7.0 Wooded Wetland Vegetation

Impacts on Quality of Inland Wetlands of the United States:
A Survey of Indicators, Techniques, and Applications of Community Level Biomonitoring Data
Excerpts from Report #EPA/600/3-90/073
(now out of print)

Discussions in this section focus on trees and shrubs that normally characterize wetlands. In many cases, community structure of wood vegetation is less effective as an indicator of short-term anthropogenic stress than is structure of herbaceous plant communities. This is because species composition of wooded wetlands responds slowly to stress, and is suitable mainly as an integrator of conditions occurring over many months and years.

7.1 USE AS INDICATORS

Enrichment/eutrophication. Changes in community composition of wooded wetlands were attributed to increased nutrients in a Michigan wetland exposed to wastewater, by Kadlec and Hammer (1980), but most studies have been too short to detect significant change. Moreover, changes in nutrient concentration are often associated with changes in hydroperiod, and distinguishing the effects of the two can be difficult. Effects of nutrient increases have more often been detected at the level of the individual plant (e.g., growth, foliage and root nutrient concentrations) than at the community level. However, effects at the individual-plant level can often be eventually translated into effects on community composition. Community-level measurements of woody vegetation may be a poorer indicator of eutrophication than are algal or herbaceous plant communities, which respond more quickly.

Organic loading/reduced DO. Existing literature often does not adequately distinguish the effects on woody plants of organic loading/reduced DO, from the effects of nutrients (discussed above) or inundation (discussed below). For example, in one Minnesota wetland experimentally exposed to wastewater, tree mortality could have resulted from hydrologic changes or methodological variation, and so was not attributed specifically to the effluent (Schimpf 1989).

Feedlot effluent entering an Illinois swamp caused increases in species richness, invasion by new species, and changes in species dominance (Pinkowski et al. 1985). Straub (1984) looked for changes in growth rates in isolated swamps with added fertilizer or wastewater, but after five years found none. Increased tree growth has been noted in Florida wetlands exposed to secondarily treated effluent, but untreated effluent appears to be detrimental (Brown and van Peer 1989, Lemlich and Ewel 1984). Florida regulations for treated wastewater discharges to wetlands specify that "the importance value of any of the dominant plant species (excluding some exotics) occupying the canopy or subcanopy shall not be reduced by more than 50 percent at any monitoring station, or 25 percent overall in the wetland." Exceptions may be allowed if changes can be attributed to catastrophic natural events such as hurricanes or fire. Dominant plant species are defined as those that have a total relative importance value of at least 90 percent during the baseline monitoring period (Schwartz 1987).

Contaminant Toxicity. Few if any studies of have been conducted of community-level response of woody vegetation to contaminants in wetlands. Shallow-rooted species are generally believed to be more sensitive to contaminants than deep-rooted species, due to their greater exposure to waterborne contaminants (Sheehan 1984). A four-year study of the response of wetland species to an oil spill in a Massachusetts inland wetland (Burk 1977) reported post-spill absence of red maple (Acer rubrum), and no effect or increase in sugar maple (Acer saccharinum) and wild grape (Vitis labrusca). Additional toxicological information may be available through EPA's PHYTOTOX database (Royce et al. 1984).

Acidification. There are apparently no community-level studies of effects on wooded vegetation specifically in wetlands.

Salinization. In general, woody plants are more sensitive than herbaceous species because they are usually unable to release salts back into the soil, and must therefore rid themselves of it through leaf loss or dying branches. However, we found very little explicit information on woody wetland community response to salinization. An experimental study in Florida demonstrated stress to individual trees from chloride-enriched water (Richardson et al. 1983). Adverse impacts of road salt on forest communities (probably including some wetland species) have been frequently demonstrated. Some tolerance data also may be available from studies of freshwater tidal wetlands. In North Carolina, Brinson et al. (1985) reported reduced tree basal area and density, and greater litterfall, in forested wetlands temporarily exposed to waters of higher salinity.

Sedimentation/burial. Trees (and especially seedlings) are killed when trunks or stems are partially buried or sediment deposition is sufficient to cut off root oxygen exchange (e.g., Eichholz et al. 1979, Kennedy 1970, Harms et al. 1980, Maki et al. 1980). Floodplain trees in Florida were killed by 0.8 m or more of fill, and tree vigor was reduced by only 0.04 to 0.12 m of fill (Clewell and McAninch 1977). Also, where sedimentation is severe, the frequency and duration of inundation may change, causing shifts in community structure (see below). Siltation can also reduce stem height and diameter growth (Kennedy 1970), thus altering competition and ultimately, community structure. Relatively sediment-tolerant species include eastern cottonwood, baldcypress, water tupelo and black willow (Broadfoot 1973).

Where sedimentation creates shoals in rivers or lakes, these sometimes provide additional substrate for establishment or expansion of wooded wetlands. Moderate amounts of sediment may also have a fertilizing effect.

Turbidity/shade; Vegetation removal. Alteration of the canopy within wooded wetlands may be expected to trigger long-term shifts in community composition of woody species, particularly shrub species. Shade tolerances of most woody species are relatively well-known. Logging has an obvious immediate impact on forested wetlands, but the long-term effects on community structure are poorly known and probably dependent upon initial state and the specific silvicultural procedures used. Repeated "high-grading" (i.e., removal of largest trees of the most valuable species) results in high-density, low-biomass stands of shade-tolerant species such as elm, maple, and willow. Grazing also affects community composition, and woody plants differ in their palatability and thus sensitivity to grazing. Usually, evergreens (particularly cedar) and thorny species are less grazed than other deciduous species, but utilization also depends on local availability.

Thermal alteration. Decreases in plant species richness, basal area, and stem density have occurred in South Carolina wooded wetlands as a result of warmed waters (Scott et al. 1985).

Dehydration. Many woody plant communities in wetlands require the absence of surface water, while others require its presence. In the latter case, many of the species comprising such communities can endure brief periods (e.g., a few hours) of occasional drawdown without changing, so long as sediments remain saturated, and many such communities change little despite weeks, months, or years without surface water (e.g., Parker and Schneider 1975). However, in other wetland communities adapted to flooding, if drawdown is sustained over many days (particularly if it occurs during the growing season and results in desaturation of sediments), dehydration can trigger significant changes in soil chemistry and in wooded wetland community structure. This is largely due to the increased availability of nutrients as sediments become desaturated and oxidized, and partly due to enhanced germination of seeds of woody plants that have lain dormant for years in sediments.

In the short-term, complete drawdown of flood-adapted communities often shifts the balance of community structure in favor of woody species, and away from submersed and emergent species. For example, drainage and groundwater withdrawals near wet prairie and cypress wetlands in Florida have resulted in invasion of these areas by willows (especially in burned and logged areas), maidencane, Brazilian pepper, wax myrtle, dahoon holly, gallberry, saltbush, buttonbush, slash pine, red bay, water oak, cabbage palm, and red maple (Alexander and Crook 1974, Carlson 1982, Duever et al. 1979, Lowe et al. 1984, Richardson 1977). In southwestern riparian wetlands, flow regime alteration by dams, and its effect on sedimentation, has resulted in replacement of native riparian woodlands with non-native salt cedar (Tamarix spp.)(Brady 1985, Stevens 1989). The exact successional pattern will depend on initial state and other factors.

Density and species richness of woody species may increase following drainage and/or across a spatial gradient of decreasing inundation duration (e.g., Thibodeau and Nickerson 1985, Maki et al. 1980). From their limited data, Taylor and Davilla (1986) concluded that the effects of river flow diversion (dehydration) on California riparian communities were more distinguishable in the smallest and largest streams (orders 1 and 4) than in streams of intermediate size (orders 2 and 3).

The presence of seasonally elevated water levels can sometimes be inferred by water marks and drift lines on vegetation, presence of adventitious root "knees", signs of current scouring, subsidence and bank collapse, and other secondary features. If these are found in a wetland whose water levels currently remain low throughout the year, then some evidence is provided that dehydration (e.g., by flow diversion) has occurred.

Also, several investigators have sought to compile hydrologic tolerance or preference data into quantitative metrics. For example, the Corps of Engineers has quantified much of the tolerance data for woody plants in its "Flood Tolerance Index" (FTI), which is based on a weighting of cover estimates according to flood tolerance of the species (Theriot and Sanders 1986). A conceptually similar index is described by Wentworth et al. (1988) and tested by Carter et al. (1988). Either index might be tested to determine its potential for use as an indicator of persistant dehydration, i.e., based on the proportion of facultative species found in an area and reflected by the index value.

Inundation/impoundment. Although the existence of many woody wetland communities is absolutely dependent upon inundation, deviations of seasonal and annual hydrologic cycles from their "normal" regime (including stabilization of usually fluctuating regimes) can profoundly affect structure of woody plant communities in wetlands.

Presence of surface water is generally much more detrimental to woody seedling survival than is simple soil saturation (Hosner 1960). Flooding later in the growing season, when seedlings have leafed out, has the potential for greater impacts than earlier floods (e.g., Scott et al. 1985). Also, stagnant, deepwater flooded conditions may be more detrimental than aerated conditions, e.g., where water is shallow and flowing, organic loading is light, and water levels fluctuate according to a natural seasonal pattern (Teskey and Hinckley 1977).

Species richness of woody wetland plants generally decreases with increasing flood duration (Brown and Giese 1988, Klimas et al. 1981). Several instances have been reported where frequent flooding has selectively removed smaller trees and shrubs (e.g., Ehrenfeld 1986, Maki et al. 1980, Noble and Murphy 1975) and may favor emergent vascular plants and mosses (Jeglum 1975). In western riparian areas, shallow-rooted woody species may be more sensitive to flooding than species with deep tap roots (Stevens and Waring 1985). In an eastern floodplain swamp, mortality was lowest in trees greater than 38 cm dbh (diameter) and greatest in trees less than 13 cm dbh (Harms et al. 1980).

As little as 3 days of flooding during the growing season can result in loss of some woody vegetation through suffocation and compounds toxic to roots (Boelter and Close 1974, Harms et al. 1980, Stoeckel 1967, Jeglum 1975, Davis and Humphrys 1977, Keddy 1989, Maki et al. 1980, Southern Forest Experiment Station 1958), or through alteration of physical conditions, e.g., erosion and scour. Although some species survive at least 3 years of continuous flooding (Green 1947), most cannot survive growing-season inundation for more than a year or two (Broadfoot and Williston 1973).

A wealth of other information about hydrologic tolerances of woody plants has been compiled in several reports, e.g.:

Burton 1984, Whitlow and Harris 1979, Hook 1984, Teskey and Hinckley 1977a,b, 1978a,b,c,d, 1980, Walters et al. 1980.

The U.S. Fish and Wildlife Service's "National List of Wetland Plants" and its FORFLO model (Brody and Pendleton 1987) also compiled substantial databases on hydrologic tolerances of plants in the course of their development. The FORFLO model quantitatively predicts wooded wetland community change, given data on expected hydrologic change. The USFWS and others (e.g., Harris et al. 1985, Kondolf et al. 1987) are currently developing methods for relating hydrologic tolerances of woody plants to instream flows. Intensive, site-specific procedures for quantifying the tolerated days, depths, and seasons of flooding in forested wetlands are demonstrated by Grondin and Couillard (1988).

Individual tree growth may also be affected by inundation. Deviations from normal flooding cycles can reduce tree growth (Malecki et al. 1983). However, temporary flooding by rivers may fertilize floodplain trees, increasing growth (Mitsch et al. 1979). In some cases the basal increment can be larger in the remaining trees as compared to unflooded areas. It should not be assumed that basal growth is a good indicator of flooding stress or survival (Franklin and Frenkel 1987).

Fragmentation of habitat. We found no explicit information on forested wetland community response to fragmentation of regional wetland resources. One can surmise that as the distance between wetlands with seed sources becomes greater and dispersal corridors become hydrologically disrupted, species with narrow environmental tolerances and which do not disperse easily might be most affected. This assumption was used by Hanson et al. (1990), who developed a model which predicted that fragmentation will lead to lower woody plant diversity in riparian wetlands. Those authors classified several woody species according to their seed dispersal ability.

Other human presence. In "developed" watersheds, the frequency of characteristic wetland shrub species was reported to be less than in wetlands in "undeveloped" watersheds (Ehrenfeld 1983).

7.2 SAMPLING METHODS AND EQUIPMENT

Natural factors that could be important to standardize (if possible) among wetlands when monitoring anthropogenic effects on community structure of woody plant communities include:

age of wetland (successional status), water or saturation depth, sediment type, conductivity and baseline chemistry of waters and sediments, current velocity, abundance of herbivores (particularly beaver, grazing cattle), stream order or ratio of discharge to watershed size (riverine wetlands), and the duration, frequency, and seasonal timing of regular inundation, as well as time elapsed (years) since the last severe inundation, drought, windstorm, or fire.

Seasonal timing of woody plant sampling is less critical than is seasonal timing for sampling of herbaceous plants, because most woody plants are present and identifiable throughout the year. Mid-growing season is usually the recommended time, because of the visibility of seedlings and the relative ease in identifying species then. However, access to woody plants in wetlands may be best in winter if ice is present.

The reference texts and choices for protocols described Section 6 for herbaceous plants generally apply to wooded wetlands as well. However, quadrats are usually larger (at least 1 m², and often over 10 m²) and transects may be longer. Percent cover is less often determined where woody plant canopies are larger than 1 m² in diameter. Belt transects and line-intercept methods (Canfield 1941, Mueller-Dombois and Ellenberg 1974) are more frequently employed, and dbh (diameter at breast height) of dominant and subdominant stems is commonly measured. Working in large tracts of bottomland hardwood wetland, Durham et al. (1985) recommended 0.1 ha fixed-area plots for overstory and saplings, with 0.025 ha subplots for sampling shrubs. The State of Florida's regulations for monitoring of discharge of treated wastewater into wooded wetlands specify that quadrat size shall be at least 100 m² for canopy vegetation and 50 m² for subcanopy vegetation, and that the number of quadrats shall be that number needed to provide 90% certainty of being within 15% of the mean number of species of the population. Additional guidance for sampling streamside wetlands is given by Ohmart and Anderson (1986).

7.3 SPATIAL AND TEMPORAL VARIABILITY, DATA GAPS

In general, quantitative community-level data on wooded wetland vegetation have not been uniformly collected from a series of statistically representative wetlands in any region of the country. Thus, it is currently impossible to state what are "normal" levels for parameters such as seasonal plant density, species richness, biomass, or productivity, and their temporal and spatial variability, in any type of wooded wetlands.

Perhaps the closest approximation of such a data set base is the U.S. Forest Service's Forest Inventory and Assessment database (FIA), and Continuous Forest Inventories (CFI). At least in theory, mean density and species richness could be calculated by state for each of the forest types that characteristically occur in wetlands. A large data set describing these metrics also was collected by the U.S. Army Corps of Engineers, along the lower Mississippi River (Klimas 1988), and another was collected by Jensen et al. (1989) in western riparian systems.

Data on another community metric--importance value--were presented in some of the soil-vegetation correlation studies sponsored by the U.S. Fish and Wildlife Service (e.g., Dick-Peddie et al. 1987, Erickson and Leslie 1987, Hubbard et al. 1988, Nachlinger 1988). In addition, examples of studies of multiple forested wetlands across a region, that quantify woody biomass, stem density, or basal area, include the following:

Dale 1984, Ehrenfeld 1986, Faulkner and Patrick n.d., Jones 1981, Klimas 1988, Osterkamp and Hupp 1984, Reiners 1972, Robertson et al. 1984.

A few published studies have quantified long-term successional changes in community structure of riparian or other wooded wetland communities, sometimes in the engineering context of reconstructing past flood histories. Examples include Malecki et al. 1983, Schwintzer and Williams 1974, and studies cited in Hupp (1988).

Quantitative data on community composition of wooded wetlands appears to be most available for California, the lower Mississippi basin, and Minnesota-Michigan-Wisconsin; and least for New England wooded swamps, Pacific Northwest swamps, and Midwestern riparian systems. Information is most available on impacts of hydrologic alteration, and least on impacts of partial burial, contaminant toxicity, salinization, and habitat fragmentation.

Reasonably complete, qualitative lists of "expected" wetland woody plants are available for most regions through the USFWS's "National List of Plant Species that Occur in Wetlands" (Reed 1988) and databases of The Nature Conservancy. Also, qualitative information may be available by wetland type from the "community profile" publication series of the USFWS (Appendix C).