfall regime, and the
tendency for there to be omly three to four new
shoots growing on woody branches no matter
how wet or dry the year.
The data are being analyzed by means of
correlation and regression, relating dimensional
and categorical records to weight measures, and
relating shoot demographic behavior and
growth patterns to rainfall and soil water status.
Model conceptualization and development is
under the direction of Drs. Berkat and Norton,
with assistance from Jean-Pierre Bertrand, a
French plant ecologist at IAV.
110
Plant-Animal Interactions Under Different
Grazing Intensities
Steppe vegeiation in a mosaic of Artemisia-dominated
and Stipa-dominaied communities
covers 50 million hectares of the semi-arid
zone in North Africa and the Middle East. The
primary use of these rangelands is for grazing
by small ruminants, and hence they constitute a
major economic as well as ecological resource.
Both the low Artemisiashrubs and the Stipa
grass tussocks are harvested for fuel by the
growing rural population, and the most productive
steppe sites are progressively being preempted
for farming. Heavy grazing imposes
additional pressure on the North African steppe,
and therefore, in order to manage these shruband-grass
rangelands for sustained productivity,
it is important to understand the levels of
grazing pressure for which the ruminants can
make adequate adjustment, and that the ecosystem
will tolerate. A grazing experiment was
initiated near Ain Beni Mathar in 1989 to study
plant-animal interactions as stocking rate increases.
The work is under the direction of Dr.
Ahmed El Aich.
Four stocking rates were employed,
ranging from 0.43 to 1.43 young rams per hectare,
with the stocking rate effect implemented
by adjusting paddock size for a standard herd of
five rams, in two replications. [Rams were
preferred over ewes as the experimental animals
because of the relative ease of fecal collection for
intake and diet digestibility determinations.]
The trial began in January and continued for six
months, covering the spring growing season
and terminating in the summer when supplementary
feed is normally provided.
As the stocking rate increased, the
animals spent more time grazing (468 vs. 545
mn/day at the SR extremes), less time resting
(resting time almost halved at the highest SR),
and walked further each day (6.3 vs 8.3 km at
the SR extremes). By these behavioral changes
the rams were able to maintain diet quality,
although intake declined by 9% and harvest
efficiency also suffered (22% less intake per
minute and 31% less per km walked, expressed
on a metabolic liveweight basis). As one might