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A little relief: Ecological functions and autogenesis of wetland microtopography

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Abstract Microtopography, or the small‐scale variation in ground surface height (10−1–100 m) over short (100–102 m) spatial scales, is a ubiquitous feature of wetlands globally. This variation in elevation, characterized by local high (“hummocks”) and low (“hollows”) patches, is more structured than what is observed in uplands, and is intertwined with concordantly structured spatiotemporal variability in hydrologic regimes and associated ecological processes. The importance of microtopography in wetlands is manifold, with critical influence on local hydrological, biogeochemical, and biological processes. Further, the creation and maintenance of wetland microtopography is a balance between activation processes (i.e., those that induce random elevation variation) and autogenic reinforcement processes (i.e., those that provide the feedbacks necessary for the persistence of microtopography). While there are many mechanisms that create vertical structure (e.g., tree falls, accumulation of roots and soil organic matter, and sediment deposition), they all yield a similar core feedback to enhance and sustain microtopographic structure. Finally, microtopography contributes to spatial patterning that confers emergent ecosystem‐scale functions such as hydrologic storage and flows, carbon cycling, organism dispersal, and biodiversity. There is an ongoing need to study the origins and implications of this fine‐scale variation in elevation, as well as the utility of including microtopography in model predictions and ecological restoration efforts. This article is categorized under: Water and Life > Conservation, Management, and Awareness
FExamples of wetland microtopography. (a) spruce (Picea mariana) bog with small Sphagnum spp. moss hummocks in northern MN (photo credit: Jacob S. Diamond); (b) black ash (Fraxinus nigra) swamp with hummocks associated with black ash trees interspersed among un‐vegetated hollows in northern MN (photo credit: Jacob S. Diamond); (c) sawgrass (Cladium jamaicense) ridge rising above an inundated slough (hollow) in Everglades, FL (photo credit: Danielle Watts); (d) mixed water tupelo (Nyssa aquatica) swamp with mounded hummock amidst inundated hollows, FL (photo credit: Amy Washuta); (e) small vegetated hummock associated with knees of baldcypress (Taxodium distichum) in a cypress/tupelo swamp in the Spanish Lake Basin of Mississippi River delta, USA (photo credit: Richard Keim); (f) Beriah tropical peat swamp forest in the floodplain of the Kerian River, Perak state, Malaysia (photo credit: Richard Keim)
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Conceptual model for hummock initiation and maintenance in wetlands (Reprinted with permission from Diamond et al. (2020)). Initiation mechanisms create small‐scale variation in soil elevation that is amplified by feedbacks that grow and maintain elevated hummock structures. Solid lines indicate positive feedback loops and dashed lines indicate negative feedback loops. Italics denote feedback processes hypothesized to only affect lateral hummock extent (thus hummock area), whereas nonitalics indicate mechanisms that affect both vertical and lateral hummock extent. Blue shading indicates mechanisms influenced by hydrology. Soil mass can include mineral and organic matter. GPP refers to gross primary production
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