13th International Biology Olympiad Latvia, 2002 Ministry of Education and Science University of Latvia Association of Biology Teachers
4 Kronvalda boulv., Riga LV-1586, LATVIA phone: +371-7034860, +371-7334125, Fax: +371-7223801, email: ulkoro@latnet.lv

Final Report

4. Tasks

4.1. Practical tasks

4.1.4. Laboratory IV – Dendroecology

Growth of aspen (Populus tremula) invading a clear cut (previously spruce) by seed.

Length of the practical test – 60 minutes; 40 points

Materials and instruments

You will need the instrument set that you received upon registering for the 13^thIBO!

Other materials:

A data sheet. The data sheet is identical to the answer sheet with tables, only with some of the columns (Table 1) and rows (Table 2) filled in. THIS DATA SHOULD BE COPIED ONTO THE ANSWER SHEETS. DO NOT WRITE ON THE DATA SHEETS.

10 labelled discs of aspen. DO NOT MAKE ANY MARKS ON THE DISCS

one measuring tape

one ruler

one magnifying glass

Introduction

Aspen has invaded an open area created by a clear cut harvest of a spruce forest. Not all of the trees established in the same year, but over a 8-year period. Now, 18 years after the start of development of the new forest stand, the vertical and radial growth of aspen will be investigated by retrospective analysis. Tree rings can be used to measure the tree stem diameter at different tree ages.

Q1. In the following photograph (Figure 1) of a cross-section of an aspen stem, identify the following, by writing the respective number from the Figure in the answer sheet!

Code Table

A.	bark
B.	pith
C.	latewood
D.	earlywood
E.	meristem
F.	annual ring
G.	branch
H.	vascular cambium
I.	phloem cells
J.	xylem cells
K.	cork cells

(11 points)

Figure 1.

The growth of aspen stems in the area will be used to determine response to possible intraspecific competition. It is assumed that the effect of competition increases with time, because tree density increases and the trees increase in size.

Q2. For a tree species, not necessarily aspen, what are the possible effects of intraspecific competition for light resources:

1. increased mortality rate over time,

2. decreased yearly growth increment in height of stems

3. increased variability among stems of yearly growth increment in height

4. decreased yearly growth increment in diameter of stems

5. increased variability among stems in yearly growth increment in diameter

6. accelerated yearly growth increment in height of stems in response to shading by neighbours

Which of the following is the correct combination of correct answers? Enter your choice in the answer sheet!

A. 1, 2, 4

B. 1, 3, 5

C. 1, 2, 4, 6

D. all of the above

E. 1, 2, 3, 4, 5

(1 point)

The purpose of this practical work is to determine whether radial and/or vertical growth of aspen is related to time of invasion of aspen individuals. In other words, since the time of invasion can be determined by estimating the age of stems, does the growth depend on the age of the individual?

Methods and Results

Within a forest plot (20mX20m), all trees growing were cut at tree base, labelled and stem height was measured. Each student is supplied with 10 segments cut at stem base, which were randomly sampled in the plots. Each stem is labelled, and the height of these stems in metres is provided beside the appropriate tree ring code in Table 1 of the data sheets. Copy the codes and the heights onto the answer sheets.

T1. Fill in the Table 1!

• Count the number of tree rings of each segment, and enter this data in Table 1 of the answer sheets! (2 points)

NOTE: Since determination of the first few tree rings is difficult due to rot, the first five tree rings developed have been determined under a dissecting microscope, and the position marking the end of five years is marked on each stem. Therefore, you need only to count the rings developed after this mark, and then add 5 years.

• As none of the cut segments are perfectly round (the radial growth differs depending on compass direction), the mean diameter will be estimated by calculation from the perimeter. Using the cloth tape provided, measure the perimeter of each stem, and enter this data in Table 1 of the answer sheet!(2 points)

• Using the appropriate formula, use the perimeter measurements to calculate the mean diameter, and enter this data in Table 1 of the answer sheet! (1 point)

• Using the ruler provided, measure the total width of the first five tree rings (from the centre to the mark on the segment) and enter this data in Table 1 of the answer sheet! (1 point)

• Using the ruler provided, measure the last five tree rings (at the location of the maximum diameter (marked by the line), and enter this data in Table 1 of the answer sheet!(1 point)

G1. Using the graph paper supplied in the answer sheet, produce scatter plot for Graph 1. - Stem age versus height (2 points)

G2. Using the graph paper supplied in the answer sheet, produce scatter plot forGraph 2. - Stem tree age versus diameter(2 points)

G3. Using the graph paper supplied in the answer sheet, produce scatter plot forGraph 3. - Stem age versus width of first produced five tree rings (2 points)

G4. Using the graph paper supplied in the answer sheet, produce scatter plot forGraph 4. - Stem age versus width of the last five produced tree rings. (2 points)

Remember to label the axes with the appropriate scales.

G5. As the graph of stem age versus height appears to indicate a linear relationship, calculate the linear regression equation (best-fit line through the points) using the Tables provided (Table 1 and Table 2 ) and draw the calculated best-fit line on the appropriate graph (1 point)

NOTE: In the data sheet supplied, you have been given the correct values of the sum of squared ages ( Xi²) and the value of the sum of age times height ( Xi*Yi). Therefore, you do not need to calculate these. ENTER THESE FROM THE DATA SHEET ONTO THE ANSWER SHEET.

T2. Completely fill in Table 2 on the answer sheets (8 points)

Check that Table 1 is completely filled in.

Discussion

Q3. The Graphs that you have produced suggest that intraspecific competition has resulted in:

1. reduced height of shaded individuals

2. reduced diameter of shaded individuals

3. reduced stem structural support of shaded individuals

4. increased variability in height of individuals of the same age

5. Increased variability in diameter of individuals of the same age

Which of the following is the correct combination of correct answers?

A. 1, 2, 3, 4, and 5

B. 1, 2 and 3

C. 2, 3 and 5

D. 2 and 4

E. 4 and 5

(1 point)

Q4. Which of the following comments are suggested by the Graphs that you have produced?

1. The reduced growth of individuals invading the clear cut later suggests that aspen has a stress-tolerant growth strategy.

2. The Graphs suggest a certain competitive ability, because late-coming stems are able to maintain the same growth rate in height as the stems which arrived earlier, indicated by the linear relationship between age and height.

3. In combination, the Graphs suggest that there will be potentially increased mortality of the individuals invading the stand later.

4. The Graph 4 suggests that the growth strategy of aspen appears to be a ruderal strategy: rapid growth early in succession, taking advantage of available resources

Which of the following is the correct combination of correct answers?

A. 1 and 3

B. 2 and 4

C. 2, 3 and 4

D. 4

(1 point)

Q5. Which of the following comments are suggested by Graphs 3 and 4 ?

1. The Graphs show that the effect of competition increases with time after clearcut.

2. Only the trees arriving on the site during the first few years (but not necessarily all of these individuals) have been able to support a high radial growth rate during the last few years prior to cutting.

3. Differences in the amount of shading in the occupied patches may be a reason for the high variability in Graph 3 .

4. Graphs 3 and 4 probably reflect linear relationships that are hidden by high variability.

Which of the following is the correct combination of correct answers?

A. 1, 2 and 3

B. 4

C. 1, 2, 3, and 4

D. 1 and 3

E. 2 and 4

(1 point)

Q6. Which of the following problems clearly need to be considered, as they can affect the results shown in the Graphs ?

1. The annual growth increment in tree ring width is related to the current stem diameter. For this reason, calculation of the relative growth rate (RGR) would have less bias.

2. Differences in the growth of the different stems should also be assessed in relation to differences in meteorological conditions during the years after the clear cut.

3. Biotic factors such as herbivory (insects, deer, moose which do occur in the area) and disease may have caused death or damage to a particular age class of stems, biasing the results.

4. The sample size is too low for conclusive results.

Which of the following is the correct combination of correct answers?

A. None of the comments are true

B. All of the comments are true

C. Only 1 and 4 are true

D. Only 2, 3, and 4 are true

E. Only 2 and 3 are true

(1 point)