Dr. R.H. Waring has suggested that the Pinus ponderosa site measured at Metolius (OTTER Project Site 5) is not typical of this area, since wind-blown trees had previously fallen and the land owners had subsequently removed all large trees. At a later date, Dr. B.E. Law re-measured a site 13 km north of OTTER Site 5, close to where the original 1976-1977 study took place (Metolius Research Natural Area; old-growth and younger trees aged 45 years; Gholz, 1982). Using a ceptometer and a plant canopy analyser, Dr. Law found that the mean projected leaf area index (LAI, summer maximum) was 1.6, and suggested an equilibrium LAI for P. ponderosa closer to 1.5 than to the 0.7-0.8 values recorded at the recently logged OTTER Site 5. The higher value is close to what Goward et al. (1994) indicated would be a reasonable value for ponderosa pine in the Metolius drainage (see Fig. 10, p. 333 of this reference). Likewise, Pierce et al. (1994) report LAI values based upon those estimated by Dr. H.L. Gholz (Gholz, 1982), using allometric equations relating tree diameter at breast height (DBH) to leaf mass. Estimates of net primary productivity (NPP) for the old-growth ponderosa pine site are due for publication in 1999 (Law, B.E., M.G. Ryan and P.M. Anthoni, Global Change Biology, 1999 in press). Using both a ceptometer and a plant canopy analyser about 5 km east of OTTER Site 6 (Juniper), Dr. Law later re-measured a projected LAI of trees of 0.4 for Juniperus occidentalis (in agreement with Pierce et al. 1994).

Future users of these data who might want to test remote sensing methods (e.g. Spanner et al. 1994; Law and Waring, 1994) should note that the estimates of LAI, as well as estimates of intercepted photosynthetically active radiation (IPAR), are mostly for the tree canopy only. However, LAI of understory shrubs was substantial at both OTTER Site 5 (Metolius) and Site 6 (Juniper).

Dr. Waring has also pointed out that the original estimates made by Gholz (1982) were for all-sided LAI, and need to be divided by about 2.5 to estimate one-sided LAI for needle leaves. Although Gholz's DBH/foliage mass allometric equations were considered state-of-the-art at the time, with the benefit of hindsight they are now thought to over-estimate LAI in some, if not all, situations. Dr. Gholz has commented that the selection of undamaged trees, or trees from relatively open stands, may have led to some degree of over-estimation of LAI for Douglas fir (Pseudotsuga menziesii), and that other discrepancies may arise from the fact that the different studies did not necessarily use identical study sites. Certainly the later studies of LAI in the OTTER Project area points to lower values than were obtained by the initial work by Grier, Running, Gholz and others - at least at the high end of the LAI range.

For example, by comparing estimates from sapwood basal area, leaf litterfall, and light penetration through old-growth Douglas fir forest in the H.J. Andrews Experimental Forest (to the southwest of both OTTER Site 1 [Cascade Head] and Site 2 [Waring's Woods]), Marshall and Waring (1986) showed an average LAI close to 8.0, whereas the original Gholz equations provided estimates close to 16.0 (converted to projected, one-sided LAI). However, Dr. Gholz has pointed out that LAI estimates based on light penetration tend to lose sensitivity at higher LAI, being at the "tail" of a negative exponential relationship.