Microtopography is a fundamental organizing structure in black ash wetlands

Abstract

<p><strong>Abstract.</strong> All wetland ecosystems are controlled by water table and soil saturation dynamics, so any local scale deviation in soil elevation represents variability in this primary control. Wetland microtopography is the structured variability in soil elevation, and is typically categorized into a binary classification of local high points (<q>hummocks</q>) and local low points (<q>hollows</q>). Although the influence of microtopography on vegetation composition and biogeochemical processes has received attention in wetlands around the globe, its role in forested wetlands is still poorly understood. We studied relationships among microtopography on understory vegetation communities, tree biomass, and soil chemistry in 10 black ash (Fraxinus nigra Marshall) wetlands in northern Minnesota, U.S.A. To do so, we combined a 1-cm resolution surface elevation model generated from terrestrial laser scanning (TLS) with co-located water table, vegetation, and soil measurements. We observed that microtopography was an important structural element across sites, where hummocks were loci of greater species richness, greater midstory and canopy basal area, and higher soil concentrations of chloride, phosphorus, and base cations. In contrast, hollows were associated with higher soil nitrate and sulfate concentrations. We also found that the effect of microtopography on vegetation and soils was greater at wetter sites than at drier sites, suggesting that distance to mean water table is a primary determinant of wetland biogeochemistry. These findings highlight clear controls of mictopography on vegetation and soil distributions, while also supporting the notion that microtopography arises from feedbacks that concentrate biomass, soil nutrients, and productivity on microsite highs, especially in otherwise wet conditions. We therefore conclude that microtopography is a fundamental organizing structure in black ash wetlands.</p>

Pattern and structure of microtopography implies autogenic origins in forested wetlands

“…In contrast, hummock-hollow terrain and patterns may be less evident at drier sites where soils are nearly always unsaturated and aerobic, weakening the elevation-productivity feedback (Miao et al, 2013;Miao et al, 2017). In a companion study we found support for this overall model, where we observed vegetation and soil chemistry associations with hummock structures, indicative of elevation-productivity feedbacks, and that these associations were greatest at the wettest sites (Diamond et al, 2019). Here, we add to that work by assessing the structure and pattern of hummock features and the extent to which they are influenced by the hydrologic regime.…”

“…3). The three lowland sites were control plots from a long-term experimental randomized block design on black ash wetlands (blocks 1, 3, and 6; Slesak et al, 2014;Diamond et al, 2018). We considered hydrogeomorphic variability among sites an important criterion, as it allowed us to capture expected differences in hydrologic regime and, thus, differences in the strength of our predicted control on microtopographic generation (Fig.…”

“…Black ash wetlands are typically hydrologically disconnected from regional groundwater and other surface water bodies, resulting in precipitation and evapotranspiration (ET) as dominant components of the water budget, with no indication of extreme surface flows (Slesak et al, 2014). Water levels follow a common annual trajectory of late-spring/early-summer inundation (10-50 cm) followed by ET-induced summer drawdown and belowground water levels (Slesak et al, 2014;Diamond et al, 2018). However, the degree of drawdown depends on the local hydrogeomorphic setting; we observed considerably wetter conditions at depression and transition sites than at lowland sites.…”

“…Overstory vegetation at the 10 sites is dominated by black ash, with tree densities ranging from 650 stems ha −1 (basal area of 195 m 2 ha −1 ) at the driest lowland site to 1600 stems ha −1 (basal area of 40 m 2 ha −1 ) at a much wetter depression site (the across-site mean was 942 stems ha −1 ; Diamond et al, 2019). At the lowland sites, other overstory species were negligible, but at the depression and transition sites there were minor cohorts of northern white cedar (Thuja occidentalis L.), green ash (Fraxinus pennsylvanica Marshall), red maple (Acer rubrum L.), yellow birch (Betula alleghaniensis Britt.…”

“…The relatively open canopies of black ash wetlands (leaf area index < 2.5; Telander et al, 2015) allow for a variety of graminoids, shrubs, and mosses to grow in the understory. However, the majority of understory diversity and biomass tends to occur on hummocks that are occupied by black ash trees (Diamond et al, 2019). Hollows exhibit relatively little plant cover and are typically bare soil areas, but may be covered at times of the year by sedges (Carex spp.)…”