Winter in the Rainforest

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As winter settles in on the Midwest, dramatic changes are everywhere. Leaves have fallen off the trees; a brisk, frigid wind sweeps through the streets. In the morning, plants, trees, and buildings are pale and sparkly with the frost that descends at night. The pace of life seems to slow as well, with people choosing to stay warm and cozy inside rather than go out and about. The change in season has a significant impact on life in this part of the world. As I collect socks, sweaters, and jackets in preparation for the cold months ahead, I find myself wondering what winter looks like in the Amazon Rainforest. Of course, the Amazon isn’t experiencing snow, frost, and freezing temperatures, but are there any seasonal changes that occur in the rainforest? I’ve always thought of the rainforest as a relatively static place- always green and lush no matter what the time of year. But in the spirit of understanding the Amazon as a place (albeit far away) in the same world where my everyday life and familiar experiences occur, I thought I’d investigate further.

It happens that there are actually distinct seasons in the Amazon, though they are very different from the winter, spring, summer and fall that happen further from the equator. There’s a wet season and a dry season, and it turns out that there is a really interesting connection between them.

Photo by Rachel Running, 2010

Photo by Dano Grayson, 2014

Painting by Julia Loman

In temperate climates, plants grow more and trees produce an abundance of leaves during spring, when there’s a lot of rain. This makes sense, because water is often a limiting factor on plant growth, so plants can grow more easily when there’s more of it. In the Amazon, however, this is not the case. Many mature plants have roots that extend so deep into the earth that they can reach water even during the dry season [1]. In fact, plants in the Amazon tend to grow more during the dry season: trees sprout many new leaves, which greatly increases leaf coverage.

 

What causes there to be more growth during the dry season? It turns out that I’m not the only one wondering. Scientists have been studying the phenomenon extensively, trying to figure out why the rainforest would operate in such a counter-intuitive way. It is only in the past ten years or so that this aspect of rainforest ecology has even been researched, since it is made possible by new technology in remote sensing. They have found a couple of different factors at play. First, since rainforest plants tend to have enough water and warm temperatures, their growth depends most on the amount of sunlight they receive. So, during the wet season when there are lots of clouds, plants tend to grow less than during the sunny, dry season [2].

Photo by Rachael Running, 2010

Photo by Dano Grayson, 2014

Additionally, plants in the rainforest actually have an active role in seasonal changes. The deciduous plants that grow in temperate regions have great power in being able to withstand the drastic temperature changes between winter and summer. Rainforest trees wouldn’t be able to survive those conditions. However, rainforest plants have different strengths.  Rather than reacting, the way deciduous trees do, to changes in temperature and rainfall, they play a huge part in initiating the transition from the dry season to the wet season. Towards the end of the dry season, trees have grown a good deal more leaves on their branches. This means they are releasing more water into the air through transpiration as a result of photosynthesis. The air becomes moist and buoyant, causing clouds to form, increasing air movement, and eventually initiating thunderstorms [3]. These thunderstorms are so powerful in warming the upper atmosphere that they actually reverse the entire atmospheric air flow to bring in even more warm, wet air, which perpetuates the wet season.

 

Of course, this is a simple explanation for a very complicated phenomenon. There are aspects of the change in rainforest seasons that scientists still do not fully understand. For example, leaf growth actually begins to increase during the wet season, just before the dry season starts, perhaps due to plants’ genetic adaptations to the seasons [2]. Curious minds will continue to explore the complexities of seasonal change in the rainforest, no doubt unearthing more mysteries as well!  

Clearly, the influence of the rainforest extends far beyond its own bounds, since the forest itself changes the seasons and reverses wind directions across the entire continent of South America. As we learn more about this powerful place, it only becomes more clear that it plays a crucial role in the biosphere. And there is still so much to learn!

It’s important that we get right down to protecting the Amazon Rainforest- today! Amazon Aid Foundation has a few different programs to help you do just that. Acre Care and Adopt a Tree programs support the active prevention of deforestation and the implementation of re-forestation in areas that have been destroyed. You can also check out some simple actions you can take in your daily life to reduce your carbon footprint here.

References

[1] Jensen, Mari, “Amazon Rainforest grows fastest during dry season.” Mongabay. 2006.  http://news.mongabay.com/2006/03/amazon-rainforest-grows-fastest-during-dry-season/

[2] Lindsey, Rebecca, “The Amazon’s seasonal secret.” Earth Observatory, NASA. 2007.

http://earthobservatory.nasa.gov/Features/AmazonLAI/amazon_lai.php

[3] “Initiation of Rainy Season in Southern Amazon.” Earth Observatory, NASA. 2007.

http://earthobservatory.nasa.gov/IOTD/view.php?id=7714

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