Forest Inventory: A Solid Foundation for Stand Density Management Stephen E. Fairweather Mason, Bruce and Girard, Inc. 707 SW Washington Street Portland, Oregon 97205 503-224-3445 sfairweather@masonbruce.com Introduction Most of us would agree that a good forest inventory system is essential for the support of a stand density management program. But what constitutes a "good" system? This paper presents some ideas about what the attributes of such an inventory system should be. Sometimes a Stand-Based Inventory Isn't Adequate A few years ago I was very fortunate to have the opportunity to take a hard look at a stand-based inventory system being used in Siberia. This was a very large system - nearly one million stands comprising an area of about 45,000,000 forested acres, and almost every stand had been cruised! 1 I had an opportunity to compare our own cruise estimates with what was being carried in the inventory system, and the estimates compared very well. But was the inventory system in Siberia suitable for supporting decision-making with regard to stand density management? Unfortunately, no. It was true that almost every stand had been cruised - but some had been cruised as long ago as 1945, so the estimates were old. The data available for each stand consisted of volumes (cubic meters) by species, the area of the stand (hectares), an index rating its suitability for management, average dbh, average total height, and age; but there was no detail about stand structure, or about the reliability (sampling error) of the estimates. Finally, the inventory system resided on paper, in about 600 books lining the shelves of a large room. Each stand could be identified by owner, compartment, and stand number, and compartments could be identified on a large map, but there was no ability to make use of the computer technology we've grown accustomed to using for storing and retrieving data. 1 This is in contrast to most of our stand-based inventories in the western U.S., where many stands have not been cruised, but instead are carrying a stratum average. 1
What Attributes Should the Inventory System Have? An inventory system suitable for supporting decision-making related to stand density management will have to feature detailed information. In addition to being stand-based, and in addition to being in a digital form such that the data are readily accessible, a "good" inventory system will be characterized by the following: Details about the stand; Details about variability within the stand; and Details about the data. Details About the Stand These are the types of data most of us are quite familiar with. They include the following - 1. Numbers of trees by species and size - A stand table presents basic information about stand density. 2. Crown ratios - Most individual tree growth models in the western U.S. rely on crown ratio (or some measure of crown size) for their projections of tree growth. If a growth model is going to be used to evaluate different silvicultural regimes, crown ratio should become a commonly collected attribute during cruising. 3. Past management and/or stand origin - Knowing something about how the stand has been treated in the past, and/or how it originated, will help to determine how it should be treated now. 4. Site Index - Some measure of site quality, whether it's site index, habitat type, or topographic characteristics such as slope, aspect, and elevation, is a key piece of information for most growth models. 5. Stand Age - For even-aged stands, knowing the stand age or the year the stand was established will certainly be important. Just as important is knowing that a stand is uneven-aged. 6. Competing Vegetation - The inventory system should contain information about nontree competing vegetation, including species, percent coverage, and some measure of size, such as height. Growth models are available and are being developed that will 2
make use of measures of non-tree competing vegetation to model the development of young forest stands. 7. Insect/disease/animal problems - Knowledge of the presence and severity of these types of problems in the stand will help to prioritize the stand for density management activities. 8. Spatial arrangement of the trees - Are the trees arranged uniformly, like in a plantation, or are the trees scattered randomly, or in clumps? The ecology literature suggests that trees in natural forest stands are usually clumped to some extent. Knowing the extent of clumpiness, or whether most of the trees of a certain species or size class are in one particular part of the stand, will help to prescribe the right management. 9. Direct measures of stand density - Direct measures, such as stand density index, relative density, and crown competition factor, can help to rank stands with regard to their need for thinning. Details About Variability Within the Stand It may be enough to know the average number of trees per acre by species and size class in a stand, but I would contend that knowing something about the variability may be quite important. Two stands, each with an average of 2,000 stems per acre, are quite different if the per acre values in one stand ranged from 1,000 to 3,000 stems per acre from plot to plot, and in the other stand the plot values ranged from 0 to 8,000 stems per acre. The second stand is patchy, or clumpy, and the cost and method of density control may be different than that used for the first stand. The lesson is that the inventory system should be carrying some measure of variability, such as the coefficient of variation or the standard error, for the attribute of interest. Some inventory systems, such as FPS and SIS, carry a "clumpiness index", which can be used to directly characterize the patchiness of the stand (see number 8 above). An alternative for carrying information about variability in the stand is for the inventory system to carry the cruise plot detail for each stand in the inventory, even as the stands are grown over time in the inventory system. The MBG Tools stand-based inventory system hangs onto the plot and tree 3
detail on all cruised plots in a stand as the stand is grown. This is particularly powerful if the cruise plots have been mapped, so that the forester can develop a picture of how the tree population is distributed across the stand. Details About the Data About 30 years ago I took a course in a newly emerging field known as "management information systems". One of the things that stuck with me from that course is the difference between data, and information. Simply put, information is data that has been transformed such that it can help us make a decision. For example, knowing that there are an average of 300 trees per acre is one thing ("data"); knowing that the 300 trees includes only trees greater than 5.0 inches dbh is something else - information. The inventory system being used to support density management planning and decision making should have details about the data, such as the following - 1. Are the estimates for any particular stand from an actual cruise in that stand? If so, what was the cruise design? Or are those estimates actually a stratum average for the stratum the stand belongs to? Or are those "inherited" data? Is the estimate actually based on an old cruise that has been grown to the present point in time? Knowing the source of the estimates allows us to gauge the reliability of the estimates. 2. What minimum dbh is being used for summary values in tables and reports? 3. What merchantability specs were used to generate estimates of volume? What taper system, scaling rule, log length, top diameter, etc.? 4. What base age is being used for the estimate of site index? Better yet, what is the reference for the site index value? 5. Is that total age or breast-height age? If it's total age, was there an assumption made about the number of years to reach breast height? Summary A good inventory system with regard to supporting stand-density management programs will be used to point foresters toward stands which are candidates for a treatment, and/or to feed a growth model to let the forester evaluate different silvicultural alternatives. In either case, the 4
system must carry details about the stand, about variability within the stand, and about the data, to really provide a solid foundation for density management. Presentation made in Canyonville, Oregon at the "Managing Stand Density: The Science Behind the Art" symposium, March 16, 2005. 5