Plant Canopy competition
Because manipulations of organisms in the real world (as opposed to simple observations
of organisms in the field) often take a relatively long time, some labs are started before you have
had time to really think about what the exercise is about in terms of ecological insights or
principles. One of the purposes of this handout is to give you some idea of what the plant canopy
experiment is designed to study. In this lab we will first set up a long term experiment in plant
competition, but only get the data after some time has passed. Its only relatively long term
though, we will finish by the end of the semester. Think about forestry experiments that are set
up by a group of workers who will not be alive when the results are in!
I Plant competition:
Koa Haole [Leucaena leucocephala (Lam.) de Wit] is an abundant, fast growing
leguminous shrub or tree found in most dry areas in the Hawaiian Islands. His plant is not native,
but was introduced for cattle fodder. It has spread widely through the archipelago, propagated for
the most part by its seeds. The plant produced many large, elongate seed pods containing
numerous large seeds. You are already familiar with these seed pods, now you can get up close
and personal with the plant itself!
When the pods are ripe, they split open and the seeds fall to the ground beneath the parent
tree. The number of seeds produced per plant, per year can be enormous. When we get to the
study site, pick up some loose soil under a mature plant and look for the seeds. Think about how
you would determine how many seeds there are in the study area?
Seeds in the soil like this are often termed the seed bank i.e. a reserve of potential plants
waiting for an opportunity. If all these seeds produced adult trees what do you think the density
of Koa Haole would be? Does the situation in the study area suggest that this is what is
happening? If not what might be some possible explanations for the difference between the
potential for population growth and what is realized in nature? {This is one of the big questions
in ecology}.
One possibility is that the shade of the adult trees is so dense that the small individuals
cannot grow. {Can you think of other testable hypotheses? Are yours exclusive of the shade
hypothesis or could many factors be operating?} To test the shade hypotheses we will reduce the
canopy of large plants in experimental plots by sawing down the large trees that shade the
experimental plots thus allowing light to reach the under story where the small plants and seeds
are. You will also need control plots, as similar to the experimental ones as possible. Also how
might you introduce some randomness into the selection of experimental and control plots?
The shade hypothesis predicts that the increase in light that your work will provide will
cause an effect on the small Koa Haole plants in the areas you cleared. What effects might this
reduction in competition (for light) have on the population of smaller Koa Haole plants? One
possibility is that the size of the young plants that had already germinated and grown to some
extent before the experimental manipulation (we will term these saplings) might increase. How
could you find this out? Another possibility is that the number of young plants in the
manipulated area might increase (relative to the control of course). Would you expect an increase
in saplings in one semester? A more realistic metric of production of new individuals is to look
at the seedlings (which can be defined as small plants that still have (or which have just lost)
their cotyledons).
Procedure:
We will begin by choosing several pairs of plots, as similar to each other in plant
composition, soil, exposure etc as possible. These plots, 2 m on a side, will be marked out with
small stakes at each corner and a string boundary line that you lay out. Be careful not to step in
your plots (or anyone else's for that matter!). Within each plot you will census the seedlings and
mark and measure all the saplings. Think carefully about how you will take your measurements.
This will have important implications when you return to take the "after" measurements. Enter
the data on the data sheet provided. Remember to write clearly - unreadable data are worthless
data!
Next, one of the plots will be randomly selected to be the experimental plot. You can flip
a coin or throw a die (if you brought one). This kind of sampling where pairs are specifically
chosen for some feature (in this case for similarity) then, within each pair, random choices are
made is a kind of stratified randomized sampling. At the experimental plot all the canopy is
removed. You will need to think about the passage of the sun, the proximity of other plots etc.,
so this will take a bit of planning. You will also need to be careful you do not disturb any of the
plants inside the plots (except the ones you remove.)
Towards the end of the semester we will return to the site and you will census seedlings
and measure saplings in the experimental and control plots, do statistical testing of your data and
discuss plant competition in the light of your results.
Some references that might be useful
Akamine, E. K. 1952. Germination of koa haole. Pacific Science 6: 51-52
Ford, E. D. 1975, Competition and stand structure in some even-aged plant monocultures. J.
Ecology 83: 311-333.
Horn, H. 1971 The adaptive Geometry of Trees. Princeton University Press NJ 144 pp.
Linhart, Y. B. 1976 Density-dependent seed germination strategies in colonizing vs. noncolonizing plant species. J. Ecology 64: 375-380.
DATA SHEET
Quadrat #
Quadrat #
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|Sapling #
Diameter
Height
|Sapling #
Diameter
Height
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|_______________________________________________________________________|
Number of seedlings at the start___
Number of seedlings at the start___
Number of Seedlings at the end____
Number of Seedlings at the end____