Responses of forested wetland vegetation to perturbations of water chemistry and hydrology

WETLANDS, Vol. 13, No. 2, Special Issue, June 1993, pp. 122-129 © 1993, The Society of Wetland Scientists

RESPONSES OF FORESTED WETLAND VEGETATION TO PERTURBATIONS OF WATER CHEMISTRY AND HYDROLOGY

Joan G. Ehrenfeld and John P. Schneider ~ Institute of Marine and Coastal Sciences

Rutgers University New Brunswick, NJ 08903

Present address: U. S. Environmental Protection Agency Region II

536 S. Clark St. (5SGIS) Chicago, IL 60605

Abstract: Nineteen mature Atlantic white-cedar swamps, located in four categories of undeveloped and suburban watersheds of the New Jersey Pinelands, were studied to determine the relationship between perturbations of water quality and hydrology and changes in species composition and community structure. Rank-orders of the 19 sites were compared for key variables (ground-water and surface-water NH4 and PO4, mean water-table level and water-table range). Rank orders for the sites were different for the various parameters, suggesting little congruence among water quality and hydrologic changes at wetlands within urban basins. Changes in species composition, measured as the number of invading species, were correlated with the number of perturbed chemical and hydrologic parameters and were not related to the absolute magnitude of any one parameter. Sites in developed watersheds supported a larger fraction of facultative upland and upland species than did sites in undisturbed watersheds; this change could affect wetland delineation of urban wetlands. Urbanization thus increases variability in environmental quality among sites of a given type of wetland and fosters an increase in proportion of non-hydrophytic vegetation within such wetlands.

Key Words: Atlantic white-cedar, forested wetland, urbanization, water quality, hydrology, disturbance, species richness

INTRODUCTION

Wetland losses in the mid-Atlantic region, as else- where in densely populated regions of both coasts, are often attributable to urbanization (Tiner 1985, Dahl et al. 1991). Even if state and/or federal regulations prevent outright wetland destruction during urban and suburban development, the proximity of urban communities to wetlands may seriously degrade wetland quality. Numerous studies have documented the changes in water chemistry and hydrology that may result from urbanization (Bryan 1970, Browne et al. 1982, Hoffman el al. 1985). In addition, urban development can lead to increases in habitat fragmentation, trampling ofsubstrate, cutting of trees, and other forms of damage. However, there have been very few studies that examine the effects of urbanization on the environmental and biotic characteristics of wetlands.

We have studied the response of Atlantic white-cedar (Chamaecyparis thyoides (L.) BSP) swamps to urbanization in the New Jersey Pinelands (Ehrenfeld and

122

Schneider 1990, 1991). The Pinelands, a region in southern New Jersey of 4 x 10 s ha of extremely sandy soil, low topographic relief, and a shallow ground-water aquifer (Rhodehamel 1979), supports extensive forested wetlands (Good and Good 1984). These wetlands, including pitch pine lowlands (Zampella 1990), hardwood swamps (Ehrenfeld and Gulick 1981), and white-cedar swamps (Laderman 1989), border the drainage network and are also found in small to large undrained depressions. Together, the wetlands cover about one-third of the land surface in the region. Sur- face water in Pinelands wetlands is typically very acidic (pH 3.5-4.5) and contains very low concentrations of NH4 + and nearly undetectable levels of NO3- and o-PO4 3 (Durand and Zimmer 1982). Cedar wood is a highly valued economic resource, and these swamps provide habitat for a disproportionate number of rare and endangered species (Synder and Vivian 1981). Be- cause white-cedar swamps are considered the most im- portant wetland type in the Pinelands, we have focused on them.

Ehrenfeld & Schneider, FORESTED WETLAND RESPONSE TO PERTURBATION 123

We have compared water quality, hydrology, plant community composition, community structure, and tree growth at a series of replicate sites representing four levels of urban impact. The study was designed to test the hypotheses that (1) differences in environmental parameters, such as surface-water chemistry, ground-water chemistry, and water-table fluctuations, and in plant community characteristics would be found among the groups of replicate sites for each level of urbanization (defined below), and (2) that the observed changes in community composition and structure could be associated with particular environmental factors. The analyses reported previously (Ehrenfeld and Schneider 1990, 1991) supported the notion that the different levels of urban effect could be associated with specific patterns of change in both environmental and biotic parameters. However, many trends suggested by graphical display of the data were not supported by the statistical analyses of the groups of sites because of the very high degree of variability of both environmental and biotic variables within each category of urban effect. We therefore compared the variability of environmental parameters with that of the biotic characteristics in order to determine whether variability in environmental change is itself a main effect of urbanization, separate from specific quantitative changes in concentrations of nutrients or in water-table levels. While many studies describe the correspondence of vegetation with variations in water chemistry and flow (e.g., Vitt and Bayley 1984, Patterson and Mendels- sohn 1991, Rheinhardt 1992), there are few that examine the relationships between variability of these environmental parameters and patterns of vegetation.

METHODS

Nineteen sites (Figure 1) were selected for the study that met criteria of similar canopy composition and landscape position. All stands had a closed canopy of only mature cedars (density 2,264-5,676 stems>2.5 cm DBH ha-t), and all were located adjacent to pe- rennial streams near the headwaters areas. Small numbers of hardwoods (Acer rubrum L., Nyssa sylvatica Marsh., Magnolia virginiana L.) were present in the understory (Schneider 1988).

Based on the current understanding of the hydrology of the region (Rhodehamel 1979, Means et al. 1981), all sites could be expected to have similar hydrologic settings (i.e., primarily influenced by ground water). Rhodehamel (1970) estimated that 89% ofstreamflow is from ground-water discharge through the fringing wetlands. Streamside cedar swamps are thought to be fed mostly by ground-water discharge, with variable amounts of streamwater entering the wetland during periods of high flow (J. Barringer pers. comm3. In one

Figure 1. Location of study sites within the Pinelands Na- tional Reserve.

of the only detailed studies of wetland hydrology in the region, both ground-water recharge and discharge, varying with season, have been demonstrated for the McDonald's Branch basin, one of the sites within the Control group of sites in this study (J. Barringer pets. comm.). None of the study sites were within undrained basins that could be associated with perched water tables. Fur thermore , surface runoff is negligible throughout the region (Rhodehamel 1979). While the C and N sites were located closer to the regional divide than were the D and R sites, all sites were in the upper reaches of low-order streams within their respective basins and thus were unlikely to be more strongly influenced by regional discharge than by local flow. Thus, we could expect that the wetlands would be primarily influenced by ground-water discharge and secondarily influenced by surface-water inflow.

Nine sites were located within undisturbed watersheds in Lebanon State Forest; the remaining ten sites were located within suburban housing developments in towns within 20 km of the State Forest (Figure 1). Site locations and descriptions are given more fully in Schneider (1988). The sites within the protected watersheds were divided into two groups. Sites in one

124 WETLANDS, Volume 13, No. 2, Special Issue, 1993

group, identified as "control" o r "C" sites, were located within swamps distant from any road or other human structure. The other group, identified as "near" or "N" sites, contained sites within the same basins but adjacent to and upstream of the unpaved dirt roads within the Forest. The sites within developments were like- wise divided into two groups. In one, identified as "developed" or "D" sites, the study areas were sur- rounded by houses on septic systems whose drain fields were contiguous with the swamp and were a minimum of 10-yr old. In the other group, identified as "runoff ' or "R" sites, the sites were similarly situated with respect to housing, but in addition, storm sewers brought road runoff from the development directly into the study area. The sites thus represented four levels of potential urban impact.

In each site, a study area of 20 m × 30 m was established, and 4 wells were installed to a depth of 2 m. The wells were constructed of 4-cm I.D. PVC pipe screened throughout their length in order to monitor water levels. Wells were sited in hollows, and water levels were recorded relative to the ground surface in the hollow surrounding each well. Water levels were determined at biweekly (growing season) or monthly (winter) intervals. Samples of ground water and surface water were taken for chemical analysis at monthly intervals using a peristaltic pump and acid-rinsed poly- ethylene bottles that were rinsed with ground or surface water prior to collection of the sample. Wells were pumped dry and allowed to refill before obtaining ground-water samples. During periods of inundation, swamp surface-water was collected from the hollow within which the monitoring well was placed. Ground- water samples were intended to characterize water chemistry within the root zone on the assumption that this water would most strongly affect the diversity and relative abundance of plants; they were not intended to characterize changes in water quality with depth in the peat. Samples were kept at 4°C during transport to the laboratory and were filtered immediately. Species composition within each plot was determined by re- peated surveys during the growing season of 1982, and each species was assigned a cover-abundance score fol- lowing the Braun-Blanquet system (Moore and Chap- man 1986). Water samples were analyzed for temper- ature, dissolved oxygen and pH by meters, ammonium (Solorzano 1969), phosphate (ascorbic acid method, APHA 1981), and chloride (mercuric nitrate method, APHA 1981).

Previous analyses of the data (Ehrenfeld and Schnei- der 1990, 1991) emphasized the statistical comparison of the four site groups and sought to identify trends of impact to environmental parameters and community composition associated with the increasing intensity of urban influence represented by the sequence of site

groups from C to R. Chloride concentrations, which were significantly higher in ground water in the D sites, and in both ground and surface water in the R sites, supported our initial assumptions in defining the groups of sites (Pye et al. 1983). The statistical analyses demonstrated that significant degradation of water quality occurred, especially in the R sites, and that it was associated with a large increase in the abundance of weedy and wide-ranging species not normally found in Pine- lands wetlands. However, the analyses also revealed that there was a very high degree of variability among the sites within each group, particularly within the two groups of sites within developments (Ehrenfeld and Schneider 1990), so that some of the trends suggested by the data were not statistically significant in com- parisons of group means. For many of the water quality parameters, for example, coefficients of variation for group means were > 100%.

In order to explore the patterns of variability in environmental and biotic parameters among the sites, we ranked the sites from highest to lowest based on annual means, without regard to their initial ' classification within groups, and compared the ranking for several parameters of particular interest. Sites are identified by a two-letter abbreviation of the site name. Addi- tional analyses were undertaken as described below in order to interpret the comparison of the site rankings.

RESULTS AND DISCUSSION

One-way analyses of variance of the water quality data, together with principal components analyses had identified NH4 ÷ and P O 4 -3 a s critical water quality parameters that are altered in urbanized watersheds (Ehrenfeld and Scheider 1991). Figures 2 and 3 show the ranking of the sites for mean annual concentrations of each parameter in ground and swamp surface water. One site is higher than the others for both ground- water (lk) and surface water (br) NH4 ÷ (Figure 2). The remaining sites show gradual declines in mean annual concentrations over a broad range. The C sites vary over an order of magnitude (6-80 ~g 1-' in ground water; 0-10 #g 1-' in surface water); the D and R sites also vary over an order of magnitude, but lower and upper limits (249-762 ~tg 1 -l in ground water; 24-337 ~g 1-1 surface water) are both an order of magnitude higher than the range of the C sites. Surface-water concentrations are in general about half those of ground- water concentrations.

Ranks of ground-water and surface-water concentrations of o-PO4 -3 are distributed in patterns similar to those ofNH4+: 2 sites (db, am) have much higher concentrations than the other sites, whose concentrations decline gradually (Figure 3). Ground-water and surface-water concentrations are more closely corre-


RANKED SITES: SURFACE-WATER NH4 RANKED SITES: GROUND-WATER NH4 400 800

,- . [ ] C sites

[] N sites 600 = 300 Z [ ] D sites

< ~ 200 Z 100

[qm_ 0 . . . . . . . . . . . . . . . . . . ~m lff'l l~'q ~ -- 0 . . . . . . . . . . . . . . . . . --o ,

br vg Ib db hd jk Ik bbam df hnmndn ht cn sf mf pn Ik amdb br jk vg hd bbcn Ib hnmndf sf mf pn dn hf

SITE SITE Figure 2. Ranking of sites for mean annual ground-water and surface-water NH4 concentrations.

lated with each other (r=0.94; P<.0001), and this is reflected in the similarity of the rankings of the sites for the two parameters (Figure 3). Unlike NH4 + concentrations,the same two sites have high o - P O 4 -3 c o n - c e n t r a t i o n s in both ground and surface water. The remaining sites are distributed over a small range of values for both parameters, and the four types of sites are well interspersed. However, the rankings of sites for phosphate are different from those of either ground- or surface- water ammonium. Correlations between ammonium and phosphate concentrations in both ground and surface water were not statistically significant.

The sites were also ranked with respect to the mean water level during the study and to the range of water levels (max imum-min imum observed levels); the rankings are shown in Figure 4. The water level fell up to 25 cm below the surface of the sediment in the hollows during the autumn, usually reaching its lowest level during late September--early October. Maximum flooding depths (25-30 cm above the sediment surface in the hollows) were reached in early spring. The rank-

ing of sites with respect to mean level (Figure 4) showed that, with the exception of one outlier site (hi), there was little differentiation of the sites, and the various site types were completely intermixed. The hl site was a swamp at the upper end of an impoundment caused by the construction of a road and had about 50 cm of standing water present year-round.

In contrast, in the ranking of sites with respect to water-level range (Figure 4), the C and N sites occupy a central position; one group of urban sites has a small water-level range compared to the control group, whereas another group of urban sites has a large water- level range. As with NH4 + and o-PO4 -3 concentrations, two of the R sites (db, am) have extreme values of water-level range compared to all other sites. In these sites, water levels fell to much lower levels (about 50 cm below the surface) during the dry period due to the presence of man-made ditches through the swamps, and the sites had several brief episodes in which the water table dropped below the surface during the wet portion of the year, unlike the C sites.

Our previous analyses of the species composition of

RANKED SITES: GROUND-WATER PO4 RANKED SITES: SURFACE-WATER PO4 150 lO0 ....

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db am jk hn br hd cn bb Ik dn vg mf Ib sf hf mnpn df dbam hn cn df Ib br pn vg jk dn hf Ik mfrnnbb hd sf SITE SITE

Figure 3. Ranking of sites for mean annual ground-water and surface-water PO4 concentrations.


6 0 -

so~ 4o

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rid 1 0 ,

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MEAN WATER LEVEL

[] c s,e, [] N sites

[ ] D sites

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SITE Figure 4. Ranking of sites for mean water-table level (cm) and for water-table range during the study period (April 1982- October 1983).

these swamps showed that in urbanized swamps, species characteristic of, and restricted in distribution to these acidic, nutrient poor wetlands, such as Carex stricta Lam., Drosera rotundifolia L., Orontium aqua- ticum L., Aster nemoralis Ait., Helonias bullata L., Utricularia spp., Eriophorum virginicum L., and Alnus serrulata (Ait.) Willd. were absent and were replaced by cosmopolitan, weedy, and occasionally exotic species (Schneider 1988, Ehrenfeld and Schneider 1991). Total number of species per site type and mean number of species per site were lowest in the C sites (46 and 26.8, respectively) and highest in the R sites (94 and 44.8, respectively). The changes in species richness were attributable to the invasion of large numbers of weedy species in the D and especially the R sites. Half of all species occurred at only one or two sites, so there was a low degree of similarity among sites within each site group.

Two approaches were taken to further explore the nature of the differences in species composition among the sites. First, total species richness at each site and the total number of species occurring uniquely at each site were regressed against the environmental parameters discussed above in order to determine whether the species occurrence could be simply related to environmental changes. Examination of the species occurring at only one site showed that 39 of the 40 species in this category were not native to Pinelands wetlands (Stone 1911), so the number of species unique to each site was used as a simple measure of"si te invasibility." Second, the wetland indicator status of each species was ascertained from Reed (1988), and the distribution of wetland plant types was determined for each site.

We hypothesized that, given the very low nutrient content of undisturbed swamp water, an increase in nutrient concentrations in surface and/or ground-water would be correlated with species composition. How- ever, no significant relationships were obtained in the

regressions of either species variable against ground- or surface-water NH4 +. Although significant regressions were obtained for the relationship of both species indicators to both ground- and surface-water o-PO4 -3 using all sites, the significance disappeared when the extreme sites were excluded from the analysis. Thus, this analysis suggested that changes in species composition are not simply related to increases in nutrient availability in the wetlands.

The wetland indicator status of the plants of a site, as given in Reed (1988) can be used to measure the degree to which the flora is adapted to wetland con- ditions (Anon. 1989). In wetlands in which standing water is present for 9 + months/yr, it could be expected that most species would be obligate hydrophytes or species designated as facultative-wet. We therefore determined the hydrophyte ranking of all species identified in the study and computed the fraction of the flora in each class for particular groups of sites (Figure 5). Species occurring only in the undisturbed sites (C and N sites) are, as expected, entirely wetland species; indeed, 87% are obligate wetland plants. Species occurring in all sites, a category that includes most of the trees and shrubs, are also predominantly wetland plants; fewer than 10% of of this component of the flora are facultative-upland or upland plants. However, plants occurring only in the developed watersheds (D and R sites) include a much larger fraction (30%) of faculta- five-upland and upland species, while a lower fraction (25%) are obligate wetland plants. The flora of the R sites continues the trend: 36.5% are facultative-upland or upland, while 21% are obligate wetland plants. As shown above (Figure 4), there was no decrease in mean water level that accompanied this shift in the flora of D and R sites towards non- hydrophytes, nor was there a decrease in percentage of time flooded during the study at the suburban sites (75% in the C sites vs. 79% in the R sites). Furthermore, at the driest site, a site


100"

t/I _ w ' O I1,1 IZ.

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Figure 5.

O B L FACW FAC FACU

INDICATOR STATUS

U P

• C & N (1§)

r"A C, N, D (231

[3 D ~. n (62)

[ ] ALL SITES (~1

Percent of flora (133 species) in each wetland indicator class for different groups of sites. Numbers in pa- rentheses are number of species Included in each site group.

in the N group in an undisturbed watershed that was flooded only 37% of the time, 82% of the species were obligate or facultative-wet hydrophytes.

As pointed out above, one of the salient features of the ranking of sites is that the sites are differently or- dered for each physical parameter, and there is no correlation between species richness or the number of invading species and the absolute value of any of the environmental parameters. Examination of the data suggested that these two measures of community structure were, rather, related to the number of environmental parameters that were significantly perturbed compared to the controls. In order to test this obser- vation, a mean rank order was calculated for each site. The rank orders for mean water-table level were omit- ted from the analysis because the control sites were widely dispersed among the group of sites, and therefore rank order had no significance. Because the control sites were found in the center of the distribution of sites ranked by water-table range, the rank order was

determined from the absolute value of centered data (IR,-RI, where R, is the range of site i and R is the mean range of all sites). The mean rank order was then regressed against total species number and number of unique species. Total species richness is significantly linearly related to mean rank (P<0.001; Figure 6), but with an r 2 value only 0.43. The number of invading species, however, increases logarithmically with the mean rank (P<0.001 and r2=.80; Figure 6). The plot suggests that there is a rapid rise in the number of invasive species as mean rank decreases. These results suggest that it is the number of perturbed factors, rather than their absolute magnitude, that affects species richness and the likelihood that plants foreign to the particular habitat of cedar swamps will invade.

CONCLUSIONS

The comparison ofrankings of the sites for the various ground- and surface-water quality and hydrologic variables showed clearly that urbanization within watersheds may have very different effects on individual wetlands within each basin. Although degradation of water quality and changes in hydrology clearly result on average from urbanization, the perturbations ex- perienced by any given site are not predictable, despite an apparent similarity in the extent and kind of urban development and in the hydrologic siting of the wetlands. One consequence of this result is that cedar wetlands in urbanized watersheds can be expected to be more variable, as a group, in physical parameters than they are at undisturbed sites. The high degree of variability among sites in abiotic characteristics could ex- plain the fact that 28% of species in the R sites occurred at only one site, as opposed to 4% in the C sites. The variation among disturbed sites underscores the need for adequate replication of sites in any study of wet-

Ul m O ul D.

. I < I - o I -

Figure 6.

60 ~ y = 47.8 - 1.6x R^2 = 0.43

5 0

4 0

3 0 - :i 0 2 4 6 8 10 12 14 16 18 20

MEAN RANK

2O

1 8 - U~ W 18" 6 I~1 1 4 ' n (/) 12 o z lO

a 6 < :> 6- z m

4 :It

2

0 0

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2 4 6 8 1 0 1 2 1 4 1 6 1 8

MEAN RANK

Regressions of total species richness and number of invading species against mean rank (see text for description).


lands in urban catchments if misleading results are to be avoided.

We speculate that the lack of sensitivity of the vegetation to changes in mean water level is attributable to a pronounced hummock/hollow microtopography. Hummocks associated with large structural roots of the cedar trees and larger shrubs (e.g., Kalmia angus- tifolia L., Vaccinium corymbosum L., Rhododendron viscosum (L.) Torr.), and covered with Sphagnum spp. are, on average, elevated 50-75 cm above the lowest points in the hollows (Ehrenfeld, unpub, data from three of the C sites). In this study, the maximum observed water-table levels were in the range of 20-25 cm above the lowest part of the hollows; thus, flooded, saturated, and unsaturated microhabitat patches are all available within each wetland through most or all of the year. This effectively buffers the wetlands against even substantial changes in water-tab!e level.

The non-native, weedy species invading disturbed sites not only displace the characteristic flora but cause a shift in the relative abundance of wetland and upland species. Because wetland delineation for regulatory purposes is contingent on the relative contribution of obligate, facultative-wet, and facultative species to the flora (Anon. 1989), an increase in the frequency of non- wetland species could result in decreases in the area delineated as wetland, particularly in areas with altered hydrology or along wetland margins.

Thus, urbanization may alter some or all of the im- portant aspects of water chemistry and hydrology in cedar swamps. The ensuing changes in vegetation, in- volving a loss of native species and incursion of weedy or exotic species, occur in proportion to the number of parameters that are perturbed and not their absolute magnitude. The ensuing changes may also result in an increase in the proportion of the flora that is not clas- sified as wetland vegetation, potentially resulting in changes in delineation of jurisdictional boundaries within urbanized areas.

ACKNOWLEDGMENTS

This study was supported by funds from the Office of Water Policy, U. S. Dept. of Interior, the New Jersey Department of Environmental Protection, and the Center for Coastal and Environmental Studies. We thank the undergraduate students who assisted with the extensive field and laboratory work involved in the study.

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Manuscript received 26 February 1992; revision received 12 March 1993; accepted 23 March 1993.

Documents

Responses of forested wetland vegetation to perturbations of water chemistry and hydrology