Re: Why does bracken grow best in the middle of a wood

You have made an observation, and that is the starting point of scientific 
investigations.  Only by conduction some research would we ever know the 
whole answer to your question.  However, based upon botanical knowledge 
and the biology of the organism in question, the bracken fern, I can 
construct a plausible hypothesis for your observation.

Your word choice does present a small problem.  What is best?  From your 
perspective the taller, more "branched" ferns were growing best, but from 
the fern's perspective, were they really growing better?  Taller or 
shorter, but both were growing successfully, so best is just your 
judgement.  We could consider which ferns were gaining mass the quickest, 
which were spreading the fastest, or which were making the most spores, 
any of which might be considered better.  Lastly, since the only part of 
the bracken fern you see are the fronds, I assume that your observation 
about "more branched" refers to larger fronds.  

Most ferns are well-adapted to the low light conditions of forest floors.  
Except for tree ferns, ferns have either short or horizontal stems, and 
although a few can have very large fronds, in comparison to trees, ferns 
are short plants that cannot compete for light in the forest canopy.  
Light has an effect on elongation of stems and leaves; light arrests 
elongation, while dark promotes elongation.  You can verify this by 
germinating some bean seeds in both a bright and dark location.  Further 
bracken fern is a clonal plant.  A single individual can spread its stems 
widely forming some huge patches.  You might have observed fronds from 
only a single individual fern, but otherwise you might just be looking at 
genetic differences that just happen to be growing in different locations. 
Although we don't think about plants this way, plants can be taller or 
shorter because of genetics just like people.

My hypothesis would be that the developing fern fronds react to the 
differing light conditions by elongating the leaf stalk and making a 
broader frond under the dimmer light conditions within the forest.  The 
fronds developing on the forest margin would have their growth arrested by 
the higher light conditions and remain smaller, but they might capture 
more solar energy for photosynthesis than the larger leaves in dimmer 
light.  If my hypothesis is true, then if I took cuttings from the same 
plant and raised them under different light conditions, but kept all other 
factors the same, I would expect to get taller, broader fronds with a low 
light treatment than with a high light treatment.  

You might decide on a different hypothesis about which ferns are growing 
better.  You might wish to determine if ferns on the sunny margin were 
photosynthesizing at a faster rate than those within the forest.  Of 
course you would have to take into account differences in leaf area and 
probably other factors as well.  But it could be done.  

When our predicted results are obtained we gain confidence in our 
hypothesis, and if really well verified, we consider it true.  So my 
answer to your question just tells you how to answer such a question, but 
anything is just an educated guess.  Lastly before I began such research 
myself, I would conduct a thorough search of the scientific literature to 
see what studies on bracken fern and other plants had been done already.  
This might answer my question, and it would certainly help me construct a 
better research project.