1,721,034 research outputs found
Canopy Exchange and Modification of Nitrogen Fluxes in Forest Ecosystems
Full text linkPurpose of Review We provide an overview of the main processes occurring during the interactions between atmospheric nitrogen and forest canopies, by bringing together what we have learned in recent decades, identifying knowledge gaps, and how they can be addressed with future research thanks to new technologies and approaches.
Recent Findings There is mounting evidence that tree canopies retain a significant percentage of incoming atmospheric nitrogen, a process involving not only foliage, but also branches, microbes, and epiphytes (and their associated micro-environments). A number of studies have demonstrated that some of the retained nitrogen can be assimilated by foliage, but more studies are needed to better quantify its contribution to plant metabolism and how these fluxes vary across different forest types. By merging different approaches (e.g., next-generation sequence analyzes and stable isotopes, particularly oxygen isotope ratios) it is now possible to unveil the highly diverse microbial communities hidden in forest canopies and their ability to process atmospheric nitrogen through processes such as nitrification and nitrogen fixation. Future work should address the contribution of both foliar nitrogen uptake and biological transformations within forest canopies to whole ecosystem nitrogen cycling budgets.
Summary Scientists have studied for decades the role of forest canopies in altering nitrogen derived from atmospheric inputs before they reach the forest floor, showing that tree canopies are not just passive filters for precipitation water and dissolved nutrients. We now have the technological capability to go beyond an understanding of tree canopy itself to better elucidate its role as sink or source of nutrients, as well as the epiphytes and microbial communities hidden within them
Atmospheric nitrogen inputs, soil nitrogen cycling, and soil respiration across the greater Boston area
No full textThis dataverse repository contains data from May to November of 2014 at fifteen locations across Metropolitan Boston for (1) throughfall nitrogen, (2) fossil fuel carbon dioxide emissions, (3) human population density, (4) land cover class, (5) ISA, (6) soil solution nitrogen and soil nitrogen cycling rates (mineralization and nitrification) and (7) soil respiration. Details of the methodology are provided in the following publications.
Decina SM, PH Templer, LR Hutyra, CK Gately, P Rao. 2017. Variability, drivers, and effects of atmospheric nitrogen inputs across an urban area: emerging patterns among human activities, the atmosphere and soils. Science of the Total Environment 609:1524-1534.
https://doi.org/10.1016/j.scitotenv.2017.07.166
Decina S, LR Hutyra, CK Gately, JM Getson, AB Reinmann, AG Short Gianotti, and PH Templer. 2016. Soil respiration contributes significantly to urban carbon fluxes. Environmental Pollution 212:433-439.
https://doi.org/10.1016/j.envpol.2016.01.012<br
Atmospheric nitrogen inputs, soil nitrogen cycling, and soil respiration across the greater Boston area
No full textThis dataverse repository contains data from May to November of 2014 at fifteen locations across Metropolitan Boston for (1) throughfall nitrogen, (2) fossil fuel carbon dioxide emissions, (3) human population density, (4) land cover class, (5) ISA, (6) soil solution nitrogen and soil nitrogen cycling rates (mineralization and nitrification) and (7) soil respiration. Details of the methodology are provided in the following publications.
Decina SM, PH Templer, LR Hutyra, CK Gately, P Rao. 2017. Variability, drivers, and effects of atmospheric nitrogen inputs across an urban area: emerging patterns among human activities, the atmosphere and soils. Science of the Total Environment 609:1524-1534.
https://doi.org/10.1016/j.scitotenv.2017.07.166
Decina S, LR Hutyra, CK Gately, JM Getson, AB Reinmann, AG Short Gianotti, and PH Templer. 2016. Soil respiration contributes significantly to urban carbon fluxes. Environmental Pollution 212:433-439.
https://doi.org/10.1016/j.envpol.2016.01.012<br
Drivers of nitrogen oligotrophication in temperate deciduous forests
No full text2025Declining nitrogen (N) availability relative to plant demand, known as N oligotrophication, is a widespread phenomenon in rural terrestrial ecosystems around the globe, and has been particularly well documented in the temperate forests of the northeastern U.S. My dissertation research provides insight into the drivers of N oligotrophication across urban and rural temperate deciduous forests. First, I examined the combined effects of urbanization and forest fragmentation on mixed temperate forests by measuring soil N cycling rates and foliar N concentrations along urban to rural and edge to interior gradients over two years. To accomplish this work, I worked in eight forested sites along an urbanization gradient from Boston, MA in eastern MA to Harvard Forest in central MA. I found that urban forests had higher net rates of ammonification and mineralization, as well as higher foliar N concentrations than rural forests, but that these differences were canceled out at the forest edge. Together these results indicate that urban forests are experiencing less N oligotrophication than nearby rural forests, but that differences in N supply are diminished by forest fragmentation. In my next chapter, I carried out a snowpack manipulation experiment to accelerate or delay spring snowmelt in the White Mountains of New Hampshire to reveal how shifting snowmelt timing affects soil N cycling, fine root production, and N utilization by trees. I found that fine root biomass, soil solution nitrate (NO3-), and net nitrification were lower under shallower snowpack and earlier snowmelt compared to ambient or delayed snowmelt conditions. In Acer saccharum trees, foliar N as well as natural abundance 15N values, a key indicator of N supply relative to demand, were lower when snow melted early snowmelt under shallow snowpack conditions, suggesting that accelerated snowmelt induced by shrinking snowpack decreases N availability relative to plant demand in snow adapted forests. Finally, I used a controlled litter manipulation experiment in a northern hardwood forest at Hubbard Brook to isolate the effects of litter quality (C:N ratio) on soil N cycling rates and fine root production and test the hypothesis that low quality litter drives a positive feedback loop that ultimately reduces available N. I found that rates of net ammonification and mineralization were positively associated with litter N content and negatively associated with litter C:N, particularly after two years under low quality litter. Through my research, I demonstrated that forest fragmentation and changing seasonality decrease N cycling rates, available N pools, and N uptake by trees, thereby widening the gap between N supply and demand in temperate forest ecosystems.2026-03-05T00:00:00
Nutrient allocation and conservation mechanisms in trees: intraspecific variation, reproductive costs, and global scale comparisons
Full text linkNitrogen and phosphorus have individually or jointly been demonstrated to limit primary productivity in most of Earth’s forested systems. Nutrient limitation of forest primary productivity is important because terrestrial systems currently store large amounts of carbon and partially mitigate carbon dioxide emissions from anthropogenic activities. There is also evidence that nutrient availability relative to demand is decreasing in forested systems. Trees have complex responses to nutrient availability, including changes in allocation of nutrients to different organs and mechanisms that aide in recycling of nutrients within the plant and ecosystem. In this work I provide new insight related to nutrient allocation and conservation mechanisms in trees, demonstrating that these mechanisms affect nutrient limitation of primary productivity. In Chapter 2, I provide evidence that tree reproductive organs have nutrient resorption processes that transfer nutrients from fruit to seeds and I also demonstrate that tree fruit are capable of photosynthesis – in the absence of such processes the carbon and nutrient costs of tree reproduction would likely be higher. In Chapter 3, I report on the results of a community science project through which I identified variation in biogeochemically relevant leaf traits across much of the geographic distribution of Acer rubrum, one of North America’s most broadly distributed tree species, demonstrating that foliar nitrogen resorption is highest in colder high latitudes and leaf litter %N is highest at warmer low latitudes. In Chapter 4, I compare leaf and reproductive litterfall nitrogen and phosphorus metrics worldwide and demonstrate that reproductive litterfall is a significant contributor to tree nutrient budgets, comprising a median of 13.0% and 16.1% of nitrogen and phosphorus fluxes, respectively, when combining leaf and reproductive litterfall. Overall, the results of my dissertation enable me to identify several understudied aspects of tree nutrient allocation and conservation processes by considering the biogeochemistry of reproductive and foliar organs and associated variation across the natural distribution of trees
Effects of climate change across seasons on litterfall mass and chemistry in a northern hardwood forest
Full text linkNorthern hardwood forests are expected to experience an increase in mean annual air temperatures, and a decrease in winter snowpack and greater frequency of soil freeze/thaw cycles (FTCs) by the end of the century. As a result of these anticipated changes, northern hardwood forests in the northeastern U.S. will also have warmer soil temperatures in the growing season and colder soils in winter. Prior studies show that warmer soils in the growing season increase net primary productivity (NPP) and C storage as a result of increased soil net N mineralization, while increases in soil freezing in winter reduces plant uptake of N and C as a result of root damage. However, the combined effects of warmer soils in the growing season and increased soil freeze/thaw cycles in winter on tree litter mass and chemistry are unknown. We report here results from the Climate Change Across Seasons Experiment (CCASE) at Hubbard Brook Experimental Forest in New Hampshire, USA to characterize the response of leaf litter mass and chemistry to growing season warming combined with soil freeze–thaw cycles in winter. Across the years 2014-2017, litterfall mass and chemistry (%C, %N, C:N) were not significantly affected by changes in soil temperature; however, there was a trend of higher total litterfall mass and litter N mass from plots where soils were warmed in the growing season, but this increase disappeared with the addition of FTCs in winter. These results indicate that while rates of NPP and the total mass of N could be increased with rising soil temperatures over the next century in northern hardwood forests, the combination of warmer soils in the growing season and colder soils in winter may ultimate have little to no impact on litter mass or chemistry. We conclude that considering the combined effects of climate changes in the growing season and in winter is vital for the accurate determination of the response of litterfall mass and chemistry in northern hardwood forests
Effects of air quality, urbanization, and fragmentation on aboveground carbon storage of temperate forest ecosystems
Full text linkUrbanization has diminished intact forest cover worldwide, leaving behind fragmented forests. Temperate forest fragments encounter unique stressors and enhancements at forest edges, including increased temperature, light, and nitrogen (N) emissions. Anthropogenic additions of reactive N to the atmosphere and enhanced atmospheric deposition to the biosphere have dramatically altered the N cycle. While rates of N emissions and deposition have recently decreased in the form of nitrate in rural areas across the U.S., they remain elevated in urban centers. High rates of oxidized N (NOx) emissions to the atmosphere can negatively impact vegetation through the production of tropospheric ozone, which damages plant tissues and reduces the capacity for carbon (C) sequestration. Although elevated N deposition and tropospheric ozone have been studied individually, much less is known about their combined effects on C storage in trees, especially in urban ecosystems. To examine the combined effects of urbanization and air quality on C storage in vegetation, we selected seven sites along a 120 km rural to urban gradient across Massachusetts. At all sites, we established a 90 meter transect from forest edge to interior to evaluate within-site forest edge effects and urbanization effects across the entire gradient on C and N dynamics. To assess air quality, we measured concentrations of ozone and NOx using Ogawa passive samplers and measured N deposition using mixed ion exchange resin collectors under the forest canopy. To characterize standing C stocks, we measured tree diameter and scaled these values to aboveground biomass using allometric equations. Results demonstrate that standing biomass C is higher at the forest edge than interior in rural areas, but that urban areas do not have differences between edge and interior biomass. Concentrations of ozone and NOx are higher at urban than rural sites and at the forest edge compared to forest interior. Rates of total atmospheric N inputs in throughfall are not significantly greater in urban than rural sites, but nitrate inputs in throughfall at forest edges are higher in urban areas. Our study suggests that edge enhancements of biomass C are present in rural areas, but that diminished air quality may suppress potential stimulatory effects of forest edges in urban areas. This work builds upon our understanding of the quantities and spatial heterogeneity of air pollutants in the greater Boston area to better understand their consequences for tree health and the terrestrial carbon sink. As carbon dioxide concentrations continue to rise, plants will continue to play an important role in removing carbon dioxide from the atmosphere through photosynthesis and plant growth; investigating their response to additional environmental factors such as elevated N deposition and tropospheric ozone is essential to understanding vegetation and global carbon dynamics
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Water and carbon uptake and soil nitrogen cycling in a northern hardwood forest under a changing climate
Full text linkProjected changes in climate for the northeastern U.S. over the next century include increased air temperatures and reduced snowpack, leading to increased frequency of soil freeze/thaw cycles (FTC) in winter. Forests of the northeastern U.S. currently offset up to 35% of regional carbon (C) emissions and water uptake by trees in these forests constitutes the majority of evapotranspiration. In addition, nitrogen (N) is an essential element and often limiting nutrient for net primary production in temperate ecosystems, but recent declines in atmospheric deposition of N and changes in climate have led to concerns about N supply not meeting demands by temperate forest trees in the future. While much is known about the effects of climate change in winter and the growing season independently on water, C, and N cycling in temperate forests, little is known about the combined effects on these processes. In Chapter 2, I utilize a soil temperature manipulation experiment and demonstrate that rates of transpiration and leaf-level C uptake by Acer rubrum increase with rising growing season soil temperatures, but increased rates of C uptake are offset by increased frequency of FTCs, while increased transpiration rates are maintained. In Chapter 3, I demonstrate that net N mineralization and foliar N in trees are elevated with soil warming and not affected by FTCs. In Chapter 4, I show that trees access shallow water ( 90 cm depth) that has greater water potential in the late growing season. In Chapter 5, my synthesis of the published literature demonstrates that the majority of studies utilizing stable isotopes of water to determine water sources for vegetation occurred using no experimental manipulation, in forests and grasslands, and in arid climates. Overall, results of my dissertation demonstrate that biogeochemical cycling of C, N, and water are affected by projected changes in climate across seasons in ways that would not have been apparent from examining only one season alone.2022-02-03T00:00:00
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