What is the Experimental Unit in Forestry – and – Why do we care!!!
by Dr. James D. Arney
In September 1972, I organized a two-day forest biometricians meeting in Olympia, Washington, to explore ways to transition traditional printed “yield tables” into computer-based, dynamic “growth models”. This had never been attempted previously. Twenty-five forest biometricians from across the country participated. No formal papers were presented. The entire meeting was impromptu and exploratory. (See full article about this meeting on FBRI website)
As a result of this workshop, in 1973 Don Monro presented a paper at the IUFRO Meeting in Vancouver, British Columbia, on potential structures and approaches to building “growth models”. His Labels used to identify types of growth models are still being used 46 years later (i.e., Whole-stand, Tree list-Distant independent, Tree list-Distant dependent).
The interesting point of this discussion is that “No one realized what had transitioned” as a result of those meetings in 1972 and 1973. When reading current forestry research articles, it is obvious that it has yet to be recognized. Don Monro recognized the transition in his IUFRO presentation when he separated “Whole stand” models from “Tree list” models. The Experimental Unit was transitioning from the individual “Plot” (stand) to the individual “Tree”!
Why is this important? In the early 1970s, the forest industry was beginning to recognize that stand structure, species composition and spatial patterns in forest stands had a significant impact on potential growth and yield outcomes. Log values were different by species and dimension. Most stands in the West were of mixed structure and species. Yield tables of single-species, even-aged whole stand parameters were no longer sufficient for stand-based forest planning.
Previous to 1973, field research trials were established using small plots with sufficient numbers of trees measured for Dbh and height to obtain a reliable set of stand parameters – #Trees, Basal Area, Quadratic Mean Dbh, Top Height, and Volume on a per acre basis. This most often resulted in a one-tenth acre fixed-area plot where all trees greater than about 5-inches were tallied and typically only about ten percent of the largest trees were measured for height.
As a result of the 1973 Olympia Workshop, most of us went away recognizing that a major shift in all aspects of Growth & Yield research was about to occur. The traditional objective to produce a printed yield table had ending. The new objective was to produce a tree list which would characterize a forest stand under any kind of silviculture and species composition.
The inputs to Yield Tables were strata averages of site, age, primary species and density. Stand-based inventories in the 1970s began to carry actual tree lists of cruised stands. These tree lists were the desired inputs to Growth Models in any attempt to forecast future growth and yield. (The categories of individual growth models, resulting from the 1973 workshop, are presented at the end of this article.)
The growth models developed by Arney (1981) and Curtis (1981) were the result of a USFS PNW Experiment Station and Weyerhaeuser Cooperative Agreement in 1973 – 1975. As part of that agreement, Arney had two four-person crews measure 2,500 permanent research plots on fourteen separate ownerships across Oregon, Washington and British Columbia. Every sample tree was stem-mapped and measured for both Dbh and height (no subsampled heights).
The reason for presenting this article is to take a critical look at how forestry research, development and application is occurring 45 years after what was learned in the 1973 workshop. Part of the emphasis for this review has been almost a tectonic shift away from single-species, even-aged forest management. This shift became prominent after the Clinton-induced Northwest Forest Plan in the mid 1990s, due to concern over “endangered species”.
While operational silviculture has shifted to mixed-species, mixed-structure, clumpy spatial pattern stands, the general approach in field studies for growth & yield research has not shifted from the pre-1973 methods for building yield tables (not growth models).
Based on this author’s first-hand observations, most publically-supported federal, state and university field research trials for silviculture, growth and yield analyses are under-designed. These trials are minimally suited to provide a basis for building or calibrating any robust growth model.
To build or calibrate a robust mixed-species, mixed-structure tree list growth model, the field research plots must be large (over ½ acre), all trees must be stem-mapped, and every tree must be tagged and measured for both Dbh and height at every re-measurement interval. These conditions are almost universally non-existent in forestry research.
A fundamental reason for the stalled condition of forestry research is the lack of understanding about what now constitutes the “Experimental Unit”.
It is appropriate to end with a few quotes from Albert Einstein’s journal:
To raise new questions, new possibilities, to regard old problems from a new angle, requires creative imagination and marks real advance in science.
Insanity: doing the same thing over and over again and expecting different results.
What is right is not always popular and what is popular is not always right.
 Don Monro. 1974. Forest Growth Models: A prognosis. IN: Growth Models for Tree and Stand Simulation. Royal College, Stockholm, Sweden. Forest Research Note 30.
For more about the Experimental Unit, contact Dr. Jim Arney by using the form below.