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A WALK THROUGH

The Forest Biome

The forest.  What we likely all think about as soon as the word "tree" comes to mind.  Now, you certainly wouldn't be wrong about the importance of trees in these biomes, but what you might not know is that it's very hard for everyone to agree on a solid definition for the biome.  We all agree on the importance of trees, and generally agree on the land cover associated to those trees, but in our changing landscape, there are a number of places deemed forests based on their historic purpose that no longer serve or function as forests.  The reverse of that is true where places aiming to function as a forest are called forests prior to being functioning forests.  Certainly a lot to think about!​

​Even beyond function, what you may consider a forest could be vastly different from the forests that someone else is familiar with.  All around the world, we have these amazing biomes, providing the planet with 75% of its gross primary production, or the total amount of carbon mass produced by our autotrophic friends.  No matter where you are, you're impacted by the work these places do, so let's go over what they are and where we find them.

Tropical

Okay, so the first thing I always thought of growing up is the idea of "the tropical rainforest" and immediately picturing the Amazon.  I know, very original.
Imagination
Plants in forest biomes are responsible for roughly 75% of the world's gross primary production
Now that's not a bad start, but would you know that's only a portion of the types of tropical forests in the world?  Even among wet tropical forests, there are some interesting outliers.  Consider the tropical montane forest.  This type of forest typically resides on mountainsides with low cloud cover.  This close relationship between tree and atmosphere allows for pulling water vapor straight from clouds without need of precipitation.  When it does rain, the rainfall can occur below the forest canopy, allowing the ground to capitalize on more rain than other tropical forests.  This cloud-canopy relationship often attributes the name "cloud forests" with the biome, which is both super cool and pretty darn accurate.​
 Cloud Forests, a unique type of wet tropical forest, is situated within cloud cover, providing non-precipitation access to fresh water
Where we have one extreme, we're actually able to experience another.  As cloud forests have a unique atmospheric water reservoir, tropical flooded forests have theirs at ground level.  This biome isn't necessarily specific to tropical areas, but are very typical of them, creating idiosyncratic hot, wet habitats that we typically associate with swamps.  Of course, this starts straddling the line of wetland vs. forest, so we'll stop there for this breakdown.

In addition to wet tropical forests, we have dry tropical forests as well.  That's kind of a misnomer, since dry tropical forests can still garner hundreds of centimeters of rainfall per year, but they experience a dry season.  During this season, broadleaf forest trees will lose their leaves, allowing for retention of water until the next wet season.  The other type of dry tropical forest is coniferous, whose trees retain their leaves year-round, and are comprised of conifers that have adapted to variable climate conditions and low-to-moderate rainfall.

Temperate

​Temperate forests are a little more streamlined, falling into 2 primary categories: coniferous and deciduous.  You'll notice that both of these forest types applied to tropical regions as well.  The primary differences to note here are in climate and hydrology.  Temperate forests generally fall between the Earth's poles and equatorial regions, making things fairly cool and receiving moderate rainfall.  One key point to make is that deciduous trees in temperate areas don't necessarily lose their leaves in a "dry" season.  While this is still somewhat true, temperate zones are generally drier in their winter months, but precipitation still occurs, it's just locked in snowfall.  Locked may be a harsh term, since the water it still accessible given warming conditions, but for the most part, this reservoir is mostly untapped in the cold.
Additionally, the closer we get to the poles, the more extreme seasonal sunlight availability becomes.  During these regions' winter months, sunlight is limited, perhaps only being provided for several hours of the day.  Coupled with the drier conditions and lower availability of water stores, trees tend to go dormant, dropping their leaves and taking their resources back into their trunks and root systems until wetter, warmer seasons return.

Boreal

​Which brings us to our final broad category, the boreal forest.  Growing up, I was taught this region as a taiga, but boreal gives us a little more information based on its etymology.  The word comes Latin origins meaning "northern" or "of the north" occasionally in reference to mythical gods of the north wind.  Taking this origin, we'd probably assume these forests appear in the northern parts of Earth, and this would be true!  The boreal forests span the northern reaches of the planet, mostly Canada and Russia.
Temperate deciduous trees drop their leaves in cold climate seasons when water isn't as readily available, generally being locked in snow and ice
These regions are particularly harsh, receiving limited sunlight and rainfall.  As such, the hardy coniferous trees make up the brunt of these biomes, where only the southern-most areas having a sparse smattering of deciduous trees.  This southern area does have a dense tree cover and is considered a subzone of the boreal forest.  This blend with the temperate regions is what we can call an ecotone, where two biomes or ecosystems meet.  Alternatively, in the northern aspects of the boreal forest, we have the northern subzone, sporting a more open canopy, as the blend with tundra forms another ecotone at the other end of the spectrum.​
Boreal forests, also known as taiga, get their name from Latin origins and mythological gods of the northern wind

Common Characteristics

Now that we have a decent grasp on the spectra of forests that exist on our lovely planet, how is it that we know we're looking at a forest?  How do we tell the forest from the trees?

Where I live, in the midwestern United States, I've learned that one way is to gauge the biome by its abundance of tree cover.  That is to say, for a given amount of terrestrial area, how much of that area is covered by mature trees?  The general rule of thumb I was taught getting into woodland ecology is 0-10% cover is a prairie, 10-30% is a savanna, 30-80% is a woodland, and 80-100% is a forest.  You'd guess correctly that there is likely a lot of debate over where to draw the line between a woodland and forest, and I tend to agree that this distinction is hard to make.  Suffice to say, if you're having difficult time seeing sunlight under the trees, you're likely in a forest!​
One set of characteristics I like, beyond statistical tree mass, is how the species who call the biome home are situated.  A typical forest is connected enough by its trees that it has 4 layers in which its inhabitants reside: the forest floor, the understory, the canopy, and the emergent layer.

​At the bottom rung, we have the forest floor, home to our soil-driven species, ground cover foliage and shrubs, and some of our bigger animal friends.  As we travel up, we reach the understory, where we meet our adolescent to adult trees and plants as well as our lower tree-abiding creatures.  We still have some large animals in this layer, like tree-loving big cats, but a majority will be avian species and a good smattering of our less-grounded primates. 
​We rise up another layer to the canopy, mostly comprised of avian species.  As this is the highest portion of the forest for most of the trees, a lot of fruit growth appears here, attracting many species.  The canopy can be used as a bit of a highway for creatures able to traverse the trees.  This may sound a bit contradictory, but while the canopy sounds like the topmost layer, it's just where the majority of trees form a shield against sunlight from filtering into the forest.  Due to this, we can understand that, while getting less sunlight, anything beneath the canopy is also protected from particularly high heat. 

Beyond the canopy, we have the emergent layer, which, true to its name, is comprised of the trees that have emerged beyond the canopy of the forest.  This layer is relatively inhospitable, being comparatively dry to the rest of the forest.  In tropical forests, this area is also remarkably hot, not benefiting from the shade of the dense interlocking of trees granted by the canopy.  Here, we'll find our tertiary avian predators, who will dwell away from the rest of the forest, diving into the canopy layer to feed on other smaller birds and other animals.

Despite the type, these 4 layers of habitat should be distinguishable in a forest biome!
Forests have 4 major layers teeming with different types of life which include the forest floor, understory, canopy, and emergent layer

Being able to identify these layers means you're likely looking at a forest
I'd say "and there we have it" but these are really only general concepts regarding one the Earth's most integral types of biomes.  When it comes to identifying ecosystems, there is a lot to be taken into account, but hopefully these thoughts can help you identify forests as you come across them in your adventures.  Though, more importantly, as you find them, hopefully these (and your) thoughts help you appreciate them all the more.
~ And, as always, don't forget to keep wondering ~
Prismatic Planet
Sources
* https://ucmp.berkeley.edu/exhibits/biomes/forests.php​
* "Forest definition and extent" (PDF). United Nations Environment Programme. 27 January 2010. Archived from the original (PDF) on 26 July 2010. Retrieved 16 November 2014.

 Pan, Yude; Birdsey, Richard A.; Phillips, Oliver L.; Jackson, Robert B. (2013). "The Structure, Distribution, and Biomass of the World's Forests" (PDF). Annu. Rev. Ecol. Evol. Syst. 44: 593–62. doi:10.1146/annurev-ecolsys-110512-135914. Archived (PDF) from the original on 7 August 2016.
https://www.nrs.fs.fed.us/pubs/jrnl/2013/nrs_2013_pan_001.pdf

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