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Effects of controlled weed densities and soil types on soil nitrate accumulation, spruce growth, and weed growth
Forest Ecology and Management 133 (2000) 135±144
Effects of controlled weed densities and soil types on soil nitrate accumulation, spruce growth, and weed growth Naresh V. Thevathasana, Phillip E. Reynoldsb,*, Ralf Kuessnerc, Wayne F. Belld a
Department of Environmental Biology, University of Guelph, Guelph, Ont., Canada N1G 2W1 Natural Resources Canada, Canadian Forest Service, 1219 Queen St. East, Sault Ste. Marie, Ont., Canada P6A 5M7 c University of Dresden, Institute for Silviculture and Forest Protection, Pienner Str. 8, D-01737, Tharandt, Germany d Ontario Ministry of Natural Resources, Ontario Forest Research Institute, 1235 Queen St. East, Sault Ste. Marie, Ont., Canada P6A 5N5 b
Accepted 6 October 1999
Abstract Soil nitrate (NO3ÿ) accumulation rates were assessed among seven weed species grown in small plots during the summer of 1997, at a northern Ontario location. Objectives were (1) to quantify soil nitrate accumulation rates at varying weed densities established on three soil types (clay, loam, and sand) and (2) to assess the effects of soil nitrate levels on weed and black spruce (Picea mariana (Mill.) B.S.P.) seedling growth and weed growth. Controlled densities (0, 0.5, 2, 8 stems per m2) of red raspberry (Rubus idaeus L. var. strigosus (Michx.) Maxim.), ®reweed (Epilobium angustifolium L.), and a grass species (Calamagrostis canadensis (Michx.) Nutt.) were planted along with spruce seedlings (1 m spacing) early in the summer of 1994. Concurrently, four more deciduous weed species, trembling aspen (Populus tremuloides Michx.), white birch (Betula papyrifera Marsh.), green alder (Alnus crispa (Ait.) Pursh), and willow (Salix humilis Marsh.) were also established at four densities (0, 0.5, 2, 4 stems per m2). In mid-summer (July) 1997, soil samples collected from the respective weed densities were placed in polyethylene bags and the bags were incubated in the respective plots for 34 days at a depth of 12 cm. Nitrate accumulation rates (mg per 100 gm of dry soil per day), weed heights, and the effective leaf area index (LAIe, in m2 mÿ2, measured with a Licor LAI-2000 Plant Canopy Analyzer) of competing vegetation were consistently higher (1.4±3.0 times) for clay soil. Nitrate levels were signi®cantly (p < 0.05) higher in clay soils obtained from plots planted with raspberry, grass, aspen, and willow. Carbon : nitrogen (C : N) ratios were higher for loam soils than those for clay or sand soils, and these higher ratios may have resulted in more immobilization than mineralization of soil nitrogen. Nitrate levels for clays and loams correlated signi®cantly with the LAIe of competing vegetation (r 0.62±0.78) and with weed growth (r 0.75±0.96), but non-signi®cantly, and often negatively, with spruce stem volume. To date, soil nitrate accumulation appears to have primarily bene®ted cultivated weeds and not to have improved spruce growth. These ®ndings af®rm that weeds must be controlled for seedlings to bene®t nutritionally. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Soil nitrogen mineralization; Soil nitrate (NO3ÿ) accumulation; Vegetation management; Competition; Boreal forest species; Weed thresholds; Growth; Black spruce (Picea mariana); Fireweed (Epilobium angustifolium); Red raspberry (Rubus idaeus); Canada bluejoint grass (Calamagrostis canadensis); Trembling aspen (Populus tremuloides); Birch (Betula papyrifera); Alder (Alnus crispa); Willow (Salix humilis); Microclimate
* Corresponding author. Tel.: 1-705-949-9461. E-mail address: [email protected] (P.E. Reynolds)
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 3 0 4 - 7
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1. Introduction In the Boreal forest, a number of key herbaceous and woody plant species compete with newly planted conifer species for light, water, and nutrients (Larson and Schubert, 1969; Sands and Nambiar, 1984; Zutter et al., 1986; Elliot and White, 1987; Morris et al., 1993; Nambiar and Sands, 1993). The degree of competition often varies with site quality, including soil type, moisture and nutrient availability, the number of years elapsed between harvesting and planting or between planting and subsequent release from competition, and the extent of site disturbance. Normally, competition is more severe (Brady, 1990) on ®ner-textured soils (e.g. clays, silts) characterized by higher water-holding and cation exchange capacities than on coarser-textured soils (i.e. sands). Due to these differences, sands bind fewer nutrients and are more prone to leaching and nutrient loss than ®ner-textured soils. Since soils exhibit considerable variability in moisture retention and nutrient content, potential plant competitors also vary in their distribution across the landscape. Species with greater drought tolerance and lower nutrient demands are often found growing on sands, whereas species with low drought tolerance or high nutrient requirements may be restricted to clays or silts. Species possessing broader ranges of tolerance for both water and nutrients are capable of growing on a range of soil types, although their growth may be clearly better where water and nutrients are less restricted. Seedling microclimate is affected by the amount of competition present on a forest site. Changes in light, temperature, relative humidity and moisture produced by varying amounts of competing vegetation affect microsite suitability for crop production (Reynolds et al., 1997a, b). Reduced competition increases solar radiation reaching the forest ¯oor, results in soil warming (Brand and Janas, 1988 and Wood and von Althen, 1993), and raises soil moisture levels (Stone, 1973; Bosch and Hewlett, 1982; Kochenderfer and Wendel, 1983). Increased soil moisture (Stanford and Epstein, 1974; Matson and Vitousek, 1981; Burger and Pritchett, 1984) coupled with soil warming (Powers, 1990) produces conditions favorable for nutrient turnover (N and C mineralization), and improved soil moisture enhances nutrient uptake (Nambiar and Sands, 1993). Soil N mineralization
is often positively correlated with soil related parameters, such as soil moisture and temperature, and inversely correlated with C : N ratio (Stevenson, 1982; Gordon, 1986; Haynes, 1986; Thevathasan, 1998). These parameters are also in¯uenced by soil types, vegetation present, and climate (Haynes, 1986 and Reynolds et al., 1997a). Improved moisture and nutrient status combined with increased light, generally results in increased photosynthesis (Lieffers et al., 1993 and Eastman and Camm, 1995) and ultimately, improved crop survival and growth (Radosevich and Osteryoung, 1987 and Newton et al., 1992). Since competing vegetation affects the availability of site resources (light, water, nutrients) for use by planted seedlings, the question arises as to when and how often competitors need to be reduced in number. This is both a temporal question (Wagner et al., 1996) and also very much related to which competitor and which soil type are concerned. Not all competitors are expected to compete equally for light, water, or nutrients, and the degree of competition is expected to vary with soil type and timing (i.e. number of years of competition establishment). Broad objectives of the current research are (1) to determine the severity of competition by various species in affecting black spruce survival and growth and (2) to determine how this varies with soil type, weed density, and the number of years of weed establishment. Speci®c objectives for this study were (1) to quantify soil nitrate (NO3ÿ) accumulation rates at varying weed densities established on three soil types (clay, loam, sand) and (2) to assess the effects of soil nitrate levels on weed and black spruce seedling growth. 2. Methods 2.1. Research site The present research site was established in 1994 by the Ontario Forest Research Institute (OFRI) to examine the effects of various herbaceous and woody competitors, planted at a range of densities on crop performance (Bell et al., 1998). The research site is at the OFRI Arboretum which is located on the western outskirts of Sault Ste. Marie, Ontario. Ten potential competitors of black spruce (Picea mariana (Mill.) B.S.P.) including red raspberry (Rubus idaeus L. var.
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strigosus (Michx.) Maxim.), ®reweed (Epilobium angustifolium L.), Canada bluejoint grass (Calamagrostis canadensis (Michx.) Nutt.), bracken fern (Pteridium aquilinnum (L.) Kuhn), large-leafed aster (Aster macrophyllus L.), trembling aspen (Populus tremuloides Michx.), white birch (Betula papyrifera Marsh.), green alder (Alnus crispa (Ait.) Pursh), willow (Salix humilis Marsh.), and jack pine (Pinus banksiana Lamb.) were planted admixed with black spruce in 7 7 m cells. Six of these cells aligned sideby-side in a row 42 m in length was designated as a weed series. Each of the 10 weed series was replicated on clay, loam, and sand soils for a total of 30 series. Within each series, herbs and woody competitors were established in additive densities. These were 0.0, 0.5, 1.0, 2.0, 4.0, and 8.0 plants per m2 for herbaceous competitors, and 0.0, 0.25, 0.5, 1.0, 2.0, and 4.0 plants per m2 for woody competitors.
were pooled prior to statistical analysis. Data were examined using basic statistics where means were calculated and standard deviations (5% level of signi®cance determined (Snedecor and Cochran, 1967). Statistical analysis and graphics were performed using CSS Statistica (StatSoft, Tulsa, OK).
2.2. Soil nitrogen mineralization studies
In early August 1997, foliar samples were bulk sampled for paired spruce and plant competitors at nine locations in each 7 7 m cell. All soil types, densities, and competitors, except pine, were sampled. Those plants sampled were the same as those where leaf area (LAIe) and plant growth measurements were made. Spruce and weed foliar samples were subsequently quanti®ed for macro- and micro-nutrient concentrations. Total Kjeldahl results are reported here.
In mid-summer (July) 1997, soil samples collected at a depth of 12 cm (i.e. mid-point of spruce rooting zone) from the respective weed densities were placed in 0.025 mm polyethylene bags and the bags were incubated in the respective plots for 34 days at a depth of 12 cm (Gordon et al., 1987). Samples were split into two bags ± a pre-sample and a second sample for burial. Pre-samples, which established N, C, NH4, and NO3ÿ levels prior to burial, were placed on dry ice and kept frozen prior to chemical analysis. Post- or exhumed samples established ammonium and nitrate levels after incubation, and were processed the same as pre-samples. In the laboratory, 20 g pre- and post-samples were extracted with 60 ml of 2N KCl on a shaker at 3500 rpm for 1 h. After agitation, the extract was allowed to settle prior to ®ltering. Frozen soil solutions were kept frozen prior to analysis on a Technicon Autoanalyzer II system. Daily rates of NH4 or NO3ÿ (mg per 100 gm of dry soil per day) were calculated. Mineralization rates were determined by subtracting the un-incubated sample values from the incubated sample values, and dividing by the number of days (i.e. 34) incubated. Since our prime objective in this report was to examine soil differences, and not to compare treatment density differences, data in each weed/soil series
2.3. Carbon : nitrogen ratios Since soil nitrogen mineralization is often inversely correlated with C : N ratio (Stevenson, 1982; Gordon, 1986; Haynes, 1986; Thevathasan, 1998), total N (%) and total carbon C (%) were determined for presamples for major treatments using standard procedures. Data were treated statistically the same as mineralization data. 2.4. Foliar nutrient analyses
2.5. Meteorological parameters Meteorological parameters were monitored continuously throughout the incubation period. Soil moisture and temperatures at 12 cm depth (cell center) were measured at the beginning, mid-point, and conclusion of the incubation period for each burial location from buried ®berglass/resistance soil cells (ELE International, Lake Bluff, ILL). Mean incubation temperatures were calculated using the three measurement dates for the 34-day incubation period. Moisture levels at times of burial (i.e. bags allow for gas exchange, but not moisture exchange) were used. Cell resistance data were converted to soil moisture data (%) using calibration curves developed for each major soil type at the arboretum research site. Statistical analysis and graphics of these data were performed using CSS Statistica software as described above.
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2.6. Determination of leaf area indices of competing vegetation The effective leaf area indices (LAIe) of weed competitors (clay and loam sites only) were measured at each location (i.e. center of each cell) where bags were buried, and adjacent to soil temperature cells, in August 1997 using a Licor LAI-2000 Plant Canopy Analyzer. Measurements were made at ground level, and recomputed to exclude rings 3±5. The latter insured that optical measurements were con®ned to within weed cells. In addition, LAIe was measured at three spruce crown heights (base, mid, top of leader) for nine paired spruce and competitor plant locations in each 7 7 m cell. Mid-crown LAIe's were subsequently correlated with spruce or weed growth. 2.7. Determination of weed and spruce growth In late August and September 1997, height (cm) and baseline diameters (mm) for spruce and weed competitors for all sites and densities were measured. Plants measured were the same as those where LAIe measurements were made in August, and included nine paired spruce/weed competitor locations per density cell. Spruce stem volumes (cm3) were computed using the formula for a cone (Avery, 1975). 2.8. Correlation of soil mineralization data with environmental parameters and growth data Soil nitrogen mineralization rates were correlated with soil temperature and/or moisture by means of linear/multiple regression. Mean temperatures for the incubation period or at the mid-point of the incubation period were used. Moisture levels for the start of the incubation period were used. Nitrate levels were correlated with weed growth (height or basal diameters), LAIe, and with spruce stem volumes. Analysis and graphics were performed using CSS Statistica software. 3. Results Nitrate accumulation rates (mg per 100 gm of dry soil per day) were higher for clay soils for grass,
Fig. 1. Daily (mg per 100 gm of dry soil per day) rates of nitrate (NO3ÿ) production during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Seedlings were admixed with differing weed competitors at differing weed densities. Legend: Sand clear bar; loam diagonal bar; clay dark bar. Mean values followed by the same letter do not differ.
raspberry, aspen, and willow (Fig. 1). Concurrently, C : N ratios were highest (22 : 1) for loam soils (Fig. 2), lowest (15 : 1) for clay and sand soils and did not differ for clay and sand soils. Ratios were higher for loam soils for aspen and alder. Ammonium production was positively correlated with increasing soil temperatures at the mid-point of the incubation period (Fig. 3) for ®reweed (r 0.73, P 0.039, y ÿ13.48 0.53042x) and for raspberry (r 0.62, P 0.104, y ÿ9.236 0.25749x). The effective leaf area indices (LAIe's) of competing vegetation were consistently higher (1.4±3.0 times) for clay soils compared with loams (Fig. 4). Concurrently, both weed and spruce heights were higher on clays than on loams, with the sole exception of spruce planted admixed with birch. However, when ratios of clay heights versus loam heights were compared for both spruce and weeds (Fig. 5), it was evident that ratios for weeds (mean 1.53) were greater than those for spruce (mean 1.20), suggesting that weed growth had bene®ted proportionately more from available nutrients than spruce. Since weed heights were positively and signi®cantly correlated with LAIe (r 0.98 willow, 0.95 ®reweed, 0.87 rasp-
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Fig. 2. Carbon : Nitrogen (C : N) ratios during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Seedlings were admixed with differing weed competitors at differing weed densities. Legend: Sand clear bar; loam diagonal bar; clay dark bar. Mean values followed by the same letter do not differ.
Fig. 3. NH4 production vs. soil temperatures (12 cm depth) during the fourth (1997) post-treatment growing season after black spruce seedlings and weed competitors were planted in 1994. Data are for clay and loam soils for the fireweed (A) and the red raspberry (B) competitive series. Predictive equations are (A) y ÿ13.48 0.53042x and (B) y ÿ9.236 0.25749x.
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Fig. 4. Effective leaf area indices (LAIe in m2 mÿ2) of major weed competitors during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Data are for clay and loam soils. Legend: loam diagonal bar; clay dark bar.
berry, and 0.85 for aspen and birch), this hypothesis was further tested by correlating LAIe, weed heights, and spruce stem volumes with NO3ÿ production rates (Table 1). LAIe for birch (r 0.78), ®reweed (r 0.69), grass (r 0.64), and raspberry (r 0.62) were signi®cantly correlated with NO3ÿ production (Table 1, Fig. 6). More directly, mean weed heights for willow (r 0.96), raspberry (r 0.90), and grass (r 0.76) were also signi®cantly correlated with NO3ÿ produc-
Fig. 5. Ratios (Ratio mean clay height/mean loam height) of weed and spruce heights during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Legend: weed competitors diagonal bar; spruce seedlings dark bar.
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Table 1 Correlations of daily (mg per 100 gm of dry soil per day) soil NO3ÿ accumulation rates with leaf area indices (LAIe) of competing vegetation, weed heights, and black spruce stem volumes during the fourth (1997) post-treatment growing season after weeds and seedlings were planted in 1994 Weed series
LAIe vs. NO3ÿ
Weed height vs. NO3ÿ
Spruce stem volume vs. NO3ÿ
Calamagrostis Epilobium Rubus Populus Betula Alnus Salix
0.64a 0.69a 0.62a 0.71 0.78a 0.13 0.62
0.76a 0.45 0.90a 0.72 0.82 ÿ0.40 0.96a
0.36 ÿ0.47 0.04 0.54 ÿ0.22 ÿ0.66 0.69
a
Significant at 0.05.
tion (Table 1, Fig. 7). For aspen, basal stem diameter was signi®cantly correlated (r 0.91) with NO3ÿ production (Table 1). By contrast, spruce stem volume was negatively and nonsigni®cantly correlated with NO3ÿ production (Table 1, Fig. 8). The latter ®nding was further reinforced by further signi®cant, and negative correlation (r ÿ0.74 birch, ÿ0.61 birch,
Fig. 7. Mean red raspberry (A), Calamagrostis (B), and willow (C) heights vs. soil NO3ÿ accumulation rates during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Red raspberry and Calamagrostis were planted admixed with spruce seedlings at differing densities. Data are for clay and loam soils. Predictive equations are: (A) y ÿ4. 993 1.292x; (B) y 116.43 0.34412x and (C) y 66.261 1.0374x.
and ÿ0.59 ®reweed) of spruce stem volume with increasing LAIe of competitors (Fig. 9). Total Kjeldahl N analyses of foliar spruce and weed samples further con®rmed that most available N had been taken up by weed competitors rather than spruce (Fig. 10). 4. Discussion
Fig. 6. LAIe (m2 mÿ2) of fireweed (A) and red raspberry (B) vs. soil NO3ÿ accumulation levels during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Fireweed and raspberry were planted admixed with spruce seedlings at differing weed densities. Data are for clay and loam soils. Predictive equations are (A) y ÿ4.469 0.1318x and (B) y ÿ2.735 0.8397x.
Observed differences in soil nitrogen mineralization rates and nitrate accumulation were presumably caused by differences in C : N ratios for the major soil types and by density-dependent vegetation differences and ensuing microclimatic (i.e. soil temperature and moisture) differences (Reynolds et al., 1997a, b, 1999). Distinct soil differences in C : N ratios and crucial site differences for soil temperatures may have
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Fig. 8. Mean black spruce stem volumes vs. soil NO3ÿ accumulation rates during the fourth (1997) growing season after seedlings and weed competitors were planted in 1994. Fireweed (A) and alder (B) were planted admixed with spruce seedlings at differing densities. Data are for clay and loam soils. Predictive equations are (A) y 165.56ÿ1.7506x and (B) y 177.76ÿ1.3406x. ÿ
played equally important roles in determining NO3 accumulation rates. C : N ratios tended to be lower for clay and sand soils than for loams, and may have favored increased mineralization for clay and sand sites (Stevenson, 1982; Gordon, 1986; Haynes, 1986; Brady, 1990; Thevathasan, 1998). Conversely, C : N ratios were higher for loams than for clays and sands, a condition which likely resulted in more immobilization than mineralization of soil nitrogen (Brady, 1990). Soil moisture levels increased in a gradient fashion moving from sands, to loams, and ®nally to clays (Reynolds et al., 1998a), this moisture gradient also likely favored increased mineralization rates for the clay sites. Finally, greater weed density or aboveground biomass of competing vegetation rapidly reduced soil temperatures (Reynolds et al., 1998a, b), a condition more favorable to N immobilization than mineralization. As competing vegetation was reduced, temperatures rose, and presumably this favored greater N mineralization. Vegetation reductions led to increased soil moisture and solar radiation (Kuessner et al., 1998) reaching the forest ¯oor, which warmed the soil. Both conditions favored increased soil N mineralization (Powers, 1990). Vegetation reductions were greatest where
Fig. 9. Mean black spruce stem volumes vs. LAIe of competing raspberries (A), fireweed (B), and birch (C) during the fourth (1997) growing season after seedlings and weed competitors were planted in 1994. Raspberries, fireweed, and birch were planted admixed with spruce seedlings at differing densities. Data are for clay and loam soils. Predictive equations are (A) y 187.15ÿ 26.826x; (B) y 101.9611.726x and (C) y 76.278ÿ7.8426x.
no weeds were planted and where unwanted weeds were eliminated. By contrast, solar radiation reaching the forest ¯oor was reduced most, and soil temperatures coolest, where weed densities were greatest. Differing weed species affected shading and soil temperatures differentially (Kuessner et al., 1998 and Reynolds et al., 1998b). In 1997, photosynthetically active radiation (PAR) and soil temperatures declined as the LAIe of competing vegetation increased (Reynolds et al., 1998b). The decline in PAR in mid-summer was most correlated with ®reweed (r ÿ0.97), followed in order by grass (r ÿ0.95), and raspberry (r ÿ0.84). Light competition was most severe on clay soils. Declines in soil temperatures at 12 cm depth were most correlated (Reynolds et al., 1998a, b) with alder (r ÿ0.96), followed in order by raspberry, ®reweed, willow, Canada bluejoint grass, birch, and aspen (r ÿ0.72). Soil temperatures were highest on sand soils and intermediate
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Fig. 10. Foliar total nitrogen (TKN) concentrations (%) for major weed competitors and black spruce seedlings during the fourth (1997) growing season after seedlings and competitors were planted in 1994. All competitive species was planted admixed with spruce seedlings at differing densities. Legend: weed species diagonal bar; spruce seedlings dark bar.
for loams. Black spruce stem volume increased in the order of r 0.90 or greater as PAR or soil temperatures increased (Reynolds et al., 1998a, b). For clay versus sand soils, higher NO3ÿ accumulation rates for clay soils likely resulted from low C : N ratios, favorable soil temperatures, and higher soil moisture. Conversely, lower rates for sand were probably due to lower soil moisture and possibly less favorable (too high) soil temperatures. For clay versus loam soils, higher NO3ÿ rates for clay were probably due to lower C : N ratios, higher soil moisture, and possibly more favorable (lower) soil temperatures. Conversely, lower rates for loam were likely due to higher C : N ratios, lower soil moisture, and possibly less favorable (higher) soil temperature. Finally, for loam versus sand soils, no signi®cant soil related differences in NO3ÿ rates were observed for soils obtained from the aspen, alder, and willow series. This likely resulted from higher C : N ratios for loam
soils; in turn, this likely resulted in more immobilization of N than mineralization (Brady, 1990). By contrast, C : N ratios for loam and sand soils did not differ for the birch series, and higher NO3ÿ rates were observed for loam soils. The latter may be more related to higher moisture levels for loam soils. In addition, no differences in NO3ÿ rates were observed for loam and sand soils obtained from the grass and ®reweed series, and corresponded with no differences in C : N ratios for these soils. Higher NO3ÿ rates for loam soils obtained from the raspberry series may have also been related to higher moisture levels for these soils as compared with sands, since C : N ratios for the two soil types did not differ. No difference in NO3ÿ accumulation rates was observed for sand, clay, or loam soils obtained from the alder series. This may have resulted due to atmospheric N ®xation, and suggests that alder's ability to ®x N may have been greater in low fertility soils than in higher fertility soil. Therefore, the use of alder as a nurse crop for low fertility soils may be desirable. Increased soil nitrates likely bene®ted both weeds and spruce seedlings, but weed competitors clearly bene®ted the most. Increased soil moisture and higher soil temperatures where weed densities were kept low, coupled with improved nitrogen turnover, probably greatly improved weed nutrition at the expense of spruce seedlings admixed with more competitive species (Nambiar and Sands, 1993). Seedling gains were greatest where weed densities were lowest, especially, where competitors were kept controlled. These ®ndings af®rm that weeds need to be controlled to maximize water/nutrient availability to seedlings. Maximum nutrient uptake is dependent upon reducing competition for water by competitors. Weeds must be controlled for seedlings to bene®t nutritionally to the maximum extent possible (Powers and Ferrell, 1996). 5. Conclusions We conclude that: 1. Nitrate accumulation rates are in¯uenced by soil type, soil parameters (moisture and temperature), and C : N ratio. 2. This study suggests that C : N ratio may be used as an indicator to predict N release patterns.
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3. Weed control by itself may not improve spruce growth in all conditions. 4. Weed control strategies need to be site specific. 5. Cultivation of alder as a nurse crop on low fertility soils to improve N nutrition, and spruce growth, should be considered. 6. Soil NO3ÿ accumulation has benefited cultivated weeds, and not improved spruce growth. 7. In most cases, improved spruce growth is dependent upon weed control. 8. Weeds must be controlled for seedlings to benefit nutritionally. Acknowledgements Funding for this research was received from the Ontario Ministry of Natural Resources (OMNR) and the Canadian Forest Service. We thank Nancy Tibbels, Terri Lutes, David Curry, and John Ralston for technical assistance. We thank Drs Terry Taylor and Bill Meades for their continued support for this project. References Avery, T.E., 1975. Natural Resources Measurements, McGrawHill, New York. Bell, F.W., Wagner, R.G., Ter-Mikaelian, M.T., 1998. Relative competitiveness of boreal plant species with jack pine and black spruce. In: Wagner, R.G., Thompson, D.G. (Comps.), Popular Summaries of Third International Conference on Forest Vegetation Management. Ontario Ministry of Natural Resources, Ontario Forest Research Institute, Forest Research Information Paper No. 141, Sault Ste. Marie, Ont., Canada, pp. 51±53. Bosch, J.M., Hewlett, J.D., 1982. A review of catchment experiments to determine the effects of vegetation changes on water yield and evapotranspiration. J. Hydrol. 55, 3±23. Brady, N.C., 1990. The Nature and Properties of Soils, Macmillan, New York. Brand, D.G., Janas, P.S., 1988. Growth and acclimation of planted white pine and white spruce seedlings in response to environmental conditions. Can. J. For. Res. 18, 320±329. Burger, J.A., Pritchett, W.L., 1984. Effects of clearfelling and site preparation on nitrogen mineralization in a southern pine stand. Soil Sci. Soc. Am. J. 48, 1432±1437. Eastman, P.A.K., Camm, E.L., 1995. Regulation of photosynthesis in interior spruce during water stress: changes in gas exchange and chlorophyll fluorescence. Tree Physiol. 15, 229±235. Elliot, K.J., White, E.S., 1987. Competitive effects of various grasses and forbs on ponderosa pine seedlings. For. Sci. 33, 356±366.
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Gordon, A.M., 1986. Seasonal patterns of nitrogen mineralization and nitrification following harvesting in the white spruce forests of interior Alaska, Ph.D. Thesis, University of Alaska, Fairbanks. Gordon, A.M., Tallas, M., Van Cleve, K., 1987. Soil incubation in polyethylene bags: effects of bag thickness and temperature on nitrogen transformations and CO2 permeability. Can. J. Soil Sci. 67, 65±75. Haynes, R.J. (Ed.), 1986. Mineral Nitrogen in the Plant-Soil System. Academic Press, Orlando, FL. Kochenderfer, J.N., Wendel, G.W., 1983. Plant succession and hydrologic recovery on a deforested and herbicided watershed. For. Sci. 29, 545±558. Kuessner, R., Reynolds, P.E., Bell, F.W., 1998. Growth response of black spruce seedlings as affected by competition for radiation. In: Wagner, R.G., Thompson, D.G. (Comps.), Popular Summaries of Third International Conference on Forest Vegetation Management. Ontario Ministry of Natural Resources, Ontario Forest Research Institute, Forest Research Information Paper No. 141, Sault Ste. Marie, Ont., Canada, pp. 145±147. Larson, M.M., Schubert, G.H., 1969. Root competition between ponderosa pine seedlings and grass. U.S.D.A. Forest Service Research Paper RM-54, 12pp. Lieffers, V.J., Mugasha, A.G., MacDonald, S.E., 1993. Ecophysiology of shade needles of Picea glauca saplings in relation to removal of competing hardwoods and degree of prior shading. Tree Physiol. 12, 271±280. Matson, P.A., Vitousek, P.M., 1981. Nitrogen mineralization and nitrification potentials following clearcutting in the Hoosier National Forest, Indiana. For. Sci. 27, 781±791. Morris, L.A., Moss, S.A., Garbett, W.S., 1993. Competitive interference between selected herbaceous and woody plants and Pinus taeda L. during two growing seasons following planting. For. Sci. 39, 166±187. Nambiar, E.K.S., Sands, R., 1993. Competition for water and nutrients in forests. Can. J. For. Res. 23, 1955±1968. Newton, M., Cole, E.C., McCormack Jr., M.L., White, D.E., 1992. Young spruce-fir forests released by herbicides II. Conifer response to residual hadwoods and overstocking. Northern J. Applied For. 9, 130±135. Powers, R.F., 1990. Nitrogen mineralization along an altitudinal gradient: interactions of soil temperature, moisture, and substrate quality. For. Ecol. Manage. 30, 19±29. Powers, R.F., Ferrell, G.T., 1996. Moisture, nutrient, and insect constraints on plantation growth: the `Garden of Eden' experiment. New Zealand J. For. Sci. 26 (1/2), 126±144. Radosevich, S.R., Osteryoung, K., 1987. Principles governing plant-environment interactions. In: Walstad, J.D., Kuch, P.J. (Eds.), Vegetation Management for Conifer Production. Wiley, New York, pp. 105±156. Reynolds, P.E., Simpson, J.A., Lautenschlager, R.A., Bell, F.W., Gordon, A.M., Gresch, D.A., Buckley, D.A., 1997a. Alternative conifer release treatments affect below- and near-ground microclimate. The Forestry Chronicle 73, 75±82. Reynolds, P.E., Bell, F.W., Simpson, J.A., Gordon, A.M., Lautenschlager, R.A., Gresch, D.A., Buckley, D.A., 1997b. Alternative conifer release treatments affect leaf area index of competing
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vegetation and available light for seedling growth. The Forestry Chronicle 73, 83±89. Reynolds, P.E., Bell, F.W., Kuessner, R., Thevathasan, N.V., 1998a. Determining the effects of controlled weed densities and soil types on black spruce microclimate, physiological responses, and growth. In: Pitt, D.G., Bell, F.W. (Comps.), In-Conference Tour Guide of Third International Conference on Forest Vegetation Management. Ontario Ministry of Natural Resources, Ontario Forest Research Institute, Forest Research Information Paper No. 141a, Sault Ste. Marie, Ont., Canada, pp. 31±34. Reynolds, P.E., Kuessner, R., Bell, F.W., 1998b. Effects of controlled weed densities and soil types on seedling microclimate. In: Wagner, R.G., Thompson, D.G. (Comps.), Popular Summaries of Third International Conference on Forest Vegetation Management. Ontario Ministry of Natural Resources, Ontario Forest Research Institute, Forest Research Information Paper No. 141, Sault Ste. Marie, Ont., Canada, pp. 278±280. Reynolds, P.E., Thevathasan, N.V., Simpson, J.A., Gordon, A.M., Lautenschlager, R.A., Bell, F.W., Gresch, D.A., Buckley, D.A., 1999. Alternative conifer release treatments affect microclimate and soil nitrogen mineralization. For. Ecol. Manage. (this issue).
Sands, R., Nambiar, E.K.S., 1984. Water relations of Pinus radiata in competition with weeds. Can. J. For. Res. 14, 233±237. Snedecor, G.W., Cochran, W.G., 1967. Statistical Methods, The Iowa State University Press, Ames, IA. Stanford, G., Epstein, E., 1974. Nitrogen mineralization-water relations in soils. Soil Sci. Soc. Am. Proc. 38, 103±107. Stevenson, F.J. (Ed.), 1982. Nitrogen in Agricultural Soils. American Society of Agronomy, Madison, WI. Stone, E., 1973. In: Report of the President's Advisory Panel on Timber and the Environment. Government Printing Office, shington, DC, pp. 427±467. Thevathasan, N.V., 1998. Complementary interactions and N dynamics in tree-based intercropping systems in southern Ontario, Ph.D. Thesis, University of Guelph, Guelph, Canada. Wagner, R.G., Noland, T.L., Mohammed, G.H., 1996. Timing and duration of herbaceous vegetation control around four northern coniferous species. New Zealand J. For. Sci. 26 (1/2), 39±52. Wood, J.E., von Althen, F.W., 1993. Establishment of white spruce and black spruce in boreal Ontario: effects of chemical site preparation and post-planting weed control. The Forestry Chronicle 69, 554±560. Zutter, B.R., Gjerstad, D.H., Glover, G.H., 1986. Effects of interfering vegetation on biomass, fascicle morphology and leaf area of loblolly pine seedlings. For. Sci. 32, 1016±1031.
Effects of controlled weed densities and soil types on soil nitrate accumulation, spruce growth, and weed growth Naresh V. Thevathasana, Phillip E. Reynoldsb,*, Ralf Kuessnerc, Wayne F. Belld a
Department of Environmental Biology, University of Guelph, Guelph, Ont., Canada N1G 2W1 Natural Resources Canada, Canadian Forest Service, 1219 Queen St. East, Sault Ste. Marie, Ont., Canada P6A 5M7 c University of Dresden, Institute for Silviculture and Forest Protection, Pienner Str. 8, D-01737, Tharandt, Germany d Ontario Ministry of Natural Resources, Ontario Forest Research Institute, 1235 Queen St. East, Sault Ste. Marie, Ont., Canada P6A 5N5 b
Accepted 6 October 1999
Abstract Soil nitrate (NO3ÿ) accumulation rates were assessed among seven weed species grown in small plots during the summer of 1997, at a northern Ontario location. Objectives were (1) to quantify soil nitrate accumulation rates at varying weed densities established on three soil types (clay, loam, and sand) and (2) to assess the effects of soil nitrate levels on weed and black spruce (Picea mariana (Mill.) B.S.P.) seedling growth and weed growth. Controlled densities (0, 0.5, 2, 8 stems per m2) of red raspberry (Rubus idaeus L. var. strigosus (Michx.) Maxim.), ®reweed (Epilobium angustifolium L.), and a grass species (Calamagrostis canadensis (Michx.) Nutt.) were planted along with spruce seedlings (1 m spacing) early in the summer of 1994. Concurrently, four more deciduous weed species, trembling aspen (Populus tremuloides Michx.), white birch (Betula papyrifera Marsh.), green alder (Alnus crispa (Ait.) Pursh), and willow (Salix humilis Marsh.) were also established at four densities (0, 0.5, 2, 4 stems per m2). In mid-summer (July) 1997, soil samples collected from the respective weed densities were placed in polyethylene bags and the bags were incubated in the respective plots for 34 days at a depth of 12 cm. Nitrate accumulation rates (mg per 100 gm of dry soil per day), weed heights, and the effective leaf area index (LAIe, in m2 mÿ2, measured with a Licor LAI-2000 Plant Canopy Analyzer) of competing vegetation were consistently higher (1.4±3.0 times) for clay soil. Nitrate levels were signi®cantly (p < 0.05) higher in clay soils obtained from plots planted with raspberry, grass, aspen, and willow. Carbon : nitrogen (C : N) ratios were higher for loam soils than those for clay or sand soils, and these higher ratios may have resulted in more immobilization than mineralization of soil nitrogen. Nitrate levels for clays and loams correlated signi®cantly with the LAIe of competing vegetation (r 0.62±0.78) and with weed growth (r 0.75±0.96), but non-signi®cantly, and often negatively, with spruce stem volume. To date, soil nitrate accumulation appears to have primarily bene®ted cultivated weeds and not to have improved spruce growth. These ®ndings af®rm that weeds must be controlled for seedlings to bene®t nutritionally. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Soil nitrogen mineralization; Soil nitrate (NO3ÿ) accumulation; Vegetation management; Competition; Boreal forest species; Weed thresholds; Growth; Black spruce (Picea mariana); Fireweed (Epilobium angustifolium); Red raspberry (Rubus idaeus); Canada bluejoint grass (Calamagrostis canadensis); Trembling aspen (Populus tremuloides); Birch (Betula papyrifera); Alder (Alnus crispa); Willow (Salix humilis); Microclimate
* Corresponding author. Tel.: 1-705-949-9461. E-mail address: [email protected] (P.E. Reynolds)
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 3 0 4 - 7
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1. Introduction In the Boreal forest, a number of key herbaceous and woody plant species compete with newly planted conifer species for light, water, and nutrients (Larson and Schubert, 1969; Sands and Nambiar, 1984; Zutter et al., 1986; Elliot and White, 1987; Morris et al., 1993; Nambiar and Sands, 1993). The degree of competition often varies with site quality, including soil type, moisture and nutrient availability, the number of years elapsed between harvesting and planting or between planting and subsequent release from competition, and the extent of site disturbance. Normally, competition is more severe (Brady, 1990) on ®ner-textured soils (e.g. clays, silts) characterized by higher water-holding and cation exchange capacities than on coarser-textured soils (i.e. sands). Due to these differences, sands bind fewer nutrients and are more prone to leaching and nutrient loss than ®ner-textured soils. Since soils exhibit considerable variability in moisture retention and nutrient content, potential plant competitors also vary in their distribution across the landscape. Species with greater drought tolerance and lower nutrient demands are often found growing on sands, whereas species with low drought tolerance or high nutrient requirements may be restricted to clays or silts. Species possessing broader ranges of tolerance for both water and nutrients are capable of growing on a range of soil types, although their growth may be clearly better where water and nutrients are less restricted. Seedling microclimate is affected by the amount of competition present on a forest site. Changes in light, temperature, relative humidity and moisture produced by varying amounts of competing vegetation affect microsite suitability for crop production (Reynolds et al., 1997a, b). Reduced competition increases solar radiation reaching the forest ¯oor, results in soil warming (Brand and Janas, 1988 and Wood and von Althen, 1993), and raises soil moisture levels (Stone, 1973; Bosch and Hewlett, 1982; Kochenderfer and Wendel, 1983). Increased soil moisture (Stanford and Epstein, 1974; Matson and Vitousek, 1981; Burger and Pritchett, 1984) coupled with soil warming (Powers, 1990) produces conditions favorable for nutrient turnover (N and C mineralization), and improved soil moisture enhances nutrient uptake (Nambiar and Sands, 1993). Soil N mineralization
is often positively correlated with soil related parameters, such as soil moisture and temperature, and inversely correlated with C : N ratio (Stevenson, 1982; Gordon, 1986; Haynes, 1986; Thevathasan, 1998). These parameters are also in¯uenced by soil types, vegetation present, and climate (Haynes, 1986 and Reynolds et al., 1997a). Improved moisture and nutrient status combined with increased light, generally results in increased photosynthesis (Lieffers et al., 1993 and Eastman and Camm, 1995) and ultimately, improved crop survival and growth (Radosevich and Osteryoung, 1987 and Newton et al., 1992). Since competing vegetation affects the availability of site resources (light, water, nutrients) for use by planted seedlings, the question arises as to when and how often competitors need to be reduced in number. This is both a temporal question (Wagner et al., 1996) and also very much related to which competitor and which soil type are concerned. Not all competitors are expected to compete equally for light, water, or nutrients, and the degree of competition is expected to vary with soil type and timing (i.e. number of years of competition establishment). Broad objectives of the current research are (1) to determine the severity of competition by various species in affecting black spruce survival and growth and (2) to determine how this varies with soil type, weed density, and the number of years of weed establishment. Speci®c objectives for this study were (1) to quantify soil nitrate (NO3ÿ) accumulation rates at varying weed densities established on three soil types (clay, loam, sand) and (2) to assess the effects of soil nitrate levels on weed and black spruce seedling growth. 2. Methods 2.1. Research site The present research site was established in 1994 by the Ontario Forest Research Institute (OFRI) to examine the effects of various herbaceous and woody competitors, planted at a range of densities on crop performance (Bell et al., 1998). The research site is at the OFRI Arboretum which is located on the western outskirts of Sault Ste. Marie, Ontario. Ten potential competitors of black spruce (Picea mariana (Mill.) B.S.P.) including red raspberry (Rubus idaeus L. var.
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strigosus (Michx.) Maxim.), ®reweed (Epilobium angustifolium L.), Canada bluejoint grass (Calamagrostis canadensis (Michx.) Nutt.), bracken fern (Pteridium aquilinnum (L.) Kuhn), large-leafed aster (Aster macrophyllus L.), trembling aspen (Populus tremuloides Michx.), white birch (Betula papyrifera Marsh.), green alder (Alnus crispa (Ait.) Pursh), willow (Salix humilis Marsh.), and jack pine (Pinus banksiana Lamb.) were planted admixed with black spruce in 7 7 m cells. Six of these cells aligned sideby-side in a row 42 m in length was designated as a weed series. Each of the 10 weed series was replicated on clay, loam, and sand soils for a total of 30 series. Within each series, herbs and woody competitors were established in additive densities. These were 0.0, 0.5, 1.0, 2.0, 4.0, and 8.0 plants per m2 for herbaceous competitors, and 0.0, 0.25, 0.5, 1.0, 2.0, and 4.0 plants per m2 for woody competitors.
were pooled prior to statistical analysis. Data were examined using basic statistics where means were calculated and standard deviations (5% level of signi®cance determined (Snedecor and Cochran, 1967). Statistical analysis and graphics were performed using CSS Statistica (StatSoft, Tulsa, OK).
2.2. Soil nitrogen mineralization studies
In early August 1997, foliar samples were bulk sampled for paired spruce and plant competitors at nine locations in each 7 7 m cell. All soil types, densities, and competitors, except pine, were sampled. Those plants sampled were the same as those where leaf area (LAIe) and plant growth measurements were made. Spruce and weed foliar samples were subsequently quanti®ed for macro- and micro-nutrient concentrations. Total Kjeldahl results are reported here.
In mid-summer (July) 1997, soil samples collected at a depth of 12 cm (i.e. mid-point of spruce rooting zone) from the respective weed densities were placed in 0.025 mm polyethylene bags and the bags were incubated in the respective plots for 34 days at a depth of 12 cm (Gordon et al., 1987). Samples were split into two bags ± a pre-sample and a second sample for burial. Pre-samples, which established N, C, NH4, and NO3ÿ levels prior to burial, were placed on dry ice and kept frozen prior to chemical analysis. Post- or exhumed samples established ammonium and nitrate levels after incubation, and were processed the same as pre-samples. In the laboratory, 20 g pre- and post-samples were extracted with 60 ml of 2N KCl on a shaker at 3500 rpm for 1 h. After agitation, the extract was allowed to settle prior to ®ltering. Frozen soil solutions were kept frozen prior to analysis on a Technicon Autoanalyzer II system. Daily rates of NH4 or NO3ÿ (mg per 100 gm of dry soil per day) were calculated. Mineralization rates were determined by subtracting the un-incubated sample values from the incubated sample values, and dividing by the number of days (i.e. 34) incubated. Since our prime objective in this report was to examine soil differences, and not to compare treatment density differences, data in each weed/soil series
2.3. Carbon : nitrogen ratios Since soil nitrogen mineralization is often inversely correlated with C : N ratio (Stevenson, 1982; Gordon, 1986; Haynes, 1986; Thevathasan, 1998), total N (%) and total carbon C (%) were determined for presamples for major treatments using standard procedures. Data were treated statistically the same as mineralization data. 2.4. Foliar nutrient analyses
2.5. Meteorological parameters Meteorological parameters were monitored continuously throughout the incubation period. Soil moisture and temperatures at 12 cm depth (cell center) were measured at the beginning, mid-point, and conclusion of the incubation period for each burial location from buried ®berglass/resistance soil cells (ELE International, Lake Bluff, ILL). Mean incubation temperatures were calculated using the three measurement dates for the 34-day incubation period. Moisture levels at times of burial (i.e. bags allow for gas exchange, but not moisture exchange) were used. Cell resistance data were converted to soil moisture data (%) using calibration curves developed for each major soil type at the arboretum research site. Statistical analysis and graphics of these data were performed using CSS Statistica software as described above.
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2.6. Determination of leaf area indices of competing vegetation The effective leaf area indices (LAIe) of weed competitors (clay and loam sites only) were measured at each location (i.e. center of each cell) where bags were buried, and adjacent to soil temperature cells, in August 1997 using a Licor LAI-2000 Plant Canopy Analyzer. Measurements were made at ground level, and recomputed to exclude rings 3±5. The latter insured that optical measurements were con®ned to within weed cells. In addition, LAIe was measured at three spruce crown heights (base, mid, top of leader) for nine paired spruce and competitor plant locations in each 7 7 m cell. Mid-crown LAIe's were subsequently correlated with spruce or weed growth. 2.7. Determination of weed and spruce growth In late August and September 1997, height (cm) and baseline diameters (mm) for spruce and weed competitors for all sites and densities were measured. Plants measured were the same as those where LAIe measurements were made in August, and included nine paired spruce/weed competitor locations per density cell. Spruce stem volumes (cm3) were computed using the formula for a cone (Avery, 1975). 2.8. Correlation of soil mineralization data with environmental parameters and growth data Soil nitrogen mineralization rates were correlated with soil temperature and/or moisture by means of linear/multiple regression. Mean temperatures for the incubation period or at the mid-point of the incubation period were used. Moisture levels for the start of the incubation period were used. Nitrate levels were correlated with weed growth (height or basal diameters), LAIe, and with spruce stem volumes. Analysis and graphics were performed using CSS Statistica software. 3. Results Nitrate accumulation rates (mg per 100 gm of dry soil per day) were higher for clay soils for grass,
Fig. 1. Daily (mg per 100 gm of dry soil per day) rates of nitrate (NO3ÿ) production during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Seedlings were admixed with differing weed competitors at differing weed densities. Legend: Sand clear bar; loam diagonal bar; clay dark bar. Mean values followed by the same letter do not differ.
raspberry, aspen, and willow (Fig. 1). Concurrently, C : N ratios were highest (22 : 1) for loam soils (Fig. 2), lowest (15 : 1) for clay and sand soils and did not differ for clay and sand soils. Ratios were higher for loam soils for aspen and alder. Ammonium production was positively correlated with increasing soil temperatures at the mid-point of the incubation period (Fig. 3) for ®reweed (r 0.73, P 0.039, y ÿ13.48 0.53042x) and for raspberry (r 0.62, P 0.104, y ÿ9.236 0.25749x). The effective leaf area indices (LAIe's) of competing vegetation were consistently higher (1.4±3.0 times) for clay soils compared with loams (Fig. 4). Concurrently, both weed and spruce heights were higher on clays than on loams, with the sole exception of spruce planted admixed with birch. However, when ratios of clay heights versus loam heights were compared for both spruce and weeds (Fig. 5), it was evident that ratios for weeds (mean 1.53) were greater than those for spruce (mean 1.20), suggesting that weed growth had bene®ted proportionately more from available nutrients than spruce. Since weed heights were positively and signi®cantly correlated with LAIe (r 0.98 willow, 0.95 ®reweed, 0.87 rasp-
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Fig. 2. Carbon : Nitrogen (C : N) ratios during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Seedlings were admixed with differing weed competitors at differing weed densities. Legend: Sand clear bar; loam diagonal bar; clay dark bar. Mean values followed by the same letter do not differ.
Fig. 3. NH4 production vs. soil temperatures (12 cm depth) during the fourth (1997) post-treatment growing season after black spruce seedlings and weed competitors were planted in 1994. Data are for clay and loam soils for the fireweed (A) and the red raspberry (B) competitive series. Predictive equations are (A) y ÿ13.48 0.53042x and (B) y ÿ9.236 0.25749x.
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Fig. 4. Effective leaf area indices (LAIe in m2 mÿ2) of major weed competitors during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Data are for clay and loam soils. Legend: loam diagonal bar; clay dark bar.
berry, and 0.85 for aspen and birch), this hypothesis was further tested by correlating LAIe, weed heights, and spruce stem volumes with NO3ÿ production rates (Table 1). LAIe for birch (r 0.78), ®reweed (r 0.69), grass (r 0.64), and raspberry (r 0.62) were signi®cantly correlated with NO3ÿ production (Table 1, Fig. 6). More directly, mean weed heights for willow (r 0.96), raspberry (r 0.90), and grass (r 0.76) were also signi®cantly correlated with NO3ÿ produc-
Fig. 5. Ratios (Ratio mean clay height/mean loam height) of weed and spruce heights during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Legend: weed competitors diagonal bar; spruce seedlings dark bar.
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Table 1 Correlations of daily (mg per 100 gm of dry soil per day) soil NO3ÿ accumulation rates with leaf area indices (LAIe) of competing vegetation, weed heights, and black spruce stem volumes during the fourth (1997) post-treatment growing season after weeds and seedlings were planted in 1994 Weed series
LAIe vs. NO3ÿ
Weed height vs. NO3ÿ
Spruce stem volume vs. NO3ÿ
Calamagrostis Epilobium Rubus Populus Betula Alnus Salix
0.64a 0.69a 0.62a 0.71 0.78a 0.13 0.62
0.76a 0.45 0.90a 0.72 0.82 ÿ0.40 0.96a
0.36 ÿ0.47 0.04 0.54 ÿ0.22 ÿ0.66 0.69
a
Significant at 0.05.
tion (Table 1, Fig. 7). For aspen, basal stem diameter was signi®cantly correlated (r 0.91) with NO3ÿ production (Table 1). By contrast, spruce stem volume was negatively and nonsigni®cantly correlated with NO3ÿ production (Table 1, Fig. 8). The latter ®nding was further reinforced by further signi®cant, and negative correlation (r ÿ0.74 birch, ÿ0.61 birch,
Fig. 7. Mean red raspberry (A), Calamagrostis (B), and willow (C) heights vs. soil NO3ÿ accumulation rates during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Red raspberry and Calamagrostis were planted admixed with spruce seedlings at differing densities. Data are for clay and loam soils. Predictive equations are: (A) y ÿ4. 993 1.292x; (B) y 116.43 0.34412x and (C) y 66.261 1.0374x.
and ÿ0.59 ®reweed) of spruce stem volume with increasing LAIe of competitors (Fig. 9). Total Kjeldahl N analyses of foliar spruce and weed samples further con®rmed that most available N had been taken up by weed competitors rather than spruce (Fig. 10). 4. Discussion
Fig. 6. LAIe (m2 mÿ2) of fireweed (A) and red raspberry (B) vs. soil NO3ÿ accumulation levels during the fourth (1997) growing season after black spruce seedlings and weed competitors were planted in 1994. Fireweed and raspberry were planted admixed with spruce seedlings at differing weed densities. Data are for clay and loam soils. Predictive equations are (A) y ÿ4.469 0.1318x and (B) y ÿ2.735 0.8397x.
Observed differences in soil nitrogen mineralization rates and nitrate accumulation were presumably caused by differences in C : N ratios for the major soil types and by density-dependent vegetation differences and ensuing microclimatic (i.e. soil temperature and moisture) differences (Reynolds et al., 1997a, b, 1999). Distinct soil differences in C : N ratios and crucial site differences for soil temperatures may have
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Fig. 8. Mean black spruce stem volumes vs. soil NO3ÿ accumulation rates during the fourth (1997) growing season after seedlings and weed competitors were planted in 1994. Fireweed (A) and alder (B) were planted admixed with spruce seedlings at differing densities. Data are for clay and loam soils. Predictive equations are (A) y 165.56ÿ1.7506x and (B) y 177.76ÿ1.3406x. ÿ
played equally important roles in determining NO3 accumulation rates. C : N ratios tended to be lower for clay and sand soils than for loams, and may have favored increased mineralization for clay and sand sites (Stevenson, 1982; Gordon, 1986; Haynes, 1986; Brady, 1990; Thevathasan, 1998). Conversely, C : N ratios were higher for loams than for clays and sands, a condition which likely resulted in more immobilization than mineralization of soil nitrogen (Brady, 1990). Soil moisture levels increased in a gradient fashion moving from sands, to loams, and ®nally to clays (Reynolds et al., 1998a), this moisture gradient also likely favored increased mineralization rates for the clay sites. Finally, greater weed density or aboveground biomass of competing vegetation rapidly reduced soil temperatures (Reynolds et al., 1998a, b), a condition more favorable to N immobilization than mineralization. As competing vegetation was reduced, temperatures rose, and presumably this favored greater N mineralization. Vegetation reductions led to increased soil moisture and solar radiation (Kuessner et al., 1998) reaching the forest ¯oor, which warmed the soil. Both conditions favored increased soil N mineralization (Powers, 1990). Vegetation reductions were greatest where
Fig. 9. Mean black spruce stem volumes vs. LAIe of competing raspberries (A), fireweed (B), and birch (C) during the fourth (1997) growing season after seedlings and weed competitors were planted in 1994. Raspberries, fireweed, and birch were planted admixed with spruce seedlings at differing densities. Data are for clay and loam soils. Predictive equations are (A) y 187.15ÿ 26.826x; (B) y 101.9611.726x and (C) y 76.278ÿ7.8426x.
no weeds were planted and where unwanted weeds were eliminated. By contrast, solar radiation reaching the forest ¯oor was reduced most, and soil temperatures coolest, where weed densities were greatest. Differing weed species affected shading and soil temperatures differentially (Kuessner et al., 1998 and Reynolds et al., 1998b). In 1997, photosynthetically active radiation (PAR) and soil temperatures declined as the LAIe of competing vegetation increased (Reynolds et al., 1998b). The decline in PAR in mid-summer was most correlated with ®reweed (r ÿ0.97), followed in order by grass (r ÿ0.95), and raspberry (r ÿ0.84). Light competition was most severe on clay soils. Declines in soil temperatures at 12 cm depth were most correlated (Reynolds et al., 1998a, b) with alder (r ÿ0.96), followed in order by raspberry, ®reweed, willow, Canada bluejoint grass, birch, and aspen (r ÿ0.72). Soil temperatures were highest on sand soils and intermediate
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Fig. 10. Foliar total nitrogen (TKN) concentrations (%) for major weed competitors and black spruce seedlings during the fourth (1997) growing season after seedlings and competitors were planted in 1994. All competitive species was planted admixed with spruce seedlings at differing densities. Legend: weed species diagonal bar; spruce seedlings dark bar.
for loams. Black spruce stem volume increased in the order of r 0.90 or greater as PAR or soil temperatures increased (Reynolds et al., 1998a, b). For clay versus sand soils, higher NO3ÿ accumulation rates for clay soils likely resulted from low C : N ratios, favorable soil temperatures, and higher soil moisture. Conversely, lower rates for sand were probably due to lower soil moisture and possibly less favorable (too high) soil temperatures. For clay versus loam soils, higher NO3ÿ rates for clay were probably due to lower C : N ratios, higher soil moisture, and possibly more favorable (lower) soil temperatures. Conversely, lower rates for loam were likely due to higher C : N ratios, lower soil moisture, and possibly less favorable (higher) soil temperature. Finally, for loam versus sand soils, no signi®cant soil related differences in NO3ÿ rates were observed for soils obtained from the aspen, alder, and willow series. This likely resulted from higher C : N ratios for loam
soils; in turn, this likely resulted in more immobilization of N than mineralization (Brady, 1990). By contrast, C : N ratios for loam and sand soils did not differ for the birch series, and higher NO3ÿ rates were observed for loam soils. The latter may be more related to higher moisture levels for loam soils. In addition, no differences in NO3ÿ rates were observed for loam and sand soils obtained from the grass and ®reweed series, and corresponded with no differences in C : N ratios for these soils. Higher NO3ÿ rates for loam soils obtained from the raspberry series may have also been related to higher moisture levels for these soils as compared with sands, since C : N ratios for the two soil types did not differ. No difference in NO3ÿ accumulation rates was observed for sand, clay, or loam soils obtained from the alder series. This may have resulted due to atmospheric N ®xation, and suggests that alder's ability to ®x N may have been greater in low fertility soils than in higher fertility soil. Therefore, the use of alder as a nurse crop for low fertility soils may be desirable. Increased soil nitrates likely bene®ted both weeds and spruce seedlings, but weed competitors clearly bene®ted the most. Increased soil moisture and higher soil temperatures where weed densities were kept low, coupled with improved nitrogen turnover, probably greatly improved weed nutrition at the expense of spruce seedlings admixed with more competitive species (Nambiar and Sands, 1993). Seedling gains were greatest where weed densities were lowest, especially, where competitors were kept controlled. These ®ndings af®rm that weeds need to be controlled to maximize water/nutrient availability to seedlings. Maximum nutrient uptake is dependent upon reducing competition for water by competitors. Weeds must be controlled for seedlings to bene®t nutritionally to the maximum extent possible (Powers and Ferrell, 1996). 5. Conclusions We conclude that: 1. Nitrate accumulation rates are in¯uenced by soil type, soil parameters (moisture and temperature), and C : N ratio. 2. This study suggests that C : N ratio may be used as an indicator to predict N release patterns.
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