The Amazon, past and future

I found a paper that tries to present information about the Amazon from when it (arguably) began, up until its present and potential future. It is called “New Views on an old forest: assessing the longevity, resilience and future of the Amazon rainforest”, by Maslin et al., and the authors use palaeoclimate and palaeoecoecological records to determine exactly how far back the Amazon’s existence can be traced.

They argue that the Amazonian rainforest “has been a permanent feature of this continent for at least the last 55 million years”, meaning it has been around since at least the late Cretaceous. However, the definition of what a rainforest is today does not match up with what “rainforests” were millions of years ago, and the paper uses “megathermal moist forests” (MTMF) as a more precise term for discussing the evolution of what would eventually become modern rainforests.

Fig. 1 Reconstructions of the exposed land masses and occurrence of megathermal moist forests for six key periods. Arrows indicate possible dispersal of rainforest taxa

In the past 55 million years, tropical rainforests have existed in more parts of the globe than they do today, and their growth was subject to upper and lower boundaries in temperature and moisture similar to what those of rainforests today. These conditions exist today within the tropics, and in earlier stages of the earth existed in different latitudes and with different expansion. However, the evidence put forth by the authors is that the region where the Amazon rainforest exists today has been continually covered in MTMF throughout the whole period.

There is a third condition (besides temperature and precipitation) that is discussed, and it is levels of atmospheric carbon dioxide. There have been great fluctuations in the quantity of carbon dioxide released into the atmosphere, and the authors cite a paper by Chambers and Silver 2004, that, among others, that show that plants grown under double current carbon dioxide levels grow faster. However, they are careful to note that this does not mean that greenhouse gases are going to necessarily help forests grow. There are certain kinds of plans that are favored by different levels of carbon dioxide, and therefore if carbon dioxide levels were to double, certain plants would probably grow better, while others would be put under tremendous strain. Overall, this would endanger the balance of the Amazonian rainforest, and is unfavorable.

Fig 2. Reconstructions of past atmospheric CO2 levels over the last 600 Ma using modelling, paleosols, algae and foraminifera carbon isotopes, stomata density and boron isotopes. These are compared with global temperatures, mean tropical temperature variation and the extent of continental glaciation.

The article then goes on to discuss the future of the Amazon, and they use what I consider to be one of the most eloquent in summing up our current situation, since “we are entering a non-analogue future”. Global climate change will spur changes within the rainforest, and in turn these changes will affect regional, and consequentially, global climate. However, it is very difficult to predict what may happen. The authors of this paper focus on the same factors that allow the Amazon to exist to try and imagine what can happen in the coming centuries. With rising world temperatures, deforestation, and carbon dioxide levels, the Amazon rainforest runs the risk of disappearing within one century. The primary problem is human impact, but then the reactions of the forest to these impacts are another problem unto their own, and are, as the paper describes, “poorly understood threats”. With no past analogue to base estimates on, the consensus seems to be that mitigating negative human impact is the world’s safest bet.

Sources: Maslin, M.A., Y. Mahli, O. Phillips and S. Cowling “New views on an old forest:assessing the longevity, resilience and future of the Amazon Rainforest.” Transactions of the Institute of British Geographers 30, 4, 390-401 (2005)

 

The Amazon since the Last Glacial Maximum

For this post I read a very interesting article published in 2004 called “Responses of Amazonian ecosystems to climatic and atmospheric carbon dioxide changes since the last glacial maximum” by Mayle et al., published by The Royal Society. In it, the authors do not do any original data collection, but instead choose to analyze several already existing papers and data in order to try and paint a more comprehensive picture regarding Amazonian ecosystems since the last glacial maximum. They also go one step further and use these findings to try and predict what could be in store for the near future of the Amazonian Rainforest.

As mentioned in a previous post, the authors disagree with the possibility of the rainforest having been reduced to separate islands, as “both fossil pollen data and dynamic, process-based vegetation simulations show that most of the Amazon Basin remained forested at the LGM”. As a whole, the forest expanded after the LGM, retracted in the early to mid-Holocene due to decreased precipitation, increase in temperature, and lower carbon dioxide levels in the atmosphere. Finally, in the late Holocene the forest began to expand once again, until significant human effects kicked in and we began to make the forest shrink once again.

Figure 1. (a) Map showing the location of sites discussed in the text. The shaded area shows the current distribution of humid evergreen broad-leaf forest (rainforest). The hatched area shows the Andes mountains. Lowland unshaded areas represent seasonally dry forests and savannahs. (b) Schematic broad-scale summary trends of climatic change and/or vegetation response for each site since 21 000 cal yr BP.

The most noticeable changes, in terms of vegetation replacement such as rainforest by savannah, would have taken place in ecotonal regions. Therefore, while the Amazon Rainforest as an integrated body was most likely intact, the boundaries of the forest did change. Not only between the rainforest and savannah, but also between the rainforest and Andean cloud forests.

However, what I found to be by far the most interesting proposition of this paper is that perhaps the savannah vs. forest replacement concept is flawed. Nowadays, in the east of Brazil there are Caatinga forests, classified as seasonally dry forests. So while there seems to be a consensus that the Amazon was never replaced by savannah since the LGM, parts of the forest could have become seasonally dry. The authors of this paper mention widely-separated semi-deciduous/deciduous forests with common species, namely the Caatinga forests in the east of Brazil, Chiquitano dry forest of eastern Bolivia and the Andean piedmont dry forest in south Bolivia/northwest Argentina.

The pollen records that were always assumed to indicate the presence of a Rainforest, could just as easily indicate a primarily dry forest region. However, the similarity of these signals brings forth this possibility, but also makes it very difficult to determine the case one way or another. The authors mention efforts towards this goal, but no definitive answer has been reached. At least from what I have read, this hypothesis is at the very least plausible, not to mention very interesting.

Figure 3. SDGVM model simulations forced with the UGAMP GCM. See § 2d(i). Key: c3, C3 grasses; c4, C4 grasses; ebl, evergreen broad-leaf forest; enl, evergreen needle-leaf forest; dbl, deciduous broad-leaf forest; dnl, deciduous needle-leaf forest. Amazonian vegetation distribution: (a) Pre-Industrial; (b) Mid-Holocene; (c) LGM; NPP of Amazonian vegetation: (d) Pre-Industrial; (e) Mid-Holocene; ( f ) LGM. (t C ha_1 yr_1, tonnes of carbon per hectare per year, where 1 hectare is 104 m2.)

After analyzing a wide array of data, the paper then goes on to propose what might happen to the Amazon Rainforest in the years to come. Since there is likely to be a rise in temperature and decrease in precipitation, both likely to be aggravated by deforestation, proxy data suggests there will be more frequent wide-spread fires, leading to a replacement of current vegetation to more drought and fire tolerant vegetation, typical of savannah-type ecosystems. However, the big wild card is carbon dioxide levels, since there is nothing in the past to compare them to, especially if they reach the projected levels of twice those of the mid Holocene by the year 2050. I do not have a tremendous amount of faith in the ability of past records to accurately predict what is to come, simply because I firmly believe that, given how different the world has become since the Industrial Age, there are more factors than carbon dioxide levels which make the future difficult to model based on proxy data. One example of this, which I have discussed in previous posts, is the increasing seasonality of climate in the Amazon, which is not the same as average temperature and precipitation changes over time, considering there can be extreme variability between seasons and yet no change to the observed average yearly values.

Sources: Responses of Amazonian ecosystems to climatic and atmospheric carbon dioxide changes since the last glacial maximum, Francis E. Mayle, David J. Beerling, William D. Gosling, Mark B. Bush, Phil. Trans. R. Soc. Lond. B March 29, 2004 359:499-514; doi:10.1098/rstb.2003.1434

The Amazon in the Late Quaternary

For this post I read a paper titled “Late Quaternary Vegetation Dynamics in the Southern Amazon Basin Inferred from Carbon Isotopes in Soil Organic Mattter”, by Freitas et al., which analyzes, as the title states, carbon isotopes of soil organic matter (SOM) extracted from a 200km transect of a region in Brazil that is composed of savannas surrounded by forests. The objective of this study is to determine what kind of vegetation was previously present in this location.

 Fig. 1 Schematic diagram of the vegetation distribution and sample sites. Single lines, forest; double lines, savanna; arrows, sites of sample collection. Sample sites are identified by the distance along BR 319, from km 0 (Porto Velho) to km 200 (Humait´a).

However, I do need to point something out about this article before I go on. It was published in 2001, and for this reason the authors still entertain the hypothesis that, as our very own Mackay puts it “during intervals of full glacial conditions, increased aridity resulted in the Amazon rainforest being restricted to isolated pockets, surrounded by expanded savannah environments”. However, as Mackay’s chapter goes on to explain, this hypothesis is at odds with palaeoecological evidence, which suggests that forests made it through time periods of full glacial conditions.

Fig. 2 The ±13C range of the C3 and C4 plant species collected at: (a) km 46 (f); (b) km 142 (f); and (c) km 188 (s). The three sites have some common plant species.

The study does not lose its relevance because of this, I merely point it out as the authors mentioned this debate in the paper, and it no longer seems to be much of a debate. This study nonetheless still has valid results is that the geographical area being analyzed has well-defined natural boundaries between savanna and forest. The carbon isotopes showed that at least in this location, there were shifts between forest and savanna vegetation. From 17,000 and 9000 ¹⁴C yr B.P. the entire area was a forest, then from 9000 to 3000 ¹⁴C yr B.P. it was a savanna, and then after 3000 ¹⁴C yr B.P. there was forest expansion once again to the present mixed state. The authors also propose that the forest-dominated periods probably correlated with a wetter climate, and a drier one for the savanna-dominated time frame. They also recognize that while these results cannot say much about the changes in regional vegetation change due to past climate change, it does indicate that the Amazon rainforest has receded and expanded in at least some areas.

Sources: De Freitas, H. A., Pessenda, L. C. R., Aravena, R., Gouveia, S. E. M., De Souza Ribeiro, A. & Boulet, R. 2001 Late Quaternary vegetation dynamics in the southern Amazon Basin inferred from carbon isotopes in soil organic matter. Quatern. Res. 55, 39–46.

Mackay, A. W. (2009) - An introduction to Late Glacial-Holocene environments. In S. T. Turkey (Edt.), Holocene Extinctions, Oxford University Press, Oxford, 1-15.

Amazon basin rainfall variability

In the study “Spatio-temporal rainfall variability in the Amazon basin countries” by Villar et al., 756 pluviometric stations throughout Brazil, Peru, Bolivia, Colombia, and Ecuador were used to analyze rainfall variability in the period ranging from 1964 to 2003. What distinguishes this study from previous ones is that it includes significant data from areas other than the Amazon forest within Brazil, which tends to have the most data since most of the forest lies within its borders.

The study confirmed the fact that there are not only great differences in rainfall between the Amazon basin and other surrounding ecosystems, but also within the basin itself. As mentioned in previous posts, the differences lie between north and south, and east and west, with the wetter areas (generally) towards the north and west. However, the more interesting findings are in relation to rainfall changes over the second half of the 20^th century. The study is careful to mention that there is much disagreement with regard to whether the forest is getting drier or wetter, and cites papers that attempt to solve this question. But in favor of the paper at hand, the authors use a larger amount of pluviometric stations over a larger area, and also seek to analyze changes over a longer period.

Figure 1. Mean 1975–2003 annual rain (mm/year). The pluviometric stations mentioned in the text are indicated. Andean regions above 500 m are limited by a black and white line.

They found that there is an average decrease in rainfall at a rate of -0.32% from 1975 to 2003, and that “the decrease has been particularly important since 1982”. So, in essence, the forest as a whole is getting drier, or at least receiving less rainfall. But, in the same way there are great differences in how much water the forest receives according to different regions, the changes in rainfall trends are also region-specific. In the 1970’s and 1990’s, in the wet season there was more rainfall in the northwest of the forest, and less in the southeast. In the 1980’s, the opposite was true. Also, up until 1992, rainfall was decreasing more significantly in the dry season, and also decreasing but not as much in the wet season. Since 1992, the wet season has seen an increase in rainfall, and the dry season continues to become drier.

Figure 2. (a) 1975–2003 evolution of the average annual rainfall (mm) in the Amazon basin at the delta and trend line (significant at the 99% level). (b) 1975–2003 evolution of the average quarterly rainfall (mm) in the Amazon basin at the delta and trend lines DJF, JJA, and SON have significant trends at the 95, 90, and 99% levels, respectively. In MAM there is no significant trend.

In context with my other posts about extreme droughts and flooding, this paper shows data that agrees with the notion that there is an increase in seasonal variability, but also brings in the argument that the forest as a whole is becoming drier. Thus, it seems that the dryness of the dry season outweighs the wet season. This is sure to bring some kind of change to the ecosystem in the years to come.
Sources: Espinoza Villar, J. C., Ronchail, J., Guyot, J. L., Cochonneau, G., Naziano, F., Lavado, W., De Oliveira, E., Pombosa, R. and Vauchel, P. (2009), Spatio-temporal rainfall variability in the Amazon basin countries (Brazil, Peru, Bolivia, Colombia, and Ecuador). International Journal of Climatology, 29: 1574–1594. doi: 10.1002/joc.1791