Re: Why are trees left standing after a large floor with only their trunks?

Casey:

What an interesting question.  First off, let us just talk a moment about 
dead trees in general.  Dead trees left standing are called snags.  Snags 
are an important part of the forest ecosystem as they provide habitat for 
a variety of wildlife including woodpeckers, neotropical migratory birds, 
insects, small mammals, etc.  

Generally, under normal forest conditions, hardwood snags tend to last a 
lot longer than softwood snags due to differences in their respective wood 
properties.  Softwoods, like pines, tend to decay fairly quickly in 
comparison to say, an oak snag.  Some other factors that can influence the 
decay rate of a snag are climatic conditions like temperature and humidity 
and the amount of insect and bacterial activity working to decompose the 
wood. 

Secondly, let me address your question on why trees that have lost their 
top don’t just disappear over time.  Actually, eventually, the tree will 
indeed disappear.  It just takes a long time for some tree species as we 
just discussed.  And while it seems counter-intuitive, snags that have 
lost their tops will probably last longer than snags that have not.  This 
is because of something called windthrow.  

Tree root systems are fairly extensive.  Roots on a tree can extend out 
from the tree to a diameter of one to two times the height of the tree.  
However, most of the roots are within 8-12 inches of the soil surface.  
Because of this, a tree with an extensive crown (top) is vulnerable to 
being knocked over by high winds (an occurrence that is known as 
windthrow).  Trees without a top do not have as much surface area to catch 
the wind and therefore, are less subject to being blown over.  

Of course, trees that are surrounded by lots of other trees are less 
subject to windthrow than a tree standing out in the open by itself 
because dense surrounding vegetation acts as a windbreak and the root 
systems of the trees become intertwined which creates stability.  
Intertwined root systems are also a  reason that trees that have been dead 
for many years are not simply ‘released’ by the roots.
  
Ok, with all that said, lets talk about trees under flooded conditions.  
Floods have a very large impact on both trees and the landscape as a 
whole.  For a really good discussion on this topic see the following 
link.  It is entitled “Flood Damage to Trees” and is written by Dr. Kim 
Coder who is a highly knowledgeable tree damage specialist:
 http://www.forestry.uga.edu/warnell/html/service/library/for94-
061.html

After reading this link, you will have a good understanding of how and why 
floods damage trees.  However, this link focuses on flooding effects on 
live trees, not snags, so let me further discuss the last part of your 
question: “after many years after the tree dies, why don't the roots just 
release the tree?” 

In addition to intertwined root systems, another reason that snags will 
not just be released by the roots has to do with the decay rate of the 
roots under long-term flood conditions.  Water standing in an area for a 
long period of time creates something known as anaerobic, or oxygen-
deprived, conditions. Under anaerobic conditions, aerobic organisms are 
replaced by anaerobic bacteria.  This significantly changes the rate of 
decomposition of wood as anaerobic bacteria are not as effective as 
aerobic organisms at decomposing organic matter.    Therefore, under 
anaerobic conditions, it is possible for the root system of the tree to 
remain intact and the tree to remain standing for a longer period of 
time.  Keep in mind that no matter the conditions, a snag will eventually 
decompose- the process is just longer in some instances than others and 
sometimes, to us impatient humans, this process can seem to take forever ;)

On a side note, anaerobic conditions are so effective at preserving wood 
that some mills store logs under water in large holding ponds until they 
are ready to process the wood.  Logs stored this way can last for many 
years in perfect condition.  In fact, recovering submerged logs from lakes 
and rivers has become a profitable business.  For more information, check 
out the link below:
 http://www.
forestnet.com/archives/april_00/submerged.htm

I hope that this has answered your question.  If you would like to discuss 
the matter further, please do not hesitate to contact me directly via e-
mail.

Best regards,

Gail