Biology 373L (Gilbert/Plowes)
February 3, 2005
Field Problem:
Reading the past and future of woodlands at Brackenridge Field
Laboratory: Using size structure and community composition of trees
to develop a scenario for succession
Introduction
Several habitat types can be discerned during a walk through of BFL, each
representing the integration of biotic and environmental factors with disturbance history.
Here we look at species composition and age structure of trees that characterize some of
these habitats.
The relative abundance of different size classes of various tree species in a
forest can provide clues about the history and future status of those species in that forest.
For example, even if a tree species is abundant, yet represented only by large, older
individuals, it is likely that the requirements for seed production or seedling
establishment in the area have not been present in recent times. In order to predict the
future status of that tree species in a given area we would need to find out about its
requirements for regeneration and determine the likelihood that such conditions will be
repeated during specified time periods. Thus, for a species that requires a fire to
germinate and establish, we would need to predict when fires are likely.
In contrast, a tree species represented by a large population of saplings presumed
from their size must have recently enjoyed favorable conditions for reproduction and
establishment. Will such a species therefore dominate a future forest on the site?
Possibly, but this may be difficult to answer since factors like disease, herbivory, fire,
and/or shading by faster growing competitors may affect the survivorship of this cohort
over time. Studying the size or age structure of a population helps ecologists make such
projections. (Note that we generally assume that big trees are older than smaller
individuals of the same species, but keep in mind that individuals of the same size may
differ in age and vice versa. Local variation in soil fertility, edaphic factors and light
environment may result in rather different trajectories of growth for identical seedlings
germinating the same year.)
The goal of this project is to acquaint you with some of the kinds of data
ecologists collect in attempting to answer questions about population trends. This
exercise will also expose you to the always-messy issue of how to collect field data, and
to give you insight and experience in statistical analysis appropriate to testing hypotheses
about differences between species, and between habitats in the age/size structure of trees.
We will observe and quantify the distribution, abundance and size structure of dominant
tree species at BFL: 1) live oak, 2) hackberry, 3) mesquite, 4) juniper, 5) cedar elm, 6)
cottonwood and 7) pecan, each of whose populations have apparently responded
differently to recent habitat changes across the area.
However, in collecting data to describe densities and size structures of these species at
BFL, we are in position to test the null hypothesis: that each species has responded
identically to major variations in the habitat at BFL. Once we have collected data on the
trees with respect to variations in habitat at BFL, the patterns observed may stimulate
additional questions and hypotheses.
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Methods and Materials
Given the brief time allowed by the class period, we don’t count absolute
numbers, but instead use sampling techniques for describing tree populations and
habitats. Additionally, we will need to pool the data of all teams. To collect data we will
establish transects in three areas of interest: the lower river terrace, the old pastures
(experimental plots) and the quarry zone.
We will use the Point-Quarter Technique to collect data. The beauty of this method is
that it allows us to quickly estimate the density of selected species in the community
while gathering information on their relative abundances.
Each 2-3 person team will perform a survey in each of 3 apparently distinctive habitats.
In each habitat, each team will establish 5 sample points which are randomly determined
along a transect line. Transect lines will be set by the instructors. Note: Areas recently
cleared or plowed will be excluded.
Each sample point is to be temporarily marked with a flag. The 5 sample points
each represent the center of four quadrants with sides along and perpendicular to the
transect line. In each quadrant, you will first measure the distance from the center point
to 1) the nearest tree >40cm dbh (diameter at breast height), then; 2) 20-40cm dbh and
finally, 3) repeat the same procedure for trees between 5 and 19 cm dbh.
How does this method allow us to estimate density? Imagine that the distance x
from sample point to a tree in one of the quadrats is the side of a square containing the
tree. Since it is the closest tree in that quadrat no other tree can occur in a square with
side of length x. Thus 1/ x squared is the density of that tree species and size class in that
quadrat.
Record all required data for each tree, as follows:
a) Record tree specific data such as dbh, species, condition (living, possibly dying and
dead). Under drought conditions we will need to carefully distinguish “extreme stress”
from death. In the case of small trees watch for evidence of (and record) death by
girdling from male whitetail deer rubbing antlers during rut. Before we start we will
review examples of these special cases in the field.
b) Categorize several ecological features of the micro site which a priori might be
expected to influence establishment and growth of trees. Such things as soil (rock, clay,
sand), vegetative ground cover, density of shrub layer 0-4), and canopy cover (open field
to dense forest. 0-4) can be given qualitative, categorical values to simplify preliminary
analysis. Again, we must jointly decide (by educated guess) the most meaningful and
efficient ways to qualitatively describe sample sites.
NEW: in 2005: Record the species and distance of the shrub nearest to each tree
sampled.
Analysis
The kind of data collected will allow you to answer these questions:
1. Describe the patterns of distribution and abundance of trees across the sites
1a. For Large trees in each habitat (River Terrace RT, Old Pasture OP, and Old Quarry
OQ), calculate the relative abundance (frequency) of each tree species (Cox p68).
Provide charts showing frequencies of species in descending order for each habitat.
1b. For Large trees, calculate the total density (individuals/ha) of all species for each
habitat (RT, OP, OQ separately). (Cox, p.66)
1c. Describe the trends that characterize the different habitats.
2. What are the projected successional trends for the three habitats considering
differences between large and small size classes, assuming size reflects a tree’s age?
2a. Now, compare the relative abundances of Large trees and Saplings of live oak,
juniper, cedar elm and hackberry (common trees). This will allow us to comment on
whether the adult trees are proportionately represented as saplings in the three habitats.
First, make charts to visualize the data:
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Chart 1 for adult trees. Y-axis = relative abundance, X-axis = species. Data
sorted by OP, OQ and RT so that you get four data series on the chart.
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Chart 2 for saplings. Same as above, but use the Sapling counts.
2b.In a particular habitat, are the total proportions of Adults to Saplings the same for
each common species? Construct three Contingency Tables, one for each habitat (RT,
OQ & OP) with 2 rows (adult, sapling) and 4 columns (live oak, juniper, cedar elm &
hackberry). Calculate the chi-square value for the tables and the p-values. Is there a
significant difference in the abundances of Adults and Saplings in each habitat?
2c. Discuss your predictions for the future dominant species in the three habitats, based
on the trends in 2a and 2b.
Discussion
As you take data and later write it up:
a) Look for clues concerning the ecology and natural history of these tree species
which may help account for results of the study (as you take data, be observant of the
status of the trees you are measuring … Are they alive? Is there evidence of herbivore
damage? In there evidence of disease, insect or wind damage?)
b) Develop a likely scenario for the past and future decades of tree population
dynamics in the woodlands of BFL.
c) Pose some additional questions for future study.
Note: If you choose to elaborate upon parts of this field problem for an independent
study, you could choose an additional tree species for study, you could investigate one or
more of the same species at other locations, and/or you could do a literature search on
past ecological studies of some or all of these tree species to put our study into a broader
context.
EXCEL HINT: To sort the dataset
1. Click the TOP LEFT unlabelled margin cell (next to the column labels A, B, C
and row labels 1,2,3,..) This will select the entire worksheet.
2. Click on Data/Sort and a dialog box opens.
3. Check the box “Header Row” at the bottom of the dialog box. The names of each
column should now appear in the top listing box.
4. In the top listing, choose the column you wish to sort by.
5. Add other sorting options if needed in the 2nd and 3rd boxes. Click OK.