Table 3.— Periodic mean"" annual diameter growth and periodic mean annual height growth for all species, western white pine. Douglas-fir, and western larch 

 at Priest River Experimental Forest 





All 



species 



Western white pine 



Douglas-fir 



Western 



larch 





Diameter 



Height 



Diameter 



Height 



Diameter 



Height 



Diameter 



Height 



Treatment^ 



0-53 5-10 



0-5 5-10 



0-5 5-10 



0-5 5-10 



0-5 5-10 



0-5 5-10 



0-5 5-10 



0-5 5-10 





— Inches — - 



- Feet 



— Inches — 



Feet 



— Inches — 



Feet 



— Inches — 



Feet 



1 



O.aea" 0.31a 



2.00a 2.13a 



0.46a 0.39a 



2.58a 3.06a 



0.40a 0.34a 



1.84a 1.95a 



0.23a 0.27a 



1.98a 2.05a 



2 



.37a .31a 



2.18b 2.05a 



.37a .37a 



2.68a 2.64a 



.42a .33a 



2.15b 1.88a 



.28a ,26a 



1.98a 2.19a 



'All means are least square means. Diameter growths are adjusted for diameter at beginning of the study and height growths are adjusted for height at the beginning of the 

 study. 



^Treatment 1 = no fertilizer, 2 = 200 lb N/acre. 



^Time since treatment in years. There are no significant differences (p s 0.05) in the means between time periods. 

 ■"Different letters following means indicate significant differences (p s 0.05) between treatments. 



between the periodic annual diameter growth means for 

 Douglas-fir for either the first or second 5 years after 

 treatment. 



TREE SIZE 



At DCEF, nitrogen fertilization did not significantly 

 influence tree size of the entire mixed stand (table 4). No 

 significant differences (p < 0.05) in either diameter or 

 height were detected among the treatments 10 years 

 after fertiUzation. The grand fir fertilized with 400 lb per 

 acre had a mean diameter 15 percent larger than the 

 unfertilized grand fir, and those fertilized with 200 lb of 

 nitrogen per acre were 12 percent larger than unfertilized 

 grand fir. But no differences in mean heights were found 

 between the treatments of grand fir at the end of 10 

 years. 



There were also no differences in mean heights or 

 mean diameters between the fertilized and unfertilized 

 trees for the entire stand at PREF (table 5) 10 years 

 after fertilization. Among the species at PREF, the only 

 significant difference in mean diameter 10 years after 

 treatment was for western white pine. In that case, the 

 unfertilized trees had a larger diameter mean than the 

 fertilized ones. No differences in mean heights or 

 diameters existed 10 years after treatment for either 

 western larch or Douglas-fir. 



DISCUSSION 



The application of nitrogen fertilizer in the form of 

 urea to young stands of mixed conifers growing on the 

 Tsuga heterophylla/Pachistima myrsinites habitat type 

 in northern Idaho does not appear to be that beneficial. 

 Growth responses were highly variable and short-lived. 

 For the most part, the only significant growth response 

 to the application of nitrogen fertihzer occurred on grand 

 fir at the DCEF study site. The results were even less 

 impressive at the PREF site, where only a moderate 

 height growth response to fertihzer was noted. In addi- 



tion, at PREF the fertilized western white pines were 

 growing more slowly than unfertHized trees. 



From these results, it appears that nitrogen was not 

 limiting at either location. Because both are good grow- 

 ing sites with deep soils and good organic reserves, the 

 sites probably have adequate nitrogen available. In addi- 

 tion, the stand cleaning that occurred prior to the fertili- 

 zation was done in a timely manner, resulting in a uni- 

 formly spaced stand of dominant and codominant trees, 

 each having adequate growing space. This probably 

 resulted in a microenvironment for each tree that had 

 sufficient nitrogen for good growth and development of 

 the species present. 



The exception to poor growth responses from applica- 

 tion of fertilizer was grand fir. At DCEF, grand fir 

 responded well, but with a duration of less than 10 

 years. Grand fir is a shade-tolerant, late-successional spe- 

 cies compared to the white pine and Douglas-fir that 

 also occupy these sites. Grand fir may be similar to 

 other late-successional species in that it can tolerate low 

 levels of son nitrogen, but may be more nitrogen 

 demanding during aggressive growth. The amplitude of 

 its growth response curve to nitrogen nutrition may be 

 high over a wide range of nitrogen levels. If grand fir 

 could become nitrogen demanding, it would increase its 

 growth rates when nitrogen fertilizer is applied under 

 good growing conditions. In contrast, a species such as 

 western white pine may have a growth response curve 

 with a low amphtude, making it tolerant to a narrow 

 range in levels of nitrogen. Therefore, on a good site, 

 western white pine would be less hkely to respond to fer- 

 tilizer than grand fir. 



Although this study adds to the literature on fertiliza- 

 tion of forest stands in the Northern Rocky Mountains, 

 it continues to show that a wide range of growth 

 responses can be expected. On good sites in northern 

 Idaho, appUcation of fertilizer to grand fir appears to be 

 of some value, but application to other species does not 

 appear to be warranted. Additional work on poor sites is 

 currently under way. 



4 



