results. McCaughey and Schmidt (1982) demonstrated 

 that taller (older) Engelmann spruce and subalpine fir 

 responded slower to release than shorter (younger) trees. 

 Taller suppressed grand fir did not release as well as 

 shorter trees of equal suppression, and younger trees 

 released better than older ones (Ferguson and Adams 

 1980). Initial diameter in our study was important only 

 in the Douglas-fir model, where larger-diameter trees 

 grew faster than the smaller ones. 



Tree age was not used as an independent variable in 

 our study. Unfortunately, much of the age data taken in 

 1974 was lost and sample size of trees for which ages were 

 available was too small for age to be of use in the analy- 

 ses. Age was an important variable in several previous 

 studies on release of understory (Ferguson and Adams 

 1980; McCaughey and Schmidt 1982), and we regret the 

 omission of this variable in our analyses. Plot trees will 

 be remeasured in 1994 when we plan to destructively 

 sample, so age will be determined then and included in 

 subsequent analyses. We did not extract increment cores 

 or disks during the 1985 remeasurement because we did 

 not want to jeopardize future growth of the plot trees. 



Trees usually do not respond to reduced competition for 

 3 or 4 years after treatment. Because we used height and 

 diameter growth as dependent variables over the entire 

 period since cutting, the responses may be conservative. 

 Perhaps we should have used only the most current 5- 

 year height and diameter growth as dependent variables. 

 Such data were not taken during this measurement but 

 will be in 1994. Thus, we expect that the future models 

 will be improvements over the current ones. 



Even though most studies on release of advance regen- 

 eration report significant increases in height and diame- 

 ter growth following harvest, the absolute growth is 

 small — usually less than 0.5 foot per year for height and 

 less than 0.2 inch per year for diameter growth during the 

 release period even on productive sites. We looked at 

 results from several different studies done in the Western 

 United States (table 8). (See appendix for notes on how 

 table 8 was derived.) Data selected for inclusion in table 8 

 represent trees of moderate to good vigor, with crown 

 ratios greater than 50 percent — in other words, the better 

 trees left after harvest. In comparison, good-vigor west- 

 em larch under even-aged management on average sites 

 will grow about 1 foot in height and 0.28 inch per year in 

 diameter during the first 20 years of growth (Schmidt and 

 others 1976). 



Predictive value of most models dealing with posthar- 

 vest release of residual trees is relatively low, even 

 though the models are statistically significant (table 8). 

 Our models are no exception; coefficients of multiple de- 

 termination for height growth ranged from 0.15 for Engel- 

 mann spruce and 0.20 for Douglas-fir to 0.27 for subalpine 

 fir. CD's for diameter growth were higher, ranging from 

 0.46 for Engelmann spruce and 0.50 for subalpine fir to 

 0.53 for Douglas-fir in our studies. Similarly, predictive 

 value was not very strong for most of the models devel- 

 oped from other studies on advance regeneration. Coeffi- 

 cients of determination developed by Seidel (1980) ranged 

 from 23.0 to 44.1 for diameter growth and 17.5 to 44.2 for 

 height growth; he did not consider them reliable. Oliver's 



Table 8 — Height and diameter growth of advance regeneration following harvest, according to several western North American 

 studies 



Study/location 



Species 



Mean annual height growth 



Preharvest 



Postharvest 



Mean annual diameter growth 



Preharvest 



Postharvest 



Current/ 



NW Montana 



Subalpine fir 

 Engelmann spruce 

 Douglas-fir 



Feef- 



0.10 

 .16 

 .12 



0.45 

 .16 

 .12 



Inches- 



0.04 

 .04 

 .04 



0.04 

 .15 

 .09 



Johnstone (1978)/ 

 Alberta, Canada 



Subalpine fir 

 Spruce 



.20 

 .22 



.52 

 .40 



NA 

 NA 



NA 

 NA 



Seidel (1980)/Oregon Grand fir 



Ferguson and Adams Grand fir 



(1980)/ldaho 



.21 

 .13 



.18 

 .42 



.05 



NA 



.15 



NA 



McCaughey and Schmidt 

 (1982)/Utah 



Engelmann spruce .10 

 Subalpine fir .17 



.50 

 .58 



NA 

 NA 



NA 

 NA 



Helms and Standiford 

 (1985)/Califomia 



White fir 



.46 



.82 



NA 



.24 



Oliver (1986)/Califomia 



Red fir 



.46 



.29 



NA 



.15 



10 



