Using modelling to predict expected timber yields in red pine monocultures and in mixed species stands to assess timber losses due to Annosum root rot in the Midwestern United States A dissertation submitted in partial fulfilment of the requirements for the degree of Master of Science (MSc) in Environmental Forestry, Bangor University By Bryn Sitkiewicz BSc Forestry Management (2014, University of Wisconsin – Stevens Point) School of Environment, Natural Resources and Geography Bangor University Gwynedd, LL57 2UW, UK www.bangor.ac.uk Submitted in September, 2015
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Using modelling to predict expected timber yields in red pine monocultures
and in mixed species stands to assess timber losses due to Annosum root rot
in the Midwestern United States
A dissertation submitted in partial fulfilment of the requirements for the degree
of Master of Science (MSc) in Environmental Forestry, Bangor University
By Bryn Sitkiewicz
BSc Forestry Management (2014, University of Wisconsin – Stevens Point)
School of Environment, Natural Resources and Geography Bangor University
Gwynedd, LL57 2UW, UK www.bangor.ac.uk
Submitted in September, 2015
[i]
DECLARATION This work has not previously been accepted in substance for any degree and is not being
concurrently submitted in candidature for any degree.
Forest landowners are required to make decisions about species compositions based upon their goals
and their accepted level of risk. Previous studies have shown that landowners in the Midwestern
United States will plant red pine monocultures if they desire a high profit margin. A model-based
study was performed to illustrate the actual timber yield of stands with differing species compositions
in the presence and absence of Annosum root rot. Timber stands were cruised to determine basal
areas. These basal areas were used as a base to create a model used to simulate scenarios of future
timber yields of differing species compositions. It was found that when Annosum root rot is present in
a timber stand, stands containing a diverse species composition and have undergone several thinnings
will have a significantly higher actual timber yield than identically managed red pine monocultures. It
was further found that when trees are spaced closer together, there is a higher loss of timber due to
Annosum root rot. It is likely that landowners who have a high tolerance of risk will continue to plant
red pine monocultures, regardless of an impending Annosum root rot infection. Landowners who have
a lower risk tolerance are more likely to plant a mixture of species to counter the risk of a species-
specific disease.
[iii]
Acknowledgements
Thank you, Dr. Mark Rayment, for the guidance and support that you have given me throughout
this process. The motivation and the pep talks that you gave me in your office were much
appreciated, and the way that you were able to make sense of the jumble of questions and
concerns that I had in my head was invaluable. It was an honour and a pleasure working with you
this year.
A special thanks to Kevin Burns and UW-Stevens Point for allowing me to access school owned
tree stands at the Tree Haven research station and for providing me with equipment to complete
my research.
Also, thank you to Kyoto Scanlon of the Wisconsin Department of Natural Resources for
providing me with current information on the Annosum root rot situation in Wisconsin and for
connecting me with landowners with infected stands.
[iv]
Table of Contents
DECLARATION .................................................................................................................................................. I
ABSTRACT ......................................................................................................................................................... II
ACKNOWLEDGEMENTS .............................................................................................................................. III
2 – LITERATURE REVIEW .............................................................................................................................. 2
2.1 – BACKGROUND ............................................................................................................................................ 2 2.1.1 - History of Heterobasidion annosum in the United States ................................................................... 2 2.1.2 - Heterobasidion annosum Life Cycle ................................................................................................... 3
2.1.2.1 - Heterobasidion annosum Reproduction ........................................................................................................ 3 2.1.2.2 – Process of Infection ...................................................................................................................................... 4
2.1.3 – Role of Beetles.................................................................................................................................... 5 2.2 - SIGNS AND SYMPTOMS ................................................................................................................................ 5
2.2.1 Fungus Identification ............................................................................................................................ 5 2.2.2 Tree Symptoms ...................................................................................................................................... 5
3.1 –COLLECTING DATA IN THE FIELD .............................................................................................................. 19 3.1.2 – Description of Sites .......................................................................................................................... 19 3.1.3 – Timber Cruise Preparation .............................................................................................................. 20
3.1.4 – Timber Cruises of Sites .................................................................................................................... 22 3.1.5 – Determination of Basal Area ........................................................................................................... 22
3.2 –USING THE FVS MODEL TO COLLECT DATA ............................................................................................. 23 3.2.1 – Model Preparation ........................................................................................................................... 23 3.2.2 – Creation of the Models ..................................................................................................................... 24 3.2.3 – Running the Model ........................................................................................................................... 25 3.2.4 – Additional Models ............................................................................................................................ 26 3.2.5 – Limitations of FVS Modelling .......................................................................................................... 26
4.1 – MODEL OUTPUT FOR DATA COLLECTED IN FIELD .................................................................................... 27 4.2 – MODEL OUTPUTS FOR THEORETICAL STANDS .......................................................................................... 28
4.2.1 – Stands Spaced at 2.4m x 2.4m .......................................................................................................... 28 4.2.2 – Stands Spaced at 2.1m x 2.1m .......................................................................................................... 30 4.2.3 – Basal Area Losses across Both Spacings ......................................................................................... 32 4.2.4 – Additional Scenarios ........................................................................................................................ 34
LITERATURE CITED ...................................................................................................................................... 44
List of Figures and Tables
Figure 1 9
Figure 2 20
Figure 3 26
Figure 4 28
Figure 5 31
Figure 6 33
Figure 7 34
Figure 8 35
Figure 9 36
Figure 10 37
Figure 11 40
Figure 12 41
Table 1 38
Table 2 39
[1]
1 – Introduction
Making a forestry related decision is a high pressure activity since a single decision can
potentially impact timber yield and economic return for generations to come. Forest
managers must predict future environmental conditions such as climate change and pest
breakouts as well as future economical characteristics such as the demand for different
species of timber. As the field of forestry is constantly evolving, more tools are becoming
available to aid forest managers in making sound decisions. A spatially explicit forest
modelling programme is one such tool that can be used to portray expected future timber
yields of tree stands under differing environmental conditions. This allows forest managers to
weigh different management strategies and select which option best meets his or her
objectives depending on the risk the landowner is willing to take.
This study aims to investigate the effectiveness of a forest model used to predict potential
disease impacts prior to planting. In Wisconsin, red pine (Pinus resinosa) is a desirable
timber species, but it is associated with Annosum root rot (Heterobasidion annosum). Despite
high red pine timber losses due to this disease in the past decade, landowners continue to
plant red pine monocultures. The model developed in this study may be used to determine
whether or not planting red pine in mixed species stands results in a higher expected timber
yield than a red pine monoculture when considering the impact of Annosum root rot.
Four different species compositions are simulated in this study. The first is a red pine
monoculture consisting of 100% red pine. The second is a mixed stand containing 60% red
pine, 15% northern pin oak, 10% balsam fir, 7.5% quaking aspen, and 7.5% red maple. The
third is a mixed stand containing 40% red pine, 20% northern pin oak, 15% balsam fir, 12.5%
quaking aspen, and 12.5% red maple. The fourth is a stand containing 50% red pine and 50%
quaking aspen. The percentage of red pine in each stand represents a different level of risk,
based off of risk return curves created by Knocke (2008) as detailed further in chapter 2. In
order to validate the model created in this study, data collected in the field is used as
indicators of how accurate the model’s outputs are. In this study, several hypotheses are
examined:
[2]
1). A red pine monoculture that has been infected by Heterobasidion annosum will
have less expected timber yield at the end of a 70 year rotation than a red pine
monoculture that has not been infected by Heterobasidion annosum.
2). Planting red pine in a stand of mixed species will result in greater expected timber
yield at the end of a 70 year rotation than the expected timber yield of a red pine
monoculture if Heterobasidion annosum enters both stand types in the same years.
3). Performing a single thinning in a stand will reduce the rate of Heterobasidion
annosum infection and will result in a higher expected timber yield than infected
stands that have been thinned more than once.
4). Planting trees at a wider spacing will reduce the rate of Heterobasidion annosum
infection and will result in higher expected timber yield than stands that have trees
spaced closer together.
There are several published scientific articles that juxtapose the yields of monocultures and
mixed stands infected by Annosum root rot using models and data collected in the field.
However, these articles examine species that were identified as hosts to the disease, such as
Norway spruce and Scots pine, decades before red pine hosts were discovered. There is very
little material that examines expected timber yield in mixed stands containing red pine
infected with Annosum root rot. This study may benefit forest managers in areas where red
pine is a primary merchantable timber species since it provides predicted timber yield for a
variety of planting options in a landscape where Annosum root rot is a threat.
2 – Literature Review
2.1 – Background
2.1.1 - History of Heterobasidion annosum in the United States
Heterobasidion annosum is recognised as one of the most devastating diseases that affects
conifers in the north temperate region of the world (Scanlon 2008). Heterobasidion annosum
was first discovered in the United States in 1909 by E.P. Meineke. Meineke observed the first
documented case of Heterobasidion annosum in the United States on a Monterey pine (Pinus
radiata) in California (Smith 1989). The disease remained in the northwest United States
[3]
until the United States entered World War II. During the war, woody material from the
northwest was introduced to military camps across the country, primarily to the southeast.
Much of this transported woody material was infected with the Heterobasidion annosum
fungus (Asiegbu et al. 2008). As the disease expanded across the country, an increased
interest in tree disease prevention arose. The discovery and the awareness of Heterobasidion
annosum coincided with the evolution of the relatively new field of forestry (Smith 1989),
and stemmed research and interest in forest pathology. The diseases remained in the north-
western and the south-eastern parts of the United States for most of the 20th
century, and
infected the forests of the Lake States towards the end of the century. In 1993, the first case
of Heterobasidion annosum was discovered in Wisconsin (Scanlon 2008) and became the
destructive force that it is today in red pine plantations.
2.1.2 - Heterobasidion annosum Life Cycle
In order to comprehend the impact that Heterobasidion annosum is having on conifer species
in the United States and the effect that the fungus has on forest management, it is important to
understand how Heterobasidion annosum reproduces and how the fungus infects its host.
This knowledge can aid in formulating strategies to prevent or reduce the likelihood of the
disease entering a tree stand.
2.1.2.1 - Heterobasidion annosum Reproduction
The Heterobasidion annosum fungus is heterothallic (sex resides in different individuals),
multiallelic (affected by multiple genes), and unifactorial (an inherited characteristic is
dependent on a single gene) (Stambaugh 1989). These characteristics result in a fungus that is
very genetically diverse. This genetic variation means that it is difficult to find hosts that are
resistant to the disease. There are two strains of the fungus. The strain that is found in
Wisconsin is the P-strain, which primarily affects pines. There is also the S-strain in other
parts of the country which affects fir, hemlock, and Douglas-fir (Frankel 1998).
Heterobasidion annosum sexual reproduction begins when individual basidiospores give rise
to both male and female homokaryotic material. Male and female homokaryotic material fuse
together through the hyphae to form mycelium that is capable of fruiting. The mycelium
created is called a dikaryon, which form clamped septa during mitotic division of nuclei
(Chase 1989).
[4]
2.1.2.2 – Process of Infection
Heterobasidion annosum infects its host by releasing basidiospores upon maturity. The
spores are most often produced when the temperature is between 23 – 26oC and can be
dispersed up to 90m from the fungal source once airborne (Schwingle et al. 2003). Once
temperatures reach 35 oC, the fungus becomes inactive and can no longer produce
basidiospores (Otrosina and Cobb 1989).
The fungal disease enters a tree plantation once the basidiospores land on tree stump surfaces
(Chase and Ullrich 1983). A stump can remain susceptible to basidiospores invasion for up to
45 days upon being cut (Otrosina and Cobb 1989). The disease can survive in the stump for
up to 62 years after the tree has been felled (Asiegbu et al. 2008). From the stump, the
disease will then move downward to the root collar and to the roots, since Heterobasidion
annosum can penetrate and degrade woody tissue, lignin, and cellulose (Schwingle et al.
2003). Living trees surrounding the infected stump and roots can then become infected by the
disease if their roots are grafted or touching the infected roots (Chase and Ullrich 1983).
Once inside a living tree, the fungus moves up through the roots and enters the bole of the
tree as seen in Figure 1. The fungus then spreads at an average growth rate of 20cm to 50cm
annually (Asiegbu et al. 2008). Since Heterobasidion annosum spreads vegetatively from tree
to tree, the disease can be passed from generation to generation (Lygis et al. 2004).
Figure 1: Spread of Heterobasidion annosum through a tree stand. Source: G. Stanosz, U. Wisc - Madison
[5]
2.1.3 – Role of Beetles
Species of beetles (Dendroctonus valens and Hylastes porculus) have been observed to
facilitate Heterobasidion annosum invasion (Erbilgin and Raffa 2001). Beetles act as vectors
since they can transport basidiospores between stumps of felled trees. They spread the fungus
underground from the roots of an infected tree to those of an uninfected tree (Otrosina and
Cobb 1989).
Heterobasidion annosum can also facilitate a beetle invasion. Pine engraver beetles attack
trees that have been stressed by biotic or abiotic factors, and colonise conifers that have been
infected with Heterobasidion annosum. This is because the fungal root disease causes a
reduction in the tree’s ability to withstand pest invasions (Erbilgin and Raffa 2001).
2.2 - Signs and Symptoms
It is important to be able to identify Heterobasidion annosum by its appearance and by the
symptoms of infected trees. Detecting the pathogen early is crucial in creating an action plan
against the disease.
2.2.1 Fungus Identification
Fruiting bodies of Heterobasidion annosum begin to appear near a tree’s root collar in the
beginning of July (Schwingle et al. 2003). These fruiting bodies – called conks – can be
found in or on stumps from felled trees, under the forest’s duff layer on the root collar, or on
the roots of windthrown trees. The conks are shelf-like in appearance and have distinct
furrows along the edges. The furrows are dark brown with creamy white margins. The lower
surface contains many tiny pores (Frankel 1998). The conks are found on the stump or
directly under the duff layer on the root collar. On the exterior of roots of infected trees, dull
white ectotrophic mycelium can be found. The mycelium is one of the mechanisms that is
used to spread the disease through root connectionism (Schmitt 1989).
2.2.2 Tree Symptoms
Trees that have been infected with Heterobasidion annosum produce resinous white streaks
speckled with black flecks (Schmitt 1989). Reddish brown staining can be seen on the
exterior of the roots and on the lower stem (Frankel 1998). As the disease spreads, the tree’s
growth rate becomes stunted and its crown becomes thinner (Scanlon 2008). Butt rot will
appear in some species of infected fir (Frankel 1998). The crown of an infected tree will
appear rounded in shape (Byler 1989). Crown symptoms appear 3-8 years following a fungal
invasion (Schwingle et al. 2003). As trees near death, they will produce an abundant cone
[6]
crop (Byler 1989). If Heterobasidion annosum enters a stand, these symptoms will appear in
pockets within the forest. These pockets are called zones of mortality, from which the
epicentre expands outwards as more trees die (Erbiligin and Raffa 2001).
2.3 - Susceptible Sites
Different sites of conifer forests have varying susceptibilities to fungal root diseases. The
texture of the soil, the ph level of the soil and the landscape where the forest is on all
contribute to how hazardous a site is regarding Heterobasidion annosum breakout.
Recognizing the hazards of a site is important for a forest manager who must decide on the
care that must be taken when performing silvicultural prescriptions within the forest.
2.3.1 – Soil Texture
Some sites may be at more risk of Annosum root rot than others. Soil has the strongest
influence in the development of this disease since it provides the growth mechanism for root
diseases (Stambaugh 1989). In 1989, an Annosum hazard system was created based on a
site’s soil type. A site with a low hazard contains soils with clay and clay loams. An
intermediate hazard site contains loams and silt loams. A high hazard site contains any type
of sandy soil (Alexander 1989). A site with sandy soil is at most risk when there is an A
horizon (Alexander 1989) containing sand for at least 25 centimetres into the soil horizon
(Schwingle et al. 2003).This is detrimental for red pine plantations, since red pine prefers
sandy sites to grow.
2.3.2 – ph Level
Soil ph levels also play a role in the susceptibility of sites to Heterobasidion annosum. Soil
that is alkaline (ph > 6) is considered hazardous for fungal invasion. When soils are acidic,
there is rarely a tree mortality rate of over 5% if the disease enters the site (Stambaugh 1989).
2.3.3 – The Landscape
The landscape can influence whether a site is susceptible to the disease. Heterobasidion
annosum is commonly found in forests planted on former agriculture land (Schwingle et al.
2003) as well as on forested land that contains grass cover, or similar vegetation (Alexander
1989). Old forest soils are less susceptible to inoculation (Schwingle et al. 2003). Other
conditions where Heterobasidion annosum thrive include sites that have a fluctuating water
table (Pukkala et al. 2005) and sites that are susceptible to high levels of air pollution
(Stambaugh 1989).
[7]
2.4 – Impacts of Heterobasidion annosum
Heterobasidion annosum has infected valuable timber species across the United States and
has been reported to reduce timber yields over time. Red pine monocultures in Wisconsin are
experiencing high timber losses due to the disease. The mortality of trees is unavoidable once
the fungus enters a site.
2.4.1 – Susceptible Hosts
Although there are some reports of hardwood trees acting as hosts to Heterobasidion
annosum, conifers are much more susceptible (Scanlon 2008). In Wisconsin, red pine, white
pine, and red cedar have been reported hosts of the disease (Schwingle et al. 2003). Trees of
all ages are susceptible to the fungal disease (Asiegbu et al. 2008), but infection is most likely
to occur on stands that have undergone a first rotation (Pukkala et al. 2005).
2.4.2 - Heterobasidion annosum in Wisconsin
In Wisconsin, there has been a decline in red pine monocultures that are between the ages of
30-50 years due to the susceptibility of the species to the disease (Erbilgin and Raffa 2001).
This means that these stands can potentially contain 55% less basal area than red pine stands
that have not been affected by the disease (Frankel 1998). This has negative implications for
the forestry industry in Wisconsin since pines occupy 15% of Wisconsin’s total volume of
merchantable timber (Scanlon 2008).
Once a tree becomes infected with the disease, there is no way for it to recover (Asiegbu et
al. 2008). The fungus kills its host by slowly decaying the roots as well as destroying the
cambium that surrounds the root collar (Frankel 1998). Trees infected with the disease will
stay alive for many years (Byler 1989) and it is almost impossible to control the spread of the
fungus once it is present in a site (Scanlon 2008). Norway spruce has been observed to
survive an infection for the longest period of time over any other coniferous species (Pukkala
et al. 2005).
2.5 – Management Strategies
There are several management strategies that can be implemented in order to reduce or
mitigate the impact of the disease. Chemical and biological control methods may prove to be
effective in keeping the fungus out of a stand. Also, silvicultural tools can be put into place to
defend against the disease.
[8]
2.5.1 – Chemical Control
Applying specific chemicals to the stumps of felled trees is a common practice during a tree
harvest or a thinning. Granular borax is the primary chemical used, and is sprayed on a stump
immediately after a cutting. The chemical is designed to kill any basidiospores that may try to
inoculate stump surfaces (Alexander 1989). Applying granular borax has been an effective
preventative tool. However, it incurs an additional cost during a thinning or a harvesting and
in sites that have been severely infected by Heterobasidion annosum, granular borax is
useless (Scanlon 2008).
2.5.2 – Biological Control
Another tool available on the market to help prevent Heterobasidion annosum from entering
a site is Phlebia gigantean applications in the form of a suspension spray. Phlebia gigantean
is a natural fungal competitor of Heterobasidion annosum and may help to control the
pathogen if it has entered a stand (Alexander 1989).
2.5.3 – Silvicultural Treatments
Silvicultural tools are some of the most powerful means of defence that a forest manager may
have at his or her disposal when it comes to lessening the impact that Heterobasidion
annosum has on a stand. Planting trees at an optimal space, performing minimal thinnings,
and selecting to plant resistant species are all techniques that can be carried out to protect the
site from the fungal disease.
2.5.3.1 – Spacing
Planting individual trees farther apart from their neighbours may reduce the incidence of
Annosum root rot in a stand. Initially planting the trees further apart increases the length of
time before an initial thinning is needed (Stambaugh 1989). Basidiospores from the fungus
enter the stand through stump surfaces. By pushing the first thinning forward in time by
planting individuals further apart, there is a lengthened time period in which there is an
absence of stump surfaces that can be exposed to disease (Linden and Volbrecht 2002).
Wider spaces combined with mixed species planting can result in a reduction in potential root
contacts from infected individuals (Stambaugh 1989). The mixed species serve to buffer
infected roots from coming into contact with roots from susceptible individuals that have not
been infected. Asiegbu et al. (2008) observed that combining wide spacing with mixed
species planting can result in higher yield than pure plantations with normal spacing under
diseased conditions.
[9]
2.5.3.2 – Thinning Regime
Carefully planning a thinning prescription can help mitigate the effects that Heterobasidion
annosum can have on a stand of trees. A forest manager can reduce the chance of a stand
becoming infected by the disease by performing a thinning regime outside summer months
when basidiospores are dispersed (Asiegbu et al. 2008). It may be beneficial to perform these
thinning regimes when it is hotter than 35 oC; this is the temperature when the fungus
becomes inactive. This is practical in only certain regions of the country – such as the
southeast – that experience these high temperatures (Otrosina and Cobb 1989).
A forest manager can modify the intensity of a thinning to protect against Heterobasidion
annosum. This is done by reducing the amount of thinnings within a stand’s rotation.
Decreasing the amount of thinnings reduces the amount of opportunities that a fungus has to
enter a stand (Pukkala et al. 2005). Fewer thinnings can be accomplished by widening the
spacing between trees upon initial planting (Stambaugh 1989). Petersen (1989) suggests that
a rotation length for a stand of trees should not exceed 120 years if Heterobasidion annosum
is a threat. Minimizing the wounding of individual trees during a harvest or a thinning can
also prevent opportunities for the fungal disease to cause infection (Petersen 1989).
Selecting to perform a pre-commercial thinning may also be detrimental to the health of a
tree stand. Pre-commercial thinnings are executed early on in the rotation when young trees
that do not contain any economic value are removed. Heterobasidion annosum does not
discriminate by the age of a stump during infection (Asiegbu et al. 2008), so performing pre-
commercial thinnings may increase the risk of the fungus entering a stand. One study
observed that the rate of infection within hemlock stands that had been pre-commercially
thinned were eight times higher than that of hemlock stands that were not pre-commercially
thinned (Otrosina and Cobb 1989).
2.5.3.3 – Species Choice
Planting species of trees that are resistant to Heterobasidion annosum infection may help
prevent the disease from entering the stand in the first place (Asiegbu et al. 2008), and may
cleanse an infected site from the disease in the long term (Lygis et al. 2004). Several studies
indicate that deciduous trees are less susceptible to the disease than coniferous trees (Lygis et
al. 2004). Although there are records of most pine species in Wisconsin contracting the
disease, there are few reported incidences of white pine developing Heterobasidion annosum
[10]
(Schwingle et al. 2003). Spruce trees have minimal reports of infection (Linden and Volbecht
2002).
2.5.3.4 – Fire Management
Some studies indicate that prescribing a burning on a site can reduce Heterobasidion
annosum infection. One study observed that seven years following a prescribed burn, the
occurrence of Heterobasidion annosum was 55% less in plots that had been burned than
similar plots that had not been burned (Stambaugh 1989). A prescribed burn may be suitable
for a stand depending on the species within the site as well as where the site is located.
2.5.3.5 – Salvage Harvest
If a site is infected with the disease, a salvage harvest may be the only option. Salvage
opportunities are scarce within an infected stand since the rate of trees that are killed per year
is relatively small compared to a large disturbance, such as wind, in which a salvage harvest
would be more practical (Frankel 1998).
2.6 – Monocultures versus Mixed Species Stands
Depending on the goals of a land manager, he or she may choose to plant a monoculture or a
mixed species stand. Monocultures provide the advantage of ease and simplicity, whereas
mixed stands provide protection through biodiversity. Planting a mixed species stand may
combat Annosum root rot from entering a stand.
2.6.1 – Monocultures
A monoculture is a stand of trees containing identical species. Red pine in Wisconsin is most
often planted in a monoculture (Martin and Ek 1984). The practice of planting monocultures
is popular due to the idea that it produces maximum yield for a desirable species and that it is
easy and simple to manage.
2.6.1.1 – Simplicity of Monocultures
There are several reasons why planting monocultures is the preferred choice for land owners
in the timber industry. One of the largest reasons why monocultures are so popular is because
they are very easy to manage compared to mixed stands. This is because a forest manager is
able to concentrate all of his or her resources on favouring a single desirable species. Planting
monocultures is easy because only a single species is needed from a nursery (Piotto 2007).
Stand management is simple since row thinnings are performed usually under five times
during the rotation age of the stand. This results in an ununiformed harvest (Aikman and
Watkinson 1979).
[11]
2.6.1.2 – Convenience of Monocultures
Another practical reason for planting a monoculture is that fires are easy to control within
them. This is because there are trails and rows put into place that the fire crew can access
(Gadgill and Bain 1999). Monocultures are favourable in that they can be planted in any
advantageous pattern (Gadgill and Bain 1999). For example, trees within a monoculture can
be planted in the shape of chevrons. There is evidence that suggests that wind movement
through chevron planted monocultures reduces the chances of windfall damage (Niklas
1998).
Monocultures can be planted with species that are genetically modified to perform better in
different environments. Pinus taieda (L.) Englemann is a spruce that has been genetically
modified to resist fusiform rust (Gadgill and Bain 1999). Although a genetically modified
Heterobasidion annosum species has not been developed, it is still a possibility.
2.6.2 - Mixed Species Stand
A mixed species stand differs from a monoculture in that it contains more than one species.
Mixed stands are seen as more natural than monocultures, and many landowners are
beginning to discover the benefits of carrying out a mixed planting scheme (Kelty 2006).
Biodiversity conservation is one of the largest benefits associated with a mixed species stand,
and in some cases, they can result in higher timber yields than monocultures (Piotto 2007).
2.6.2.1 – Biodiversity in Mixed Stands
A mixed species stand may be more difficult to manage than a monoculture, but it contains
many advantages. Biodiversity conservation is one of the main benefits of planting a mixed
stand (Piotto 2007). A site containing more than one species of trees serves to protect the
overall stand if a species-specific threat enters the site. If a species-specific disease enters a
mixed stand, only a portion of the overall population will suffer, rather than the entirety. It is
more difficult for a pest or a pathogen to find a proper host if the concentration of hosts is
diluted by unsusceptible species (Kelty 2006). The diversity of trees in a mixed stand also
leads to the creation of diverse habitats. A range of habitats may support a range of natural
enemies to any pest species that enters the stand (Kelty 2006).
This tactic can be applied with the strategy of increasing the space between individual trees
when planting a forest. If Heterobasidion annosum infects an individual red pine, then a wide
space as well as an unsusceptible species may serve to buffer the further contraction of the
[12]
disease (Stambaugh 1989). This is why loses from an outbreak of Heterobasidion annosum in
a mixed plantation have been recorded to be lower than an outbreak in a monoculture
(Asiegbu et al. 2008).
In southern Sweden, Norway spruce and Scots pine were planted in the same stand.
Monocultures of each species were also planted. Each of the three sites was inoculated with
Heterobasidion annosum. After ten years, the mixed stand had a significant lower incidence
of the disease than the monoculture counterparts, due to the lack of root contact between each
of the two species. The best results were achieved when the mixtures was 50% Norway
spruce and 50% Scots pine. (Linden and Volbrecht 2002).
2.6.2.2 – Facilitation and Interspecific Competition
In some circumstances, a higher timber yield has been reported in mixed species plantations
over monocultures. This stems from higher diameter growth as a result of facilitation and
interspecific competitive production. Facilitation from nitrogen fixing species, such as alder,
has been observed in mixed species stands. According to the research of Piotto (2007), non-
nitrogen fixing species grow at a greater rate in the presence of these nitrogen fixing species.
2.7 – Risk
2.7.1 – Risk Awareness
Although planting a mixed species stand may seem to be the most logical route when
considering the threat of Heterobasidion annosum, there are many factors that play a role in a
landowner’s decision to manage a forest. According to Petersen (1989), some landowners are
ignorant to the devastation that root diseases can bring to a stand of trees, and there are not
enough forest managers in the field today who have taken any formal pathology course.
As reported by Lidskog and Sjodin (2014), landowners will act differently when faced with
risk due to several reasons. In Sweden in 2005, there was a devastating storm that felled 250
million trees; 80% of these trees were spruce since spruce trees are more vulnerable in storms
than other species. After this event, awareness of wind devastation among Swedish
landowners increased from 48% to 84%. This means that landowners become more aware of
a risk once they experience the risk first hand. However, 95% of these landowners still
replanted spruce to replace the fallen timber.
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The landowners gave several reasons as to why they still planted spruce, even though their
risk awareness about wind damage increased. Economic pressure plays a factor, since it is
cheaper to plant spruce over other species. Spruce is not the only species that is threatened by
external factors; other species of trees may be susceptible to other biotic or abiotic elements.
Uncertainty about future climate change was another concern. Also, the landowners were
familiar with spruce and were not certain on how well other species would grow on their soil;
they lacked the knowledge to manage other species. Lidskog and Sjodin (2014)’s findings in
this case study can be applied to other situations across the globe, such as landowners in
Wisconsin continuing to plant red pine plantations despite the threat of Heterobasidion
annosum. It is understandable that a land manager might not want to abandon his or her way
of managing a forest in order to embrace the knowledge of an outsider (Lidskog and Sjodin
2014).
2.7.2 – Risk Management
Since an investment in timber is a long term commitment due to lengthy rotation ages, it may
be important for a forest manager to take the proper steps to manage the risk involved.
According to Gardiner and Quine (2000), there are three main steps of risk management. The
process begins with a risk analysis in which potential hazards are identified and their
likelihood is estimated. The second step is risk handling. This phase involves implementing
alternative management strategies and calculating the opportunity cost of handling the risk
through different management techniques versus not handling the risk. Lastly, the risk control
phase implements the alternative management strategies and evaluates them through time
(Gardiner and Quine 2000).
According to Hanewinkel et al. (2009), there are three primary questions that a forest
manager needs to answer in the first step of risk management. First, it is important to
determine what can go wrong. Next, a forest manager should identify how likely it is that
something can go wrong. Lastly, the consequences of something going wrong should be
identified. The answers to these questions are not straightforward. The probability of specific
hazards differs within the spatial scale, and stakeholders do not all share a similar awareness
of risk. (Hanewinkel et al. 2009).
In the risk handling phase, there are two ways that risk can be handled: cause-oriented or
effect-oriented. Cause-oriented handling involves avoiding risk. This could mean ceasing to
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harvest or thin a forest to prevent Heterobasidion annosum from entering a site, or increasing
the stability of a forest by planting species that are resistant to disease. The goal of effect-
oriented handling is to decrease the damage of a risk, but not decrease the probability of the
damage actually happening. An example of effect-oriented risk handling is to diversify the
forest by planting a mixture of species, or insuring the timber with an insurance agency
(Hanewinkel 2009).
2.7.3 – Financial Implications of Risk
Investing in standing timber is considered a risky expenditure. This is because it is a long
term investment since tree stands may have lengthy rotations. There are many uncertainties
associated with forest investments such as fluctuating timber prices and the ambiguous
assumption that interest rates are held constant over the entire time period of the investment.
Therefore, it is important for a stakeholder to understand how to determine the risks involved
in forest investments.
2.7.3.1 – Risk Integration
Calculating the risk of a financial investment in timber has several challenges that are
associated. First, there may be multiple coinciding threats that need to be assessed. Another
challenge is being able to determine the net value of a stand at different rotation periods
considering associated risks (Hanewinkel et al. 2009).
Integrating risk into a financial investment involves four phases. First, a framework must be
created and analysed. This includes a development of all potential scenarios and hazards
associated with a timber investment. Next, the probability for each hazard must be
determined. This includes an estimation of the amount of damage associated with each hazard
as well as the probability of that damaging happening. The third part of the risk integration
process is an estimation of cost. The cost of risk reducing actions is compared to the cost of
not including risk reducing actions in a management plant. Lastly, the best choice of action is
selected based off of the previous benefit-cost analyses (Kurz et al. 2008).
2.7.3.2 – Risk Return Curves
One technique to determine the optimal species composition, depending on a stakeholder’s
acceptable risk, it to create risk return curves. A risk return curve is a graph that portrays
levels of risk involved with planting different percentages of a tree species within a mixture.
The risk return curve is then displayed on a graph containing utility curves, which are
standardised curves that reflect weak, normal, and strong risk-aversion scenarios. The y-axis
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contains standard deviations of the net present value of the species, and the x-axis portrays
the net present value of the species. The slope of each curve portrays the intensity of risk-
aversion, and the optimum percentage of the tree species within a mixture is where the slope
of the risk return curve and the slope of the utility curve meet. Any point lower than this is
less risky, and any point higher than this value is more risky (Knocke 2008).
Figure 2 is an example of a risk return curve. The risk return curve where k=0 shows what
risk is to be expected when combinations of spruce/beech affect the amount of risk. This
differs from the simple straight-line curve of k=+1, where the amount of risk and return grow
proportional to the amount of Norway spruce within the mixture. The normal equal utility
curve meets the risk-return curve where k=0 (for Norway spruce) at 54%. The other 46% is
allocated to European beech in this example. Norway spruce is a more valuable species than
European beech, yet it is more prone to disease and therefore more risky to plant. Hence,
planting 54% Norway spruce is the most justifiable mixture to plant in order to obtain a high
profit from the stand without taking a large risk. Depending on the amount of risk that a
landowner is willing to accept, a strong risk-aversion curve or a weak risk-aversion curve
(not illustrated in Figure 2) can be used. (Knocke 2008).
Figure 2: Risk return curve for Norway spruce in a European beech/ Norway spruce mixture
(Knocke 2008). A forest containing 100% Norway spruce is depicted on the right hand side of
the chart, and a forest containing 100% beech is depicted on the left hand side.
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2.7.3.3 – Future Discounting
Future discounting is a rate at which future benefits and costs are converted to a net present
value. It is important to be aware of the future discount rate when investing in a forest to
determine if it is worth taking a risk on the investment. If the net present value is greater than
0, then the project is efficient. If the net present value is below zero, then the investment is
not worthwhile (Hepburn 2006). When a discount rate is high (5% or higher), a present day
investment is not economically practical since there is little incentive to replant trees after one
rotation (Samuelson 1976).
A constant discount rate is most commonly used; a discount rate of 3% would remain at 3%
until the final stand rotation. A constant discount rate is risky for long term investments since
it ignores uncertainty of the future and assumes that the yield of the forest will not be
hindered by devastating abiotic and biotic disturbances. This has caused a trend of
stakeholders investing in short-term investments rather than in long-term investments.
A future discount rate that declines through time better protects the stakeholder from such
uncertainties. This is because an unknown hazard that may reduce timber yield is balanced by
a lower discount rate in the future (Hepburn 2006).
2.8 – Using Modelling in Forestry Applications
Investing in timber may have a high risk since it is a long term investment with a range of
uncertainties. It is impossible to look into the future, but spatially explicit models are useful
in portraying the outputs of possible scenarios under a variety of management strategies and
external biotic and abiotic factors. An effective model that assists land managers in making
decisions in regards to the threat of Heterobasidion annosum will produce yield outputs
based on the primary and secondary rates of infection, root contacts, and the development of
decay within individual trees (Asiegbu et al. 2008). The Western Root Disease extension
within the Forest Vegetation Simulator is one such model. Every model has its advantages
and disadvantages for different situations, and it is important to consider a specific problem
or objective before selecting an appropriate model (Korzukhin 1995).
2.8.1 – Spatially Explicit Models
Spatially explicit models have become a valuable tool for land managers studying the
population dynamics of a forest within a specific scale. A model that is considered to be
spatially explicit incorporates a population simulation within a landscape and its spatially
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distributed features (Dunning 1995). A spatially explicit model’s output will reflect the
response of trees within a constantly changing environment and the output is tailored to
individual situations since habitat-specific information is needed to run the model. This
allows managers to consider adaptive management strategies regarding species choice and
silvicultural treatments (Walters 1986).
These types of models are useful in portraying possible outcomes of a catastrophic event that
can impact the landscape at a large scale such as a wind event, insect outbreak, or disease
outbreak (Levin 1992). Using a spatially explicit model can help a land manager to compare
management techniques within complicated landscapes and can improve one’s ability to
understand how a landscape and its features correlate with tree growth (Dunning 1995).
Some models can be non-spatially explicit. These are useful in studying isolated processes
within the landscape. The physiography of the landscape is ignored since the arrangement of
habitats and tree stands are not taken into consideration (Dunning 1995).
2.8.2 – Empirical versus Process Models
2.8.2.1 – Empirical Models
Forest managers tend to favour using empirical models to aid decision making. Empirical
modelling is implemented when predications of management strategies are needed. The
output contains quantitative answers based off of yield and growth tables of different species
that have been pre-written into the model. An empirical model is the simpler of the two
models since the answers are produced in a short amount of time and based off of levels of
precision and accuracy that have been programmed (Korzukhin 1995).
Empirical models are most useful when they are used to produce statistical relationships
among data collected in the field in order to describe gathered knowledge and relate it to
management strategies. There are several limitations to using this type of model. Empirical
models are not as flexible as process models since the specifications used to create a model
must remain the same for every new condition or object that is added (Leersnijder 1992).
Empirical models are not as effective as process models if an increased database of
knowledge is required; the data that is inserted into the model is directly measured from the
specific condition that is designed to make the prediction (Wissel 1992).
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2.8.2.2 – Process Models
While an empirical model is used as a tool for predicting relationships and describing
knowledge, a process model is used as a tool for understanding relationships and developing
knowledge. This is because a process model is a representation of a hypothesis of how forest
structure and forest processes function (Korzuhkin 1995). Because process models revolve
around knowledge that is unknown, there are many parameters that are required to run the
models. Process models are most useful in situations where principle mechanisms are known
after there is an accumulation of knowledge through the use of empirical models (Wissel
1992).
Many claim that process models are limited since their high complexity makes it difficult to
produce a clear picture or prediction. Running a process model can also be more time
consuming. They do not produce as accurate or as precise outputs as empirical knowledge
since rigorous statistical testing cannot ensue (Korzuhkin 1995).
2.8.3 – Forest Vegetation Simulator (FVS)
The Forest Vegetation Simulator (FVS) is designed to predict forest stand dynamics in
United States forests. It is the most widely used forest modelling programme in the United
States. Agencies that regularly use FVS include the United Stated Department of Agriculture
(USDA) Forest Service, the United States Department of Interior (USDI) Bureau of Land
Management, the USDI Bureau of Indian Affairs, and many other state agencies (Dixon
2002). FVS is a spatially explicit empirical model.
FVS is designed to summarise current stand conditions, predict future stand outputs under
potential environmental factors and silvicultural prescriptions, and update tree inventory
statistics. This is a valuable tool for forest managers who are constantly under pressure to
create and carry out stand management alternatives in order to meet different objectives
(Dixon 2002).
2.8.3.1 – Western Root Disease (WRD)
Western Root Disease (WRD) is an expansion within the FVS modelling system. According
to Pukkala et al. (2005), WRD is the most comprehensive Heterobasidion annosum
modelling programme available to forest managers. WRD enables the user to juxtapose
future yields of healthy stands and Heterobasidion annosum infected stands and can be
manipulated to portray outputs under different silvicultural prescriptions. The output of WRD
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is in the form of tables containing information on the basal area of stands under different
conditions, as well as in the form of visual graphics and charts. Impacts of various levels of
the disease can be portrayed throughout different stages of management regimes (Frankel
1998).
3 – Methodology
3.1 –Collecting Data in the Field
3.1.2 – Description of Sites
To determine if Heterobasidion annosum has a significant impact on red pine plantations, a
timber cruise took place in an uninfected red pine plantation and in an infected red pine
plantation. The two plantations were located on similar sites within Portage County,
Wisconsin and were planted in the mid 1960’s. The soil type of each plantation was
determined by using the USDA (United States Department of Agriculture) Web Soil Survey,
which is a soil survey tool that is free to the public and can be found at: