Dendrochronology, the study of tree-ring records to glean information about the past, is possible because each year a tree adds...

GMAT Reading Comprehension : (RC) Questions

Source: Official Guide

Reading Comprehension

Bio Sciences

MEDIUM

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Notes

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Dendrochronology, the study of tree-ring records to glean information about the past, is possible because each year a tree adds a new layer of wood between the existing wood and the bark. In temperate and subpolar climates, cells added at the growing season's start are large and thin-walled, but later the new cells that develop are smaller and thick-walled; the growing season is followed by a period of dormancy. When a tree trunk is viewed in cross section, a boundary line is normally visible between the small-celled wood added at the end of the growing season in the previous year and the large-celled spring wood of the following year's growing season. The annual growth pattern appears as a series of larger and larger rings. In wet years rings are broad; during drought years they are narrow, since the trees grow less. Often, ring patterns of dead trees of different, but overlapping, ages can be correlated to provide an extended index of past climate conditions.

However, trees that grew in areas with a steady supply of groundwater show little variation in ring width from year to year; these "complacent" rings tell nothing about changes in climate. And trees in extremely dry regions may go a year or two without adding any rings, thereby introducing uncertainties into the count. Certain species sometimes add more than one ring in a single year, when growth halts temporarily and then starts again.

Ques. 1/3

In the highlighted text, "uncertainties" refers to

dendrochronologists' failure to consider the prevalence of erratic weather patterns

inconsistencies introduced because of changes in methodology

some tree species' tendency to deviate from the norm

the lack of detectable variation in trees with complacent rings

the lack of perfect correlation between the number of a tree's rings and its age

Solution

1. Passage Analysis:

Progressive Passage Analysis

Text from Passage	Analysis
Dendrochronology, the study of tree-ring records to glean information about the past, is possible because each year a tree adds a new layer of wood between the existing wood and the bark.	What it says: Scientists can learn about history by studying tree rings because trees grow a new ring each year. What it does: Introduces the main concept and explains the fundamental principle that makes it work Source/Type: Scientific fact/definition Connection to Previous Sentences: This is our starting point - no previous information to connect to Visualization: Think of a tree trunk like stacking plates - each year adds one more plate to the stack. If you cut across the stack, you can count the plates (rings) to know how old the tree is. What We Know So Far: Trees add one ring per year, scientists study these rings to learn about the past What We Don't Know Yet: How exactly do rings tell us about the past? What do rings look like?
In temperate and subpolar climates, cells added at the growing season's start are large and thin-walled, but later the new cells that develop are smaller and thick-walled; the growing season is followed by a period of dormancy.	What it says: During spring, trees make big cells with thin walls. Later in the growing season, they make smaller cells with thick walls. Then trees stop growing for a while. What it does: Explains the biological process behind ring formation Source/Type: Scientific fact about tree biology Connection to Previous Sentences: This builds on Sentence 1's claim that trees "add a new layer each year" by explaining HOW that layer forms - through different sized cells at different times. Visualization: Spring cells: ●●●● (big, thin circles) Later cells: •••• (small, thick circles) Winter: [pause] no growth Reading Strategy Insight: The author is answering our question from Sentence 1 - we wanted to know HOW rings form, and now we're getting the step-by-step process.
When a tree trunk is viewed in cross section, a boundary line is normally visible between the small-celled wood added at the end of the growing season in the previous year and the large-celled spring wood of the following year's growing season.	What it says: When you cut across a tree trunk, you can see a clear line between last year's small cells and this year's big spring cells. What it does: Restates and simplifies the previous sentence by showing us what we would actually SEE Source/Type: Scientific observation/fact Connection to Previous Sentences: This is NOT new complexity! Sentence 2 told us about big cells and small cells. NOW Sentence 3 is helping us by explaining that this creates visible boundary lines we can actually see. Visualization: What you see in a tree cross-section: Year 1: ●●●●•••• [clear line] Year 2: ●●●●•••• [clear line] Year 3: ●●●●•••• [clear line] Reading Strategy Insight: Feel relieved here - this is simplification, not new complexity. The author is translating the technical cell information into something visual and practical.
The annual growth pattern appears as a series of larger and larger rings.	What it says: Each year's growth looks like rings that get bigger as the tree gets older. What it does: Further simplifies and restates - gives us the "big picture" view Source/Type: Scientific observation Connection to Previous Sentences: This is pure restatement! Sentences 2-3 explained the complex biology. NOW the author gives us the simple takeaway: "it looks like rings that get bigger." Visualization: Tree rings from center outward: Ring 1 (age 1): small circle Ring 2 (age 2): medium circle around Ring 1 Ring 3 (age 3): large circle around Ring 2 Reading Strategy Insight: Major confidence booster! After three sentences of technical detail, the author gives us an incredibly simple summary. This is the pattern of RC passages - complex explanation followed by simple restatement.
In wet years rings are broad; during drought years they are narrow, since the trees grow less.	What it says: More water = bigger rings, less water = smaller rings, because trees grow more with more water. What it does: Explains how rings tell us about past climate conditions Source/Type: Scientific fact/relationship Connection to Previous Sentences: This builds on our foundation by answering the key question from Sentence 1: HOW do tree rings "glean information about the past"? The answer: ring width tells us about rainfall. Visualization: Wet year ring: ━━━━━━━━ (thick line) Normal year ring: ━━━━━━ (medium line) Drought year ring: ━━━ (thin line) What We Know So Far: Trees make rings each year, ring width depends on water availability, so ring width tells us about past weather
Often, ring patterns of dead trees of different, but overlapping, ages can be correlated to provide an extended index of past climate conditions.	What it says: Scientists can study multiple trees of different ages to learn about climate patterns going back even further in time. What it does: Shows how the basic principle can be extended for longer time periods Source/Type: Scientific methodology Connection to Previous Sentences: This builds on the wet/dry year concept by showing how scientists can study not just one tree's lifetime, but hundreds or thousands of years by combining multiple trees. Visualization: Tree A (died 2020): rings from 1900-2020 Tree B (died 1950): rings from 1800-1950 Tree C (died 1880): rings from 1700-1880 Combined: climate data from 1700-2020 Reading Strategy Insight: This extends rather than contradicts our understanding - it's showing the power of the basic wet/dry principle we just learned.
However, trees that grew in areas with a steady supply of groundwater show little variation in ring width from year to year; these "complacent" rings tell nothing about changes in climate.	What it says: Trees near steady water sources have similar-sized rings every year, so they don't give us useful climate information. What it does: Introduces the first limitation/exception to the basic principle Source/Type: Scientific limitation/caveat Connection to Previous Sentences: This contrasts with our wet/dry year principle. If a tree always has water available, then ring width won't reflect natural rainfall patterns. Visualization: Normal tree rings: thick-thin-medium-thin-thick (shows climate variation) "Complacent" tree rings: medium-medium-medium-medium (no climate info) Reading Strategy Insight: The author is being thorough by showing us when the method DOESN'T work, which actually reinforces our understanding of when it DOES work.
And trees in extremely dry regions may go a year or two without adding any rings, thereby introducing uncertainties into the count.	What it says: In very dry places, trees might skip growing rings for one or two years, which makes counting rings unreliable. What it does: Introduces a second limitation to dendrochronology Source/Type: Scientific limitation/caveat Connection to Previous Sentences: This is another exception, like the "complacent" rings. Both show situations where the basic "one ring per year" principle breaks down. Visualization: Normal tree: Ring-Ring-Ring-Ring (4 rings = 4 years) Extremely dry region tree: Ring-[no ring]-[no ring]-Ring (2 rings = 4 years → counting error) Reading Strategy Insight: We're seeing a pattern of limitations. The author is systematically showing us the boundaries of when dendrochronology works vs. when it doesn't.
Certain species sometimes add more than one ring in a single year, when growth halts temporarily and then starts again.	What it says: Some types of trees can grow multiple rings in one year if their growth stops and restarts. What it does: Introduces a third and final limitation to dendrochronology Source/Type: Scientific limitation/caveat Connection to Previous Sentences: This completes the pattern of limitations. We now have three ways the "one ring per year" rule can break down: 1) complacent rings (no climate info), 2) missing rings (undercounting), 3) multiple rings per year (overcounting). Visualization: Normal tree year: Ring (1 ring = 1 year) Problem tree year: Ring-Ring (2 rings = 1 year → counting error) Overall Pattern Recognition: The passage follows a classic RC structure: 1) Introduce concept, 2) Explain how it works, 3) Show its applications, 4) Acknowledge limitations. This is reinforcement, not random complexity!

2. Passage Summary:

Author's Purpose:

To explain how dendrochronology works and identify the conditions where this scientific method is reliable versus unreliable

Summary of Passage Structure:

In this passage, the author walks us through dendrochronology by explaining the science and then showing its limitations:

First, the author introduces dendrochronology and explains the basic biological process of how trees form rings each year
Next, the author shows how ring width reflects climate conditions, with wet years creating broad rings and dry years creating narrow rings
Then, the author demonstrates how scientists can extend this method by combining data from multiple trees to study longer time periods
Finally, the author systematically presents three major limitations that can make dendrochronology unreliable: trees with steady water supplies, trees that skip years without forming rings, and trees that form multiple rings in one year

Main Point:

While dendrochronology can provide valuable information about past climate conditions by analyzing tree ring patterns, scientists must be aware of specific situations where this method breaks down and produces unreliable results

3. Question Analysis:

The question asks us to identify what "uncertainties" refers to in the highlighted text about trees in extremely dry regions that "may go a year or two without adding any rings, thereby introducing uncertainties into the count."

Connecting to Our Passage Analysis:

From our passage analysis, we know that:

The fundamental principle of dendrochronology relies on trees adding "one ring per year" (from Sentence 1)
The passage systematically presents three limitations where this principle breaks down
The "uncertainties" sentence introduces the second limitation, specifically about trees that skip growing rings in extremely dry conditions
Our visualization showed: Normal tree (Ring-Ring-Ring-Ring = 4 rings = 4 years) vs. Problem tree (Ring-[no ring]-[no ring]-Ring = 2 rings = 4 years → counting error)

Prethinking:

The passage structure shows us that "uncertainties" refers to problems with the basic counting method of dendrochronology. When trees skip years without forming rings, scientists counting rings will get a number that doesn't match the tree's actual age. This creates uncertainty about the correlation between ring count and age - the fundamental assumption that makes dendrochronology possible.

Answer Choices Explained

dendrochronologists' failure to consider the prevalence of erratic weather patterns

Why It's Wrong:

The passage doesn't discuss dendrochronologists' methodology or their consideration of weather patterns
The "uncertainties" specifically refers to a biological phenomenon (trees not forming rings), not human analytical failure
This choice mislocates the source of the problem as human error rather than natural tree behavior

Common Student Mistakes:

Did I assume that scientific problems must come from human error rather than natural limitations?
→ Focus on what the passage actually describes as causing the uncertainties
Am I confusing the concept of "erratic weather patterns" with the specific dry conditions mentioned?
→ Note that the passage discusses "extremely dry regions," not erratic patterns

inconsistencies introduced because of changes in methodology

Why It's Wrong:

The passage makes no mention of changes in methodology or methodological inconsistencies
The "uncertainties" stem from natural tree behavior, not scientific methodology
This choice introduces concepts not present in the passage

Common Student Mistakes:

Am I bringing outside knowledge about scientific method issues instead of focusing on the passage?
→ Stick to what the passage explicitly states as the source of uncertainties
Did I confuse "uncertainties in the count" with "uncertainties in the method"?
→ The passage specifies the count is uncertain, not the methodology

some tree species' tendency to deviate from the norm

Why It's Wrong:

While the passage mentions species that "sometimes add more than one ring," this appears in a separate sentence after the "uncertainties" reference
The "uncertainties" specifically refers to the problem of missing rings, not multiple rings per year
This choice conflates two different limitations mentioned in the passage

Common Student Mistakes:

Did I combine information from different sentences without noting their distinct contexts?
→ The "uncertainties" sentence discusses missing rings; the species deviation sentence discusses extra rings
Am I assuming all the limitations mentioned create the same type of "uncertainty"?
→ Track which specific limitation the highlighted text refers to

the lack of detectable variation in trees with complacent rings

Why It's Wrong:

"Complacent" rings are discussed in a separate sentence before the "uncertainties" reference
Complacent rings don't create uncertainties in counting; they simply provide no climate information
The passage states complacent rings "tell nothing about changes in climate," not that they create counting uncertainties

Common Student Mistakes:

Am I confusing "lack of climate information" with "counting uncertainties"?
→ Complacent rings are still countable; they just don't reflect climate variation
Did I assume all limitations mentioned in the passage relate to the highlighted "uncertainties"?
→ Each limitation creates different types of problems for dendrochronology

the lack of perfect correlation between the number of a tree's rings and its age

Why It's Right:

The passage establishes that dendrochronology works because "each year a tree adds a new layer," creating a direct correlation between rings and age
When trees "go a year or two without adding any rings," this breaks the fundamental ring-count-equals-age relationship
The visualization from our analysis shows exactly this problem: 2 rings representing 4 years of age creates uncertainty about the tree's actual age

Key Evidence: "trees in extremely dry regions may go a year or two without adding any rings, thereby introducing uncertainties into the count"

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