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DIVISION S-7 - FOREST & RANGE SOILS

Nutrient Retranslocation Response of Picea mariana Seedlings to Nitrogen Supply

Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON M5S 3B3, Canada

Corresponding author (vic.timmer{at}utoronto.ca)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The hypotheses that retranslocation is controlled by soil nutrient availability or plant nutrient reserves were tested under field conditions for one growing season using nutrient-loaded and non-loaded (conventional) black spruce [Picea mariana (Mill.) BSP] seedlings planted on a poor, medium, and rich fertility soil created by equivalent applications of 0, 200, and 400 kg N ha^-1, respectively. Growth and nutrient uptake increased with N supply, and was consistently higher in loaded than conventional seedlings, demonstrating the advantage of nutrient loading practices to accelerate seedling growth across the range of soil N tested. Compared to the poor soil, new shoot biomass of loaded seedlings increased by 34 and 134% on the medium and rich soils, suggesting loaded seedlings may be more efficiently transplanted on more fertile sites. Net retranslocation of N, P, and K increased by 569, 185, and 102% by nutrient loading in the nursery, supporting the hypothesis of translocation driven by the magnitude of plant nutrient reserves. However, net N retranslocation diminished with time due to root system expansion that promoted uptake and reduced the need for N redistribution. Net retranslocation of N (the most limiting nutrient) declined with soil N supply, but that of P and K were relatively independent of soil fertility. Increased N availability in the soil enhanced N accumulation in the plants but lowered N retranslocation. We conclude that higher net retranslocation of N on poor soils is a phenotypic adjustment by P. mariana seedlings to maximize N use at low availability.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
NEWLY PLANTED SEEDLINGS depend highly on retranslocation of internal nutrient reserves to sinks of new growth soon after transplanting because initial slow root development limits uptake from the soil (Burdett et al., 1984; van den Driessche, 1985; Burdett, 1990). Retranslocation can be defined as the amount of an element depleted from older plant components and made available for new growth (Lim and Cousens, 1986), and has demonstrated prime significance in the nutrient economy of plants (Kim et al., 1987).

New cultural techniques, such as nutrient loading, have been designed specifically to build higher plant nutrient reserves in seedlings during nursery culture (Timmer and Aidelbaum, 1996). These reserves are depleted later to support current growth in newly transplanted seedlings. For example, nutrient-loaded seedlings depleted 67% N from old shoots to support new growth in black spruce (Malik and Timmer, 1998), and 51 to 60% in Chinese fir [Cunninghamia lanceolata (Lamb) Hook] (Xu and Timmer, 1999). Nutrient-loaded seedlings also grew faster, took up more nutrients, and suppressed weeds more effectively in the field (Malik and Timmer, 1998). Since nutrients often limit growth of seedlings in boreal forests under interspecific competition (Nilsson et al., 1986; Orlander et al., 1986), the higher internal reserves acquired by nutrient-loaded trees may ameliorate nutrient shortages when transplanted on such sites (Timmer, 1997; Timmer and Aidelbaum, 1996).

Malik and Timmer (1998) contend that net retranslocation is a key mechanism driving the improved transplanting growth response and competitiveness of nutrient-loaded seedlings on weed prone sites. However, the retranslocation response seems to decline with soil N availability (Timmer and Munson, 1991; Xu and Timmer, 1999), raising the question whether nutrient-loaded seedlings should be more efficiently transplanted on poor rather than rich soils? Furthermore, variability of retranslocation with soil nutrient availability is still controversial (Hawkins et al., 1999). Current work suggests that retranslocation may be regulated by soil nutrient supply, nutrient uptake rates, the size of plant nutrient reserves, and age of trees (Munson et al., 1995; Hawkins et al., 1998, 1999; Malik and Timmer, 1998), although the mechanisms involved are not well understood (Hawkins et al., 1999). Furthermore, the relative contributions of nutrients from retranslocation and from external uptake to sinks of new growth are unknown.

In this study, we used a conceptual model (Lim and Cousens, 1986) to quantify relative contributions of nutrients from net retranslocation and external uptake to new growth under field conditions and to explain the mechanisms driving retranslocation on a N supply gradient. In addition, we examined site specific planting for loaded trees and determined whether net retranslocation is controlled by plant nutrient reserves. We focused mainly on N, because this element is usually most limiting for growth of black spruce seedlings on Boreal mixedwood sites (Malik and Timmer, 1996). Our research addressed four major questions:

1. Should nutrient-loaded trees be more efficiently transplanted on poor rather than rich soils for enhanced net retranslocation for growth of newly established seedlings?
   
2. Does N retranslocation in spruce decrease in response to N supply? Such a decrease has been demonstrated with Chinese fir under greenhouse conditions (Xu and Timmer, 1999) and in other studies (Miller et al., 1976; Lim and Cousens, 1986), but does it also occur with spruce under field conditions?
   
3. Is greater retranslocation and growth response in transplanted nutrient-loaded seedlings controlled by higher plant N reserves acquired during nursery culture?
   
4. Will increased root growth over time following transplanting enhance nutrient uptake from external soil sources and reduce the need for internal redistribution of nutrients as Millard and Neilsen (1989) suggested?
   

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Plant Material and Prior Nursery Culture
Black spruce seedlings were reared in commercial greenhouses at North Gro Development located at Kirkland Lake in northeastern Ontario. Seedlings were spring sown (April, 1998) in styro-block trays containing 40-mL cavities (165 cavities per tray), filled with peat/vermiculite (3:1, v/v) growing media, and reared under similar environmental conditions (moisture, light, and temperature) except for N fertilizer treatments. A commercial water soluble fertilizer (20-20-20, Plant Products Co. Ltd., Brampton, ON) was supplied in solution form with the irrigation. A seasonal total of 10 mg N seedling^-1 was delivered to one group of seedlings at a constant weekly rate, starting 1 wk after germination to represent industry's normal practice for containerized black spruce seedlings production in northern Ontario (called the conventional treatment) (Timmer and Aidelbaum, 1996; Timmer, 1997). The other treatment (referred to as the loaded treatment hereafter) received a seasonal total of 64 mg N seedling^-1. Weekly loading rates for this latter treatment were calculated according to exponential functions described in detail by Timmer (1997) and Timmer and Aidelbaum (1996) for steady-state nutrient culture (Ingestad and Lund, 1986; Ingestad and Agren, 1995). Fertilization started 1 wk after germination and was carried out for 18 wk using traveling boom-sprayers. After each spray, the seedlings were washed with water to avoid possible burning. Seedlings were reared until budset in summer, hardened, and cold stored (-2°C) for overwintering.

Site Description and Transplanting Procedure
Experimental plots were established on a clear-cut area that had lain fallow for 3 yr in Kirkland Lake, ON, Canada (48°10'N, 88°01'W). The site type is classified as ST3b, characterized by mixedwood stands on fresh to moderately moist, sandy to coarse loamy soils (McCarthy et al., 1994). The soil belongs to the Orthic Humo-Ferric Podzol subgroup, equivalent to the Cryorthods (Soil Classification Working Group, 1998). Mechanical analysis (Day, 1965) of composite mineral soil samples collected from the 0- to 10-cm depth at 20 random points within the study site showed particle-size distribution averaged 72, 26, and 2% sand, silt, and clay, respectively. Total N and soil organic matter (Ball, 1964) averaged 0.4 and 19%, respectively. Mean values for available P (Bray and Kurtz, 1945) and pH_(water) were 63 mg kg^-1 soil and 4.0. Previous overstory vegetation included trembling aspen (Populus tremuloides Michx.), jack pine (Pinus banksiana Lamb.), black spruce, and white spruce [Picea glauca (Moench) Voss]. Shrubs included bush honeysuckle (Diervilla lonicera Mill.) and velvetleaf blueberry (Vaccinium myrtilloides Michx.), while mosses were schreber's moss [Pleurozium schreberi (Brid.) Mitt] and plume moss [Ptilium crista-castrensis (Hedw.) De Not.].

Prior to plot establishment, a reconnaissance survey was conducted to locate a uniform area devoid of vegetation to avoid possible confounding effects of competing vegetation on treatment responses. Twelve 6 by 12 m² blocks (main plots) separated by 2-m buffers were established within this area. Each main plot was then split into two subplots (6 by 6 m²). One subplot was planted with 25 conventional seedlings and the other with 25 nutrient-loaded seedlings at 1 by 1 m spacing. A nutrient gradient was created by incorporating 0, 30, and 60 mg N tree^-1 (20-20-20 water soluble fertilizer) around each seedling within a 0.5-m radius, equivalent to 0, 200, and 400 kg N ha^-1, respectively, on the main plots at planting. These treatments are referred to as the poor, medium, and rich soils, respectively, hereafter. After fertilization, water was sprayed on the seedlings to avoid possible burning. Planting was conducted on 28 May 1999 with visually presorted seedlings of similar size to minimize possible confounding due to initial size differences.

Experimental Design, Plant Sampling and Nutrient Analysis
The experiment tested three levels of N supply (0, 30, and 60 mg N seedling^-1), two levels of nursery nutrition (loaded and conventional), at two time periods (60 and 120 d) after transplanting (Table 1). The experimental design was a split plot design, with a 3 by 2 by 2 factorial treatment structure replicated four times, for a total of 48 experimental units. At planting, five batches of four seedlings (a total of 20 seedlings) per loading regime were sampled, roots were washed free of soil, and the plants were separated into stems and roots. A random sample of three seedlings within the 6 by 6 m² plot, for a total of 12 seedlings per treatment, were harvested 60 and 120 d after establishment. Seedlings were washed free of soil and separated into new shoots, old shoots, and roots. Samples were composited by replicate for chemical analysis, but measured individually and averaged for growth analysis. Plant material was oven-dried for 72 h at 68°C, ground, and wet-digested in a block digester using an H₂SO₄-H₂O₂ solution (Lowther, 1980). Total tissue N was determined using a Technicon II Autoanalyzer System (Technicon Industrial Systems, Tarrytown, NY) (Eastin, 1978), P by the molybdate blue method (Allen, 1974), and K by atomic absorption (Model 3100, Perkin-Elmer, Norwalk, CT).

(1)

where Re is the amount of nutrient retranslocated, A is the nutrient content in shoot at Day 0 (preplant status), B is the nutrient content in old shoots at Day 60 or 120 after transplanting, D is the nutrient content of dead tissues between Day 0 and Days 60 or 120, and Le is the amount of nutrient leached. Since D + Le is assumed negligible as noted by others (Miller et al., 1976; Chapin and Kedrowski, 1983; Lim and Cousens, 1986; Helmisaari, 1992), and as for short-term trials like ours, percentage net retranslocation was calculated as

(2)

Net uptake from external source (U) was quantified by

(3)

where Req was nutrient content in new tissues at Day 60 or 120 after transplanting.

Statistical Analysis
Analysis of variance (ANOVA) was conducted on growth and nutrient variables using SAS and where appropriate means were ranked according to Waller-Duncan's multiple range tests (SAS Institute, 1989). The Anderson and McLean's (1974) linear model for the ANOVA is given as:

(4)

where Y_ijklm is seedling biomass, nutrient content or concentration of the mth replicate (m = 1), estimated at lth time (l = 1, 2), for the kth nursery nutrition (k = 1, 2), within the ith soil N supply (i = 1, 2, 3) from the jth block (j = 1, 2, 3, 4); µ is overall mean; S_i is fixed effect of the ith soil N supply; _(i)j is whole plot error; _(ij) is first restriction error; F_k is fixed effect of the kth nursery nutrition; '_(ijk) is second restriction error; T_l is fixed effect of the lth time; followed by the interactions among the main and block effects; and is error associated with measured seedling biomass, nutrient content, or concentration from bulk replicates.

Vector Diagnosis
Vector diagnosis involves comparing nutrient concentration, nutrient content, and biomass of plants or plant components in a graphic format known as a vector nomogram (Timmer, 1991, Fig. 1). Plant tissues sampled at time intervals are usually compared to a control or reference (R) usually on a relative basis, or on an absolute basis as in our case. The technique has undergone periodic refinement to enhance diagnostic precision, capacity, and flexibility. New concepts, such as steady-state nutrition characterized by stable internal tissue nutrient concentration with time (Shift B), were introduced by incorporating temporal parameters in the system (Imo and Timmer, 1997). Recently a new vector (Shift G) was added to expand the diagnostic power of vector analysis to examine retranslocation processes (Malik, 1998). Shift G depicts depletion of nutrients (see box under Fig. 1) since nutrient concentration and contents decline in plant components with or without change in biomass often associated with net nutrient retranslocation from older plant parts to sinks of new growth (Malik and Timmer, 1998). In the nomogram (Fig. 1), slanting lines or diagonals represent lines of equal biomass; thus, Shifts C and G may contrast markedly in nutrition at similar biomass. Shifts to the left and right of the reference (R) treatment depict respective net depletion and gain of nutrients. For comprehensive reviews of vector diagnosis, see Timmer (1991), Haase and Rose (1995), and Imo and Timmer (1997). The technique is employed here to elucidate the net retranslocation response and examine temporal biomass and nutrient allocation dynamics during seedling development.

View larger version (36K): [in this window] [in a new window]   Fig. 1. Nutritional interpretations of directional changes with time in dry mass, nutrient content, and nutrient concentration during plant development. The reference point (R) represents an initial plant sample taken at time T0 (usually normalized to 100). Vector orientation and magnitude characterize parameter relationships [increase (+), decrease (-), or unchanging (0)] relative to seedlings sampled at subsequent time intervals. Arrow direction also depicts time progression from time T0 to T1, T2... A new Shift G, characterizing net depletion often associated with retranslocation, is included in the box (Modified from Imo and Timmer, 1997; Malik, 1998)

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

View larger version (29K): [in this window] [in a new window]   Fig. 2. Component dry mass production of nutrient-loaded (L) and conventional (C) black spruce seedlings 0, 60, and 120 d after transplanting on poor (P), medium (M), and rich (R) soils

Vector Diagnosis of Biomass and Nitrogen Allocation Dynamics
Temporal changes in biomass and N status in old shoots of P. mariana seedlings are shown in Fig. 3 . Rapid early growth of new shoots (Fig. 2) was at the expense of old shoots that exhibited N depletion, apparently severe for loaded (Fig. 3, top), and mild for conventional trees (Fig. 3, bottom), as reflected by differential vector length (Shift G, Fig. 1 and 3) during the initial 60 d only. These shifts demonstrated typical net retranslocation of N from source organs in old shoots to sinks of new growth (Malik, 1998; Malik and Timmer, 1998; McAlister and Timmer, 1998). The loaded seedlings exhibited an exploitive nutrient use strategy (Malik and Timmer, 1996, 1998) by depleting higher N reserves for investment in active metabolic sinks in new shoots and roots that promoted growth in these components (Fig. 2). In contrast, conventional seedlings showed a conservative strategy characterized by less N depletion from lower reserves (Fig. 3, bottom). For example, biomass and N uptake in new shoots increased by 336 and 366%, respectively, in loaded, compared with only 100 and 82% in conventional seedlings at final sampling (Fig. 2). Moreover, biomass increased by 1300 mg in old shoots of loaded, compared with 300 mg in conventional seedlings planted on the same soil at final harvest with respect to Day 0 (Fig. 3). Steady-state nutrition (Imo and Timmer, 1992; Quoreshi and Timmer, 2000) characterized by stable internal tissue N concentration with time was achieved in old shoots of loaded seedlings on the rich soil after Day 60 (Shift B, Fig. 1 and 3, top) suggesting growth rate matched N uptake rate. In contrasts, conventional seedlings on the rich soil were N deficient as shown by increased dry mass, N uptake and concentration after Day 60 (Shift C, Fig. 1 and 3, bottom) presumably because of the lower N reserves and uptake in these seedlings (Imo and Timmer, 1997).

View larger version (23K): [in this window] [in a new window]   Fig. 3. Vector nomograms of absolute change in dry mass, N uptake, and concentration in old shoots of nutrient-loaded (L, top) and conventional (C, bottom) spruce seedlings after transplanting on poor (P) and rich (R) soils. Vectors reflect progressions in time (d) from T0 to T60, and T60 to T120. Note scale difference between nomograms

View larger version (31K): [in this window] [in a new window]   Fig. 4. Estimated amounts of N, P, and K mobilized from old shoots (net retranslocation/10) and from the soil (uptake) for new growth in nutrient-loaded (L) and conventional (C) black spruce seedlings [left]; transplanted on poor (P), medium (M) and rich (R) soils [right]. Vertical bars are standard errors of mean. Similar bars marked with different letters are significantly different according to DMRT, P < 0.05

View larger version (17K): [in this window] [in a new window]   Fig. 5. Estimated amounts of N, P, and K mobilized from old shoots (net retranslocation/10) and from the soil (uptake) for new growth in black spruce seedlings 60 and 120 d after transplanting. Vertical bars are standard errors of mean. Similar bars marked with different letters are significantly different according to DMRT, P < 0.05

Higher net retranslocation in loaded seedlings suggests N reserves in these seedlings were probably less structurally bound, hence readily available for depletion to active metabolic sinks (Chapin, 1990; Miller et al., 1979; Fagerstrom and Lohm, 1977). Nitrogen in plant tissues is distributed in either the mobile or structurally bound forms (Fagerstrom and Lohm, 1977). The mobile pool is thought to consist of nonfunctional amino acids and proteins that are readily depleted from source organs to support new growth (Chapin, 1990). Amino acid analysis showed that arginine, glutamic acid, and proline were the major N forms in the mobile pool in black spruce seedlings (Kim et al., 1987). We speculate that in addition to building N reserves, nutrient loading may also increase the mobile N pool in plant tissues that are then rapidly remobilized to sinks of new growth soon after transplanting as demonstrated in this study and others (Malik and Timmer, 1998; Xu and Timmer, 1999). The degree of N retranslocation (50%) by loaded seedlings 60 d after transplanting in this study is lower than 55% reported for Chinese fir seedlings (Xu and Timmer, 1999) and 55% for radiata pine (Pinus radiata D. Don) (Nambiar and Fife, 1987) and white pine (Pinus strobus L.) (Munson et al., 1995), suggesting that N retranslocation capacity may differ between species (Ralhan and Singh, 1987; Schlesinger et al., 1989; Munson et al., 1995; Keenan et al., 1995).

Loaded seedlings also exhibited higher net retranslocation of P (P < 0.0001) and K (P < 0.0195) compared with conventional seedlings, but response magnitude was lower than occurred with N (Fig. 4, left and Table 1). Compared with conventional seedlings, N, P, and K net retranslocation increased by 569, 185, and 102% in loaded seedlings (Fig. 4, left), exemplifying the ability of nutrient-loaded seedlings to redistribute nutrients for sinks of new growth soon after transplanting.

Net Retranslocation as Influenced by Soil Nutrient Supply
Studies suggest nutrient retranslocation may be enhanced on poor sites (Miller et al., 1976; Johnson et al., 1982) or may be independent of nutrient gradients (Millard and Proe, 1992; Nelson et al., 1995), or may increase with nutrient availability (Proe and Millard, 1994; Munson et al., 1995). We found that N net retranslocation declined (P < 0.0001) with N supply (Fig. 4, right), supporting a hypothesis of reduced retranslocation on rich soils (Miller et al., 1976; Turner, 1977; Johnson et al., 1982; Shaver and Milillo, 1984; Lim and Cousens, 1986; van den Driessche, 1991). Increased N uptake at higher availability, 86 and 149%, respectively, on the medium and rich soils compared with the poor soil (Fig. 4, right), seemed to lower net retranslocation as noted by Turner (1977). In contrast, P and K net retranslocation were independent of soil nutrient supply, suggesting that for less-limiting nutrients retranslocation is independent of nutrient gradients (Fife and Nambiar, 1982, 1984; Chapin and Kedrowski, 1983; Birk and Vitousek, 1986; Nambiar and Fife, 1991; Chapin and Moilanen, 1991; Del Arco et al., 1991; Millard and Proe, 1992; Nelson et al., 1995).

Net Retranslocation as Influenced by Time since Transplanting
Net retranslocation of N, P, and K significantly diminished (P < 0.0001) with time (Fig. 5, Table 1); supporting the hypothesis that retranslocation is influenced by seedling development (Clark, 1983; Potter et al., 1987; Hawkins et al., 1998; Xu and Timmer, 1999). Apparently, N, P, and K uptake from external sources significantly increased (P < 0.0001) with time (Fig. 5, Table 1), presumably because of greater root establishment and growth (Fig. 2). This supports the contention of Millard and Neilsen (1989) that an established root system would allow greater exploitation of soil N to occur and lower the need for N retranslocation. For example, greater root growth (Fig. 2) resulted in increased N, P, and K uptake from the soil by 223, 493, and 166% (P < 0.0001, Table 1) at final harvest with respect to Day 60 (Fig. 5). Interaction effects were also significant (data not presented). Compared with the rich soil at Day 60, for example, N and P uptake increased by 336 and 433% at Day 120 on the same soil, exemplifying significant (P < 0.001– 0.005, Table 1) soil N supply x time interaction effects. Similarly, greater N acquisition by loaded (313%) compared with conventional (131%) seedlings at final sampling with respect to Day 60 status exemplifies significant (P < 0.001, Table 1) nursery nutrition x time interaction effects.

Retranslocation from old foliage provided about 66% of the nutrients required for new growth in closed conifer stands (Miller, 1995). Also, 43% of N required for new growth in Italian stone pine (Pinus pinea L.) (Rapp et al., 1979), and between 50 and 60% in Austrian pine (Pinus nigra Arnold) (Miller, 1984) were met by retranslocation. Furthermore, retranslocation met about 39% N, 60% P, and 22% K demand for new growth of a 20-yr-old Loblolly pine (Pinus taeda L.) plantation (Switzer and Nelson, 1972). The estimates of respective N and K net retranslocation in newly planted black spruce seedlings were 50 and 40% in this study, and retranslocation was most critical for growth soon after transplanting when nutrient stress was most severe (Xu and Timmer, 1999). Similarly, about 32 to 40% of N used for leaf growth in young conifers was derived from retranslocation (Millard, 1996). We propose that net retranslocation is critical at seedling establishment, but declines with root system development because of increased nutrient uptake from external sources. It may become significant at crown closure where two-thirds of nutrients required for growth could be met by retranslocation from tight nutrient budgets (Miller, 1995).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Results show that nutrient loading practices promoted depletion of higher plant N, P, and K reserves to sinks that stimulated rapid new growth and nutrient uptake in transplanted seedlings. In contrast, conventional fertilization practices induced nutrient conservation in seedlings and reduced new growth and nutrient uptake. Consistently greater growth and nutrient uptake on the simulated N supply gradient suggest loaded seedlings may be transplanted on the full range of soil N supply tested, with enhanced growth response on rich soils. Net retranslocation of N and P increased by 569 and 185% in nutrient-loaded vs. conventional seedlings, and higher preplant nutrient reserves accounted for the greater retranslocation in loaded seedlings, supporting a hypothesis of retranslocation driven by the amount of plant nutrient reserves. Root growth in loaded seedlings increased N, P, and K uptake from external sources by 223, 493, and 166%, respectively, and lowered the need for internal redistribution of nutrients over time. Net N retranslocation declined with increased soil N availability due to enhanced plant uptake that lowered the need for internal redistribution of N. Nutrient loading has been shown to offset early ambient deficiencies, promote vigorous growth, and may accelerate early dominance (McAlister and Timmer, 1998), which is essential for the development of an even-aged stand (Miller, 1995). We speculate that the greater physiological plasticity exhibited by nutrient-loaded seedlings in net retranslocation, nutrient uptake, and biomass allocation kinetics soon after transplanting will contribute to enhanced future survival, growth, and nutrition when compared with conventional seedlings.

	ACKNOWLEDGMENTS

 
This research was financially supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC). Support from University of Toronto open Fellowships, Faculty of Forestry and E.E. Johnson Postgraduate Forestry Award are gratefully acknowledged. We also thank North-Gro Development Ltd. for supplying seeds, assistance in growing seedlings, and for providing storage facilities for hardening and over-winter storage of seedlings. Finally, we thank the anonymous reviewers and associate editor whose contributions have significantly improved the manuscript.

Received for publication June 6, 2000.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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