BI237 Woody Plants
Laboratory 10
SEED DISPERSAL AND REGENERATION NICHE IN MAPLES
Seed dispersal is an important stage in the life cycle of seed plants. Even for
species such as aspen and beech that have extensive vegetative reproduction, seed
dispersal is necessary for the colonization of new habitats. Theoretical models show that
non-dispersing species will always be replaced by those with some capability for
dispersal.
Seeds of trees can be dispersed by gravity, wind, water, or animals. In the
temperate forest, wind dispersal is probably the predominant mechanism, being used by
conifers, ashes, maples, elms, birches, willows, and aspens. The ability to stay airborne
is an important feature of such seeds; wind dispersal is not terrifically effective for seeds
that plummet. The rate of descent is determined largely by the relationship between the
mass of the dispersal unit and its surface area. In fact, it has been shown for a variety of
wind-dispersed seeds that the rate of descent is proportional to the square root of wing
loading, where wing loading is weight divided by area.
Seeds with a low wing loading will travel very far in the wind, but this great
dispersal ability comes at a cost. Low wing loading is only feasible when seeds
themselves are quite light. There is a built-in tradeoff, therefore, between seed
provisioning, which can help in the critical life stage of establishment, and likely
dispersal distance. Species with different regeneration niches will have contrasting types
of diaspores (the general word for dispersal units). "Pioneers" need to constantly
colonize newly opened, sunny sites. They tend to not need large seeds, but instead rely
upon rapid photosynthesis to get through the establishment stage. Pioneers usually have
light seeds and diaspores with low wing-loading. "Grey beards", in contrast, are already
in appropriate habitats but need sufficient resources to establish in the shade. They tend
to have larger seeds with higher wing-loading.
There are a number of different aerodynamic categories of diaspores: rolling,
autogyration, tumbling, etc. Since each morphological group is aerodynamically distinct,
it is difficult to compare dispersal of different species using only wing-loading data. In
the maples, however, we are blessed with a large number of species that span the
spectrum of regeneration niches: from the shade-tolerant "greybeard" sugar maple, to the
opportunistic and weedy red maple. In this laboratory, we will compare the flight
characteristics of diaspores of species of maple that differ in regeneration ecology.
HYPOTHESIS FORMATION
We have obtained samaras of Acer saccharum, Acer saccharinum and Acer rubrum.
What are the regeneration niches of these species? How would you expect their wingloading and descent rates to compare? With your group, look ahead to the analysis
questions and decide which two you would like to address. Note that the red maple
samaras are near the limit of our resolution of differences in mass.
BI237 Woody Plants
Laboratory 10
METHODS
1. Rate of descent for individual samaras.
Write numbers on each of ten or more samaras, so that they can be individually
identified.
Work in pairs in one of the Olin or Arey stairwells.
Measure the vertical distance of the drop.
Position one team-mate above and the other below. The person above makes an
utterance to indicate when s/he is dropping the samara. The person below times the drop
time using a stopwatch. Record the drop time along with the samara number.
You will now be able to calculate rate of descent in m/s for each samara.
2. Wing-loading for individual samaras.
Calculate the area of each samara. First trace the outline of the samara onto a piece of
paper. Then (if it was graph paper) count the squares. If it was regular old paper, weigh
it on an analytical balance and calculate the area by comparison with the weight of a
piece of the same paper of known size. Record with samara number.
Weigh each samara. Record with samara number.
You will now be able to calculate wing-loading for each samara.
ANALYSIS -- ADDRESS AT LEAST TWO OF THE FOLLOWING QUESTIONS
1. Within a species, is there a correlation between the square root of wing-loading and
the descent rate? (Hint: for this question, you may want to collect data on more than ten
individual samaras per species.)
2. How does the wing loading of the three species compare? Is the comparison what you
would expect? Why or why not?
3. How do the descent rates of the three species compare? Is this what you would
expect? Why or why not?
This lab was inspired by:
Thomson, J. D., and P. R. Neal. 1989. Wind dispersal of tree seeds and fruits. American
Biology Teacher 51: 482-486, which was in turn inspired by Carol Augspurger's work on
the aerodynamics of wind-dispersed diaspores of tropical trees.