The thermal environment of northern spotted owls in northwestern California: possible explanations for use of interior old growth and coastal early successional stage forest

Spotted Owls (Strix occidentalis) appear heat intolerant. One of the hypotheses to explain the close association of Spotted Owls with old-growth coniferous forests is that the multilayered and complex structure of these forests provides a cooler microclimate for roosting owls than early seral-stage forests. Therefore, I compared temperatures at summer roosts of 22 Northern Spotted Owls (S. o. caurina) with temperatures at random sites within stands where owls roosted and at random sites within adjacent younger forests on the Willow Creek Study Area (WCSA) in the interior of northwestern California. Temperatures at actual roost sites within mature/old-growth forests were significantly lower than those at random sites within adjacent younger stands. Temperatures at random sites within owl roost stands were intermediate between roost site temperatures and those at random sites within younger forests. Spotted Owls selected cooler microclimates each time they changed their roost when ambient temperatures were above their upper critical temperature of 25.2°C. In an effort to explain the use of second-growth forests by Spotted Owls in the coastal redwood zone of northwestern California, I also examined temperature differences and temperature profiles between microsites in mature/old-growth and younger forests at both WCSA and Redwood National Park (RNP). Forests, including mature/old-growth and younger forests, at RNP were much cooler than those at WCSA. Due to the marine influence on summer climate, there was little variation in temperature profiles between mature/old-growth and younger forests at RNP. In addition, mature/old-growth forests at WCSA had a cooler microclimate and a less variable temperature profile than younger forests at WCSA. My study demonstrated microclimate selection in Spotted Owls, and the importance of multilayered stand structure in creating cool microclimates and stable thermal environments from the forest floor to canopy level. Finally, in comparison with the interior, the much cooler summer climate at the coast may account for the relaxed Spotted Owl roost selection in early successional stage forests.

Thesis (M.S.)--Humboldt State University, Wildlife Management, 1998