James S. Wakeley

Factors controlling the formation of oxidized root channels: A review

Although root plaques and associated oxidized root channels are used for wetland identification as field indicators of wetland, hydrology, little information is available concerning their reliability as related to the environmental and biotic factors controlling their formation. Therefore, this review describes and evaluates the current state of knowledge of the factors controlling the formation of iron plaques and recommends research to address information gaps.Both abiotic and biotic factors control the presence and degree of iron plaque formation. The most important abiotic factor is the availability of soil iron. However, the effect of site variation in soil physico-chemical characteristics (e.g., texture, organic matter, pH, Eh, and soil fertility), on iron availability and microbial activity can influence the formation and persistence of root plaques and oxidized root channels. Although the oxidizing capacity of the plant root is the most important biotic factor controlling plaque formation, only a limited number of wetland species have been evaluated for this ability, so species-specific differences are generally unknown.Unlike some of the other hydrologic indicators used in wetland delineation (e.g., water marks on trees or sediment deposits) root plaques and oxidized root channels indicate soil saturation for a sufficient period to produce anaerobic soil conditions. Additionally, when found in conjunction with a living root, oxidized root channels indicate that the anaerobic conditions occurred within the life span of the plant root. Therefore, the presence of oxidized root channels and iron plaque surrounding living roots is a relatively good indicator of current wetland hydrologic conditions. However, research is needed to elucidate the relative abilities of different plant species to produce oxidized root channels, the temporal persistence of the root iron plaque and the role that soil physico-chemical condition plays in controlling plaque formation. Without a better understanding of the controls on iron plaque formation and disappearance, the absence of oxidized root channels, in itself, should not be used to indicate the absence of a wetland.

Bird distributions and forest zonation in a bottomland hardwood wetland

During spring of 1988 and winter of 1988–89, we sampled songbird populations and habitat characteristics along two belt transects extending across the broad, forested floodplain of the Cache River, Arkansas, USA. Objectives were to compare avian abundance and species richness among floodplain forest zones and to investigate bird species distributions in relation to the wetness gradient. Forest zones differed in structure, flooding regime, and use by birds. The tupelo/baldcypress zone, in particular, provided habitat unlike that in the higher oak-dominated zones and supported a number of bird species that were much less abundant elsewhere. Distributions of chimney swifts (Chaetura pelagica), prothonotary warblers (Protonotaria citrea), and great crested flycatchers (Myiarchus crinitus) were skewed toward wetter sites, whereas summer tanagers (Piranga rubra), red-eyed vireos (Vireo olivaceus), and others were skewed toward drier sites.

Environmental gradients and identification of wetlands in north-central Florida

Vegetation composition, soil morphology, and hydrology were characterized along wetland-to-upland gradients at six forested sites in north-central Florida to compare results of Federal wetland delineation methods with 3–5 yr of hydrologic data. Wetland and non-wetland identifications were supported by hydrology data in eight of nine plant communities. Lack of hydric soil indicators and hydrophytic vegetation in two upland communities (scrub and mixed mesic hardwoods) agreed with a deep water table. Six wetland communities (cypress dome, cypress strand, bayhead, cypress/bayhead, red maple/oak swamp, and cedar swamp) with field indicators of wetland hydrology, hydrophytic vegetation, and hydric soils were inundated or had water tables at or near the ground surface at least 5% of the growing season in most years., Flatwoods communities, however, occurred at intermediate positions on the moisture gradient and could not be consistently identified as wetland or upland communities. Identification of flatwoods as wetlands depended on wetland delineation method and was not usually supported by hydrologic measurements. In the flatwoods community, soil properties and vegetation composition were correlated with the mean and standard deviation of water-table depths, as well as the depth continuously exceeded by the water table at least 5% of the growing season in most years. Various hydrologic parameters need to be considered in addition to the 5% exceedence level currently used in Federal wetland delineation guidance when characterizing wetland conditions in low-gradient areas such as flatwoods.

Relationships among wetland indicators in Hawaiian rain forest

We applied established methods for wetland identification in lowland and montane wet forests (rain forests) on the island of Hawaii to determine whether rain forests exhibited wetland indicators specified in delineation manuals and to examine relationships among indicators of hydrophytic vegetation, hydric soils, and wetland hydrology. Morphological characteristics and ferrous iron tests indicated pockets of hydric organic soils within areas mapped as Folists. Hydrophytic vegetation decisions based on prevalence values agreed with hydric soil determinations more often than did decisions based on dominant plant species. None of the rain forest types we studied exhibited wetland indicators throughout, but some sites contained scattered small wetlands occupying microtopographic lows created by cracks, folds, and undulating flow patterns in the lava bedrock. Further work is needed to identify reliable wetland indicators that can be used during drier portions of the year and to distinguish hydric from nonhydric organic rain forest soils.

Disagreements between plot-based prevalence indices and dominance ratios in evaluations of wetland vegetation

Methods for wetland identification and delineation require the investigator to determine whether vegetation is hydrophytic. Two widely used techniques for making hydrophytic vegetation decisions involve dominance ratios (i.e., the percentage of dominant species that are rated obligate (OBL), facultative wetland (FACW), and facultative (FAC) and prevalence indices (i.e., the weighfed-average wetland indicator status of all plants present). We sampled 338 vegetation plots on sites throughout the United States and calculated the dominance ratio and a plot-based prevalence index for each plot. We found that hydrophytic vegetation decisions based on the two methods disagreed on 16% of field plots. Analysis of simulated plot data (n=80,000) indicated that frequencies of disagreement increase as vegetation complexity (i.e., number of strata and number of species per stratum) increases. We conclude that the two methods for hydrophytic vegetation decisions disagree too often to be considered equivalent. Additional studies are needed in differnent biogeographic regions and plant community types to determine the conditions under which prevalence indices, dominance ratios, or some other treatment of vegetation data provide more reliable indicators of wetland vegetation.