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The H for DBH
Forest Ecology and Management 129 (2000) 89±91
The H for DBH Nicholas Brokawa,*, Jill Thompsonb a Manomet Center for Conservation Sciences, P.O. Box 1770, Manomet, MA 02345, USA Institute for Tropical Ecosystem Studies, University of Puerto Rico, P.O. Box 363682, San Juan, PR 00936, USA
b
Received 1 January 1999; accepted 13 April 1999 Keywords: Basal area; DBH; Diameter at breast height; Diameter-class distribution; Tree biomass
A common way to measure trees is to record `diameter at breast height', or DBH. DBH is then used to calculate tree growth, basal area, and biomass. Thus, the accuracy of various quantities used in forest ecology depends on accurate and consistent measurement of DBH. Because tree stems taper, `breast height' (BH), the height above ground where diameter is measured, should in¯uence DBH recorded. It thus seems clear that BH must be speci®ed and consistent if researchers wish to compare data between study sites and between study periods at one site. We surveyed the literature to determine how frequently the value of BH is given and if BH is standard. We also measured trees at two BHs to see how much effect a difference in BH has on DBH and on the derived values of basal area and tree biomass. In our literature survey we recorded if a value for BH was stated and, if so, what value was given, in all studies using DBH and published over the last 10 years (1988±1997) in ®ve journals. These included three devoted to general ecology: Biotropica, Ecology, and Journal of Tropical Ecology; and two devoted to forest
*Corresponding author. Tel.: +1-508-224-6521; fax: +1-508224-9220. E-mail address: [email protected] (N. Brokaw).
ecology and forestry: Forest Science and Forest Ecology and Management. For our analysis we only included studies in which DBH was a variable that could have in¯uenced a study's results and conclusions, as in studies of tree growth, diameter-class distribution, or biomass. We did not, for example, include studies where the number of trees 10 cm DBH was given as a site descriptor for work on seedlings, or phenology studies in which DBH was used to de®ne focal trees. Nor did we include reviews of tree data from various sources. In the literature of the past 10 years surveyed by us, more than half (58%) of the appropriate papers did not report BH used (Table 1), and in the last year, 1997, 59% did not. Of those that did report BH used, values ranged from 120 to 160 cm, while 130 cm was the mode (62% of reported values). Using a BH of 130 cm was customary in continental Europe (Robbie, 1955), whereas the seemingly odd value of 137 cm is an approximation of 4.5 feet, the usual BH where English units were employed (Grubb et al., 1963). There were some minor differences among the ®ve journals; for example, papers in forestry journals report BH 137 cm relatively more often. However, in each of the journals more than 50% of relevant papers did not report BH. In our ®eld survey we measured diameters at both BH 130 and BH 140 cm on the same trees in a
0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 1 4 1 - 3
90
N. Brokaw, J. Thompson / Forest Ecology and Management 129 (2000) 89±91
Table 1 The distribution of values stated for `breast height' (BH), as used for measurement of `diameter at breast height' (DBH), in papers published over the period 1988±1997 in selected journals (see text). Most commonly, papers did not state BH. Among values stated, BH 130 cm was the mode. Breast height stated (cm) Journal type
120
135
137
140
150
160
None
Total
General ecology Forest ecology
1 ±
130 40 73
± 2
4 24
12 15
7 3
1 ±
80 178
145 295
Total
1
113
2
28
27
10
1
258
440
subtropical wet forest at El Verde, Puerto Rico (188200 N 658490 W), as part of research on tree allometry. The study trees were the 100 we have measured so far in a sample strati®ed to include both a random sample among all diameters and an adequate sample in each of ®ve diameter classes (10±20, 20±30, 30±40, 40±50, >50 cm DBH; BH 130 cm) of 12 common species. The study trees ranged from 10.4 to 85.6 cm DBH and had boles that were free of vines, buttresses, or major stem irregularities at BH. The mean difference between diameters measured at both BH 130 and BH 140 cm was 3.5 mm (SD 5.8, n 100). Seventy-three of the trees had a larger diameter at BH 130 than at BH 140 cm; 16 were smaller at BH 130; and 11 had the same diameter at both BHs. Total basal area for the 100 trees was 2394 cm2 higher (2.6%) and biomass was 3.61 metric tons higher (4.0%) when calculated using the diameter at BH 130 than at BH 140 cm (biomass: ln(dry weight) ÿ1.966 1.242ln(DBH2) (Overman et al., 1994). Our study showed that in the journals we surveyed most researchers publishing papers using DBH did not give the value of BH they used. And we cannot assume they used some well-accepted standard value, since there was much variation among the BH that was reported. Furthermore, the BH used can affect the diameter recorded and therefore the derived values of basal area and biomass. The effect on diameter recorded will differ among tree species, depending on characteristic trunk form and growth stage. The effect on derived values will differ among forests according to the density of large trees, because basal area and biomass are exponential functions of diameter. Regardless, our results indicate that failing to
report the value of BH, or using different values of BH, may lead to erroneous comparisons of diameter-class distributions, growth, basal area, or biomass. Errors concerning biomass can mislead us about such important phenomena as global carbon storage in forests. To avoid erroneous comparisons among sites and to ensure consistency within longitudinal studies, we urge forest researchers to report the BH used. Because BH 130 cm was the most commonly reported value, we suggest using BH 130 cm when initiating studies (Philip, 1994; Condit, 1998). Lastly, since `DBH' is ambiguous, we propose abandoning that term and instead using `D' and a subscript to indicate diameter measured at a speci®c height. Thus `D130' would indicate tree diameter at 130 cm. `DBH' may still be needed as a generic term. Acknowledgements We thank Dawn Gabree for compiling part of the data, Elizabeth Mallory for suggesting the title, and Franc,ois Houllier for suggesting the use of `D130'. Staff in the library at the Arnold Arboretum of Harvard University and the Biological Sciences Library at the University of New Hampshire helped us with the literature survey. This research was supported by grant BSR-8811764 from the National Science Foundation to the Institute for Tropical Ecosystem Studies, University of Puerto Rico, and to the International Institute of Tropical Forestry, as part of the Long-Term Ecological Research Program in the Luquillo Experimental Forest. The US Forest Service (Department of Agriculture) and the University of Puerto Rico provided additional support.
N. Brokaw, J. Thompson / Forest Ecology and Management 129 (2000) 89±91
References Condit, R., 1998. Tropical Forest Census Plots: Methods and Results from Barro Colorado Island, Panama and a Comparison with Other Plots. Springer, Berlin. Grubb, P.J., Lloyd, J.R., Pennington, T.D., Whitmore, T.C., 1963. A comparison of montane and lowland forests in Ecuador I. The forest structure, physiognomy, and floristics. J. Ecol. 51, 567±601.
91
Overman, J.P.M., Witte, H.J.L., Saldarriaga, J.G., 1994. Evaluation of regression models for above-ground biomass determination in Amazon rainforest. J. Trop. Ecol. 10, 207±218. Philip, M.S., 1994. Measuring Trees and Forests, 2nd ed. CAB International, Wallingford, UK. Robbie, T.A., 1955. Teach Yourself Forestry. English Universities Press, London.
The H for DBH Nicholas Brokawa,*, Jill Thompsonb a Manomet Center for Conservation Sciences, P.O. Box 1770, Manomet, MA 02345, USA Institute for Tropical Ecosystem Studies, University of Puerto Rico, P.O. Box 363682, San Juan, PR 00936, USA
b
Received 1 January 1999; accepted 13 April 1999 Keywords: Basal area; DBH; Diameter at breast height; Diameter-class distribution; Tree biomass
A common way to measure trees is to record `diameter at breast height', or DBH. DBH is then used to calculate tree growth, basal area, and biomass. Thus, the accuracy of various quantities used in forest ecology depends on accurate and consistent measurement of DBH. Because tree stems taper, `breast height' (BH), the height above ground where diameter is measured, should in¯uence DBH recorded. It thus seems clear that BH must be speci®ed and consistent if researchers wish to compare data between study sites and between study periods at one site. We surveyed the literature to determine how frequently the value of BH is given and if BH is standard. We also measured trees at two BHs to see how much effect a difference in BH has on DBH and on the derived values of basal area and tree biomass. In our literature survey we recorded if a value for BH was stated and, if so, what value was given, in all studies using DBH and published over the last 10 years (1988±1997) in ®ve journals. These included three devoted to general ecology: Biotropica, Ecology, and Journal of Tropical Ecology; and two devoted to forest
*Corresponding author. Tel.: +1-508-224-6521; fax: +1-508224-9220. E-mail address: [email protected] (N. Brokaw).
ecology and forestry: Forest Science and Forest Ecology and Management. For our analysis we only included studies in which DBH was a variable that could have in¯uenced a study's results and conclusions, as in studies of tree growth, diameter-class distribution, or biomass. We did not, for example, include studies where the number of trees 10 cm DBH was given as a site descriptor for work on seedlings, or phenology studies in which DBH was used to de®ne focal trees. Nor did we include reviews of tree data from various sources. In the literature of the past 10 years surveyed by us, more than half (58%) of the appropriate papers did not report BH used (Table 1), and in the last year, 1997, 59% did not. Of those that did report BH used, values ranged from 120 to 160 cm, while 130 cm was the mode (62% of reported values). Using a BH of 130 cm was customary in continental Europe (Robbie, 1955), whereas the seemingly odd value of 137 cm is an approximation of 4.5 feet, the usual BH where English units were employed (Grubb et al., 1963). There were some minor differences among the ®ve journals; for example, papers in forestry journals report BH 137 cm relatively more often. However, in each of the journals more than 50% of relevant papers did not report BH. In our ®eld survey we measured diameters at both BH 130 and BH 140 cm on the same trees in a
0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 1 4 1 - 3
90
N. Brokaw, J. Thompson / Forest Ecology and Management 129 (2000) 89±91
Table 1 The distribution of values stated for `breast height' (BH), as used for measurement of `diameter at breast height' (DBH), in papers published over the period 1988±1997 in selected journals (see text). Most commonly, papers did not state BH. Among values stated, BH 130 cm was the mode. Breast height stated (cm) Journal type
120
135
137
140
150
160
None
Total
General ecology Forest ecology
1 ±
130 40 73
± 2
4 24
12 15
7 3
1 ±
80 178
145 295
Total
1
113
2
28
27
10
1
258
440
subtropical wet forest at El Verde, Puerto Rico (188200 N 658490 W), as part of research on tree allometry. The study trees were the 100 we have measured so far in a sample strati®ed to include both a random sample among all diameters and an adequate sample in each of ®ve diameter classes (10±20, 20±30, 30±40, 40±50, >50 cm DBH; BH 130 cm) of 12 common species. The study trees ranged from 10.4 to 85.6 cm DBH and had boles that were free of vines, buttresses, or major stem irregularities at BH. The mean difference between diameters measured at both BH 130 and BH 140 cm was 3.5 mm (SD 5.8, n 100). Seventy-three of the trees had a larger diameter at BH 130 than at BH 140 cm; 16 were smaller at BH 130; and 11 had the same diameter at both BHs. Total basal area for the 100 trees was 2394 cm2 higher (2.6%) and biomass was 3.61 metric tons higher (4.0%) when calculated using the diameter at BH 130 than at BH 140 cm (biomass: ln(dry weight) ÿ1.966 1.242ln(DBH2) (Overman et al., 1994). Our study showed that in the journals we surveyed most researchers publishing papers using DBH did not give the value of BH they used. And we cannot assume they used some well-accepted standard value, since there was much variation among the BH that was reported. Furthermore, the BH used can affect the diameter recorded and therefore the derived values of basal area and biomass. The effect on diameter recorded will differ among tree species, depending on characteristic trunk form and growth stage. The effect on derived values will differ among forests according to the density of large trees, because basal area and biomass are exponential functions of diameter. Regardless, our results indicate that failing to
report the value of BH, or using different values of BH, may lead to erroneous comparisons of diameter-class distributions, growth, basal area, or biomass. Errors concerning biomass can mislead us about such important phenomena as global carbon storage in forests. To avoid erroneous comparisons among sites and to ensure consistency within longitudinal studies, we urge forest researchers to report the BH used. Because BH 130 cm was the most commonly reported value, we suggest using BH 130 cm when initiating studies (Philip, 1994; Condit, 1998). Lastly, since `DBH' is ambiguous, we propose abandoning that term and instead using `D' and a subscript to indicate diameter measured at a speci®c height. Thus `D130' would indicate tree diameter at 130 cm. `DBH' may still be needed as a generic term. Acknowledgements We thank Dawn Gabree for compiling part of the data, Elizabeth Mallory for suggesting the title, and Franc,ois Houllier for suggesting the use of `D130'. Staff in the library at the Arnold Arboretum of Harvard University and the Biological Sciences Library at the University of New Hampshire helped us with the literature survey. This research was supported by grant BSR-8811764 from the National Science Foundation to the Institute for Tropical Ecosystem Studies, University of Puerto Rico, and to the International Institute of Tropical Forestry, as part of the Long-Term Ecological Research Program in the Luquillo Experimental Forest. The US Forest Service (Department of Agriculture) and the University of Puerto Rico provided additional support.
N. Brokaw, J. Thompson / Forest Ecology and Management 129 (2000) 89±91
References Condit, R., 1998. Tropical Forest Census Plots: Methods and Results from Barro Colorado Island, Panama and a Comparison with Other Plots. Springer, Berlin. Grubb, P.J., Lloyd, J.R., Pennington, T.D., Whitmore, T.C., 1963. A comparison of montane and lowland forests in Ecuador I. The forest structure, physiognomy, and floristics. J. Ecol. 51, 567±601.
91
Overman, J.P.M., Witte, H.J.L., Saldarriaga, J.G., 1994. Evaluation of regression models for above-ground biomass determination in Amazon rainforest. J. Trop. Ecol. 10, 207±218. Philip, M.S., 1994. Measuring Trees and Forests, 2nd ed. CAB International, Wallingford, UK. Robbie, T.A., 1955. Teach Yourself Forestry. English Universities Press, London.