2.6 Provincial Forest Inventory in British Columbia. What's next
? Sam Otukol, Ulf SoderbergSummary
The Province of British Columbia, Canada, is planning a multi-resource vegetation inventory. The objectives of the new inventory are to provide a foundation for estimating broad summary statistics at the provincial level and to provide a framework for management planning, long-term change detection, and monitoring. Two, competing sampling designs have been proposed, i.e., 1) grid-based sampling, and 2) sampling from a sorted polygon list.
The grid-based sampling would consist of two components. The first component would be the establishment of a systematic grid system over the province. Ground measurements would be obtained from a cluster of plots at each grid point. The second component would be management-unit based, and it would involve photo classification of the vegetation cover types, attribute estimation, and supplementary ground sampling. Maps would be produced at the management unit level.
The sampling from a polygon list would be management unit based, and would have two phases. The first phase would be the photo classification, polygon delineation, and attribute estimation. The formed polygons, representing different vegetation cover types, would then be compiled into a list which would be sorted. Sample polygons would then be selected from the list, and visited on the ground. Ratios of ground estimated attributes to the photo estimated attributes would be used to adjust the photo attributes for the polygons which are not visited on the ground.
The grid approach seems to have several advantages over sampling from a polygon list, but depending on what objective is considered to be most important, the method selected can change.
Keywords: multi-resource inventory, systematic grid, sorted list
2.6.1 Introduction
Forest inventory in British Columbia on the provincial level has a fairly recent history.
The first complete provincial inventory was carried out in 1951-1957 (MALCOLM 1957).
The objective was to estimate average volumes for strata. Strata were defined as groups of related forest types (summary types) by age, height and stocking class. The second and so far the last complete inventory was done in 1961-1977 with the objective to estimate total volume for aggregate forest types in each management unit (Public Sustained Yield Unit).
The target population (mature timber) was divided into strata which consisted of 42 inventory type groups, nine age classes, eight height classes and four site classes. This results in more than 1 2 000 strata for which a minimum of ten samples were required for each. Over all 50,000 samples were collected. About 77% of the current inventory information originates from these data (THROWER 1992).
The aim of the inventory was to use stratified random sampling. This could not be achieved due to logistic, budgetary and administrative difficulties. These difficulties have also reduced the ambition of other inventory efforts performed so far. The random sampling was abandoned for a subjective selection of strata of older age classes and of
strata with the largest areas. The magnitude of the bias introduced by the subjectively located plots is not known and also makes the calculated sampling errors and other statistics of uncertain value.
After the last provincial inventory the inventory efforts were directed towards getting more stand specific estimates of volume and other attributes. This required changes in classification, sampling, data bases, compilation including methods of estimating volumes for individual stands. Another major change in the period from 1978 to present time was the conversion of the inventory data base to continuous variables instead of the formerly used class-based descriptions. During this period the amount of ground sampling has been limited. An approach with multi-phase stratified sampling using 70 mm low level photography and ground samples was tried, but later abandoned due to logistic problems.
Most activities have concentrated on a history update program for major depletions from harvesting, fire, insects and a re-inventory program for reclassification.
A number of deficiencies of the inventory has been revealed during the last years and demands for additional information that so far has not been collected (considered) are requested from several users of the inventory and from the general public. It is realized that future inventories must not only consider present and future utilization of timber, but also the need for more information about many other components of the forest. As in most countries information about ecological and environmental characteristics of the land base are needed. Different solutions are used, incorporation and enlargement of existing forest inventories (SODERBERG 1993) or establishment of totally new inventories (SCOTT et al.
1993).
Between 1988 and 1991, there was a major initiative to establish the status of the current inventory (SPANDLI 1994). The British Columbia Forest Resource Commission (FRC) was asked to examine the state of the province's land base and to recommend improvements to the way it is managed (FRC 1991). Among other things, the FRC recommended the formation of a Resource Inventory Committee (RIC). The RIC, through the Terrestrial Ecosystem Task Force (TETF) is responsible for the new inventory initiative. The primary objective of the new vegetation inventory is to develop a multi
resource inventory to include disciplines such as range, ecology, wildlife, soil, timber, recreation, etc. The other objectives include:
1) To provide a foundation for estimating broad summary statistics at the provincial and regional level.
2) To develop a framework for long term change detection and monitoring.
3) To provide a framework for projecting selected stand parameters.
4) To create common sample locations for multi-resource inventories.
5) To be a flexible structure to meet a range of client needs, i.e., a wide range of users could use the same sample points to obtain information for their needs, but also allow them the option to collect additional information to supplement these needs.
6) To provide attribute estimates with precision, or accuracy statements
7) To provide a frame work for obtaining information to allow validating and testing growth and yield models.
8) To provide a value neutral vegetation classification system, i.e., avoid use of descriptions such as 'productive', 'non-productive', 'operable', 'non-operable', etc.
9) To provide an objective and statistically based approach for measuring and estimating vegetation cover attributes.
The New Provincial Vegetation Inventory Design should meet these objectives, with a particular view to satisfying the needs of the inventory information users.
Information needs
The mandate of the British Columbia Inventory Program is to provide current and reliable inventory information (FRC 1991). Other resource inventory organizations within all government agencies also have similar expectations from this inventory system. Some users such as the Resource Planning Branch of the MOF are interested in getting accurate, broad-based management unit and provincial information. Other users, such as the Regional Offices, and the Industry users are interested in more accurate information on the productive part of the forest lands at a polygon level.
The inventory information is needed for a wide range of purposes, including:
a) provincial strategic planning (both short term and long term), and generating summary reports;
b) annual allowable cut (AAC) determination;
c) local resource planning;
d) to support other government initiatives, e.g., Old Growth Strategy, protection of sensitive environmental sites;
e) change detection and reporting
f) to provide information to national and international agencies involved in monitoring and providing reports and statistics on forest based natural resources.
Designing a provincial inventory that would satisfy such a wide range of objectives and to meet the needs of the users is a challenge. Recognizing this, the Resource Inventory Committee (RIC) in 1991, formed a working group (the Vegetation Inventory Working Group, VIWG) to design the new inventory. The VIWG members consists of academics, forestry consultants, forest industry executives and Ministry of Forests personnel. This working group has been deliberating over the past two years, and is currently planning operational testing of two sampling designs.
The two proposed sampling designs are:
a) the management unit based, sampling from a sorted polygon list b) the two-component, grid based design.
2.6.2 Sampling from a Sorted Polygon List Classification
The sampling design based on a sorted polygon list is management unit based, and consists of two stages. In the first stage, 1:15,000-scale aerial photography of the target management unit is obtained. The photographs are used to identify and delineate homogeneous vegetation cover types, forming polygons. For each polygon, vegetation cover attributes are estimated based on photo interpretation. The delineated polygons, the associated attribute estimates, and the maps produced constitute the sampling frame.
Sample Selection
A list of the polygons is compiled and sorted by some specific criterion, e.g., by species.
There may be 500,000 to 6,000,000 polygons in a management unit. A sample of the sorted
polygons is then obtained, for ground visitation, through systematic selection, with a random start. Stratification may be involved in the process.
The second stage in the polygon based design is the establishment of ground sample plots within the selected polygons. A 100 m systematic grid is established within the selected polygons. A number of the grid points is selected for sample plot establishment, and ground measurements are obtained from the plots.
Adjustment of photo based polygon attributes
For each sample polygon, two sets of data will be collected: the photo based data, and the ground based data. It is assumed that the photo based data is inaccurate (biased), and that the ground data is accurate (unbiased).
To eliminate the 'bias' in the photo based estimates, it is proposed to adjust the photo based attributes using either a regression or ratio estimator. The regression estimator would be in the form:
Y; = b0 + b1x;
and the ratio estimator would be in the form:
where: Y;
R
= regression estimate of ground attribute y for polygon i,
= photo based estimate of attribute x for polygon i,
= ratio estimate of ground attribute y for polygon i,
= regression coefficients estimated from sample data,
= }: Y; I }: X; from sample data.
Computation of Sample Variances
It is hoped that this adjustment will improve the accuracy of the photo based polygon attributes. This adjustment process is controversial in that, some members of the VIWG feel, it would not be effective at the individual polygon level, if the correlation between the ground measurements and the photo based attribute estimates is not high ( COCHRAN 1963, CHACKO 1965). Furthermore, the computation of the variances of the adjusted values would be more complex, and a misleading results could arise.
Some questions on the computation of sample variances are still begging to be an
swered, such as:
a) How will the variance of the adjusted values be computed and interpreted?
b) How will the variance of the ratio or regression estimator be computed and interpreted?
c) Under what circumstances does the regression or ratio estimator provide efficiency in sampling?
2.6.3 Two Component Grid Based Design The provincial Grid component
The first component of the inventory would be a Provincial grid system. The grid would be designed to capture vegetation cover diversity across the Province, and to serve as a source of information to deal with broad planning issues at the Provincial and at the management unit levels. The Province would be divided into zones using biogeoclimatic type criteria. A different grid intensity would be applied to each zone depending on the diversity of cover type. For example; the following zones are proposed (see Map):
Zone 1 - Coastal and Islands area, Zone 2 - South-Eastern area, Zone 3 - Central-Interior area Zone 4 - Northern area.
A grid system would be established in each zone in such a way as to uniformly cover the entire zone. For instance, the following intensities could be applied: a 5 km grid in zones 1 and 2 because there is considerable vegetation cover diversity in those zones, and 10 km and 15 km grids in zones 3 and 4, respectively.
The actual grid size for each zone could be established through pilot testing in each of the zones. The grid points would be laid out at regular intervals using UTM co-ordinates (e.g., 5 km apart). The grid points would be permanent sample points which would be revisited at regular time intervals. At each grid point a cluster of points would be estab - lished. The number of plots in each cluster and the distance between them would be deter
mined after assessment of the results from the pilot tests.
The grid would be revisited to measure change and to provide a basis for adjusting and projecting the inventory information.
A cluster of plots will be established at each grid point. The size of the cluster and the nature of the plots will be designed to allow completion of the field work within one day.
ZONE 4
FORT NELSON T. S . A.
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CD
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ZONE 2
Fig.l . An illustration of probable zoning for the proposed Provincial grid based inventory.
An effective plot layout should cover at least two directions and the plots should be placed some distance apart. A cluster of plots will consist of three plot locations with an L
shaped layout. The plot location centers will be 200 m apart. This configuration is efficient and provides a considerable amount of information.
200 m
200 m
Fig. 2. An illustration of the plot configuration at a grid point.
Each plot location within a cluster will be independent - providing unique information to the sampling process. The cluster information will be post stratified in the compilation process.
The first plot in the cluster will be established at the grid intersection. The second plot will be located 200 metres north of the grid intersection, and the third, 200 metres east of the intersection.
At each plot location, three nested, fixed-area, plots and a variable radius plot will be established. The plot sizes may vary from cluster to cluster, but the same size will be maintained within a cluster (Figure 3). Point sampling will be used to select sample trees for height and age measurement, call grading, and net factoring.
Fig. 3. An illustration of plot nesting at a plot location.
General data collection
Plot radius 10.00 m 5.64 m 1.78 m
In addition to the information collected for the timber or forest cover inventory, information will be collected for all participating disciplines. Generally, the data will include:
a) Range data,
b) Ecological, site data,
c) Vegetation list and occurrence coverage, d) Soil information,
e) Tree attributes, f) Coarse woody debris, g) wildlife tree data, etc.
All vegetation characteristics will be measured to the standards developed by the Vegetation Inventory Working Group when and where applicable.
The management unit component
The second component of the inventory would be unit- or TSA-based, and would involve mostly classification and polygon attribute estimation. This component of the grid based inventory would be aimed specifically at satisfying the information requirements at management unit level and for local management applications at the district and regional levels. Deficiencies in available information would be assessed and additional data would be collected if necessary. The first component of the inventory would provide the information required to determine whether additional data would be required for the different cover types of interest.
This component would primarily be a classification process based on aerial photography. It would involve the delineation of the vegetation cover types and Classification ( estimation) of attributes. It would also involve some sampling from a polygon list, and ground data collection, if after reviewing the grid data, a need for more information on a specific cover type or cover types is identified.
The grid and the sample polygon information will be used for estimation of strata averages for ground based attributes at the unit (TSA} level. Samples obtained from the provincial grid system would be the a source of ground based information at the management unit level as well.
Variables to be measured
Generally, the following types of records or measurements will be included:
• referencing of the plot locations,
• ownership and status,
• topography,
• vegetation and land cover,
• site quality and ecological classification,
• soil characteristics
• individual tree measurements,
• additional measurements requested by other participants.
The measurements will be made using a common set of standards and terminology developed by the Vegetation Inventory Working Group.
2.6.4 Discussion of the Merits of the two Designs
Both the grid based and the sorted polygon list designs have feature which make them attractive under different condition. There are no well established criteria for assessing conclusively the advantages of one over the other. The choice of one design over the other depends more on subjective judgments of how it meets the objectives of the proposed inventory, than on the quantitative analysis of the statistical advantages.
There are several criteria which could assist a forest biometrician in choosing a sam
pling design for a specific situation. Some of these criteria are discussed in the following sections.
Simplicity
A polygon list can be sorted by several criteria. For the forest timber participants, the list could be sorted by species, height, age, volume, etc .. The criteria by which the list is sorted would influence the polygon selection. The question then is: what combination of the criteria would be used, and what influence would the other participants in the inventory have in the choice of combinations?
The sorted polygon list design would be simple and straight forward if the sorted list consisted of a small number of polygons, and if the variation among the vegetation cover types were small. In addition, the sample obtained would be representative of the popu
lation if the proportions of the different cover types within the population were nearly equal (WARWICK and LININGER 1975).
The vegetation cover types in British Columbia are quite diverse, and there are some cover types which form only small proportions of the vegetation in most management units.
Consequently, using the sorted lists without stratification would not produce desirable results.
With the introduction of stratification, the determination of sample size, and the choice an allocation rule to ensure adequate representation of each stratum, complicate the planning process for the sampling. For this reason, the sorted polygon design is not easy to implement.
The grid based design on the other hand, only requires a base map for a sampling frame.
The grids are established on the base map and ground sampling follows without the complex sorting and stratification processes.
Flexibility
The design based on sample selection from a sorted list would be based on management units. One or more management units would be sampled at a time. Statistics for the province as a whole would be obtained by rolling up the individual management unit values after all the units are sampled. There are several types of management units, i.e., Timber Supply Areas (TSAs), Tree Farm Licenses (TFLs), Parks, and other privately and publicly owned lands. Overall, the number of management-unit-type areas may be as large as 70.
Conducting inventories in each of these management units through the two stage process of photo classification followed by ground sampling, would be cumbersome and slow. It would take 15 to 20 years to obtain the final provincial statistics.
The grid based design, however, would provide provincial statistics within 3 to 5 years, because sampling at the grid points would not require the time consuming photo classification component.
For the second component, which is management unit based, the sampling would target specific vegetation cover types that are not sufficiently represented by the grid points within the management unit. This feature would offer the flexibility of allowing more sampling depending on local planning needs. The criteria for increasing sample size may include the economic value of the vegetation cover type, under-representation of a cover type, or special studies on one or more vegetation cover types.
The grid based design also offers the flexibility of the use of a combination of recent and new photography to obtain photo based at�ribute estimates. This could lead to substantial cost savings since this component is the most costly in the inventory.
Forming Common Locations for Multi-Resource Sampling
The primary objective of the proposed new inventory is to design an inventory which will allow for integrated resource management. This means that the ground samples should be selected without bias toward one particular cover type, and that no prior assumptions of value or importance should be assigned to any cover type. A sampling design based on a provincial grid concept would be the most suitable design to meet this objective.
The sorted polygon list design on the other hand would require stratification to ensure representativeness. The stratification process assumes that: a) the cover type identification, and estimation of polygon areas based on photo interpretation are accurate, and that, b) some strata are more important than others.
In reality, the accuracy of cover type identification is questionable. For the forested polygons, past experience shows that identification of the leading tree species may be wrong on up to 40% of the polygons in some instances. The magnitude of the error of the polygon area estimation is not known.
Furthermore, the forest classification polygons, which would most likely be the basis for forming a sorted list may not coincide with the ecology, soil type, range, and other participants' classification polygons. This situation would lead to conflict and bitterness among the multi-resource inventory participants in the long-run.
Change Detection, updating, and Projection
The provincial grid would form permanent sampling point which would be revisited at regular intervals. In the long-run, this would provide a wealth of information on growth rates, depletion rates, and stand dynamics in general. Consequently, the inventory information would be projectable into the future with better accuracy. In addition, the grid based inventory would facilitate easier adjustment of the inventory information after logging, blow-down, fire, etc. In other words, more current inventory information would be available at all times.
Because of the nature of the plot layout within the sample polygons, it would be difficult to establish a re measurement system to monitor change, for the sampling design based on a sorted polygon list. Furthermore, it would be difficult to account for the effects of changes such as fire logging, etc., particularly when only parts of a sample polygon were affected.
This, in fact, is one of the major weaknesses of the last (1961-1977) inventory. Without a well coordinated update program, the relevance of an inventory deteriorates with time.
Old Growth and Second Growth Information
One of the major forest management issues in British Columbia, is the need for information on the quantity, and geographic distribution of old growth forests in the province. The sampling design based on a sorted polygon list is not amenable to providing information on this aspect of forestry.
The provincial grid would cover the entire land base more or less uniformly. This would allow for the determination of the geographic distribution, and estimation of the pro - portions of old growth and second growth forest
Summary Statistics
The proposed new inventory should provide broad summary statistics for provincial general and strategic planning, as well as management unit summary statistics for regional
The sorted polygon list design would probably provide inventory information more effectively at the management unit level. The two component, grid based design on the other hand, would be more efficient at the provincial level.
2.6.S Summary and Conclusion
Both the grid based and the sorted polygon list designs have characteristics which make them attractive under different conditions. To make a correct choice the objectives of the inventory have to be ranked and qualities of the two designs evaluated according to the weights of the objectives.
The grid approach seems to have several advantages over sampling from a polygon list.
This is particularly important in view of the fact that the primary objective of the new inventory is to provide information for integrated resource management. However, if emphasis shifts to producing more detailed information for local management planning, the sorted polygon list becomes more interesting.
2.6.6 References
CHAKO , VJ., 1965: A manual on sampling techniques for forest surveys. Printed at the Survey of India offices (P.Z.O), Debra Dun.
COCHRAN, W.G., 1963: Sampling techniques. 3rd ed. New York, John Wiley & Sons.
FRC, 1991: The future of our forests. British Columbia Forest Resource Commission, 700-747 Fort Street, Victoria, B.C., Canada.
MALCOLM, R. M., 1957: Provincial Inventory and Forest Surveys in British Columbia. Dept. Lds.
For., B.C. Forest Service, For. Surv. Notes No. 1, Victoria, B.C.
SCOTT, C. T.; CASSELL, D.L.; HAZARD, J.W, 1993: Sampling Design of the US National Forest Health Monitoring Program. Proceedings of the Ilvessalo Symposium, IUFRO S4.02-05.
SODERBERG , U., 1993: Monitoring Forest Health in the Swedish National Forest Inventory.
Proceedings of the Ilvessalo Symposium, IUFRO S4.02-05. Invited paper.
SPANDLI, I., 1994: Personal communication. Forest Inventory Branch, Ministry of Forests, British Columbia, Canada.
THROWER, J.S., 1992: An Historical Summary of Forest Inventory Sampling Designs in British Columbia. In: Report of the Timber Inventory Task Force on the Current Timber Inventory with Recommendations for the Future. The Resources Inventory Committee, 840 Coromant Street, Victoria, B.C.,V8W lRl, Canada.
WARWICK, D.P.; LININGER, C.A., 1975: The sample survey: Theory and Practice. New York, McGraw-Hill Book Company.
