Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2012
Publication Date: 9/1/2012
Citation: Scott, R.L., Serrano-Ortiz, P., Domingo, F., Hamerlynck, E.P., Kowalski, A. 2012. Commonalities of carbon dioxide exchange in semiarid regions with monsoon and Mediterranean climates. Journal of Arid Environments. 84:71-79. doi:10.1016/j.jaridenv.2012.03.017. Interpretive Summary: Arid and semiarid areas occupy around one third of the Earth’s land surface and store around 25% of the world’s carbon. In these regions, water is a major limiting element on the exchange of carbon between the atmosphere and land. We measured and analyzed carbon fluxes of semiarid ecosystems that were dominated by either summer or winter precipitation to understand how precipitation seasonality influences patterns of photosynthesis and net carbon dioxide flux. We found that precipitation strength and timing were the dominant controls of carbon exchange, but temperature also played an important role in regulating plant growth and net carbon uptake. While all the systems were active in the warm parts of the year if water from precipitation was available, photosynthesis and uptake were not responsive to precipitation in the coolest parts of the winter. Rather, precipitation likely accumulated in the soil and fueled springtime growth after it had warmed. The commonalities that we found across these contrasting climates bode well for later modeling efforts in that these exchanges carbon are likely regulated by similar ecosystem processes across major differences in climate.
Technical Abstract: Semiarid ecosystems with monsoon climates receive precipitation during the warm season while Mediterranean systems are characteristically wet in the cool season and dry in the summer. Comparing biosphere-atmosphere carbon exchange across these two climate regimes can yield information about the interaction between energy and water limitation. Using data collected from eddy covariance towers over grass and shrub ecosystems in Arizona, USA and Almeria, Spain, we used net ecosystem carbon dioxide exchange (NEE), gross ecosystem production (GEP), and other meteorological variables to examine the effects of different precipitation seasonality. We found that precipitation magnitude and timing were the dominant drivers of carbon exchange, but temperature also played an important in regulating GEP and NEE at all sites. The response of carbon exchange to precipitation events conformed well to the paradigm of discrete precipitation pulses giving rise to rapid event-based ecosystem flux responses. While pulse responses occurred at all sites, they were limited mainly to the warm season when temperatures were not limiting. In the coolest parts of the winter season, NEE and GEP were not responsive to precipitation. Rather, precipitation accumulated in the soil and fueled springtime growth after it had warmed. We found a strong relationship between springtime cumulative GEP and precipitation across the sites when we accounted also for antecedent winter precipitation, and cumulative summer and fall GEP was highly correlated with growing-season precipitation alone. The differences that we observed between the sites like the timing of the growing seasons and periods of net carbon uptake were explained by differences in site vegetation, precipitation amounts, and temperature climatology. The similarities between monsoon and Mediterranean carbon exchange bode well for future modeling efforts because the ecosystem processes that control these carbon exchanges were likely similar across these distinct climatic regimes.