Reading Questions
McDowell, W.H. 1998. Internal fluxes in a Puerto Rican rain forest.
1) Does this scientific paper have a hypothesis? If so, what is it? Or is this paper simply
a quantification of nutrient fluxes? Explain your answer.
a. Answers will vary
b. Author states in last paragraph of Introduction that the objective of the study was
to characterize the magnitude of internal elemental fluxes.
2) In the first full paragraph on page 522 (“Characterization of nutrient cycles in
tropical forest…”), McDowell mentions that humid tropical ecosystems can be
defined by a “tightness of nutrient cycling.” What does he mean by this?
a. Answers will potentially vary
b. Due to a high nutrient limitation in tropical systems (i.e. “paucity”) elements and
nutrients cycle tightly in space and time (i.e. used as soon as they enter). One of
the main hypotheses to explain nitrogen limitation in humid tropical systems is
that nitrogen is used extremely quickly once it enters the soils (in contrast to a
lack of input).
3) In November 1984 Hurricane Klaus hit the eastern portion of Puerto Rico which is
where the Luquillo Experimental Forest in located (see Figure 1). How might this
intense hurricane affect the results reported in this paper? Were there elements of
the experimental design that may have help control for such a disturbance?
a. Answers will vary
b. Hurricanes could provide a large pulse of nutrients (e.g. marine aerosols),
precipitation, and throughfall
c. Sampling over the course of a full year can help control for any outliers
attributable to hurricane disturbance
4) Leaf litter is another major source of nutrients in forest and stream ecosystems.
How do fluxes of certain nutrients compare between throughfall and litter fall? Why
such large differences considering that one of the main sources of throughfall
nutrients is the leaching of leaf-derived nutrients when rain drops fall onto leaves.
a. Fluxes of potassium are higher than leaf litter, calcium and magnesium are high
but 50-75% lower than leaf litter and nitrogen is only 5% of leaf litter derived
nitrogen.
b. The leaves are in a different state of phenology. Leaf litter is also no longer
metabolically active and no longer producing molecules and substances that
protect from the loss of nutrients.
c. McDowell makes the important point that even though some throughfall fluxes
are relatively small (e.g. nitrogen) they still may be ecologically important as they
are readily soluble and bioavailable.
5) At the end of the paper McDowell mentions that fluxes of inorganic N are much
higher than levels of inorganic N in the stream runoff. Why are the processes of
plant uptake and denitrification important at the ecosystem level?
a. Plant uptake is the process of removing nitrate or ammonium from the soil and
using it to build plant biomass. Denitrification is the microbial process of
converting nitrate to N2 gas which is returned to the atmosphere. Both process
remove nitrogen from the soils; however, only denitrification permanently
removes nitrogen from the system. Removal of nitrogen prior to entering the
downstream export via streams potentially can reduce eutrophication of
downstream systems.