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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.
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This content will become publicly available on June 16, 2026
Changes in soil moisture and its relationships with nitrogen cycle processes in a northern hardwood forest
Identifying which aspects of global environmental change are driving observed ecosystem process responses is a great challenge. Here, we address how long-term (10-25 year) alterations in soil moisture, and nitrogen (N) oligotrophication (i.e. decreases in soil N availability relative to plant demand), alter the production of plant-available N via net mineralization and nitrification in a northern hardwood forest. Our objectives were to determine whether soil moisture has changed over the past decade and whether N cycle processes have become less sensitive to soil moisture over time due to N oligotrophication. We used long-term data sets from several related studies to show: (i) increasing winter soil temperatures and declining summer soil moisture from late 2010 into 2024; (ii) reductions in sensitivity of N cycling rates to soil moisture, and (iii) declining moisture-adjusted N cycle processes (the ratio of rate of N process:soil moisture) over time in both summer and winter. These changes suggest continued reductions in N availability to plants in these forests, with potential effects on forest productivity and response to disturbance.
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- PAR ID:
- 10608034
- Publisher / Repository:
- Canadian Science Publishing
- Date Published:
- Journal Name:
- Canadian Journal of Forest Research
- ISSN:
- 0045-5067
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Successfully mitigating and adapting to declining N availability will require a broader understanding that this phenomenon is occurring alongside the more widely recognized issue of anthropogenic eutrophication. Intercalibration of isotopic records from leaves, tree rings, and lake sediments suggests that N availability in many terrestrial ecosystems has steadily declined since the beginning of the industrial era. Reductions in N availability may affect many aspects of ecosystem functioning, including carbon sequestration and herbivore nutrition. Shaded areas indicate 80% prediction intervals; marker size is proportional to the number of measurements in each annual mean. Isotope data: (tree ring) K. K. McLauchlan et al. , Sci. Rep. 7 , 7856 (2017); (lake sediment) G. W. Holtgrieve et al. , Science 334 , 1545–1548 (2011); (foliar) J. M. Craine et al. , Nat. Ecol. Evol. 2 , 1735–1744 (2018)more » « less
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The goal of this project is to test the overarching hypothesis that positive feedback mechanisms involving changes in seasonal cycles that diminish N availability to plants such that plant N demand is not met by soil N availability in northern forests. Specifically, we hypothesize that increasing N demand by plants (induced by increasing temperatures, longer growing seasons, and other environmental changes) leads to greater N resorption by trees in autumn, increased C:N in litter, and greater net immobilization of N by soil microbes in the following spring. However, the timing of snowmelt and soil freezing in spring may further affect net mineralization and N availability for plants. These hypotheses are being tested with a combination of observational, experimental, and modeling approaches at Hubbard Brook Experimental Forest in New Hampshire: 1) measurements at 14 previously established sites along an elevation/aspect climate gradient; 2) litter and snow manipulation experiments at six sites along the climate gradient to create variation in soil climate conditions and microbial N immobilization during spring. We leveraged 14 sites previously established along an elevation and aspect-driven climate gradient at Hubbard Brook as a “natural climate experiment" to test our hypothesis that a positive feedback between N cycling during fall senescence and spring contributes to declining N availability in northern forests. This elevation gradient encompasses variation in mean annual air temperature of ~2.5 °C that is similar to the change projected to occur with climate change over the next 50–100 years in the northeastern U.S. There is relatively little variation in soils along the gradient. We are utilizing three sites at higher elevation (~550-660 m, north facing) and three sites at lower elevation (~375-500 m, south facing) for the litter and snow manipulation experiments to maximize the differences in temperature among the 14 sites. Litterbox manipulation: The objective of the litterfall manipulation experiment is to determine whether increases in autumn litter C:N ratios contribute to greater N immobilization by microbes and reductions in net mineralization and plant N uptake in spring, and ultimately, N oligotrophication in northern forest ecosystems. We applied early (low C:N litter that is lost from from hardwood foliage in the first two weeks of autumn) and late (high C:N litter that falls in the last two weeks of autumn) season litter in October 2022 that was collected in fall 2021 at rates equal to standing mass of litter (300 g m2). We also applied native litter that was collected from the forest floor of each intensive site to represent background levels of C:N in litter samples. This litter was applied to one litterbox at each of the six intensive sites. Following application of litter, we installed deer netting around and on top of each of the litterboxes to eliminate litter loss from wind. Soil samples were collected from these plots in November 2021, April 2022, May 2022, June 2022, November 2022, April 2023, May 2023, June 2023 and anlalysed for microbial biomass and activity, as described in the methods section. Snow manipulation: The objective of the snow manipulation experiment is to determine whether the timing of spring snowmelt, length of the spring, and soil freezing in spring affect microbial N immobilization, hydrologic losses, net mineralization, and plant N uptake. The snow manipulation treatment was conducted in the spring of 2022 and 2023. We manually halved (Removal treatment) or doubled (Addition treatment) snow water equivalent (SWE) in experimental plots in March of 2022 and 2023 to accelerate or delay by an average of one week, respectively, the onset of spring snowmelt. Soil Samples were collected from these plots in November 2021, April 2022, May 2022, June 2022, November 2022, April 2023, May 2023, June 2023 and anlalysed for microbial biomass and activity, as described in the methods section. 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.more » « less
