Site preparation effects on foliar N and P use, retranslocation, and transfer to litter in 15-years old Pinus taeda

Forest Ecology and Management 129 (2000) 143±152

Site preparation effects on foliar N and P use, retranslocation, and transfer to litter in 15-years old Pinus taeda 1

Kathryn B. Piatek*, , H. Lee Allen Department of Forestry, North Carolina State University, Box 8008, Raleigh, NC 27695, USA Received 8 August 1998; accepted 23 April 1999

Abstract Intensive site preparation in loblolly pine (Pinus taeda L.) plantations may remove nutrients and lower site productivity. We evaluated the effects of nutrient removal in site preparation on mid-rotation pine foliar production, and foliar N- and P-use, retranslocation, and transfer to litter for two years. We also investigated changes in foliar nutrients one year after fertilization. Site preparation treatments were: shear±pile±disk and chop±burn, used with or without vegetation control. Mid-rotation pines were fertilized with 200 kg haÿ1 N and 50 kg haÿ1 P, or with 200 kg haÿ1 N and 50 kg haÿ1 P and micronutrients. Foliar production was estimated from litter mass. N- and P-use was estimated from N and P concentrations in green foliage and foliar production. Retranslocation was the difference in N and P between green foliage and litter, in percent. N and P transfer to litter was estimated from litter N and P concentration and litter mass. Nutrient removal in site preparation at plantation establishment did not affect mid-rotation pine foliar production, foliar N- and P-use, retranslocation, or nutrient transfer to litter. The lack of site preparation effects may be related to the length of time after treatment; the stage of decomposition of organic matter that may be removed in site preparation may determine when nutrient supply will be affected. Competition with hardwoods decreased pine foliar production by 56%, and N- and P-use by 55% and 52%, but not percent retranslocation. On shear±pile±disk/herbicide, shear±pile±disk/no-herbicide, and chop±burn/herbicide plots (none or small hardwood component), average pine foliar production was 4365 kg haÿ1 yearÿ1, N- and P-use was 53.2 and 4.5 kg haÿ1 yearÿ1, N and P retranslocation was 63.7% and 69.8%, N and P transfer to litter was 18.9 and 1.3 kg haÿ1 yearÿ1. Based on a hypothetical N budget for the total stand, an N limitation may develop on those treatments that lost more nutrients in site preparation. Fertilization increased foliage production by 26%, and N- and P-use both by 49%, indicating some luxury consumption. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Pinus taeda; Nitrogen; Phosphorus; Retranslocation; Productivity; Plantation management; Fertilization

*Corresponding author. Tel.: +1-360-357-5204; fax: +1-360357-9313. E-mail address: kpiatek/[email protected] (K.B. Piatek). 1 Present address: USDA Pacific Northwest Research Station, Olympia Forestry Sciences Laboratory, 3625 93rd Ave, SW, Olympia, WA 98512±9193, USA.

1. Introduction Current forest management in loblolly pine (Pinus taeda L.) plantations in the southeastern US targets foliage production as a strategy to raise productivity

0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 1 5 0 - 4

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K.B. Piatek, H.L. Allen / Forest Ecology and Management 129 (2000) 143±152

(Allen et al., 1990). Foliar biomass and net aboveground productivity are positively and linearly related on a range of loblolly pine sites (Teskey et al., 1987; Vose and Allen, 1988). Foliage is relatively nutrient-demanding, and assimilates between 30% and 70% of the total nutrients used annually by an established forest stand (Switzer and Nelson, 1972). Current-year foliage in loblolly pine stands ages 13 to 25-year old may use 50±83 kg haÿ1 N, and 5±11 kg haÿ1 P (Switzer and Nelson, 1972; Wells and Jorgensen, 1975; Johnson and Lindberg, 1992). Pinus sylvestris, up to 30-years old on a nutrient-poor site, used 27 kg haÿ1 N ÿand 2 kg haÿ1 P in 1-year old foliage (Rode, 1993). Primary sources of foliar nutrients are soil supply and retranslocation from older foliage. Soils in the southeastern US, where loblolly pine is widely planted, are nutrient-limited (Allen, 1987; Binkley et al., 1995). Although N availability after clearcutting and site preparation can increase in the soil to ca. 100 kg haÿ1 yearÿ1 in both, loblolly pine in the southeastern US and radiata pine in Australia (Vitousek and Matson, 1984, 1985; Vitousek et al., 1992; Smethurst and Nambiar, 1995), it also tends to decline within several years after site preparation (Vitousek et al., 1992; Piatek and Allen, 1999) when nutrient demand of developing forest stands is still increasing. The decline of N availability may occur earlier or be more severe after site preparation that removes organic matter and large amounts of organic matter-bound nutrients. Nutrient removal associated with mechanical site preparation has raised widespread concerns over future site productivity, especially on nutrient-limited sites (Burger and Kluender, 1982; Neary et al., 1984; Morris and Lowery, 1988; Fox et al., 1989; Powers et al., 1990; Thornley and Cannell, 1992; Munson et al., 1993). This study evaluated the effects of site preparation at plantation establishment on midrotation pine foliar production, N and P concentration, N- and P-use, retranslocation, and transfer to litter, and examined changes in these foliar nutrients after mid-rotation fertilization. Our goal was to determine if nutrient removal in site preparation lowered loblolly pine foliar production and N- and P-use.

2. Methods 2.1. Study site The study site was located near Henderson, Vance County, North Carolina (368 250 N, 788 300 W) in the physiographic region of the Piedmont. The average temperature is 14.88C and annual precipitation is 1133 mm, based on a 64-year record (National Climatic Data Center, 1997). Soils on the site are kaolinitic clays of the Cecil series classi®ed as thermic, kaolinitic, Typic Kanhapludults. 2.2. Treatments The original study design has been described in detail by Vitousek and Matson, 1984, 1985; and Tew et al., 1986. Brie¯y, two types of harvest (stem-only and whole-tree removal) and two types of site preparation (shear±pile±disk and chop±burn) were implemented in a factorial design in 1981 to monitor long-term site productivity under plantation management. For the current study, we used only those plots that were harvested as stem-only because previous research showed no signi®cant differences in growth due to harvesting level (NCSFNC, 1995). Stem-only harvest resulted in an estimated removal of 57 kg N haÿ1, 5 kg P haÿ1, 35 kg K haÿ1, 51 kg Ca haÿ1, and 14 kg Mg haÿ1. Site preparation using the shear± pile±disk treatment resulted in an estimated removal of 591 kg N haÿ1, 34 kg P haÿ1, 92 kg K haÿ1, 363 kg Ca haÿ1, and 64 kg Mg haÿ1 (Tew et al., 1986). In this treatment, woody debris from harvesting and some topsoil were pushed away from the planting site into windrows 47 m apart. The area between the windrows was disked to a depth of 7±12 cm (Gent et al., 1984). Site preparation using the chop±burn treatment resulted in an estimated removal of 46 kg N haÿ1, and 0 kg haÿ1, P, K, Ca, and Mg. In this treatment, woody debris was fragmented with a roller drum chopper and burned. Burning was light and patchy (Tew et al., 1986). First-generation improved loblolly pine seedlings were planted in 1982. Vegetation control (complete herbicide for the ®rst ®ve years and no herbicide) was added in a split-plot design to the main treatments. The current study was initiated in April 1994 in plots in the following treatment combinations: shear±pile±disk/

K.B. Piatek, H.L. Allen / Forest Ecology and Management 129 (2000) 143±152

herbicide, shear±pile±disk/no herbicide, chop±burn/ herbicide, and chop±burn/no herbicide, for a total of 12 plots in three blocks. A fertilization trial was added at plantation age 14 (mid-rotation). Two new plots were established in each block in areas that were treated in 1981 with shear±pile±disk/no herbicide, but never used as measurement plots. The shear±pile±disk/no-herbicide treatment (measurement plot) was used as control against which we tested the fertilizer effect. One of the two fertilizer plots received 200 kg haÿ1 N and 50 kg haÿ1 P, and is referred to as the `N ‡ P' treatment. The other plot received, in kg haÿ1, 200 N, 50 P, 100 K, 119 Ca, 100 S, 50 Mg, 20 Fe, 20 Mn, 7.5 Cu, 7.5 Zn, 1.5 B, and 0.5 Mo, and is referred to as the `complete' treatment. Fertilizers were in the form of urea, triple superphosphate, potassium chloride, gypsum, dolomitic lime, epsom salts, sulfates of iron, manganese, copper, zinc, borax, and sodium molybdate. Fertilizer was mixed in the ®eld and spread manually in the spring 1994. 2.3. Foliage production and nutrients We collected pine foliar litter to estimate foliar production. In April 1994, ®ve ®berglass traps, each with an area of 0.75 m2 and collectively covering 0.83% of the plot area, were systematically located in each plot. Four traps were placed toward (but away from) the plot corners and a ®fth one in the middle of the plot. Litter was collected bimonthly in early summer, semi-monthly in fall, and monthly during the rest of the year, for two years. Litter from the ®ve traps was composited per plot. On the no-herbicide treatments, hardwood litter was separated from pine litter in the ®eld, and also composited by plot. Herbicide plots contained pine litter only. Litter was ovendried for three days at 708C and weighed to the nearest gram. The sum of litter weights found between April 1994 and March 1995 represented the 1993 foliar production (Vose and Allen, 1988), here referred to as Year 1. Similarly, the April 1995 to March 1996 collections represented the 1994 foliar production, or Year 2. Pine foliar N- and P-use were estimated from N and P concentrations in green foliage and foliar production. For nutrient concentrations, pine foliage from the ®rst ¯ush of the previous growing season was sampled

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from the upper part of the crown in February each year from ®ve trees per plot. Previous studies showed that maximum N and P concentration in loblolly pine foliage occurs in late winter (Zhang and Allen, 1996), at the time when foliage present on branches is one-year old (produced in the previous year), and two-year-old foliage is already dropped. February foliar N and P concentrations were multiplied by annual foliar production (annual litter mass) to estimate foliar N- and P-use. Hardwood foliar N- and Puse was not estimated because N and P concentration in green hardwood leaves was not measured. N and P retranslocation was estimated from the difference in N and P between green foliage and litter. Percent N and P retranslocation in pine foliage was de®ned as: …green N and P content ÿ litter N and P content= green N and P content†  100 N and P transfer to litter was estimated from litter N and P concentration and litter mass. Litter N and P concentration was analyzed in each sample collection. Litter (pine and hardwood separately) was mixed thoroughly by hand, and subsamples were drawn. After grinding to pass a 1 mm screen, subsamples were dried. Aliquots (0.2 g) were digested in a sulfuric acid/hydrogen peroxide mix (Parkinson and Allen, 1975). Ten percent sample duplication and a pine standard tissue (Standard Reference Material 1575, National Institute of Standards and Technology) were used as quality controls. Kjeldahl N and P (total organic P and polyphosphates) were determined colorimetrically on a Lachat autoanalyzer (Lachat Quickchem; Methods 13-107-06-2-D and 13-115-01-1-B for N and P, respectively). Litter N and P content was obtained by multiplying N and P concentrations for each collection by the litter weight for that collection. Annual N and P transfer to litter was estimated by adding N and P contents from each collection for the year. 2.4. Statistical analyses Analysis of variance for split-plot design was used to examine site preparation effects on mid-rotation pine and plot total (pine plus hardwood) foliar production, and N and P concentration, use, retransloca-

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tion, and transfer to litter. Whole-plot effects were those of site preparation, with subplots of vegetation control. Fertilized plots were evaluated against the shear±pile±disk/no-herbicide treatments of the main study in a randomized complete block design with three fertilizer regimes: none, N ‡ P, and complete. Hardwoods were present in six plots (no-herbicide only) and were tested for the effect of site preparation on leaf litter production. Year-to-year variation was analyzed by analysis of covariance, by including a variable `year'. All treatment effects were considered signi®cant at p < 0.05. 3. Results 3.1. Foliar biomass production Production of pine foliar biomass ranged from 1938 to 4875 kg haÿ1 yearÿ1 on the non-fertilized plots. Pine foliar production was lower on the chop±burn/ no-herbicide treatment than on any other treatment in Year 1 and in Year 2, both treatment combinations without vegetation control had signi®cantly lower pine foliar production (Tables 1 and 2). Hardwood

foliage in the chop±burn/no-herbicide plots, comprised up to 50% of total foliar weight in Year 1, and 45% in Year 2. Hardwood foliage in the shear± pile±disk/no-herbicide plots comprised 17% of the total foliar weight in Year 1, and 13% in Year 2. 3.2. Foliar N and P concentration, and N- and P-use N concentration in green pine foliage did not differ across treatments in Year 1. N concentration in Year 2 was higher in herbicide plots than in no-herbicide plots (Table 2). N-use ranged from 43 kg haÿ1 yearÿ1 (average of Year 2) to 49 kg haÿ1 yearÿ1 (average of Year 1). N-use in Year 1 was lowest on the chop±burn/ no-herbicide and highest on the chop±burn/herbicide treatment, with a signi®cant site preparation  vegetation control interaction. N-use in Year 2 was 43% lower on the no-herbicide plots than on herbicide plots, with a signi®cant effect of vegetation control (Table 2). On average, N-use in year 1 pine foliage was 6.1 kg haÿ1 higher than in Year 2, and this difference was signi®cant. P concentration in green pine foliage exhibited no treatment effects in Year 1 or 2. P-use ranged from 3.4 (average of Year 1) to 4.4 kg haÿ1 yearÿ1 (average of

Table 1 Biomass production, N and P concentrations and use in pine foliage in a mid-rotation loblolly pine plantation under different silvicultural treatments (A) and following fertilization (B) Foliage production (kg haÿ1 yearÿ1)

N concentration (g kgÿ1)

N-use (kg haÿ1 yearÿ1)

P concentration (g kgÿ1)

P-use (kg haÿ1 yearÿ1)

1993

1994

1993

1994

1993

1994

1993

1994

1993

1994

(A) DIHRa DINOb CHHRc CHNOd SEg

4727 3891 4875 1938 264.3

4770 3551 4374 1994 306.8

12.5 12.1 13.4 12.7 0.6

11.9 10.9 12.0 11.5 0.6

58.8 46.9 65.3 24.8 3.2

56.6 38.8 52.5 23.1 3.3

1.1 1.1 1.2 1.2 0.1

0.9 0.9 0.9 0.9 0.1

5.1 4.3 5.6 2.4 0.3

4.5 3.3 4.1 1.9 0.3

(B) N ‡ Pe Completef SEg

3912 3920 196.6

5107 4792 242.0

12.3 12.9 0.8

14.9 15.0 0.4

48.1 50.6 4.3

76 71 3.1

1.2 1.2 0.1

1.4 1.4 0.4

4.4 4.6 0.3

6.9 6.5 0.3

a

Shear±pile±disk/herbicide. Shear±pile±disk/no-herbicide. c Chop±burn/herbicide. d Chop±burn/no-herbicide. e 200 N, 50 P (in kg haÿ1). f 200 N, 50 P, 100 K, 119 Ca, 100 S, 50 Mg, 20 Fe, 20 Mn, 7.5 Zn, 1.5 B and 0.5 Mo (in kg haÿ1). g Standard error. b

K.B. Piatek, H.L. Allen / Forest Ecology and Management 129 (2000) 143±152

147

Table 2 P-values for treatment effects on biomass, N and P concentration and use in pine foliage in a mid-rotation loblolly pine plantation under different silvicultural treatments (A) and following fertilization (B) Foliage production

N concentration

N-use

P concentration

P-use

1993

1994

1993

1994

1993

1994

1993

1994

1993

1994

(A) Block Site preparation Vegetation control Site preparation  vegetation control

0.849 0.208 0.002 0.017

0.811 0.267 0.004 0.132

0.387 0.376 0.453 0.857

0.159 0.250 0.005 0.167

0.516 0.352 0.001 0.011

0.680 0.327 0.002 0.152

0.133 0.411 0.511 10.00

0.024 0.527 0.678 0.781

0.274 0.361 0.003 0.025

0.353 0.219 0.003 0.115

(B) Block Fertilizer

0.278 0.994

0.448 0.022

0.673 0.747

0.846 0.003

0.562 0.827

0.385 0.002

0.063 0.752

0.170 0.004

0.371 0.803

0.704 0.002

was retranslocated with no signi®cant treatment effects, but with a larger between-treatment variation. In Year 2, percent N retranslocation was lower than in Year 1, at 59% of maximum green N content, with no treatment effects. Percent P retranslocation was 62% in Year 2, with a signi®cant effect of vegetation control; herbicide plots retranslocated P at rates 11% higher than no-herbicide plots (Table 3).

Year 2). Similar to N-use in year 1, P-use in year 1 was lowest on the chop±burn/no-herbicide and highest on the chop±burn/herbicide treatment, with a signi®cant site preparation  vegetation control interaction. In Year 2, P-use was 40% lower on the no-herbicide plots than on herbicide plots, with a signi®cant effect of vegetation control. The difference in foliar P-use between years 1 and 2 was signi®cant at 0.9 kg haÿ1 (Tables 1 and 2).

3.4. N and P transfer to litter

3.3. N and P retranslocation

The amount of N returned to the forest ¯oor in pine foliar litter varied from a low of 7.4 kg haÿ1 yearÿ1 in Year 1 to a high of 23.4 kg haÿ1 yearÿ1 in Year 2 (Table 4). In Year 1, site preparation  vegetation control interaction was signi®cant; pine foliage in

Pine foliage produced in Year 1 and dropped as litter in Year 2 retranslocated 68% of its maximum Ncontent regardless of treatment (Tables 3 and 4). Additionally, 77% of the maximum green P content

Table 3 Treatment means for percent N and P retranslocation, and amounts of N and P lost in pine foliar litter

DIHRa DINOb CHHRc CHNOd SEe a

N retranslocation (%)

P retranslocation (%)

N transfer to litter (kg haÿ1 yearÿ1)

P transfer to litter (kg haÿ1 yearÿ1)

1993

1994

1993

1994

1993

1994

1993

1994

68.9 67.3 68.6 68.7 2.1

58.6 56.3 62.5 58.9 1.4

78.1 72.2 79.0 79.3 1.8

62.9 58.7 67.6 58.2 2.4

18.2 14.9 20.5 7.4 0.9

23.4 16.9 19.7 9.5 1.8

1.11 1.17 1.19 0.48 0.07

1.66 1.35 1.34 0.81 0.12

Shear±pile±disk/herbicide. Shear±pile±disk/no-herbicide. c Chop±burn/herbicide. d Chop±burn/no-herbicide. e Standard error. b

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K.B. Piatek, H.L. Allen / Forest Ecology and Management 129 (2000) 143±152

Table 4 P values for treatment effects on pine foliar retranslocation and nutrient transfer to litter

Block Site preparation Vegetation control Site preparation  vegetation control

N retranslocation

P retranslocation

N transfer to litter

P transfer to litter

1993

1994

1993

1994

1993

1994

1993

1994

0.126 0.837 0.744 0.707

0.155 0.145 0.113 0.670

0.297 0.190 0.190 0.150

0.109 0.073 0.049 0.350

0.840 0.405 0.001 0.006

0.967 0.254 0.010 0.348

0.540 0.028 0.010 0.006

0.259 0.142 0.022 0.393

Note: Error a for bloc and site preparation is bloc  site preparation.

the chop±burn/herbicide plots lost 13.1 kg haÿ1 yearÿ1 more N in litter than in the chop±burn/noherbicide plots. In Year 2, vegetation control was signi®cant, and foliage in the herbicide plots lost 8.4 kg haÿ1 yearÿ1 more N in litter than no-herbicide plots (Tables 3 and 4). P transfer to litter ranged from a low of 0.5 kg ha-1 yearÿ1 in Year 1 to a high of 1.7 kg haÿ1 yearÿ1 in Year 2 (Table 3). In Year 1, the site preparation  vegetation control was signi®cant; P transfer to litter in the chop±burn/herbicide plots was 0.7 kg haÿ1 yearÿ1 more than in the chop± burn/no herbicide plots. In Year 2, vegetation control was signi®cant, and foliage in the herbicide plots lost 0.42 kg haÿ1 yearÿ1 more P in litter than in no-herbicide plots. 3.5. Fertilization effects on pine foliar biomass, N and P concentration and use Before fertilization (1993), pine foliar production and nutrient concentrations were not signi®cantly different between plots destined for fertilization and the control plots (shear±pile±disk/no-herbicide). One year after fertilizing (1994), the fertilizer effect was signi®cant for all measurements but percent retranslocation. The two fertilizer regimes (N ‡ P, and complete) did not differ from each other (Table 2). Fertilizing increased pine foliage production by 26% over the control, and N- and P-use by 49% each. 4. Discussion Mechanical site preparation may remove site nutrients and reduce productivity (Burger and Kluender,

1982; Neary et al., 1984; Morris and Lowery, 1988; Fox et al., 1989; Powers et al., 1990; Thornley and Cannell, 1992; Munson et al., 1993). Reductions in tree growth associated with nutrient loss after site preparation have been reported on some nutrient-poor or drought-prone sites (Minore, 1986; Graham et al., 1989), but not on other sites (Munson et al., 1993), and have not been observed at this site (NCSFNC, 1995). In this study, we also did not ®nd evidence that nutrient removal with site preparation at plantation establishment decreased pine foliage production at mid-rotation (age 15 years). At an average of 4687 kg haÿ1 yearÿ1 on plots without hardwoods, pine foliar production on these eroded Piedmont soils was comparable to that reported in other studies (Wells and Jorgensen, 1975; Lockaby et al., 1995). Foliage production is important to forest management in loblolly pine plantations because it has been shown that net aboveground productivity increases linearly with foliar biomass (Teskey et al., 1987; Vose and Allen, 1988). Foliar N- and P-use strongly re¯ected treatment differences in foliage production. Average N- and P-use by pine foliage on this site was similar to that reported in other studies (Wells and Jorgensen, 1975; Johnson and Lindberg, 1992). Also, pine foliage in the previous rotation at this site used 46 kg N haÿ1 and 3.9 kg P haÿ1 at age 22 (Tew et al., 1986), an amount comparable to the averages observed in this study on plots without hardwoods. At the current level of foliar biomass, the removal of nutrients in site preparation 14±15 years earlier did not diminish the ability of this site to supply nutrients needed in foliage at midrotation. N and P removed in harvest (stem-only) and shear± pile±disk site preparation constituted 14% N and

K.B. Piatek, H.L. Allen / Forest Ecology and Management 129 (2000) 143±152

almost 60% of ecosystem P (aboveground vegetation, forest ¯oor, and mineral soil from 0±60 cm depth included) (data from Tew et al., 1986). Chop±burn site preparation removed 2% of total ecosystem N and 7.6% of total ecosystem P. Yet neither N nor P in pine foliage at mid-rotation were affected by site preparation alone. The state of organic matter decomposition was probably more important to whether nutrient supply was affected 15 years after treatment than the absolute amount of organic matter or its nutrient content. Coarse woody debris, for example, may need decades and possibly centuries (Tyrell and Crow, 1994) to mineralize, and its removal would most likely not be re¯ected in soil nutrient supply only 15 years later. This does not change the immediate importance of coarse woody debris to other ecosystem functions, such as moisture retention, wildlife habitat, or soil physical properties. Since P was removed at a substantially higher rate relative to ecosystem P capital, we would expect treatment differences in foliar P rather than N. One explanation for the lack of apparent effect of substantial P removal on subsequent foliar P may be that N supply limits the amount of foliar biomass to below the level, in which P may become limiting. Such interaction of N and P in loblolly pine foliage was observed by Zhang and Allen (1996). Retranslocation may meet a substantial portion of the foliar nutrient needs, ranging from 18% of foliar N in Abies amabilis in the Paci®c Northwest (Keenan et al., 1995), 44% in Larix laricina in Minnesota (Tilton, 1977) to 79% in Larix laricina in Alaska (Chapin and Kedrowski, 1983). Loblolly pine retranslocated 43% of foliar N and 65% of foliar P in Tennessee (Grizzard et al., 1976), and 75% N and 73% P in the Georgia Piedmont (Zhang and Allen, 1996). Contribution of retranslocation to foliar N- and P-use in our study can be estimated by subtracting N-useYear 2 from N retranslocatedYear 1, and assuming that the retranslocated nutrients were used for foliar production only. Thus, retranslocation contributed 40.6 kg haÿ1 yearÿ1 (71.6%) to foliar N on shear±pile±disk/herbicide, 32.0 kg haÿ1 yearÿ1 (81.3%) on shear±pile±disk/noherbicide, 44.8 kg haÿ1 yearÿ1 (85.3%) on chop± burn/herbicide, and 17.4 kg haÿ1 yearÿ1 (73.8%) on the chop±burn/no-herbicide treatments. Soil N supply needed for pine foliage may now be estimated by another subtraction (N-useYear 2 ÿ N

149

from retranslocationYear 2). Soil N supply was needed at 16.0 kg N haÿ1 yearÿ1 on the shear±pile±disk/ herbicide, 6.8 kg N haÿ1 yearÿ1 on shear±pile±disk/ no-herbicide, 7.7 kg N haÿ1 yearÿ1 on chop±burn/ herbicide, and 5.8 kg N haÿ1 yearÿ1 on chop±burn/ no-herbicide treatments for the Year 2 foliage. The actual net N mineralization in mineral soil at 0±15 cm depth that year was 28.1, 19.1, 29.8, and 34.3 kg N haÿ1 yearÿ1 on the above treatments (Piatek and Allen, 1999), suf®cient to close the budget for foliar N-use. Foliar nutrients make up 30% to 70% of the total stand nutrient use in established stands (Switzer and Nelson, 1972). Root, branch, and stemwood production, which were not assessed in this study, make up the remainder. Assuming that foliar nutrient use was 70% of the total (the best case scenario), a hypothetical total stand N-use would be 80.9, 55.4, 75.0, and 33.0 kg N haÿ1 yearÿ1 on the above treatments. After subtracting N from retranslocation, soil N supply for the hypothetical stand would have to be 40 kg N haÿ1 yearÿ1 on shear±pile±disk/herbicide, 24 kg N haÿ1 yearÿ1 on shear±pile±disk/no-herbicide, 30 kg N haÿ1 yearÿ1 on chop±burn/herbicide, and 16 kg N haÿ1 yearÿ1 on chop±burn/no-herbicide treatments. Because net N mineralization was assessed in closed tubes (Piatek and Allen, 1999), atmospheric deposition at a rate of 10 kg N haÿ1 yearÿ1 (Richter and Markewitz, 1996) must be added to ®nd out the total soil N supply. Again, comparing soil N supply to the hypothetical stand N-use from above results in a de®cit of ÿ2 kg N haÿ1 yearÿ1 on shear±pile±disk/ herbicide, and a surplus of ‡5 kg N haÿ1 yearÿ1 on shear±pile±disk/no-herbicide, ‡9 kg N haÿ1 yearÿ1 on chop±burn/herbicide, and ‡28 kg N haÿ1 yearÿ1 on chop±burn/no-herbicide treatments. These calculations may indirectly suggest that despite no evidence of declining foliar production or foliar nutrient use at this time, N limitation may be developing on the shear±pile±disk treatments, where organic matter was removed before plantation establishment. The estimated de®cit would be more striking, if the hypothetical stand N-use were estimated based on foliar N-use as 30% of the total. However, relative to soil N supply, more N was returned in litter on the shear±pile±disk/no-herbicide treatment. In the next rotation, a higher litter N content may actually increase the forest ¯oor N turnover and N mineralization in the mineral soil. In general, pine foliage in the

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herbicide plots lost more nutrients to litter than in the no-herbicide plots. N transfer to litter was positively related to soil N availability in other studies (Vitousek et al., 1982; Pastor et al., 1984). Retranslocation also supplied 88.5, 94.0, 107.9, and 100.2% of P used in the Year 2 pine foliage on the shear±pile±disk/herbicide, shear±pile±disk/no-herbicide, chop±burn/herbicide, and chop±burn/no-herbicide treatments, respectively. Numbers above 100% were possible because pine foliar biomass in Year 1 was higher than in Year 2. Because more P was available from retranslocation from Year 1 than used in foliage in Year 2, factors other than P nutrition may have affected foliage production and its P-use. Again, an insuf®cient N supply seems to be a plausible explanation. Loblolly pine foliage production can be drastically reduced in competition with hardwoods. In this study, pine foliage production was lowest on the chop±burn/ no-herbicide plots where hardwood leaves made up to 50% of the total plot litter. By comparison, the shear± pile±disk treatment had only 13±17% of hardwood litter. These differences resulted because the chop± burn treatment allowed hardwood root systems from the previous rotation to persist, and root sprouts occupied the site quickly. By contrast, in the shear± pile±disk treatment, stumps were sheared, and any hardwoods present today seeded in over time from adjacent stands. Hardwoods proliferated in the noherbicide subplots of both site preparation treatments. Total foliar production in the chop±burn/no-herbicide treatment was signi®cantly lower than in any other treatment, suggesting that the site's capacity to produce foliage in general was impeded by the presence of a large hardwood component, despite a higher current N mineralization rate (Piatek and Allen, 1999). This was probably because hardwood species are more nutrient-demanding than conifers (Nadelhoffer et al., 1983; Nadelhoffer et al., 1984; Rode, 1993). The year-to-year variation in foliar nutrition seemed to be related to variation in pine foliar production, in that N- and P-use was lower when foliar production decreased in Year 2. Annual variation in foliar production itself, although not signi®cant in this study, may be related to climate variations; climatic data were not taken in this study. Large annual variations in litter production due to climatic factors were demon-

strated in conifers, including Picea abies and Pseudotsuga menziesii (Bray and Gorham, 1964). As shown by other studies in this region, midrotation fertilization increases nutrient availability, and may increase foliage and stand production (Vose and Allen, 1988; Allen et al., 1990). This site's potential for growing foliage can be increased by adding nutrients. Fertilization boosted pine foliar production, and N- and P-use over non-fertilized. Nutrients other than N or P did not seem to be limiting to growth, at least that year, because the complete fertilizer treatment did not result in any more growth or nutrient uptake than N ‡ P alone. Some storage consumption may have occurred following fertilization because biomass did not increase any further with increasing N and P concentration. This apparent luxury consumption may result in further foliar biomass increase in subsequent years because greater amounts of nutrients will be available for retranslocation. Acknowledgements This study was supported by the Forest Nutrition Cooperative at North Carolina State University and conducted on land owned by Champion International. The reviews by M. Barbercheck, D. Richter, S. Shafer, T. Wentworth, and two anonymous reviewers are gratefully acknowledged. References Allen, H.L., 1987. Forest fertilizers: nutrient amendment, stand productivity, and environmental impact. J. For. 85, 37±46. Allen, H.L., Dougherty, P.M., Campbell, R.G., 1990. Manipulation of water and nutrientsÐpractice and opportunity in southern U.S. pine forests. For. Ecol. Manage. 30, 437±453. Binkley, D., Carter, R., Allen, H.L., 1995. Nitrogen fertilization practices in forestry. In: Bacon, P.E. (Ed.), Nitrogen Fertilization in the Environment. Marcel Dekker Inc., New York. pp. 608. Bray, J.R., Gorham, E., 1964. Litter production in forests of the world. Adv. Ecol. Res. 2, 101±158. Burger, J.A., Kluender, R.A., 1982. Site preparationÐPiedmont. Symposium on loblolly pine ecosystem. December 8±10, 1982, Raleigh, NC. Chapin III, F.S., Kedrowski, R.A., 1983. Seasonal changes in nitrogen and phosphorus fractions and autumn retranslocation in evergreen deciduous taiga trees. Ecology 64, 376±391.

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