Structure and mortality of Tsuga heterophylla forests of the Olympic
Will Scharffenberger
Peninsula following a large-scale windthrow event in 1921 Capstone Advisor: James Freund
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Trees
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DBH (cm)
DNR 1 DBH Histogram
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ONF 2 DBH Histogram
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Figure 2: Diameter distributions of living trees broken down by species and year for each plot and all plots combined.
Table 1: Mortality data broken down by species, year, and plot.
TSHE
All transects DNR 1 DNR 2 ONF 1 ONF 2 ONP 1 ONP 2
Living trees in 2009
979
136
186
123
181
194
159
Living trees in 2013
959
130
173
123
181
193
159
Living trees in 2015
933
128
167
119
177
186
156
Annual Mortality 2009-2015
0.80%
1.01% 1.78% 0.55% 0.37% 0.70% 0.32%
Annual Mortality 2013-2015
1.36%
0.77% 1.75% 1.64% 1.11% 1.83% 0.95%
Table 2: General plot parameters including min and max DBH, s. dev, basal area, and density.
All Transects DNR 1 DNR 2 ONF 1 ONF 2 ONP 1 ONP 2
Min DBH
5.9
6.0
5.9 23.0
10.2
16.6
6.7
Max DBH
145.0 145.0
97.1 105.0
85.5
79.6
81.9
S. dev DBH
17.28 25.58 16.23 14.44 14.27 14.55 14.44
75.92 72.60 71.60 79.38 78.94 79.74 73.27
Basal Area in m2/ha
Tree Density in trees/ha
351.48 297.78 388.89 266.67 393.33 413.33 348.89
Snag Density in snags/ha
108.15 111.11 220.00 64.44 97.78 77.78 77.78
Results
Figure 1: Forest structure and composition of western hemlock forests 94 years following the 1921
windstorm event. a) DNR 1: This transect had the most small trees (<30 cm) and greatest variation (S. dev.
of diameter at breast height) in tree size overall. b) DNR 1: Medium diameter (30-70 cm) snags were common. c) DNR 2: A very uniform stand with most trees with similar DBHs, height class, and species composition. The stand also has little light penetration, and a understory consisting of mainly moss with a sparse
shrub layer. d) ONF 1: This stand exhibited a more developed understory as well as numerous trees with epicormic branches. e) ONP 2: Large diameter Sitka spruce were only found on this transect. f) ONP 2: A structurally complex western hemlock with a reiterated top and many witches’ brooms representative of a hemlock
dwarf mistletoe infection.
plots indicating the highest variability in diameter classes among plots (see Table 2). In contrast, ONF 1, ONF 2, and ONP 1 have the smallest standard deviation (14.25, 14.12, and
14.56 cm respectively) in their diameter at breast height (Table 2), indicating relative uniformity of size classes. While DNR 2 has one of the highest tree densities of the plots as well as
the highest snag density (388.89 trees/ha and 213.33 snags/ha), the basal area was among the
lowest observed at 71.81 m2/ha.
Mortality
The western hemlock mortality rate over the study period (2009-2015) for all the plots
was 0.8%, with individual plots varying from 0.32% to 1.78% (Table 1). Looking at change
in mortality rate, it is clear that there has been a 0.56% increase in annual mortality in the last
two years compared to annual mortality since 2009. From 2013-2015 rates of annual mortality for western hemlock range from 0.77% to 1.83% for all plots. Over all the plots, hemlock
mortality rates increased 1.7 times in the last two years compared to the last six (1.36% versus 0.80%). In ONF 1, annual hemlock mortality grew
Mortality by Cause
nearly 3 times when comparing the previous two years
to the entire study period.
15% 12%
Causes of mortality were variable, and were priUnknown
marily driven by biotic factors related to suppression or
Biotic
decay fungi such as Armillaria ostoyae. Few trees were
Mechanical
73%
killed by wind or any other mechanical breakage event
except in ONP 2 where windthrow was responsible for
2 mortalities.
Discussion
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Study site locations are composed of mostly closed canopy western hemlock stands with
little herbaceous understory. Understory composition was slightly variable among plots, but is
characterized by sword fern (Polystichum munitum), red huckleberry (Vaccinium parvifolium),
and salal (Gaultheria shallon) (Figure 1). Plots were established with dimensions of 150 m x
30 m.
Using a transect tape, laser rangefinder, and compass, tree stems were mapped from the
center line of the plot. Trees taller than breast height (1.37m) were tagged and their diameter
at breast height, alive/dead status, and species were recorded. Snags also had top diameter and
height recorded. A mortality check was conducted during 2010 and 2013 and a full re-measure
of all 6 plots was conducted during August 2015. This included measuring diameter at breast
height for living trees and snags as well as assessing mortality including causes.
Overall stand characteristics were calculated as a basic way to compare these plots to existing literature about forest structure (Table 2). Examining changes in tree growth was conducted
by creating histograms of the diameter distribution from data collected in 2009 and comparing
it with the diameter distribution from data collected in 2015. Annual mortality was calculated
for each plot and was assessed for a number of factors including which plot the mortality occurred in and the cause of death for each species. Annual mortality was calculated by the following formula:
m1 = 100 [1 — (N1 / N0)]1/t
where m1 is the one-year mortality rate, N0 is the number of stems alive in the previous measurement, N1 is the number of stems alive in the current measurement, and t is the number of
years between measurements (Lutz and Halpren, 2006).
PISI 2015
ABAM 2015
TSHE 2015
PISI 2009
ABAM 2009
TSHE 2009
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Methods
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In 1921 the largest wind storm in recent history (referred to as “the 21 blow”) leveled 1300
km2 of natural forests residing on the western Olympic peninsula between the Quillayute/
Bogachiel and the Hoh rivers (Van Pelt and Franklin, 2010). Western hemlock (Tsuga heterophylla) has become the dominant species in these stands, sometimes to the exclusion of any
other tree species. Some have suggested that stands such as these will undergo rapid mortality
during their second century of growth due to the ravages of wind as well as biological agents
like Armillaria fungus, resulting in long periods of open canopy conditions (Van Pelt and
Franklin, 2010).
Wind storms generally have a greater effect on the largest trees in a stand, especially those
with large, dense crowns, thin boles, and poor root systems (Curtis, 1943), while leaving the
suppressed, shade tolerant trees growing in the understory (Franklin et al. 2002).
In this study my objective was to better understand how western hemlock stands have developed following the large-scale blowdown event of 1921. My approach utilized 6 previously
established permanent sample plots set in areas which blew down in 1921 and exemplify hemlock dominance. Specifically, I pose three questions: (1) what are some structural characteristics of western hemlock stands which developed following a major wind disturbance? (2) how
does the level of structural development compare with the Franklin et al. (2002) conceptual
model of forest development? (3) what are the mortality rates in hemlock stands over the study
period?
Results (continued)
All Plots DBH Histogram
Trees/Plot
Introduction
Live tree diameter distributions
The all plot distribution exhibits a skewed normal distribution and relatively little variation (s. dev is 17.28cm) in the size classes of the trees among these stands (Figure 2).
Eighty-seven percent of the trees in all the plots are between 30 and 70 cm. However, individual plots do indicate some variation especially in the smaller trees (<30cm) and the larger
(>80cm) (Figure 2). Notably, ONP 2 has two large Sitka spruce, which are the only spruce individuals in the plots. DNR 1 was also rare in that it has many of its stems in the < 30cm size
range. ONF 1 and DNR 1 have the greatest number of large trees (>80cm) in their plots (at
55.6 trees/ha and 48.9 trees/ha respectively), including two large Pacific silver firs in DNR 1.
Indeed, both DNR plots were the only plots to have living silver firs including some saplings
in each (<10cm).
DNR 1 has the largest diameter at breast height standard deviation (23.00 cm) of all the
Stand structure
All sites exhibited similar growth patterns of increasing diameters which was particularly
concentrated in the larger size classes (70–90 cm). However, recruitment of trees into the smaller size classes is not readily occurring. Low recruitment of small diameter stems may be due to
competitive influences related to stand density, low light levels in the understory, or a lack of
sufficient substrate in the form of nurse logs. While the basal area of these stands (66-80 m2/
ha) falls generally in the range of old-growth Douglas-fir—western hemlock forests (64-74 m2/
ha) observed tree density (351 trees/ha mean) falls within the range of mature stands (Spies and
Franklin 1991).
Comparison to conceptual model of Franklin et al. 2002
These stands have moved past the canopy closure stage into the biomass accumulation/competitive exclusion phase and are moving onto the maturation phase of development (Franklin et al. 2002). Snag recruitment in the 50-70cm size class is evidence of previous developmental stages where competitive exclusion occurred among the hemlocks. Relatively low mortality rates among all sites over the study period are also evidence that these stands are beyond the
competitive exclusion phase and are in the mature forest phase. Overall low rates of mortality (~
1% annually) further support the categorization of these stands in the biomass accumulation/mature stage because this mortality rate is more similar to old growth Douglas-fir forests, with little
density dependent mortality (Borden, 2013).
Mortality
If annual mortality rates continue the trend observed from the 2013-2015 period, it may be
reasonable to conclude that these stands will develop a largely open canopy as described in Van
Pelt and Franklin (2010). However, this conclusion is limited because of the fact that in some of
these plots, (DNR 1 and DNR 2) there was an observed decrease in mortality. The evidence for
these stands suggests that they are attaining a mature forest state where mortality declines and
the understory re-establishes. Examining the mortality rates over the short period of study is insufficient to conclude that these stands are currently unraveling (2010).
As these forests continue to acquire biomass and move deeper into the maturation phase
they may have a lower density of trees, especially if small tree recruitment remains low. A lower density of trees might make these stands less resistant to wind because gaps allow wind to act
on a larger surface of exposed tree, leading to greater mortality (Curtis, 1943). The increased
effect of wind might be compounded by the abundance of trees which are predisposed to windthrow because of root diseases (eg. Armillaria ostayoe). Conversely, the development of larger
diameter stems and increased rooting surface area might give trees more wind resistance (Curtis,
1943).
Mortality of younger, smaller western hemlocks (~40 years) (Lutz and Halpren, 2006) was
much higher than those in old-growth stands (Borden 2013). The mean western hemlock annual mortality rates I observed from 2009 to 2015 are more similar (0.8%) to those observed in the
old-growth Wind River Forest Dynamics Plot from 2010 to 2012 (Borden, 2013), further evidence that these stands might be more stable that suggested in Van Pelt and Franklin (2010).
References
Borden, B. 2013. Assessing tree mortality in an old-growth forest: Wind River Forest Dynamics Plot 2010-2012. Senior Project Report, School of Environmental and Forest Sciences,
UW. Advisor: James A Lutz
Curtis, J. D., 1943. Some observations on wind damage. Journal of Forestry. 41, 12. 877–882.
Franklin, J.F., Spies, T.A., Van Pelt, R., Carey, A.B., Thornburgh, D.A., Berg, D.R., Lindenmayer, D.B., Harmon, M.E., Keeton, W.S., Shaw, D.C., Bible, K., Chen, J., 2002. Disturbances
and structural development of natural forest ecosystems with silvicultural implications, using Douglas-fir forests as an example. For. Ecol. Manage. 155, 399–423.
Lutz J.A., Halpern CB. 2006. Tree mortality during early forest development: a long-term study of rates, causes, and consequences. Ecological Monographs 76: 257–275.
Spies, T.A., Franklin, J.F., 1991. The structure of natural young, mature, and old-growth Douglas-fir forests. In: Ruggiero, L.F., Aubry, K.B., Carey, A.B., Huff, M.H. (Eds.), Wildlife and
Vegetation of Unmanaged Douglas-fir Forests. USDA For.
Serv. Gen. Tech. Rep. PNW-GTR-285, pp. 91–110.
Van Pelt, R. and J.F. Franklin. 2010. Forest structure and succession in stands initiated after the wind storm of January 29th, 1921.
Acknowledgements
This project was supported by funding and generous support from Kenneth L. Fisher. Many thanks to Dr. James A Freund for his mentorship and guidance
throughout this project. I would like to recognize Jerry Franklin and Robert Van Pelt for their scientific vision and hard work establishing these plots. A
special thanks to Andrew Jensen for his quality field work and to Scott Horton for logistical support. This work could not have been completed without the
help of many staff members from Olympic National Forest, Olympic National Park, and Washington State Department of Natural Resources.