Because the flowering and fruiting phenology of plants is sensitive to environmental cues such as temperature and moisture, climate change is likely to alter community-level patterns of reproductive phenology. Here we report a previously unreported phenomenon: experimental warming advanced flowering and fruiting phenology for species that began to flower before the peak of summer heat but delayed reproduction in species that started flowering after the peak temperature in a tallgrass prairie in North America. The warming-induced divergence of flowering and fruiting toward the two ends of the growing season resulted in a gap in the staggered progression of flowering and fruiting in the community during the middle of the season. A double precipitation treatment did not significantly affect flowering and fruiting phenology. Variation among species in the direction and magnitude of their response to warming caused compression and expansion of the reproductive periods of different species, changed the amount of overlap between the reproductive phases, and created possibilities for an altered selective environment to reshape communities in a future warmed world.climate change ͉ global warming ͉ precipitation P henology is a sensitive biosphere indicator of climate change (1, 2). Long-term surface data and remote sensing measurements indicate that plant phenology has been advanced by 2-3 days in spring and delayed by 0.3-1.6 days in autumn per decade (3-6) in the past 30-80 years, resulting in extension of the growing season. An extended growing season leads to increased production in terrestrial and marine ecosystems (7,8), widens amplitudes of the annual CO 2 cycle in the atmosphere (9), and prolongs production of allergic pollens (10). Although changes in vegetative phenology have considerable consequences for ecosystem functioning, we lack information on responses of reproductive phenology due to climate change, especially in a community setting (11,12). Reproductive events usually determine population and community dynamics in future generations, affecting evolutionary processes. Because the flowering and fruiting phenology of plants is very sensitive to environmental cues such as temperature, moisture, and photoperiod (13), it is imperative to understand the impact of climate change on reproductive phenology.Reproductive phenology of assembled species in a plant community is often staggered in an unbroken progression over the growing season (14-17). This temporal distribution of community-level reproductive events is largely generated by the different developmental trajectories and life forms of the different species and may be shaped by their resource needs during reproduction and ecological sorting (18). Phenological differences in reproductive events among species over the growing season may reduce competition by spreading primary resource use over different temporal pools (19)(20)(21). Differential changes in phenology and growth between species in response to climate change could lead to new patterns of spec...

Carbon, nutrient, and water balance as well as key plant and soil processes were simultaneously monitored for humid tropical plant communities treated with CO(2)-enriched atmospheres. Despite vigorous growth, no significant differences in stand biomass (of both the understory and overstory), leaf area index, nitrogen or water consumption, or leaf stomatal behavior were detected between ambient and elevated CO(2) treatments. Major responses under elevated CO(2) included massive starch accumulation in the tops of canopies, increased fine-root production, and a doubling of CO(2) evolution from the soil. Stimulated rhizosphere activity was accompanied by increased loss of soil carbon and increased mineral nutrient leaching. This study points at the inadequacy of scaling-up from physiological baselines to ecosystems without accounting for interactions among components, and it emphasizes the urgent need for whole-system experimental approaches in global-change research.

Global climate change is expected to result in a greater frequency of extreme weather, which can cause lag effects on aboveground net primary production (ANPP). However, our understanding of lag effects is limited. To explore lag effects following extreme weather, we applied four treatments (control, doubled precipitation, 4 1C warming, and warming plus doubled precipitation) for 1 year in a randomized block design and monitored changes in ecosystem processes for 3 years in an old-field tallgrass prairie in central Oklahoma. Biomass was estimated twice in the pretreatment year, and three times during the treatment and posttreatment years. Total plant biomass was increased by warming in spring of the treatment year and by doubled precipitation in summer. However, double precipitation suppressed fall production. During the following spring, biomass production was significantly suppressed in the formerly warmed plots 2 months after treatments ceased. Nine months after the end of treatments, fall production remained suppressed in double precipitation and warming plus double precipitation treatments. Also, the formerly warmed plots still had a significantly greater proportion of C 4 plants, while the warmed plus double precipitation plots retained a high proportion of C 3 plants. The lag effects of warming on biomass did not match the temporal patterns of soil nitrogen availability determined by plant root simulator probes, but coincided with warming-induced decreases in available soil moisture in the deepest layers of soil which recovered to the pretreatment pattern approximately 10 months after the treatments ceased. Analyzing the data with an ecosystem model showed that the lagged temporal patterns of effects of warming and precipitation on biomass can be fully explained by warming-induced differences in soil moisture. Thus, both the experimental results and modeling analysis indicate that water availability regulates lag effects of warming on biomass production.

The net ecosystem CO 2 exchange (NEE) between a Mojave Desert ecosystem and the atmosphere was measured over the course of 2 years at the Mojave Global Change Facility (MGCF, Nevada, USA) using the eddy covariance method. The investigated desert ecosystem was a sink for CO 2 , taking up 102 AE 67 and 110 AE 70 g C m À2 during 2005 and 2006, respectively. A comprehensive uncertainty analysis showed that most of the uncertainty of the inferred sink strength was due to the need to account for the effects of air density fluctuations on CO 2 densities measured with an open-path infrared gas analyser. In order to keep this uncertainty within acceptable bounds, highest standards with regard to maintenance of instrumentation and flux measurement postprocessing have to be met. Most of the variability in half-hourly NEE was explained by the amount of incident photosynthetically active radiation (PAR). On a seasonal scale, PAR and soil water content were the most important determinants of NEE. Precipitation events resulted in an initial pulse of CO 2 to the atmosphere, temporarily reducing NEE or even causing it to switch sign. During summer, when soil moisture was low, a lag of 3-4 days was observed before the correlation between NEE and precipitation switched from positive to negative, as opposed to conditions of high soil water availability in spring, when this transition occurred within the same day the rain took place. Our results indicate that desert ecosystem CO 2 exchange may be playing a much larger role in global carbon cycling and in modulating atmospheric CO 2 levels than previously assumedespecially since arid and semiarid biomes make up 430% of Earth's land surface.

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.

The objectives of this paper were to review the literature on the responses of root systems to elevated CO # in intact, native grassland ecosystems, and to present the results from a 2-yr study of root production and mortality in an intact calcareous grassland in Switzerland. Previous work in intact native grassland systems has revealed that significant stimulation of the size of root systems (biomass, length density or root number) is not a universal response to elevated CO # . Of the 12 studies reviewed, seven showed little or no change in root-system size under elevated CO # , while five showed marked increases (average increase 38%). Insufficient data are available on the effects of elevated CO # on root production, mortality and life span to allow generalization about effects. The diversity of experimental techniques employed in these native grassland studies also makes generalization difficult. In the present study, root production and mortality were monitored in situ in a species-rich calcareous grassland community using minirhizotrons in order to test the hypothesis that an increase in these two measures would help explain the increase in net ecosystem CO # uptake (net ecosystem exchange) previously observed under elevated CO # at this site (600 vs 350 µl CO # l −" ; eight 1.2-m# experimental plots per CO # level using the screen-aided CO # control method). However, results from the first 2 yr showed no difference in overall root production or mortality in the top 18 cm of soil, where 80-90% of the roots occur. Elevated CO # was associated with an upward shift in root length density : under elevated CO # a greater proportion of roots were found in the upper 0-6-cm soil layer, and a lower proportion of roots in the lower 12-18 cm, than under ambient CO # . Elevated CO # was also associated with an increase in root survival probability (RSP ; e.g. for roots still alive 280 d after they were produced under ambient CO # , RSP l 0.30 ; elevated CO # , RSP l 0.56) and an increase (48%) in median root life span in the deepest (12-18 cm) soil layer. The factors driving changes in root distribution and longevity with depth under elevated CO # were not clear, but might have been related to increases in soil moisture under elevated CO # interacting with vertical patterns in soil temperatures. Thus extra CO # taken up in this grassland ecosystem during the growing season under elevated CO # could not be explained by changes in root production and mortality. However, C and nutrient cycling might be shifted closer to the soil surface, which could potentially have a substantial effect on the activities of soil heterotrophic organisms as CO # levels rise.

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.

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