Inventory Deflated
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Inflated Inventory Estimates Deflated [1]
Vince Taylor, March 5, 2006

In 1989, Jackson Forest replaced its original inventory system, used for 25 years, with a new system. The Intensive Forest Inventory (IFI) system had many more plots spread across the forest than its predecessor,  the Continuous Forest Inventory (CFI) system. Everyone had high hopes for better inventory estimates.

The first estimates based on the 1989 inventory measurements were outlandishly high. Measured just 5 years after the prior inventory, the new inventory was estimated to be almost 50% higher. The forest grows at only a bit over 2% per year, before harvesting, and the intent of forest policy was to cut all growth. Thus, the big increase couldn't represent forest growth. What did it represent?

The staff of Jackson Forest evidently debated the causes in the early 1990s, but never seriously tackled the validity of the estimates; so they were allowed to stand. With the passage of time, they became the gospel. They are incorporated in the 2002 Draft Forest Management Plan and the newly revised Environmental impact report for Jackson Forest. The projected harvest levels in the management plan are based on these estimates.

We now know that these estimates of inventory and forest growth are so greatly in error that they invalidate the management plan and environmental report.[2]

How could such an important error go uncorrected for over fifteen years? The only plausible explanation is that the inflated estimates were too attractive. No one in the state forestry establishment was willing to get to the bottom of the discrepancy between the IFI estimates and the earlier ones because the new results made them look so good.

Apparently in 1991, the staff at Jackson Forest did an analysis that raised serious doubts about the validity of the 1989 estimates, but no one in upper management wanted to pursue it further. Then, in 1998, I told the department of forestry that the new estimates could not be reconciled with the prior 25 years of inventory history and must be in error. I have repeatedly urged them since that time to find out what went wrong with the 1989 estimates. My arguments have been dismissed as invalid, no matter how many times I try to explain.

How do we know the 1989 estimates are in error? The proof is quite simple, although apparently difficult for people to understand. The foundation of the proof is that inventories in a forest will grow bigger if forest growth exceeds the harvests and decline if harvests exceed growth. This is a simple mathematical truth. It must hold true. And , as we shall see, this truth proves that the IFI estimates are in error.

When challenged on how the IFI estimates could exceed by 50% the estimate of 5 years earlier, the state responded that the earlier estimates were too low; that the actual forest inventory and growth were much higher than the earlier system estimated. But, this argument is contradicted by the history of growth, harvests, and inventories under the prior system.

If actual growth had been higher than estimated growth during the earlier period, only part of the growth would have been cut in each period, and so both actual and measured inventories would have increased significantly from period to period. This result is shown graphically in Figure 1 in an example that approximates the situation proposed in defense of the accuracy of the IFI estimates.

Figure 1

But, in the earlier period inventories didn't increase, they declined. Jackson Forest policy has always been to set the harvest level equal to the estimated rate of forest growth. If all estimates were exactly accurate and the policy exactly carried out, the inventory would have remained constant. In fact, from 1964 to 1984, it declined by an average of 4 million board feet per year, a little more than 10 percent of the harvest rate. Estimated growth exceeded the actual growth. Setting harvest levels equal to the (over)estimated growth caused a modest overcutting of the forest.

The much higher IFI estimates cannot be accurate, because they imply contrary to empirical fact, that measured inventories would have been increased. They did not. Q.E.D.

Since 1998, I have repeatedly asked the California Department of Forestry (CDF), the managers of Jackson Forest, to repudiate the 1989 inventory estimates and to do the analysis necessary to identify the sources of errors. They have as steadfastly refused, insisting against all logic and empirical evidence, that the IFI estimates were correct. They argued that new equations used to estimate the inventory in 1989 were more accurate than the 1984 equations. The different estimating equations, according to CDF publications, only explained a small part of the discrepancy, but CDF would look no further.

Frustrated that CDF would not look further, I determined to answer the questions that CDF was unwilling to answer. CDF agreed to provide me with electronic files containing the inventory data. This past summer, with the help of a programmer, I analyzed the 1984 and 1989 inventory data.

What I found was shocking, even to me. To remove the effects of changes in the estimating equations between the 1984 and 1989 inventories, I applied the 1989 estimating equations to both the 1984 and 1989 inventory sample data and compared the estimates. The results are shown in Table 1.

Table 1.  Estimated 1984-1989 Inventory Growth, using 1989 estimating equations
(millions of board feet gross)

 

1984 Inventory

1989 Inventory

Estimated Growth 1984-89

All Conifers

1,747

2,011

264

Harvest 1985-89

 

 

214

Total Forest Growth

 

 

480

The estimated net growth plus harvests (total forest growth) in the five years 1984-89 equaled 480 million board feet! The exclamation point is appropriate. This is an absurd figure. It amounts to 96 million board feet per year, 5.5% per year. It is more than double the 42 million board feet per year that CDF uses as the basis for estimating the allowable cut in the draft management plan, a figure that is extremely questionable itself. It is triple the most reliable estimate of volume growth: 32 million board feet per year estimated by the 1984 CFI inventory.

I emphasize that the results in Table 1 reflect inventory estimates made with one set of estimating equations. None of the difference is due to change in the estimating equations between the two inventory years. The difference reflects differences in the basic tree data collected in the two different years. Further, these data were collected from essentially the same 140 plots of the historical Continuous Forest Inventory system; so sampling errors could not account for an appreciable amount of the difference. Something went seriously wrong with some parts of the 1989 inventory design, plot measurements, data collection and processing.

Hopefully, before CDF publishes the results of new inventory measurements that it recently completed, it will first determine the source of errors in the 1989 inventory; so that it does not repeat the same mistakes again.


[1] This article excerpts from the detailed analysis of Jackson Forest inventory estimates contained in my comments on the 2005-06 Draft Environmental Impact Report for Jackson Demonstration State Forest, February 27, 2006. This can be accessed, together with other filed comments, at: http://www.jacksonforest.com/EIR2/Comments/comments_2_top.htm

[2] The inflated growth figures led the management plan to propose harvest levels that would violate the mandate for managing forests for long term sustained yield. See Section 6 of my comments on the Draft EIR:
http://www.jacksonforest.com/EIR2/Comments/taylor_EIR2.pdf