A preliminary venture into the relationships of tropical tree wood density, leaf specific area, leaf
removal force, and data collection in the Peruvian Amazon
Sami Rifai
Introduction
The primary research goal of this project was to determine if a positive relationship exists
between the force required to remove a leaf from a tree and that tree’s wood density. To do this, I
was able to use the funds from the Stone/Tinker Summer Travel Grant to travel to Iquitos, Peru
and attach myself to another ongoing research project within my advisor’s lab. Trees, especially
tropical trees exhibit a great range of wood density, leaf sizes and leaf thicknesses. Multiple
species of tropical trees actually produce wood with densities greater than water. Recently I had
been wondering how leaves respond to high intensity winds. It is well established that high
density wood is generally stronger and more flexible than low density wood. Furthermore, it is
easily observable that leaves of a similar size from different species of trees require differing
amounts of force to remove them (pull them off) from the tree. Despite these observable traits, I
have been unable to find any documentation concerning this topic in either of the scientific
literature databases Web of Science or Google Scholar. From preliminary trials using local tree
species around the New Orleans area, I observed a positive correlation between wood density
and the force per unit leaf area required to remove a leaf. From this, I developed the hypothesis
that high density wood trees would require greater force per unit leaf area to remove leaves. My
reasoning for the given hypothesis is that trees with lower wood density are more likely to snap
under high velocity winds due to the low density wood’s lack of strength. Thus, if these low
wood density trees are more apt to “let go” of their leaves under high winds, less drag would be
exerted upon the tree’s canopy and the tree would have a better chance of remaining intact
through the windstorm. Secondly, I hypothesized that leaves with higher leaf specific area (leaf
area / leaf mass) would require greater leaf removal force. The reasoning for this is because
leaves with higher specific leaf area contain more carbon, and would generally require a greater
proportion of the tree’s resources to produce them than leaves with a lower specific leaf area,
thus making it a resource expensive endeavor for trees to let go of leaves with a high specific leaf
area. Given that my advisor’s lab now works in the Amazonian tropical forests of Peru, I decided
to travel with my lab and test this hypothesis in the Iquitos region.
Methods
Trees of varying wood density were selected along a portion of a 3-km forest wind disturbance
gradient transect in the Allpahuayo-Mishana forest reserve, located about 25 km south of the city
Iquitos in Peru. A tenon/plug cutter attached to a 36V Bosch electric drill was used to extract 12
mm diameter wood plugs from the boles of trees at a height of about 1.3 m. Our accompanying
“matero”, a.k.a. forester/woodsman, identified tree species and then climbed the trees to obtain
leaf samples. When weather conditions permitted, a portable gun trigger gauge (not waterproof)
was used to measure the force required to remove a leaf from a branch as soon as branches were
removed from the tree. Depending upon leaf size, five to twenty leaves were then stored in
Ziplock bags and transported to New Orleans for leaf area determination using a flatbed scanner
and the open source imagery analysis program, ImageJ. In total, wood cores and leaves were
collected from fifty different trees, while leaf force measurements were only taken from nineteen
trees due to persisting rain.
Results and Discussion
No significant relationship was observed between wood density and the force required for leaf
removal (R2 = 0.0062), nor specific leaf area and force for leaf removal (R2 = 0.0136).
Furthermore, no significant relationship was generated from wood density and specific leaf area
(R2 = 0.0454).
Despite the complete lack of any significant relationship with the data collected from this
field season, I am not completely deterred from further testing of this hypothesis because of the
very small sample size used in this study. Ideally, I would have sampled a greater number of
trees but at the current time it was not possible, predominately because our matero and my
participation were needed to assist in other research efforts for a joint Tulane University and
Universidad Nacional Amazonía Peruana (UNAP) project. For this research effort, a number of
UNAP faculty, staff and students in addition to another Tulane employee and myself developed a
straight 3km transect through a gradient of wind damage which will serve as the infrastructure
for addressing future hypotheses. The data for my experiment was only able to be collected
during the last week of our four weeks in Iquitos, however it will serve as the beginning of a
much larger dataset which I hope to greatly augment in the coming year.
I would like to note that in line with the goals of the travel grant, I was able to better
determine my possibilities of further research in Peru. Upon our arrival in Iquitos, we spent the
first week on introductions to people and facilities as well as the gathering of equipment. The
next two weeks were spent developing a trail to the center of the 3km transect, construction of
the 3km transect itself, and on one occasion searching for a lost team member (who was found
the next day in good health). It was during our last week in Iquitos that the resources were
available for us to begin collecting the auxillary data that supported my own project. I believe
that my experiences in Iquitos will be greatly beneficial for my future research for three primary
reasons. First, the opportunity to work and live for a month allowed me to test my Spanish
language skills as well as determine what parts I need to improve to better communicate with
people in Peru. Secondly, the travel grant allowed me to meet and work with faculty members,
staff and students at UNAP. Not only was I able to learn about their methods for forest research,
but I was able to begin developing a professional relationship with many faculty and staff
members at UNAP for future research endeavors. For example, it was with the help of three
different UNAP faculty that we managed to navigate the Peruvian bureaucracy for exporting
plant samples from the country. I do not believe exporting the samples would have been possible
had it not been for the guidance of our UNAP collaborators. Finally, by traveling to Iquitos I was
finally able to experience the very challenging working conditions of a tropical forest. While in
the past I have been privileged to work in ecosystems ranging from Canadian Boreal forests to
the high desert regions of the Mojave, I had never been able to work in a tropical forest until this
past summer. Working in tropical forests has long been my goal because of my great interest in
the biodiversity and forest dynamics of the Amazonian tropics, and now that I have been able to
experience the language, culture, and working conditions of the Iquitos region, I am confident
that I will be capable of conducting further research in the Amazonian forests of this area.