How do scientists measure trees? What is DBH?

Transcription

1 Hw d scientists measure trees? What is DBH? Purpse Students develp an understanding f tree size and hw scientists measure trees. Students bserve and measure tree ckies and explre the relatinship between tree circumference and diameter. Students cmpare the estimates f diameter made frm circumference measurements (and vice versa). Overview In this activity students will bserve and explre the relatinship between circumference and diameter using tree ckies (crss sectins frm real trees). Cntent Questin Hw d scientists measure trees? Sub-questins What is the relatinship between circumference and diameter? What is diameter-at-breast-height? Student Outcmes Students will bserve the physical characteristics f and suggest ways t measure tree ckies (i.e., crss sectins f real trees). Students will wrk with a partner t measure the circumference and diameter f ne tree ckie. Students will wrk with a partner t calculate circumference r diameter f the tree ckie based n the circle equatin. Students will cmpare and cntrast the calculated circumference and diameter values t the actual measurements f these parameters. Science Cncepts 5-8 & 9-12 Change, cnstancy, measurement Abilities necessary t d scientific inquiry Time minutes Level Middle & High Schl Materials and Tls Several different sized tree ckies (1 per student pair) Flexible measuring tape (metric) (1 per student pair) Calculatr (1 per student pair) Ntebk and pencil (1 per student) Items fr circumference height tl [Optinal] Sticks and permanent marker OR Strings and scissrs (see part 2) Preparatin Write the main essential questin n the bard. Gather all materials fr the activity. Teacher Guide 1 f 7 Hw d scientists measure trees?

2 Pre- requisites Skill: Calculating average (mean) numbers Backgrund Scientists use a standard methd t measure the size f trees, diameter-at-breast height (DBH), t ensure cnsistency ver time, acrss plts and between data cllectrs. DBH means the diameter f each tree is measured at breast height, defined as 1.35m up frm the highest pint f grund at the tree s base (See the Tree Circumference Guide fr sme pictrial examples). DBH measurements can be used t estimate the vlume, bimass, and carbn strage f trees - t learn mre abut the relatinship between DBH and bimass refer t the activity, Understanding Allmetry. Keep in mind that circumference and DBH are the first tw steps in the prcess f understanding bimass and carbn strage in lcal ecsystems. Frm gemetry class, we knw that diameter is a line that passes thrugh the center f a circle, with the endpints f the line lcated n the edge f the circle. Hw then can fresters and scientists measure tree diameter withut cutting dwn the tree and measuring its crss sectin? Scientists measure the circumference f a tree and calculate the diameter using equatin 1 shwn belw. Scientists smetimes use tape measures that are calibrated r adjusted fr diameter based n this equatin. These tapes are referred t as DBH tapes. During the GLOBE Carbn Cycle field data cllectin, hwever, students will measure tree circumference rather than diameter (due t tl restrictins); therefre it is imprtant fr them t knw hw circumference and diameter are related. Circumference = π*diameter (where π = 3.14) r Diameter = Circumference/ π [equatin 1] Field Cncepts Diagram Teacher Guide 2 f 7 Hw d scientists measure trees?

3 What T D and Hw T D It ENGAGE Student Gruping: Partners (Part 1) Time: 5 minutes In grups f tw, students select a tree ckie and cmplete Part 1a f the Student Directins. Class discussin f student s bservatins and suggestins fr tree ckie measurement. Intrduce the essential questin, equatin 1, and set the expectatins fr Part 2. Demnstrate t measure the circumference and diameter f a tree ckie. Remind students hw t calculate an average if necessary. EXPLORE EXPLAIN ELABORATE/ INVESTIGATE EVALUATE/ WRAP- UP Student Gruping: Partners (Part 2) Time: 20 minutes Students cnduct measurements and calculatins as directed n the student sheet. This is a prf f cncept exercise t shw students hw diameter and circumference are related. Student Gruping: Whle Class Time: 15 minutes Interpret the measurements and discuss answers student questins #4a- c. Use questin 4c as a lead in t the next part f the activity: Hw might scientists (and yu) use the circumference/diameter relatinship t study live trees? Discuss the standard height at which circumference (diameter) is measured, 1.35m, called diameter- at- breast- height (DBH). This is a gd pint t have a discussin abut accuracy and precisin. Yu may want t brainstrm sme ideas abut what thse terms mean befre students read abut them in Student Directins: Part 3. Student Gruping: Partners (Part 3) Time: 30 minutes Students determine the height f 1.35m against their wn bdy. Students fllw teacher directins t create a height measurement tl. [Optinal] Use a measuring tape r meter stick, measure ut 1.35m: n a stick and mark with a permanent marker OR n a string and cut the string s it is exactly 1.35m tall Students perfrm activities t investigate accuracy and precisin. Student Gruping: Whle Class Time: 10 minutes Intrduce the Field Cncepts Diagram and highlight the first step, which this activity supprts. **See activity example in HwTMeasureTrees_example.xls Assessment Students shuld answer the essential questin individually and explain the cnnectin between this activity and the upcming field wrk. Resurces Teacher Guide 3 f 7 Hw d scientists measure trees?

4 TEACHER ANSWERS Student Wrksheet: Hw d scientists measure trees? What is DBH? Part 1: Tree Ckie Observatins 1) Recrd the physical traits f yur selected tree ckie. Draw and/r describe. Cylindrical like, tp and bttm have rings f differing clrs and widths, bark is the uter mst layer, the bark has sme bumps/ridges 2) Suggest sme ways that yur tree ckie culd be measured t learn abut its size. Cunt the number f rings Measure circumference Measure diameter Measure height Weight the ckie Displace water t find density Essential Questin: Hw d scientists measure trees? Part 2: Circumference versus Diameter 1) Measure 1 tree ckie per grup. a) Pull the measuring tape tightly arund the tree ckie and recrd its circumference in the data table belw. Sharing measurement respnsibilities between grup members, repeat the circumference measurement 2 mre times n the same tree ckie. b) Measure the tree ckie s diameter and recrd in the data table. Repeat the diameter measurement 2 mre times. Nte: Measure the diameter in several directins acrss the surface f the tree ckie, as they will nt be perfectly rund and yu want t make sure yur measurements represent the verall shape f the tree ckie. Circumference Trial Trial Trial Diameter Average (1+2+3)/ ) Calculate an average (mean) circumference and average (mean) diameter fr the tree ckie. Average (mean) = Sum all f the values by the # f values. [Equatin 1] Teacher Guide 4 f 7 Hw d scientists measure trees?

5 3) Use equatins 2 & 3 and the apprpriate calculated averages (f circumference and diameter) t find a calculated circumference and diameter. Calculated Circumference = π*average diameter (where π = 3.14) [Equatin 2] C = 3.14*7.8 Calculated Diameter = average circumference/ π [Equatin 3] D = 25.3/3.14 Calculated (using equatin 2 & 3) Circumference Diameter 4) Cmpare calculatins t averages by answering the fllwing questins: a) Hw similar are yur measured circumference values? Circumference values shuld be pretty clse, as lng as all students are practicing gd measurement technique, which they may nt be. Measurement accuracy and precisin are discussed at greater depth in Part 3, s here just get students t examine the measurements they have made. b) Hw similar are yur measured diameter values? Diameter measurements will vary mre widely especially if yu have dd shaped tree ckies. Students will prbably decide t measure diameter f the tree ckie n several different axes s the mre dd shaped the ckie the mre different the measurements culd be. c) Why might calculated values be different frm measured average values? Students will prbably make nte f the challenges they encuntered when chsing hw t measure diameter. Sme may bserve that in rder t get a better average value f diameter yu wuld want t measure the ckie n mre than 3 axes, perhaps 10 r 20 measurements wuld get an average that wuld be clser t the calculated value. This wuld be the case because the calculated value basically assumes that the tree ckie surface is a perfect circle and des nt accunt fr bumps, ridges, grwths, etc. that may exist n the tree ckie. Students may als suggest measurement errr as a surce f difference if yu made a number f errrs measuring either circumference r diameter yur measured average will nt be the same as the calculated value. d) Hw might scientists (and yu) use the circumference/diameter relatinship t study live trees? Teacher Guide 5 f 7 Hw d scientists measure trees?

6 Yu shuld explain t students that when scientists/fresters want t knw abut the mass f trees, hw much carbn they stre, hw many bard feet they cntain r hw much wd prduct exists in a whle frest they must knw at least ne aspect, tree diameter at 1.35meters. Why tree diameter? The primary reasn is pretty simple; it is easier fr peple t cnceptualize a tree with a diameter f 8cm than ne that has a circumference f 24.5cm. Over the years there have been many methds develped t measure tree diameter, such as the Biltmre stick and tree calipers. The mst cmmnly used methd tday, especially fr highly accurate measurements that will be used in scientific studies, is the diameter tape. This is a calibrated tape where every 1inch marked n the tape is actually a distance f 3.14inches. This means that althugh yu are measuring circumference yu are reading the diameter. 5) Discuss Part 2 as a class. Part 3: Prepare t measure tree circumference in the field. 1) Hw high is 1.35m? a) Measure 1.35m frm the grund and determine where this falls n yur bdy (nse/neck/shulder/etc) as a basic reference. This reference pint will be used when making field measurements. By standing near a tree, and using this reference pint, yu will nt have t measure frm the grund at each tree t determine the 1.35m height. b) Fllw yu re yur teacher s instructins fr hw t create a height measurement tl. 2) Measuring trees greater than 15 centimeters circumference. a) When measuring trees in the field yu will nly recrd infrmatin fr trees greater than r equal t 15cm circumference. While this value may seem arbitrary it is equal t 2 inches diameter, a cutff used by scientists in many ther frest inventry prgrams. A cutff value is used fr several reasns: i) It is nt practical t measure every small sapling n a sample site, ii) In a frest, saplings d nt cntribute a significant amunt f bimass, iii) Many saplings smaller than 15cm die ff due t lack f available light. In sme frest inventries saplings are cunted separately n a sub- plt, hwever, since we are nly trying t assess basic carbn strage we will nt include saplings in ur study. 3) Tw key cncepts scientists must cnsider when they measure trees in the field, are accuracy and precisin. a) Accuracy: Accuracy is the degree t which a measured r calculated value matches the true value. In the case f circumference measurements this can be influenced by: i) Placement f the measuring tape: Is the measurement 1.35m frm the grund? Was the tape perpendicular t the main axis f the tree? Was the tape twisted? Teacher Guide 6 f 7 Hw d scientists measure trees?

7 ii) Reading and recrding data: Was the crrect number read frm the tape? Was this number crrectly entered n the datasheet? By clsely fllwing the rules in the Tree Circumference Guide, and carefully recrding data, ne will be able t make accurate circumference measurements. Accuracy becmes particularly imprtant if trees are measured in future years and cmpared t previus measurements. b) Precisin: Precisin is the degree t which repeated measurements f the same tree are in agreement. Yu can determine hw precise circumference can be measured by making repeated measurements f the same tree either by ne persn, r by several peple. i) With yur grup, re- examine yur tree ckie circumference results frm Part 2. Hw precise were yur measurements? Example: Our measurements were relatively precise. All measurements were taken t the 10ths place and they were separated by nly ne tenth. ii) Why might they be different? Hw tight did yu pull the tape t read the measurement? What decimal place did they recrd t? Was the measuring tape flat r twisted as it was pulled arund ckie? Hw many measurements d yu think shuld be made in rder t knw if yu have a precise measurement? Why? Example: I think that ur measurements were s similar because we pull the tape arund the tree ckie using the same tightness all 3 times. We als all agreed ahead f time that we wanted t recrd the circumference t the nearest tenth f a centimeter. I think there shuld be at least three trials cmpleted fr each measurement type. It is best t use mre than tw trials because if the results were different we wuld nt knw which ne is mre accurate. c) Make a list f at least 4 things yu can d in the field t make sure all f yur measurements are made accurately and precisely? Fllw the Tree Circumference Guide fr marking 1.35m, circumference at breast height, n each tree. Re-mark any trees where the 1.35m line is fading t ensure the measurement is made at the exact same height each year. Make all circumference measurements t the nearest tenth f a centimeter. Wrk as a team t make sure the measuring tape is hrizntal arund the tree (nt lwer in the back where it is hard t see) Make sure there are n twists in the tape. Practice making a few measurements befre beginning t recrd the data t make sure everyne agrees hw tightly t pull the tape and knws hw t read the measurement ff f the measuring tape. Have the recrder repeat the measurement back t the measurer t ensure it is recrded crrectly. If yu have any dubt that a measurement yu just made was nt executed well, make the measurement again befre reprting it t the recrder. Teacher Guide 7 f 7 Hw d scientists measure trees?

8 Name: Date: Student Wrksheet: Hw d scientists measure trees? What is DBH? Part 1: Tree Ckie Observatins 1) Recrd the physical traits f yur selected tree ckie. Draw and/r describe. 2) Suggest sme ways that yur tree ckie culd be measured t learn abut its size. Student Wrksheet 1 f 5 Hw d scientists measure trees?

9 Essential Questin: Hw d scientists measure trees? Part 2: Circumference versus Diameter 1) Measure 1 tree ckie per grup. a) Pull the measuring tape tightly arund the tree ckie and recrd its circumference in the data table belw. Sharing measurement respnsibilities between grup members, repeat the circumference measurement 2 mre times n the same tree ckie. b) Measure the tree ckie s diameter and recrd in the data table. Repeat the diameter measurement 2 mre times. Nte: Measure the diameter in several directins acrss the surface f the tree ckie, as they will nt be perfectly rund and yu want t make sure yur measurements represent the verall shape f the tree ckie. Trial 1 Trial 2 Trial 3 Average (1+2+3)/3 Circumference Diameter 2) Calculate an average (mean) circumference and average (mean) diameter fr the tree ckie. Average (mean) = Sum all f the values by the # f values. [Equatin 1] 3) Use equatins 2 & 3 and the apprpriate calculated averages (f circumference and diameter) t find a calculated circumference and diameter. Calculated Circumference = π*average diameter (where π = 3.14) [Equatin 2] Calculated Diameter = average circumference/ π [Equatin 3] Calculated (using equatin 1) Circumference Diameter Student Wrksheet 2 f 5 Hw d scientists measure trees?

10 4) Cmpare calculatins t averages by answering the fllwing questins: a) Hw similar are yur measured circumference values? b) Hw similar are yur measured diameter values? c) Why might calculated values be different frm measured average values? d) Hw might scientists (and yu) use the circumference/diameter relatinship t study live trees? 5) Discuss Part 2 as a class. Student Wrksheet 3 f 5 Hw d scientists measure trees?

11 Part 3: Prepare t measure tree circumference in the field. 1) Hw high is 1.35m? a) Measure 1.35m frm the grund and determine where this falls n yur bdy (nse/neck/shulder/etc) as a basic reference. This reference pint will be used when making field measurements. By standing near a tree, and using this reference pint, yu will nt have t measure frm the grund at each tree t determine the 1.35m height. b) Fllw yu re yur teacher s instructins fr hw t create a height measurement tl. 2) Measuring trees greater than 15 centimeters circumference. a) When measuring trees in the field yu will nly recrd infrmatin fr trees greater than r equal t 15cm circumference. While this value may seem arbitrary it is equal t 2 inches diameter, a cutff used by scientists in many ther frest inventry prgrams. A cutff value is used fr several reasns: i) It is nt practical t measure every small sapling n a sample site, ii) In a frest, saplings d nt cntribute a significant amunt f bimass, iii) Many saplings smaller than 15cm die ff due t lack f available light. In sme frest inventries saplings are cunted separately n a sub- plt, hwever, since we are nly trying t assess basic carbn strage we will nt include saplings in ur study. 3) Tw key cncepts scientists must cnsider when they measure trees in the field, are accuracy and precisin. a) Accuracy: Accuracy is the degree t which a measured r calculated value matches the true value. In the case f circumference measurements this can be influenced by: i) Placement f the measuring tape: Is the measurement 1.35m frm the grund? Was the tape perpendicular t the main axis f the tree? Was the tape twisted? ii) Reading and recrding data: Was the crrect number read frm the tape? Was this number crrectly entered n the datasheet? By clsely fllwing the rules in the Tree Circumference Guide, and carefully recrding data, ne will be able t make accurate circumference measurements. Accuracy becmes particularly imprtant if trees are measured in future years and cmpared t previus measurements. b) Precisin: Precisin is the degree t which repeated measurements f the same tree are in agreement. Yu can determine hw precise circumference can be measured by making repeated measurements f the same tree either by ne persn, r by several peple. i) With yur grup, re- examine yur tree ckie circumference results frm Part 2. Hw precise were yur measurements? Student Wrksheet 4 f 5 Hw d scientists measure trees?

12 ii) Why might they be different? Hw tight did yu pull the tape t read the measurement? What decimal place did they recrd t? Was the measuring tape flat r twisted as it was pulled arund ckie? Hw many measurements d yu think shuld be made in rder t knw if yu have a precise measurement? Why? c) Make a list f at least 4 things yu can d in the field t make sure all f yur measurements are made accurately and precisely? Student Wrksheet 5 f 5 Hw d scientists measure trees?

13 Tree Circumference Guide Tree Circumference Guide 1 f 1 Hw t Measure Trees