Predicted response of forest above ground biomass (AGB) and composition to an increase in dryseason length (DSL). (A) Change in AGB after 100 years as a result of increasing DSL for forests with historic DSLs of 2, 4, and 6 months for the range of soil textures simulated in the ensemble model simulations. The magnitude of the change in AGB is influenced by soil clay fraction: The mean (solid line), 1σ deviation (shaded region), and minimum and maximum values (dashed lines) are shown. Graphic: Levine, et al., 2015 / PNAS

By Shreya Dasgupta
29 December 2015

(mongabay.com) – By the end of this century, as climate continues to warm, dry seasons could become longer and more intense in the Amazon region. Droughts could become more commonplace. But the fate of the Amazon forest — home to around 300 billion trees, and crucial to the Earth’s water and carbon cycle — in this drier future remains largely uncertain.

Some studies have predicted that the Amazon could suffer from a catastrophic die-back post 2050. Others have suggested that the region would mostly remain intact. Now, scientists say that the models used in these studies are flawed. The vast Amazonia is unlikely to respond to environmental changes in the same way, researchers say in a study published Monday in the Proceedings of the National Academy of Sciences. Instead, different parts of the Amazon forest will react differently and with varying intensities.

The problem with previously used climate-change models, researchers say, is that they treat the immensely diverse Amazon rainforest as a vast swath of monotonous green. By doing so, these models fail to capture the complexity of the Amazonian ecosystem.

To avoid these pitfalls, co-author Paul Moorcroft, an ecologist at Harvard University, and his colleagues, developed and used a new model called the Ecosystem Demography Biosphere model that allows scientists to track the response of individual trees to climate change. To predict how the Amazon could change in the future, the team combined field observations and remote sensing estimates to the model.

The researchers did not find any evidence supporting previous studies that the Amazon forests would either collapse, or be unresponsive, in a warmer, drier climate in the future. Instead, their results suggest that the Amazon would show varied response to changes in the climate. And these changes will be gradual, the team found.

The model predicted, for example, that as dry seasons become longer, forests will lose more biomass. Gradually, high-biomass rainforests will transition to low-biomass dry forests and savannah-like states.

Parts of the Amazon that have a four-month long dry season could lose around 20 percent of their biomass with a two-month increase in dry season length, the model predicts. Drier forests, which have six-months long dry seasons, would respond more rapidly to changes in climate, losing around 29 percent of their biomass with a one-month increase in dry season length.

“Fire, logging, and other anthropogenic disturbances may, however, exacerbate these climate change-induced ecosystem transitions,” the authors write in the paper. [more]

Amazon rainforests could transition to savannah-like states in response to climate change, new study predicts


ABSTRACT: Amazon forests, which store ∼50% of tropical forest carbon and play a vital role in global water, energy, and carbon cycling, are predicted to experience both longer and more intense dry seasons by the end of the 21st century. However, the climate sensitivity of this ecosystem remains uncertain: several studies have predicted large-scale die back of the Amazon, whereas several more recent studies predict that the biome will remain largely intact. Combining remote-sensing and ground-based observations with a size- and age-structured terrestrial ecosystem model, we explore the sensitivity and ecological resilience of these forests to changes in climate. We demonstrate that water stress operating at the scale of individual plants, combined with spatial variation in soil texture, explains observed patterns of variation in ecosystem biomass, composition, and dynamics across the region, and strongly influences the ecosystem’s resilience to changes in dry season length. Specifically, our analysis suggests thatincontrasttoexistingpredictionsofeitherstabilityorcatastrophic biomass loss, the Amazon forest’s response to a drying regional climate is likely to be an immediate, graded, heterogeneous transition from high-biomass moist forests to transitional dry forests and woody savannah-like states. Fire, logging, and other anthropogenic disturbances may, however, exacerbate these climate change-induced ecosystem transitions.

Ecosystem heterogeneity determines the ecological resilience of the Amazon to climate change

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