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  1. null (Ed.)
    Mercury is toxic to wildlife and humans, and forests are thought to be a globally important sink for gaseous elemental mercury (GEM) deposition from the atmosphere. Yet there are currently no annual GEM deposition measurements over rural forests. Here we present measurements of ecosystem–atmosphere GEM exchange using tower-based micrometeorological methods in a midlatitude hardwood forest. We measured an annual GEM deposition of 25.1 µg ⋅ m −2 (95% CI: 23.2 to 26.7 1 µg ⋅ m −2 ), which is five times larger than wet deposition of mercury from the atmosphere. Our observed annual GEM deposition accounts for 76% of total atmospheric mercury deposition and also is three times greater than litterfall mercury deposition, which has previously been used as a proxy measure for GEM deposition in forests. Plant GEM uptake is the dominant driver for ecosystem GEM deposition based on seasonal and diel dynamics that show the forest GEM sink to be largest during active vegetation growing periods and middays, analogous to photosynthetic carbon dioxide assimilation. Soils and litter on the forest floor are additional GEM sinks throughout the year. Our study suggests that mercury loading to this forest was underestimated by a factor of about two and that global forests may constitute a much larger global GEM sink than currently proposed. The larger than anticipated forest GEM sink may explain the high mercury loads observed in soils across rural forests, which impair water quality and aquatic biota via watershed Hg export. 
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  3. Foliage was collected in 2015 and 2017 from red maple trees at the Climate Change Across Seasons Experiment (CCASE) as part of the Hubbard Brook Ecosystem Study (HBES). Analyses of foliar metabolites include polyamines, amino acids, chlorophylls, carotenoids, soluble proteins, soluble inorganic elements, sugars, and total nitrogen and carbon. There are six (11 x 14m) plots in total in this study; two control (plots 1 and 2), two warmed 5 degrees (°) Celsius (C) above ambient throughout the growing season (plots 3 and 4), and two warmed 5 °C in the growing season, with snow removal during the winter to induce soil freezing and then warmed with buried heating cables to create a subsequent thaw (plots 5 and 6). Each soil freeze/thaw cycle includes 72 hours of soil freezing followed by 72 hours of thaw. Four kilometers (km) of heating cable are buried in the soil to warm these four plots. Together, these treatments led to warmer growing season soil temperatures and an increased frequency of soil freeze-thaw cycles (FTCs) in winter. Our goal was to determine how these changes in soil temperature affect foliar nitrogen (N) and carbon metabolism of red maple trees. These data were gathered as a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station. 
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    Abstract The expansion of an urban tree canopy is a commonly proposed nature-based solution to combat excess urban heat. The influence trees have on urban climates via shading is driven by the morphological characteristics of trees, whereas tree transpiration is predominantly a physiological process dependent on environmental conditions and the built environment. The heterogeneous nature of urban landscapes, unique tree species assemblages, and land management decisions make it difficult to predict the magnitude and direction of cooling by transpiration. In the present article, we synthesize the emerging literature on the mechanistic controls on urban tree transpiration. We present a case study that illustrates the relationship between transpiration (using sap flow data) and urban temperatures. We examine the potential feedbacks among urban canopy, the built environment, and climate with a focus on extreme heat events. Finally, we present modeled data demonstrating the influence of transpiration on temperatures with shifts in canopy extent and irrigation during a heat wave. 
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  8. Abstract

    Mean annual air temperatures are projected to increase, while the winter snowpack is expected to shrink in depth and duration for many mid‐ and high‐latitude temperate forest ecosystems over the next several decades. Together, these changes will lead to warmer growing season soil temperatures and an increased frequency of soil freeze–thaw cycles (FTCs) in winter. We took advantage of the Climate Change Across Seasons Experiment (CCASE) at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire, USA, to determine how these changes in soil temperature affect foliar nitrogen (N) and carbon metabolism of red maple (Acer rubrum) trees in 2015 and 2017. Earlier work from this study revealed a similar increase in foliar N concentrations with growing season soil warming, with or without the occurrence of soil FTCs in winter. However, these changes in soil warming could differentially affect the availability of cellular nutrients, concentrations of primary and secondary metabolites, and the rates of photosynthesis that are all responsive to climate change. We found that foliar concentrations of phosphorus (P), potassium (K), N, spermine (a polyamine), amino acids (alanine, histidine, and phenylalanine), chlorophyll, carotenoids, sucrose, and rates of photosynthesis increased with growing season soil warming. Despite similar concentrations of foliar N with soil warming with and without soil FTCs in winter, winter soil FTCs affected other foliar metabolic responses. The combination of growing season soil warming and winter soil FTCs led to increased concentrations of two polyamines (putrescine and spermine) and amino acids (alanine, proline, aspartic acid, γ‐aminobutyric acid, valine, leucine, and isoleucine). Treatment‐specific metabolic changes indicated that while responses to growing season warming were more connected to their role as growth modulators, soil warming + FTC treatment‐related effects revealed their dual role in growth and stress tolerance. Together, the results of this study demonstrate that growing season soil warming has multiple positive effects on foliar N and cellular metabolism in trees and that some of these foliar responses are further modified by the addition of stress from winter soil FTCs.

     
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