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As the interface between plants and soil, the organic horizon is the foundation of forest ecosystems. Two potential predictors of O-horizon properties, vegetation and mineral soil type, are difficult to separate because they typically covary. We conducted a factorial study involving four canopy tree species and two soil types with different hydrology and topographic position to parse patterns in chemistry and microbiota of the O-horizon in a north-temperate deciduous forest. There were frequent strong effects of tree species. Organic horizon properties under white ash frequently differed from the other trees: e.g., lower cation exchange capacity and exchangeable acidity, thinner Oi horizon, lower %C and C:N, and, from phospholipid fatty acids, more AM fungi and less gram positive bacteria. These patterns, presumably due to species-specific attributes of leaf litter quality, root exudates, and microbial associations, must arise over decades, given that the forest stands that we studied were established only 85—100 years ago. We also found patterns in the O-horizon related to underlying soil type, independent of tree species: e.g., Bh podzols, compared to typical podzols, had higher trace metals, thicker Oa horizon, and more AM fungi. Relationships between mineral soil type and the organic horizon could arise because landscape features that influence hydrology and therefore soil formation over centuries also influence biogeochemistry of the organic horizon over decades. It could also involve bioturbation by organisms across horizons. There is basic and applied value in better understanding of properties of the O-horizon based on vegetation and soil types. 

Forest ecosystems are increasingly threatened by interacting biotic and abiotic stressors, yet how insect defoliation interacts with climate to influence tree growth remains poorly understood. In this study, we hypothesized that biogeography modulates the influence of defoliation on tree growth responses to climate, with two possible outcomes: defoliation may amplify climate sensitivity and increase vulnerability to stress, or alternatively, it may neutralize or even reverse typical climate–growth relationships. We compared pine forests in four biogeographic regions: two water-limited Mediterranean lowland regions in Portugal, and two temperature-limited regions, one in a Mediterranean mountain in Portugal, and the other in a humid subtropical forest in New Jersey (USA). Using dendroecological techniques, we reconstructed defoliation events, marking the first such reconstruction for Portuguese pine forests, and assessed growth responses to seasonal climate variables. In the absence of defoliation, tree growth aligned with the primary climatic limitation at each site, whether temperature or water availability. However, during defoliation events, these typical climate-growth relationships were disrupted. In temperature-limited regions, defoliation reversed the positive effects of warming and increased vulnerability to moisture stress. Conversely, in water-limited regions, defoliation reduced growth sensitivity to warming and did not increase drought vulnerability, suggesting a buffering effect. These results demonstrate that defoliation can decouple tree growth from climatic drivers, with biogeographically distinct outcomes shaped by underlying environmental constraints. Our findings emphasize the need to integrate biotic disturbances into models of forest climate sensitivity to improve predictions of forest resilience under global change. 

As the interface between plants and soil, the organic horizon is the foundation of forest ecosystems. Two potential predictors of O-layer properties, vegetation and mineral soil type, are difficult to separate because they typically covary. We conducted a factorial study involving four canopy tree species and two soil types with distinctly different hydrology and topographic position to parse patterns in chemistry and microbiota of the O-layer in a north-temperate deciduous forest. There were frequent strong effects of tree species. White ash frequently differed from the other trees: e.g., lower cation exchange capacity and exchangeable acidity, thinner Oi layer, lower %C and C:N, and, from phospholipid fatty acids, more AM fungi and less gram+ bacteria. These patterns, presumably due to species-specific attributes of leaf litter quality, root exudates, and microbial associations, must arise over decades, given that the stands in the study age between 85 and 100 years. We also found patterns in the O-layer related to underlying soil type, independent of tree species: e.g., Bh podzols, compared to Typical podzols, had higher trace metals, thicker Oa layer, and more AM fungi. Relations between mineral soil type and the organic layer, which were larger than expected, could arise because landscape features that influence hydrology and therefore soil formation over millennia also influence biogeochemistry of the organic layer over decades. It could also involve bioturbation by organisms across horizons. There is basic and applied value in models that can predict properties of the O-layer based on vegetation and soil types. 

Numbers and lengths of Lepidoptera larvae (caterpillars, all species) were censused on shrub level foliage at biweekly intervals from late May/early June through late July/early August each year. Measurements were conducted on the Main bird plot in the Hubbard Brook Experimental Forest and on three additional plots within the White Mountain National Forest from 1986-1997. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

Mycorrhizal fungi are important drivers of soil organic matter dynamics, but it can be difficult to isolate the effects of the fungi themselves from co-varying traits of their host trees. For example, many trees with an evergreen leaf habit associate with ectomycorrhizal (ECM) fungi, while many deciduous tree species associate with arbuscular mycorrhizal (AM) fungi. Because leaf habit influences the quantity and quality of organic matter inputs to soil, it is often an important factor in soil carbon and nitrogen dynamics, and thus can mask the effects of mycorrhizal fungi on soil organic matter processes. We evaluated how tree mycorrhizal associations and leaf habit separately influence the amount and composition of mineral-associated organic matter (MAOM) and particulate organic matter (POM) in forest soils in New Hampshire and Vermont, USA. We measured carbon (C) and nitrogen (N) concentrations and C:N ratios of three soil density fractions beneath six tree species that vary in mycorrhizal association and leaf habit. We found lower concentrations of MAOM C and N beneath evergreen vs. deciduous trees, but only for tree species associating with AM fungi. Further, MAOM C:N was higher beneath evergreen trees and beneath trees with ECM fungi rather than AM fungi. These results add to the growing body of support for mycorrhizal fungi as mediators of soil organic matter dynamics, suggesting that the MAOM fraction is more sensitive to leaf habit beneath AM-associated versus ECM-associated trees. Because MAOM decomposition is thought to be less responsive than POM decomposition to changes in soil temperature and moisture, differences in the tendency of AM- and ECM-dominated forests to support MAOM formation and persistence may lead to systematic differences in the response of these forest types to ongoing climate change. 

As the interface between plants and soil, the organic horizon is the foundation of forest ecosystems. Two potential predictors of O-layer properties, vegetation and mineral soil type, are difficult to separate because they typically covary. We conducted a factorial study involving four canopy tree species and two soil types with distinctly different hydrology and topographic position to parse patterns in chemistry and microbiota of the O-layer in a north-temperate deciduous forest. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station. 

Relations among territoriality, abundance and habitat suitability are fundamental to the ecology of many animal populations. Theory suggests two classes of possible responses to increasing abundance in territorial species: (1) the ideal free distribution (IFD), which predicts smaller territory sizes and decreased fitness as individuals adaptively pack into suitable habitats, and (2) the ideal despotic distribution (IDD), which predicts stable territory sizes and fitness in preferred habitats for dominant individuals and increased use of marginal habitats, reduced fitness and changes in territory sizes for subordinate individuals. We analysed the territory sizes and locations of seven migratory songbird species occupying a 10‐ha plot in the Hubbard Brook Experimental Forest, New Hampshire, USA over a 52‐year period. Species varied in abundance over years from twofold to 22‐fold, and all species displayed clear patterns of habitat preference within the study plot. Consistent with IFD, and contrary to IDD, territory sizes decreased with local abundance for all species, irrespective of habitat preferences. There was at least a twofold variation in territory size within years. Conformity of territory size to predictions of the IFD argues for the efficacy of territorial defence in songbirds and has general consequences for population dynamics. 

In this study, we analyzed territory sizes of seven migratory songbirds occupying a 10-hectare plot in the Hubbard Brook Experimental Forest, New Hampshire, USA over a 52-year period (1969-2021). All species varied in abundance over the duration of the study, some dramatically. Changes in territory sizes were inversely related to changes in abundance within the study plot despite differences in habitat preference, supporting the ideal free distribution. Territory sizes varied two-fold within a year across species. Results contribute to understanding how variation in territory size relates to 1) how habitat use changes with bird abundance and 2) the evolution of territory size. This dataset includes data, R code, and spatial files supporting this study. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station. Associated datasets in the data catalog: Holmes, R.T., N.L. Rodenhouse, and M.T. Hallworth. 2022. Bird Abundances at the Hubbard Brook Experimental Forest (1969-present) and on three replicate plots (1986-2000) in the White Mountain National Forest ver 8. Environmental Data Initiative. https://doi.org/10.6073/pasta/6422a72893616ce9020086de5a5714cd (Accessed 2023-12-17). Zammarelli, M.B. and R.T. Holmes. 2023. Hubbard Brook Experimental Forest: 10-ha bird plot territory maps, 1969 - 2021 ver 1. Environmental Data Initiative. https://doi.org/10.6073/pasta/df93595ba8df60570d472f6e6f58839e (Accessed 2024-01-11). 

In this study, we analyzed territory sizes of seven migratory songbirds occupying a 10-hectare plot in the Hubbard Brook Experimental Forest, New Hampshire, USA over a 52-year period (1969-2021). All species varied in abundance over the duration of the study, some dramatically. Changes in territory sizes were inversely related to changes in abundance within the study plot despite differences in habitat preference, supporting the ideal free distribution. Territory sizes varied two-fold within a year across species. Results contribute to understanding how variation in territory size relates to 1) how habitat use changes with bird abundance, 2) the evolution of territory size, and 3) the role of territoriality in population dynamics. This dataset includes data, R code, and spatial files supporting this study. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station. Associated datasets in the data catalog: Holmes, R.T., N.L. Rodenhouse, and M.T. Hallworth. 2022. Bird Abundances at the Hubbard Brook Experimental Forest (1969-present) and on three replicate plots (1986-2000) in the White Mountain National Forest ver 8. Environmental Data Initiative. https://doi.org/10.6073/pasta/6422a72893616ce9020086de5a5714cd (Accessed 2023-12-17). Zammarelli, M.B. and R.T. Holmes. 2023. Hubbard Brook Experimental Forest: 10-ha bird plot territory maps, 1969 - 2021 ver 1. Environmental Data Initiative. https://doi.org/10.6073/pasta/df93595ba8df60570d472f6e6f58839e (Accessed 2024-01-11).