Transcript soilslab.cfr.washington.edu

Long-term Results of Nitrogen Fertilization in Pacific Northwest Coastal Douglas-fir Plantations
Amy R. Sidell, Robert B. Harrison, University of Washington; David G. Briggs, Randol Collier, Robert Gonyea, Stand Management Cooperative; and Robert J. Luxmoore, Environmental Sciences Division, Oak Ridge National Laboratory
CURRENT ANALYSES
After 20 years, the original RFNRP and other SMC installations have undergone a
number of different treatments with multiple N fertilizer applications at varying time
intervals. This variation in treatment regime severely complicates meaningful data
analysis using the methods previously employed to summarize the growth response of
RFNRP plots. Therefore, in this analysis, the data are summarized as growth
differences relative to cumulative N fertilizer application. While there is some growth
data available over time periods up to 26 years, the number of sites with regular growth
data falls off dramatically above 20 years of growth data as the majority of the stands
monitored had reached harvestable size after 20 years of monitoring. Therefore,
summaries 20 years of growth are presented in this poster.
Figure 2: RFNRP Fertilizer Trials; unthinned (•) and thinned (o) stands
Figure 3a: Fertilizer application
Figure 3b: SMC stand measurement event
200
400
--23%
53
16%
13%
1.09
18%
40
12%
3%
1.39
24%
50
15%
3%
--18%
42
13%
19%
--24%
47
17%
14%
Volume growth (ft3/ac)
10 00 0
Volume growth (ft3/ac)
10 00 0
10000
8000
60 00
40 00
20 00
0
40 00
20 00
20 0
40 0
60 0
80 0
10 00
0
C umu lative N Fertilize r Appli ed (lb/ac)
4000
60 00
40 00
20 00
400
600
800
1000
80 0
10 00
80 00
There are a number of different factors which contribute to the wide variation in response.
These factors include differences in:
Initial stocking
Tree size distribution
Competitive status of trees
Species composition
Microsite and or soil
Physical damage, disease, insects
both within and between sites. The effect of species composition was minimized by
focussing on growth trends in Douglas-fir stands only. Stands suffering significant
numbers tree deaths were also excluded from the analysis to minimize the influence of
physical damage, disease and pests on the summary statistics. Although by expressing
the growth in terms of an absolute or relative growth increment over a given time period,
the focus is shifted to the growth rate over a given time period, the effect of initial stand
size differences is not eliminated as the volume added to a tree is typically proportional to
its size.
60 00
40 00
20 00
0
0
200
60 0
10 00 0
80 00
0
0
40 0
12 00 0
10 00 0
Site Index
I: >140ft
II: 120-140ft
III: 100-120 ft
IV: 80-100 ft
V: <80 ft
20 0
C umu lative N Ferti lize r Ap plie d ( lb/a c)
12 00 0
6000
0
20 0
40 0
60 0
80 0
10 00
0
C umu lative N Ferti lize r Ap plie d ( lb/a c)
Cumulativ e N Fertilizer Applied (lb/ac)
20 0
40 0
60 0
80 0
10 00
C umu lati ve N Ferti lize r Ap plie d ( lb/a c)
12 00 0
Volume growth (ft3/ac)
Figure 5b: Harvesting at SMC Installation 17
60 00
0
0
2000
Figure 5a: Fertilizer Application
80 00
Figure 8a (above): Average 20 Year Volume
Growth by Site Class for Unthinned Stands
10 00 0
80 00
60 00
40 00
20 00
The effect of initial differences in stand size on the representation of growth response can
also be diminished by adjusting the difference in treated and untreated stand size by the
pre-treatment difference in stand size. The differences in stand volume between treated
and untreated plots at each installation were calculated and also adjusted for initial
differences in stand volume (Figures 11a and 11b).
0
Data Treatment
Plot summary data from Douglas-fir stands which have been fertilized with nitrogen
only were selected for analysis. The cumulative amount of N fertilizer applied was
determined for each measurement event at a plot. The absolute stand size and growth
increment for a given time period were calculated for each treated and untreated plot.
For each installation, the stand size and growth increments for the treated plots were
compared to those for the installation’s untreated (control) plot, and the average
differences between treated and untreated plots at each installation were calculated.
In general, the absolute volume increment over 20 years increased significantly (=0.1)
with increasing levels of cumulative N fertilizer applied for unthinned plots (Figure 6).
There was no apparent trend for thinned plots.
2016
2251
1985
1853
1522
1740
1971
1542
1542
1347
Figure 6 (left): Average Volume
Growth over 20 Years. Average
value inside bars, standard deviation
above error bars
20 0
40 0
60 0
80 0
C umu lative N Fertilize r Appli ed (lb/ac)
10 00
Diff erence in Fert ilized and Untreated Unthinned
Stand Volume
4000
3000
2000
The average absolute difference in growth was calculated as the difference in growth
increment over a 20 year period between the fertilized and control plots at a given installation.
The average difference in growth increment can be considered to be the average additional
growth caused by N fertilization, and tended to increase with increasing levels of total N
fertilizer application over 20 years. Fertilized plots at less fertile sites (lower site class) tended
to exhibit greater volume response than those of higher site class (Figure 9a).
Average Difference in 20 Year Unthinned Volume Growth
(Fertilized-Control)
100%
I
II
I
II
III
2000
IV
1500
V
1000
Site Index
I: >140f t
II : 120-140f t
II I: 100-120 f t
IV: 80-100 f t
V: <80 f t
500
0
-500
400
600
800
80%
III
IV
60%
V
I
II
II I
IV
V
4000
3000
2000
1000
0
Site Index
I: >140ft
II: 120-140ft
III: 100-120 ft
IV: 80-100 ft
V: <80 ft
-1000
-2000
0
200
400
600
800
1000
1200
Cumulativ e N Applied (lb/ac)
0
-1000
-2000
0
200
400
600
800
1000
Site Index
I: >140ft
II: 120-140ft
III: 100-120 ft
IV: 80-100 ft
V: <80 ft
1200
Cumulativ e N Fer tilizer Applied (lb/ac)
Figure 11b: Difference in Fertilized and
Untreated Unthinned Stand Volume
Adjusted by Initial Stand Differences
Site Index
I: >140f t
II : 120-140f t
II I: 100-120 f t
IV: 80-100 f t
V: <80 f t
20%
0%
-20%
0
1000
200
400
600
800
Cumulativ e N Fertilizer Applied (lb/ac)
.
Figure 9a: Average Absolute Difference in
Unthinned Volume Growth (Fertilized -Control)
By adjusting the treatment response, as conveyed by difference in stand size, by the
initial stand size difference, the variability was lowered, bringing most average
treatment response values into the positive, and lowering the peak response values.
40%
1000
IMPLICATIONS
1200
.
Figure 9b: Average Difference in Unthinned Volume
Growth Relative to Control (Fertilized Control)/Control
The differences in response between site classes is more pronounced when expressed as a
percent of the control (Figure 9b). These results emphasize the difference in response to N
fertilization based on site class. As with other measures of growth, there was no distinct
pattern of fertilizer response among thinned and fertilized sites.
1446
1523
10
Figure 7 (below): Average
Difference in 20-Year Growth
(Fertilized-Control)
Diff erence in Fert ilized and Untreated Unt hinned
Stand Volume (adjusted for initial differ ences)
Average Difference in Volume Growth Relative to Control
Over 20 Years (Fertilized-Control)
3000
2500
I
II
III
IV
V
1000
Figure 11a: Difference in Fertilized and
Untreated Unthinned Stand Volume
Cumulati ve N Fer til izer Appli ed (lb/ ac)
Average 20 Year Growth
0
Figure 8b (right): Variability associated with Average
20 Year Volume Growth. Average volume growth ± 1
standard deviation
Average Growth by Time
Pr e vious RFNRP Data Analys e s
6
8
200
400
200
400
--26%
59
18%
14%
I
II
II I
IV
V
200
Summaries of the RFNRP growth data reporting Douglas-fir response to single and
multiple applications of N fertilizer have been published periodically since the inception of
the project. Growth response has been typically expressed in terms of the periodic
annual increment (PAI) in volume and basal area, the difference between the growth rates
of treated and untreated stands. The untreated growth rate was estimated for each
treatment plot based on a model relating untreated growth rate as a function of breast
height age, site index, initial basal area and stems per acre. The initial data analyses
reported that 80% of Douglas-fir stands responded to 200 or 400 lb N/ac with an increase
of least 10% in basal area, and that the response tended to increase in magnitude and
consistency as site quality decreased. (RFNRP, 1974). The reported growth response
values are presented in Table 1 for 4, 6, 8 and 10 years following N fertilization. The
growth response to N fertilization was reported to peak during the 3rd or 4th growing
season following application of 200 lb N/ac (RFNRP, 1978) but was still statistically
significant at 10 years for basal area increment. (RFNRP, 1980).
12 00 0
80 00
12000
The data for thinned and unthinned installations were analyzed separately and also
divided into each of the following Douglas-fir site classes, based on average tree height
at age 50 years, as defined by King (1966):
I : >140 ft
II : 120-140 ft
III : 100-120 ft
IV: 80-100 ft
V : < 80 ft
For each site class, the growth data was further separated by the total amount of N
fertilizer applied to the plot into five fertilization levels: 200 lb N/ac, 400 lb N/ac, 600 lb
N/ac, 800 lb N/ac, 1000 lb N/ac. The average response for each fertilization level and
site class was calculated and expressed in terms of absolute stand size and growth of
fertilized trees relative to the control.
Previous Analyses
TABLE 1: Sum m ar y of
Year Follow ing N Fertilization
4
N Fertilizer Applied (lb N/ac)
200
400
Grow th Response
Basal Area
f t2/ac/yr
----%
20%
22%
Volume
f t3/ac/yr
56
64
%
18%
21%
Standard Error
%
Average 20 Year Volume Growth
12 00 0
Volume growth (ft3/ac)
The Regional Forest Nutrition Research Project
(RFNRP), now incorporated into the Stand Management
Cooperative (SMC), was formed in 1969 with a primary
objective of providing forest resource managers with
accurate growth data on fertilization of second growth
Douglas-fir and western hemlock stands on a range of
sites throughout the Pacific Northwest. As part of the
RFNRP, 348 installations (225 unthinned and 123
thinned) receiving varying levels of N fertilizer have been
established in 5 phases and monitored regularly for
growth response to different N fertilization regimes
(Figure 2). These data have been compiled and are
currently maintained in a database by the SMC. The
SMC database contains plot treatment regimes (Figure
3a) and summaries of all data collected during each
measurement event (Figure 3b) for each measurement
plot at each SMC installation.
Volume (ft3/ac)
Figure 1: Douglas-fir stand at
SMC Installation 134
Variability
The average absolute growth over a 20 year time period was calculated for different site
classes and levels of fertilization. The average absolute volume growth for unthinned
installations is presented in Figures 8a and 8b. The absolute volume growth over 20 years
increased with increasing levels of N fertilizer for all site classes; the largest difference was
observed in site class V. No significant differences between fertilization levels could be
detected due to the high variability (Figure 8b). There were no obvious trends in the 20 year
growth of thinned stands due to high variability and the small number of sites which were both
thinned and fertilized.
Volume Growth (%)
Growth of trees in Pacific Northwest forests is typically thought to be
limited by available nitrogen (N). Nitrogen fertilization of forest
plantations is a common practice throughout the Pacific Northwest,
with the aim of increasing productivity of these N limited ecosystems.
The response of second and third-growth Douglas-fir stands to N
fertilization is typically positive with reports of approximately 60% of
stands responding with an increase in volume of at least 10% (Miller
et al, 1986).
Average Growth by Site Class
Volume growth (ft3/ac)
BACKGROUND
The average annual growth increments for unthinned plots receiving only a single application
of N fertilizer (200 lb N/ac and 400 lb N/ac) were calculated. The average annual volume
increment tended to decrease over time. The average annual volume increment declined
slightly more quickly with the lower N fertilizer application rate as presented in Figure 10.
The long-term future of the forest industry in the Pacific
Northwest depends in part on the productivity of the region's
forests and on the choice of silviculturally sound and costeffective management regimes. The results of this analysis
indicate a trend toward increasing growth increment with
increasing rates of fertilization. While the absolute volume is
greatest on the most fertile sites, the greatest additional
response appears to be in the less fertile sites. The response
of unthinned Douglas-fir to N fertilization tends to increase
with increasing total levels of N fertilizer application, with no
indication of the growth rate leveling off as more fertilizer is
added. This suggests that greater growth yields may be
possible with even higher levels of cumulative N fertilizer
application. Although this analysis found no apparent trends
in 20 year growth of thinned sites, there is no reason to
believe that the response of thinned sites to fertilization would
not follow the same trends as unthinned sites. Past RFNP
reports have stated that thinned sites have exhibited higher
growth response than unthinned sites, suggesting that the
average growth yields for thinned and fertilized sites could be
even greater than those reported fertilized unthinned sites.
Source: RFNRP Biennial Report s (1974-1982)
Figure 4: Example of Douglas-fir growth response to N
fertilization
The difference in 20 year
growth increment between
treated and untreated plots
can be thought of as the
additional growth as a result
of N fertilization over the 20
year period. For both
unthinned and thinned plots,
the average difference in growth between treated and untreated growth tended to
increase with increasing levels of total N fertilizer applied (Figure 7). The average
differences in growth among thinned stands was slightly greater than in unthinned
stands although the differences were not significant (=0.1).
200 lb N/ac
350
Average Annual Volume
Increment (ft3/ac/yr)
An analysis of RFNRP data along with
data from the British Columbia Ministry
of Forests reported that 60% of DF
stands in western Washington, western
Oregon, and western British Columbia
responded to N fertilization (Figure 4)
and pure Douglas-fir stands responded
better than mixed Douglas-fir – western
hemlock stands. N fertilization was
also reported to accelerate losses of
smaller trees in unthinned stands
(Miller et al, 1986).
Ave rage Annual Volume Increme nt by Time Since N Fe rtilizer
Application of Unthinned Stands
400 lb N/ac
Figure 10: Average Annual Volume
Increment for Unthinned Stands by
Time Since N Fertilizer Application
Col leg e of For est Resources
Uni versi ty of Washi ng ton
Box 352100
Seattl e Washington 98195-2100
300
250
200
150
100
50
0
0
2
4
6
8
10
12
14
16
Ye ars Since N Fertilizer Application .
20
24
SI LVI C UL TUR E
NU TR I TI ON
W OOD QU ALI TY
M ODE LI N G
206-543-5355 phone
206-685-3091 fax