The effects of flooding on bottomland hardwood seedlings planted on lignite mine spoil in east Texas.
During the 1950's to 1970's bottomland hardwood (BLHW) wetlands suffered losses exceeding 120,000 ha per year (MacDonald et al. 1979; National Research Council 1982; Wilen & Frayer 1990). In Texas, over 60% of an original estimated 6.5 million ha of bottomland vegetation (i.e., western riparian and bottomland hardwoods) were lost by the 1980's, primarily due to conversion to agriculture and reservoir construction. Additional reservoir construction is expected to directly eliminate another 20% of east Texas bottomland hardwoods and degrade substantial acreage of the remainder (Frye 1986). Although forested wetlands are declining more slowly now (Tansey & Cost 1990), the magnitude of their decline, and of wetlands in general, prompted the Bush administration to issue a "no-net loss" wetland policy (Anonymous 1989) later endorsed by the Clinton administration (Anonymous 1993).
Surface-mining companies can assist in fulfilling this policy through wetland creation efforts because of regulatory requirements for soil stabilization, sediment retention, and treatment of mine drainage on reclaimed surface-mined lands (Nawrot & Klimstra 1989; Kepler 1990; Brenner 1992). In Texas, over 405,000 ha of land eventually will be surface-mined (Hossner 1980; McKnight 1992), thereby creating numerous opportunities for BLHW wetland creation. Techniques for creating BLHW systems, however, are considered both inefficient and unpredictable (Clewell & Lea 1990), particularly on surface-mined lands where altered soil properties may affect growth and survival of BLHW species. Relative to soils from BLHW systems, mine spoil from east Texas is typically low in organic matter, highly compacted, and has poor soil structure (Dixon et al. 1980; Patrick 1981). There is a need for basic and applied research directed at the creation and restoration of BLHW wetlands on surface-mined lands.
The objectives of this study were to determine the effects of flooding on the growth and survival of seedlings of BLHW wetland species planted on lignite mine spoil in east Texas and develop management strategies for the creation and/or restoration of BLHW wetlands on surface-mined lands in east Texas.
STUDY AREA AND METHODS
The study was conducted in a series of nine experimental ponds constructed on 1-yr old lignite mine spoil on Texas Utilities' Big Brown Mine in Fairfield, Texas. Each experimental pond consisted of a shallow basin and a planting shelf that was approximately 12 m long and 2.5 m wide. A gravitational flow system connected each pond to a 0.5 ha reservoir.
Spoil material from this site is heterogeneous but generally is of intermediate texture and often approximates silty clay loams and clay loams (Dixon et al. 1980). Spoil was moderately acidic (pH = 6.7 [+ or -] 1.1; x [+ or -] 1 SD; n = 18) and low in nitrogen (N) (1.4 [+ or -] 0.6 ppm; n = 18), and depth of the water table was > 30 cm during drawdown. Due to the low nitrogen levels, the shelves of the experimental ponds were fertilized with 112 kg/ha of 30-10-0 fertilizer in April 1991. Preliminary flooding experiments indicated that three ponds had significant leakage; therefore, bentonite clay was added in summer 1991 to seal the shallow basins within these ponds.
Five species were chosen for study based upon flooding tolerance (Hook 1984) and wildlife value. In order of decreasing flood tolerance the species were: baldcypress (Taxodium distichum), overcup oak (Quercus lyrata), Nuttall oak (Q. nuttalli), willow oak (Q. phellos), and Shumard oak (Q. shumardii). Bare-root seedlings were obtained from a commercial nursery located in Leland, Mississippi, where they were grown from locally collected seeds. Seedlings were placed in cold storage for one month prior to planting.
[FIGURE 1 OMITTED]
On 7 February 1992, 25 seedlings of each species were planted on the shelf of each experimental pond (n = 225 per species). To ensure maximum dispersion (Hurlbert 1984) and to prevent clumping of seedlings on highly acidic microsites (Arora et al. 1980), each shelf was divided into five square blocks each consisting of five seedlings of each species randomly placed at 0.5 m intervals.
Three experimental flooding regimes were designed to test the effects of flooding on seedling growth and survival (Figure 1). Treatment 1 contained five flooding episodes of long duration ([greater than or equal to] 15 days each) with the initial flooding episode occurring in early spring (10 April). Treatments 2 and 3 each consisted of four flooding episodes [less than or equal to] 16 days each; however, the initial flooding episode of treatment 3 was longer (16 days vs 10 days) and earlier in the spring (16 April vs 14 May) than the initial flood period of treatment 2. Flooding regimes were equally and randomly replicated among the nine ponds (i.e., 3 replicates/flood trt). Seedlings were not flooded until April to allow time for root establishment. Ponds were flooded approximately 5-10 cm deep according to their designated schedule except for rare one- to three-day deviations. Following drawdown, the upper 5 cm of soil remained saturated < 5 days.
Seedling survival was monitored every 7 to 10 days. Height from the ground to the highest point of the stem with a living leaf was measured on 7 February, 12 May, 20 July, and 25 September. Survival data were analyzed with a fixed-effects analysis of variance; preplanned contrasts (Freund & Wilson 1993) were used to compare treatment and species differences. Analysis of covariance (Lentner & Bishop 1986) was used to determine treatment effects on height growth with initial height as the covariate.
[FIGURE 2 OMITTED]
Final survival and the timing of mortality were related to the flood tolerance of a given species. The order of highest to lowest survival was baldcypress, overcup oak, Nuttall oak, willow oak, and Shumard oak in all treatments except treatment 2, where both baldcypress and overcup oak had 100% survival (Table 1 and Figure 2). Baldcypress also exhibited 100% survival in treatments 1 and 3, whereas overcup oak survival exceeded 93% and did not differ significantly (P > .05) from baldcypress in any treatment.
With the exception of Shumard oak, survival of all species was [greater than or equal to] 90% in treatment 2 and [greater than or equal to] 85% in treatment 3. Survival did not differ (P > .05) between these treatments for any species. However, Shumard oak seedlings demonstrated lower survival and three to four times greater variability in survival than most other species in these treatments.
The lowest survival rates of all hardwood species were in treatment 1 (Table 1). Shumard oak (0% survival) was unable to tolerate the frequent and prolonged flooding conditions of treatment 1. Willow oak exhibited a mean survival rate of 20% which was not significantly (P > .05) different from that of Shumard oak. In addition, the variability of willow oak survival was exceedingly high with the standard error almost as large as the mean (x = 19.1; SE = 16.8).
The relationship between the flood tolerance of a given species and the timing of mortality was most visible in treatment 1. Survival of Shumard oak dropped to < 80% following the first 22 day flooding period (10 April to 1 May) and dropped to < 30% following the next flooding period (8 May to 31 May) (Figure 2). In contrast, the more water-tolerant willow oak exhibited > 90% survival following the first flooding period, 65% following the second flooding period, and 20% after the third flooding period. Survival of Nuttall oak equaled willow oak after the second flooding episode (65%), but unlike willow oak which experienced further declines, Nuttall oak survival remained relatively constant for the rest of the study.
Analysis of covariance indicated that first-year growth rates were significantly related to initial height (f = 12.5; P = .0096; df = 1), species (f = 6.63; P = .0157; df = 4), and treatment-species interactions (f = 3.71; P = .0006; df = 7). Baldcypress exhibited significantly ([proportional] < .05) greater growth than the hardwood species in all treatments, with the exception of Shumard oak in treatment 2 (Table 2). Baldcypress growth was significantly greater in treatment 1 than in treatments 2 and 3, but no other treatment differences were observed for any species. Low growth rates were common for the hardwood species. Leaf abscission often resulted in negative growth rates since height was measured from the ground to the highest point of the stem with a living leaf. No consistent differences among the hardwood species were observed within treatments.
DISCUSSION AND CONCLUSIONS
This study indicates the importance of hydroperiod in regulating growth and survival of BLHW seedlings. Furthermore, the results of this study and others (Gorsira & Risenhoover 1994; Holl & Cairns 1994) suggest that the potentially slow growth rates of BLHW seedlings planted on lignite mine spoil, the complexity of BLHW systems, and the time required to reach maturity even under more favorable conditions (Walbridge 1993) pose significant problems to creation and restoration efforts on surface-mined lands (Gorsira & Risenhoover 1994; King 1994).
The consistent trend within the rank-order of survival suggested that all flooding regimes used in this study were of relatively long duration for the less water-tolerant species. This was particularly true for Shumard oak, which did not exceed 63% survival in any treatment. Other studies also reported that Shumard oak is weakly water tolerant and that prolonged flooding can cause substantial mortality (Hosner 1960; Krinard & Johnson 1981).
The generally low growth rates of all hardwood species may have been related to factors such as flood-stress and nutrient limitations. Several studies note that growth of BLHW is sensitive to soil moisture and nutrient levels (Kennedy 1970; Harms 1973). Soil analyses in fall 1992 indicated that soil N was extremely low ([less than or equal to] 1.0 ppm; n = 13) in all ponds. A lack of N can inhibit growth and/or result in the allocation of the available N to the root system at the expense of the shoots, thereby reducing height growth (Dickson & Broyer 1972). Low soil pH, such as that associated with surface-mining operations in other regions of the United States (Williston & LaFayette 1978) can alter the availability and uptake of nutrients (Haines & Carlson 1989) and thereby limit tree growth. The soil pH observed in this study, however, falls within the preferred ranges of the test species (Williston & LaFayette 1978) and presumably did not affect seedling growth and survival.
Created BLHW wetlands will require a considerable period of time before they can begin to replace the values associated with most natural stands (Walbridge 1993; King 1994). Gorsira & Risenhoover (1994) noted that reclaimed woodlots on Big Brown Mine will require at least 27 yr before vertical structure, canopy closure, and other habitat features resemble those of non-mined woodlots. Permitting agencies should, therefore, carefully review applications for permits to impact BLHW wetlands and should demand long-term commitments ([greater than or equal to] 25 yr) to insure successful mitigation through BLHW creation or restoration. Mitigation banks may provide a plausible alternative (Hammer et al. 1994). When feasible, reclamation officials should be granted the flexibility to experiment with BLHW wetland creation techniques in the normal reclamation process (Klimstra & Nawrot 1985).
The long-term success of a creation or restoration project will necessitate designing areas so that the mean and variation of the duration, frequency, depth, and timing of flooding at each point in the floodplain can be determined with a high level of confidence, thereby enabling species to be planted at the appropriate point along the flood gradient. Securing soils from existing BLHW wetland areas also may increase the probability of success of BLHW wetland creation on surface-mined lands. Added soils can enhance the establishment of understory vegetation that can increase the vertical structure and plant diversity of the wetland site and provide substrate stabilization if mass mortality of seedlings occurs (Clewell 1988; Brenner & Hofius 1990; Brenner 1992, Gorsira & Risenhoover 1994). Furthermore, the higher organic matter and nutrient content of BLHW wetland soils (Patrick 1981) as compared to lignite mine spoil (Dixon et al. 1980) may improve growth rates and survival of planted seedlings.
Table 1. Between-species comparisons of treatment effects on mean first- year seedling survival of bottomland hardwood wetland species planted in experimental ponds on Texas Utilities' Big Brown Mine, Fairfield, Texas, 1992. Mean Percent Treatment Taxon Survival (SE) * n 1 baldcypress 100.0% (0.0) A (68) Nuttall oak 41.4% (11.6) B (57) overcup oak 93.2% (2.8) A (74) Shumard oak 0.0% (0.0) C (58) willow oak 19.1% (16.8) BC (58) 2 baldcypress 100.0% (0.0) A (75) Nuttall oak 93.9% (1.7) A (66) overcup oak 100.0% (0.0) A (74) Shumard oak 63.0% (20.9) B (61) willow oak 90.7% (4.9) AB (61) 3 baldcypress 100.0% (0.0) A (74) Nuttall oak 95.5% (4.5) A (67) overcup oak 97.5% (1.3) A (78) Shumard oak 31.0% (16.2) B (51) willow oak 85.3% (5.0) A (50) * Means sharing a letter within the same treatment do not differ (P > 0.05). n Represents the total number of seedlings alive at the initiation of the study (April 1991) from a total of 75 seedlings/species/treatment planted in February 1991. Table 2. Between-species comparisons of treatment effects on mean first- year height growth of seedlings of bottomland hardwood wetland species planted in experimental ponds on Texas Utilities' Big Brown Mine, Fairfield, Texas, 1992. Height Treatment Taxon Growth (SE) * n 1 baldcypress 16.5 cm (1.3) A (68) Nuttall oak -0.6 cm (2.1) C (23) overcup oak 4.9 cm (1.2) B (69) Shumard oak -- -- -- -- willow oak 3.4 cm (3.0) BC (11) 2 baldcypress 8.5 cm (1.2) A (75) Nuttall oak 3.0 cm (1.3) B (62) overcup oak 4.9 cm (1.1) B (74) Shumard oak 5.4 cm (1.6) AB (41) willow oak -2.1 cm (1.3) C (56) 3 baldcypress 10.4 cm (1.2) A (74) Nuttall oak 1.0 cm (1.3) C (64) overcup oak 5.6 cm (1.1) B (76) Shumard oak 0.7 cm (2.4) BC (17) willow oak -1.0 cm (1.5) C (43) * Means sharing a letter within the same treatment do not differ (P > .05). n Represents the total number of seedlings surviving the treatment.
Funding for this study was provided by Texas Utilities Electric Company through the Texas Utilities Environmental Research Program. I wish to thank M. W. Weller, C. Jackson, J. Richards, J. Thomas, S. E. King, W. R. Harms, A. Clewell, W. H. Conner, B. Keeland, B. Vairin and W. H. McKee for assistance during the study and manuscript reviews.
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Sammy L. King
Department of Wildlife and Fisheries Sciences
Texas A & M University, College Station, Texas 77843
Present address: National Biological Service, Southern Science Center
700 Cajundome Boulevard, Lafayette, Louisiana 70506
Direct reprint requests to SLK at: firstname.lastname@example.org
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|Author:||King, Sammy L.|
|Publication:||The Texas Journal of Science|
|Date:||Feb 1, 1996|
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