Maine’s Forest Resources, 2006 Research Note NRS-51

This publication provides an overview of forest resource attributes for Maine based on an annual inventory 

conducted by the Forest Inventory and Analysis (FIA) program at the Northern Research Station of the U.S. 

Forest Service. These annual estimates, along with web-posted core tables, will be updated annually. For 

more information please refer to page 4 of this report.

Table 1.- Annual estimates, uncertainty, and change.

Figure 1. - Area of timberland and forest land by year.

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Figure 2. - Area of timberland for the top four foresttype groups by stand-size class. Note: When available, sampling errors/bars provided in Figures 

and Tables represent 68 percent confidence intervals

Estimate 

(2006)

Sampling 

error (%)

Change (%) 

since 2003

Forest Land Estimates

Area (1,000 acres) 17,695.4 0.4 -0.1

Number of live trees > 1-inch 

diameter (million trees) 22,687.5 1.5 4.9

Dry biomass of live trees > 

1-inch diameter (1,000 tons) 660,893.1 1.0 1.0

Net volume of live trees > 

5-inch diameter (1,000,000 ft3

) 25,565.6 1.2 -0.1

Annual net growth of live trees > 

5-inch diameter(1,000 ft3

/year)

548,271.4 3.1 -3.7

Annual mortality of live trees 

(1,000 ft3

/year)

374,817.0 3.6 33.6

Annual removals of live trees 

(1,000 ft3

/year) 607,390.6 7.2 13.3

Timberland Estimates

Area (1,000 acres) 17,145.7 0.5 -0.3

Number of growing-stock trees 

(million trees) 2,692.1 1.5 -1.2

Dry biomass of live trees > 

1-inch diameter (1,000 tons) 642,041.5 1.0 0.5

Net volume of live trees > 

5-inch diameter (1,000,000 ft3

)

24,800.8 1.3 -0.7

Net volume of growing-stock 

trees (1,000,000 ft3

) 23,163.4 1.3 -0.8

Annual net growth of growingstock trees (1,000 ft3

/year)

518,760.0 2.7 1.2

Annual mortality of growingstock trees (1,000 ft3

/year)

288,659.0 3.9 17.4

Annual removals of growingstock trees (1,000 ft3

/year) 559,434.1 7.2 6.4
Table 2. - Top ten species by statewide species group volume estimates, 2006

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Figure 3. - Area of forest land area (1000 acres)

by ownership group.

Figure 4. - Area of timberland by stand size and

year.

1 Red spruce 3,252.5 3.8 10.1 8,201.4 4.6 1.1

2 Red maple 3,116.9 2.8 -0.8 4,646.7 4.7 -0.4

3 Eastern white pine 2,646.2 5.4 -0.1 9,673.0 6.0 0.2

4 Sugar maple 2,188.5 5.3 0.5 6,008.7 6.6 1.1

5 Balsam fir 2,185.5 3 1.1 2,735.8 5.1 -0.8

6 Northern white-cedar 2,173.3 4.8 2.1 4,447.8 5.7 3.3

7 Eastern hemlock 1,841.7 5.3 0.5 5,046.5 5.9 1.1

8 Yellow birch 1,638.5 3.9 2.1 3,898.9 6.0 3.7

9 Paper birch 1,215.1 4.3 0.1 1,290.3 7.5 2.5

10 American beech 968.7 5.8 -3.3 1,299.8 9.0 -3.6

Other softwoods 1,432.8 4.7 1.8 3,156.7 5.4 2.9

Other hardwoods 2,905.9 4.1 -0.3 5,999.0 7.4 2.5

All Species 25,565.6 1.3 56,404.6 1.9

Species Sampling 

error (%)

Rank Sampling 

error 

(%)

Volume of live 

trees on forest land 

(1,000,000 ft3

)

Change (%) 

since 2003

Volume of sawtimber 

trees on timberland 

(1,000,000 bdft)

Change (%) 

since 2003
Continuous Improvement Process for Volume, Biomass, and Carbon Estimation

Implementation of the annual inventory requires ongoing evaluation of existing methods and 

implementation of improved system components as they become available. Recently, significant changes 

were made to the methods for estimating tree-level volume and biomass (dry weight) for Northeastern 

states, and the calculation of change components (net growth, removals, and mortality) was modified for 

national consistency. These changes have focused on improving the ability to report consistent estimates 

across time and space--a primary objective for FIA.

Tree-level Volume Estimation

As the remeasurement phase of the annual inventory began, it became apparent that the 5-year 

measurement cycle was limiting the ability to track changes in volume over time because “noise” in the 

system outweighed the actual “signal” of the ecosystem. Sources of noise include: measurement 

variability, field protocol changes, field crew cohort changes, definition changes, processing algorithm 

changes, and model error. This noise impacts estimates of volume, biomass, carbon, and associated 

change component estimates.

Volume estimation is critically important because individual tree volume estimates form the basis for 

biomass and carbon estimates. In the past, estimates of individual tree gross volumes (cubic foot and 

board foot) were estimated using equations based on field-measured diameters and lengths (bole and 

sawlog). Net volume of the tree was computed by subtracting field-measured cull (Scott 1981, Scott 

1979). Over time, it was found that noise in the measurements of tree lengths and cull outweighed the 

true signal. To ameliorate this, a taper model was used to estimate length (Westfall 2009). In addition, a 

model was developed for determining the percent of total tree volume represented by cull. The modeled 

length and cull estimates are used in conjunction with Scott’s volume equations; thus providing a 

consistent estimation procedure across the Northeast. In general, the new volume estimates will be 

higher because previous estimates were based on length measurements that included reductions based 

on merchantability, e.g., prior to 2007, the measurement of the top of the bole may have been below the 

4-inch top due to form deficiencies that prevents use for pulpwood. In 2007, the merchantability limits 

were removed from the field protocols. In conjunction with the removal of merchantability limits, changes 

in cull measurements were implemented, such that net tree volume would continue to represent only the 

merchantable portion of the tree. The modeled lengths and culls are aligned with these new measurement 

protocols.

Component Ratio Method (CRM) for Biomass/Carbon Estimation

Biomass and carbon have become increasingly important to FIA clients as demand for bio-energy has 

increased and carbon sequestration has emerged as a vital component of climate change analyses. As 

such, a method for harmonizing volume, biomass, and carbon estimates was needed. Prior to 

implementation of the CRM method, volume and biomass were estimated using separate sets of 

equations. The CRM method is comprised of the following steps (Heath and others 2009):

• Conversion of sound-wood volume in the merchantable bole to biomass using species-level specific 

gravities (dry mass per unit of green volume)

• Calculation of bark biomass using a set of percent bark and bark specific gravities

• Calculation of biomass in tops and limbs as a proportion of merchantable bole biomass based on 

component proportions using equations from Jenkins and others (2003)

• Calculation of stump biomass using equations from Raile (1982)

• Summation of biomass components to obtain total aboveground biomass

Estimates of carbon are derived through simple conversion of the biomass variables.

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Continuous Improvement Process for Volume, Biomass, and Carbon Estimation

Change Component Estimation

Change components are perhaps the most important volumetric variables reported by FIA because they are 

indicative of sustainability (net growth-to-removals ratio) and forest health (mortality). To summarize, gross 

growth is equal to ingrowth (volume of trees reaching the 5.0-inch minimum diameter threshold for 

merchantable volume) plus accretion (growth of surviving trees that were above the threshold at previous 

measurement). Net growth is equal to gross growth minus mortality (volume of trees meeting the threshold 

at previous measurement that died). Removals are equal to the volume of trees meeting the threshold at the 

previous measurement that were harvested or on forest land that changed to nonforest land. Net change in 

inventory is equal to net growth minus removals.

The National Inventory Management System (NIMS) is the compilation system for all FIA estimates. 

Implementation of NIMS has included some adjustments to regional change-component algorithms to 

achieve national consistency. A new method, termed the “midpoint method,” has introduced some 

fundamental differences in methods (Westfall and others 2009). Essentially, the new approach involves 

growing trees to the midpoint of the inventory cycle (2.5 years for a 5-year cycle). Although the overall net 

change component is equivalent under the previous and new theory evaluations, estimates for individual 

components will be different. For ingrowth, the midpoint method produces a smaller estimate because 

volumes are calculated at the 5.0-inch threshold instead of using the actual diameter measurement, which 

may be larger. The estimate for accretion is higher because growth on ingrowth, mortality, and removal trees 

is included. As such, the removals and mortality estimates will also be higher than before.
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Citation for this Publication

McCaskill, G.L.; McWilliams, W.H.; Butler, B.J.; Barnett, C.J.; Hansen, M.H. 2010. Maine’s forest resources, 2006. Res. 

Note. NRS-51. Newtown Square, PA. U.S. Department of Agriculture, Forest Service, Northern Research Station. 5 p.

Bechtold, W.A.; Patterson, P.L. 2005. The Enhanced Forest Inventory and Analysis Program: national sampling design and 

estimation procedures. Gen. Tech. Rep. SRS-80. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern 

Research Station. 85 p.

Heath, L.S.; Hansen, M.H.; Smith, J.E.; Smith, W.B.; Miles, P.D. 2009. Investigation into calculating tree biomass and carbon

in the FIADB using a biomass expansion factor approach. In: McWilliams, W.; Moisen, G.; Czaplewski, R., comps. 2008 

Forest Inventory and Analysis (FIA) Symposium; 2008 October 21-23; Park City, UT. Proc. RMRS-P-56CD. Fort Collins, CO: 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 1 CD.

Jenkins, J.; Chojnacky, D.C.; Heath, L.S.; Birdsey, R.A. 2003. National scale biomass estimators for United States tree 

species. Forest Science. 49(1): 12-35.

Raile, G.K. 1982. Estimating stump volume. Res. Paper NC-224. St. Paul, MN: U.S. Department of Agriculture, Forest 

Service, North Central Research Station. 4 p.

Smith, J.E.; Heath, L.S.; Skog, K.E.; Birdsey, R.A. 2006. Methods for calculating forest ecosystem and harvested carbon with 

standard estimates for forest types of the United States. Gen. Tech. Rep. NE-343. Newtown Square, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Research Station. 216 p.

Scott, C.T. 1981. Northeastern forest survey revised cubic-foot volume equations. Research Note NE-304. Broomall, PA: 

U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 2 p.

Scott, C.T. 1979. Northeastern forest survey board-foot volume equations. Research Note NE-271. Broomall, PA: U.S. 

Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. 2 p.

Smith, W.B. 2002. Forest inventory and analysis: a national inventory and monitoring program. Environmental Pollution. 116: 

233-242.

USDA Forest Service. 2009. Forest inventory and analysis national core field guide, Vol. 1, field data collection procedures for

Phase 2 plots, Ver. 5.0. Available at www.fia.fs.fed.us/library/field-guides-methods-proc (verified July 2009). 

Westfall, J.A.; Frieswyk, T.; Griffith, D.M. 2009. Implementing the measurement interval midpoint method for change 

estimation. In: McRoberts, R.E.; Reams, G.A.; Van Deusen, P.C.; McWilliams, W.H., eds. Proceedings of the Eighth Annual 

Forest Inventory and analysis Symposium. 2006 October 16-19; Monterey, CA. Gen. Tech. Rep. WO-79. U.S. Department of 

Agriculture, Forest Service: 231-236.

FIA Program Information

Contact Information

FIA Maine Analyst: George McCaskill, (610) 557-4045, georgemccaskill@fs.fed.us

Data Management: Carol L. Alerich, 610-557-4068, calerich@fs.fed.us

Website: fiatools.fs.fed.us

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