Abstract
Conversion of grasslands to woodlands may alter the sensitivity of CO 2 exchange of individual plants and entire ecosystems to air temperature and precipitation. We combined leaf-level gas exchange and ecosystem-level eddy covariance measurements to quantify the effects of plant temperature sensitivity and ecosystem temperature responses within a grassland and mesquite woodland across seasonal precipitation periods. In so doing, we were able to estimate the role of moisture availability on ecosystem temperature sensitivity under large-scale vegetative shifts. Optimum temperatures (T opt) for net photosynthetic assimilation (A) and net ecosystem productivity (NEP) were estimated from a function fitted to A and NEP plotted against air temperature. The convexities of these temperature responses were quantified by the range of temperatures over which a leaf or an ecosystem assimilated 50% of maximum NEP (Ω 50). Under dry pre- and postmonsoon conditions, leaf-level Ω 50 in C 3 shrubs were two-to-three times that of C 4 grasses, but under moist monsoon conditions, leaf-level Ω 50 was similar between growth forms. At the ecosystems-scale, grassland NEP was more sensitive to precipitation, as evidenced by a 104% increase in maximum NEP at monsoon onset, compared to a 57% increase in the woodland. Also, woodland NEP was greater across all temperatures experienced by both ecosystems in all seasons. By maintaining physiological function across a wider temperature range during water-limited periods, woody plants assimilated larger amounts of carbon. This higher carbon-assimilation capacity may have significant implications for ecosystem responses to projected climate change scenarios of higher temperatures and more variable precipitation, particularly as semiarid regions experience conversions from C 4 grasses to C 3 shrubs. As regional carbon models, CLM 4.0, are now able to incorporate functional type and photosynthetic pathway differences, this work highlights the need for a better integration of the interactive effects of growth form/functional type and photosynthetic pathway on water resource acquisition and temperature sensitivity.
Original language | English (US) |
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Pages (from-to) | 1389-1400 |
Number of pages | 12 |
Journal | Global change biology |
Volume | 18 |
Issue number | 4 |
DOIs | |
State | Published - Apr 2012 |
Keywords
- Eddy covariance
- Mesquite (Prosopis velutina)
- Net ecosystem exchange
- Photosynthesis
- Respiration
- Temperature optima
- Vegetative change
- Woody plant encroachment
ASJC Scopus subject areas
- Global and Planetary Change
- Environmental Chemistry
- Ecology
- General Environmental Science