| dc.description.abstract | A semi-arid grassland was investigated for biomass
production and the factors that influence its productivity.
Live aboveground biomass ranged from 73 g m-2 in
October 1984 to 338 g m-2 in April 1985. Peak live
biomass was attained during the rainy season. Peak dead
aboveground biomass was attained during the dry season.
Dead aboveground biomass ranged from 66 g m-2 in
December 1984 to 651 g m-2 in September 1985. From
September 1985 dead aboveground biomass kept a constant
trend fluctuating between 400 and 600 g m-2. Standing
dead biomass ranged betwee? 19 and 457 g m-2 while
litter biomass ranged,b,etween 50 and 190 g m-2.
Stratified clipping revealed that most of the live
aboveground, biomass (64%) occured in the first 0-20 cm
canopy layer above the soil surface. Individually, T.
triandra contributed 35% of total aboveground live
biomass, P. mezianum 26%, other grasses combined
- 21% and dicots with sedges 18%.
Belowground live biomass was highest during . the dry season and ranged between 60 and 260 g m-2.
Dead belowground biomass ranged between 12 and 345 '.:";- .
Relative decomposition rate ranged between 0.023
~nd 0.18 g g-l mon-1 for aboveground dead herbage and
0.076 and 0.335 g g-l mon-1 for belowground dead.
Net primary productivity was 1332.4 g m-2 yr-1
(3.65 g m-2 d-1) for aboveground compartment while
it was 965.8 g m-2 y r+! (2.65 g m-2 d+ v ) for belowground
compartment. Monthly net primary production
ranged between 9 and 324 g m-2 for aboveground material
and between 8 and 357 g m-2 for belowground material.
Turnover rates for different plant materials
ranged between 0.4 for aboveground dead and 2.5 for the
litter.
Leaf area index ranged from 0 to 3.09 while
stem and sheath indices ranged from 0 to 0.95 and 0 to
1.53 respectively. Total area index ranged between 0
and 5.57.
Total solar radiation averaged 19.7 MJ m-2
d-1 at the top of plant canopy. 64% (or 12 MJ m-2
d-1) was intercepted by the plant canopy during the
growing season. The efficiency with which the intercep-
,
ted energy was used to produce newdpY matter ranged
from 0 to 0.31 ~ MJ-l. ~... .
Soil moisture content ranged f~om 6% (during the
-
dry season) to 35% (during the wet season) in the first
o - 5 cm of the soil column. Rainfall amounts ranged from
o to 212.6 mm.
Field measurements of physiological variables
showed that seasonal rates of photosynthesis ranged ...
from 0 to 26.8 Vmol m-2 S-1 in T. triandra and 0 to
27.1 pmol m-2 S-1 in P. mezianum at midday. At the same
time seasonal rates of transpiration ranged from 0.83
to 9.51 mmol m-2 S-1 in T. trindra and 0.93 to 16.2
mmol m-2 S-1 in P. mezianum. Stomatal conductance
ranged from 0.095 to 0.533 cm S-1 in T. triandra and
0.148 to 1.1 cm S-1 in P. mezianum. Leaf water
potential ranged from -2 MPa to <-4 MPa at midday. Air
temperatures were optimum and ranged between 30 and
J50C at midday while vapour pressure deficit ranged
from 30 to 40 mbars.
Top leaves in the plant canopy possessed the
highest ra~~s of photo~ynthesis (22 - 27 pmol m-2 S-1)
followed by the middle canopy leaves (10 - 12 pmol m-2 ,
S-1) and finally by the bottom canopy leaves (8 - 10 pmol
m-2 s-1).Diurnal course in photosynthetic rates showed
that the highasL values were ~ttained between 11.00 and
12.00 hours during the growing season. During the early
dry season peak values occured early in the morning
hours (10.00 hours) and eveniq$ hours (16.00 hours)
with a midday stomatal closure. The same trend of onepeaked
trend during the growing season and two-peaked
trend during the qEY season was observed for transpiration
rate and stomatal conductance.
Laboratory work was carried out on stomatal count,
chlorophyll determination and photosynthesis by
different plant structures (leaf blades, stems, sheaths
and inflorescences). Stomatal count revealed that
all structures contained stomata which ranged
between 836 cm-2 in stems of T. triandra to
29866 cm-2 on the adaxial surface of leaf blade of T.
triandra. Total chlorophyll ranged from 0.18 mg g-l
fresh weight in the stem of R. repens to 2.49 in the
leaf blade of T. triandra. Ratio of chlorophyll a/b
was lowest (2.55) in the sheath of C. caesius and
highest (5.73) in the leaf blade of P. mezianum. Mean
photosynthetic rates ranged from 0.38 pmol m-2 S-1 in
the inflorescence of C. caesius to 28.68 pmol m-Z S-1
in the le&f blade of ,T. triandra.
Water stress experiment showed that photosynthesis
decreased with water stress effects. Mean rates of
C02 assimilation decieased by 47% in T. triandra.
The decrease-was by 100% in P. mezianum.
With rewatering, photosynthetic rates increased by
49% in T. triandra and by 109% in P. mezianum. Both
leaf water potential and chl~rophyll levels decreased
with increase in water stress. Leaf water potential
decreased from between -1.08 and -1.48 MPa to between
-1.6 and <-4 MPa~while total chlorophyll decreased from
between 1.14 and 2.52 mg g-1 to between 0.99 and 1.76
mg g-1. However, after rewatering leaf water potential
increased to between -1.39 and 3.24 MPa while total
chlorophyll increased to between 1.05 and 2.1 mg g-l.
Leaf anatomy of the grass species used in the
water stress experiment revealed that the grass species
were either NADP-me or PEP-ck C4 Kranz sub-types. NADP-me
species included T.triandra, P. mezianum, C. caesius
and C. ciliaris while PEP-ck species were R. repens, E.
paspaloides and D. aegyptium. Scanning electron micrographs
of two grasses; T. triandra and P. mezianum
confirmed that stomata occured on both adaxial and
abaxial surfaces of leaf blade. Stomata were often
found in crypts in T. triandra. Except in the stem,
epidermal cells of T. triandra possessed protrusions
called papillae. Epidermal surfaces of leaf blade,
stem, sheath and inflorescence possessed numerous
",
stomata, prickles, microhairs and waxy cuticle.
Spectral reflectance ratio (SRR) data revealed
that t.here wa s a good correla!-ion (r = > -0.8) between
SRR data and live aboveground biomass during the
growing season and that the method was a good predictor
of live aboveground biomass. However, during the dry
season when plant leaves were senescent, the
correlation was 'poor (r = <-0.5) and SRR was not a good
method to p~~dict the amount of live aboveground plant
biomass from the SRR data . | en |
