Temperature rising would slow down tropical forest dynamic in the Guiana Shield

Abstract : Increasing evidence shows that the functioning of the tropical forest biome is intimately related to the climate variability with some variables such as annual precipitation, temperature or seasonal water stress identified as key drivers of ecosystem dynamics. How tropical tree communities will respond to the future climate change is hard to predict primarily because several demographic processes act together to shape the forest ecosystem general behavior. To overcome this limitation, we used a joint individual-based model to simulate, over the next century, a tropical forest community experiencing the climate change expected in the Guiana Shield. The model is climate dependent: temperature, precipitation and water stress are used as predictors of the joint growth and mortality rates. We ran simulations for the next century using predictions of the IPCC 5AR, building three different climate scenarios (optimistic RCP2.6, intermediate, pessimistic RCP8.5) and a control (current climate). The basal area, above-ground fresh biomass, quadratic diameter, tree growth and mortality rates were then computed as summary statistics to characterize the resulting forest ecosystem. Whatever the scenario, all ecosystem process and structure variables exhibited decreasing values as compared to the control. A sensitivity analysis identified the temperature as the strongest climate driver of this behavior, highlighting a possible temperature-driven drop of 40% in average forest growth. This conclusion is alarming, as temperature rises have been consensually predicted by all climate scenarios of the IPCC 5AR. Our study highlights the potential slowdown danger that tropical forests will face in the Guiana Shield during the next century. The tropical forests cover accounts for 25% of the terrestrial carbon pool, and therefore plays an essential role on carbon cycle and storage 1,2. Higher atmospheric CO 2 concentration might increase carbon uptake, maintaining the carbon sink historical role of tropical forests 3. But recent droughts linked to El Nino phenomenon have weakened this carbon sink 4-7 , highlighting the dependence of tropical forest dynamics on the global Earth climate. On the other hand, tropical forest dynamic, through tree growth and mortality, itself impacts carbon storage and cycle, and provides important feedbacks on climate change. In this context, more and more efforts are being made to describe the long-term impact interplays between climate change and tropical forest functioning 8-13. Recently, the impacts of exceptional droughts have been coaching more attention, first because droughts are predicted to be more frequent and severe in the tropics 14 , and second because tropical forests have already suffered from past severe droughts 15-17. Massive tree mortality have been observed after droughts 18,19 , potentially caused by hydraulic failure and/or carbon starvation 20 , and affecting more severely large trees 19,21. Beyond exceptional droughts and other long-term changes in water availability, temperatures are also expected to rise and the dry season length to increase over the next century in Amazonia 14,22. These changes will likely impact tree dynamics 23,24 , and dynamic global vegetation models (DVGMs) sometimes predict a shift toward drier forests or even savannas 25. Coarse scale DGVMs allow efficient large-scale carbon cycle prediction with little input data, relying on a wide set of mechanistic assumptions 26. These models were initially developed to simulate ecosystem carbon fluxes, they
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Melaine Aubry-Kientz, Vivien Rossi, Guillaume Cornu, Fabien Wagner, Bruno Hérault. Temperature rising would slow down tropical forest dynamic in the Guiana Shield. Scientific Reports, Nature Publishing Group, 2019, 9 (1), ⟨10.1038/s41598-019-46597-8⟩. ⟨hal-02185274⟩

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