Managing Second-Growth Forests in the Redwood Region for Accelerated Development of Marbled Murrelet Nesting Habitat
Jerry F. FranklinAndrew B. CareySteven P. CourtneyJohn M. MarzluffMartin G. RaphaelJohn C. TappeinerD. A. Thornburgh
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Witt Distribution of the marbled murrelet in southwestern Oregon. Northwest Science. 1998; 72(2): 96-101
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The marbled murrelet (Brachyramphus marmoratus) is a threatened seabird that nests in old-growth forests in the Pacific Northwest. Despite concern for this species, little has been published on murrelet nesting habitat in the coast redwood (Sequoia sempervirens) region. Here we present the first comprehensive study of marbled murrelet nesting characteristics in redwood forests based on confirmed nest sites. In this study, we 1) described habitat characteristics at 17 murrelet nest sites in the Santa Cruz Mountains, California, USA, located using radiotelemetry and visual searches, and 2) compared nest sites with random sites located in nearby stands and centered on trees ≥120 cm diameter at breast height (dbh [potential nest trees or PNTs]). All 17 nests were located in stands of old-growth redwood forest and the mean dbh of nest trees was 210 cm (SD = 91 cm). Eighty-two percent of nests (90% of telemetry-found nests) were in unharvested stands and 18%, all on private property, had been lightly harvested but did not contain significantly fewer trees ≥120 cm dbh than unharvested nest sites. Twelve of 15 (80%) nests for which we were able to locate the nesting platform were on limbs and the remaining 3 (20%), all in redwood trees, were located on broken tops. Nest trees were significantly larger than PNTs and tended to be Douglas-fir (Pseudotsuga menziesii) despite the fact that nest stands were dominated by redwoods, perhaps because of greater nest platform availability in Douglas-fir trees. Nest sites were located closer to streams, had a greater basal area of trees ≥120 cm dbh, and were located lower on slopes than random sites based on analysis of variance models. We classified 71% of nest sites correctly with a simple logistic regression model that included only nest tree dbh and distance to stream—a model that could be used by managers in the region to identify potentially suitable nesting habitat. Our findings indicated that murrelets in central California, USA, primarily use old-growth redwood stands for nesting but will use partially harvested stands if a significant residual component remains; stands that have experienced some harvest but retain old-growth characteristics should be considered potential murrelet habitat in redwood forests.
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We measured offshore Marbled Murrelet (Brachyramphus marmoratus) abundance from April through October between 1989 and 1998, in northern California and southern Oregon and investigated its relationships with marine and terrestrial habitats. We found that higher murrelet abundance offshore was strongly related to the presence of large, clustered and unfragmented old-growth forests on nearby inland areas. Murrelets were most abundant offshore of contiguous old-growth forest adjacent to relatively abundant medium-sized, second-growth coniferous forests. Compared to the forest habitat, marine habitat was relatively unimportant in determining murrelet abundance offshore; high marine primary productivity and nutrients were not associated with high murrelet numbers. Tidal flat shorelines were weakly associated with more murrelets, independent of inland habitat. Our findings suggest management efforts to conserve the Marbled Murrelet should focus on protecting or creating large, contiguous blocks of old-growth habitat, features which currently are rare in the study area.
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The Marbled Murrelet is legally listed as a Threatened species in Canada primarily due to loss of nesting habitat. The Marbled Murrelet Nesting Habitat Suitability Model for the British Columbia Coast is a model developed to predict the amount and distribution of Marbled Murrelet nesting habitat. This BC Model is an aggregate of predicted suitable habitat from a widescale algorithm using elevation, distance inland and mapped forest cover attributes of tree height and age that are important for nesting habitat as well as separate regional models in Haida Gwaii and Clayoquot Sound. It will be used to monitor progress towards meeting the Canadian Marbled Murrelet Recovery Team goal of maintaining at least 70% of the suitable nesting habitat that existed in 2002. The BC Model predicted approximately 2 million hectares of potential Marbled Murrelet nesting habitat. Twenty-six percent of the total habitat was in protected areas, which is equivalent to 37% of the habitat required to meet the recovery goal. The amount of nesting habitat predicted from the model algorithm was compared with aerial survey habitat predictions. There was no significant difference between the predictions from the two methods of identifying potential nesting habitat. The BC Model is recommended for broad-scale, strategic accounting of habitat, status assessments, and to aid in recovery and action planning.
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Identifying and managing nesting habitat for the threatened Marbled Murrelet (Brachyramphus marmoratus) is difficult because it nests secretively, high in the canopies of large, old conifers of the Pacific Northwest. In British Columbia, low-level surveying from a helicopter is now a recommended standard method of assessing forested landscapes for key microhabitat features—such as availability of potential platforms and developed moss pads for nests, foliage cover above the nest, and accessibility—that are not distinguishable on air photos, satellite images, or forest cover maps. Using a sample of 111 nest sites and 139 random sites within forests greater than 140 years old and distributed across three study areas in south coastal British Columbia, we confirmed the effectiveness of the aerial survey method for classifying overall habitat quality of murrelet nesting habitat. The minimum map units were 3-ha patches. Overall, 40% of the 111 nest sites were in patches classed as Very High, 36% were in High, 15% were in Moderate, 6% were in Low, and 3% were in Very Low. Our ranking of habitat quality was most strongly influenced by estimates of platform availability and moss development. Using an information-theoretic approach, we identified that the Resource Selection Function scores of nest patches improved as elevation decreased, slope grade increased, and the proportion of emergent and canopy trees with mossy pads increased. We also confirmed that study area location affected the strength of model application. Our findings support the potential utility of the low-level aerial survey method for identifying or confirming Marbled Murrelet nesting habitat for land-management purposes.
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