fERTILIZE WESTERN HEMLOCK -- YES OR NO? -
- DEAN S. DEBELL
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Being assigned the rather specific title, Fertilize
Western Hemlock - Yes or No?, I have been forced
to arrive at some definite conclusions regarding fertil-,
ization of western hemlock stands. I will provide
what might be considered an interim answer to the
question of fertilizing western hemlock. First, howev­
er, I want to review some of the background informa­
tion that has influenced my answer and can influence
your thinking as well.
A decision to fertilize or not to fertilize hemlock
stands depends on several factors.
These factors include:
1. the amount of money an organization has for
silivcultural investments,
2. the alternatives available to the organization for
investing that money, and
3. how well hemlock fertilization compares with
the other alternatives regarding:
a. estimated magnitude of response - since this
determines the return that can be expected
from the expenditure, and
b.the probability that the expected return will
be achieved.
This paper will deal primarily with the last two
factors: (i) the magnitude of response we can expect,
and (ii) the probability that we can achieve an ade­
quate response. In doing so, I will talk about the fol­
lowing:
1. what has been done to date - to provide an idea
of the extent of research and the general conclu­
sions we can draw from it,
2. what we know or think we know - some more
specific trends and speculations regarding
growth response of hemlock stands to fertilizers;
also, a review of some silvical differences between
western hemlock and Douglas-fir, with sugges­
tions as to how these differences might affect
response to fertilizers, and
3. some conclusions about what we should do now
and in the future regarding this aspect of hem­
lock management.
Several organizations working on hemlock fertil­
ization and providing data and thoughts on the topic
include: Crown Zellerbach Corporation, University of
Washington, Weyerhaeuser Company, MacMillan
Bloedel Limited, Washington State Department of
Natural Resources, Canadian Forestry Service, and
the U.S. Forest Service. Thus, the ideas and interpre­
tations presented in this paper are based on the work
of many other people.
WHAT HAS BEEN DONE
Let's consider the extent of research effort on fertil­
ization of west.ern hemlock and the general results
140
and conclusions to date. A large number of fertilizer
trials have been established in hemlock stands. They
encompass a variety of sites on about 1,000 plots.
Urea has been the major fertilizer applied, but a few
tests have included other sources of nitrogen and
other elements. Most of the organizations involved
in this work are listed in Table 1 with numbers of
plots and the general magnitude of basal area growth
responses.
Table 1 Fertilization of western hemlock - ex­
tent of research in the Pacific Northwest: orga­
n i z a t i o n s, n u m b e r o f p l o t s a n d g e n e r a l
responses
Number
of
Organization
plots
Crown Zellerbach . .... . ... ... 406
MacMillan Bloedel Ltd..... ..... . 60
University of Washington
Regional Project ... ..... ... .. 238
Washington State Department of
Natural Resources . . .. .... . . . 92
Weyerhaeuser.................. ,195
.
.
.
.
Tot!!l or average ........... 991
Basal area growth
response
after 2 to 6 years
Average Low High
Percent
5
16
-20'
2
47
28
5
-10
45
NA'
NA"
NN
5
-17
36
--
6
--
-11
39
----
1 Negative values represent growth reductions.
, Response data not yet available.
It is obvious from these data that growth response
to fertilizer is not consistently good. On the average,
it is rather low; moreover, negative effects on growth
have been observed in numerous instances. On the
other hand, the high responses do indicate that
growth of the species can be accelerated by applica­
ton of nitrogen in certain situations.
In summary, the work to date indicates that: ( 1)
average response is low « + 10 per cent); (2) varia­
tion is great (+47 to -20 percent); and (3) because
of this variation (most of which is unexplained), we
are unable to predict response.
WHAT WE KNOW OR THINK WE KNOW
To develop a prescription for fertilizing hemlock,
we must first understand the reasons for differences in
the growth behavior of individual hemlock stands
after fertilizer is applied. At this time, reasons for
the growth response differences are largely unknown.
There are, however, some general trends in response
patterns, which appear reasonable from a biological
standpoint. However, the trends are slight and rather
speculative in some instances; they certainly deserve
more thorough testing before being accepted as valid.
Geographic Location
The strongest relationship uncovered to date is
with respect to geographic location (Table 2). Lowest
responses (actually negative response or reduced
growth) were obtained in hemlock stands along the
southern and central coast of Oregon. Response was
better in the Lower Columbia River Valley and on
up the Washington coast. The highest responses were
obtained in the Washington Cascades and on Van­
couver Island.
Thus, the chances for obtaining a good response
increase with latitude along the coastal hemlock belt.
Also, response is considerably better in the Washing­
ton Cascades than along the central coast of Wash­
ington.
Table 2 Fertilization of western hemlock - mag­
nitude and consistency of growth response to
urea by geographic location
Geographic
Number of
location
plots fertilized
.
Coast .
Lower Columbia and
.
.
.
.
.
.
.
.
.
Washington Coast
Washington Cascades
Noithern Vancouver
Island
.
.
.
.
.
.
.
.
Proportion of
growth
response
10% response
plots with
PERCENT
Southern Oregon
.
Basal area
.
.
.
.
.
.
.
48
-10
8
30
67
.
.
.
.
.
.
80
72
+ 4
+12
.
.
.
45
+16
90+
What is the reason for differences due to geograph­
ical location? We do not know; however, we can spec­
ulate that basic factors are differences in soil parent
material and precipitation and temperature patterns.
These factors could influence growth response
through inherent nutrient supplies, nutrient cycling
rates, the fate of applied fertilizer, and nutrient up­
take and assimilation processes.
Tree or Stand Conditions
There are slight trends between response and a
number of tree and stand conditions which affect tree
vigor. These trends are based on specific analyses of
selected data from plots established by Crown Zeller­
bach Corporation and the University of Washington's
Regional Forest Nutrition Research Project:
1. Crown class
Trees in trials showing an
overall positive response were examined individually
to determine how the growth was distributed within
the stand. Upper crown class trees showed a good
response (40 + per cent); the lower crown class trees
showed little or no response to fertilizer.
-
Response to fertilizer decreased as
2. Stocking
number of trees increased, but no definite relation­
ship existed with initial stand basal area. The rela­
tionship of growth response to growing space, com­
petitive stress, or some other integrated stocking
index remains to be examined.
3. Age No generall'elationship between response
and ag is now apparent. However, some of the high­
est responses have occurred in young (15- to 3D-year­
old) stands that have been thinned.
4. Thinning
Effects of thinning were examined
in sevel'al 20- to 40-year-old stands near Seaside, Ore­
gon. Although the trend is rather weak, it appears
that trees growing in thinned stands generally
responded slightly ( + 4 percent) to fertilizer, whereas
trees in unthinned stands showed no response or a
negative response (-4 percent).
Using these trends, I believe it is reasonable to
propose the following general hypothesis:
Growth response to nitrogen fertilization is due
primarily to an increase in effective leaf area.
In some cases a hemlock stand may be of suffi­
cient density or trees may be of such low vigor that
the required buildup in leaf area can occur only
slowly or not at all.
In such cases, response to fertilizer application
will be correspondingly slow, low, or negligible.
-
-
-
Site Index
There was a tendency for response to decrease as
site index increased. A similar relationship exists for
Douglas-fir; such a trend is to be expected if nitrogen
is a major factor limiting the inherent productivity
of hemlock sites.
Fertilizer Applications
Response data were also examined for trends asso­
ciated with the nature of the fertilizer application
itself: application dosage, nitrogen source, and other
elements.
For urea applications,
1. Application dosage
the effect of application level is inconsistent. There
is a tendency for response to decrease with higher
dosages of nitrogen along the Oregon coast and in
densely-stocked stands. In the Cascades and in Brit­
ish Columbia, response increased with application
level.
2. Nitrogen sources
The nitrogen source ques­
tion needs to be pursued more vigorously. Because
of urea's inherent advantages (high N content, cost,
availability) and the initial success with urea in
Douglas-fir stands, work in hemlock also has been
concentrated on urea; however, other forms of nitro­
gen fertilizer may be more desirable for hemlock.
Crown Zellerbach does have a large test of urea,
ammonium nitrate, and ammonium sulfate at one
location near Seaside, Oregon. After 4 years, urea had
increased basal area growth of dominant and codo­
minant trees by 15 percent. Response with ammon­
ium sulfate was slightly better (20 percent), but ef­
fects of ammonium nitrate were negligible. In other
locations, however, response to ammonium nitrate
has been higher than to urea. We suspect that dif­
-
-
141
ferences in rainfall pattern after fertilizer application
are involved in the response differences.
3. O ther elements - The question of whether
other elements are limiting growth or response to ni­
b'ogen remains to be answered. A number of complete
fertilizer plots have been established, and the prelimi­
nary assessments do not point to deficiencies of other
elements. However, much of the existing data has
not been adequately analyzed. Moreover, some re­
searchers suspect the phosphorous and micronu­
trients may play a role in the variation in response.
Comparison of Western Hemlock and Douglas­
fir
There are some basic biological, silvical, and
growth differences between western hemlock and
Douglas-fir that may affect the species' response to
standard applications of urea fertilizer (Table 3). The
poorer response in hemlock stands could be due to
one or more of the following factors:
1. Native nitrogen supply and tree requirements
may be more in balance for a climax species; some
comparisons in mixed coastal stands have indicated
a good response for Douglas-fir, whereas response of
hemlock growing beside the Douglas-fir was consider­
ably less or negligible.
2. In more tolerant, highly dense stands, it may
be more difficult to increase effective leaf area.
3. For a shallow-rooted species, fertilizer placement
and source may be more critical than the species with
roots distributed throughout the profile. At the least,
we should be somewhat skeptical about extrapolating
fertilization prescriptions (or any silvicultural prac­
tices, for that matter) developed for Douglas-fir to
western hemlock.
Table 3. Comparison of western hemlock and
Douglas-fir with respect to silvical and growth
traits
Item
Hemlock
Successional stage
Shade tolerance
Average stand density
Root distribution
climax
very tolerant
more dense
shallow
Douglas-fir
subclimax
relatively intolerant
less dense
throughout profile
CONCLUSIONS
As a result of the facts and speculations presented,
my answer to the question - Fertilize Western Hem­
lock - Yes or No? - is
NO - however, I would like to qualify that as No,
not yet,
Until we obtain a better understanding of the
causes of response variation and develop guidelines,
an organization will probably be wasting its money
if it fertilizes hemlock stands in the coastal areas of
Oregon and perhaps Washington. On the other hand,
chances for obtaining a profitable response are con­
siderably better in the .Washington Cascades and on
Vancouver Island in British Columbia. However, one
must consider the possible alternative investments.
Even in these locations, an organization may find
that other practices (e.g., precommercial thinning)
offer a greater likelihood of profitable returns on the
invested dollars.
However, I am optimistic about the possibility of
developing methods for achieving good response to
fertilizer on much of the hemlock acreage. My opti­
mism is based upon the high response obtained in
Table 4. O ther possible causes of variation in estimated
growth response of hemlock to fertilizer
application.
Possible cause of variation
Mensurational technique
Nitrogen supply
Other limiting factors
Tree uptake of fertilizer N
Toxicity
Genetics
142
E xplanation
Conventional measurational and analytical methods may not
be adequate for estimating short-term growth response in
hemlock. It may be necessary to account for differences in
spatial relationships within hemlock stands. Also, measure­
ments of upper stem diameters may be needed to accurately
assess volume growth.
The capacity of the soil to supply native nitrogen is adequate
on those sites which do not respond to N-fertilizer.
The supply of nitrogen is not sufficient for maximum tree
growth but other factors (soil, other nutrients, climate, or
plant) are more limiting.
With present methods of application and fertilizer materials,
added N is not sufficiently absorbed and utilized by the tree.
May be inten-elated with climate, soil, and root distribution
patterns.
Some fertilizers may create a rhizosphere environment that
is toxic (e.g., high N H3, high ionic concentration, or initi l
high pH) to feeder roots and/or mycorrhizal associations of
hemlock. May also be interrelated with climate, soil, and root
distribution patterns.
There may be genetic differences in the ability of hemlock
trees to absorb and utilize fertilizer nitrogen.
several trials and also in our technical ability to eval­
uate some of the fundamental aspects of hemlock nu­
trition. In the past, considerable sums of money have
been invested in field trials. This approach has given
us some preliminary guidelines for fertilizing Doug­
las-fir and some indications of possibilities with hem­
lock; it has not yet provided guidelines for western
hemlock and I doubt that field trials alone will pro­
.
vide the needed information.
We need to critically examine some of the trends
and speculations I have posed in my previous com­
ments. We must also look at some of the other factors
Reproduced from
/I
that may be involved in response or lack of response
(Table 4). At present, we know little or nothing about
the basic processes underlying hemlock nutrition and
response to fertilizer application. If researchers in
various industry, government, and educational orga­
nizations attack some of these· matters on a coopera­
tive basis, I'm confident that guidelines for successful
fertilization of hemlock stands can be developed and in a fairly short time.
In closing, my answer to the question our modera­
tor posed on fertilizing western hemlock is:
No, not yet but yes, someday.
G.LOBAL FORESTRY AND THE WESTERN ROLE, /I
Permanent Association Committees Proceedings,
and Conservation Association,
SERVICE,
U.S.
Portland,
Department of Agriculture,
1975
Western Forestry
Oregon, by the FOREST
for official use.
143