Pattern and structure of microtopography implies autogenic origins in forested wetlands
2019
Abstract. Wetland microtopography is a visually striking feature, but also critically
influences biogeochemical processes at both the scale of its observation
(10 −2 –10 2 m 2 ) and at aggregate scales (10 2 –10 4 m 2 ). However, relatively little is known about how wetland microtopography develops or the factors influencing its structure and
pattern. Growing research across different ecosystems suggests that reinforcing processes may be common between plants and their environment,
resulting in self-organized patch features, like hummocks. Here, we used
landscape ecology metrics and diagnostics to evaluate the plausibility of
plant–environment feedback mechanisms in the maintenance of wetland
microtopography. We used terrestrial laser scanning (TLS) to quantify the
sizing and spatial distribution of hummocks in 10 black ash (Fraxinus nigra Marshall) wetlands in northern Minnesota, USA. We observed clear elevation bimodality in our wettest sites, indicating microsite divergence into two states: elevated hummocks and low elevation hollows. We coupled the TLS dataset to a 3-year water level record and soil-depth measurements, and showed that hummock height (mean = 0.31±0.06 m) variability is
largely predicted by mean water level depth ( R2=0.8 at the site scale, R2=0.12 –0.56 at the hummock scale), with little influence of subsurface microtopography on surface microtopography. Hummocks at wetter sites exhibited regular spatial patterning (i.e., regular spacing of ca. 1.5 m,
25 %–30 % further apart than expected by chance) in contrast to the
more random spatial arrangements of hummocks at drier sites. Hummock size
distributions (perimeters, areas, and volumes) were lognormal, with a
characteristic patch area of approximately 1 m 2 across sites. Hummocks
increase the effective soil surface area for redox gradients and exchange
interfaces in black ash wetlands by up to 32 %, and influence surface
water dynamics through modulation of specific yield by up to 30 %. Taken
together, the data support the hypothesis that vegetation develops and
maintains hummocks in response to anaerobic stresses from saturated soils,
with a potential for a microtopographic signature of life.
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