The mechanisms controlling ecological responses to ongoing climate change are important to understand as we strive for accurate predictions of future environmental conditions. A major source of uncertainty in the predictions of how ecosystems will respond to climate change is a limited understanding of belowground patterns and processes. This is particularly critical in high-latitude ecosystems, where belowground components have a disproportionate influence on ecosystem function and services, such as carbon storage and the regulation of elemental cycles. I study belowground processes through the lenses of microbial ecology, community ecology, ecosystem ecology, and biogeochemistry. I am specifically excited by research that addresses how fine-scale belowground processes contribute to broad-scale ecosystem responses to a warming climate and associated changes in disturbance regimes. My research investigates how plant and microbial traits and interactions shape community structure and ecosystem function in three main areas: 1) the influence of belowground communities on ecosystem and regional patterns of vegetation transitions (e.g., treeline shifts), 2) the effects of plant-fungal interactions on ecosystem recovery from disturbances such as wildfire or permafrost thaw, and 3) the interplay between nutrient limitation and plant allocation to roots and symbionts on regional carbon balance and elemental cycling. I use skill sets from molecular ecology, plant physiological ecology, and statistical modeling to infer how mechanisms at the plot-level translate to broad-scale processes. I use isotopes as tracers of plant and microbial resource use and molecular tools to characterize microbial communities.
The Arctic Underground: a network for the synthesis of root and rhizosphere processes in cold soils.
Belowground ecosystem properties are poorly understood, but likely one of the most important drivers of Arctic ecosystem response to climate change. I am part of an interdisciplinary team working to synthesize what is known about root traits and rhizosphere processes in cold ecosystems with soil profiles dominated by thick organic horizons - tundra, boreal forest, and peatlands. Our group is composed of belowground ecologists spanning molecular biologists investigating rhizosphere processes, to plant ecologists and evolutionary biologists who use a trait framework to understand vegetation patterns and function, to ecosystem ecologists measuring the interplay between terrestrial ecosystem function and the climate system. Our goal is to produce conceptual and data-driven synthesis products that will be useful to more accurately predict how cold ecosystems will respond to a changing climate.
Funding: International Arctic Science Committee, Terrestrial Working Group funding, The Arctic Underground: a network for the synthesis of root and rhizosphere processes in cold soils. 2019 & 2020 (co-Lead)
Effects of soil temperature and nutrient constraints on Arctic treeline dynamics
The northern limit of the boreal forest in Alaska is formed within the Brooks Range. The eastern Brooks Range is an area well known to dendrochronologists as an epicenter of divergent tree growth responses to climate warming. Divergence refers to the deterioration of historically strong positive correlations between temperature and tree growth. While divergence is a well-known phenomenon, its implications for changes in tree abundance and shifts in treeline position remain unknown. Recent findings in four watersheds along a west-to-east gradient in the Brooks Range suggest colder, more permafrost-affected soils limit tree access to soil nutrients and may be the cause of divergence in the eastern Brooks Range; whereas, in the west relatively warmer soils may have greater nutrient availability and support a positive growth response to warming. In this study, we are examining the causes and consequences of divergence in the Brooks Range. Specifically, I am focusing on quantifying tree resource allocation belowground to ectomycorrhizal fungi across a soil temperature and nutrient availability gradient.
Funding: NSF, Arctic Natural Sciences, Collaborative Research: soil temperature, mycorrhizal association and tree nutrition as determinants of divergent changes in tree growth and abundance in arctic Alaska. 2018-2022(co-PI) Award #1748847
- Ellison, S.E., P. Sullivan, S. Cahoon, and R.E. Hewitt. 2019. “Poor nutrition as a potential cause of divergent tree growth near the Arctic treeline in northern Alaska.” Ecology. DOI: https://doi.org/10.1002/ecy.2878
Fire severity effects on larch successional trajectories
Larch forests overlie extensive areas of the Siberian arctic. Although larch is a fire-dependent tree species, previous research suggests that increased fire activity may limit larch forest recovery and potentially trigger forest loss and a shift to an alternative vegetation state dominated by shrubs or grasses. Forest loss could have large consequences for climate because of changes in carbon storage and reflection of heat and light (albedo). Larch forests occur across much of the Arctic latitudinal treeline, and future larch recruitment dynamics will be a primary determinant of whether boreal forests respond to climate warming via treeline migration. The many mechanisms governing post-fire larch recruitment and the consequences for system-level feedbacks to regional and global climate remain untested. I am studying the role of ectomycorrhizal fungi in larch recruitment success or failure.
Funding: NSF, Arctic System Science, Collaborative Research: Fire influences on forest recovery and associated climate feedbacks in the Siberian Arctic. 2017-2021 (co-PI) Award #1708344
Plant use of newly thawed permafrost nutrients
Plant acquisition of nitrogen from thawing permafrost has the potential to play a critical role in the trajectory of future climate change. Nitrogen released from deep, thawing soils may stimulate productivity, offsetting carbon lost from these soils as greenhouse gasses, and thus regulate the pace and magnitude of the permafrost carbon-climate feedback. This regulatory role of nitrogen depends on the opportunistic capacity of arctic plants and their obligate fungal symbionts to forage as seasonally unfrozen ground deepens. I am investigating the role of belowground rooting traits and mycorrhizal associations in tundra and boreal plant acquisition of permafrost-derived N.
Funding: NSF, Arctic System Science, Collaborative Research: The roles of plant roots, mycorrhizal fungi and uptake of deep nitrogen in the permafrost carbon feedback to warming climate. 2015-2019 (Postdoc) Award #1504312
National Geographic Society, Plant acquisition of deep nitrogen and the permafrost carbon feedback to climate. 2017-2018. (co-PI)
- Hewitt, R.E., M.R. DeVan, I. Lagutina, H. Genet, A.D. McGuire, D.L. Taylor, and M.C. Mack. 2020. “Mycobiont contribution to tundra plant acquisition of permafrost-derived nitrogen.” New Phytologist, 226: 126-141. DOI:10.1111/nph.16235 (See also the Commentary on this article by Robinson et al., (2020), 226: 8–10.)
- Hewitt, R.E., D.L. Taylor, H. Genet, A.D. McGuire, and M.C. Mack. 2018. “Below-ground plant traits influence tundra plant acquisition of newly thawed permafrost nitrogen.” Journal of Ecology, 00:1–13. DOI: https://doi.org/10.1111/1365-2745.13062
Post-fire plant-fungal interactions shape biome shifts
The mechanisms controlling treeline advance are a major knowledge gap that reduces our ability to adequately project the trajectory of climate change at high latitudes and associated feedbacks to the climate system. Seedling recruitment is facilitated by wildfire in the boreal forest. However, wildfire can strongly influence the inoculum potential and community composition of ectomycorrhizal fungi, the obligate symbionts of all boreal tree species. While wildfire activity has increased at high latitudes with climate warming, the role of fire in facilitating treeline advance is not well understood. I am investigating the influence of ectomycorrhizal fungi on vegetation transitions between tundra and boreal forest after wildfire.
- Hewitt, R.E., F.S. Chapin III, T.N. Hollingsworth, and D.L. Taylor. 2017. “The potential for mycobiont sharing between shrubs and seedlings to facilitate tree establishment after wildfire at Alaska arctic treeline.” Molecular Ecology, 26: 3826– 3838. DOI: https://doi.org/10.1111/mec.14143
- Hewitt, R.E., T.N. Hollingsworth, F.S. Chapin III, and D.L. Taylor. 2016. “Fire-severity effects on plant-fungal interactions after a novel tundra wildfire disturbance: implications for arctic shrub and tree migration.” BMC Ecology, 16: 25. DOI: https://doi.org/10.1186/s12898-016-0075-y
- Hewitt, R.E., A.P. Bennett, A.L. Breen, T.N. Hollingsworth, D.L. Taylor, F.S. Chapin III, and T.S. Rupp. 2015. “Getting to the root of the matter: landscape implications of plant-fungal interactions for tree migration in Alaska.” Landscape Ecology, 31: 895–911. DOI: https://doi.org/10.1007/s10980-015-0306-1
- Hewitt, R.E., E. Bent, T.N. Hollingsworth, F.S. Chapin III, and D.L. Taylor. 2013. “Resilience of arctic mycorrhizal fungal communities after wildfire facilitated by resprouting shrubs.” Ecoscience, 20:296-310. DOI: https://doi.org/10.2980/20-3-3620