Sap, Leaves and a Shrinking Stream

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Sap, Leaves and a Shrinking Stream

Follow water through a forest to see what sunlight and gravity do.

Rain on the leaves

Read the passage and Table 1. Then answer questions 1–3.

A Week at Birch Hollow

1Each July, Birch Hollow Research Forest in the Appalachian Mountains hosts a science camp for middle school students. The forest and its numbers are made up, but they are based on real research forests. This year, camper Leilani Brandt spent a week with Dr. Tobias Arguello, a hydrologist, which is a scientist who studies how water moves over and under the land. The forest fills one small valley, and a single stream, Hollow Run, drains it. At the mouth of the valley, a gauge measures how many liters of water flow out each second.

2The first stop was a pair of rain gauges. One stands in an open clearing, and the other stands under the oak and maple trees. A plastic collar wrapped around several tree trunks catches the water that runs down the bark, and Dr. Arguello converts it into millimeters over the forest floor. After a July storm, Leilani noticed that the leaves above her were still dripping, and by the next afternoon they were dry.

3Next, Dr. Arguello showed Leilani the sap flow sensors on a large tulip tree. Trees pull water from the soil into their roots and up through their trunks to their leaves. Most of that water leaves the tree as water vapor through tiny openings in the leaves, a process called transpiration. The sensors measure how many liters of water move up the trunk each day.

4Last, the group visited the stream gauge. Between storms, Hollow Run is fed by groundwater that seeps slowly out of the hillside soil into the streambed. During a dry week in July, the flow rose and fell each day even though no rain fell. Dr. Arguello said that in March, before the trees had leaves, the gauge showed almost the same flow all day and all night.

Table 1. One July Storm
Where measured	Water (mm)
Clearing gauge (all the rain)	40
Gauge under the trees	32
Water down the trunks	2

1.

Use Table 1. Count the water down the trunks as water that reached the forest floor.

Of the 40 mm of rain, mm reached the forest floor. The other mm stayed on the leaves.

2.

What percent of the storm's rain stayed on the leaves and never reached the forest floor? %

3.

Use paragraph 2. What most likely happened to the water that stayed on the leaves?

A. Energy from sunlight warmed the wet leaves, and the water evaporated.B. It sank through the leaves and joined the groundwater that night.C. It stayed on the leaves as drops until the next storm washed it off.D. It soaked into the leaves and was stored in the wood until the fall.

Water through a tree

Use paragraph 3 and Table 2 to answer questions 4–6.

Table 2. Water Moving Up the Tulip Tree Each Day
Day	Weather	Sap flow (L)
Monday	Sunny, 28 °C	150
Tuesday	Sunny, 30 °C	160
Wednesday	Cloudy, 22 °C	60
Thursday	Rain all day, 19 °C	20
Friday	Partly sunny, 26 °C	110

4.

Do not round. Monday's sap flow was times Wednesday's sap flow.

5.

Which explanation fits Table 2 best?

A. On cloudy days, gravity pulls harder on the water, so it stays down in the roots.B. Cool air makes leaves give off more water vapor, so the tree pulls up more water.C. The rain on Thursday filled the tree, so it needed less water on the next day.D. On sunny days, more energy reaches the leaves, so more water evaporates from them.

6.

Underline each step at Birch Hollow in which water moves mainly because of gravity.

A stream that rises and falls

Use paragraph 4 and the graph. The graph shows one dry day in July, with no rain. Then answer questions 7–12.

7.

From 6 a.m. to 6 p.m., the flow of Hollow Run dropped by L/s.

8.

Which explanation of the daily drop in flow fits the passage, Table 2 and the graph best?

A. Warm afternoon air condensed into dew on the stream, which slowed the water down.B. In the sunny afternoon, trees pulled up more soil water, so less seeped into the stream.C. Gravity is weaker in the afternoon, so less groundwater flows downhill into the stream.D. Afternoon rain showers soaked into the soil instead of running into the stream.

9.

A camper thinks the daily drop happens because sunlight evaporates water straight off the stream's surface. Which evidence best shows that the trees cause most of the drop?

A. In March, before the trees had leaves, the flow barely changed all day.B. No rain fell on the forest during the dry week in July.C. The flow was lowest at 6 p.m., several hours after noon.D. The stream gauge sits at the mouth of the valley, below the trees.

10.

Which order shows a path that some water at Birch Hollow could take?

A. Rain falls → water soaks into the soil → water moves up the trunk → roots take it up → water vapor leaves a leaf → vapor cools into a cloudB. Rain falls → roots take it up → water soaks into the soil → water moves up the trunk → water vapor leaves a leaf → vapor cools into a cloudC. Rain falls → water soaks into the soil → roots take it up → water moves up the trunk → water vapor leaves a leaf → vapor cools into a cloudD. Rain falls → water vapor leaves a leaf → water soaks into the soil → roots take it up → water moves up the trunk → vapor cools into a cloud

11.

Suppose the trees in a nearby valley of the same size were cut down, and only short grass grows there now. The two valleys get the same rain. Compared with Hollow Run, what would that valley's stream most likely do in July?

A. Carry less water, because without shade the soil would soak up much less rainB. Carry more water, because less soil water would go back into the air as vaporC. Carry about the same water, because the valleys get the same amount of rainD. Carry less water, because grass catches more rain on its blades than trees do

12.

A camper says, "Trees take water out of the water cycle. The water they drink is used up and gone." Do you agree? Write a claim, give evidence from Table 2, the graph and the passage, and explain your reasoning. Name the roles of sunlight and gravity in your answer.