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It’s easy to imagine the Amazon Basin as a monotonous valley of deep green forests and bustling wildlife. The reality is a bit different. The Amazon largely consists of dense tropical forests but also supports savannah grasslands, and the latter could grow in size as dry seasons lengthen during the beginning of this century. Will the latter expand with climate change, reaching a tipping point when forested regions are no longer stable? A new study1 published in PNAS provides insight into this question and partially resolves a conflict between theoretical models and experimental data with important implications about the future of the planet’s most valuable rainforest.
An uncertain future
The health of the Amazon has global implications. The basin stores half of all carbon in tropical forests and governs worldwide water and carbon cycles. The rainforest regions of the basin store most of the carbon, important for limiting carbon dioxide levels in the atmosphere. Recent evidence of longer dry seasons, however, have raised concern among scientists that forested regions could be replaced by savannahs that support ecosystems requiring less precipitation but trap less carbon.
Southern Amazon forests are already coping with longer dry seasons,2 but unfortunately we don’t yet know how these climatic changes will affect forest health. Scientists use theoretical models to predict how forests will respond, but these models suggest wildly different views: either the Amazonian forests will remain stable or completely die off in a drier climate!3-4 These all-or-nothing predictions don’t match up with a recent experiment indicating that recent, short-term droughts only lead to a partial transition to savannahs.5 Which one is right – the theoretical extremes or experimental moderation – and why don’t they match up?
Taking a closer look
Theoretical models often don’t match experimental results because important variables are not included in the analysis. These types of omissions miss important relationships between the so-called ‘hidden variables’ and the outcomes of interest.
Levine and colleagues suspected this to be the case concerning predictions about how dry seasons impact Amazonian forest cover. In particular, previous models averaged quantities like precipitation, soil content (sand, silt, or clay), and biomass levels across large regions of the forest. This averaging effectively wipes out any information about variations in plant health or soil texture at the local level, which could be important in understanding how the forest responds to drier seasons. The researchers thought that by gathering more detailed information at the level of individual plants or small groups of biomass, the theoretical model could capture important regional relationships and show very different behavior.
It turns out that they were exactly right! The researchers tested two models. Both models used variables like soil content, plant water stress, and precipitation as input and predicted how levels of biomass change in the rainforest over the 21st century. However, one model used a very coarse grid to average these quantities over large regions of the rainforest, whereas the second used a very fine grid that could capture regional changes in these variables.
The coarse-grid model predicts a similar future as seen in previous theoretical studies. In particular, as dry seasons increase with climate change, plant water stress increases and leads to a tipping point where biomass disappears. The Amazon irreversibly transitions into wide-spread savannah. However, when using the fine-grid model, only local transitions to savannah ecosystems are seen while large regions still remain intact as tropical rainforest. This moderate transition matches the experimental data that droughts create only a slow loss of forest cover.
Why does the model capturing locally diverse information suggest such a different future? The answer lies in the heterogeneity of the rainforest ecosystem that is only captured by the fine-grid model. In particular, the study found that soil content is spatially diverse and has a significant impact on plant survival. Regions with low clay content are largely unaffected by the drier climate, whereas plants in soil with high clay content that experience high water stress cannot survive the change in climate. Approximately three years after the climate changes, the model predicts that these high-clay areas will transition toward savannah ecosystems. However, due to a large heterogeneity in biomass type and soil content, this transition remains localized and does not affect the rest of the forest.
These new findings give a clue as to how diverse ecosystems are more resilient to sudden shifts in climate Whereas homogeneous systems will all suffer in the same way from climatic changes and lead to large-scale shifts in plant productivity and survival, heterogeneous ecosystems feel only localized changes that prevent the system as a whole form changing irreversibly. Some areas will support high-biomass forests while others will be savannahs. Similar reasoning has been used to criticize widespread use of monocultures in agricultural industries. It seems that we are only beginning to appreciate the importance of diversity that Nature has long understood.
It should be noted that the study purposefully did not include other forcing factors like fires and human disturbances that could affect the Amazon’s future. These factors should be studied in conjunction with the detailed grid used in this study. However, the authors wanted to focus on vegetation and soil variation and their unique effect on ecosystem survival. This is an excellent case of emergent phenomena – macroscopic changes that can only be seen after accounting for the behavior of individual plants and trees. Hopefully this connection between small and large scales will continue to be studied in Asian and African rainforests to see if it is a universal trend.
- Levine, NM et al. “Ecosystem heterogeneity determines the ecological resilience of the Amazon to climate change.” PNAS Early Edition, accessed online January 8 2015.
- Fu R et al. “Increased dry season length over southern Amazonia in recent decades and its implication for future climate projection.” PNAS, 110(45):18110, 2013.
- Cox PM, et al. “Amazonian forest dieback under climate carbon cycle projections for the 21st century.” Theor Appl Climatol, 78(1-3): 137-156, 2004.
- Thompson SL, et al. “Quantifying the effects of CO2-fertilized vegetation on future global climate and carbon dynamics.” Geophys Res Lett, 31(23): L23211, 2004.
- Phillips OL, et al. “Drought sensitivity of the Amazon rainforest.” Science, 323(5919): 1344-1347, 2009.
Levine NM, Zhang K, Longo M, Baccini A, Phillips OL, Lewis SL, Alvarez-Dávila E, Segalin de Andrade AC, Brienen RJ, Erwin TL, Feldpausch TR, Monteagudo Mendoza AL, Nuñez Vargas P, Prieto A, Silva-Espejo JE, Malhi Y, & Moorcroft PR (2015). Ecosystem heterogeneity determines the ecological resilience of the Amazon to climate change. Proceedings of the National Academy of Sciences of the United States of America PMID: 26711984