Hamer 



Chapter 17 



Inland Habitat Associations in Western Washington 



inadequate to accurately reflect the true structure of the 

 stand. These potential problems could be avoided by 

 increasing the number of survey visits to a stand used to 

 determine occupancy and increasing the vegetation sampling 

 effort. More vegetation information from a larger number 

 of independent occupied and unoccupied stands needs to be 

 collected to validate the model. 



The results of the statistical model suggested that any 

 land management activity that reduced or affected the number 

 of potential nest platforms/ha, composition of low elevation 

 conifers, moss cover on tree limbs, stem density of dominant 

 trees, or canopy closure, would reduce the probability of 

 occupancy of old-growth, and thus the suitability of an old- 

 growth stand as nesting habitat for murrelets. Results from 

 studies of murrelet habitat use to date have been derived 

 from comparisons of stands occupied by murrelets to 

 unoccupied stands, comparisons of stands receiving high 

 use versus low use, or comparisons of nest trees and nest 

 plots to random trees and plots. Although these can provide 

 extremely useful descriptions and definitions of suitable 

 habitat, they do not provide information on the habitat 

 characteristics associated with successful nests. Information 

 on the landscape and within-stand habitat characteristics 

 that influence reproductive success is needed to fully 

 understand murrelet nesting ecology and to model optimum 

 habitat suitability for this species. Reproductive success should 

 be used as a measure of habitat suitability in future studies 

 by intensively studying occupied stands that have high 

 detection rates of Marbled Murrelets and locating a sample 

 of active nests to observe. A discussion of each variable used 

 by the model follows. 



Total Platforms Results suggest that if any variable 

 were to be used solely to assess habitat quality, total platforms 

 would be the best indicator. More potential nest platforms 

 within a stand mean more nesting and hiding opportunities 

 and a higher diversity of nest choices for the murrelet. Although 

 the total number of platforms was important, I currently have 

 few measures of platform quality. A examination of the limb 

 diameters of Marbled Murrelet nests indicated higher use and 

 possible selection for platforms >35 cm in diameter (Hamer 

 and Nelson, this volume b). Some stands may have an 

 abundance of smaller potential nest platforms that are only 

 10-20 cm in diameter. These stands may be marginal nesting 

 habitat because of the limitations of platform size. Future 

 studies should include a measure of mean platform size when 

 quantifying forest vegetation. 



The total number of potential nest platforms would be 

 especially important if nest platforms within a stand were 

 limited, the number of nesting stands available on the 

 landscape were limited, or intraspecific competition occurred 

 for nest platforms within a given area. It is unknown whether 

 platforms meeting all the requirements for nesting are 

 limited in availability in a typical old-growth stand. It has 

 been assumed that nest platforms may be unlimited in old- 

 growth stands (Sealy 1974), but an understanding of the 

 structural requirements needed for a platform to be used by 



murrelets is required before an analysis of platform 

 availability is possible. 



Total Moss The presence of moss in the tree canopy 

 was another important indicator of murrelet habitat. Although 

 murrelets do not absolutely require moss as a nest substrate, 

 the majority of nests have been located on moss (Hamer and 

 Nelson, this volume b); the presence of moss may increase 

 the number of potential platforms within a stand. Limbs with 

 little or no moss coverage result in nest locations close to the 

 trunk of a tree, which is usually the only area on a tree where 

 debris such as needles and duff collect in sufficient quantities 

 to form a thick substrate suitable for nesting, or where 

 branches are large enough in diameter to create suitable nest 

 platforms (pers. obs.). Other areas on the tree are usually too 

 exposed to wind and other environmental influences to collect 

 enough substrate to form a platform of suitable size. Thick 

 mistletoe blooms are sometimes the exception to this 

 observation. A high cover of moss creates a multitude of 

 nest platform choices by providing substrate on many locations 

 throughout a single limb, especially where there is suitable 

 overhead cover and the limb is large enough to support a 

 nest. In addition, the presence of a moss carpet essentially 

 thickens the diameter of limbs, transforming limbs of marginal 

 size into suitable nesting platforms. Moss is therefore related 

 to the number of potential nest platforms of a stand. It is not 

 known if one species of moss is preferred over others. 



Mean D.b.h. Although not selected by the final 

 regression model, mean tree d.b.h. had one of the highest 

 initial chi-square values (16.2) and the chi-square values 

 showed high stability through the selection process. The 

 mean number of platforms/tree increased rapidly with an 

 increase in tree diameter from 50 to 200 cm (fig. 3). No 

 increase in the mean number of platforms was evident for 

 larger trees that ranged from 220 to 300 cm in diameter. 

 Suitable platforms were most commonly found in stands 

 with larger tree sizes, as evidenced by a correlation of total 

 platforms to mean tree d.b.h. (r = 0.60), but the relationship 

 of these two variables was complex. The presence of larger 

 trees alone did not always explain the presence of nest 

 platforms. In Washington, there were abundant examples of 

 large trees >176 cm in diameter that contained no platforms. 

 Other factors that can create platforms may include wind 

 and insect damage, mistletoe brooms or other plant parasites, 

 moss or larger quantities of duff, multiple overlapping tree 

 limbs, natural limb deformities, and disease. Examples of 

 80-year-old stands of western hemlock that are heavily infested 

 with mistletoe and occupied by murrelets have been found in 

 Oregon (Nelson, pers. comm.). Therefore, total platforms 

 was the best indicator of suitable murrelet nesting habitat 

 because it directly measures the nesting structures required 

 by this alcid, whereas mean tree diameter measures the 

 availability of platforms indirectly and with less accuracy or 

 predictability. Still, most agencies and private timber 

 companies have measures of mean tree diameter available 

 for their stands, but no measures of platforms or structure. In 

 attempts to force the model to use mean tree diameter, the 



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



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