Terrestrial ecosystems control carbon dioxide fluxes to and from the atmosphere through photosynthesis and respiration, a balance between net primary productivity and heterotrophic respiration, that determines whether an ecosystem is sequestering carbon or releasing it to the atmosphere. Global and site-specific data sets have demonstrated that climate and climate variability influence biogeochemical processes that determine net ecosystem carbon dioxide exchange (NEE) at multiple timescales. Experimental data necessary to quantify impacts of a single climate variable, such as temperature anomalies, on NEE and carbon sequestration of ecosystems at interannual timescales have been lacking. This derives from an inability of field studies to avoid the confounding effects of natural intra-annual and interannual variability in temperature and precipitation. Here we present results from a four-year study using replicate 12,000-kg intact tallgrass prairie monoliths located in four 184-m(3) enclosed lysimeters. We exposed 6 of 12 monoliths to an anomalously warm year in the second year of the study and continuously quantified rates of ecosystem processes, including NEE. We find that warming decreases NEE in both the extreme year and the following year by inducing drought that suppresses net primary productivity in the extreme year and by stimulating heterotrophic respiration of soil biota in the subsequent year. Our data indicate that two years are required for NEE in the previously warmed experimental ecosystems to recover to levels measured in the control ecosystems. This time lag caused net ecosystem carbon sequestration in previously warmed ecosystems to be decreased threefold over the study period, compared with control ecosystems. Our findings suggest that more frequent anomalously warm years, a possible consequence of increasing anthropogenic carbon dioxide levels, may lead to a sustained decrease in carbon dioxide uptake by terrestrial ecosystems.

Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO 2 ]. Net ecosystem CO 2 exchange (NEE) was measured in the eighth year of CO 2 enrichment at the Nevada Desert Free-Air CO 2 Enrichment (FACE) Facility between the months of December 2003-December 2004. On most dates mean daily NEE (24 h) (lmol CO 2 m À2 s À1 ) of ecosystems exposed to elevated atmospheric CO 2 were similar to those maintained at current ambient CO 2 levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO 2 ] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO 2 ]. Mean daily NEE varied seasonally across both CO 2 treatments, increasing from about 0.1 lmol CO 2 m À2 s À1 in December to a maximum of 0.5-0.6 lmol CO 2 m À2 s À1 in early spring. Maximum NEE in ecosystems exposed to elevated CO 2 occurred 1 month earlier than it did in ecosystems exposed to ambient CO 2 , with declines in both treatments to lowest seasonal levels by early October (0.09 AE 0.03 lmol CO 2 m À2 s À1 ), but then increasing to near peak levels in late October (0.36 AE 0.08 lmol CO 2 m À2 s À1 ), November (0.28 AE 0.03 lmol CO 2 m À2 s À1 ), and December (0.54 AE 0.06 lmol CO 2 m À2 s À1 ). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO 2 uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free-living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO 2 treatments were net CO 2 sinks in 2004, but exposure to elevated CO 2 reduced CO 2 sink strength by 30% (positive net ecosystem productivity 5 127 AE 17 g C m À2 yr À1 ambient CO 2 and 90 AE 11 g C m À2 yr À1 elevated CO 2 , P 5 0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO 2 -along with the extensive coverage of arid and semi-arid ecosystems globally -points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.

The Penman-Monteith combination equation, which is most frequently used to derive the surface conductance to water vapour (G s ), implicitly assumes the energy balance to be closed. Any energy imbalance (positive or negative) will thus affect the calculated G s . Using eddy covariance energy flux data from a temperate grassland and a desert shrub ecosystem we explored five possible approaches of closing the energy imbalance and show that calculated G s may differ considerably between these five approaches depending on the relative magnitudes of sensible and latent heat fluxes, and the magnitude and sign of the energy imbalance. Based on our limited understanding of the nature of the energy imbalance, we tend to favour an approach which preserves the Bowenratio and closes the energy balance on a larger time scale.

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Accepted Article This article is protected by copyright. All rights reserved Ecosystems integrity and services are threatened by anthropogenic global changes. Mitigating and adapting to these changes requires knowledge of ecosystem functioning in the expected novel environments, informed in large part through experimentation and modelling. This paper describes 13 advanced controlled environment facilities for experimental ecosystem studies, herein termed ecotrons, open to the international community. Ecotrons enable simulation of a wide range of natural environmental conditions in replicated and independent experimental units whilst simultaneously measuring various ecosystem processes. This capacity to realistically control ecosystem environments is used to emulate a variety of climatic scenarios and soil conditions, in natural sunlight or through broad spectrum lighting. The use of large ecosystem samples, intact or reconstructed, minimises border effects and increases biological and physical complexity. Measurements of concentrations of greenhouse trace gases as well as their net exchange between the ecosystem and the atmosphere are performed in most ecotrons, often quasi continuously. The flow of matter is often tracked with the use of stable isotope tracers of carbon and other elements. Equipment is available for measurements of soil water status as well as root and canopy growth. The experiments run so far emphasize the diversity of the hosted research. Half of them concern global changes, often with a manipulation of more than one driver. About a quarter deal with the impact of biodiversity loss on ecosystem functioning and one quarter with ecosystem or plant physiology. We discuss how the methodology for environmental simulation and process measurements, especially in soil, can be improved and stress the need to establish stronger links with modelling in future projects. These developments will enable further improvements in mechanistic understanding and predictive capacity of ecotron research which will play, in complementarity with field experimentation and monitoring, a crucial role in exploring the ecosystem consequences of environmental changes.