Initial responses of woody vegetation, water quality, and soils to harvesting intensity in a Texas bottomland hardwood ecosystem

Fores~~;ology

Management ELSJWIER

Forest Ecology and Management ‘X(1997)201-215

Initial responses of woody vegetation, water quality, and soils to harvesting intensity in a Texas bottomland hardwood ecosystem Michael G. Messina aP * , Stephen H. Schoenholtz b, Matthew W. Lowe a, Ziyin Wang b, Dolores K. Gunter b, Andrew J. Londo a a Deparhnent ofForest b Department of Forestry,

Science, Texas A&M University, College Mississippi State University, P.O. Drawer

Station, TX 77843-2135, USA FR, Mississippi, MS 39762, USA

Abstract Sustainable management of bottomlandhardwoodforestecosystems requiresa knowledgeof responses to management impacts,including timber harvesting.The effects of clearcuttingand partial cutting on woody vegetationregeneration dynamics,surfaceand groundwaterquality, soil physical properties,and soil respirationwere tested in a bottomland hardwoodecosystemin southeastern Texas,USA, throughcomparisonwith non-cutcontrol areas.Overstory removalonly slightly affected compositionof woody vegetation regeneration1 year after harvesting comparedwith pre-harvest composition.Initial compositionin both cutting treatmentsappearedto be the strongestdeterminantof post-harvest composition,at leastfor the first year after harvesting.There werefew significantdifferencesin groundwaterproperties when harvestingtreatmentswere comparedwith control areasduring a 17-monthperiod following harvest.Turbidity, temperature,electricalconductivity, dissolved0,, NH,-N, NO,-N, and PO,-P of streamwaterdid not vary significantly amongtreatments.Slightdecreases in total andmacroporositywereobservedin association with higherbulk densitiesat O-5 cm depthin the cleat-cutandpartialcut treatments.Saturatedhydraulicconductivity valuesdid not declinesignificantlywith treatmentintensity.No significantdifferencesamongtreatmentsin measured soilphysicalpropertieswereobservedat 5-10 cm depth.Although in situ soil respirationincreasedwith harvestintensity, treatmenthadno significanteffect on mineral soil respiration.In summary,mostvariablesshowedonly slight responseto harvesting,thereby indicatingthat harvesting practicescanbe conductedwith minimalinitial impactson measured response variables. Keyworcls: Wetlands; Species diversity; Soil respiration; Clearcutting; Regeneration

1. Introduction

Southern bottomland hardwood forests provide many values including flood and sediment control, removal of nutrients originating from upland sources, recreation, wildlife habitat, and as a source of forest products. Timber removals across the South from these forests totaled 22.1 million m3 in 1984, and are projected to increase to 36.3 million m3 by 2030 * Corresponding author.

(USDA Forest Service, 1988). However, the area of bottomland hardwoods in the South is predicted to decrease from 12.2 million ha in 1990 to 10.6 million ha in 2030, and timber removals are expected to exceed growth for nearly this entire period (USDA Forest Service, 1988). It is likely that bottomland hardwood forests will come under increased harvesting pressure well into the 21st century. Thus, it is imperative that the influence of timber harvesting operations on bottomland hardwood ecosystem functions and values be investigated.

0378-I 127/97/$17.00 Copyright0 1997ElsevierScience B.V. All rightsreserved. PII SO3781 127(96)03895-9

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A study was conducted in southeast Texas, USA, to examine the effects of standard bottomland hardwood forest harvesting activities on a variety of ecological functions. The objectives of the research were to test the effects of harvesting intensity on woody vegetation regeneration dynamics, groundwater chemistry, surface water quality, soil physical attributes, and soil respiration. The null hypotheses tested were: 1. early post-harvest composition does not reflect pre-harvest composition, and any shifts in species diversity will not reflect the degree of harvest disturbance; 2. harvesting would not initially increase soil temperature and moisture, nor alter the relationship between decomposition and plant uptake of nutrients, and therefore not result in higher nutrient content of surface- and groundwater; 3. soil physical properties are not altered by harvesting intensity; 4. degree of stand removal does not affect subsequent soil microbial activity as measured by soil respiration rates both in situ and in laboratory incubations.

2. Materials

and methods

The study was located in the floodplain of the Neches River in Tyler County, Texas, USA, (30”39’N, 94Y’W) on land owned by Temple-Inland Forest Products Corporation, Diboll, Texas. The site occurs within the temperate deciduous forest biome with a humid continental climate. The Neches River originates in northeast Texas within the Coastal Plain province and flows south to join the Sabine River immediately before draining into the Gulf of Mexico near Beaumont, Texas. Overbank flooding, both spatially and temporally, is uncommon on the study area since the flow of the Neches River is controlled by two upstream dams. No flooding was observed during the study period. The general study area is a broad, level flat within the first bottom of the Neches River with all treatment plots situated within 2 km of the main river channel. Elevation above sea level ranges from about 17 to 19 m with microsite variation sufficient to influence plant species occurrence.

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The climate is warm and humid with an average annual temperature of about 19.4”C with a mean monthly range from lO.O”C in January to 27.2”C in July. The frost-free season is 241 days. Annual precipitation averages 132 cm and is generally well distributed (Griffiths and Bryan, 1987). Soils are varied but are predominantly Aeric Dystraquerts (Ozias series), Fluvaquentic Dystrochrepts (Iulus series), and Fluvaquentic Eutrochrepts (Laneville series) (R. Dolezel, personal communication, 19921. Soils varied in texture from clay to loam. but were predominantly clay loams and loams. Personnel from the Natural Resources Conservation Service determined that not all soils on the study site are hydric, and therefore not all of the study area was jurisdictional wetland. The study area was heavily logged in the early 1920s but has been largely undisturbed since (Norman Davis, personal communication. 1992). The overstory is principally sweetgum and water oak, whereas the midstory is predominated by ironwood. Understory composition varied considerably and will be described in detail later. Appendix A contains scientific and preferred common names of woody plant species encountered on the study site. The effects of clearcutting (CC), partial cutting (PC), and a non-cut control (CT) were tested on 8. l-ha approximately square plots. The clearcuts had all standing woody vegetation severed with a mechanical harvester fitted with a rotating sawhead. One skid trail was established on each side of the streams to make crossings unnecessary. The PC plots had a basal area reduction of approximately 50% and were marked by Temple-Inland foresters to improve stand composition and condition, and to promote advance reproduction of desirable species (e.g. oak species and sweetgum). The treatments were arranged in three blocks, each containing three contiguous plots with one treatment per plot. Blocks were located along separate first-order, intermittent headwater streams within the Neches River floodplain which approximately bisected the blocks. Stream channel and treatment plot slopes were less than 1%. Treatments were arranged in downstream order as CT. PC, CC to avoid confounding effects of stream position on treatments. Streamside management zones extending about 20 m from each stream bank were left largely undisturbed with occasional selective harvesting.

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Harvesting occurred in September 1992 during dry conditions and according to best management practices (BMP). Dry soil conditions were defined as those in which tire tracking and rutting did not occur. No post-harvest site preparation was performed. The woody vegetational community was sampled immediately before and approximately 1 year after harvesting. The objective of this portion of the research was to monitor harvesting effects on both advance woody regeneration dynamics and new regeneration developing after harvesting. Nine permanent subplots were systematically established on a 3 X 3 grid in each treatment plot, with each subplot consisting of three concentric plots: a 0.004-ha plot for vegetation less than 4 cm diameter-at-breastheight (DBH), a 0.02-ha plot for vegetation 4- 11 cm DBH, and a 0.0%ha plot for vegetation over 11 cm DBH. The grid pattern of subplot establishment was abandoned where subplots would have been established in or close to the streamside management zones, in which case they were established randomly in a location deemed free of border influence. Prior to harvesting, data recorded for vegetation over 4 cm DBH included species composition and DBH. Data recorded for trees less than 4 cm DBH included species composition and origin (sprout or seedling). Trees in this size class were also marked for remeasurement with colored plastic-coated wire signifying year of origin and height class. Data were collected 1 year after harvest only on the 0.004-ha plots and included tallying of trees that survived harvesting as well as those that appeared since harvesting. A tree ring count on stumps of 754 trees over 11 cm DBH was also performed for description of age class distribution. Data analysis on vegetation composition consisted of using importance value and diversity indices for comparison of pre-harvest and post-harvest composition as well as composition among treatments for both pre- and post-harvest. The importance value index (IVI) used was a combined expression of relative frequency, relative density, and relative dominance (Krebs, 1985), where Relative frequency of sp A =

number of plots containing sp A number of plots containing all spp

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Relative density of sp A =

number of individuals of sp A number of individuals of all spp

Relative dominance of sp A =

basal area of SD I A basal area of all spp

Shannon’s diversity index was used as an expression of woody plant species diversity (Magurran, 1988). Streamwater and groundwater quality was monitored in streams and in piezometers, respectively, to determine effects of harvesting. It was hypothesized that harvesting initially would increase soil temperature and moisture, altering the relationship between decomposition and plant uptake of nutrients, and result in higher nutrient content of surface- and groundwater. Nine 2-m piezometers with screens at the bottom 30 cm were systematically installed across each plot two months before harvesting (July 1992) at which time monthly monitoring of water table depth and groundwater quality was initiated. Water temperature, pH, electrical conductivity, and dissolved oxygen were measured directly in piezometers with a portable analyzer (ICM Series 51000 Water Analyzer, Hillsboro, OR) after bailing and recharge. Groundwater samples were also analyzed for NO,-N, NH,-N, and PO,-P calorimetrically with an autoanalyzer (American Public Health Association, 1992). Number of groundwater subsamples in each plot ranged from none during dry periods to nine during period of high water tables. Results are reported for sampling dates with at least one groundwater subsamples in each plot. Streamwater was sampled at permanent grab-sampling stations located at plot borders. When streams were flowing, streamwater quality was measured at the same time and by the same techniques as those for groundwater, with the addition of turbidity measured with a portable turbidity meter (Monitek Model 2lPE, Hayward, CA) (American Public Health Association, 1992). Results are reported for sampling dates with at least two flowing streams. Sampling of soil physical properties was conducted during the first growing season (May through August 1993) following harvest. One hundred 5-cm diameter X 5-cm length intact cores were collected

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throughout each treatment plot on a 26 X 26 m grid at depths of O-5 cm and 5- 10 cm. At each sampling point, the forest floor was classified as (1) disturbed, (2) somewhat disturbed (forest floor present, but evidence of alteration), or (3) severely disturbed (forest floor completely absent or mixed with mineral soil). For each intact core, water content was measured by gravimetry with oven drying (Gardner, 19861, bulk density was measured by the core method (Blake and Hartge, 19861, and saturated hydraulic conductivity (K,,,) was measured by the constant head method (Klute and Dirksen, 1986). Macroporosity (pore diameter over 0.06 mm> of intact cores was measured using a tension table with a 50-cm column of water and comparing saturated core weight with drained core weight. Microporosity was calculated as core weight loss between macroporosity measurement and 105°C oven-dry measurement. Total porosity was the summation of macroporosity and microporosity measurements. Soil respiration rates both in situ and in laboratory incubations were measured. Eight sampling points were randomly located in each treatment plot on a common soil series (Ozias) such that no point was within 3 m of a large slash pile, stump, or tree. At each point, soil CO, efflux was measured for 24 h with the static-chamber soda lime absorption technique (Edwards, 1982; Edwards and Ross-Todd. 1983). Tin cans (2.9 1) with a surface area of 0.019 m2 were used as incubation chambers and were set upside down approximately 1 cm into the soil to minimize outside contamination and root severing which may increase respiration rates. Jars containing 30 g of soda lime were placed on a hardware cloth platform under the cans. To reduce unnatural heating of the soil surface, the cans were coated with silver paint and covered with small plywood shelters. Unnatural cooling was not considered to be a problem. Temperature of the top 15 cm of soil was measured when chambers were placed in the field. Two sealed cans per treatment resting on the soil surface functioned as blanks. Respiration was measured approximately monthly with cans located in the same place each time to minimize spatial variation. Soil samples were collected to a depth of 15 cm with a bucket auger after removal of forest floor from points near the incubation chambers, but not close enough to affect in situ respiration, for determi-

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nation of the mineral soil contribution to total soil respiration rates (Edwards and Ross-Todd, 1983). Temperatures of the top 15 cm of soil were determined for each sample at collection. In the laboratory, the soil was passed through a 2 mm sieve to remove roots and large soil organisms. After sieving. a 50 g sample of soil was oven dried at 100°C for 24 h and reweighed to determine gravimetric moisture content. The sieved soil was incubated for 10 days by treatment group at average field soil temperature using the wet alkali absorption procedure for CO, (Anderson, 1982). One sample per field point was incubated giving 72 samples at each coilection, plus blanks. Soil (50 g) was placed in screw-top jars (946 ml) containing vials with 30 ml of 0.5 N NaOH as an alkali trap, which was later titrated with 0.5 M NC1 after addition of BaCl?.

3. Results

and discussion

3.1. Vegetation composition

In this discussion, trees over 11 cm DBH will be referred to as ‘overstory’, trees 4- 11 cm DBH as ‘midstory’, and trees less than 4 cm DBH as “understory’. However, the overstory category was quite broad and included distinct strata reflecting species composition. For instance, ironwood was consistently overtopped by other species and was never dominant or codominant. Therefore, ‘overstory’ will refer to those trees over 1 I cm DBH, and not exclusively to trees that comprised the main upper canopy stratum. The diameter distribution for all overstory tree species combined was a negative exponential function sometimes construed to indicate an uneven-aged stand (Fig. 1). However, individual species distributions were sometimes quite distinct and suggested an even-aged stand (Fig. 2). The age distribution was characteristic of an even-aged stand with a mean age of about 55 years (Fig. 3). The negative exponential diameter distribution and symmetric age distribution are common in naturally-regenerated mixed hardwood stands (Nixon et al., 1977). The pre-harvest stand attributes were typical of a well-stocked, species-rich, bottomland hardwood riverine forest (Marks and Harcomb, 198 1; Sharitz

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40

f ; I

:Lw

-

35

20

25-

k a

20 15.

-

;

lo-

-

10

E

5

-

5

-

0

25

15

0 0

25

50

Tree

75

100

DBH

(cm)

125

0

150

and Mitsch, 1993). Overall, 37 species or speciesgroups were identified. Species numbers varied in the order overstory (32) > understory (29) > midstory (23) (Appendix A). Importance value indices showed the overstory to be dominated by sweetgum, water oak, and ironwood, although ironwood was considerably smaller in size than both sweetgum and water oak (Table 1). Ironwood occurred in an overtopped stratum in both the midstory and overstory categories whereas water, willow, swamp chestnut, and cherrybark oaks occupied the upper stratum of the overstory category. Sweetgum was slightly subordinate to the oaks. Ironwood’s high IVI was due to its high relative frequency and density (Table 1). Even though ironwood had the largest number of stems in the overstory category, its small size made it a relatively minor contributor to total basal area. Mean DBH for this category was 28 cm, but the range was substantial as ,

c

- -.--.--. .---

Oaks Blockgum Ironwood Sweetgum

20

40

60

Tree

80

DBH

100

120

140

25 2.

t

0

20

40 Age

Fig. 1. Diameter distribution for all trees of at least 11 cm DBH (overstory) in a Texas bottomland hardwood forest.

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(cm)

Fig. 2. Diameter distribution for oak spp., ironwood, sweetgum, and blackgum of I1 cm DBH (overstory) in a Texas bottomland hardwood forest.

60

60

100

120

(Yd

Fig. 3. Age distribution for all trees of at least (overstory) in a Texas bottomland hardwood forest.

11 cm DBH

some trees exceeded 100 cm. Average basal area for all species tallied was 30.2 m2 ha-’ in 355 trees ha-‘. Ring-count data indicated a relatively small range among ages of those species that comprised the main upper canopy stratum (Table 1). The most important of these species averaged about 60 years old. Those species that occupied lower strata of the overstory category (ironwood, blackgum, red maple, American elm, American holly) were 5-20 years younger. The distribution of IVIs among species in the midstory was much more uneven than that for the overstory (Table 2). Ironwood dominated this size class with an IV1 much greater than that of any other species. The midstory was comprised mainly of species characteristic of the lower strata of bottomland hardwood stands in this area (Nixon et al., 1977). The decrease in species richness from the overstory to the midstory was likely due to characteristic deep shade, interrupted only occasionally by tree-fall gaps. Mean DBH in this category was 6 cm. Density was not much greater than that in the overstory with 473 trees ha-” comprising 1.7 m* ha-’ basal area. Similar densities in the overstory and midstory categories reflected the history of minor canopy disturbance in these stands. The understory category (less than 4 cm DBH) contained 33 068 trees ha- ’ representing 29 species (Table 3). Composition was dominated by water and willow oaks due to their relative densities. Relative frequencies in this category were much less variable, because only some plots had large numbers of water and willow oaks and these were not common across

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Table I Attributes a of ten highest importance value species with mean DBH Pre-harvest data for all three blocks combined Species

IV1

DBH (cm)

(SE)

Sweetgum Water oak lronwood Cherrybark oak Blackgum Swamp chestnut oak Red maple American elm Willow oak American holly Size class values b

51.8

30 (0.6)

46.8 43.4

47Cl.l) 16 (0.2)

18.3 17.1 16.1 14.7 13.0 12.5 11.4

45 (2.5)

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in a Texas bottomland

Density (trees ha- ’ 1

Crown

class

hardwood Age

6.2 8.8 1.9

72 44 91

3.0 2.5 4.0

is---59 4?

32C2.1)

2.6 0.7 1.8

13 25 16

2.6 3.9 3.3

61 .5i 64

23 (0.9)

0.9

18

3.8

48

24c1.21

0.7 2.0 0.4

13 9 13

3.8 2.5 4.0

54 53 s7

-

55

19 (0.5)

48 (3.0) 19 (0.7) 28

30.2

a IV1 is importance value index; see text for explanation b Values for all 32 species in this size class.

355

of IVI and crown

forest.

class.

the sites. Most of the oak seedlings were congregated beneath conspecific adults, as is typical of oak species in these ecosystems (Streng et al., 1989). Germination of large numbers of oak seedlings in bottomland hardwood forests is common, but subsequent survival and growth into the midstory without canopy disturbance is not (Hodges and Gardiner, 1993). This explains the dominance of oak species in the understory, and the lack of these generally shade intolerant species in the midstory (Clatterbuck and Meadows, 1993).

Shannon’s Diversity Index was used to account for harvesting effects on both species richness and evenness (Magurran, 1988) (Table 4). Prior to harvesting, Shannon’s Index varied among the three treatments significantly ((Y = 0.05) in the order PC > CC > CT. Depth to mottling was 22, 13, and I2 cm in the CT, PC and CC, respectively, with the CT differing significantIy (at = 0.05) from the PC and CC, which did not differ from each other. Depth to mottling was likely greater in the CT because the systematic arrangement of treatment plots placed the

Table 2 Attributes a of ten highest importance value species within DBH range 4- 11 cm (midstory) in a Texas bottomland hardwood forest. Pm-harvest data for all three blocks combined

Table 3 value species Pm-harvest attributes a of ten highest importance with DBH less than 4 cm (uttderstoryl in a Texas bottonthmd hardwood forest. Pre-hat-vest data for all three blocks combined

Species

Species

IV1

Ironwood 120.3 Blackgum 38.1 Swamp privet 33.6 Silverbells 24.7 American holly 13.6 Red maple 13.6 Sweetgum 13.1 Deciduous holly 10.0 9.4 Sweetleaf 7.0 American elm Size class values b -

(cm)

DBH (SE)

Basal area Density (m* ha- ‘) (trees ha- ‘)

7tO.l) 6tO.l)

0.90 0.21

190 60

5 (0.1)

0.15

66

5 (0.1)

0.10

44

7 (0.41 7 (0.41 7 (0.51

0.07 0.08

15 18

0.06

15

6CO.3) 4to.21 7CO.7)

0.06 0.03 0.03

20 20 7 473

6

1.74

a IVI is importance value index; see text for its calculation. b Values for all 23 species in this size class.

IV1

Density

Relative density

Relative frequency

31.4 21.9

9.5 9.5

(trees ha- ’ 1 Water oak Willow oak Ironwood American holly Cherrybark oak Sweetgum Diamondleaf oak Deciduous holly Swamp privet Red maple

40.9

31.4 16.0

14.1 10.7 10.6

10.4 8.7 8.7

8.1

IO392 7248 264 1 1820 1628 1156 2250 765 592 589

8.0 5.5 4.9 3.5 6.8 2.3 1.8 1.8

8.0 8.6 5.8 7. I 3.6 6.4 6.9 6.3

a IVI is importance value index; see text for its cakulation. Relative values based upon entire species count of 29.

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Table 4 Shannon Diversity Index for species with DBH less than 4 cm (understory) arranged by cutting treatment a in a Texas bottomland hardwood forest Treatment

Shannon

Control Partial cut Clearcut Mean

1.950a 2.299b 2.2 13c 2.211

before

Shannon 1.980a 2.331b 2.331b 2.241

after

Change

b

NS NS 0.01

a Values within columns followed by the same letter are not statistically different at (Y = 0.05. b Statistical significance of change from before cutting to 1 year after; NS, P > 0.05.

CT upstream from the PC and CC, and therefore on slightly higher ground. The CT contained 51 329 trees per hectare, whereas the PC and CC had 23 428 and 24 443, respectively. One year after harvesting, Shannon Index increased in all treatments, but the increase was statistically significant ((Y = 0.01) only in the CC compared with pre-harvest values. Shannon Index varied among treatments in the order CC = PC > CT. Total species count across all three blocks increased by two due to the appearance of one boxelder in the PC and one Chinese tallow in the clearcut after harvesting. Changes in rankings among those species with the ten highest IVIs were minor indicating that preharvest composition strongly controlled post-harvest composition, at least 1 year following treatment (Table 5). Oaks (water, willow, diamondleaf, and cherrybark), sweetgum, and ironwood dominated postharvest composition. Furthermore, the sum of IVIs represented by the top ten species did not vary greatly among treatments before harvest (164.6168.0) or after harvest (160.5-165.9), or between years (Table 5). Survival of advance regeneration 1 year after harvesting was 64%, 50%, and 26% in the CT, PC, and CC, respectively, thereby directly reflecting degree of harvest disturbance. However, seedling germination and sprouting caused the total number of trees in 1993 to be 97%, 99% and 50% of those counted in 1992 in the CT, PC, and CC, respectively. The low percentage in the CC was not only due to physical eradication of trees through harvesting, but also to inhibition of establishment beneath the large slash piles. Therefore, roughly half of the trees counted 1 year following harvesting in the PC and

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one-quarter in the CC were survivors from before harvesting, whereas survivors in the CT accounted for roughly two-thirds of the inventory. Survival varied significantly ( (Y= 0.01) among treatments. Survival appeared to depend upon the distribution of trees between seedlings and sprouts. Prior to harvest, sprouts comprised 37%, 65% and 48% of the inventory in the CT, PC, and CC, respectively. However, survivors of the harvesting that were classified as sprouts were 99%, 98%, and 99% of the inventory in CT, PC and CC, respectively. Sprouts Table 5 Treatment effects on importance value index for top ten species with DBH less than 4 cm in a Texas bottomland hardwood forest Pre-harvest

Post-harvest

controt Water oak Willow oak Sweetleaf American holly Sweetgum Ironwood Diamondleaf oak Cherrybark oak Swamp privet Hickory Total Partial

47.8 34.2 13.0 12.9 10.9 10.1 9.5 9.0 8.7 8.5 164.6

Water oak Willow oak American holly Sweetleaf Diamondleaf oak lronwocd Cherrybark oak Sweetgum Hickory Red maple

45.5 35.8 12.6 12.2 10.6 10.3 8.7 8.5 8.3 8.0 160.5

33.6 27.6 23.5 18.2 14.4 12.3 10.5 10.2 9.5 8.2 168.0

Water oak Ironwood Willow oak Cherrybark oak American holly Red maple Deciduous holly Elm Sweetgum Hickory

38.8 23.1 21.5 14.3 13.5 11.8 10.9 10.8 9.8 7.9 162.4

33.5 32.9 20.3 16.3 12.6 10.4 10.0 9.8 9.8 9.3 164.9

Water oak Willow oak Diamondleaf oak lronwood Sweetgum Deciduous holly Elm Persimmon American holly Red maple

37.1 24.3 20.3 16.7 15.2 14.8 10.3 10.3 10.2 6.1 165.9

cut

Water oak lronwood Willow oak American holly Cherrybark oak Sweetgum Deciduous holly Red maple Elm Hickory Total Clearcut

Water oak Willow oak Diamondleaf oak Ironwood American holly Red maple Silverbells Cherrybark oak Swamp privet Elm Total

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that developed after harvesting comprised 23%, 46%, and 83% of the inventory in the CT, PC, and CC, respectively. The increase in sprouts as a percentage of inventory in the CC (48% before vs. 83% after) is expected due to the complete removal of a seed source in this treatment and the propensity of hardwoods to sprout from stump and roots after cutting. 3.2. Streamwater

and groundwater

quality

There were few statistical ((Y = 0.05) differences in groundwater properties among harvesting treat-

0 JASONOJFYAYJJASONOJF ~...........*......L...........,...,,. 1992

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ments during 17 months following harvest (Fig. 4). Water table levels were below the 2-m piezometer detection limit during the 3 months immediately following treatment and again near the end of the first growing season (August-November 1993), so groundwater quality assessments were not possible at those times (Fig. 4A). Nonetheless, several trends and treatment responses were observed. Groundwater temperature and dissolved 0, showed strong seasonal trends with relatively low temperature and high dissolved 0, during the winter and relatively high temperature and low dissolved 0, during the sum-

JASONDJFYAYJJASONDJF lS92 lD93

Fig. 4. Effects of harvesting on groundwater attributes in terms of water depth (A), temperature dissolved oxygen (E), and concentrations of ammonium (F), nitrate (G), and phosphate (H) Asterisks indicate statistical significance at (2 = 0.05.

-0.1 1994

(B), pH CC), electrical in a Texas bottomland

conductivity (II@, hardwood forest.

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mer (Fig. 4B and Fig. 4E). Water table levels tended to be lower in the uncut control areas following harvest (Fig. 4A); but only in January 1994, 16 months after harvest, was this difference statistically significant. During 17 months following treatment, average measurable monthly water table levels in the CT, PC, and CC treatments were 110,95, and 94 cm below the surface, respectively. This genera1 lack of harvesting effect on water table level in a bottomland hardwood forest contrasts results reported by Lockaby et al. (1994) who observed lower water tables in clearcut wetlands in Alabama, attributed to increased rates of evaporation which compensated for lower transpiration rates. Nitrate-N levels were consistently higher in the clearcut plots but significant differences occurred only during the fourth and fifth months after treatment (Fig. 4G). Nitrate-N levels 4 months after harvesting were 0.14, 0.58, and 1.29 mg 1-l for the CT, PC, and CC, respectively; and were 0.13, 0.14 and 0.48 mg 1-r 5 months after harvesting. Average NO,-N levels during 17 months following harvesting were 0.06, 0.19, and 0.64 mg 1-l for the CT, PC, and CC, respectively. This suggests that higher rates of nitrification and lower plant uptake of NO,-N may have occurred with increasing harvesting intensity. However, NO,-N levels for any of the three treatments in this study are well below the 10 mg 1-l water quality standard. Few harvesting impact studies have evaluated groundwater quality responses. However, Lockaby et al. (1994) reported no statistically significant harvesting effects on surface or groundwater NO,-N and PO,-P in an oligotrophic floodplain forest. They attributed lack of harvesting effects to the inherently low nutrient status of the study site and minimal physical soil disturbance using helicopter logging. Electrical conductivity, NH,-N, and PO,-P tended to be lower in the CC but treatment effects were not statistically significant. Phosphate-P, NH,-N, and NO,-N levels did not exceed 0.4, 0.7, and 1.4 mg l- ’ , respectively, for any treatment during the 17 months after harvesting, suggesting that treatments had minima1 overall impacts on measured groundwater quality. Turbidity, temperature, electrical conductivity, dissolved O,, NH,-N, NO,-N, and PO,-P of streamwater did not vary significantly among treat-

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ments following harvesting (Fig. 5). Shepard (1994) reviewed nine studies of effects of silvicultural practices on surface water quality in wetland forests and reported that water quality criteria were rarely exceeded by silvicultural operations and any effects were transient. In April 1993, streamwater pH was significantly higher in the PC and in July 1993, streamwater pH was significantly different among all three treatments (Fig. 50. Streamwater pH in April was 5.5, 5.6, and 5.5 for the CT, PC, and CC, respectively. In July, these treatments had streamwater pH levels of 5.9, 6.0, and 6.1. It is questionable whether these pH differences are ecologically significant, since all other measured streamwater attributes which more directly affect the aquatic system were not significantly altered by harvesting. Increased erosion from logging roads and resultant sediment delivery to streams is a concern in sloping landscapes (Brown and Binkley, 1994). However, in this floodplain forest, slope was less than 1% and erosion was not evident. 3.3. Soil physical attributes

Post-harvest bulk density at O-5 cm depth was 7% and 9% higher than control areas in the partial cut and clearcut treatments, respectively (Table 6). Slight decreases in total and macroporosity were observed in association with the higher bulk density of these treatments. Saturated hydraulic conductivity values did not decline significantly with treatment intensity. No significant difference in the measured soil physical properties were observed at 5-10 cm depth (Table 6). Logging operations during this study were halted when soil moisture increased. The lack of treatment effects suggests the value of conducting logging activities when soils are dry and less susceptible to compaction (Greaten and Sands, 1980; Karr et al., 1987). Approximately 75% of the PC and CC showed some evidence of forest floor disturbance and were classified as somewhat disturbed. These areas were observed where tree cutting and removal occurred and where visible alterations of forest floor such as crushed branches and broken stems were evident. Only one percent of the PC forest floor area was severely disturbed, as indicated by mixing of mineral soil with surface litter or by an absence of forest

210

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90 (1997)

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lowed by a general decline through winter (Fig. 6). Mean respiration rates were 6.81, 5.50, and 4.38 g m -’ day-’ in the CC, PC, and CT treatments, respectively. These rates varied significantly ((u = 0.05). There was a significant period X treatment interaction, likely because the pattern of CC > PC > CT occurred more commonly in the warm months (April-October) th an in the cool months (November-March). Furthermore, separation among treatments was greater in the warm months. Soil temperature also followed a seasonal pattern (Fig. 6B), with an exception bemg an abnormally

floor materials and exposure of mineral soil. Seven percent of the CC was severely disturbed. These areas of relatively severe disturbance had significantly higher bulk density at O-5 cm (Table 71, but other measured soil properties in these areas were not significantly altered. Soil physical properties at 5-10 cm were not different among forest floor disturbance classes (Table 7). 3.4. Soil respiration

In situ soil respiration rates exhibited a seasonal pattern with values peaking in early summer fol100

6

60 60 40 20 0

0

25

1.2 1.0

20

0.8

15

0.6 0.4

I; I-

10 5

0

1

:wrl-

Y

C

.

.i,

G

CA E z ,

e-m-e+ c 0.2 0.0

; z

-0.2

0 t‘

s‘ ,

‘1.

1

0.05 s0.04 \ m

I

6

P

0.03

E

0.02

z

O.O’ 0.00

2 =

I

5

-0.01

140

0.06

120

0.06

2

100

0.04

a

60

0.02

’

0.00

;: p.

'T;

E

60 40 J,,,,,,.,.,.,,,..,....,‘,,,,,,,.,,,,..,.,..,,,~ YJJASOHDJFUAMJJASONOJFL(JJASCJNOJFMAMJJAS0NOJF 1992 1993 1994

-0.02 1992

1993

Fig. 5. Effects of harvesting on streamwater attributes in terms of turbidity (A). temperature dissolved oxygen (E), and concentrations of ammonium (F), nitrate (G), and phosphate (H) Asterisks indicate statistical significance at cz = 0.05.

1994

(B), pH CC), electrical in a Texas bottomland

conductivity hardwood

CD), forest.

M.G.

Table 6 Harvesting

effects

on soil physical

Messina

properties

Treatment O-5

Ecology

at two depths in an east Texas bottomland

90 (1997)

211

201-215

hardwood

forest

Bulk density (Mg mW3)

Total porosity

Microporosity

Macroporosity

(So)

(%)

(%I

0.45a 0.26a 0.18a

1.03b 1.10a 1.12a

61.23a 60.62a 59.03a

39.85a 41.01a 41.86a

21.36a 19.39a 17.17a

0.21a O.lla 0.12a

1.26a 1.31a 1.31a

52.5 1a 52.86a 52.28a

35.94a 37.75a 38.4Oa

16.58a 15.11a 13.88a

a For each depth, means within

columns

followed

by different

letters are significantly

high temperature measured in the February 6 sampling. This was caused by air temperatures of 2530°C during the preceding week and the lack of vegetation shielding the soil surface. There was a significant (cy = 0.05) treatment effect on soil temperature. Temperatures averaged 22S”C, 21.0°C, and 19.7’C in the CC, PC, and CT, respectively. Mean temperatures during the study ranged from 8 to 29°C. Variation among treatments was greatest in the first growing season after harvesting due to the large amounts of exposed soil in the harvested areas. However, as the second growing season progressed, temperature differences diminished in response to the vigorous regrowth of herbaceous vegetation. Surface soil moisture content (O-15 cm> also varied seasonally with values decreasing steadily during the growing season until soil moisture recharge began in October (Fig. 6C). In most periods, treatments affected soil moisture content in the Table 7 Disturbance

class effects

Disturbance

class

on soil physical

properties

different

at Q = 0.05.

order CT > PC > CC. Average soil moisture was 19.5%, 22.5%, and 22.1% in the CC, PC, and CT, respectively. The CC was significantly ((u = 0.05) drier than the CT and PC, which did not differ significantly from each other. Soil moisture during the study ranged from about 10 to 32%. Differences during the first year after treatment tended to steadily diminish as the growing season progressed and became slight during recharge in the cool months. As the second growing season progressed, the aforementioned pattern of CT > PC > CC again became apparent. This pattern was caused by soil surface evaporation in the harvested areas during the first growing season, especially in the clearcuts. This likely played a role in the second growing season along with the transpirational drying power of the vigorous herbaceous regrowth in the harvested plots. Surface CO, efflux increased linearly with soil temperature (Fig. 7). The regression relationship was

at two depths in an east Texas bottomland

hardwood

forest

Total porosity

Microporosity

Macroporosity

kmin-‘)

Bulk density (Mg m-3)

(%I

(96)

(%)

0.24a 0.24a 0.08a

I .08b 1.1Ob 1.24a

59.45a 60.30a 58.4Oa

39.60a 41.87a 43.22a

19.74a 18.34a 15.19a

0.12a O.lla 0.07a

1.30a 1.31a 1.35a

5 1.79a 52.74a 52.71a

36.04a 38.46a 39.76a

15.72a 14.28a 13.72a

cm depth

Undisturbed Slightly disturbed Severely disturbed 5-10

Management

cm depth

Control Partial cut Clearcut

O-5

and

cm depth

Control Partial cut Clearcut 5-10

et al./Forest

cm depth

Undisturbed Slightly disturbed Severely disturbed

a For each depth, means within

columns

followed

by different

letters are significantly

different

at o = 0.05.

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et al./ Forest Ecology

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90 (1997)

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18

Clearcut Partial

16

cut

P>i

14

:r 0.006

12 10

a 6 4 2 0 b

,

30

5

10

25 20 15

15

Soil

~--~-

20

Moisture

25

. ----. c 0 30

35

(%)

Fig. 8. Soil moisture effects on surface CO, cfilux in a Texas bottomland hardwood wetland forest after harvesting+ Data points are from ail treatments and sampling periods.

10 5 30 25 20 15 10

T

5

‘M’A’M’J’J’A’S’O’N’D’J’F’M’A’M’J’J’

1993-94

Month

Fig. 6. Soil surface CO, efflux (a>, soil temperature (b), and soil moisture content (c) for a Texas bottomland hardwood wetland forest after harvesting. Vertical lines indicate standard errors.

but soil temperature explained only 34% of the variation in surface CO, efflux. Surface CO, efflux decreased lineariy with soil moisture (Fig. 81, but the influence of soil moisture was less than that

significant,

of temperature with only 18% of the variation in efflux explained by soil moisture. Alternative regression models failed to significantly improve our ability to explain variation in surface CO, efflux. Treatment had no significant (cz = O.OS>effect on mineral soil respiration (Fig. 9). There was a stight seasonal trend in mineral soil respiration. Lack of treatment effect may have been caused partly by some samples containing more organic matter than others, and storage of samples in a 2°C cooler before incubation due to limited incubator space. We found that harvesting in bottomland hardwood wetlands can significantly affect the activity of soil organisms and plant roots as measured by soil respiration. In general, CO, efflux from the soil surface varied directly with degree of harvest intensity due to the influence of harvesting on SiFe microclimate and availability of carbon sources. Furthermore, harvesting did not affect the normal seasonal cycle of ; g 7 03 8 E”

.-

0

5

Soil

10

15

Temperature

20

25

30

(‘C)

Fig. 7. Soil temperature effects on surface CO, efflux in a Texas bottomland hardwood wetland forest after harvesting. Data points are from all treatments and sampling periods.

a0 70 60 50 40 30 to”

5 0 WI z -10 ‘C -20 : -30 IT

-

-10 -20

F’M’A’M’J’J’A’S’O’N’O’J’F’M

1993-94

-30

Month

Fig. 9. Mineral soil respiration during laboratory incubation for a Texas bottomland hardwood wetland forest after harvesting. Vertical lines indicate standard errors.

M.G.

Messina

et d/Forest

Ecology

higher rates of soil respiration in the warm months compared with rates during cooler months. Our results agree with those of other workers performing similar research. Edwards and Ross-Todd (1983) found that harvesting in a mixed deciduous forest significantly increased CO, efflux in situ in some parts of the year, but not in others. They attributed this lack of long-term effect to the counteracting influences of greater efflux immediately following harvest disturbance, but less efflux afterwards in harvested plots due to decreased live root activity. Efflux in our system was normally higher in harvested plots than in the CT, and was still significantly higher nearly 2 years after treatment. Vegetation regrowth in our harvested plots has been extremely vigorous so root activity as well as increased soil temperatures likely combined to effect larger rates of soil surface CO, efflux. Our data are near the lower end of the range of published values. Weber (1985) measured rates of 4.2 g CO, m-* day-’ under jack pine in Ontario, whereas Cropper et al. (1985) recorded 18.9 g CO, m -’ day-’ under slash pine in Florida. Although this range may imply a direct effect of soil temperature, rates averaging only 7.0 CO, me2 day-’ were recorded under mixed deciduous forest in Tennessee (Edwards and Ross-Todd, 1983). Although we showed significant influences of soil temperature and soil moisture on soil surface CO, efflux, predictive ability of both of these variables was weak. This was likely due to several factors. Spatial variation in both CO, and temperature data existed since we sampled only 72 points distributed over a 73-ha study site. In addition, soil temperature sometimes varied diurnally, particularly in the cool months, due to substantial diurnal range in air temperature. Therefore, temperature data sometimes varied more due to hour than to treatment. Research elsewhere on the influence of soil temperature and moisture on soil respiration has given mixed results. Wildung et al. (1975) found that both soil temperature and moisture regulated soil respiration in an arid grassland soil. They were able to account for 70% of the variation in soil respiration using a simple regression equation which included only a temperature-moisture multiplicative term. However, Mathes and Schriefer (1985) found no significant influence of moisture on soil respiration,

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90 (1997)

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213

but a significant temperature effect. Edwards (1975) concluded that temperature accounted for more of the variation in respiration rates in a mixed deciduous forest, but moisture had its greatest influence on CO, evolution rates because of aeration effects on soil and litter. Furthermore, moisture effects were short-lived and usually important only when extreme. We concur with Edwards and conclude that soil temperature can be used to predict soil respiration in our system, but with varying accuracy under extreme soil moisture conditions. Our results concerning mineral soil respiration were less clear than those for soil surface CO, efflux. Substantial temporal variation was found in the CC data likely due to changing populations of soil microorganisms and perhaps the recolonization of pre-harvest biota following harvesting (Tate, 1987). A clearer pattern of treatment effect was observed between the CT and PC (Fig. 9). The higher rates of growing season mineral soil respiration in the CT than in the PC seems inconsistent with the results for surface CO, efflux since soil temperatures were normally higher in the PC. This effect may have resulted from greater root activity in the CT during the first growing season after harvesting. Even though roots would have been sieved out of the soils before laboratory incubation, one of the sources of carbon for soil microorganisms is root exudates and soughed off root tissues (Richards, 1987). These may have been higher in the CT soils which would have supported greater microbial activity upon incubation. The steady increase in mineral soil respiration in the PC (Fig. 9) may reflect the recolonization of the soil by roots and microbes. However, since these factors were not measured directly, this remains speculative. The increased efflux of CO, from the soil surface following harvesting will likely be a short-lived effect. Influence on atmospheric CO, is minimal. On an ecosystem level, the first few months following harvesting may show a net loss of CO, from our sites to the atmosphere. However, the vigorous vegetative regrowth we have observed in the second growing season coupled with data on forest succession from adjacent sites (Greene and Lowe, 19931, indicates that our sites will become CO, sinks for many years following harvesting (Edwards and Ross-Todd, 1983).

214

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et al./ Forest Ecology

4. Summary Our data indicated that the consequences of partial cutting and clearcutting in this Texas bottomland hardwood ecosystem were minimal and relatively ephemeral in the first year following treatment. Our null hypotheses concerning harvest effects on woody species composition were rejected. That is, overstory removal only slightly affected composition of woody vegetation in the understory category one year after harvesting. Initial composition appeared to be the strongest determinant of post-harvest composition, at least for the first year after harvesting. Even though they were relatively minor, effects were most apparent in the clearcuts, indicating that shifts in diversity did reflect the degree of harvest disturbance. We accepted our null hypothesis regarding nutrient content of surface- and groundwater, since very few sampling periods showed any treatment response. The overall absence of treatment effects on groundwater and streamwater quality suggests the following: 1. the silvicultural systems used in this study which do not rely on site preparation or prescribed burning are compatible with maintenance of water quality; 2. bottomland hardwood systems of this nature are resistant to disturbance; 3. use of BMP, including avoidance of wet-weather logging and maintenance of 20-m streamside management zones were effective in protecting water quality. Our null hypothesis that soil physical properties would not reflect the intensity of harvesting was rejected. Even though impacts were minor, they were most pronounced in the partial cuts and clearcuts. Although forest floor disturbance was minimal, it too directly reflected degree of harvest removal. Our null hypothesis concerning microbial activity stated that microbial activity as measured by soil respiration would not be affected by degree of stand removal. This hypothesis was rejected for in situ respiration. Our results indicated that, for some periods, in situ CO, efflux was significantly increased in the clearcuts and partial cuts when compared with the control. Results for the mineral soil were too variable for adequate hypothesis testing.

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Overall, cutting effects were minor for the attributes measured. We conclude that this resulted directly from use of BMP during harvesting. Also, the inherent productivity of this ecosystem enabled a rapid recovery of vegetation which influenced all of our measured attributes.

Appendix A. Scientific names of woody plant species encountered on the study site Red maple Boxelder River birch Ironwood Hickory Bitter pecan

Acer rubrum L. Acer negundo L. Bet& nigra L. Caipinus caroliniana Walt. Carya spp. Carya aquatica (Michx. f.)

Nutt. Shagbark hickory

Carya

ovum

(P. Mill.1

K.

Koch. Sugarberry Hawthorn Persimmon American beech Swamp privet

Celtis laevigata Willd. Crataegus spp. Diospyros virginiana L. Fagus grandifolia Ehrh. Forestiera acuminata (Michx.)

Poir. Carolina ash Deciduous holly American holly Sweetgum Chinaberry Red mulberry Blackgum Loblolly pine Sycamore Carolina laurelcherry

Fraxinus caroliniana Mill. Ilex decidua Walt. Ilex opaca Ait. Liquidambar styracijluo L. Melia azedarach L. Morus rubra L. Nyssa sylvatica Marsh. Pinus taeda L. Platanus occidentalis L. Prunus caroliniana (P. Mill.)

White oak Cherrybark oak

Quercus alba L. Quercus falcata var. pagodaefoliu Eli. Quercus hemisphaerica &arW. Quercus lyrata Walt. Quercus michauxii Nutt. Quercus nigra L. Quercus nu?ta&i Palmer Quercus phellos L. Robinia pseudoacacia L.

Ait.

Diamond-leaf oak Overcup oak cow oak Water oak Nuttall oak Willow oak Black locust

M.G.

Chinese tallow Sassafras

Messina

et al./ Forest

Ecology

Supium sebiferum CL.) Roxb. Sussufius ulbidum (Nutt.)

Nees. Silverbells Sweetleaf Baldcypress American elm Toothache tree

Styrax umericuna Lam. Symplocus rincforiu L’Her. Tuxodium distichum (L.) Rich. Ulmus americana L. Zanthoxylum clava-herculis L.

References American Public Health Association, 1992. Standard Methods for the Examination of Water and Wastewater, 18th edn. APHA, Washington, DC. Anderson, J.P., 1982. Soil respiration. In: A.L. Page (Editor), Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties, 2nd edn. American Society of Agronomy, Madison, WI, pp. 831-871. Blake, G.R. and Hartge, K.H., 1986. Bulk density. In: A. Klute (Editor), Methods of Soil Analysis. Part I. Physical and Mineralogical Methods, 2nd edn. American Society of Agronomy, Madison, WI, pp. 363-375. Brown, T.C. and Binkley, D., 1994. Effect of management on water quality in North American forests. Gen. Tech. Rep. RM-248, USDA Forest Service, Rocky Mountain Forest Range Experiment Station, Fort Collins, CO, 27 pp. Clatterbuck, W.K. and Meadows, J.S., 1993. Regenerating oaks in the bottomlands. In: D.L. Loftis and C.E. McGee (Editors), Oak Regeneration: Serious Problems, Practical Recommendations. Knoxville, TN, 8-10 September 1992, 319 pp. Cropper, W.P., Jr., Ewel, K.C. and Raiche, J.W., 1985. The measurement of soil carbon dioxide respiration in situ. Pedobiologia, 28: 35-40. Edwards, N.T., 1975. Effects of temperature and moisture on carbon dioxide evolution in a mixed deciduous forest floor. Soil Sci. Sot. Am. J., 39: 361-365. Edwards, N.T., 1982. The use of soda-lime for measuring respiration rates in terrestrial systems. Pedobiologia, 23: 321-330. Edwards, N.T. and Ross-Todd, B.M., 1983. Soil carbon dynamics in a mixed deciduous forest following clear-cutting with and without residue removal. Soil Sci. Sot. Am. J., 47: 1014- 1020. Gardner, W.H., 1986. Water content. In: A. Klute (Editor), Methods of Soil Analysis. Part I. Physical and Mineralogical Methods, 2nd edn. American Society of Agronomy, Madison, WI, pp. 493-544. Greaten, E.L. and Sands, R., 1980. Compaction of forest soils: A review. Aust. J. Soil Res., 18: 163-189. Greene, T.A. and Lowe, W.J., 1993. Effects of chemical and mechanical site preparation on bottomland hardwood regeneration after ten years. In: Proc. of the 7th Biennial Southern Silvicultural Research Conference, Mobile, AL, 17-19 November 1992. Gen. Tech. Rep. SO-93, USDA Forest Service, pp. 425-428.

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Griff~ths, J. and Bryan, J., 1987. The climates of Texas counties. Monogr. Ser. 2, Office of the State Climatologist, Department of Meteorology, Texas A&M University, College Station, 569 PP. Hodges, J.D. and Gardiner, ES., 1993. Ecology and physiology of oak regeneration. In: D.L. Loftis and C.E. McGee (Editors). Oak Regeneration: Serious Problems, Practical Recommendations. Knoxville, TN, 8- 10 September 1992, 3 19 pp. Karr, B.L., Hodges, J.D. and Nebeker, T.E., 1987. The effect of thinning methods on soil physical properties in North Central Mississippi. South. J. Appl. For., 11: 110-l 12. Klute, A. and Dirksen, C., 1986. Hydraulic conductivity and diffusivity: Laboratory methods. In: A. Klute (Editor), Methods of Soil Analysis. Part I. Physical and Mineralogical Methods, 2nd edn. American Society of Agronomy, Madison, WI, pp. 687-734. Krebs, C.J., 1985. Ecology: The Experimental Analysis of Distribution and Abundance, 3rd edn. Harper and Row, New York, 440 PP. Lockaby, B.G., Thornton, F.C., Jones, R.H. and Clawson, R.G., 1994. Ecological responses of an oligotrophic floodplain forest to harvesting. J. Environ. Qual., 23: 901-906. Magurran, A.E., 1988. Ecological Diversity and its Measurement. Princeton University Press, Princeton, NJ, 179 pp. Marks, P.L. and Harcomb, P.A., 1981. Forest vegetation of the Big Thicket, southeast Texas. Ecol. Monogr., 51: 287-305. Mathes, K. and Schriefer, T., 1985. Soil respiration during secondary succession: Influence of temperature and moisture. Soil Biol. Biochem., 17: 205-211. Nixon, E.S., Willett, R.L. and Cox, P.W., 1977. Woody vegetation of a virgin forest in an eastern Texas river bottom. Castanea, 42: 227-236. Richards, B.N., 1987. The Microbiology of Terrestrial Ecosystems. Wiley, New York, 399 pp. Shari& R.R. and Mitsch, W.J., 1993. Southern floodplain forests. In: W.H. Martin, S.G. Boyce and AC. Echtemacht (Editors), Biodiversity of the Southeastern United States: Lowland Terrestrial Communities. Wiley, New York, pp. 3 1 l-372. Shepard, J.P., 1994. Effects of forest management on surface water quality in wetland forests. Wetlands, 14: 18-26. Streng, D.R., Glitzenstein, J.S. and Harcombe, P.A., 1989. Woody seedling dynamics in an east Texas floodplain forest. Ecol. Monogr., 59(2): 177-204. Tate, R.L., III, 1987. Soil Organic Matter. Biological and Ecological Effects. Wiley, New York, 291 pp. USDA Forest Service, 1988. The South’s fourth forest: altematives for the future. For. Resourc. Rep. No. 24, USDA Forest Service, Washington, DC. Weber, M.G., 1985. Forest soil respiration in eastern Ontario jack pine ecosystems. Can. J. For. Res., 15: 1069-1073. Wildung, R.E., Garland, T.R. and Buschbom, R.L., 1975. The interdependent effects of soil temperature and water content on soil respiration rate and plant root decomposition in arid grassland soils. Soil Biol. B&hem., 7: 373-378.