Dendrochronology, the study of tree-ring records to glean information

Question

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.

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.   1 
 1. In the highlighted text, " uncertainties " refers to

(A) dendrochronologists' failure to consider the prevalence of erratic weather patterns
(B) inconsistencies introduced because of changes in methodology
(C) some tree species' tendency to deviate from the norm
(D) the lack of detectable variation in trees with complacent rings
(E) the lack of perfect correlation between the number of a tree's rings and its age

2 
 2. The passage suggests which of the following about the ring patterns of two trees that grew in the same area and that were of different, but overlapping, ages?

(A) The rings corresponding to the overlapping years would often exhibit similar patterns.
(B) The rings corresponding to the years in which only one of the trees was alive would not reliably indicate the climate conditions of those years.
(C) The rings corresponding to the overlapping years would exhibit similar patterns only if the trees were of the same species.
(D) The rings corresponding to the overlapping years could not be complacent rings.
(E) The rings corresponding to the overlapping years would provide a more reliable index of dry climate conditions than of wet conditions.

3 
 3. The passage is primarily concerned with

(A) evaluating the effect of climate on the growth of trees of different species
(B) questioning the validity of a method used to study tree-ring records
(C) explaining how climatic conditions can be deduced from tree-ring patterns
(D) outlining the relation between tree size and cell structure within the tree
(E) tracing the development of a scientific method of analyzing tree-ring patterns

GMATNinja · Accepted Answer

Question 3

The author describes the method in the first paragraph and then, in the second paragraph, explains a couple  limitations  of that method. The author does not imply that the method is invalid or that it should not be used. Rather, the author explains  how  climatic conditions can be deduced from tree-ring patterns and then describes conditions under which that method would not be reliable. In other word, the author believes that the method is valid but that it doesn't work under certain circumstances.

So the main purpose is to describe how (and under what conditions) the method can be used, not to question the validity of the method.

So choice (C) is more accurate than choice (B).

I hope this helps!

GMATNinja · Answer

For the first question, refer to the following portion: "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."

So if we look at the ring patterns of dead trees, we should be able to tell which years were dry and which years were wet based on the width of the rings. If we only have one tree, then we would obviously only be able to gather data for the years in which that tree was alive. But now imagine we have two trees. One of the trees died in 1960 at the age of 50, and the other was born in 1950 and lived for 60 years (a 10 year overlap). In that case, we would have ring data from 1910 - 2010 and could use that data to help figure out which years were wet and which were dry.

Now consider the 10-year overlap. Because we are talking about different trees, the actual widths of the rings from those years might not be exactly the same. However, during wet years, we would expect RELATIVELY broad rings. During dry years, we would expect relatively narrow rings. So even though the precise widths might not be exactly the same for both trees, the PATTERNS (i.e. narrow-narrow-broad-narrow-broad-broad-broad-etc) will likely be the same. Now the second paragraph does explain why this is not ALWAYS the case, but the passage suggests that the patterns will OFTEN exhibit similar patterns.

Hopefully that helps you arrive at the correct answer!

The highlighted portion specifically refers to trees that may go a year or two without adding rings. According to the first paragraph, it is often possible to determine a tree's age by counting the rings. Why? Because there is typically an annual boundary line at the end of the growing season. With a regular annual growth pattern, you can count the rings to determine the age.

But what if a tree goes a year or two without adding any rings? In that case, if you count the rings to determine the age, your number will be LESS than the tree's actual age. For example, if you count 40 rings, the tree could actually be 41, 42, 45, 50, or 60+ years old! We would have no way to determine the number of years in which rings were not added. This corresponds to choice (E).

As for choice (D), the highlighted portion refers to the uncertainty in ring count, not to the lack of variation in ring width in complacent trees. We are indeed told that trees with "complacent" rings show little variation in ring width from year to year. However, you should still be able to count rings to determine the AGE of complacent trees. You would not be able to tell which years were wet/dry, but you should still be able to determine the age.

Choice (E) is the best answer.

GMATNinja · Answer

Passage breakdown 
In the first paragraph (P1), the author  describes a method  to learn about past climatic conditions

Dendrochronology, or "the study of tree-ring records to glean information about the past."

In the second paragraph, the author  lists several limitations  of the method introduced in P1.

For more on the process of breaking down RC passages, check out  this article  and  our live RC videos .

Explanations for individual questions 
 
  Question 1  
  Question 2  
  Question 3

thevenus · Answer

In 2:55 minutes,
The rings are the main clues for understanding the age of a tree, the various shapes of the rings formed changes with the change of weather (water supplied to it, in different climatic condition).So, clearly it triggers the fact that the rings are the only link to find the age and if due to some reason the rings are  uncertain  to figure out by the researchers then it brings lack of correlation , i.e tree rings and its age.

jaiswalamrita · Answer

In the highlighted text, "uncertainties" refers to

(A) dendrochronologists' failure to consider the prevalence of erratic weather patterns
(B) inconsistencies introduced because of changes in methodology
(C) some tree species' tendency to deviate from the norm
(D) the lack of detectable variation in trees with complacent rings
(E) the lack of perfect correlation between the number of a tree's rings and its age

In opening line it says "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."------- means every year tree adds a new layer (ring)

In last but one line it says "And trees in extremely dry regions may go a year or two without adding any rings, thereby introducing uncertainties into the count." ------ means the regular pattern of "one ring per year" phenomenon may be not shown due to extremely dry weather.

So if scientists count the age of tree by counting number of rings and if the tree is having sometimes two rings in a year or no ring at all in any given year, this may lead to wrong age determination of the tree.

Hence E.

HTH

DIII · Answer

The passage is primarily concerned with

(A) evaluating the effect of climate on the growth of trees of different species
(B) questioning the validity of a method used to study tree-ring records
(C) explaining how climatic conditions can be deduced from tree-ring patterns
(D) outlining the relation between tree size and cell structure within the tree
(E) tracing the development of a scientific method of analyzing tree-ring patterns

Can someone explain? 
I don't see how C Is the main point.
First dendrochronology was introduced and then it was questioned in the second paragraph.
if you take C as the answer it is missing the most of the second paragraph.

Shivikaa · Answer

hi, 
I chose D for Q2. Please help why is it E? 
Also, I didn't understand Q1.

thanks in advance

Unknown_ · Answer

GMATNinja

For Question2:

Basis this line in the passage "  Certain species   sometimes add more than one ring in a single year, when growth halts temporarily and then starts again."

Shouldn't the answer be :
(C) The rings corresponding to the overlapping years would exhibit similar patterns only if the trees were of the  same species.

As (A) does not talk about any species?

zoezhuyan · Answer

hi  SajjadAhmad ,  u1983 ,  GMATNinja ,  GMATNinjaTwo ,  workout ,  Gnpth

what's the problem of the B in Q3?

IMOP, P1 states general patterns of rings under different environments, 
while, P2 says patterns are similar under certain environment, and under some other certain environments, pattern has "uncertain" pattern, question the accuracy of the age if count the rings. 
because I think Q2 questions the patterns, so I picked up B, 
I think C does not include what P2 says, so I cross off C
I saw B is the second ranking, suggesting not few people agree with B.

GMATNinja · Answer

To choose (C), we need to find support in the passage indicating that ring patterns between overlapping trees are ONLY similar if those trees are of the same species.

In other words, NO two trees of different species would exhibit similar patterns in the years in which they overlap.

As you point out, the passage mentions that "certain species" have different ring growth patterns. But does that mean that ring patterns are ONLY similar amongst trees of the same species?

Not necessarily. It is entirely possible that  some , or even  most , tree species exhibit similar patterns in the same year -- even if  certain  trees have different patterns. Because the passage doesn't indicate that ring patterns are similar ONLY if trees are of the same species, (C) is out.

I hope that helps!

mimishyu · Answer

Why (B) cannot be correct answer???
I have the same problem as zoezhuyan
 
we can see from the first word “however” in para2, doesn’t it indeed “ question” the validity of Dendrochronology,a method to study the tree-ring records for gleaning information about the past, in para1???

We can easily see from this passage that (C) correspond to para1’s main idea, and for (B), it correspond to para2

Or maybe we can think for another way, it is that—which one, para1 or 2, is this passage’s main focus???however, its not quite obvious in this whole passage for us to discern which one is the main focus
so in Q3, it just hard for us to choose between (B) and (C)

GMATNinja · Answer

When answering "main idea" questions on RC, looking for the answer choice that "covers what all the paragraphs said" will cause you endless frustration and often lead you to the wrong answer choice.

That's because the question is not, "Which answer choice seems to cover the content of every paragraph?"

Rather, the question is, "Why did the author write this passage in the first place?"

Again,  why  did the author write the passage?

The author wrote P1 in order to introduce the technique of dendrochronology and show how this method can reveal past climate conditions. 
  The author wrote P2 in order to explain a few scenarios where dendrochronology's results can be compromised. In other words, the author wrote P2 to qualify the statements made in P1, rounding out the overall explanation of how dendrochronology works.

The author wanted to explain how dendrochronology can be used to deduce climate history (including when it might not work).

Let's take a look at choice (B) one more time:

Did the author write this passage  in order to question the validity of a method ?

Nope. While reading P2 in isolation might lead us to choose (B), remember that  the reason the author wrote P2 in the first place  is to expand on P1. None of the language here signals a desire by the author to convince us that these exceptions should make us question the validity of dendrochronology overall.

The overarching purpose is to present a well-rounded explanation of how and when it works.  When it might not work  is presented  as part of that explanation , not a challenge to the method's validity.

That's why choice (C) is the best fit and choice (B) can be eliminated.

mSKR · Answer

Hi  VeritasKarishma   GMATNinja   bm2201

Can you please give your opinion on Q1: why not D

And trees in extremely dry regions may go a year or two without adding any rings, thereby introducing uncertainties into the count. 
What I understand:
Add no rings then it will be difficult to calculate the age.

If No rings then what happens ? Then we get wrong count of rings. It means lack of something ( ring) that could have been considered to determine the age.
so what is uncertainity to the count here? I think uncertainity count means somethign that was detectable ( such as ring)

So I choose D. lack of something visible that could have added in the count to compensate missing count of rings.

Please suggest  VeritasKarishma   GMATNinja   bm2201

KarishmaB · Answer

1. In the highlighted text, "uncertainties" refers to

(A) dendrochronologists' failure to consider the prevalence of erratic weather patterns
(B) inconsistencies introduced because of changes in methodology
(C) some tree species' tendency to deviate from the norm
(D) the lack of detectable variation in trees with complacent rings
(E) the lack of perfect correlation between the number of a tree's rings and its age

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.

Complacent rings appear in trees with steady supply of ground water. So climate has no impact on their rings.

The "uncertainties" mentioned refers to an inability to determine the exact age because in very dry regions (These trees do not have steady supply of ground water and hence have no complacent rings), trees may skip adding any rings in some years. Hence, it refers to the lack of perfect correlation between the tree's rings and its age.

mSKR · Answer

hello ma'm  VeritasKarishma

So here the count refers to this perfect correlation ?( perfect correlation between the tree's rings and its age)
uncertainty == lack

please confirm

KarishmaB · Answer

Yes. Since the trees add 1 ring every year, we can count the rings to get their age. But if they skip adding rings some years, the perfect correlation goes for a toss.

mSKR · Answer

Actually I don't understand much highlighted in red. what controls ? Who who ? Anyways, thanks for the definition.

Sneha2021 · Answer

GMATNinja   KarishmaB

1. In the highlighted text, "uncertainties" refers to
 
(C) some tree species' tendency to deviate from the norm
why c is incorrect?

And trees in extremely dry regions may go a year or two without adding any rings, thereby introducing uncertainties into the count.

If the trees go without adding any rings, they are deviating from the normal behaviour (norm), so it should explain uncertainty.

KarishmaB · Answer

Question 1.

1. In the highlighted text, "uncertainties" refers to

(A) dendrochronologists' failure to consider the prevalence of erratic weather patterns
(B) inconsistencies introduced because of changes in methodology
(C) some tree species' tendency to deviate from the norm
(D) the lack of detectable variation in trees with complacent rings
(E) the lack of perfect correlation between the number of a tree's rings and its age

Paragraph 1 explains the norm. Paragra And trees in extremely dry regions may go a year or two without adding any rings, thereby introducing uncertainties into the count.

The author mentions the word 'uncertainties' to talk about how trees in dry region may not add any rings in some years causing uncertainty in the count. So there are 100 rings but does it mean that tree was 100 years old when it died? What if in some years it did not add any rings and we have not accounted for those in our count of 100? This is what 'uncertainties' refers to.

Hence answer is (E).

Why is it not (C)?
We are given that "Certain species sometimes add more than one ring in a single year, when growth halts temporarily and then starts again" which is another deviation but the word 'uncertainties' is not used to talk about this. We need to give the exact function of the word.

Dendrochronology, the study of tree-ring records to glean information

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Dendrochronology, the study of tree-ring records to glean information

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