In Brazil, pastures for cattle ranching are being established in areas that
were previously forested. To investigate some consequences of this change in
land use we measured fluxes of CO2 and water vapour over
a typical pasture, dominated by the introduced C4 grass
Brachiaria brizantha. In addition, we compared the
CO2, water vapour fluxes and canopy stomatal
conductances observed with those obtained simultaneously over a nearby
undisturbed rain forest. Measurements were made near the end of the wet season
under conditions of ample soil moisture. Leaf area index of the pasture was
3.9.
The pasture had a lower canopy stomatal conductance than the forest (typically
0.2–0.3 mol m-2 s-1 versus
0.4–0.9 mol m-2 s-1 at high
photon irradiance) and was less responsive to the canopy-to-air vapour
pressure difference. As a consequence of these lower canopy stomatal
conductances, the pasture used much less water than the forest with average
values over the period examined being 153 mol H2O
m-2 d-1 and 249 mol
H2O m-2 d-1
for pasture and forest respectively (2.74 and 4.48 mm
d-1 respectively). This was also reflected by differing
fractions of the absorbed energy being dissipated as evaporation. This
proportion was typically 0.56 for the pasture and 0.74 for the forest.
After allowing for soil and plant respiration, average daily photosynthetic
rates were 0.67 mol C m-2 d-1 for
the pasture and 0.57 mol C m-2
d-1for the forest (8.0 and 6.8 g C
m-2 d-1, respectively). Thus,
despite an appreciably lower rate of water use the pasture assimilated more
carbon on a daily basis. Nevertheless, Brachiaria
displayed a somewhat lower rate of photosynthesis than expected for a
C4 grass, perhaps because of a low nutrient status.
Indeed, at low and medium photon irradiance the pasture and forest showed
remarkably similar photosynthetic performance. There was, however, less
tendency for CO2 assimilation rates of the pasture
canopy to saturate at high photon irradiance. The respiratory fluxes from the
two ecosystems at night were quite similar, 6–8 µmol
m-2 s-1.
The ratio of intercellular CO2 concentration to ambient
CO2 concentration was usually 0.4 to 0.6 for the
pasture, a range which is higher than that often reported for
C4 plants but possibly not unusual for tropical grasses
in their natural environment.
Photosynthetic performance, nutrient status and lipid yield of microalgae Chlorella vulgaris and Chlorococcum humicola under UV-B exposure
