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Ice storms are a type of extreme winter weather event common to north temperate and boreal forests worldwide. Recent climate modelling studies suggest that these storms may become more frequent and severe under a changing climate. Compared to other types of storm events, relatively little is known about the direct and indirect impacts of these storms on forests, as naturally occurring ice storms are inherently difficult to study. Here we describe a novel experimental approach used to create a suite of ice storms in a mature hardwood forest in New Hampshire, USA. The experiment included five ice storm intensities (0, 6.4, 12.7 and 19.1 mm radial ice accretion) applied in a single year, and one ice storm intensity (12.7 mm) applied in two consecutive years. Results demonstrate the feasibility of this approach for creating experimental ice storms, quantify the increase in fine and coarse woody debris mass and nutrients transferred from the forest canopy to the soil under the different icing conditions, and show an increase in the damage to the forest canopy with increasing icing that evolves over time. In this forest, little damage occurred below 6.4 mm radial ice accretion, moderate damage occurred with up to 12.7 mm of accretion, and significant branch breakage and canopy damage occurred with 19.1 mm of ice. The icing in consecutive years demonstrated an interactive effect of ice storm frequency and severity such that some branches damaged in the first year of icing appeared to remain in the canopy and then fall to the ground in the second year of icing. These results have implications for National Weather Service ice storm warning levels, as they provide a quantitative assessment of ice-load related inputs of forest debris that will be useful to municipalities creating response plans for current and future ice storms. [ABSTRACT FROM AUTHOR]

Ice storms are important but understudied disturbances that influence forest structure and function. In 1998, an ice storm damaged forest canopies and led to increased hydrologic losses of nitrogen (N) from the northern hardwood forest at the Hubbard Brook Experimental Forest (HBEF), a Long-Term Ecological Research (LTER) site in New Hampshire, USA. To evaluate the mechanisms underlying this response, we experimentally simulated ice storms with different frequencies and severities at the small plot scale. We took measurements of plant and soil variables before (2015) and after (2016, 2017) treatments using the same methods used in 1998 with a focus on hydrologic and gaseous losses of reactive N, as well as rates of soil N cycle processes. Nitrogen cycle responses to the treatments were insignificant and less marked than the responses to the 1998 natural ice storm. Pools and leaching of inorganic N, net and gross mineralization and nitrification and denitrification rates, and soil to atmosphere fluxes of nitrous oxide (N2O) were unaffected by the treatments, in contrast to the 1998 storm which caused marked increases in leaching and watershed export of inorganic N. The difference in response may be a manifestation of N oligotrophication that has occurred at the HBEF over the past 30 years. Results suggest that ecosystem response to disturbances, such as ice storms, is changing due to aspects of global environmental change, challenging our ability to understand and predict the effects of these events on ecosystem structure, function, and services. [ABSTRACT FROM AUTHOR]

Soils are the largest terrestrial pool of mercury (Hg), a neurotoxic pollutant. Pathways of Hg accumulation and loss in forest soils include throughfall, litterfall, soil gas fluxes, and leaching in soil solution, all of which will likely be altered under changing climate. We took advantage of three ongoing climate-change manipulation experiments at the Hubbard Brook Experimental Forest, New Hampshire, USA: a combined growing-season warming and winter freeze-thaw cycle experiment, a throughfall exclusion to mimic drought, and a simulated ice storm experiment to examine the response of the forest Hg cycle to climatic disturbances. Across these three experiments, we compared Hg inputs in throughfall and leaf litterfall and Hg outputs in soil gas fluxes. Soil solution was measured only in the simulated ice storm experiment. We found that northern forest soils retained consistently less Hg by 16–60% in the three climate manipulations compared to the undisturbed controls (~ 7.4 µg Hg m−2 year−1), although soils across all three experiments still served as a net sink for Hg. Growing-season soil warming and combined soil warming and winter freeze-thaw cycles had little effect on litterfall and throughfall flux, but they increased soil Hg0 evasion by 31 and 35%, respectively, relative to the control plots. The drought plots had 5% lower litterfall Hg flux, 50% lower throughfall Hg flux, and 21% lower soil Hg0 evasion than the control plots. The simulated ice storm had 23% higher litterfall Hg flux, 1% higher throughfall Hg flux, 37% higher soil Hg0 evasion, and 151% higher soil Hg leaching than the control plots. These observations suggest that climate changes such as warmer soils in the growing season or more intense ice storms in winter are likely to exacerbate Hg pollution by releasing Hg sequestered in forest soils via evasion and leaching. [ABSTRACT FROM AUTHOR]

While much research over the past 30 years has focused on the deleterious effects of excess N on forests and associated aquatic ecosystems, recent declines in atmospheric N deposition and unexplained declines in N export from these ecosystems have raised new concerns about N oligotrophication, limitations of forest productivity, and the capacity for forests to respond dynamically to disturbance and environmental change. Here we show multiple data streams from long-term ecological research at the Hubbard Brook Experimental Forest in New Hampshire, USA suggesting that N oligotrophication in forest soils is driven by increased carbon flow from the atmosphere through soils that stimulates microbial immobilization of N and decreases available N for plants. Decreased available N in soils can result in increased N resorption by trees, which reduces litterfall N input to soils, further limiting available N supply and leading to further declines in soil N availability. Moreover, N oligotrophication has been likely exacerbated by changes in climate that increase the length of the growing season and decrease production of available N by mineralization during both winter and spring. These results suggest a need to re-evaluate the nature and extent of N cycling in temperate forests and assess how changing conditions will influence forest ecosystem response to multiple, dynamic stresses of global environmental change. [ABSTRACT FROM AUTHOR]

The integrated forest ecosystem model ForSAFE-Veg was used to simulate soil processes and understory vegetation composition at three—sugar maple, beech, yellow birch—hardwood forest sites in the Northeastern United States (one at Hubbard Brook, NH, and two at Bear Brook, ME). Input data were pooled from a variety of sources and proved coherent and consistent. While the biogeochemical component ForSAFE was used with limited calibration, the ground vegetation composition module Veg was calibrated to field relevés. Evaluating different simulated ecosystem indicators (soil solution chemistry, tree biomass, ground vegetation composition) showed that the model performed comparably well regardless of the site's soil condition, climate, and amounts of nitrogen (N) and sulfur (S) deposition, with the exception of failing to capture tree biomass decline at Hubbard Brook. The model performed better when compared with annual observation than monthly data. The results support the assumption that the biogeochemical model ForSAFE can be used with limited calibration and provide reasonable confidence, while the vegetation community composition module Veg requires calibration if the individual plant species are of interest. The study welcomes recent advances in empirically explaining the responses of hardwood forests to nutrient imbalances and points to the need for more research. [ABSTRACT FROM AUTHOR]