650 



TIMBER RESOURCES FOR .^JVIERICA'S FUTURE 



All Data Are Not Equally Reliable 



Among the major groups of national estimates, 

 some overall comparisons of reliability can be 

 made. In general, the most reliable data are the 

 estimates of forest land area, timber volume, and 

 ownership of forest land. The data for foreign 

 countries, the estimates of past trends in timber 

 volume and growth, and the estimates of tree 

 planting and growth impact, though adequate for 

 this report, are considerably less reliable. Inter- 

 mediate between these two groups are the data on 

 growth and utilization, productivity of recently 

 cut lands, and ownership of timber volume. 



The estimates of future demand and supply are 

 in a different category. Unlike current or past 

 data, their reliability depends almost entirely on 

 the assumptions upon which they are based. 

 Studies of past trends help in selecting assump- 

 tions and making projections, but estimates of 

 conditions that will not occur until 1975 or 2000 

 cannot be made with anywhere near the same 

 assurance as estimates of present conditions, which 

 can be measured. Despite this limitation, pro- 

 jections form an essential part of any appraisal 

 such as this. It is believed that the assumptions 

 chosen are reasonable ones and that the future 

 demand, growth, and inventory data are suffi- 

 ciently reliable and adequate for the purpose of 

 this Review. 



FOREST LAND, TIMBER VOLUME, 

 AND GROWTH 



The 1953 estimates of forest land area and 

 timber volume and the 1952 estimates of net 

 annual growth are adequate and reliable enough 

 for describing the national timber resource situ- 

 ation and for making regional comparisons within 

 the continental United States and Coastal Alaska. 

 For State by State comparisons, many of these 

 data are also adequate, but some are not. The 

 comparable 1945 estimates, on the other hand, 

 are not adequate enough for similarly detailed 

 analysis. The estimates for Interior Alaska are 

 also crude and should be taken as no more than 

 indicators of the timber situation there. 



The 1953 estimates of forest land area and 

 timber volume and the 1952 estimates of timber 

 growtli were based mainly on the Forest Survey, 

 a continuing, nationwide project of the Forest 

 Service. Data were available from initial surveys 

 of 484 million acres of forest land, and resurveys 

 of 171 million acres. Most Forest Survey data 

 are obtained from aerial photographs and from 

 ground observations. Aerial photographs provide 

 some of the area data, but these are always 

 checked and amplified by ground measurement. 

 Sample ground plots provide all of the volume 



estimates, such as volume by species, volume by 

 log grade, and volume by tree size. 



Growth estimates are obtained by boring sample 

 trees, measuring radial growth for a short period 

 of years, and determining the dimensions of the 

 tree at the beginning of the period. The difference 

 between past volume of the tree and present 

 volume is periodic growth. In some regions, 

 average annual periodic growth of a species is 

 taken as current annual growth; in other regions, 

 growth by species is calculated for each diameter 

 class and applied to stand tables from w^hich 

 current annual growth is then calculated. The 

 average annual volume of trees that died during 

 the period is deducted from gross growth to 

 arrive at a net growth estimate.'* 



Forest Survey Provided Data for 33 

 States 



Because the Forest Survey had been completed 

 or partially completed in 33 States, and had not 

 been started in others, procedures for determining 

 area, volume, and growth varied, depending upon 

 the Forest Survey situation in each State. 



Forest surveys or resurvej's were complete in 23 

 States containing 256 million acres of commercial 

 forest land, 52 percent of the total commercial 

 forest area in the United States and Coastal 

 Alaska. Where these surveys antedated 1953. 

 adjustments w^ere made for known changes, such 

 as in area by land-use class or stand-size class, and 

 new volume estimates were calculated by adding 

 net annual growth and deducting annual cut year 

 by year. The growth estimate used in the calcu- 

 lations, as well as 1952 growth, was obtained by 

 using species growth rates determined at the time 

 of the survey. 



In ten States, the Forest Survey was incom- 

 plete. Although 87 million acres of commercial 

 forest area had been covered, 70 million acres had 

 not. In three of these States, the unsurveyed 

 part was examined and classified on aerial photo- 

 graphs, and the ground plot data from the sur- 

 veyed part was then applied to the remainder of 

 the State on the basis of this examination. In 

 two States, partial resurvey estimates of varying 

 but recent data were adjusted as necessary to a 

 common year by allowing for growth and cut: 

 average timber volumes per acre and other rela- 

 tions were calculated ; and the results were applied 

 to the entire commercial forest area of the State 

 as determined by original surveys and as adjusted 

 for known area changes. In the other five un- 

 completed States, Forest Survey procedures 'vere 

 used in the unsurveyed parts but ground sampling 

 was lighter than usual or was limited to sample 

 counties. 



* The adequacy of the mortality data i.s discussed undef 

 Poorest Protection, p. 658. 



