Biological constraints on the growth of hardwood regeneration in upland Piedmont forests

Forest Ecology and Management 175 (2003) 545±561

Biological constraints on the growth of hardwood regeneration in upland Piedmont forests Mark A. Romagosa1, Daniel J. Robison* Department of Forestry, Box 8008, Room 3118, Jordan Hall, North Carolina State University, Raleigh, NC 27695 USA Received 26 November 2001; accepted 6 May 2002

Abstract The effects of aboveground fungi, insects, browsing mammals and weeds on the growth (height, diameter and volume index) and density of hardwood stems were studied on three upland sites in the Piedmont of North Carolina, USA during the ®rst two growing seasons following clearcutting. Competition from weeds was the most detrimental to hardwood growth. Pesticide treatments alone (broad-spectrum insecticide, fungicide and mammal repellent) did not signi®cantly increase growth. Compared to the control (no weeding or pesticides), stems receiving the pesticide ‡ weeded treatment increased in height 1.6±4.5 times, diameter 1.7±5.3 times and volume index 3.4±5.1 times by the end of second growing season, across all sites. Stem density did not exhibit clear treatment effects, although individual tree mortality and recruitment in the plots were not recorded. Exclosures, with weeding, to eliminate the browsing impact of white-tailed deer (Odocoileus virginianus Zimmermann), installed only during the second year of study, yielded a 1.9, 1.6 and 3.2 times increase in mean stem height, diameter and stem volume index, respectively, as compared to weeded only plots. Results indicate that very young hardwood regeneration can respond quickly to release from biological constraints and the rate of stand establishment and development may be enhanced with stand manipulations in the ®rst two growing seasons. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Natural regeneration; Hardwoods; Competition; Release; Weeds; Insects; Fungi; Deer; Productivity

1. Introduction Rapid growth of regeneration following harvest remains one of the most limiting factors in the management of natural hardwood forests for timber production. The impacts of biological constraints to young hardwoods, such as insects, fungi, browsing mammals and competition from weeds have not been suf®ciently * Corresponding author. Tel.: ‡1-919-515-5314; fax: ‡1-919-515-6193. E-mail address: [email protected] (D.J. Robison). 1 Present address: Palm Beach County Department of Environmental Resources Management, 3323 Belvedere Road, West Palm Beach, FL 33406, USA.

evaluated. This information is necessary to develop silvicultural approaches to accelerate early stand development and thereby reduce rotation length. While each natural regeneration system has merits (Kellison et al., 1981, Kelty, 1988), clearcut and shelterwood methods (Kirkham, 1988) are most commonly recommended for regenerating USA southern hardwoods (McGee and Hooper, 1970; Sander, 1980; Frederick, 1983). In addition to their low cost (Dutrow, 1980; Shropshire, 1980), natural regeneration methods, especially even-aged, tend to result in a mix of commercial and non-commercial, shade-tolerant and intolerant species, that meet the multi-use management objectives common to many landowners (Boyce, 1977).

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 2 2 4 - 4

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In the USA south alone, there are about 81 million ha of natural hardwood and mixed hardwood±pine timberlands (66% of the regions total timberland and 60% of the nations' hardwood harvest) (Powell et al., 1993; Shef®eld and Dickson, 1998), where management would bene®t from such knowledge. Compared to the advances in pine production, comparatively little has been done to advance hardwood production in natural stands or plantations (Kellison, 1977; Buckner, 1980; Robison et al., 1998). Most natural hardwoods are managed under passive approaches (Bechtold and Phillips, 1983), mismanaged or not managed at all (Birch, 1997), resulting in lost production potential (Bechtold and Phillips, 1983; Birch, 1997; DuBois et al., 1991; Waldrop, 1997). While hardwood inventories in the USA south are large (Araman and Tansey, 1991), removals are projected to increase as much as 51% from 1990 to 2040 (Araman, 1989; Cubbage and Abt, 1998; Haynes et al., 1995). To meet demand, productivity and production ef®ciency must be raised (Robison, 2001). These conditions are common worldwide in natural forests. Attention has traditionally been focused on insect and disease pests of mature trees, rather than on young natural regeneration (National Academy of Sciences, 1975; Verrall, 1982) and this continues to be true. Most work on young trees has been done in nurseries and plantations, with young natural stands generally ignored, even though the potential impact of pests on stand development may be substantial. Stanosz (1994) provided some insight into the constraining effects of fungal and insect pests on young hardwoods by demonstrating increased survival of 1-year-old sugar maple (Acer saccharum) by using fungicides and insecticides. Mammal browsing can also impact the development of young natural stands and this has been well studied (Harlow and Downing, 1970; Jordan, 1967; Tierson et al., 1966; Trumbull et al., 1989). Forest managers have recognized the importance of weed competition in North American forests since the early 1900s (Walstad, 1981). However, little attention has been paid to forest weed impacts on the regeneration process (Leak, 1988). Recent studies in natural stands and with planted seedlings have explored the effects weeds have on competition for light (Gottschalk, 1994; Gardiner and Hodges, 1998), nutrients (Zutter et al., 1999) and water (Kolb and Steiner,

1990; Hopper et al., 1993). These suggest that weed control may accelerate young stand development (Luken, 1990), leading to increased production. The individual and combined effects of biological constraints on young natural regeneration may lead to undesired changes in species composition, stocking patterns and production. It will be useful to develop and improve management practices, based on understanding the ecological roles of these factors, to optimize favorable conditions for ef®cient timber production. The current study examined responses of natural hardwood regeneration to relaxation of biological constraints, 1 and 2 years after clearcutting in central North Carolina, USA. 2. Methods 2.1. Site descriptions This study was conducted in the lower Piedmont of North Carolina, USA on three commercial (salvage) clearcuts, harvested during winter 1996/1997, in response to damage from Hurricane Fran (6 September 1996). The sites were all on North Carolina State University (NC State) forests; the Schenck Memorial forest in Wake County (sites A and B in the current study) and the Hill Demonstration forest in Durham County (site C). The growing season in both areas is approximately 200 days (April±October), with mean annual precipitation of 108±119 cm and temperature of 16 8C (Cawthorn, 1970; Kirby, 1976). The predominant soils at sites A and B were in the Cecil soil series (clayey and/or sandy-loam; kaolinitic, thermic Typic Kanhapludults, with 2±10 % slopes, eroded) (Cawthorn, 1970) and on site C the Mecklenburg soil series (loamy, ®ne, mixed, thermic Ultic Hapludalfs; 6±10 % slopes) (Kirby, 1976). These soils have low fertility and organic matter, due to topsoil erosion while under agriculture before the early 1900s. The current study began in spring 1997; a few months after clearcutting, when the sites were occupied by advance regeneration and new seed- and sprout-origin stems, and various weed species. Site A (Schenck forest compartment no. 3) is 2.5 ha and before clearcutting was comprised of 89-year-old natural loblolly pine (Pinus taeda L.). Site B (Schenck

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forest compartment no. 4) is 1.4 ha and had been a natural stand of 88-year-old hardwoods (predominantly oak, Quercus spp.). Site C (Hill forest compartment no. A-3) is 1.2 ha and had been a 42-year-old loblolly pine plantation. During the middle of the 1998 growing season, the species composition of herbaceous plants on the untreated 10 m2 study plots (see later) was determined. Plants considered weed species (nomenclature from Radford et al., 1968) in this study wereÐherbaceous: ragweed (Ambrosia artemisiifolia), broom sedge (Andropogon virginus), Indian hemp (Apocynum cannabinum), columbine (Aquilegia canadensis), frost aster (Aster pilosus), sedge (Carex spp.), wild sensitive plant (Cassia nictitans), beggar's ticks (Desmodium sp.), Dichanthelium sp., crab grass (Digitaria sanguinalis), lettuce (Erechtites hieracifolia), horseweed (Erigeron canadensis), Eupatorium album, dog-fennel (Eupatorium capillifolium), Eupatorium serotinum, strawberry (Fragaria virginiana), rabbit tobacco (Gnaphalium obtusifolium), sun¯ower (Helianthus atrorubens), St. John's wort (Hypericum sp.), rush (Juncus spp.), Lespedeza procumbens, Microstegium vimineum, Panicum sp., Perilla frutescens, pokeweed (Phytolacca americana), plantain (Plantago rugelii), Polygonum cespitosum, Christmas fern (Polystichum acrostichoides), ®ve-®ngers (Potentilla canadensis), Prunella vulgaris, Pycnanthemum ¯exuosum, foxtail grass (Setaria sp.), nightshade (Solanum carolinense), wooly mullein (Verbascum thapsus); woody-type: mimosa (Albizia julibrissin), groundsel tree (Baccharis halimifolia), trumpet creeper (Campsis radicans), Virginia creeper (Parthenocissus quinquefolia), loblolly pine, winged sumac (Rhus copallina), smooth sumac (R. glabra), wild rose (Rosa carolina), blackberry (Rubus argutus), dewberry (R. ¯agellaris), black raspberry (R. occidentalis), greenbrier (Smilax glauca), muscadine (Vitus rotundifolia). 2.2. Determining sample plot size To develop an estimate of hardwood species composition and stem density on sites A, B and C, to guide the experimental approach in this study, inventories were taken in June 1997 on nearby sites. These were in 2-year-old regeneration following clearcutting at Schenck forest (prior stand was principally natural loblolly pine) and 3-year-old regeneration following

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clearcutting at Hill forest (prior stand was principally oaks and yellow-poplar, Liriodendron tulipifera L.). A 1 m  1 m square sampling quadrat was placed at random within each clearcut. Species composition and stem number were recorded for hardwood regeneration (seed- and sprout-origin) within the quadrat and then the quadrat was ¯ipped to expand the sample area in a sequential manner. This procedure was repeated until 4 and 6 m2 areas were sampled at Schenck (n ˆ 8) and Hill (n ˆ 10) forests, respectively. The number of species found increased with increasing quadrat size at both sites, with a slower rate of increase at quadrat sizes larger than 2±3 m2 (Fig. 1A). Given this change in slope and logistical constraints, the 10 m2 plot was used in this study. The number of stems and variance within each sample quadrat increased in a linear fashion with increasing quadrat size at both sites (Fig. 1B), indicating a relatively constant relation between sample plot area, hardwood stem density and variation. Hardwood species counts (and the proportion originating as sprouts) from this study's 10 m2 treatment plots (see later), on sites A, B and C, ranged from 15 to 19 (Table 1), from a total of 23 hardwood species found (52% on all sites, 70% on two of three sites). Yellow-poplar and red maple (Acer rubrum L.) were the most abundant species on all sites, with redbud (Cercis canadensis L.) uniquely common on site C. 2.3. Plot design, treatments and measurements Four treatments were applied to 3:16 m  3:16 m (10 m2) plots with 0.5±1.0 m treated buffers in a randomized complete block (across topographic features) design on each site. Treatments began in July 1997 on sites A and B, and in August 1997 on site C. Plots were installed away from the effects of stand edges and where slash was not deep. The following treatments were applied to plots throughout the 1997 (July/August±October) and 1998 (May±September) growing seasons. (1) Pesticide treatment: every six weeks a mammal repellent was applied to all foliage and stems: in 1997 the product applied was Ro-pel1 (purchased 1997) and in 1998 Deer-Away1 (purchased 1998). Biweekly, a tank mixture of the broad-spectrum

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Fig. 1. Mean number of species/area (A) and mean number of stems/area (B) for hardwood regeneration 2 and 3 years after clearcutting in the North Carolina Piedmont at Schenck and Hill forests, respectively.

insecticide acephate (Orthene1 at 1.16 g/l water) ‡ the broad-spectrum fungicide benomyl (Benlate1 at 0.69 g/l water) was also applied to all foliage and stems. (2) Weeded treatment: biweekly, all herbaceous and coniferous plants and woody vines were sheared at ground level with hand-pruners.

(3) Pesticide (as described earlier) ‡ weeded (as described earlier) treatment. (4) Untreated control. Whenever plots receiving the pesticide treatment were sprayed, an equivalent amount of water was applied to the weeded-only and untreated control

Table 1 Mean percent of stems, including all stems from sprout clumps, of hardwood species on 10 m2 plots at three study sites in year 1 (August 1997) and year 2 (September 1998) following clearcutting (in winter 1996/1997) of mature overstories in central North Carolina Site A

Site B

Site C

Common name

Scientific name

Year 1 stems (%)

Year 2 stems (%)

Year 1 stems (%)

Year 2 stems (%)

Year 1 stems (%)

Year 2 stems (%)

Yellow-poplar Red maple Sweetgum Sourwooda Mockernut hickory Blackgum Northern red oak White oak Willow oak White ash Ironwooda American beech Hophornbeama Black cherry Common persimmona Sassafrasa Black willowa Flowering dogwooda Redbuda Red mulberrya Winged elma American sycamore American hollya

L. tulipifera L. A. rubrum L. L. styraciflua L. Oxydendrum arboreum (L.) DC. C. tomentosa (Poiret) Nuttal N. sylvatica Marshall Quercus rubra L. Q. alba L. Q. phellos L. Fraxinus americana L. Carpinus caroliniana Walter Fagus grandifolia Ehrhart Ostrya virginiana (Miller) K. Koch Prunus serotina Ehrhart Diospyros virginiana L. Sassafras albidum (Nuttall) Nees Salix nigra Marshall C. florida L. C. canadensis L. Morus rubra L. U. alata Michaux Platanus occidentalis L. Ilex opaca Aiton

61.2 24.4 6.3 (36) 4.8 (96) 0.6 0.6 0.6 0.4 ± 0.4 0.2 0.2 0.2 0.2 ± ± ± ± ± ± ± ± ±

69.8 10.2 9.3 (80) 0.7 0.4 7.6 (83) 0.4 0.6 ± ± ± ± ± 0.4 (50) 0.2 0.4 0.2 ± ± ± ± ± ±

70.3 12.8 4.2 ± 0.4 ± 0.3 5.0 ± 0.8 ± 0.5 3.8 ± ± ± ± 1.6 0.3 ± ± ± ±

49.8 15.0 (22) 7.7 (18) ± 0.7 ± 1.4 12.9 ± 2.1 ± 0.7 4.2 0.5 ± 0.7 ± 1.4 1.6 1.2 0.2 ± ±

19.7 23.0 2.2 ± 4.9 1.1 1.1 ± 0.4 0.2 ± 0.9 0.6 1.3 0.6 ± ± 15.5 15.7 1.3 11.0 0.4 ±

19.1 15.9 2.3 ± 4.9 4.3 0.6 ± 0.9 0.3 ± ± 0.3 2.3 1.2 0.6 ± 6.4 35.9 0.6 2.6 0.9 0.9

(35) (3)

(14)

(25)

2

(83) (81) (83)

(100) (43) (100) (90) (18) (71) (92)

(76) (50) (47) (99)

(38) (75) (68) (5) (78) (99)

Numbers in parentheses indicate percent of stems of each species from sprouts. Mean total number of stems per 10 m plot: site AÐ32 (year 1), 34 (year 2); site BÐ95 (year 1), 54 (year 2); site CÐ44 (year 1), 29 (year 2). a Indicates species that are typically found as mid-/understory trees in mature North Carolina forests.

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Species

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plots. Site A had suf®cient space to accommodate four replications of all four treatments. Sites B and C were smaller and included four replications of treatments 1 and 4, and 1, 3 and 4, respectively. Monthly during the two growing seasons, the following measurements were recorded from each plot on all woody stems: (1) height (h) (0.1 cm)Ðfrom ground-line to the terminal bud of every stem (including individual stems within coppice clumps); (2) diameter (d) (0.1 mm)Ðat ground-line with calipers; (3) stem densityÐstem count per 10 m2 plot (including individual stems within coppice clumps); (4) species and origin (seed or sprout). Stems were considered sprouts if they originated from stumps or when root suckering was easily identi®ed (e.g. multiple stems of a species growing in a linear fashion away from a stump). Initial measurements in each growing season were conducted prior to the ®rst treatment application for the season. A stem volume index was calculated as d2h for each stem in the plots. In June 1998 (second growing season), two additional treatments (n ˆ 4), as paired plots, were installed on 10 m2 plots with buffers along the edge of site B (in the regenerating stand but next to the adjacent mature forest). These treatments were: (1) weeded treatment (as the control)Ðmonthly all herbaceous, coniferous and woody vine vegetation was sheared at ground level from June to September. (2) weeded ‡ exclusion treatmentÐweeded (as described earlier) plus fenced to exclude whitetailed deer (Odocoileus virginianus Zimmermann), by surrounding and covering the plot with wire mesh. The same tree measurements described previously were taken monthly on these plots in 1998. These additional plots were added to the study because there was substantial deer browse in the ®rst growing season, even in mammal repellent treated plots. 2.4. Soil sampling and weed assessment A battery-operated probe (Aquaterr1 Temp 200, Aquaterr Instruments Inc., Fremont, CA) was used to measure plot soil moisture and temperature (three measurements randomly in each plot) in 1998 on sunny days; sites A and B on 3 June and site C on 17 June. The

moisture sensor spanned a depth of 5±14 cm and the temperature sensor was 3 cm deep. A composite sample of the top 20 cm of soil was collected with a 7 cm diameter soil auger from two random locations in each plot on 30 September 1998. A 100 g subsample was analyzed for total cation exchange capacity, pH, percent organic matter, estimated nitrogen release and Mehlich III extractable sulfur, phosphorus, calcium, magnesium, potassium and sodium (Brookside Laboratories Inc., New Knoxville, OH). A weed density index meter was constructed by painting a brightly colored 10 cm  10 cm grid on a 1:5 m  1:0 m board (Romagosa and Robison, unpublished technique). It was placed vertically at two random locations within each of the unweeded experimental plots (control and pesticide-only plots) and from 1 m away, with eye-level at the center of the board, the number of cells fully or partially obscured by weeds were counted. The mean number of tallied cells per plot was used an index of weed density. 2.5. Statistical analyses Sites A, B and C, and the deer exclosure study were evaluated separately. Data were analyzed using the general linear model procedure of SAS (SAS, 1985). Height, diameter, d2h index, stem density and weed competition index means were calculated by block, treatment and month. Analysis of variance (ANOVA) was performed on initial measurements on each site each year and for the deer exclusion study. Two-way repeated measures ANOVA was used to test if treatments and/or blocks had signi®cant effects on these means after each month of treatment. The repeated measures ANOVAs were conducted for the following time intervals: in 1997ÐJuly±August, July±September and July±October (site C time intervals began in August); in 1998ÐMay±June, May±July, May± August and May±September (deer exclosure time intervals began in June). When repeated measures ANOVA or ANOVA results were signi®cant (P  0:05), treatment means were compared with the Ryan±Einot±Gabriel±Welsch (REGWQ) multiple comparison procedure (P  0:05) (SAS, 1994). Two-way ANOVA was used to determine if treatments and/or blocks had signi®cant (P  0:05) soil temperature, moisture or chemical differences (using the PROC GLM procedure) (SAS, 1985).

M.A. Romagosa, D.J. Robison / Forest Ecology and Management 175 (2003) 545±561

3. Results 3.1. Treatment effects on hardwood growth and density There were no signi®cant differences among treatment plots for stem height, diameter, d2h index or density on any of the sites at the beginning of year 1 or 2 (Figs. 2±9). Blocking effects were not signi®cant for h, d or d2h on any site during the 2-year period. Figs. 2, 4, 6 and 8 illustrate growth responses for the stem volume index and Figs. 3, 5, 7 and 9 show stem density responses. Height and diameter responses are described only in the text. 3.1.1. Site A Yellow-poplar, red maple and sweetgum (Liquidambar styraci¯ua L.), in that order, dominated the hardwood species on site A in years 1 and 2 (Table 1),

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comprising about 90% of stems in all plots. Of these species, only sweetgum included sprout-origin stems. Signi®cant treatment effects on mean stem volume index (d2h) were evident by October of the ®rst growing season (F3;9 ˆ 4:30, P ˆ 0:0385), and in July±September of the second growing season (range of F3;9 ˆ 7:84±11.95, P ˆ 0:0070±0.0017) (Fig. 2). The divergence of treatment effects on stem size was consistent through years 1 and 2, with the exception of a very slight decline in the mean d2h of pesticide treated stems during year 1. By October of year 1, treatment effects on mean stem heights were signi®cant, ranging from 10 cm for the control to 18 cm for the pesticide ‡ weeded (F3;9 ˆ 5:88, P ˆ 0:0167). In year 2, signi®cant treatment differences in height were found beginning in July. In September of year 2, mean stem heights ranged from 15 cm for the control to 46 cm for the pesticide ‡ weeded (range of F3,9 from July to September ˆ 9:01±11.68, P ˆ 0:0045±0.0019). Mean

Fig. 2. Mean volume index (d2h) of all hardwood stems during the ®rst two growing seasons after clearcutting at site A in the North Carolina Piedmont. Treatments with the same letter, by month, are not signi®cantly different (GLM-protected REGWQ multiple comparison procedure, P  0:05). Treatments described in Section 2.3.

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Fig. 3. Mean stem density of all hardwoods during the ®rst two growing seasons after clearcutting at site A in the North Carolina Piedmont. Treatments with the same letter, by month, are not signi®cantly different (GLM-protected REGWQ).

stem diameter followed the same pattern, with signi®cant treatment effects ®rst evident in October 1997, ranging from 1.5 mm for the control to 4.3 mm for the pesticide ‡ weeded (F3;9 ˆ 6:89, P ˆ 0:0104). Signi®cant diameter differences were found among treatments from July to September of year 2 (range of F3;9 ˆ 6:12±10.90, P ˆ 0:0148±0.0024), with mean stem diameter in September ranging from 3.7 mm for the control to 9.2 mm for pesticide ‡ weeded. Treatment effects on stem density were ®rst evident in June at the start of year 2 and continued through September 1998 (range of F3;9 ˆ 6:57±8.04, P ˆ 0:0120±0.0065) (Fig. 3). Blocking effects were not signi®cant in 1997, but were signi®cant throughout 1998 (range of F3;9 ˆ 4:64±11.92, P ˆ 0:0318± 0.0017). 3.1.2. Site B Yellow-poplar, red maple and sweetgum, in that order, dominated the hardwood tree species on site B

in years 1 and 2 (Table 1); comprising about 87 and 72% of all stems among all plots in years 1 and 2, respectively. Red maple and sweetgum were represented by substantial numbers of sprout-origin stems. Signi®cant treatment effects on stem volume index on site B were evident at the end of the ®rst season (F1;3 ˆ 11:66, P ˆ 0:0420) and found in June, July and September of year 2 (range of F1;13 ˆ 10:53±26.33, P ˆ 0:0477±0.0143) (Fig. 4). Mean heights among treatments did not differ signi®cantly during year 1 (end of year 1, 12 cm for the control, 23 cm for the pesticide ‡ weeded). However, in July and September of year 2 heights differed signi®cantly (F1;3 ˆ 19:67, P ˆ 0:0213 and F1;3 ˆ 10:59, P ˆ 0:0474, respectively). At the end of year 2, mean height was 20 cm for control and 55 cm for pesticide ‡ weeded treatments. Mean diameter among treatments differed signi®cantly within 1 month of treatment in year 1 (August 1997 F1;3 ˆ 12:60, P ˆ 0:0381) and continued to differ among treatments in year 1 through October, when

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Fig. 4. Mean volume index (d2h) of all hardwood stems during the ®rst two growing seasons after clearcutting at site B in the North Carolina Piedmont. Treatments with the same letter, by month, are not signi®cantly different (GLM-protected REGWQ).

diameter was 2.3 mm for the control and 4.9 mm for the pesticide ‡ weeded treatments. At the end of year 2, stems receiving the pesticide ‡ weeded treatment had mean diameter of 9.4 mm, while the control was 3.3 mm and signi®cant treatment effects were also found in June, July and September (range of F1;3 ˆ 12:96±28.92, P ˆ 0:0368±0.0126). Signi®cant treatment and blocking effects on mean stem density were not detected during the study (Fig. 5). On site B, among the deer exclosure plots, 17 hardwood species were found, with yellow-poplar the most abundant species and combined with red maple and sweetgum amounting for 93% of the total number of stems. No sprout-origin stems were found in these plots. Signi®cant deer exclosure treatment effects on mean stem volume index were detected in August and September 1998 (F1;2 ˆ 32:75, P ˆ 0:0292 and F1;2 ˆ 18:61, P ˆ 0:0498, respectively) (Fig. 6). Signi®cant exclosure effects on mean stem height were detected in August and September 1998 (F1;2 ˆ 59:08, P ˆ 0:0165 and F1;2 ˆ 24:29, P ˆ 0:0388, respectively),

with September height of 28 cm for weeded and 45 cm for weeded ‡ exclosed. Signi®cant effects on mean stem diameter were detected in August 1998 (F1;2 ˆ 19:68, P ˆ 0:0472), with end of season September diameters of 5.3 and 7.1 mm for weeded and weeded ‡ exclosed plots, respectively. Signi®cant exclosure treatment effects on mean stem density were detected in August and September 1998 (F1;2 ˆ 61:73, P ˆ 0:0158 and F1;2 ˆ 121:32, P ˆ 0:0081, respectively) (Fig. 7), when blocking effects on density were also signi®cant (August: F1;2 ˆ 59:16, P ˆ 0:0166; September: F1;2 ˆ 51:27, P ˆ 0:0191). In August, blocking accounted for 65% and the exclosure treatment 34% of the variation in stem density, while in September, blocking accounted for 45% and the treatment 54% of the variation in stem density. 3.1.3. Site C Yellow-poplar and red maple were the most abundant hardwoods among all plots on site C, each accounting for about 16±23 % of all stems during years 1 and 2 (Table 1). Other common timber species

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Fig. 5. Mean stem density of all hardwoods during the ®rst two growing seasons after clearcutting at site B in the North Carolina Piedmont. Treatments with the same letter, by month, are not signi®cantly different (GLM-protected REGWQ).

were mockernut hickory (Carya tomentosa (Poiret) Nuttal), blackgum (Nyssa sylvatica Marshall) and sweetgum. Among these ®ve species, all but yellow-poplar were represented by substantial numbers of spout origin stems and combined these ®ve accounted for 46±51 % of all stems in years 1 and 2. The other group of abundant species, were nontimber types, ¯owering dogwood (Cornus ¯orida L.), redbud and winged elm (Ulmus alata Michaux), together accounting for 42±45% of all stems in years 1 and 2, with substantial number of sprouts. Signi®cant treatment effects on mean stem volume index were evident in year 1 at site C one month after the study began, in September 1997 (F2;6 ˆ 11:59, P ˆ 0:0087), but then not again until September 1998 at the end of year 2 (F2;6 ˆ 14:80, P ˆ 0:0048) (Fig. 8). Treatment effects on stem height and diameter followed nearly the same pattern in years 1 and 2, as well. At the end of year 1, mean heights ranged from 20 cm for the pesticide treatment to 30 cm for the pesticide ‡ weeded treatment, with height on the

control plots intermediate at 25 cm (F2;6 ˆ 9:82, P ˆ 0:0128). Similarly, only at the end of year 2 did heights differ among the treatments, with the range of mean heights among treatments from 34 cm for the pesticide treatment to 67 cm for the pesticide ‡ weeded treatment and the control at 51 cm (F2;6 ˆ 8:51, P ˆ 0:0177). Treatment effects on mean stem diameter were signi®cant early in year 1 (September 1997: F2;6 ˆ 10:83, P ˆ 0:0102; October 1997: F2;6 ˆ 6:35, P ˆ 0:0330) and then not again until the end of year 2 (F2;6 ˆ 16:76, P ˆ 0:0035). At the end of year 1, diameters were 2.5, 3.6 and 4.6 mm for pesticide, control and pesticide ‡ weeded treatments, respectively. At the end of year 2, diameters were 4.5, 7.6 and 10.0 mm, for pesticide, control and pesticide ‡ weeded, respectively. Treatment effects on stem density at site C were not signi®cant during year 1, but became signi®cant in the latter half of year 2 (range of F2;6 ˆ 5:77±13.63, P ˆ 0:0401±0.0059) (Fig. 9). Blocking effects on density were not signi®cant in either year.

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3.2. Soil analyses and weed assessment

Fig. 6. Mean volume index (d2h) of all hardwood stems during the second growing season following clearcutting, for the deer exclusion study, in the North Carolina Piedmont. Treatments with the same letter, by month, are not signi®cantly different (GLMprotected REGWQ).

Treatment effects on soil temperature were signi®cant only at site A, ranging from 35 8C in the control to 38 8C in the weeded plots (F3;41 ˆ 6:99, P ˆ 0:0007). Average soil temperature among treatment plots on site B was 33 8C and on site C it was 29 8C. Signi®cant treatment effects on percent soil moisture were detected at site B; 37% in pesticide ‡ weeded plots and 51% in control plots (F1;19 ˆ 7:75, P ˆ 0:0118), but not at sites A (ranging from 58 to 63%) or C (ranging from 73 to 79%). Soil chemical analyses did not reveal signi®cant differences among the treatments at site A. Only one signi®cant difference was detected at site B, where potassium was greater in the control than in the pesticide ‡ weeded treatment (F1;3 ˆ 17:73, P ˆ 0:0245). At site C, percent organic matter and estimated nitrogen release were signi®cantly greater in the pesticide treatment, than in the control or the pesticide ‡ weeded treatment plots (F2;6 ˆ 6:82, P ˆ 0:0285 and F2;6 ˆ 6:73, P ˆ 0:0293, respectively). Also at site C, magnesium was signi®cantly greater in the control and the pesticide treatment, than in the pesticide ‡ weeded treatment (F2;6 ˆ 5:47, P ˆ 0:0445). Weed density index values among unweeded plots in year 1 ranged from 1 to 36, 27 to 67, and 21 to 45 at sites A, B and C, respectively. In year 2, the values in unweeded plots ranged from 45 to 106, 83 to 120, and 21 to 77, for sites A, B and C, respectively. On average, weed density index increased 4.2, 2.4 and 1.4 times from year 1 to 2 for sites A, B and C, respectively. No statistical analyses were conducted on this data. 4. Discussion

Fig. 7. Mean stem density of all hardwoods during the second growing season after clearcutting for the deer exclusion study, in the North Carolina Piedmont. Treatments with the same letter, by month, are not signi®cantly different (GLM-protected REGWQ).

Signi®cant growth responses (height, diameter, volume index) to the treatments were evident in the ®rst and second growing seasons at all three sites (Figs. 2, 4, 6 and 8). These rapid responses indicate that hardwood regeneration can quickly be released from biological constraints, and that the rate of stand development may be enhanced with silvicultural manipulations in extremely young stands (Romagosa, 1999). Had the treatments been applied earlier in year 1, in May

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Fig. 8. Mean volume index (d2h) of all hardwood stems during the ®rst two growing seasons following clearcutting at site C in the North Carolina Piedmont. Treatments with the same letter are not signi®cantly different (GLM-protected REGWQ).

rather than July, growth responses may have been more pronounced. Diameter growth responded to the treatments more rapidly than height growth. At all three sites, signi®cant diameter differences among treatments were evident at the end of the ®rst growing season, while only site A had signi®cant height differences among treatments at that time. Positive growth responses continued throughout the study. At the end of year 2, mean stem volume index in the pesticide ‡ weeded treatment was 33.3, 10.5 and 1.9 times greater than the mean volume index in the control plots on sites A, B and C, respectively (Figs. 2, 4 and 8). Pesticide ‡ weeded treatments had 3.0, 2.8 and 1.2 times greater mean heights and 2.5, 2.9 and 1.3 times greater mean diameters than the controls at sites A, B and C, respectively. Stems released by the weeding treatments consistently had larger growth responses than those in unweeded treatments (control, pesticide) at all sites. Growth responses of species that in a mature forest

would likely be in the overstory, might have been even more pronounced had the other (mid-/understory-destined) species been removed. The general effect of pesticides (pesticide, pesticide ‡ weeded treatments) was less clear. While the pesticide treatment did not result in greater growth than the control (sites A and C), the trend at site A suggests that the combination of pesticides and weeding had a synergistic effect on tree growth. The pesticide component of the pesticide ‡ weeded treatment may have afforded some protection from insects, diseases and/or browsing mammals and reduced these growth constraints. This inference is only valid for site A, where all treatments were applied. On site C the pesticide treatment produced a smaller growth response at the end of the study than did the control and responses in the control plots were not signi®cantly different from the pesticide ‡ weeded treatment (Fig. 8). It is unlikely that the pesticide treatment had phytotoxic effects responsible for this, as such an effect was not found at sites A or B.

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557

Fig. 9. Mean stem density of all hardwoods during the ®rst two growing seasons after clearcutting at site C in the North Carolina Piedmont. Treatments with the same letter, by month, are not signi®cantly different (GLM-protected REGWQ).

Nor were there visual signs of phytotoxicity or reason to suspect that the compounds used would suppress growth or differentially affect various species. It is likely that these unexpected growth responses were due partly to a higher representation of sprout-origin stems in the control plots (by random chance) than in the pesticide ‡ weeded or pesticide plots on this site, 43% versus 21 and 27 %, respectively. Sprouts, with their well-developed root systems grow more rapidly than seedlings (Johnson, 1993) and that likely enhanced growth responses of trees in the control plots on site C. When site C data were analyzed without sprout-origin stems, the control and pesticide treatments were not statistically different and the pesticide ‡ weeded treatment was larger than the others (data not shown), as expected given the ®ndings on site A (Fig. 2). Other analyses from site C do not suggest a soil-based explanation for these ®ndings.

Evidence of insect herbivory and fungal infection on the regenerating hardwoods on the study sites was rare. A leaf spot disease, putatively Tubakia leaf spot (Tubakia dryina), was observed on sweetgum at sites A and B (C. Hodges, 1997, Plant Disease and Insect Clinic, North Carolina State University Cooperation Extension Service, personal communication). Another leaf spot disease, putatively Cristulariella leaf spot (Cristulariella moricola) (C. Hodges, 1997, Plant Disease and Insect Clinic, North Carolina State University Cooperation Extension Service, personal communication) and interveinal chlorosis and necrotic spots (associated with hot/dry conditions) (Sinclair et al., 1987) were observed on yellow-poplar leaves at all three sites. While these infections were of low intensity at the three sites, they could lead to reduced photosynthetic surface area and premature leaf drop. This might partly explain the positive effect

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of the pesticide treatment at site A, although this was not measured. Growth responses in the pesticide treated plots (alone or with weeding) might re¯ect reduced deer browsing due to the mammal repellents applied (Michael, 1989), even though browsing damage was still observed in all of the treatments at Schenck forest (sites A and B) and to a lessor extent at Hill forest (site C). The Schenck forest is estimated to have 12±15 deer per km2, while Hill forest has 8±11 deer per km2 (Osborne, 1998). The mammal repellent used in year 1 (Ro-pel1), was changed in year 2 to Deer-Away1, due to the discovery of a report verifying positive results for this compound (Graveline et al., 1998). The ef®cacy of the repellents in pesticide treated plots cannot be ascertained, due to their combination with other pesticides. Damage from deer browsing in the ®rst growing season, even in mammal repellent treated plots, was the impetus for the deer exclosure study installed on site B in year 2. Signi®cant growth responses (mostly on yellow-poplar, the dominant tree in these plots) (h, d and d2h) to the weeded ‡ excluded treatment were evident after two months of exclusion (Fig. 6). At the ®nal measurement, mean stem volume index was 3.2 times greater in the weeded ‡ excluded treatment, than in the weeded treatment. Mean stem height and diameter were 1.6 and 1.3 times greater in the weeded ‡ excluded treatment than in the weeded treatment, respectively. Deer browsing in the weededonly plots may have been greater as a result of the treatment itself. These plots were directly adjacent to the visually apparent exclosure (paired plots along the stand edge) and deer may have browsed the area more intensively, thereby enhancing the difference between the weeded-only and the weeded ‡ exclosure plots. Stange and Shea (1998) showed an increase in browse frequency as a result of conspicuous weed mats placed around planted oak seedlings and memory may have an important role in deer foraging activities (Gillingham and Bunnell, 1989). The actual amount of deer browsing in the study plots was not determined, but clearly it was signi®cant, given visual observations and the ancillary information derived from the limited deer exclosure study on site B in year 2. However, despite the apparent intensity of browsing, tree growth responses to weeding, in particular, were signi®cant.

Treatment effects on stem density were dif®cult to evaluate in the current study. Since individual stems were not marked, it was not possible to quantify the effect of individual stem mortality or recruitment on density. However, it was visually apparent that during each of the years there were small trees that died and others that emerged. The net effect of these changes, and perhaps the impact of the treatments on density are re¯ected in Figs. 3, 5, 7 and 9. Among these graphs, the effect of treatment on stem density cannot be discerned due to changes in relative density within and between years (Figs. 3 and 5) and no intuitive explanation is clear as to why some treatments provided for higher densities than others (Fig. 9). However, there were some statistically signi®cant density differences among treatments, especially later in the study on each site. It is probable that treatments which substantially enhance tree growth, will in time exert a substantial in¯uence on stem density through competition for resources and self-thinning. If the process of self-thinning (stand development and density) is advanced through these types of treatments, then their effects may be long-lived. Soil temperature and moisture in¯uence early hardwood growth, and are affected by vegetation cover (Bonner, 1968; Larson, 1970; Larson and Whitmore, 1970; Loftus Jr., 1975; Stephens, 1965). In the current study, soil temperature and moisture were recorded only once, and differences found only for some treatments on sites A and B. It is unlikely that these effects in¯uenced measurable tree growth responses. Similarly, soil chemical analyses did not reveal plot or treatment differences related to the tree responses. Readings from the weed density index meter showed an increase in weed density from year 1 to 2. In year 1, the most dominant weed at all sites was E. hieracifolia (L.) Raf., a species of lettuce that can reach 2 m tall. This species was abundant, but was widely spaced and did not produce large density index values, even though it may have been very competitive. Whereas in year 2, horseweed (E. canadensis L.), which grows densely, dominated all three sites and contributed substantially to the density index values. There was no indication that differential weed competition among non-weeded plots confounded treatment effects in this study. Although weed indices are dif®cult to reconcile among studies, they provide an

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ef®cient means to assess the relative levels of competition within a study (Salonius et al., 1991). Biological constraints, especially non-hardwood plant competition, were demonstrated in the current study to reduce hardwood growth and delay stand development. Weeding alone provided signi®cant growth advantages through the initial two years of stand development. Weeding (releasing) very young stands from herbaceous competition on a large scale, relying on a broadcast herbaceous plant herbicide, could substantially enhance the production of natural stands. A preliminary herbicide screening study supports this conclusion (HRC, 2001). Acknowledgements We recognize N. Hascoat, K. Hess, L. Jervis, H. Moberly, D. Nishida, D. Parker, J. Rapp and J. Smith of North Carolina State University Department of Forestry, for providing ®eld and investigative assistance, an early review of this manuscript by T. Shear of NC State, and referee comments. This research was funded by the NC State Hardwood Research Cooperative, a partnership of NC State, industry and state forestry agencies. References Araman, P.A., 1989. Hardwood export markets: a look at the past, a look at the future. In: National Hardwood Lumber Association Annual Report and Proceedings of the 91st Annual Convention, pp. 29±32. Araman, P., Tansey, J., 1991. US has plenty of hardwood but much of it's not for sale. For. Ind. 118 (9), 16±17. Bechtold, W.A., Phillips, D.R., 1983. The hardwood resource on non-industrial private forest land in the southeast Piedmont. Southeastern Forest Experimental Station, USDA Forest Service Research Paper SE-236, 19 pp. Birch, T.W., 1997. Private forest land owners of the southern United States, 1994. Northeastern Forest Experimental Station, USDA Forest Service Resource Bulletin NE-138, 195 pp. Bonner, F.T., 1968. Responses to soil moisture de®ciency by seedlings of three hardwood species. Southern Forest Experimental Station, USDA Forest Service Research Note SO-70, 3 pp. Boyce, S.G., 1977. Management of eastern hardwood forests for multiple bene®ts (DYNAST-MB). Southeastern Forest Experimental Station, USDA Forest Service Research Paper SE-168, 116 pp.

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