Self-amplified Amazon forest loss due to vegetation-atmosphere feedbacks

Reduced rainfall increases the risk of forest dieback, while in return forest loss might intensify regional droughts. The consequences of this vegetation-atmosphere feedback for the stability of the Amazon forest are still unclear. Here we show that the risk of self-amplified Amazon forest loss increases nonlinearly with dry-season intensification. We apply a novel complex-network approach, in which Amazon forest patches are linked by observation-based atmospheric water fluxes. Our results suggest that the risk of self-amplified forest loss is reduced with increasing heterogeneity in the response of forest patches to reduced rainfall. Under dry-season Amazonian rainfall reductions, comparable to Last Glacial Maximum conditions, additional forest loss due to self-amplified effects occurs in 10-13% of the Amazon basin. Although our findings do not indicate that the projected rainfall changes for the end of the twenty-first century will lead to complete Amazon dieback, they suggest that frequent extreme drought events have the potential to destabilize large parts of the Amazon forest

Vegetation and climate changes during the last 22,000 yr from a marine core near Taitao Peninsula, southern Chile

International audienceHigh-temporal resolution of pollen analyses from marine core MD07-3088 (46°S) documents regional and coastal vegetation changes in the mid-latitude of southern Chile during the last 22 kyr BP. The coastal margin was partly ice-free during the last glacial period with the presence of scattered vegetation. After ~ 17.6 kyr BP, the expansion of Nothofagus woodland highlights a warming trend and marks the beginning of the last deglaciation. The deglacial forest expansion is interrupted by the development of Magellanic moorland simultaneously with the Antarctic Cold Reversal. This vegetation change illustrates a pause in the warming trend with an abrupt increase in precipitation which is related to strong intensity of Southern Westerly Winds. After 12.8 kyr BP, the retreat of Magellanic moorland illustrates a second-step warming that is strengthened after 11.5 kyr at the onset of the Holocene. Expansion of heliophytic taxa illustrates warmest and driest conditions of the Holocene from 11 to 7.4 kyr BP. Later, the decrease of heliophytic taxa shows a return to cooler and wetter conditions, illustrated by the presence of the North Patagonian rainforest that reaches modern condition after 5 kyr. Climate changes inferred from the marine pollen record are consistent with geochemical data changes from the same samples and illustrate a clear synchronicity with δD variations of EPICA Dome C ice core during the last deglaciation and Holocene. Comparison between vegetation changes from the marine core and regional palaeoclimatic records highlights southward-northward shifts of the Southern Westerly Wind belt and supports a close link between Southern Westerly Wind belt and atmospheric CO2 variability during the last deglaciation

Vegetation and climate changes in Patagonia (46°S) during the last 20 kyr cal. BP from South East Pacific MD 070388 Core.

International audienc

Teleconnection between the Intertropical Convergence Zone and southern westerly winds throughout the last deglaciation

Comparison of environmental changes between northeastern Brazil and western Patagonia during the last deglaciation reveals concomitant trends in moisture from the Intertropical Convergence Zone (ITCZ) and southern westerly winds (SWW). The data confirm an atmospheric teleconnection between the ITCZ and SWW, associated with Atlantic Meridional Overturning Circulation (AMOC) variations. When the AMOC decreases, both the ITCZ and the SWW shift southward; they shift northward when the AMOC increases. Climate simulations in which the AMOC is made to vary agree with this general pattern. Additional experiments performed with an atmosphere-only model show that the tropical Atlantic is a key area in promoting relationships between the AMOC, ITCZ, and SWW. Our data show that this mechanism, which transfers climate changes between low and middle latitudes to high latitudes in the Southern Hemisphere, acted throughout the abrupt climatic events of the last deglaciation

7300 years of vegetation history and climate for NW Malta: a Holocene perspective

International audienceThis paper investigates the Holocene vegetation dynamics for Burmarrad in Northwest Malta and provides a pollen-based quantitative palaeoclimatic reconstruction for this centrally located Mediterranean archipelago. The pollen record from this site provides new insight into the vegetation changes from 7280 to 1730 cal BP which correspond well with other regional records. The climate reconstruction for the area also provides strong correlation with southern (below 40 40 degrees N) Mediterranean sites. Our interpretation suggests an initially open landscape during the early Neolithic, surrounding a large palaeobay, developing into a dense Pistacia scrubland ca. 6700 cal BP. From about 4450 cal BP the landscape once again becomes open, coinciding with the start of the Bronze Age on the archipelago. This period is concurrent with increased climatic instability (between 4500 and 3700 cal BP) which is followed by a gradual decrease in summer moisture availability in the late Holocene. During the early Roman occupation period (1972-1730 cal BP) the landscape remains generally open with a moderate increase in Olea. This increase corresponds to archaeological evidence for olive oil production in the area, along with increases in cultivated crop taxa and associated ruderal species, as well as a rise in fire events. The Maltese archipelago provides important insight into vegetation, human impacts, and climatic changes in an island context during the Holocene

Stability of a Neotropical microrefugium during climatic instability

Aim The primary objectives of this study were (1) to assess, in the light of palaeoecological reconstruction, the climate stability hypothesis used by evolutionary biologists to explain high diversity in historically stable areas, and (2) to identify the response mechanisms of a tropical rain forest microrefugium to climatic variability. Location North-eastern Brazil, Serra de Maranguape. Methods Vegetation and climatic changes were reconstructed using a pollen record in a sediment core from a forest hollow, and the chronology was based on accelerator mass spectrometry radiocarbon analyses. Results Past vegetation dynamics consisted of three main forest types, shown by major compositional changes in rain forest assemblages between 5000 and 1000cal. yr bp. Dense ombrophilous forest was abruptly replaced by heliophilous early successional tree taxa at 4275cal. yr bp. These early successional tree taxa were established over a period of c.100years, and their dominance lasted for c.750years and was associated with dry conditions until 3525cal. yr bp. Subsequently, the expansion of secondary successional tree taxa over a period of c.550years enabled the recovery of ombrophilous forest. Main conclusions The vegetation changes in the Serra de Maranguape provide evidence for the high sensitivity of this rain forest microrefugium to climatic variability on a multidecadal to millennial time-scale during the mid- to late Holocene. Despite the substantial compositional and climatic changes, this microrefugium apparently was continuously forested and responded to climatic instability by recruiting key species to its highly diverse stock. This evidence helps to address the joint concerns of evolutionary biologists and palaeoecologists regarding how forests can persist during periods of climatic variability by showing that some tropical regions can remain continuously forested despite reorganization during abrupt and short-term climatic changes