Long-term effects of nitrogen fertilization on nitrogen availability in coastal Douglas-fir forest...

Effects of N fertilization… page 1

Long-term Effects of Nitrogen Fertilization on the Productivity of Subsequent 1

Stands of Douglas-fir in the Pacific Northwest 2

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P.W. Footen1*, R.B. Harrison1 and B.D. Strahm2 4

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1University of Washington, College of Forest Resources, Box 352100, Seattle, WA 98195-2100 6

2Cornell University, Department of Ecology and Evolutionary Biology, Ithaca, NY 14853 7

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*Corresponding author: 9

TEL: 206-543-4978 10

FAX: 206-685-3091 11

Email: [email protected] 12

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Long-term Effects of Nitrogen Fertilization on the Productivity of Subsequent 14

Stands of Douglas-fir in the Pacific Northwest 15

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Abstract 17

The carryover effects of N-fertilization on five coastal Pacific Northwest Douglas-fir 18

(Pseudotsuga menziesii [Mirb.] Franco) plantations was studied. "Carryover" is defined as the 19

long-term impact of N fertilizer added to a previous stand on the growth of a subsequent stand. 20

Average height and diameter at 1.3 m above-ground (DBH) of 7-9 year old Douglas-fir trees and 21

biomass and N-content of understory vegetation were assessed on paired control (untreated) and 22

urea-N fertilized plots that had received cumulative additions of 810-1120 kg N ha-1 to a 23

previous stand. Overall productivity was significantly greater in the fertilized stands compared to 24

the controls. In 2006, the last growth measurement year, mean seedling height was 15% greater 25

(p = 0.06) and mean DBH was 29% greater (p = 0.04) on previously fertilized plots compared to 26

control plots. Understory vegetation biomass of fertilized plots was 73% greater (p = 0.005), and 27

N-content was 97% greater (p = 0.004) compared to control plots. These results show that past N 28

fertilization markedly increased seedling growth in these plantations as well as biomass and N-29

content of understory vegetation in a subsequent rotation. These findings suggest that N 30

fertilization could potentially increase site productivity of young Douglas-fir stands found on low 31

quality sites in the Pacific Northwest 15-22 years after application by a carryover effect. These 32

plantations have not yet reached the age where marketable materials can be harvested from them, 33

and the growth of trees should be monitored over a longer time period before potential impacts 34

on older stands, if any, can be determined. 35

Key Words: Carryover, urea, understory, biomass, nitrogen retention, productivity 36

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1. Introduction 37

Growth of coniferous forests in the Pacific Northwest is commonly limited by the supply 38

of plant-available nitrogen (N) (Gessel et al., 1973; Chappell et al., 1991). As demand for wood 39

products increases, commercial timberlands in the region continue to shrink in response to land-40

use restrictions and conversion into other uses, putting further pressures on the sustainable 41

supply of forest products from the Pacific Northwest (Alig et al., 2003). In many areas 42

reductions in the supply of wood has resulted in the closing of lumber and paper mills, and long 43

distances to suitable wood supplies often make new mills unprofitable. Increasing current and 44

future productivity of timberlands by means of N fertilization could help to increase the 45

sustainability of forest productivity in the Pacific Northwest. 46

Several studies have shown that N fertilization of second and third-rotation Douglas-fir 47

stands can increase tree growth, and such fertilization has become common for increasing forest 48

productivity (Gessel and Walker, 1956; Edmonds and Hsiang, 1987; Stegemoeller and Chappell, 49

1990; Chappell et al. 1991). Prior studies have shown that observable increases in N availability 50

from N fertilizer generally last no longer than 5-10 years (Binkley and Reid, 1985; Binkley, 51

1986; Miller, 1988; Strader and Binkley, 1989; Prescott et al., 1995; Priha and Smolander, 1995; 52

Smolander et al., 1998; Nohrstedt et al., 2000). Nitrogen fertilization also initially increases the 53

amount of foliar N (Heilman and Gessel, 1963; Turner 1977) and total biomass N (Pang et al., 54

1987) in Douglas-fir trees and understory vegetation (Abrams and Dickmann, 1983; Matsushima 55

and Chang, 2007; VanderSchaaf et al., 2004). 56

Post-harvest retention of organic material with increased N content, such as needles and 57

branches, may be an important pool of nutrients available to subsequent stands (Mann et al., 58

1988). However, few studies have specifically examined the potential carryover effects of 59


repeated N fertilization and organic matter retention on subsequent stand productivity. Most 60

studies of long-term impacts of N fertilization have only followed the effects of fertilization 61

within a rotation (i.e. effects on the stand originally fertilized). It is well known; however, that 62

the nutritional requirements and ability to acquire nutrients change greatly as stands develop over 63

time (Strahm et al., 2005). 64

The objective of this study was to quantify the carryover effects of previous N 65

fertilization on new stands of Douglas-fir. Understanding the carryover effects of N-fertilization, 66

organic mater retention and the combined secondary effects of both practices could provide 67

forest managers with better treatment strategies for maintaining and increasing long-term 68

productivity and sustainable yields. 69

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2. Materials and Methods 71

2.1 Study sites 72

All five of the study sites began as fertilization trials of the former Regional Forest 73

Nutrition Research Project (RFNRP), now integrated into the Stand Management Cooperative 74

(SMC). In the late 1960's and the early 1970's, N fertilization trials were established by the 75

RFNRP to study the effects of repeated N fertilization on stand growth with the goal of 76

determining whether forest fertilization could increase wood supply in the region economically. 77

The SMC continues to monitor multiple replicated fertilization sites that are now reaching 78

harvest age after having demonstrated the impact of N fertilization on forest processes and 79

productivity. Instead of protecting these stands from harvest well beyond their commercial 80

rotation age, this study was initiated to examine the potential long-term effects of fertilization on 81

subsequent stand rotations. 82


The study sites are located in the Puget Sound Area of Western Washington, and belong 83

to the Tsuga heterophylla [Raf.] Sarg. zone described by Franklin and Dyrness (1988). This 84

zone is characterized by a wet, mild maritime climate, with moderate moisture stress during 85

summer. Mean annual air temperatures average 9-10 °C, depending on the topographic location. 86

The study sites differ considerably in annual precipitation, elevation, slope and aspect (Table 1), 87

representing the range of intensively managed forests in the region. However, all five sites are 88

similar in that they have low site indexes, and four of the five sites have soils derived from 89

glacial outwash parent materials (Table 1). 90

At four sites (Coyle, Hank's Lake, Simpson Log Yard and Camp Grisdale), the parent 91

material is exclusively or predominantly glacial outwash. Soils are Dystric Xerochrepts of the 92

Everett series with sandy, skeletal texture. At Pack Forest, where the parent material is Andesite 93

colluvium, the soil is a fine-loamy Ultic Haploxeralf of the Wilkeson series. At Camp Grisdale, 94

where parent material is old alluvium and glacial drift, fine-loamy Umbric Dystrochrepts of the 95

Hoquiam series have been formed. All study sites were fully stocked plantations of pure 96

Douglas-fir seedlings . 97

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2.2 Experimental treatments 99

The RFNRP/SMC installed multiple plots 0.04 ha in size with 5 m treated buffers at each 100

study site. All sample plots were well-monumented at the initiation of the study, and all trees 101

permanently tagged during the study period. Each installation included at least one (untreated) 102

control plot and at least one adjacent plot fertilized repeatedly with urea. Treatment plots were 103

fertilized with an initial application of 448 kg N ha-1 followed by repeated applications of 224 kg 104

N ha-1 at consecutive intervals of eight, four and four years. In total, 1120 kg N ha-1 were applied 105


to Camp Grisdale, Coyle, and Simpson Log yard sites, while Pack Forest received 896 kg N ha-1 106

total (Table 1). The mature timber on all plots was harvested between 1997 and 1999 and within 107

the same year replanted with Douglas-fir seedlings to a density of 1235 stems per hectare. 108

At the time of harvest the study plots were re-monumented, and all corners re-marked. 109

Tops to 5 cm and foliage were removed from harvested timber, returned to the plots of origin and 110

scattered evenly within the boundaries of the study areas by hand to simulate the retention of 111

branches and needles that commonly fall off during harvest. In 2006, at the time of this study, 112

crop trees ranged in age from 7-9 years. The fact that each installation did not have replication 113

for each treatment at a given site means that this study is a paired-plot design with one 114

replication at each site, and n = 5. 115

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2.3 Vegetation sampling and analysis 117

A stratified random sample point location was used to identify understory vegetation 118

sampling points. Two random numbers were selected from a random number table. The first 119

number chosen was used as the distance along the x-axis (east-west) and the second was used as 120

the distance along the y-axis (north-south) from the southwest plot corner. Five 0.25 m2 random 121

sample point locations (subplots) were identified for each plot. 122

Vegetation was clipped to the ground using hand shears or scissors. Vegetation for 123

determining biomass was harvested from the three dimensional volume of the quadrat (height x 124

width x length). Parts of the plants that were rooted within the quadrat but did not occupy space 125

within the cubic volume were not harvested, while plants that were not rooted within the quadrat 126

but overlapped into the volume were harvested to the point of overlap with the sample boundary. 127


Understory plant material from the biomass sampling was dried to a constant weight at 70 128

°C and weighed to determine biomass on a sub-plot level. A separate study showed that the 129

constant weight at 105 °C was less than 0.1% different than at 70 °C. The mean of five sub-plot 130

vegetation samples were then extrapolated up to represent understory biomass on the kg ha-1 131

level. Afterward, all samples were finely ground to <1 mm for subsequent N analysis. Total N 132

concentration was determined by dry combustion (Perkin-Elmer CHN Analyzer Model 2400, 133

Norwalk, CT). Nitrogen content for each sample was determined by multiplying the average N 134

concentration by the biomass. Plot level N contents were calculated by summing all the subplots 135

in each plot. Seedling or tree heights and diameter at breast height (DBH) were recorded in the 136

study plots after each growing season (every Fall) from 1997-2006 (except in 2004). 137

Statistical analyses were performed on data at the installation-level using the Student's 138

paired-samples t-test (n = 5). Since each installation contained only one control plot and one 139

treatment plot, only a regional analysis was available and analyses for individual sites were 140

limited to descriptive statistics. 141

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3. Results and Discussion 143

Initially, all means of plots had the same average heights and basal diameters, as all plots 144

at a given location were planted with the same planting stock. Mean-tree height and DBH on the 145

N-fertilized plots were significantly greater than the unfertilized controls (Figure 1) beginning in 146

2001 and 2005, respectively. The differences in height were statistically significant (p < 0.1) 147

every year measured from 2001 to 2006 with a mean tree height 15% greater (p = 0.06) in the 148

fertilized plots in 2006. Mean DBH was significantly different in 2005 and 2006. In the last 149

growth measurement year (2006), mean DBH was 29% greater (p = 0.04) on the previously 150


fertilized plots than on the controls. The differences in growth have increased over the time of 151

this study causing the carryover plots to be at least one full growing season ahead of the controls. 152

This could mean earlier first commercial entry and harvest on the carryover plots. Seedling 153

survivorship was 98% or better on all plots and no significant differences were found between 154

previously fertilized and controls. 155

There was a significant carryover effect on understory biomass and N-content. 156

Understory biomass on the previously fertilized sites was 73% greater (p = 0.005) in the 157

fertilized plots and N-content in the understory was 97% greater (p = 0.004) than the control 158

(Figure 2). 159

Previous studies indicate that the effects of N fertilization on stand productivity are 160

temporary, generally lasting only 5-10 years (Binkley and Reid, 1985; Strader and Binkley, 161

1989; Prescott et al., 1995; Priha and Smolander, 1995; Smolander et al., 1998; Nohrstedt et al., 162

2000). However, all of these studies researched changes of the impact of fertilization within one 163

rotation. This study shows that N fertilization, coupled with organic matter retention from bole 164

only tree removal harvest practices, increases site productivity of subsequent, plantations 15-22 165

years after the last application. This carryover phenomenon, to our knowledge, has not been 166

studied and the mechanisms driving these prolonged effects of N fertilization are not well 167

known. 168

Fertilization has been found to increase needle production as well as needle N content 169

(Heilman and Gessel, 1963; Pang et al., 1987). During harvesting, branches were removed from 170

harvested trees and scattered evenly throughout the plots, resulting in greater and/or richer forest 171

floor biomass almost immediately after harvest. Depending on harvest method (i.e. central or 172

dispersed delimbing, "shovel" yarding, etc.) such redistribution of biomass may or may not be 173


actually done in any particular commercial forest harvest. Tree bolewood and bark, which are 174

typically low in N and may even be lower following fertilization (Pang et al., 1987), were almost 175

entirely removed during harvest operations. 176

The logging residues (branches and foliage) retained on-site contain substantial quantities 177

of readily decomposable organic matter that is relatively rich in nutrients. Since competition by 178

trees for nutrients, water and sunlight have been eliminated by harvesting, and root respiration is 179

reduced or eliminated, conditions are better for microbial populations and nitrifying bacteria in 180

particular (Van Miegroet et al., 1990). Post-harvest conditions that favor increased microbial 181

activity are likely to mineralize a large portion of the forest floor humus and increase levels of 182

plant available N (NH4+ and NO3

-). Previous research at the sites investigated in this study found 183

that fertilization decreased C/N ratios and increased the N mineralization potential of the organic 184

horizon (Prietzel et al., 2004). Fertilization was also found to increase N concentrations and the 185

total N pool of the organic horizon (Canary et al., 2000; Adams et al., 2005). Sites with low site 186

productivity, like the ones in this study, often exhibit very tight N cycling and have an increased 187

ability to retain N (Van Miegroet et al., 1990; Mead et al., 2008). One limitation of this study is 188

the similarity in soil types throughout the sites. All five sites have relatively low site indexes, and 189

four of the five sites have glacial outwash as the primary soil parent material (Table 1). These 190

sites are not representative of all sites in the PNW region. Therefore, Douglas-fir plantations 191

with higher site indexes and different parent materials may not show the same carryover effects 192

observed in this study. 193

An increase in the N-content of understory plants also potentially increases the N-quality 194

of litter and could result in higher microbial activity and mineralization. The retention of greater 195

amounts of higher quality organic matter due to bole-only harvest practices coupled with 196


seasonal inputs of similarly affected litter (Prescott et al., 1993; Prescott et al., 1999) from the 197

previously fertilized plots may be driving the observed increase in productivity of Douglas-fir 198

trees. In this study the 2006 mean height and DBH were 15% and 29% greater respectively than 199

the unfertilized controls. Mean height was significantly greater annually from 2001 to 2006 and 200

mean DBH in 2005 and 2006. These increases in growth response should continue over time. 201

Other studies have shown assessment of existing understory vegetation can be used to 202

estimate soil N availability (Klinka et al., 1989). In this study biomass and N-content of 203

understory vegetation on previously fertilized plots increased 73% and 93%, respectively (Figure 204

2). These results agreed with observations from other studies. Heilman and Gessel (1963) 205

reported increased N content of understory vegetation following fertilization. Turner and Gessel 206

(1988) measured increased understory biomass and N content, while Canary (1994) observed 207

increased aboveground production with fertilization. The increase in biomass and N-content of 208

understory vegetation not only produces greater quantities and higher quality liter fall, but also 209

increases N uptake which can decrease N leaching and help retain N on site (Chang and Preston, 210

2000). 211

Use of N fertilization could potentially satisfy multiple natural resource management 212

objectives; from increasing growth and yield of primary and secondary forest products to 213

increasing the quality and abundance of wildlife habitat, as evidenced by richer and higher-214

nutrient content understory plants. Further study of this phenomenon will ultimately give forest 215

managers a better understanding of how to use the tool of N fertilization to reach specific 216

objectives. 217

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5. Conclusions 219


• Repeated N fertilization of previous stands coupled with post harvest organic matter 220

retention increased DBH by 29% and total height by 15% for a new Douglas-fir 221

plantation in the Pacific Northwest compared to an unfertilized control. 222

• Biomass and N-content of understory vegetation increased by 73% and 93%, respectively 223

in N-fertilized compared to unfertilized stands. 224

• The effects of repeated N fertilization may last much longer than previously understood 225

when considering impacts of that fertilization on young, replanted second growth stands 226

instead of first rotation stands. 227

• Increases in tree growth witnessed on carryover plots should continue to increase with 228

time; meaning earlier first commercial entry and harvest than on the paired controls. 229

230

6. References 231

Abrams, M. D., Dickmann, D. I., 1983. Response of understory vegetation to fertilization on 232

mature and clear-cut jack pine sites in Northern lower Michigan. Amer. Mid. Natur. 110, 233

No. 1. 194-200. 234

Adams, A.B., Harrison, R. B., Sletten, R.S., Strahm, B.D., Turnblom, E.C., Jensen, C.M., 2005. 235

Nitrogen-fertilization impacts on carbon sequestration and flux in managed coastal 236

Douglas-fir stands of the Pacific Northwest. For. Ecol. Manage. 220, 313-325. 237

Alig, R.J., Plantinga, A.J., Ahn, S.E., Kline, J.D., 2003. Land use changes involving forestry in 238

the United States: 1952 to 2997, with projections to 2050. Gen Tech. Rep. PNW-GTR-587. 239

Portland, OR: U.S. Dept. Agriculture, Forest Service, Pacific Northwest Research Station. 240

92 p. 241

Binkley, D., 1986. Forest Nutrition Management. Wiley, New York. 242


Binkley, D., Reid. P., 1985. Long-term increase of nitrogen from fertilization of Douglas-fir. 243

Can. J. For. Res. 15, 723-724. 244

Canary, J.D., 1994. Carbon and nitrogen storage following repeated urea fertilization of a second 245

growth Douglas-fir stand in western Washington. MS Thesis. Univ. of Washington. 246

Canary, J.D., Harrison, R.B., Edmonds, R.E., Chappell, H.N., 2000. Carbon sequestration 247

following repeated urea fertilization of second-growth Douglas-fir stands in western 248

Washington. For. Ecol. Manage. 138, 225-232. 249

Chang, S.X., Preston, C.M., 2000. Understorey competition affects tree growth and fate of 250

fertilizer-applied 15N in a coastal British Columbia plantation forest: 6-year results. Can. J. 251

For. Res. 30, 1379-1388. 252

Chappell, H.N., Cole, D.W., Gessel S.P., Walker, R.B., 1991. Forest fertilization research 253

and practice in the Pacific Northwest. Fertilizer Research 27, 129-140. 254

Edmonds R.L., Hsiang, T., 1987. Forest floor and soil influence on response of Douglas-fir to 255

urea. Soil Sci. Soc. Am. J. 51, 1332-1337. 256

Franklin, J.F., Dyrness, C.T., 1988. Natural vegetation of Oregon and Washington. Oregon State 257

Univ. Press, USA. 258

Gessel, S.P., Walker, R.B., 1956. Height growth response of Douglas-fir to nitrogen fertilization. 259

Soil Sci. Soc. Am. Proc. 20, 97-100. 260

Gessel, S.P., Cole, D.W., Steinbrenner, E.C., 1973. Nitrogen balances in forest ecosystems of the 261

Pacific Northwest. Soil Biol. Biochem. 5, 19-34. 262

Heilman, P.E., Gessel, S.P., 1963. The effect of nitrogen fertilization on the concentration and 263

weight of nitrogen, phosphorus, and potassium in Douglas-fir trees. Soil Sci. Soc. Am. 264

Proc. 27, 102-105. 265


Klinka, K., Krajina, V.J., Ceska, A., Scagel, A.M., 1989. Indicator plants of coastal British 266

Columbia. University of British Columbia Press, Vancouver. 267

Mann, L., Johnson, D., West, D., Cole, D., Hornbeck, J., Martin, C., Riekerk, H., Smith, C., 268

Swank, W., Tritton, L., Lear, D., 1988. Effects of whole-tree and stem-only clearcutting on 269

postharvest hydrologic losses, nutrient capital, and regrowth. Forest Science. 24, 410-428. 270

Matsushima, M., Chang, S.X., 2007. Effects of understory removal, N fertilization, and litter 271

removal on soil N cycling in 13-year-old white spruce plantation infested with Canada 272

bluejoint grass. Plant Soil. 292, 243-258. 273

Mead, D.J., Chang, S.X., Preston, C.M. 2008. Recovery of 15N-urea 10 years after 274

application to a Douglas-fir pole stand in coastal British Columbia. For. Ecol. Manage. 256, 694-275

701. 276

Miller, H.G., 1988. Long-term effects of application of nitrogen fertilizers on forest sites. In 277

Cole, D.W., Gessel, S.P., (ed.) Forest site evaluation and long-term productivity. Univ. of 278

Washington Press, Seattle, 97-106. 279

Nohrstedt, H.O., Jacobsen, S., Sikstrom, U., 2000. Effects of repeated urea doses on soil 280

chemistry and nutrient pools in a Norway spruce stand. For. Ecol. Manage. 130, 47-56. 281

Pang, P.C., Barclay, H.J., McCullough, K.M., 1987. Aboveground nutrient distribution within 282

trees and stands in thinned and fertilized Douglas-fir. Can. J. For. Res. 17, 1379-1384. 283

Prescott, C.E., Kischchuk, B.E., Weetman, G.F., 1995. Long-term effects of repeated nitrogen 284

fertilization and straw application in a jack pine forest .3. Nitrogen availability in the forest 285

floor. Can. J. For. Res. 25, 1991-1996. 286


Prescott, C.E., McDonals, M., Gessel, S.P., Kimmins, J.P., 1993. Long-term effects of sewage 287

sludge and inorganic fertilizers on nutrient turnover in litter in a coastal Douglas-fir forest. 288

For. Ecol. Manage. 59, 149-164. 289

Prescott, C.E., Kabzems, R., Zabek, L.M., 1999. Effects of fertilization on decomposition rate of 290

Populus tremuloides Michx. Foliar litter in a boreal forest. Can. J. For. Res. 29, 393-397. 291

Prietzel J., Wagoner G.L., Harrison R.B., 2004. Long-term effects of repeated urea fertilization 292

in Douglas-fir stands on forest floor nitrogen pools and nitrogen mineralization. For. Ecol. 293

Manage. 193, 413-426. 294

Priha, O., Smolander A., 1995. Nitrification, denitrification and microbial biomass N in soil from 295

two N-fertilized and limed Norway spruce forests. Soil Biol. Biochem. 27, 305-310. 296

Smolander, A., Priha, O., Paavolainen, L., Steer, J., Malkonen, E., 1998. Nitrogen and carbon 297

transformations before and after clear-cutting in repeatedly N-fertilized and limed forest 298

soil. Soil Biol. Biochem. 16, 957-962. 299

Stegemoeller, K.A., Chappell, H.N., 1990. Growth response of unthinned and thinned Douglas-300

fir stands to single and multiple applications of nitrogen. Can. J. For. Res. 20, 343-349. 301

Strader, R.H., Binkley, D., 1989. Mineralization and immobilization of soil nitrogen in two 302

Douglas-fir stands 15 and 22 years after nitrogen fertilization. Can. J. For. Res. 19, 798-303

801. 304

Strahm, B.D., Harrison, R.B., Flaming, B.L., Terry, T.A., Licata, C.W., Petersen, K.S., 2005. 305

Soil solution nitrogen concentrations and leaching rates as influenced by organic matter 306

retention on a highly productive Douglas-fir site. For. Ecol. Manage. 218, 74-88. 307

Turner, J., 1977. Effect of nitrogen availability on nitrogen cycling in a Douglas-fir stand. Forest 308

Science. 23, 307-316. 309


Turner, J., Gessel, S.P., 1988. Forest productivity in the southern hemisphere with particular 310

emphasis on managed forests. In: Gessel, S.P., Lacate, D.S., Weetman, G.F., Powers, R.F. 311

(Eds.). Proceedings of the 7th North American Forest Soils Conference, University of 312

British Columbia, Vancouver, British Columbia, pp. 23-39. 313

Van Miegroet, H., Cole, D.W., Homann, P.S., 1990. The effect of alder forest cover and alder 314

forest conversion on site fertility and productivity. pp. 333-354 In: S.P. Gessel, D.S. 315

Lacate, G.F. Weetman, and R.F. Powers (eds.) Sustained Productivity of Forest Soils. 316

Proceedings of the 7th North American Forest Soils Conference, University of British 317

Columbia, Faculty of Forestry Publication, Vancouver, B.C. 318

VanderSchaal, C., Moore, J., Kingery, J., 2004. The effect of multi-nutrient fertilization on 319

understory vegetation nutrient concentrations in inland Northwest conifer stands. For. Ecol. 320

Manage. 190, 201-218. 321

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Table 1 333

Descriptions and experimental treatment regimes for the Carryover study sites. 334

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342 343 Figure 1. Mean height (m) and mean DBH (cm) of Carryover study Douglas-fir trees. The 344

differences in height were statistically significant (p < 0.1) every year measured from 2001 to 345

2006. In 2006 mean tree height was 15% greater on the previously fertilized plots than on the 346


control. The differences in DBH were statistically significant (p < 0.1) in years 2005 and 2006. 347

In 2006 mean DBH was 29% greater on the previously fertilized plots than on the control. 348

Control points are offset to show (+/- 1) standard error bars. 349

350


350 351 352

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Figure 2. Biomass and N content for understory vegetation on Carryover study sites. The 354

differences in biomass of understory were statistically significant (p < 0.1) in 2006. 355

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