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- Title
Topographical Controls on Hillslope‐Scale Hydrology Drive Shrub Distributions on the Seward Peninsula, Alaska.
- Authors
Mekonnen, Zelalem A.; Riley, William J.; Grant, Robert F.; Salmon, Verity G.; Iversen, Colleen M.; Biraud, Sébastien C.; Breen, Amy L.; Lara, Mark J.
- Abstract
Observations indicate shrubs are expanding across the Arctic tundra, mainly on hillslopes and primarily in response to climate warming. However, the impact topography exerts on hydrology, nutrient dynamics, and plant growth can make untangling the mechanisms behind shrub expansion difficult. We examined the role topography plays in determining shrub expansion by applying a coupled transect version of a mechanistic ecosystem model (ecosys) in a tundra hillslope site in the Seward Peninsula, Alaska. Modeled biomass of the dominant plant functional types agreed well with field measurements (R2 = 0.89) and accurately represented shrub expansion over the past 30 years inferred from satellite observations. In the well‐drained crest position, canopy water potential and plant nitrogen (N) uptake was modeled to be low from plant and microbial water stress. Intermediate soil water content in the mid‐slope position enhanced mineralization and plant N uptake, increasing shrub biomass. The deciduous shrub growth in the mid‐slope position was further enhanced by symbiotic N2 fixation primed by increased root carbon allocation. The gentle slope in the poorly drained lower‐slope position resulted in saturated soil conditions that reduced soil O2 concentrations, leading to lower root O2 uptake and lower nutrient uptake and plant biomass. A simulation that removed topographical interconnectivity between grid cells resulted in (1) a 28% underestimate of mean shrub biomass and (2) over or underestimated shrub productivity at the various hillslope positions. Our results indicate that land models need to account for hillslope‐scale coupled surface and subsurface hydrology to accurately predict current plant distributions and future trajectories in Arctic ecosystems. Plain Language Summary: Several observations have shown that shrubs are expanding across the Arctic tundra. Most of these observations indicate that shrubs are expanding mainly on hillslopes and the processes through which topography controls shrub expansion remain unclear. We showed here, from our modeling analysis, that topographic controls on lateral surface and subsurface fluxes of water, nutrients, and energy affect the productivity and distributions of shrubs across the Kougarok watershed, Seward Peninsula, Alaska. Consistent with field measurements, the fast‐growing deciduous shrubs were modeled to dominate the hillslope position with intermediate soil water content and higher nutrients. We conclude that surface and subsurface drainage hinders model performance in topographically diverse tundra landscapes. Key Points: Topography and landscape hydrology are key environmental controls on observed Arctic shrub expansion of the past three decadesLateral flows of water, nutrients, and energy across slopes control nitrogen (N) mineralization, root O2 and plant N uptakes, and thus plant compositionN2‐fixing deciduous shrubs were shown to benefit more from a higher resource environment driven by enhanced mineralization
- Subjects
SEWARD Peninsula (Alaska); SHRUBS; WOODY plants; CLIMATE change; TOPOGRAPHY
- Publication
Journal of Geophysical Research. Biogeosciences, 2021, Vol 126, Issue 2, p1
- ISSN
2169-8953
- Publication type
Article
- DOI
10.1029/2020JG005823