To investigate seasonal and annual influences of environmental conditions on
leaf net CO2 uptake (A), three
codominant species from the north-western Sonoran Desert differing in
photosynthetic pathway and leaf phenology were examined: the
C3 deciduous sub-shrub
Encelia farinosa, the C4 deciduous
bunchgrass Pleuraphis rigida, and the CAM evergreen leaf
succulent Agave deserti. To allow interspecific
comparisons and to predict field responses from 1974 through 1995, an
environmental productivity index (EPI) model previously developed for CAM
plants was used, which scaled the responses of A to
water, temperature, and photosynthetic photon flux (PPF) over 24-h periods to
individual dimensionless values. The net CO2 uptake
predicted using the EPI approach agreed well with field measurements.
Agave deserti was the most drought-tolerant and
E. farinosa was the least; the optimum day/night air
temperatures and the PPF requirement for A were highest
for P. rigida and lowest for
A. deserti. For 1974 through 1995, daily EPI averaged
over a year was highest for E. farinosa, indicating that
it operates closest to its photosynthetic optimum. However, the predicted
A was highest for P. rigida.
Variations in A were annually bimodal, with the greatest
differences among the three species in wet years. Afor
all three species increased linearly as annual rainfall increased. Leaf area
per plant for E. farinosa was highest in the winter and
early spring and did not respond appreciably to summer rainfall; leaf area for
P. rigida was also highest in the winter. For the
evergreen A. deserti, which based on ground cover is the
dominant species at the field site, new leaves unfolded in response to both
winter and summer rainfall but most photosynthetic area was contributed by
older leaves, leading to the highest annual plant net
CO2 uptake.
Winter daytime warming and shift in summer monsoon increase plant cover and net CO2 uptake in a central Tibetan alpine steppe ecosystem
