Douglas-fir dwarf mistletoe spread, intensification, and tree growth impact : thirty-eight year remeasurement

2008·5.1 MB·English

by  Jackson, Marcus Benjamin

Historic, Archive Document
Do not assume content reflects current
scientific knowledge, policies, or practices.

aSB608
.D6J32
Numbered Report 08-09 December 2008
Douglas-fir Dwarf Mistletoe Spread, Intensification, and Tree Growth Impact:
Thirty-Eight Year Remeasurement
Marcus B. Jackson, Plant Pathologist
USDA Forest Service, Forest Health Protection
Missoula, MT
DO iN
INTRODUCTION
A study was established in 1970 to measure originally proposed, but only two were installed
Douglas-fir dwarf mistletoe (Arceuthobium for a total of eight plots or 24 subplots (four
douglasii Engelm.) spread and intensification silvicultural treatments X three dwarf mistletoe
and dwarf mistletoe impact on Douglas-fir treatments X two replicates). Because a check
growth and mortality in four precommercially plot was inadvertently thinned, an additional
thinned treatments (Dooling 1970 and Dooling, silvicultural treatment plot (plot 25) was
et al. 1986). All plots are located near the South established.
Fork of Lolo Creek, Lolo National Forest,
Montana on a Douglas-fir/Ninebark Berg (1974) noted the original overstory was
(PSME/PHMAS) Habitat Type (Figure 1). The ponderosa pine, but he didn’t describe the
study was designed to help determine levels of Douglas-fir component of the original stand.
dwarf-mistletoe removal through thinning and Steve Slaughter, Lolo National Forest
pruning that would reduce wood volume losses Silviculturist, provided stand histories from
below an economic threshold and help FACTS (Forest Service Activity Tracking
understand the spread and intensification of System) reports (Slaughter 2008). Some history
Douglas-fir dwarf mistletoe in thinned stands prior to study establishment were provided for
(Dooling 1970). 15 of the 25 plots. No pre-1970 stand
information was available for plots 1, 3, 7, 11,
12919720 2 16227 and 25; /Plots 10 and.18.were
METHODS planted in 1947. Plots 2, 4, 5, 6, 8, 9, 23, and 24
were sanitized/salvaged in 1964 and a
In 1970, one of four silvicultural treatments was shelterwood cut was completed on plots 13, 14,
applied to each plot with dwarf mistletoe 15, 16, and 17 in that same year. From 1971
treatments superimposed as subplots on each plot through 1987 a variety of treatments
(Figure 1 and Table 1). Three replicates were (precommercial thin, sanitation/salvage,
Figure 1. Douglas-fir dwarf mistletoe plot distribution.
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liberation cut, site preparation and planting) were (dwarf mistletoe rating) greater than one were
variously applied to stands in which the plots are retained. Some trees on partial-high control
situated. Some of these treatments were plots were pruned to reduce DMR rating to one
apparently applied to the plots and/or plot if necessary to maintain stocking levels. Trees
buffers, while others didn’t include the plots but with DMRs up to three were retained on the
only the stand area outside of the plots partial-low control plots. No pruning was done
(Anonymous 1988). on the partial-low control plots. No mistletoe
was removed on the check subplots. Although
the maximum levels of mistletoe allowed per
TREATMENTS subplot were established, there were no
minimums; therefore, some of the subplots for
Silvicultural Treatments. Each one-quarter each of the three dwarf mistletoe treatments
acre plot was precommercially thinned to an 8 X could be free from dwarf mistletoe and indeed
8, 14 X 14, or 20 X 20 foot spacing or left that was the case. See Table 1 for assignment of
unthinned (check). With the exception of the silvicultural and dwarf mistletoe treatments to
check, only crop trees at appropriate spacing plots.
were retained on each plot and all other trees
were removed in 1970. Crop trees were defined One-quarter acre subplots were monumented at
by Dooling (1970) as: four comers with steel fence posts and buffered
by a one-half chain (33 feet) strip that received
Trees selected as potential crop trees the same treatment as the respective subplot. All
should be dominants and codominants. treatments were completed in 1970. Since one of
The trees should be vigorous growing, the silvicultural check plots was mistakenly
free of other major diseases or thinned to 14 X 14 foot spacing, plot 25 was
deformities, and of good form. Trees added as a substitute in 1971. However, a brief
with trunk swellings, burls, twin tops, summary provided by a field crew member after
malformed leaders, crooks, or showing taking data in plot 25 in 1988 stated that this plot
evidence of mechanical injury will not be “does not reflect the characteristics of the rest of
selected. Crop tree selection will the plots” (Anonymous 1988). Review of 1970
conform with the practice of maintaining data and observations in 2008 support this
mixed stands of species suited to the site. conclusion. The trees in plot 25 were
substantially larger and more heavily infected
Crop trees included ponderosa pine in most plots with dwarf mistletoe than those in other plots.
and western larch and lodgepole pine in a few
plots each; however, Douglas-fir was the primary In May and June 2008, tree height, dbh, height to
crop tree species in all subplots. The project crown, live crown ratio, dwarf mistletoe rating
proposal (Dooling 1970) stated 100 trees would (for each crown third) using the Hawksworth
be identified and tagged as sample trees in the method, and crown class were recorded for each
check (unthinned) plots. All crop trees in the tagged tree on all 25 plots. In addition, a rapid
thinned stands and sample trees in the check assessment of Douglas-fir ingrowth was
were tagged with wire or nails at breast height. completed by tallying all trees less than nine feet
Wired tags were nailed in subsequent years as in height. The ingrowth assessment was carried
trees reached adequate size. out to help determine if ingrowth (see below)
provided by FVS (Forest Vegetation Simulator)
Dwarf Mistletoe Treatments. Dwarf mistletoe was accurate.
treatments consisted of a check, partial-high
control, and partial-low control. In the partial- Dwarf Mistletoe Rating (DMR). Dwarf
high control subplots, no trees with a DMR mistletoe infections were measured on all tagged
trees in 1970, 1973, 1978, 1983, 1988, and 2008. A review of the 1988 data shows half of the
At some point between the writings of the twelve subplots that contained dwarf mistletoe
proposal (Dooling 1970) and the writing of the had at least one tree with different ratings when
1986 report (Dooling et al), the dwarf mistletoe the tree was rated using the original six-point
rating system changed from a 6-point system system compared to the Hawksworth system.
used by Leaphart, Graham, and Wicker (no Since “each tree was rated for dwarf mistletoe
citation provided in proposal) to Hawksworth’s infection using the 6-Class dwarf mistletoe rating
6-point dwarf mistletoe rating system (DMR) system (Hawksworth 1977)” (Dooling, et
(Hawksworth 1977). Hawksworth (1977) does al. 1986) contradicts the 1988 project file, the
not cite a previous 6-point system used by dwarf mistletoe rating system used may have
Leaphart, Graham, and Wicker as described in changed sometime between 1970 and 1983. The
Dooling’s proposal. The two systems are very latter year is the last time data was taken under
similar; they both divide the crown of an infected Dooling’s oversight. Since we were unable to
tree into thirds, rate each third for dwarf determine which system was used in 1970, 1973,
mistletoe, and add up the thirds for tree ratings 1978, and 1983, the only changes in dwarf
that could range from zero (no infection) to six mistletoe ratings we can compare with any
(most severely infected). The difference confidence are between 1988 and 2008 using the
between the two systems is the distinction Hawksworth system.
between a rating of one or two in each third.
Whereas the Leaphart, Graham, and Wicker FVS Analyses. Data from nine plots were
system assigns a two when more than one-third entered into FVS to determine if the model
of the branches are infected in a crown third, the would accurately predict the spread and
Hawksworth system requires one-half or more of intensification of dwarf mistletoe on each of
the branches to be infected to move a rating up these plots with each of these treatments. The
from a one to a two. In 1998, all trees were rated other sixteen plots were not used in the model
using both systems. The project file from 1988 for three reasons. First, twelve plots were
(Anonymous) noted the following: removed because there was no dwarf mistletoe
on those plots in 1988 (plots 2, 5, 6, 9, 10, 11,
During this remeasurement, the question 12, 13, 14, 15, 16, 18). Three additional plots
of changing the mistletoe rating system were not used because they were control thinning
was explored. Using Hawksworth’s treatments (plots 19, 21, 25) without any way to
method, that which is currently used by determine plot density at the beginning of a
the [Forest [S]ervice, was proposed to model cycle. Finally, plot 1 was not included
replace the 6 point system designed by because too many trees were lost due to road
Leaphart, Graham, and Wicker which has widening and Christmas tree cutting. Since
been used in previous measurements. ingrowth was not measured any time before
Both systems were recorded on the data 2008, ingrowth was modeled from 1970 to 1990
sheets... Many differences were noted and the modeled ingrowth was applied to the
between the old and the new systems. | 1998 data. Ingrowth was modeled for twenty
feel that it would significantly change rather than eighteen years because the model’s
results 1f Hawksworth’s method was used default is a 10 year cycle and it performs best on
from now on. The old system is not hard 10 year cycles (David 2005 and Keyser 2008).
to learn. If the system were changed The ingrowth modeled from 1998 to 2008 was
now, all data previously collected on then compared to the rapid assessment of
mistletoe could not be used in data Douglas-fir ingrowth completed in 2008. Since
analysis. The old rating system’s a walk-through tally of ingrowth shows that
numbers can not be assumed to reflect ingrowth was overestimated by the model for all
Hawksworth’s rating. plots (data not shown), the model was run with
and without the ingrowth data with the the brooms were shaded out. Most of these dead
assumption that the 2008 field collected dwarf brooms remained visible on the trees for the
mistletoe spread and intensification data should 2008 remeasurements. Many live brooms had
fall somewhere between the modeled data with several feet of shaded-out dead branches above
ingrowth and the modeled data without them and some contained live dwarf-mistletoe
ingrowth. shoots.
Very little within-tree dwarf mistletoe
RESULTS AND DISCUSSION intensification occurred from 1988 to 2008, with
a trend towards reduced DMRs for infected trees
Spread and Intensification. Thinning and (Table 3). Over ninety percent of the trees
pruning treatments done three and eight years remained within one DMR of the previous
after plot establishment confounded analysis of rating. The very low dwarf mistletoe spread and
spread and intensification of dwarf mistletoe intensification in the study area may have been
early in the study (Dooling et al. 1986). The due to poor dwarf mistletoe seed production,
project proposal (Dooling 1970) stated plots poor seed viability, shading out of Douglas-fir
would be established and treated in 1970 and re- branches, or a combination of these factors.
measured after three years followed by five year Dooling et al (1986) attributed little spread and
re-measurement intervals. However, it appears intensification differences between treatments in
pruning and thinning treatments continued 1983 to young stand age and low dwarf mistletoe
during the first two re-measurements. No intensity. A primary reason for little spread and
thinning or pruning was done after 1978. intensification within this stand is the lack of a
Douglas-fir dwarf mistletoe infected overstory
In 1988, 13 of the 25 plots contained dwarf (Hadfield, et al. 2000).
mistletoe-infected Douglas-fir trees (Table 1)
with all plots except 3, 24, and 25 containing less A trend toward increasing DMR developed in
than ten infected trees. In 2008, one additional the 14 X 14 thinning treatment from 1988 to
plot (plot 9) contained an individual infected tree 2008, while a trend toward decreasing DMR
and all plots except 20, 24, and 25 contained less developed in the control and 8 X 8 treatment.
than ten infected trees. In plot 3, 10 of the 16 The higher densities in the control and 8 X 8
infected trees in 1988 lost all live infections by treatments appeared to shade out dwarf mistletoe
2008 and one tree not infected in 1988 was infected branches more effectively.
infected by 2008 resulting in a net loss of nine
infected trees. Three of the dwarf mistletoe Growth and Yield. Given stand DMRs
check plots (no dwarf mistletoe removed) remained below a rating of one for all stands
contained no dwarf mistletoe and dwarf (except the dissimilar plot 25) and inadequate
mistletoe levels across most plots remained very replication due to no dwarf mistletoe infections
low during the 38-year period. in some plots, growth and yield impacts on crop
trees could not be expected. However, a trend
Percent infected Douglas-fir decreased in most towards reduced height growth after 38 years
of the infected plots (Table 1) over the last was apparent as DMR increased above a rating
twenty years. Plot DMR increased in about half of 3 or 4 (Tables 4 and 5). This is consistent
of the infected plots and decreased in half the with other studies that have shown no reduced
infected plots, while plot DMI (dwarf mistletoe height and diameter growth in lightly infected
ratings averaged over infected trees only) was Douglas-fir compared to uninfected Douglas-fir
unchanged, increased, and decreased in about (Mallams 2007).
one-third of the infected plots each (Table 2). In
plots where dwarf mistletoe decreased, some of
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