56 | New Scientist | 25 July 2020
Spreading roots
If the height of a tree is limited by
capillary action, why can’t trees
grow to an unlimited width?Guy Cox
St Albans,
New South Wales, Australia
This takes me back to my plant
science degree. First of all,
capillary action doesn’t limit the
height of trees. If it did, they would
be nothing like the magnificent
specimens we see all around us.
Water transport in trees
depends partially on water
molecules sticking together to
form a column in hollow cells
within the bulk of the trunk. This
is distinct from capillary action
and therefore no fundamental
limit exists on the height of a tree.
It isn’t clear that anything
limits the girth of trees either,
but the trunk functions to support
the crown. In a forest, the primary
consideration is to get the crown
high enough to catch the sun.
Ancient, slow-growing trees
can have very broad trunks –
this is seen in the redwoods of the
Sierra Nevada mountain range in
the US, which can be more than
3000 years old and have trunks
about 10 metres in diameter.
In Australia, we have the largest
broadleaved trees in the world,
reaching heights in excess of
100 metres, but they are relatively
fast growing and short lived.
Trees in isolation can spread
very widely. Two oaks in England –
the Newland Oak, Gloucestershire,
and the Cowthorpe Oak, North
Yorkshire – reportedly had trunks
more than 5 metres in diameter.
Both were relics of ancient forests,
but once freed from vertical
competition, they spread
horizontally. Though they are
now both dead, the Newland
Oak was about 1000 years old
and the Cowthorpe Oak lived
for up to 1800 years.
In the case of the Newland
Oak, its large horizontal spread
was probably encouraged by
pollarding – the practice of
harvesting branches for timber.Mike Follows
Sutton Coldfield,
West Midlands, UK
The function of a tree trunk
is to provide support for the
branches, which carry leaves for
photosynthesis, as well as being
a conduit for water. The trunk
won’t grow wider than it needs
to be as building that extra tissue
would be a waste of energy.
It also won’t be too narrow,
however. If a tree were too thin
for its height, it would buckle.
The minimum diameter required
to prevent a cylindrical column
buckling under its own weight is
proportional to its height to the
power of 1.5. Trees largely obey
this relationship, but with some
variation between species and
among individual trees within
each species.
Trees can also develop wider
trunks if exposed to the wind.
This should mean that trees
on the edge of a forest have thicker
trunks. Indeed, it is often the
spindlier trees away from the edge
of a forest that are toppled during
severe storms. This is probably
because these trees aren’t adaptedto the strong wind that can
penetrate further into a forest
on such occasions.Cyclic logic
A triplet bike is lighter and has
less resistance per person, so is
more efficient than a tandem,
which is more efficient than a
regular bike. Does this trend
hold however long the bike?Cedric Lynch
Potters Bar, Hertfordshire, UK
Multi-seat bicycles get more
efficient the more seats they have,
up to the point where the builder
is forced to adopt motorcycle tyres
instead of bicycle tyres in order to
accommodate the weight.
There is then a large drop in
efficiency because motorcycle
tyres typically have a rolling
resistance coefficient – the
horizontal force needed to rollthe wheel on a level surface,
expressed as a fraction of the
weight on the wheel – about
four times that of bicycle tyres.
The bicycle in your photo is for
three riders, one or two of whom
are children. Thankfully, there are
bicycle tyres available that can
comfortably support this weight.N. C. Friswell
Horsham, West Sussex, UK
It depends on how “efficiency”
is measured. In the 1950s, I owned
a tandem bike and my fiancée
and I decided to go on a touring
holiday to Land’s End in Cornwall.
When we got to the Cornish hills,
we found that they were
impossible to cycle up, even with
the combined weight of the two
of us standing on the pedals.
When I mentioned this to
a cycling enthusiast, he said:
“What do you expect? Hill-
climbing trials bikes have very
short wheelbases. The tandem is
long.” But he was unable to explain
why short-wheelbase bikes were
able to get up hills easier. Clearly,
in this respect, the tandem isn’t
more efficient and the triplet
bike in your illustration must be
completely useless in hilly areas.Eric Kvaalen
Les Essarts-le-Roi, France
As a bike gets longer and longer,
it eventually has to get wider
and taller as well. It wouldn’t
be possible to make a bike that
was a mile long using struts with
the same diameter as those for
a normal bike.
Eventually, the weight would
rise faster than is proportional to
the length, whereas the number of
riders would still be proportional
to the length. As such, the weight
per rider would increase. ❚This week’s new questions
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