Dams and flooded forests

Often, when we focus on dams and their effects on terrestrial ecosystems, the focus is on deforestation, mainly due to the reservoir creation. This is an important impact of dams, and definitely one that cannot be neglected. But for my PhD research, I keep thinking about the forests that are left behind. The Amazon is known for its beautiful seasonally flooded forests which evolved under fairly predictable flood conditions. The trees of these flooded forests spend much of the year submerged (some are underwater for as many as 9 months), and have developed many mechanisms to deal with the stress of living life sunken under the flood waters of the many Amazonian tributaries.

I was able to briefly visit the a flooded forest on the Rio Urubu near Manaus during my anniversary last year. It was an amazing site, which reminded me of the scene in Lord of the Rings where Frodo and Sam are walking through a bog filled with submerged bodies. Just below the surface of the water, trees could be seen peeking up. Most of them looked completely in tact, like reflections of their terrestrial selves. A few months after my trip, the waters would recede, and the trees would be back on terra firme, as if the river had never engulfed them to begin with.

So what becomes of these trees when the rivers dry up? Given that they have evolved to deal with such long periods of flooding, what happens when they are no longer subjected to regular floods? Or if the floods stop altogether? This is what I would like to understand.

A recent study in the Proceedings of the National Academy of the Sciences (PNAS) suggests forests may represent a sort of Achilles’ heel for the Amazon’s resilience. These forests are more vulnerable to wildfire and may be the first to collapse under the pressures of climate change. This collapse would allow the savannas to expand into the terra firme forest biomes and threaten the resilience of the Amazon as a whole. Of course, climate change is a large, global problem, whereas dams represent a smaller, local to regional scale threat.

This begs the question: if global climate change increases drought conditions which lead to increases in forest fires in the Amazon, can damming the rivers do the same? Will these dams increase the drought in the flooded forest, introducing a localized climate change that will allow another entry point for increased fire? If they do, how much more forest will be lost than what has been accounted for? How much deforestation could dams be responsible for beyond the reservoir? Though my research does not directly answer these questions, I hope it will be a sign post on the way there, determining whether the dams are affecting flooded forests at all. These are important and unique ecosystems that need to be considered beyond numbers of raw hectares deforested from building the dams.