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Landbirds were censused "in the Tanana River floodplain during the summers of 1995 and 1996. Two 10.5 hectare bird census plots were placed in willow-alder, white spruce, and black spruce forest. We used spot mapping data from these plots to learn about bird habitat associations at two spatial scales. At the scale of habitat type, we investigated the relative influences of structural complexity of vegetation, stage of succession, and primary productivity on the breeding bird community. Bird species richness and territory density in the three habitat types were highly positively correlated with primary productivity. At the spatial scale of an individual territory, detailed vegetation measurements recorded within 42 subplots on each census plot were used to formulate logistic regression models. These models describe habitat characteristics associated with territory presence for 11 bird species that were most common. Vegetation structure and plant species composition were important determinants of territory presence"--Iii.
The boreal forest is the northern-most woodland biome, whose natural history is rooted in the influence of low temperature and high-latitude. Alaska's boreal forest is now warming as rapidly as the rest of Earth, providing an unprecedented look at how this cold-adapted, fire-prone forest adjusts to change. This volume synthesizes current understanding of the ecology of Alaska's boreal forests and describes their unique features in the context of circumpolar and global patterns. It tells how fire and climate contributed to the biome's current dynamics. As climate warms and permafrost (permanently frozen ground) thaws, the boreal forest may be on the cusp of a major change in state. The editors have gathered a remarkable set of contributors to discuss this swift environmental and biotic transformation. Their chapters cover the properties of the forest, the changes it is undergoing, and the challenges these alterations present to boreal forest managers. In the first section, the reader can absorb the geographic and historical context for understanding the boreal forest. The book then delves into the dynamics of plant and animal communities inhabiting this forest, and the biogeochemical processes that link these organisms. In the last section the authors explore landscape phenomena that operate at larger temporal and spatial scales and integrates the processes described in earlier sections. Much of the research on which this book is based results from the Bonanza Creek Long-Term Ecological Research Program. Here is a synthesis of the substantial literature on Alaska's boreal forest that should be accessible to professional ecologists, students, and the interested public.
Primary productivity and animal species richness are associated across spatial scales and extents. The productivity hypothesis had been credited for explaining these correlations by asserting that places with more energy fixed in the system by plants should support more individuals, whereby the community accumulates more species. Many have interpreted the spatial association between richness and productivity as support for the hypothesis. If true, bird richness should track productivity temporally, and there should be spatial and temporal relationships between productivity and both bird abundance and richness.We tested these predictions in the breeding season and winter across space and time. Using a remotely-sensed primary productivity proxy (NDVI) and avian data from two large North American citizen science surveys, the Breeding Bird Survey (BBS) and Christmas Bird Count (CBC), we evaluated the response of avian richness and abundance to interannual changes in plant productivity across 25 years. In the breeding season we found positive spatial relationships between richness and NDVI each year, but when evaluated temporally no support for the hypothesis was found. Richness and NDVI were positively associated at only half (49.3%) of BBS sites (mean r2 = 0.09). Further, total abundance and productivity were unrelated.Despite summer findings, it is possible that richness depends more on productivity in winter when birds are stressed by harsh conditions. Using CBC data we found that across several thousand communities the number of individuals was not spatially associated with productivity, providing no support for the productivity hypothesis.In these and earlier tests of the hypothesis, trait-neutral partitioning of resources across individuals is assumed, though it may be more realistic that partitioning is in unequal shares. To test for energetic trait differences, we regressed total avian biomass at sites against NDVI and found that avian biomass is independent of productivity.We conclude that primary productivity is unlikely the driver of bird diversity. Spatial relationships between productivity and bird richness may be spuriously arising via covariance between productivity and vegetation structural complexity, and the latter may be driving bird communities, consistent with the MacArthurs' classic hypothesis that the vertical profile of foliage drives bird species diversity.
"A combination of both the horizontal and vertical approaches to the study of avian succession was applied over a 20-year period in research on the breeding birds of fallow farmland in Long Island, New York. Eight fields, representing stages in secondary plant succession from bare soil to a field 45 years old, varied in size from 0.41 to 1.88 hectares. Botanical data on average cover of herbaceous species and upright woody species, and on the density of trees by height class and by diameter characterize the vegetation of each study area. Breeding bird censuses were counts of all individuals, not indices to or estimates of population size. Particular attention was given to nest location and construction and to light readings taken at the nests to provide an index of relative nest cover. The sequence in which bird species became established in these fields was: red-winged blackbird, song sparrow, field sparrow, indigo bunting, common yellowthroat, blue-winged warbler, gray catbird, brown thrasher, and rufous-sided towhee. This is the same sequence of species derived by an arrangement in order of increasing cover at the nest. Likewise, this is the order in which these same species disappear from the continuum of communities that characterizes old field succession on Long Island. The concordance among these three sequences suggests that species-specific requirements with respect to cover at the nest plays an important role in determining which stages of old field succession a species finds attractive for nesting. There was a rapid increase in species diversity with an increase in age of the fields, and a leveling-off in numbers of territorial species from about 15 years (open shrubland) to 40 years (dense shrubby woodland) after cultivation. A more mature oak woodland in the same region (60 years after clear-cutting) had a greater species diversity than that of any earlier stage of old field succession. This progressive increase in species diversity with succession from herbaceous fields to mature forest is consistent with the findings of others. The density of breeding birds continued to increase beyond that age at which species diversity began to level off (about 15 years after cultivation) and did not reach peak density until another 15 years had elapsed. Density within the more mature oak woodland was significantly less than that of the 30- to 40-year old shrubby woodland. Whether or not there is a decline in density of breeding birds as succession approaches climax probably is determined by the relative availability of moisture. Other investigators have confirmed that in the more xeric successions, such as described in this study, a decline in density is the rule. In the more mesic successions, density is higher in the forest than in the intermediate shrubland stage. Species diversity and density were significantly lower in study areas where succession was arrested than in those permitted to revegetate naturally"--P. 5.