Species selection trials and silvicultural techniques for the restoration of bottomland hardwood forests

Ecological Engineering 15 (2000) S35 – S46

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Species selection trials and silvicultural techniques for the restoration of bottomland hardwood forests Kenneth W. McLeod * Sa6annah Ri6er Ecology Laboratory, Uni6ersity of Georgia, PO Drawer E, Aiken, SC 29802, USA Received 19 March 1999; received in revised form 21 May 1999; accepted 7 September 1999

Abstract Since 1990, a series of experiments has examined the appropriateness of 24 tree species for restoring a bottomland and swamp forest in the delta of Fourmile Branch in the coastal plain of South Carolina, USA. In addition, various silvicultural techniques used to maximize the survival of tree plantings, have been appraised for effectiveness. While the topographic relief on the delta is small (dm differences between most sites), it is sufficient to utilize woody species with different flood tolerances. Hence, a diverse forest community can be established, using these elevation differences. In the wettest sites where water persists virtually continuously and may be one to two meters deep during large flood events, only the most flood-tolerant species, such as Taxodium distichum and Nyssa aquatica, can survive. These species will also survive very well at higher elevations, because the permanent water table never fell below one meter deep. In sites that are not flooded, unless the entire swamp is flooded, several additional species can be used. These include Fraxinus pennsyl6anica, Carya aquatica, and Quercus lyrata. Finally, in slightly higher areas, Q. michauxii, Q. nuttallii and Q. phellos would have adequate survival. To minimize herbivory and maximize survival, tree shelters should be used if herbivore pressure is high. The prime herbivore of concern is beaver. Thus, any plantings that are frequently flooded may require protection. Other silvicultural techniques, including fertilization and control of herbaceous and willow competition, were not essential to ensure growth and survival. Finally, survival of the least expensive planting stock, bare root saplings, was nearly as good as balled and burlapped stock. Thus, unless absolute maximal survival is required, bare root stock will produce good survival at a fraction of the cost. One critical characteristic of the bare root stock is height, which must exceed the flood depth during the growing season. In the case of the Fourmile Branch delta, this was at least 45 – 60 cm tall. Published by Elsevier Science B.V. Keywords: Wetland restoration; Bottomland forest; Hydrology; Savannah River; Pen Branch

1. Introduction

* Tel.: +1-803-7255309; fax: + 1-803-7253309. E-mail address: [email protected] (K.W. McLeod).

Clewell and Lea (1989) reviewed the state of our knowledge of creation and restoration of forested wetlands in the southeastern United States, concluding that critical information gaps

0925-8574/00/$ - see front matter Published by Elsevier Science B.V. PII: S 0 9 2 5 - 8 5 7 4 ( 9 9 ) 0 0 0 7 0 - 1

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existed. More important, they noted that many extant projects were not being monitored, hence we were not even learning what worked and what did not. At that time, most of these projects were conducted on abandoned agricultural land in the Mississippi River drainage (Allen and Kennedy, 1989; Newling, 1990) or in reclaiming surface phosphate mines in Florida. In the past 10 years, many more creation and restoration projects have been initiated in a variety of situations across the southeast. Even with the larger number of projects, it is still difficult to point to a successful project that displays the structural and functional attributes of a natural community. One aspect which lead to this uncertainty is the great time required for a forested wetland to develop. Many current projects just have not had enough time to assess their ultimate success or failure. Conflicting results are also obtained in creation and restoration projects which are dependent solely on natural water sources, which may vary seasonally, annually and on longer time frames and may now be regulated by flood control structures. The restoration project discussed in this paper summarizes the results from a series of species trials, using different species and silvicultural techniques, in the hopes of providing useful information to improving the success of other restoration projects. The experimental sites had been impacted over a 30-year time period by thermal effluent from a nuclear production reactor which had virtually sterilized the site. Once the disturbance was removed, it was apparent that the former plant community, including the soil seed bank, could not contribute to the natural recovery. Initial recovery was similar to that of sandbar succession, described by Ware and Penfound (1949), such that wind-dispersed plant species initially dominated the site and no species established from the existing soil seed bank. This restoration project was unusual in that the former vegetation could make no contribution to the vegetational recovery, and in this regard it was similar to a creation project which must rely on establishing desirable species in an early successional habitat. The project differs from a strict creation project in that the site was once a bottomland forest and

that soil disturbance must be minimized due to low-level radionuclide contamination. Hence, the site could not be mechanically contoured to fit a specific planting scheme.

2. Site description of Fourmile Branch The experimental sites were in the Fourmile Branch delta which had been impacted over a 30-year time period by thermal effluent from a nuclear production reactor. It was one of three streams that received thermal effluents on the Savannah River Site. The delta of this stream following cessation of thermal effluent release was drier than the deltas of the other thermally impacted streams (Pen Branch and Steel Creek). Research was initially begun in Fourmile Branch, due to the similarity to Pen Branch and in anticipation of a large-scale restoration project in the Pen Branch corridor and delta. Fourmile Branch (also known as Four Mile Creek) is a third order tributary of the Savannah River on the upper coastal plain of South Carolina, with a base flow of  1 m3/s (Jensen et al., 1984). Fourmile Branch empties into a 3020 ha forested swamp which parallels the Savannah River. Fourmile Branch received thermal effluent from a nuclear production reactor from 1955 to 1985 (Jensen et al., 1984). During this time, flow rate increased to  11 m3/s, raising the water depth in the channel by 15–30 cm (Repaske, 1981). Effluent temperature at the outfall was 60°C, decreasing to 40–45°C at the delta, depending on the operation level of the reactor, the season of the year, and the specific meteorological conditions. The increased flow rate also increased erosion in the upper reaches of the stream with deposition of this eroded material occurring in the delta (Ruby et al., 1981) averaging 60 cm of newly deposited sand on top of the former substrate. Maximum deposition occurred at the mouth of the delta with decreasing amounts being deposited further out in the deltaic fan. By 1961, the combination of factors began to cause tree mortality. A canopy loss rate of  2 ha/year was estimated. By 1978, 92 ha of the forest were destroyed. In late June of 1985, the

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reactor was shut down and subsequently retired. Secondary succession of the vegetation in the Fourmile Branch corridor and delta began immediately. In 1987, permanent plots were established and the initial survey for tree, shrub, and herbaceous composition showed that the stream corridor and delta had been virtually sterilized by the 30 years of thermal effluent (R.R. Sharitz, personal communication). The former woody vegetation could not resprout, nor did a viable woody seedbank exist in the sediment. Natural recovery of the impacted delta was dominated by early successional, wind-dispersed species (Andropogon sp. [broomsedge] and Pinus taeda L. [loblolly pine] in dry sites to Salix nigra Marsh. [black willow] in wetter sites). Hydrology of Fourmile Branch delta has been highly variable since 1985, due to the combination of rainfall and management of the river by the US Army Corps of Engineers. When the water level of the Savannah River is low, local rainfall may raise the water level in Fourmile and temporarily pond water on the delta. Likewise, low rainfall and high evapotranspiration will drive the water table below the soil surface. Any extensive and long duration flooding of the delta is controlled

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by the height of the Savannah River which is regulated by the management of the upstream Clark’s Hill Dam (Schneider et al., 1989). The combination of rainfall and river management has produced very different hydrology during the growing seasons of each of the last 6 years (Fig. 1). Growing seasons of 1990, 1993, 1994, 1995 and 1996 had rainfall below the 1950-to-1981 average, but above average rainfall occurred during 1991 and 1992 (NOAA, 1990–96). Little flooding occurred during the growing seasons of 1990, 1992, 1993, 1995 and 1996, but floodwater accumulated on the delta several times during the growing seasons of both 1991 and 1994. Extensive floods occurred during the dormant winter seasons of 1992/93, 1994/95 and 1995/96. Dormant season flooding frequently exceeded one m in depth and covered the delta from late autumn through early spring. Dormant season flooding usually does not negatively affect survival since the trees are leafless and metabolically less active. In contrast, flooding during the growing seasons of 1991 and 1994 while the trees were metabolically active was deep enough to overtop seedlings, leading to mortality of individuals of certain species.

Fig. 1. Hydrology at the Fourmile Branch delta from 1990 through to the middle of 1997. Positive values are water standing above the soil surface and negative values are depth to the water table. Dots are values from a staff gauge and the solid line is data from a continuous recording pressure transducer. Gaps in the data during the winter of 1992/93 were due to extreme high water which threatened to inundate the recording equipment.

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Table 1 Description of the ten field trials which make up the Fourmile Branch Restoration Program Trial I II III IV V VIII IX X XI XII a

No. of species 9 4 3 10 10 3 3 3 6 4

Soil wetness

Variable tested

Seedling type

Dry Wet Wet Dry and wet Dry and wet Water Water Water Wet Wet

9Fertilizer 9Fertilizer 9Fertilizer Transplant height 9Tree shelters Seedling type/tree shelters Acclimation/tree shelters Root pruning Herbaceous control Willow control

Containerized Containerized Bareroot Bareroot Bareroot Bareroot/HB/BBa HB or BB Bareroot Bareroot Containerized

HB, hand bagged; BB, balled and burlapped.

3. Materials and methods The initial experiments focused on replanting latter successional species into the existing dense early successional vegetation. Because some sites appeared to be dry, as evidenced by the abundance of Andropogon sp. and P. taeda, other sites were dominated by species of Scirpus, Juncus and Erianthus (wet site species), and other sites were almost continuously covered by standing water (ponds and braided stream segments), it was felt that microtopographic variation was responsible for the current successional communities and could be used to favor the reintroduction of different species into different habitats. Therefore, the experiments were designed to examine a variety of individual species and silvicultural techniques in specific habitats (Table 1). Experiments I, II and III determined the response of 11 species to fertilization in dry and wet sites. Experiments IV and V used several transplant sizes (heights) and tree shelters to appraise planting stock characteristics of ten species and the necessity of protection from herbivores. Our ability to successfully establish seedlings of three species in standing water habitats was evaluated in experiments VIII – X. Latter experiments were concerned with the biotic impacts, i.e. competition of six woody species with the existing herbaceous species (experiment XI) and the success of four species planted under an early successional S. nigra canopy (ex-

periment XII). Overall, 4500 individuals of 24 different woody species were planted in 12 experiments. Two of these experiments were exploratory to field experiments and in artificial environments and will not be discussed here. Flood and shade tolerance for each of the 24 species is listed in Table 2, according to McKnight et al. (1981). Most experimental field trials were planted in the winter or early spring of a given year. Following planting, survival was determined on a 1–2 week basis for the first growing season to determine whether the plants were alive at planting and to determine the success of initial establishment. Thereafter, survival and seedling height was usually only determined in the autumn of each year. The field trials were sequentially established over several years and hence results of individual trials vary in length.

4. Results and discussion Many studies stress that knowledge of the site hydrology is critical in the success of any restoration efforts (Allen and Kennedy, 1989; Clewell and Lea, 1989; Newling, 1990). In this particular example, the site hydrology existing from 1955 to 1985 was confounded by the effluent release and construction of flood-control dams. Site hydrology existing prior to 1955 was now also irrelevant, since the dam construction has permanently altered the site hydrology. In addition, during reac-

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tor operations, the delta was extensively recontoured by the effluent. Thus, neither the current site hydrology nor topography are characteristic of the pre-1950 conditions. The sediments are now largely dominated by recently deposited sands (up to 60 cm deep). Sands have low cation exchange and water holding capacities and are generally considered to be infertile and potentially droughty for plant establishment without frequent rainfall. Although the upper soils are sandy and could be droughty, a permanent water table exists at  1 m depth. The management of the Savannah River by the US Army Corps of Engineers has changed the volume, duration, and frequency of flood events downstream of the Clark’s Hill Dam. The flood events and their timing are critical to natural regeneration of downstream areas. The vegetation response will vary tremendously depending on the characteristics of the flood (Kozlowski, 1984). If a prolonged flood event occurs during the dormant

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season, minimal damage may occur, but when these flood events occur during the growing season (an unusual event under natural conditions), serious damage may result. Conducting this research over multiple years was critical to determine the true nature of the site. Woody plantings will have to survive the environmental extremes of decades or even centuries. Experiments of 1 or 2 years duration cannot provide this perspective. The first objective of this research program was to determine the appropriate species for reintroduction. Of the 24 species used in the experiments, only eight had survival of \ 50% in any single experiment (Tables 3–6). Five species (Carya aquatica [Michx. f.] Nutt. [water hickory], Fraxinus pennsyl6anica Marsh. [green ash], Nyssa aquatica L. [water tupelo], Quercus lyrata Walt. [overcup oak], and Taxodium distichum [L.] Richard [baldcypress]) had survival of 90% or greater in one experiment. All of these species are moderately tolerant to tolerant of flooding (Table

Table 2 Species used in Fourmile Branch restoration trials, with their respective flood and shade tolerances, according to McKnight et al. (1981) Scientific name

Common name

Flood tolerance

Shade tolerance

Acer rubrum Betula nigra Carya aquatica Fraxinus pennsyl6anica Liquidambar styraciflua Liriodendron tulipifera Nyssa aquatica Nyssa syl6atica Platanus occidentalis Quercus alba Q. coccinea Q. falcata var. pagodaefolia Q. laurifolia Q. lyrata Q. margaretta Q. marilandica Q. michauxii Q. nigra Q. nuttallii Q. rubra Q. phellos Q. shumardii Q. stellata Taxodium distichum

Red maple River birch Water hickory Green ash Sweetgum Tulip tree Water tupelo Swamp tupelo Sycamore White oak Scarlet oak Cherrybark oak Laurel oak Overcup oak Scrubby post oak Blackjack oak Swamp chestnut oak Water oak Nuttall oak Red oak Willow oak Shumard oak Post oak Bald cypress

M. tolerant M. tolerant M. tolerant M. tolerant M. tolerant Intolerant Tolerant Tolerant M. tolerant W. tolerant to intolerant Unlisted W. tolerant to intolerant W. to m. tolerant M. tolerant Unlisted Unlisted Weakly tolerant W. to m. tolerant M. tolerant W. to m. tolerant W. to m. tolerant W. tolerant Unlisted Tolerant

Tolerant Intolerant Intermediate Intermediate Intolerant Intolerant Intolerant Intermediate M. intolerant to intolerant Intermediate Very intolerant M. intolerant. to intolerant Intermediate to intolerant M. intolerant Unlisted Unlisted M. intolerant Intolerant Intolerant Intermediate Intolerant Intolerant Intolerant Intermediate

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Table 3 Percent seedling survival in 1994, 5 years after planting in the Fourmile Branch deltaa Species

Experiment number I

Fraxinus pennsyl6anica Quercus alba Q. coccinea Q. margaretta Q. marilandica Q. michauxii Q. nigra Q. phellos Q. shumardii Q. stellata Taxodium distichum

5). This enforces the obvious conclusion that the Fourmile Branch delta is wet, potentially very wet. The excellent survival of T. distichum also infers that the water table is never very deep, even when the rainfall is slight and the surface soils appear dry. Thus, it is unlikely that once established, drought tolerance of species would ever be an important characteristic. The success of T. distichum over the entire delta also infers that other woody species with similar flood tolerance would be widely successful. This is supported by examining the survival of N. aquatica, F. pennsyl6anica, C. aquatica and Q. lyrata from a number of experiments. Even with high flood tolerance, survival of F. pennsyl6anica was reduced by planting in a deep-water habitat (30–60 cm deep) versus a shallow water habitat (0–30 cm deep) (Table 5). F. pennsyl6anica was also very sensitive to any root pruning prior to planting in muck soil (Table 5). The same intensity of root pruning did not affect survival of N. aquatica or T. distichum, except when unrooted cuttings were actually planted. In temporary to permanent water habitats, protection from beaver was essential for F. pennsyl6anica, N. aquatica and T. distichum (Table 5).

II – 4 0 0 0 36 30 0 – 11 68

III

– 0 – – – 50 4 – – – 79

93 – – – – 10 – – 0 – –

a

See Table 1 for a description of the experimental variables. Fertilizer treatments were combined due to the lack of statistical significance. Data from McLeod et al. (1994).

2), an obviously important characteristic. Whenever T. distichum was used in the trials in Fourmile Branch, it had the best survival (Tables 3 – 6), including when the root system was severely pruned to allow planting by inserting the seedlings into muck soil without even digging a hole (Table

Table 4 Percent seedling survival in 1996, 6 years after planting in the Fourmile Branch deltaa Species

Experiment number IV

V

Initial heightb (cm)

Tree shelters

A Acer rubrum Betula nigra Liquidambar styraciflua Liriodendron tulipifera Nyssa syl6atica Platanus occidentalis Quercus alba Q. nigra Q. phellos Taxodium distichum a b

17 7 7 0 0 13 0 10 23 83

B – – 7 0 – – 0 – – 87

C 0 30 – 0 0 13 0 10 23 90

D 0 – – 0 0 – 0 – 10 –

E 0 20 0 0 – 17 0 7 – –

With 10 3 0 0 0 17 0 10 47 83

See Table 1 for a description of the experimental variables. Data from McLeod and Ciravolo (1999). Initial height: A, 30–45 cm; B, 45–60 cm; C, 60–90 cm; D, 90–120 cm; and E, \120 cm.

Without 17 7 7 0 0 13 0 10 23 83

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Table 5 Percent seedling survival 3 years after planting in the Fourmile Branch deltaa Species (habitat; shelter)

Experiment number VIII

IX

X

Transplant typeb

Acclimated

Root pruning

BB Fraxinus pennsyl6anica Backwater; no shelter Backwater; shelter Stream; no shelter Stream; shelter

HB

DBR

SBR

Yes

No

Cutting

Severe

Moderate

95 100 45 75

25 30 30 30

10 0 0 0

65 100 45 85

0 0 16 53

8 5 20 35

– 0 – –

– 0 – –

– 0 – –

Nyssa aquatica Backwater; no shelter Backwater; shelter Stream; no shelter Stream; shelter

– – – –

65 70 50 75

60 90 25 65

60 80 35 75

8 69 16 54

0 70 0 70

– 13 – –

– 78 – –

– 100 – –

Taxodium distichum Backwater; no shelter Backwater; shelter Stream; no shelter Stream; shelter

100 100 100 100

100 100 95 100

100 100 100 100

95 100 55 100

100 100 100 100

100 100 95 100

– 33 – –

– 100 – –

– 100 – –

a See Table 1 for a description of the experimental variables. Data for experiments VIII and X from Reed and McLeod (1994), Conner et al. (2000b). b BB, balled and burlapped seedlings; HB, handbagged seedlings; DBR, bareroot seedling in water 30–60 cm deep; and SBR, bareroot seedlings in water 30–60 cm deep.

Three other oak species (Q. michauxii, Q. nuttallii and Q. phellos) have been shown to be marginally appropriate for restoration of the delta, depending on the elevation of the planting locations. Q. nuttallii Palmer (Nuttall oak) and Q. phellos L. (willow oak) had survival of greater than or equal to 70% in at least one experiment (Table 6). These species are moderately and weakly to moderately tolerant to flooding, respectively (Table 2). Survival of Q. phellos was reduced in experiment V when the seedlings were not protected by tree shelters (Table 4). Because the planting locations in this experiment were largely terrestrial and not in water, beavers were not likely responsible. Other terrestrial herbivores may have been responsible for the mortality or the microenvironment created within the tree shelters was responsible for enhanced survival. Only one other species (Q. michauxii Nutt. (swamp chestnut oak)) had survival of between 50

and 70% in any experiment (Table 3). Survival was lower with containerized seedlings planted in drier sites of Experiment I and with large bare root seedlings in Experiment III (Table 3). This species is weakly flood-tolerant (Table 2). While six of the eight species (C. aquatica, F. pennsyl6anica, N. aquatica, Q. lyrata, Q. nuttallii and T. distichum with greater than 50% survival were at least moderately flood-tolerant, seven other species (Acer rubrum, Betula nigra, Liquidambar styraciflua, N. syl6atica, Q. laurifolia, Q. nigra, Platanus occidentalis), used in the field trials were also at least moderately flood-tolerant (Table 2) yet had poor survival (Tables 3–6). An examination of these species can illustrate other factors that must be considered in species selection for this type of restoration. N. syl6atica Marsh. (black tupelo) was planted in experiments IV and V and had very poor survival (Table 4). Seedling stock was purchased

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flood-tolerant, but since these ecotypes are recognized, the flood tolerance must likewise be variable. A dry site ecotype may have been used and hence the poor survival in a wet habitat. B. nigra L. (river birch) and L. styraciflua L. (sweetgum) are both moderately flood-tolerant species, yet had poor survival in experiments IV and V (Table 4). These species are also shade intolerant and generally known to be poor competitors. The delta has an open canopy, since the previous forest had been eliminated, but the herbaceous and grass layer is very dense and frequently several meters tall, with species such as Leersia oryzoides (L.) Swartz (cut grass) and Erianthus giganteus (Walter) Muhl. (plume grass). Survival of taller planting stock of B. nigra was greater, suggesting that the competition for light may have restricted survival (Table 4). Successful establishment of species in the delta is dependent on flood tolerance, shade tolerance and competitive ability. P. occidentalis L. (sycamore) was another species which would be expected to colonize the delta, yet did not do well when outplanted. It is moderately flood-tolerant and intermediate in shade tolerance. It most commonly establishes in

commercially and specified to be N. syl6atica var. biflora (Walt.) Sarg. (swamp tupelo), the wet site variety of this species. In this study and another study with N. syl6atica provided by a different commercial nursery (McCarron et al., 1998), this species did not survive well in wet or flooded soils. Two explanations are possible. First, the wrong variety was provided in both cases. N. syl6atica Marsh. var. syl6atica (black tupelo) is well known as an upland variety which does not survive well in wet habitats (McGee and Outcalt, 1990). The two varieties are normally differentiated by the habitat in which they are found (McGee and Outcalt, 1990). Alternatively, because the seedlings did poorly in two very different experiments, provided by two different nurseries, it may well be that this species is not as flood-tolerant as previously thought. Regardless, careful selection of planting stock is critical and if possible, local seed sources should be used to produce the seedling stock. A very similar situation exists for A. rubrum L. (red maple) which was also planted in experiments IV and V, except that wet and dry site ecotypes are recognized and have not been elevated to a variety status. The species is listed as moderately

Table 6 Percent seedling survival 3 years (experiment XI) and 4 years (experiment XII) after planting in the Fourmile Branch deltaa Species

Carya aquatica Nyssa aquatica Quercus falcata var. pagodaefolia Q. laurifolia Q. lyrata Q. nuttallii Q. phellos Taxodium distichum a

Experiment number XI

XII

Herbaceous controlb

Willow controlc

HW

HR

– 63 10 – 97 67 50 90

– 67 10 – 77 57 50 80

C – 73 20 – 83 67 67 83

PR

PW

Cut

– 60 3 – 80 63 67 90

– 47 27 – 80 57 37 93

73 – – 0 78 – – 75

Control 90 – – 0 90 – – 95

Herb 85 – – 0 90 – – 70

Data for experiments XI and XII from McLeod et al. (2000a), McLeod, et al. (2000b). See Table 1 for a description of the experimental variables. b HW, whole plot treated with herbicide; HR, planting row treated with herbicide; C, not treated; PR, planting row mowed; and PW, whole plot mowed. c Cut, willows cut and removed; control, willow canopy intact; and herb, herbaceous vegetation with no willow canopy.

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open soil habitats and is not known as a good competitor. In the delta, survival was low but those individuals that did establish grew very well (height of 3–4 m by 1997; Table 4). Another moderately flood-tolerant species that unexpectedly had poor survival was Q. laurifolia Michx. (laurel oak) (Table 6). This illustrates another common difficulty in restoration, the extreme atypical year. These seedlings were outplanted in late winter of 1994. During the first growing season, two flood events occurred with maximum water depths of 1 m. These flood depths totally submerged the seedlings for several days duration. It would be very surprising for survival of this species to be unaffected by these hydrological events, which occurred only once during a 7-year time period (1990 – 1996). While there was low survival of this species at higher elevation planting sites following the first growing season, flooding during the late winter of 1995 killed the remaining individuals. Dormant season flooding usually does not harm deciduous tree seedlings, but Q. laurifolia is semi-evergreen and hence had leaves present when flooded during the winter. The poor survival of Q. nigra L. (water oak) was somewhat unexpected, but considering the weak to moderate flood tolerance, shade intolerance, slow growth, and poor competitive ability, it might require ideal environmental conditions to become established. The other nine species (Liriodendron tulipifera L. [yellow-poplar], Q. alba L. [white oak], Q. coccinea Muenchh. [scarlet oak], Q. falcata var. pagodaefolia Ell. [cherry bark oak], Q. margaretta Ashe [scrubby post oak], Q. marilandica Muenchh. [black jack oak], Q. rubra Michaux [northern red oak], Q. shumardii Buckl. [Shumard oak], and Q. stellata [post oak] Wangenh.) used in the experimental trials have poor flood tolerance and were chosen for the initial field trials, based on an apparent misconception of the delta hydrology existing from the late 1980s. From 1985 to 1990, the delta had been very dry and early successional vegetation was dominated by old-field successional species (Andropogon sp. and P. taeda). Several experiments demonstrated that small differences in elevation of the planting sites can

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result in significant differences in survival. For example, F. pennsyl6anica can withstand being planted in sites with 0–30 cm of standing water, but not in sites with 30–60 cm of water (Table 5). Elevation differences within the herbaceous competition control field plots, shown in Fig. 2, were much more influential in affecting survival than the different methods and intensities of controlling the herbaceous vegetation (experiment XI, McLeod et al., 2000b). Several species used in experiment XII also showed differential survival based more on elevation differences than on the removal of a willow canopy (McLeod et al., 2000a). Elevation influences the depth of water during flooding and depth to the water table during drought, both of which ultimately influence survival. Several other important silvicultural aspects have been demonstrated by these studies, in addition to the importance of species selection and planting elevation. Tree shelters do not guarantee complete protection from herbivores, but they do provide a good level of protection. This protection is especially necessary in standing water habitats where evidence of beaver activity is present (Table 5). Conner et al. (2000a) also showed this result in plantings in the Pen Branch delta. In drier habitats, tree shelters can provide increased protection for seedlings from terrestrial herbivores (Table 4) and may be necessary where floodwater depth can be expected to exceed the height of the tree shelters, exposing the seedlings to beaver herbivory. Dulohery et al. (2000) also indirectly showed the need for herbivory control in the Pen Branch corridor, although the herbivore differed (feral hogs) and the control mechanism differed (fencing). Some initial plantings had to be replanted due to immediate herbivory and were not successfully established until fences were established. It would be very informative to see how tree shelters would fare against feral hog herbivory. Bare root planting stock generally did as well as or better than more expensive planting stock (Table 5). Seedling height which would keep the seedlings from being totally submerged by any growing season flood was also beneficial (Table 4). This minimal height necessary would be influ-

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Fig. 2. Topography of the herbaceous competition control field trial site (experiment XI). Twenty-five individual 15 × 15 m2 plots were laid out in this site.

enced by the flood characteristics and the floodtolerance of the species. For example, total mortality of Q. laurifolia occurred as a consequence of the same events that C. aquatica, Q. lyrata, and T. distichum survived (Table 6). Other silvicultural aspects, such as fertilization (Table 3), acclimating the seedlings prior to planting (Table 5), or attempting to control the herbaceous or willow competition (Table 6) were not influential in affecting growth or survival and incurred additional costs. Future restoration plantings should involve multiple species, with possible differentiation of planting sites by elevation with wetter or standing water sites planted with T. distichum, N. aquatica, and C. aquatica and slightly drier sites planted with F. pennsyl6anica and the Quercus species. A mixed species approach to restoration is desirable for diversity and is accomplishable in the delta. This does not mean that small scale monotypic patches will not occur or are undesirable. In fact, in some very wet areas, there are so few species that can tolerate these conditions that only mono-

typic communities can be expected to develop. This type of planting (species selected for each planting location) requires that each area and habitat type be evaluated, and that the planting crew must be well trained and supervised. In extrapolating this research to other stream systems, including Pen Branch, matching the species with the hydrology is obviously very critical. Pen Branch delta is wetter which further restricts the choice of appropriate species and also makes the use of tree shelters even more critical to prevent beaver herbivory. This can be observed in the results of Conner et al. (2000a) who showed survival of T. distichum and N. aquatica better than that of N. syl6atica and F. pennsyl6anica. Dulohery et al. (2000) working in the Pen Branch corridor, which generally did not have standing water as was found in the Pen Branch delta and less subject to flooding than the Fourmile Branch delta, showed that T. distichum and F. pennsyl6anica had better survival that N. aquatica and Q. michauxii. The different results in survival of N. aquatica and F. pennsyl6anica between Dulohery

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et al. (2000) and Conner et al. (2000a) has no obvious explanation except that the former study was done in the corridor and the latter in the delta. Results of this study more closely match those of Conner et al. (2000a). Success of restoration should be measured not only at the level of survival of individual plantings, but by how the community develops over the future decades. For instance, although individuals have been established successfully, will the resultant community existing in 10 or 20 years be different as a result of the specific plantings? Will the communities converge on a similar community type, guided by the environmental constraints of the delta? Will this community differ from the result of natural succession? The answer to these questions, although long-term, would provide extremely useful data with which to evaluate the entire process of wetland restoration and appraise its value relative to natural succession. The research over the past 8 years has contributed greatly to the general paucity of knowledge about bottomland forest restoration. In addition, we have successfully established a great many individuals of a number of tree species in the delta. Once they reach maturity and begin producing seed, plant succession in the delta will be greatly accelerated. For several of these species, including F. pennsyl6anica and T. distichum, this has already begun. Thus, when the vegetation of the Fourmile Branch delta is examined at some future date, tree establishment will be observed to have originated from a great number of points from within the delta, in contrast to the linear succession usually observed only from the margins of a disturbed area. In summary, species selection is most critical, followed by protection from herbivores. Fertilization and control of herbaceous and/or willow competition were not necessary. Bare root planting stock will produce good survival if seedling height is considered relative to the maximum flood depth that is anticipated.

Acknowledgements This research was supported by the Savannah River Technology Center (Westinghouse Savannah

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River Company) and Financial Assistance Award Number DE-FC09-96SR18546 from the US Department of Energy to the University of Georgia Research Foundation. Support of Drs E. A. Nelson and L. D. Wike (technical representatives at SRTC) was very much appreciated. Technical assistance was provided by J. Barnes, K. Barnett, J. Burkett, T. Ciravolo, B. Dietsch, P. Drannon, A. Ellis, T. Frantom, P. Griffin, R. Lumpkin, B. Moyer, V. Parrish, M. Reed, R. Risher, C. Robert, A. Schumpert and M. Steed.

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