Pattern and structure of microtopography implies autogenic origins in forested wetlands

Abstract

Abstract. Wetland microtopography is a visually striking feature, but also critically influences biogeochemical processes at both the scale of its observation (10−2–102 m2) and at aggregate scales (102–104 m2). 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 m2 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.

Microtopography is a fundamental organizing structure of vegetation and soil chemistry in black ash wetlands

“…That is, wetland microtopography can result from feedbacks among hydrology, vegetation, and soil processes that induce soil elevation divergence into two modes: (1) a high-elevation mode (hummocks) and 2a low-elevation mode (hollows) (Rietkerk et al, 2004;Eppinga et al, 2008;Heffernan et al, 2013). In previous work, we observed clear microtopographic patterns that we propose arise from these types of ecohydrological feedbacks (Diamond et al, 2019). If these microtopographic patterns are in fact a result of proposed ecohydrological feedbacks, we therefore expect there to be concordant microtopographic differences in vegetation and soils.…”

“…Importantly, the monotypic nature of black ash systems coupled to existential threats from the invasive emerald ash borer (Agrilus planipennis) has led to black ash being listed as critically endangered (Jerome et al, 2017), making the study of their ecology a timely matter. In a complementary study (Diamond et al, 2019) of black ash wetlands, we observed an evident hummock-hollow structure that was more pronounced at wetter sites. Here, we further investigate the ecological importance of black ash microtopography and ask the following question: to what extent do the integrated controls of water table regimes and microtopography determine spatial variation in vegetation and soil properties?…”

“…In previous work (Diamond et al, 2019) and as part of a larger project (D'Amato et al, 2018), we categorized and grouped each wetland site by its hydrogeomorphic characteristics as follows: (1) depression sites (D; n = 4) characterized by a convex, pool-type geometry with geographical isolation from other surface water bodies; (2) lowland sites (L; n = 3) characterized by flat, gently sloping topography; and (3) transition sites (T; n = 3) characterized as flat, linear features between uplands and black spruce (Picea mariana Mill. Britton) bogs.…”

“…Britton) bogs. Additional detail on site characteristics is provided in Diamond et al (2019), but we provide a brief summary of their characteristics here. The studied wetlands varied in soil type, organic layer depth, hydrology, and vegetation but were all characterized by having a black ash canopy dominance of at least 75 %.…”

“…This productivityelevation positive feedback is ultimately constrained and stabilized by increased decomposition rates of accumulated organic matter as hummocks become more aerobic (Belyea and Clymo, 2001;Watts et al, 2010). The resulting microtopography often displays a clear structure, with observations of marked spatial patterns in open bog (Eppinga et al, 2009) and marsh systems (Casey et al, 2016) and potentially in swamp systems as well (Diamond et al, 2019). If these feedbacks are operating in wetlands, the expectation is greater vegetation biomass and productivity on hummocks rather than hollows.…”