Crowded Radishes and Sunny Gaps: When Plants Share Resources

Use a garden trial and a forest survey to explain how resources affect plants and populations.

Answer Key

The radish crowding trial

Read the passage and study Table 1. A question mark shows a value for you to work out.

Room to Grow

1Saanvi Rao, a seventh grader at Elm Ridge Middle School, runs on the cross-country team and helps in the school garden after practice. This fall, her science teacher, Mr. Abdi Warsame, asked the garden club a question that farmers and foresters often face: what happens when many plants have to share the same light, water and soil nutrients?

2The club tested radishes, which grow from seed to harvest in about six weeks. They marked out ten garden plots, each 1 meter by 1 meter, in the same sunny bed. All plots had the same soil and the same compost, and a drip hose gave each plot the same amount of water every morning. Club members pulled weeds by hand twice a week, so no other plants grew in the plots. The only difference was how many seeds were sown: 25, 50, 100, 200 or 400 seeds per plot, with two plots for each number. Almost every seed sprouted. After six weeks, the club counted the plants still alive in each plot, pulled them and weighed the radish roots. Table 1 shows the average of the two plots for each number of seeds.

3Saanvi saw the second half of the study on her training runs. Three summers ago, a storm knocked down several large oaks beside the trail at Pinecrest Nature Center, opening a sunny gap in a shady forest. Right after the storm, the center's naturalist, Ms. Beatriz Orozco, marked small plots with sugar maple seedlings in three places: under the full forest canopy, at the edge of the gap and in its center. She fenced every plot so that deer could not eat the seedlings, and her soil-moisture readings were about the same in all three places. Each summer she measured how much light reached the ground and counted and measured the seedlings (Table 2). Growth in Table 2 means how much taller the surviving seedlings got each year, on average.

Table 1. Radish Plots After 6 Weeks (each plot 1 m², average of 2 plots)
Seeds sown	Plants alive	Root mass per plant (g)	Total root mass (g)
25	25	40	1,000
50	50	30	1,500
100	98	18	1,764
200	188	10	?
400	320	6	1,920

Use Table 1. Total root mass = plants alive × root mass per plant.

The total root mass in a plot sown with 200 seeds was 1880 g.

Use paragraph 2 and Table 1. Almost every seed sprouted.

In a plot sown with 400 seeds, about 80 plants died before the harvest.

Which pattern does Table 1 show as more seeds were sown?

A. Each radish got lighter, and the total root mass fell from 1,000 g to about 600 g.
B. Each radish got heavier, and the total root mass rose by about the same amount each time.
C. Each radish stayed about 30 g, and the total root mass rose along with the seeds sown.
D. Each radish got lighter, and the total root mass rose at first, then leveled off near 1,900 g.

From 100 to 400 seeds, the plots held many more plants, but the total root mass rose only a little. Which explanation fits paragraph 2 best?

A. Radish seeds sown close together sprout later, so the crowded plants had less time to grow.
B. Each plot got the same light, water and nutrients, so more plants split the same supply.
C. The big radishes in the uncrowded plots took nutrients from the crowded plots next to them.
D. The crowded plots got less water from the drip hose, so their plants grew more slowly.

Suppose the club sows a plot with 800 seeds and treats it the same way. Based on Table 1, which result is most likely?

A. About 800 plants alive, each about 10 g, for a total of about 8,000 g
B. About 790 plants alive, each about 6 g, for a total of about 4,700 g
C. Many more plants dying, each radish under 6 g, and a total near or below 1,900 g
D. About 320 plants alive, each about 40 g, because only the strongest plants survive

A market gardener sells radishes only if each one has a mass of 25 g or more. Which number of seeds per plot gives the greatest total root mass while the average radish still has a mass of 25 g or more?

A. 50 seeds, because its 30 g radishes made a total of 1,500 g
B. 25 seeds, because its radishes were the largest, at 40 g each
C. 400 seeds, because its plot had the most plants and root mass
D. 100 seeds, because its plot had a total of 1,764 g of roots

Seedlings in the storm gap

Use paragraph 3 and Table 2 to answer the questions.

Table 2. Sugar Maple Seedling Plots at Pinecrest (3 years)
Place	Light (% of open sky)	Seedlings at start	Alive after 3 years	Growth (cm per year)
Under full canopy	3	40	6	2
Edge of the gap	20	38	17	9
Center of the gap	60	41	29	25

Use Table 2.

After 3 years, 23 more seedlings were alive in the center of the gap than under the full canopy.

Which resource best explains the differences in Table 2, and why?

A. Protection from deer, because only the plots in the gap were fenced
B. Light, because more light let seedlings make more food to grow and survive
C. Space, because the gap center started with far fewer seedlings than the canopy
D. Water, because the seedlings in the gap center got more rain than the others

Over the next 10 years, young trees in the gap will grow tall and shade the ground again. Which prediction for new maple seedlings in the gap center fits the data best?

A. They will grow more slowly, and fewer will survive, as the light at the ground falls.
B. They will grow faster than 25 cm per year, because older trees nearby protect them from storms.
C. They will grow more slowly, but more will survive, because shade keeps the soil cooler.
D. They will grow about 25 cm per year, because the soil there will stay just as moist.

10.

How are the crowded radish plots and the shaded maple plots alike?

A. In both, each plant got less of a resource it needed, so plants grew less and fewer survived.
B. In both, the plants ran short of water, so the plots with the most plants dried out first.
C. In both, the plants with less light grew taller, so they could reach the light above them.
D. In both, the numbers of plants alive at the end matched the numbers at the start.

More seeds, more radishes?

Write your answer in full sentences.

11.

A teammate tells Saanvi, “If you want more radishes by weight, just sow more seeds. The more seeds you sow, the bigger the harvest.” Do you agree? Write a claim, use evidence from Table 1, and explain how the supply of resources affects individual plants and the plant population in a plot. You may also use Table 2.

Claim: I disagree. Sowing more seeds raises the harvest only up to a point, because the plants in a plot share a limited supply of light, water and nutrients. Evidence: going from 25 to 50 seeds raised the total root mass from 1,000 g to 1,500 g, but going from 100 to 400 seeds, four times as many, raised it only from 1,764 g to 1,920 g. At the same time, the average radish shrank from 40 g to 6 g, and 80 of the 400 plants died. Reasoning: each 1 m² plot got the same light, water and compost, so when more plants shared it, each one got less and grew smaller, and some could not survive. The plot could only support about 1,900 g of radishes in all. The maple seedlings show the same idea with light: under the full canopy, with 3% light, only 6 of 40 seedlings survived and they grew 2 cm a year, while in the gap center, with 60% light, 29 of 41 survived and grew 25 cm a year.