Seed Dispersal and Regeneration Niche

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 wing-loading 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.

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?

BI237 Woody Plants Laboratory 10

BI237 Woody Plants Laboratory 10

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.