Global Biogeography & Biogeochemistry of C₄ Vegetation

The goal of this project is to improve our understanding of C₄ photosynthesis in the global carbon cycle. C₄ plants are functionally different from C₃ plants in several important respects, including stomatal conductance, responses to light, temperature, nitrogen, and CO₂, as well as carbon and oxygen isotope fractionation during photosynthesis. As a result, global change affects C₃ and C₄ plants differentially, with important implications for such essential human needs as food and fiber production and fuel gathering. Climatic variations and land use change heavily impact C₄-dominated ecosystems, and C₄ crops are critical for the nutritional support of several hundred million people. The carbon isotope signature of biomass burning products in low latitudes depends critically on the relative amounts of C₃ and C₄ vegetation and their interannual variations. Finally, knowledge of seasonal and interannual C₄ carbon fluxes is essential to accurate partitioning of land and ocean carbon sinks through inversion of atmospheric ¹³CO₂ and CO₂ data because of the similar isotopic imprint that C₄ and oceanic carbon exchanges leave on the atmosphere. Unique isotope disequilibrium fluxes attend these variations and must be accounted for in order to fully exploit the use of ¹³C for interannual land:ocean partitioning.

The photosynthetic pathway composition (C₃/C₄ fraction) is a fundamental ecological distinction in tropical and subtropical ecosystems, as well as in many temperate grasslands. C₄ plants likely will respond quite differently than C₃ plants to the suite of anthropogenic global changes expected in the coming decades. In addition to the well-known differences imposed by rising atmospheric CO₂ on photosynthesis (Poorter 1993), C₃ and C₄ plants are expected to respond differentially to climate change. This is because C₄ plants have higher photosynthetic rates at high temperatures and under high light (Collatz et al. 1992; Sage and Monson 1999) and higher water-use efficiency than do C₃ plants (Farquhar et al. 1989). The generally lower nitrogen requirements of C₄ plants (associated with reduced leaf Rubisco content) produce different responses to nitrogen deposition (Wedin and Tilman 1996; Collins et al. 1998). The ¹²CO₂ molecule is assimilated preferentially relative to ¹³CO₂ during photosynthesis in all vascular land plants. However, C₄ photosynthetic assimilation is depleted in ¹³C relative to the background atmosphere by roughly 0.4%, whereas C₃ photosynthetic assimilation is depleted in ¹³C by ~2% (corresponding to fractionations of 4 and 20‰, respectively, and δ¹³C values of -12‰ and -28%).

The different isotope fractionations of C₃ and C₄ plants are relevant for inversion studies that solve for surface carbon fluxes from atmospheric measurements of ¹³CO₂ and CO₂. This approach is used to partition net uptake between the land and the ocean, since the physical-chemical processes of air-sea gas exchange don't strongly affect the δ¹³C of atmospheric CO₂ (Keeling et al. 1989; Tans et al. 1993; Enting et al. 1995; Francey et al. 1995; Battle et al. 2000). However, because C₄ carbon exchanges are similar isotopically to ocean fluxes, knowledge of C₄ flux variations is essential for accurate partitioning, particularly at low latitudes where this plant type is most abundant (Lloyd and Farquhar 1994; Ciais et al. 1995a,b; Fung et al. 1997; Still et al. 2003a).

Another approach to land:ocean partitioning relies on measuring variations in the atmospheric O₂ content and comparing these data to expected variations based on fossil fuel burning and cement production (Keeling et al. 1996; Bender et al. 1998). Because net ocean uptake of carbon does not affect atmospheric O₂ levels, differences between measured and expected O₂ concentrations can then be attributed to changes in the size of the terrestrial biosphere, since O₂ is consumed and released by respiration and photosynthesis. This approach complements ¹³C partitioning because it is better suited to decadal partitioning, while the ¹³C approach is useful for diagnosing interannual sink attribution (e.g., Keeling et al. 1995; Francey et al. 1995; Langenfelds et al. 1999, 2002; Le Queré et al. 2003). However, in order to fully exploit the ¹³C tracer for diagnosing interannual variations in land and ocean sinks, interannual variations in C₄ vegetation activity must be diagnosed accurately. In addition to the isotopic similarity of C₄ and air:sea exchanges, new and unaccounted for isotopic disequilibrium fluxes from mixed C₃/C₄ ecosystems are likely as a result of the large interannual production variations in such systems.

Interannual Variability in Mixed C₃/C₄ Ecosystems

Ecosystems containing mixtures of C₃ and C₄ plants experience extreme natural climate variations. Los (1998) demonstrated a link among variations in sea surface temperature (SST), land precipitation, and vegetation greenness in low latitude, semi-arid regions composed of savanna and grassland ecosystems containing an abundance of C₄ plants. Knapp et al. (2002) documented large variations in carbon cycling related to precipitation variation at the Konza Tallgrass Prairie LTER site; significantly, the total amount of precipitation was less important than the variability in its timing and amount. The carbon cycling variations were largely driven by the dominant C₄ grass species at the site. Knapp and Smith (2001) also highlighted the intrinsically high variability in aboveground net primary production (ANPP) of grassland and oldfield (herbaceous) ecosystems relative to forest ecosystems across a wide range of LTER study sites. It is precisely such warm, herbaceous ecosystems that contain an abundance of C₄ plants.

Multi-year eddy flux data also demonstrate intrinsically large carbon flux variability in grassland ecosystems containing a mixture of C₃ and C₄ plants (Frank and Dugas 2001; Sims and Bradford 2001; Suyker and Verma 2001; Suyker et al. 2003; Flanagan et al. 2002). In the study by Suyker and Verma (2003), the CV of annual net ecosystem exchange (NEE) was 78%; in the study by Flanagan et al. (2002), the CV of annual NEE was 175%; in the Sims and Bradford study it was 118%. These responses were also largely modulated by precipitation (and thus soil moisture) differences between years. By contrast, the CV of annual NEE at the well-studied Harvard forest flux site from 1991-1995 was 25%, and flux variations were driven by interannual climate variation (Goulden et al. 1996).

Large interannual variations are also apparent in the carbon isotope composition of ecosystem respiration from such ecosystems. Still et al. (2003b) sampled nighttime concentration and isotope gradients above the tallgrass prairie studied by Suyker and Verma (2001) and Suyker et al. (2003) during the 1999 and 2000 growing seasons. This sampling showed distinct differences in the δ¹³C of ecosystem respiration, which translated to large differences between years in the C₄ contribution to respiration. The carbon flux difference between years was driven by precipitation and its influence on C₃ vegetation. Recent work at Konza Prairie using the same methods of Still et al. (2003b) also documents large variability in the δ¹³C of ecosystem respiration (C.T. Lee, personal communication). There are very few studies documenting temporal variations in the δ¹³C of respiration from low latitude mixed C₃/C₄ ecosystems. Despite the paucity of such studies, it is likely that production in many mixed C₃/C₄ ecosystems varies considerably from year-to-year in response to climatic variations. Because of the intrinsically different responses that C₃ and C₄ plants exhibit in response to light, temperature and moisture, this production variability is driven mostly by changes in the production from each plant type. Thus, the C₃/C₄ composition co-varies along with the total production.

Isotope Disequilibrium Fluxes from Mixed C₃/C₄ Ecosystems

Interannual variations (IAV) in C₄ production alone will impact the ¹³C budget and the land:ocean partitioning of the global carbon sink (Fung et al. 1997). However, co-variability in flux and C₃/C₄ composition in grasslands and savannas has the potential to further complicate our interpretation of the ¹³C budget because of isotope disequilibrium fluxes accompanying these variations. Isotope disequilibrium fluxes are isotope fluxes that occur even in the absence of net fluxes (i.e., the ¹³C content of atmospheric CO₂ will change but the CO₂ concentration is unchanged - Tans et al. 1993; Ciais et al. 1995a,b; Francey et al. 1995). Combined terrestrial and oceanic disequilibrium fluxes increase the δ¹³C of atmospheric CO₂. They result from the progressive depletion of atmospheric δ¹³C (driven by fossil fuel and biomass burning of carbon with a C₃ isotopic signature - the Suess effect) interacting with long-lived carbon pools. In essence, the return flux from land or ocean to the atmosphere contains some carbon fixed in past decades when atmospheric δ¹³C was higher than at present. Since the opposite flux from atmosphere to land or ocean is composed of today's atmosphere, it will by definition have a lower δ¹³C. Thus an isotopic imbalance results, and it must be accounted for in the land:ocean partitioning equations. As shown by Randerson et al. (2002) and Scholze et al. (2003), other disequilibrium fluxes also impact the partitioning. They suggested that even small variations in C₃ photosynthetic discrimination, when coherent over large spatial scales as likely occurs during ENSO years, produce large isotopic disequilibrium fluxes in the following years that impact atmospheric δ¹³C. Such 'discrimination disequilibria' are distinct from the Suess effect disequilibria that were first identified.

There is likely to be a disequilibrium flux in mixed C₃/C₄ ecosystems that is very similar to the C₃ discrimination disequilibrium highlighted by Randerson et al. (2002). Isotopic disequilibria in these ecosystems, however, are driven by interannual co-variations in flux and C₃/C₄ mixtures, which would effectively translate as ecosystem discrimination variations. Alternating C₃:C₄ mixtures will produce disequilibria that enrich or deplete the atmosphere in ¹³C, depending on the C₃-C₄ trend direction.

The potential magnitude of global C₃/C₄ disequilibrium fluxes and their impact on land:ocean partitioning is illustrated by a simple example. Low latitude savannas and grasslands cover some 20 million sq. km, and their combined gross primary production (GPP) is roughly 50 Pg C/yr (Still et al. 2003a). Assuming a conservative annual average isotopic imbalance between photosynthesis and ecosystem respiration of 0.1‰ (or -0.1‰ in the case of a C₄-C₃ trend - this imbalance is only 0.7% of the isotopic difference between typical C₃ and C₄ plant d¹³C), the isotope disequilibrium flux would be ~5 Pg C‰/yr (-5 Pg C ‰ /yr). For comparison, the total land disequilibrium from the Suess effect is roughly 20-25 Pg C‰/yr. Including a C₃/C₄ disequilibrium of 5 Pg C‰/yr into the standard ¹³C budget equation with a global terrestrial discrimination of 18‰ would increase the inferred annual ocean sink by 0.3 Pg C (at the expense of the land sink).

Hypotheses

We hypothesize the following:

The project

Using a suite of products from sensors onboard Terra, TRMM and Aqua, I will produce seasonal and interannual maps of C₃ and C₄ vegetation activity for use in climate and carbon cycle models. I will employ these maps in an updated version of the CASA model for calculating seasonal and interannual C₃ and C₄ carbon and isotope fluxes, including this new class of disequilibrium fluxes. These outputs will also be used for forward modeling of the basis regions used in interannual, 3-D inverse modeling analyses. These analyses will use surface atmospheric CO₂ and ¹³CO₂ data collected by NOAA, as well as aircraft data from the LBA, SAFARI and North American Carbon Project (NACP) campaigns where available. The goal of these inversions will be the separation of low latitude ocean fluxes from C₄ vegetation fluxes. It is likely that previous ¹³C inversion studies attributed variable C₄ fluxes to the oceans. The proposed inversions should reconcile the divergence of such top-down estimates with bottom-up studies that suggest small equatorial ocean variability. This would be a significant advance in our understanding of the carbon cycle. A separate focus will be determining the contribution of C₄ vegetation to the North American carbon cycle in coordination with the NACP.

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