Restoring sugar pine in the Tahoe Basin: regeneration ecology and recruitment dynamics
of sugar pine under various stand structures.
I. Project Team and Contact Information
a. Principal Investigator:
Kristen M. Waring (Northern Arizona University)
Co-Principal Investigator:
Kevin L. O’Hara (Univ. of California - Berkeley)
b. Institution:
Northern Arizona University
c. Address:
School of Forestry, PO Box 15018
Flagstaff, AZ 86011
d. Phone:
(928) 523-4920
Fax:
(928) 523-1080
Email:
Kristen.waring@nau.edu
e. Grants Contact Person:
Cindy Judge, Phone: 928-523-6917
Fax: 928-523-1075, email: cindy.judge@nau.edu
f. Theme 4, Managing the Basin’s Ecological Communities, Sub-theme A.
II. Justification Statement
Sugar pine (Pinus lambertiana) is a key conifer species of the Sierra Nevada mixed-conifer
forests that has been declining due to fire suppression, logging and white pine blister rust for the
past several decades (Kinloch et al. 1996, van Mantgem et al. 2004). Due to the relative shade
intolerance and high susceptibility of small trees to white pine blister rust (Cronartium ribicola)
mortality, recruitment of sugar pine into the overstory is limited. Furthermore, knowledge of the
regeneration ecology and recruitment dynamics are limited, as is the effect of management
activities on sugar pine regeneration. This research will investigate the ecology and dynamics of
seedling and sapling sized sugar pine. Results will be applicable to restoration treatments and
management of structures that will enhance the ability of this ecologically important species to
attain mature tree status and help maintain the long-term viability of sugar pine populations in
the Lake Tahoe Basin.
A primary objective of forest management in the Lake Tahoe Basin is to move more forest
stands into late successional / old forest status (Barbour et al. 2002). Additionally, many forest
stands are extremely dense and contain ladder fuels, both of which can contribute to crown fire
potential. Silvicultural treatments, including prescribed fire, thinning and regeneration methods,
are being implemented to restore forest structure to pre-settlement structures throughout the
Sierra Nevada. Multiaged management – relatively simple structures that maintain two or more
age classes (O’Hara 1998) – can lead to vertical and horizontal heterogeneity while meeting
multiple management objectives, such as passive crown fire reduction, aesthetics and sugar pine
enhancement. The proposed research will develop tools for multiaged stand management to
enhance sugar pine populations and meet fuel reduction and restoration objectives as well as
contribute to the knowledge of Lake Tahoe Basin ecosystems.
III. Background / Problem Statement
Sugar pine can be found primarily in the mixed-conifer forest type in the Sierra Nevada
Mountains and has the potential to reach the largest size of any pine species. The most common
conifer associates of sugar pine are ponderosa pine (P. ponderosa), Jeffrey pine (P. jeffreyii),
incense-cedar (Calocedrus decurrens), Douglas-fir (Pseudotsuga menseizii) and white fir (Abies
concolor). However, beginning in the early 1900’s, a combination of logging, fire suppression
and white pine blister rust led to an overall decline of sugar pine, particularly in the smaller
(seedling/sapling) size classes (Ahlstrom 1996). White pine blister rust arrived in northern
California in the 1920’s, proceeding southward and finally arriving in the southern Sierra Nevada
Mountains by 1970 (Kinloch et al. 1996). Genetic resistance is present in sugar pine and efforts
to breed sugar pine have been moderately successful. Although these breeding efforts are
ongoing, natural regeneration of sugar pine has been declining due to unfavorable forest floor
conditions and white pine blister rust mortality (Samman et al. 2003, van Mantgem et al. 2004).
Recent research has predicted that on long time scales, sugar pine is not reproducing in sufficient
numbers to sustain the population (van Mantgem et al 2004). Silvicultural treatments may have
the potential to enhance sugar pine regeneration and recruitment and reduce passive crown fire
potential (Stephens and Moghaddas 2005, Waring and O’Hara 2005). Historically, the
disturbance regime in Sierran mixed-conifer stands was low to moderate severity fire and
occasional mortality from native insects and diseases (Ferrell and Scharpf 1996, Kilgore 1973).
Stand-replacing events were probably uncommon. Current efforts underway to restore these
stands include thinning and prescribed burning to reduce density and increase light availability
on the forest floor (Stephens and Moghaddas 2005).
Sugar pine is moderate in shade tolerance, requiring some direct light for regeneration to
establish, survive, and increase in height (Oliver 1996). Fire suppression has led to dense layers
of more shade-tolerant species in the understory and sapling size classes, outcompeting sugar
pine for any available growing space and virtually eliminating direct light on the forest floor. The
greater shade tolerance of white fir, Douglas-fir and incense cedar allow them to outcompete
sugar pine in these understory environments (Gersonde and O’Hara 2005). Many sugar pine that
do establish are susceptible to white pine blister rust infection; in western white pine, rust
cankers are infrequent above 3m height (Hunt 1991). While height of infection has not been
quantified in a similar manner for sugar pine, observations support the idea that most infections
occur close to the ground. While mortality of sugar pine from white pine blister rust has been
patchier than other species of white pine, it has spread throughout the Sierra Nevada and is
considered a major factor determining the future health of sugar pine populations (van Mantgem
et al. 2004, Ledig 1992). However, relative shade tolerance of these species varies over a
gradient in overstory leaf area (Gersonde and O’Hara 2005). Previous research has shown that
sugar pine may respond favorably to increased light availability, allowing it to maintain itself
with the stand (Oliver 1996, Oliver and Dolph 1992, Waring 2005). Oliver and Dolph (1992)
report that sugar pine height growth under a ponderosa pine canopy was most favorable when
trees received 58% above canopy light when compared to trees receiving only 15% and 34%
above canopy light. Using a different methodology, Gersonde et al (2004) found that among
various canopy structures, none provided 60% above canopy light at 10m height above the
ground. However, the canopy gaps provided around 40%, followed by the irregular structure at
~30% and the two-storied structure at slightly <30%. Sugar pine regeneration was not an
objective in the Gersonde et al. (2004) research, so it is unknown whether these light conditions
provide for favorable sugar pine height growth or vigor in the seedling and sapling size classes.
In a survey of current stands classified as old-growth, Barbour et al. (2002) found that in the
mixed-conifer series, sugar pine was the 3rd most common species occurring in the overstory.
However, in the sapling size class, the sugar pine component was absent and only represented
9% of the understory conifers, dropping to the least common. These size classes were dominated
by white fir. This further indicates the lack of sugar pine recruitment to the overstory,
particularly in unmanaged stands where fire has been suppressed.
Research on sugar pine regeneration ecology and dynamics is lacking (van Mantgem et al. 2004),
particularly in the growth and development of small sugar pine following restoration treatments.
However, silviculture can play an important role in management of introduced pests (Waring and
O’Hara 2005) and restoration of old forest and pre-settlement forest structures. Multiaged forest
structures have the potential to meet multiple objectives in the Lake Tahoe Basin, such as
reducing crown fire risk, enhancing sugar pine growth and regeneration, and aesthetics. While
complex vertical forest structures currently exist across much of the mixed-conifer type (found
along the West shore of Lake Tahoe), these stands tend to be dominated by white fir
(Weatherspoon 1996). White fir increases structural complexity by regenerating prolifically and
surviving in very low light levels, creating ‘ladders’ of various size classes of trees. This
structure increases the risk of catastrophic wildfire and reduces light on the forest floor, in turn
leading to decreasing regeneration and survival of more shade intolerant species such as
ponderosa and sugar pine. Passive crown fire risk is increased when fuel continuity is high
between stand structural elements (Stephens 1998). The key to a multiaged approach is to reduce
fuel continuity by controlling white fir and provide sufficient growing space for sugar pine to
develop to maturity. Previous modeling research has indicated that group selection followed by
slash and landscape treatments reduces extreme fire behavior (Stephens 1998, Stephens and
Moghaddas 2005). However, this research did not include multiaged management. The proposed
research will model fire behavior under various stand structures, including multiaged structures,
using both the FARSITE (Finney 1998) and the Forest Vegetation Simulator – Fire and Fuels
Extension (FVS-FFE) (Reinhardt and Crookston 2003) models. FARSITE has previously been
successful in modelling fire behavior in Sierran mixed-conifer forests (van Wagtendonk 1996,
Stephens 1998) while FVS-FFE was specifically designed to simulate both vegetation dynamics
and fire behavior and effects (Reinhardt and Crookston 2003). The use of these models will lead
to valuable management recommendations regarding multiaged management, conservation of
sugar pine, and forest restoration that necessarily require consideration of fire behavior and
effects.
IV. Goals, Objectives and statement of hypotheses
The goals of this research are to enhance conservation and recruitment of sugar pine using
multiaged stand management and restoration strategies. We will meet these goals by quantifying
sugar pine regeneration and growth requirements / rates in the Lake Tahoe Basin mixed-conifer
forests. This information will be linked to multiaged management strategies that will lead to
more crown fire resistant forest structures and restoration of old forest structure. Fire is an
important component of the disturbance regime and a necessary consideration for any forest
treatment; fire behavior and effects will be simulated for different multiaged scenarios to ensure
that recommendations based on this research will not result in structures at high risk of
catastrophic fire. Additionally, conservation of an important component of the mixed-conifer
forest type – sugar pine -- will be enhanced through increased regeneration and survival of young
trees into the main canopy.
Our objectives are to:
1. determine the growth of sugar pine under varying light levels
2) develop relationships between light levels and stand structure
3) develop preliminary stocking control/stand structure guidelines to
manage light in multiaged stands
4) evaluate fire behavior in multiaged stands
5) develop recommendations for managing multiaged mixed-conifer stands to favor sugar pine
regeneration within certain fire behavior limits
To meet these objectives, the proposed research will evaluate second-growth mixed conifer
stands in the Lake Tahoe Basin. We hypothesize that sugar pine growth will increase with
increasing levels of light and that sugar pine survival into the forest overstory is dependent upon
light availability. The results will be used to meet objectives 3 and 5, creating guidelines for
managers that will both enhance sugar pine survival and growth and reduce passive crown fire
behavior. Objective 4 will be met through the use of fire behavior and effects models.
V. Approach, Methodology and Geographic Location
We will approach this research by locating forest stands exhibiting a variety of stand structures
and light conditions. These second-growth stands will be located on the west side of the Lake
Tahoe Basin. Target structures include unmanaged stands, multiaged stands and thinned and/or
burned stands but will depend on the range of conditions available in the Lake Tahoe Basin.
Stand composition will consist of sugar pine and at least 3 other conifer species: Jeffrey pine,
Douglas-fir, incense-cedar or white fir. Fixed area plots (1/100 ha) will be located on a
systematic grid through each stand, with a minimum of 20 plots will be established in each stand.
A smaller nested plot (1/1000th ha) will be installed at each plot center to intensively sample
trees less than 1.4 m tall.
Objective 1: Determine the growth of sugar pine under varying light levels.
At each plot, light will be indirectly assessed using basal area and canopy cover. Above each
seedling and sapling, a spherical densitometer reading will be taken to determine amount of
cover in that location. Basal area will also be assessed using a variable-sized prism plot at each
seedling and sapling as an additional measure of competition. Height growth and crown size will
be measured on each tree as well. If seedling and sapling numbers prohibit 100% sampling, a
subsample of each species except sugar pine will occur. All sugar pine will be sampled on all
plots.
Objective 2: Develop relationships between light levels and stand structure.
At each plot center, basal area and densitometer readings will provide an estimate of light and
competition. Additionally, a subsample of trees from all size classes will be cored to determine
age and stand history. Species, diameter at breast height and damage/defects/biotic agents will be
recorded for each tree in both plot sizes. Total height, height to live crown base, and crown radii
will be recorded on the same trees sampled for age. A relationship will result between light
levels (inverse of canopy cover) and measures of basal area and other measures of stand
structure.
Objective 3: Develop preliminary stocking control/stand structure guidelines to
manage light in multiaged stands.
Based on the relationship developed in objectives 1 and 2, guiding relationships for allocating
growing space among cohorts will be designed following the methodology of O’Hara et al.
(2003). The relationship between sugar pine seedling/sapling growth and light (or basal area)
will be the primary equation of this model. This will allow a user to design stand structures that
control competition of overstory trees to achieve the desired level of sugar pine growth in the
understory. These guidelines will allocate basal area or canopy cover among cohorts or canopy
strata to achieve these goals.
Objective 4: Evaluate fire behavior in mulitaged stands
The fire behavior model FARSITE will be used to evaluate potential fire behavior in each
sampled stand. Additional information on fire behavior and effects and forest vegetation
dynamics will be simulated using FVS-FFE. Within the 20-plot sample grid, every other plot will
be used to collect extra data for these models. Brown’s fuel transects (Brown 1974) will be
installed to estimate ground fuels. Crown bulk density, total tree height and height to live crown
base will be measured on a subsample of trees. Potential for passive crown fire occurrence will
be assessed by quantifying crown length and overlap between size classes. At each plot center,
aspect, slope, elevation and canopy cover will be recorded. The collected data will then be used
to run the FARSITE model and FVS-FFE for each stand to predict fire behavior in different
stand structures.
Objective 5: Develop recommendations for managing multiaged stands to favor regeneration of
sugar pine within certain fire behavior limits.
Linking relationships between stand structure and sugar pine seedling/sapling growth, and
relationships between stand structure and fire behavior will allow the development of integrated
guidelines for both sugar pine regeneration in complex stand structures and avoidance of stand
structures with excessive accumulations of fuels. Additional recommendations will address
species composition as well as stocking and stand structure providing the impetus for control of
more aggressive shade tolerant species such as incense-cedar and white fir. It is anticipated that
these structures will be multiaged, open, and consist of two to three canopy strata.
VI. Deliverables / Products
One Master of Science thesis at Northern Arizona University will result from this work. Two
scientific peer-reviewed papers are anticipated to result and will be suitable for publication in
forestry journals such as the Canadian Journal of Forest Research or Forest Ecology and
Management. Presentations at scientific meetings (at least 1 per year) are also anticipated.
VII.
Schedule of Events / Reporting and Deliverables
The proposed research will begin July 1, 2007 and end with a final report during spring 2010.
The funding will support one Master of Science student at Northern Arizona University for 3
years. Presentations will occur beginning in 2008; publications are expected to be completed in
2009.
Timeline
Site selection
Data collection
Analysis and
Interpretation
Publication Prep /
Writing
2007
2008
summer fall
Spring
XX
XX
XX
2009
summer
2010
fall
Spring
summer
XX
XX
XX
fall
spring
XX
XX
XX
XX
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Sci. 13: 461-472.
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