Extreme climatic events (ECEs) such as droughts and heat waves are predicted to increase in intensity and frequency and impact the terrestrial carbon balance. However, we lack direct experimental evidence of how the net carbon uptake of ecosystems is affected by ECEs under future elevated atmospheric CO 2 concentrations (eCO 2 ). Taking advantage of an advanced controlled environment facility for ecosystem research (Ecotron), we simulated eCO 2 and extreme cooccurring heat and drought events as projected for the 2050s and analyzed their effects on the ecosystem-level carbon and water fluxes in a C3 grassland. Our results indicate that eCO 2 not only slows down the decline of ecosystem carbon uptake during the ECE but also enhances its recovery after the ECE, as mediated by increases of root growth and plant nitrogen uptake induced by the ECE. These findings indicate that, in the predicted near future climate, eCO 2 could mitigate the effects of extreme droughts and heat waves on ecosystem net carbon uptake.climate change | extreme events | elevated CO 2 | carbon fluxes | grassland ecosystem I ncreased aridity and heat waves are projected to increase in the 21st century for most of Africa, southern and central Europe, the Middle East, and parts of the Americas, Australia, and southeast Asia (1-3). These regions have a large fraction of their land covered by grasslands and rangelands, a biome covering approximately one-quarter of the Earth's land area and contributing to the livelihoods of more than 800 million people (4). There is mounting evidence that extreme climatic events (ECEs) may significantly affect the regional and global carbon (C) fluxes (3, 5-9) and potentially feed back on atmospheric CO 2 concentrations and the climate system (7). However, our knowledge concerning the outcome of the interaction between future ECEs and elevated atmospheric CO 2 concentrations (eCO 2 ) for ecosystem C stocks is equivocal (10-12). Studies focusing on plant physiological responses have shown that eCO 2 has the potential to mitigate future drought-related stress on plant growth by reducing stomatal conductance, thereby increasing water use efficiency (WUE) (13-15) and preserving soil moisture (16)(17)(18). However, to date, little is known on whether and how eCO 2 alters the consequences of ECEs for ecosystem net C uptake. Because the capacity of ecosystems to act as a C sink depends on the relative effects of eCO 2 , ECE, and their potential interaction on both plant and soil processes, an integrated assessment of all C fluxes during and after the ECEs is important if we are to estimate the overall C balance.Using the Montpellier CNRS Ecotron facility (www.ecotron. cnrs.fr), we tested with 12 large controlled environment units (macrocosms, SI Appendix, Fig. S1) whether (i) an ECE (severe drought and heat wave) predicted for the 2050s reduces ecosystem net C uptake by reducing ecosystem photosynthesis relative to ecosystem respiration (R eco ), (ii) eCO 2 buffers the negative effects of the ECE on ecosystem CO 2 fluxes ...
Summary1. Non-random spatial patterns are a common feature of plant communities. However, the mechanisms leading to their formation remain unknown. The clonal dispersal ability of a species, that is, the average length of spacers between ramets, is commonly acknowledged to influence spatial patterns in clonal plants, although this relationship remains to be demonstrated. Moreover, the clonal dispersal ability of neighbouring species may influence environmental conditions and trigger modifications in clonal characteristics of a focal species. Thus, not only the clonal dispersal ability of a species, but also that of its competitors may influence the fine-scale spatial pattern of a species. 2. In this article, we compared spatial patterns (in terms of colonization and occupation of space) of species with low (L), intermediate (I) or high (H) clonal dispersal abilities. Twelve species were classified within three groups of clonal dispersal (L, I or H) based on their average spacer lengths, and seven types of experimental assemblages consisting of species from one, two or three dispersal groups were studied. Two questions were addressed: (i) does the species clonal dispersal ability influence their spatial patterns and (ii) are species fine-scale spatial patterns affected by the clonal dispersal of neighbours? Species spatial patterns were recorded for each assemblage and were then analyzed using point pattern analysis. 3. Despite strong species-specific effects, L-species displayed the highest level of local aggregation, which is indicative of limited space colonization, and the lowest level of local co-occurrence with other species, which is indicative of a high level of space occupation. The opposite pattern was observed in H-species, while that of I-species was intermediate. The species spatial patterns were modified by the clonal dispersal ability of competitors. 4. Synthesis. This study emphasizes the importance not only of clonal dispersal but also of biotic interactions and, more precisely, of plant neighbour characteristics, in the spatial patterning of grassland plant communities.
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