Hamer 



Chapter 17 



Inland Habitat Associations in Western Washington 



76 



101 126 151 176 201 226 251 

 TREE DIAMETER (CM) 



276 



Figure 3 The mean number of potential nest platforms/tree in relation to tree 

 diameter (25-cm intervals) for western hemlock, western red cedar, Douglas-fir, and 

 Sitka spruce trees in western Washington, 1 991 -93. Trees (n = 1 ,860) were sampled 

 from 151 stands. 





model always re-selected a platform variable to replace mean 

 tree diameter. Total platforms accounted for all the variation 

 of mean tree d.b.h., but mean tree d.b.h. could not account 

 for all the variation of total platforms. These results indicate 

 that the structure of a stand is more important in predicting 

 stand occupancy by murrelets than the size of the trees 

 within the stand. 



Mean Lichens The percent cover of lichens on tree 

 limbs was negatively correlated with the percent cover of 

 moss (r = -0.23). Some common moss species such as 

 Isothecium spp. require mild and wet conditions. These 

 conditions are usually found at lower elevations in the Sitka 

 Spruce and Western Hemlock Zones. Lichens such as Alectoria 

 spp. and Bryoria spp. are most abundant at higher elevations 

 where conditions are colder and dryer (Henderson and others 

 1989). These stands usually have a high percent composition 

 of silver fir and mountain hemlock, which are not known to 

 be used as nest trees by Marbled Murrelets in the Pacific 

 Northwest. Therefore, it was not surprising that lichen cover 

 was negatively related to the probability of occupancy. 



Stem Density (trees >81 cm d.b.h.) This variable was 

 not correlated to any other variable to any great degree except 

 basal area (0.63), but it can be assumed that in general, stands 

 with a higher stem density of trees >81 cm d.b.h. would have 

 a larger number of potential nest platforms/ha and higher 

 canopy closures. A larger sample of stands in Washington 

 with lower stem densities is needed to fully understand this 

 variable and its effect on the probability of occupancy. Occupied 

 stands with stem densities of only 5 trees/ha have been 

 documented in Oregon (Nelson, pers. comm.) 



Canopy Closure It may be difficult for murrelets to 

 locate and access nest platforms in stands with extremely 

 high canopy closures, and the results of the analysis may 

 reflect this because occupied old-growth stands still had 

 mean canopy closures of 86 percent. A larger sample of 

 stands with lower canopy closures is needed to fully 

 understand this variable and its effect on the probability of 

 occupancy. Nests located in stands with very low canopy 

 closures may be subject to higher predation rates since corvids 

 are the most common nest predator and locate prey almost 

 entirely by sight. Stands with low canopy closures and low 

 tree densities would be expected to have longer sight distances 

 through the canopy. In these cases, murrelet nests would be 

 easier to locate by visual predators. 



Mean Diameter of Western Hemlock Because the 

 majority of trees infected with mistletoe were western hemlock 

 and the mean d.b.h. of low-elevation trees was useful in 

 assessing suitable habitat, the mean d.b.h. of western hemlock 

 appears to combine the variation of these two factors into 

 one variable. 



Mistletoe Number Stands that are infested with mistletoe 

 may provide a higher number of nest platforms for murrelets. 

 Mistletoe infects the branches of living trees, causing swelling, 

 deformation, and brooming, which acts to thicken smaller 

 diameter branches. This process can create suitable nest 

 platforms from otherwise marginally-sized limbs. Thick 

 secondary branching is characteristic of these mistletoe brooms 

 that create dense overhead cover, a characteristic found at 

 many murrelet nest platforms in Washington and Oregon 

 (Hamer and Nelson, this volume b). In addition, mistletoe 



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USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



