Section I and Management

Sessions A and B Inventories of Resources Inventaires des ressources forestières Inventarios de Riquezas Forestales Selecting Objectives and Methods of Estimating Forest Resources

ERIC HAGBERG Acting Director, The Forest Research Institute of Sweden, Stockholm, Sweden

The establishment of an appropriate forest policy is squares. Large distances between the lines are applied to possible only on the basis of a sound knowledge of the reduce the cost of the survey.This requires rather national resources of forest land and timber. The data dependable map material as a basis for the location of necessary for the purpose must be gathered by means of lines. Along the lines or the sides of the tract the site and inventory standards and methods that are adapted to timber conditionsareusually recorded continuously. provide the information wanted. Sometimes the growing stock, too, is continuously meas- The selection of various methods is made to conform ured by calipering the within a strip of certain width with the objectives expressed at the planning stage of the (strip survey). The latter task has lately been replaced by inventory. Three methods applicable to various objectives a registration of trees within sample plots located at even and circumstances will be discussed in the following. intervals along the survey line. The current measurement Method A is useful in areas where the objective is to of the trees has sometimes been replaced by continuous obtain information needed when planning the exploitation ocular estimatesoccasionallysupported by relascope of accessible forest resources. observations along the survey lines. The method may be Here the primary objective is to procure explicit sum- supplemeflted with regularly spaced sample plots to con- maries of the present volume of accessible timber by stand trol and guide the ocular survey. types, species and sizes for the end purpose of establishing The method based on field observations only has been their value. An inventory by means of aerial photography applied in the Nordic countries (Finland, Norway and appears to be the cheapest and most effective method in Sweden) since the beginning of the 20's. The survey has this case.Systematically spaced flight lines or randomly been repeated at even intervals to provide current infor- located sample plots provide information on the volume mation on the development of the growing stock and to of growing stock (per hectare), its distribution by species state the changes of volume that have occurred between and a rough estimate of age classes and sizes by technical the surveys. For reasons of organization the inventories means currently available. Frequently an estimate with a were extended over several years (3-4 years in Finland; precision of ± 10 percent for the individual stand types 10-15 years in Norway and Sweden).Thus, one or may be considered satisfactory. several provinces were surveyed annually to enable a Method B is applicable in areas where the objective is presentation of partial results pertaining to administrative to follow the development of the growing stock and the units. The accuracy was sufficient (standard error 2-3%) sustained utilization of the land for purposes. to be informative to the local authorities. Survey methods of point sampling nature may be A total picture of the national resources was not applied in this case either as a combination of aerial obtained until the survey comprised all the provinces, and photography with certain field observations or as a field it was then a sum of results from a number of separate observation procedure only.In the first alternative the years. The exactness of the total result was dependent aerial photos provide dependable information on the on the changes that have occurred in the partial results volume of growing stock when supplemented by field during the inventory cycle. observations. Point samples that are assumed to be applied Method C is applicable in areas where the objective iv on a certain fraction of the aerial photo material, con- to account for the balance between the timber depletion stitute control data necessary for a determination of the and the increment of current forest capital, and to register accuracy of information obtained from the aerial photo- such occurrences that provide valuable information only graphs.Details that are impossible or very difficult to if subject to an annual summary. perceive in the aerial pictures are supplemented by the This method of survey is principally similar to that fieldobservations.Point sampling may be arranged described under B for field observations. To reduce the eithersystematically or randomly.In thefirstcase, cost, the requirements of accuracy with reference to the dependable estimates may be obtained by a rather small annual result in subregions must be abandoned.The amount of data although the determination of the accuracy accuracy must be adapted to certain main data regard- may meet with certain difficulties.In the latter case the ing the total area. The survey can be carried out annually determination of the limits of error is highly facilitated. by first determining the location of survey lines and tracts An estimate based on field observations only and for the whole 10-year period. Then one-tenth is sys- obtained by systematic point sampling may be carried out tematically selected for survey during the current year either along regularly spaced lines or survey tracts, e.g., at a cost not essentially higher than a tenth of the total.

Silviculture and Management 243 The method is more expensive than that mentioned under intervals of time. However, estimates of this kind are B but it has certain advantages that will be discussed here. rather inaccurate and only great differences can be ascer- Unlike the provincial surveys this method provides a tained. Higher standards are needed when a difference of synchronized picture of the whole area. The results pre- about ten percent magnitude is to be established. On a sented for subregions after ten years certainly constitute long sight the need to plan the rehabilitation of the averages of a ten-year period but they pertain to the presents itself as a national matter. same period of time in each of the subregions. The partial The sustained yield of timber is effectively controlled results may also be computed for the growing stock of only if the extent of clear-cut area is known. It is also the final year of the period, if the increment and the necessary to collect information on the reserves of land timber depletion are reported annually. that are suitable for timber production although they have Since the account of the growing stock is synchronized previously been neglected. For this purpose the inventory and the annual cut and increment are recorded by certain of growing stock is often combined with a close account years or periods, the changes of the growing stock can be of the site conditions.If the investigation is carried out studied for the area as a whole, or for subregions. At by means of aerial photos, relatively comprehensive field certain intervals it is possible to present a balance account observations are needed asa supplementImproved showing the initial growing stock, the increment and the knowledge of the balance between the increment of the timber depletion on one side, and the final growing stock growing stock and the timber depletion (including natural on the other side. The method also provides an oppor- mortality) is another condition for the maintenance of tunity to register certain occurrences of a given year. even yield.The increment depends,i.a., on the age First, the annual cut should be mentioned. If statistics composition of the growing stock and the kind of of the felling operations in certain areas are no