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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Effects of N fertilization… page 2
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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Effects of N fertilization… page 3
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
Effects of N fertilization… page 4
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
70
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
Effects of N fertilization… page 5
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
98
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
Effects of N fertilization… page 6
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
116
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
Effects of N fertilization… page 7
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
142
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
Effects of N fertilization… page 8
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
Effects of N fertilization… page 9
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
Effects of N fertilization… page 10
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
218
5. Conclusions 219
Effects of N fertilization… page 11
• 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
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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
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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
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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
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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
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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
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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
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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
322
323
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325
326
327
328
329
330
331
332
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Table 1 333
Descriptions and experimental treatment regimes for the Carryover study sites. 334
335
336
337
338
339
340
341
342
Effects of N fertilization… page 17
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
Effects of N fertilization… page 18
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
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