Graduate Thesis Or Dissertation
 

Bird-vegetation relationships across ten years after thinning in young thinned and unthinned Douglas-fir forests

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  • Bird-vegetation associations are a base for bird conservation and management, as well as for predictions of the effects of resource management and climate change on wildlife populations. A recent shift in forest management priorities from timber production to native species' habitat conservation on federal lands has emphasized the need to understand the mechanisms underlying the effects of vegetation management on wildlife. The assumption of strong bird-vegetation relationships is rarely tested for forest birds, especially at large temporal extents, which are more likely to reveal instabilities in bird-vegetation relationships than short-term studies. This study aimed to quantify bird-vegetation relationships and investigate their strength in young thinned and unthinned Douglas-fir forest stands over ten to years post thinning. Additionally, this study investigated whether disturbance associated with forest thinning decoupled bird-vegetation relationships in the thinned and unthinned stands. I used abundance or occurrence data for eight bird species collected at 58 point count surveys, conducted during six breeding seasons over ten years following forest thinning. I obtained detailed local-scale vegetation characteristics associated with bird sampling points and modeled bird occurrence or abundance as a function of vegetation characteristics. Vegetation characteristics explaining individual species occurrence or abundance varied among species and among years for any given species. Six out of eight species showed responses to examined vegetation characteristics. For three out of six species, the effects of vegetation characteristics on bird occurrence or abundance remained consistently positive or negative over time. For the other three species the absolute effect of vegetation decreased over time to that of not statistically different from zero. The estimates of vegetation effects on bird response varied in size among years, though they were not statistically different among years. Magnitude of vegetation effect on bird occurrence or abundance did not increase with time, nor was it related to species prevalence or abundance. I found evidence of a response threshold for one species, Swainson's thrush. I suggest that changing abundance of resources, associated with thinning disturbance, demographic stochasticity associated with small population sizes, as well as large-scale demographic processes and possibly life history traits of examined species, mediate the strength of local-level bird-vegetation associations. Variability of vegetation effects on bird occurrence or abundance over time suggests a greater uncertainty of results of vegetation-related wildlife management efforts than has previously been suggested by short-term studies. Therefore, incorporation of the uncertainty of bird-vegetation relationships into predictive models and continued long-term monitoring of species response to management may be crucial for future successful and effective management decisions.
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