Relationships Between Stand Age, Stand Structure, and Biodiversity in Aspen Mixedwood Forests in Alberta
A joint publication of the
Alberta Environmental Centre, Canadian Forest Service, Alberta Land and Forest Services
Full Report in PDF Format (1.4 MB)
EXECUTIVE SUMMARY
Resource managers and the environmental community are concerned that intensive
clearcut logging of Alberta's aspen-dominated boreal mixedwood forests at 60-70
year rotations may alter the age class structure of the forest landscape and
result in a change in forest structure and biota. In response to these concerns,
we described forest structure and composition of plant and animal communities in
young (20-30 years), mature (50-65 years) and old (120+ years) aspen mixedwood
stands of fire origin in Alberta. The information collected in this study will
serve as a reference against which structure and biota in harvested forests can
be compared.
Old stands were structurally distinct from younger seral stages. Their
uniqueness was related to a combination of canopy break-up and subsequent
release of understory plants, emergence of secondary canopy species (white
spruce and paper birch), the accumulation of deadwood, i.e., snags and down
woody material (DWM), and the presence of nonvascular communities that develop
on DWM. Relative to younger seral stages, old stands had trees of many ages and
had more large-canopy trees, large snags, and advanced rot-class large DWM.
Old stands had a deeper organic soil layer and greater microtopographic relief
than young and mature stands. The accumulation of organic material in the upper
soil horizon may be related to greater input of deadwood materials from the
canopy and subcanopy or to slower decomposition rates associated with cooler
soils found in old stands.
Young stands had features associated with old stands because of residual
materials from the previous stand cohort that escaped combustion during the fire
or because of light conditions that were similar to old stands. Attributes that
escaped combustion, or were created during fires, included large canopy trees,
large snags, and coarse DWM.
Relative to young and old stands, mature stands were simple in structure with a
closed canopy of aspen of relatively similar age, height, and diameter. In
addition, mature stands had a more open understory with fewer shrubs and
saplings.
Successional changes in stand heterogeneity were observed. For example, fires
produced spatially heterogeneous patterns in the densities of trees and deadwood
materials in young stands. The degree of spatial heterogeneity within and among
stands was lowest in mature stands. In old stands, densities of trees and
deadwood materials retained spatial homogeneity within stands; however, these
structures became more heterogeneous among stands.
Microclimate varied among stand age by season. During summer, young stands were
colder at night and warmer during the day than other stand ages. During winter,
old stands were warmer during the day and young stands were warmer during the
night than other stand ages. Soil temperatures below ground varied among season
and by stand age. Old stands had lower soil temperatures than other stand ages
in summer while young stands had the coldest soils in winter. Photosynthetically
active radiation (PAR) varied seasonally and was related to canopy and subcanopy
foliation and defoliation events. Mature stands had lower levels of PAR and
higher wind speeds than did young and old stands.
Herbaceous cover during summer was positively correlated with temperatures above
and below the ground surface. Shrub/sapling cover and depth of organic material
(DOM) were negatively correlated with air and soil temperatures during summer.
Soil temperatures were highest beneath forest canopies characterized by tall
sparse trees and decreased as shrub/sapling cover and DOM increased. PAR was
positively correlated with tall sparse trees, shrub/sapling cover, and DOM, and
negatively correlated with grass and herb cover.
Compositional changes in understory vascular plants during succession in aspen
mixedwood forests followed an initial floristics, rather than a relay floristics,
pattern. Species turnover (introduction/extinctions) from young to old stands
were relatively uncommon. Turnovers occurred mostly among uncommon species and
were independent of stand age. Approximately half of the species differed in
abundance among stand age. Of the species that changed, most either increased or
decreased monotonically with stand age. Successional changes and patterns in
understory plants were the result of changes in the relative frequency of core
species. The core group produced the different physiognomic profiles that were
associated with different stand ages.
Old aspen mixedwood stands had the greatest variety of woody substrates
available in different decay stages; consequently they supported the greatest
number of moss, liverwort, and fungi species. High variability of habitat types
in old stands may also explain why old stands had more rare nonvascular species
than either young or mature stands. These species were categorized into three
ecological groups: epiphytes (species commonly found on live trees); epixylics
(species typical of rotting wood). and terricolous species (species that use the
ground as their primary habitat). Different decay stages of DWM did not have
unique nonvascular assemblages due to high intergradation of species. Epiphytes
were more abundant on early decay stages, epixylics were more abundant on
intermediate decay stages, and terricolous species were more abundant on
advanced decay stages. The sensitive liverwort group was restricted to
intermediate stages of decay. Nonvascular species assemblages on similar decay
stages of DWM were different among stand ages, indicating that time, as well as
structural attributes, was important in determining species assemblages.
A rich biota in aspen mixedwood forests was identified including 47 lichen
species, 39 mosses, 7 liverworts, 24 fungi, 11 pteridiophytes (horsetails, club
mosses, and ferns), 65 herbs, 27 shrubs, 6 trees, 3 amphibians, 76 birds, and 33
mammals.
Old stands had greater bird species
richness than young stands, which had greater richness than mature stands.
Variation in abundance among stand age was evaluated for 33 bird species that
were detected ten or more times. Twenty-one species had their highest abundance
in old stands, nine species had their highest abundance in young stands, and
only three species had their highest abundance in mature stands.
Variation in bird abundance among stand age may have been caused by variation in
vegetation. During succession within aspen mixedwood forests, density of live
trees decreased, height of canopy trees increased, and density of large live
trees, large dead trees, and large DWM increased. As a secondary pattern during
succession, canopy and understory heterogeneity were greatest in old stands,
intermediate in young stands, and lowest in mature stands. Many attributes of
deadwood (snags and DWM) and understory plant communities were correlated with
variation in canopy heterogeneity. For most species of birds, abundances were
related to variation in one, or both, of these two major successional patterns
of forest community attributes. Abundance of 20 bird species was correlated with
successional stage, and abundance of 14 bird species was correlated with canopy
heterogeneity. Ten of the 21 species that were most abundant in old stands
preferred conifer-dominated mixedwood forests during the breeding season; they
may have been most abundant in old stands because those stands contained many
conifer trees.
Mammal species richness in summer was significantly higher in old stands than in
young or mature stands. During winter, mammal species richness was higher in old
and young stands than in mature stands. Variation in relative abundance among
stand age was evaluated for 19 mammal species that were detected ten or more
times. During summer, most species had their highest abundance in old stands
(9), followed by young stands (4), then by mature stands (3). Differences in
mammal abundance during winter were less obvious (2, 3, 1 species had their
highest abundances in old, young, and mature stands, respectively).
Changes in mammal species abundance were associated with forest community
attributes that varied in relation to successional stage (stand age) and canopy
heterogeneity. Complex old stands supported more species at high relative
abundance, structurally simple mature stands supported few species at high
relative abundance, and young stands that were intermediate in structural
complexity due to pre-fire residuals (snags, large live trees, DWM) supported an
intermediate number of species at high relative abundance.
Three mammal groups were examined in greater detail because of their economic
importance (ungulates) or because of their perceived sensitivity to logging
practices (bats, flying squirrel). During winter, forest structures that offer
thermal protection may limit deer abundance, while browse availability may limit
moose abundance. For example, relative abundance of white-tailed deer during
winter was significantly higher in old than in young and mature stands, a trend
that may be explained by greater availability of thermal protection (white
spruce) found proximal to shrubby browse in old stands. Relative abundance of
moose during winter was significantly higher in young and old stands than in
mature stands, a trend that may be explained by high abundance of shrub and
sapling browse in young and old stands. For both white-tailed deer and moose,
abundance did not differ among stand age during summer; thus, their abundance
seems to be affected by stand age only in winter.
Bat abundance was evaluated using echolocation calls and feeding buzzes, while
bats fitted with radio transmitters were used to locate roost trees. Adult and
juvenile little brown bat, silver-haired bat of both sexes, and one adult of
both hoary bat and northern long-eared bat were captured. Nine little brown bat
and five silver-haired bat were radiotagged and followed to 23 roosts, all in
Populus spp. trees. Mean roost tree height and DBH were 21.8 m and 41.4 cm,
respectively, for little brown bat, and 22.4 m and 41.3 cm for silver-haired
bat.
Myotis spp., silver-haired bat, hoary bat, and big brown bat were detected in all stand ages. Myotis spp. constituted the highest proportion of total activity (#passes/hr) in both years. During 1993, more old stands than young stands were used by Myotis spp. and all bats in the first hour post-sunset; during 1994, more old stands than both young and mature stands were used. The distribution of bat activity in the boreal forest was related to the density of trees in the forest, the presence of large gaps in the canopy, and the abundance and distribution of roost trees in stands of different ages.
Flying squirrel abundance was higher in
old stands than other stand ages and was positively related to conifer and
shrub/sapling densities. Although flying squirrels frequently used cavities with
small entrances in large diameter aspen snags, preference for den trees,
relative to availability, appeared to be based on the following characteristics:
live trees, low decay stage, high percent of bark remaining, a single cavity in
the tree, and a large cavity entrance.
Much of the structural complexity (residual green trees, snags, DWM) found in
young stands is related to the variable manner in which fire burns through aspen
mixedwood forests. Furthermore, these structures appear to be highly related to
the presence and abundance of many different species of plants and wildlife. If
this general connection between fire, stand structure, and biodiversity exists,
then resource managers must compare structures retained by natural disturbances
(e.g., wildfire) to those left by human-caused disturbances (e.g., clearcuts).
The current and dominant form of forest management in Alberta's mixedwood
forests includes unstructured clearcut logging, a narrow range of rotation ages
leading to a constant-age class merchantable forest, silvicultural enhancement
for conifer regeneration, and the administrative separation of the hardwood and
softwood landbase. These forestry practices are fundamentally different from
fire as a stand-initiating event and, when implemented over periods as short as
one rotation, will lead to a forest landscape that is both different and more
uniform than those found presently. Changes to the forest landscape caused by
current forestry practices include the loss of structural complexity (green
trees, snags, DWM) in young forests, loss of older forest stages, and the likely
spatial separation of aspen and conifers. The transformation of the boreal
forest landscape, in turn, is likely to alter the composition and abundances of
biota and the nature of ecological processes. Current forest management
practices, therefore, are inadequate to ensure the ecological integrity of
Alberta's boreal mixedwood forests. Results of this study offer a template for
incorporating the natural dynamics of snags, DWM, and conifer regeneration into
forestry operations.
Provincial government agencies and the forest industry are therefore encouraged
to:
The implementation of these recommendations may affect future production and availability of tree fiber for harvest from Alberta's aspen mixedwood forests. Changes to forest harvest levels will be affected by the degree to which the recommendations are adopted (i.e., how much structure is retained on cutblocks, how variable rotation ages become, how much wildfire occurs on the landscape, how much area is assigned to unharvested reserves). For example, the retention of high densities of live aspen trees on cutblocks for purposes of wildlife habitat or ecological processes will likely reduce post-harvest aspen regeneration and, hence, future fiber production. A review of the social and economic consequences of the proposed recommendations is required to arrive at sustainable management goals that meet ecological, social, and economic criteria. However, resource managers must recognize that all forest-based land-uses and social benefits (i.e., commercial forestry, hunting, aesthetics, bird-watching, etc.) ultimately depend on functional forest ecosystems; hence wise forest stewardship should be the primary and over-riding societal objective.
J. BRAD STELFOX, Project Leader,
Forest Ecologist; Alberta Environmental Centre, Vegreville, AB.
LAURENCE D. ROY, Research Scientist, Wildlife Biologist; Alberta
Environmental Centre, Vegreville, AB.
PHILIP C. LEE, Research Scientist, Plant Community Ecologist; Alberta
Environmental Centre, Vegreville, AB.
JIM SCHIECK, Research Scientist, Ecologist; Alberta Environmental Centre,
Vegreville, AB.
SUSAN CRITES, M.Sc. Candidate, Plant Ecologist; Department of Biological
Sciences, University of Alberta, Edmonton, AB.
MARIE NIETFELD, Research Scientist, Wildlife Biologist; Alberta
Environmental Centre, Vegreville, AB.
JACK NOLAN, Senior Technician, Wildlife Biology; Alberta Environmental
Centre, Vegreville, AB.
LISA H. CRAMPTON, M.Sc. Candidate, Ecologist; Department of Biological
Sciences, University of Calgary, Calgary, AB.
LISA MCDONALD, M.Sc. Candidate, Ecologist; Department of Biological
Sciences, University of Alberta, Edmonton, AB.
DAVID CHESTERMAN, M.Sc. Candidate, Geographer; Department of Earth and
Atmospheric Sciences, University of Alberta, Edmonton, AB.
LEN PELESHOK, Technician, Wildlife Biology; Alberta Environmental Centre,
Vegreville, AB.
KELLY STURGESS, Technician, Wildlife Biology; Alberta Environmental
Centre, Vegreville, AB.
DELINDA RYERSON, Research Technician, Database Coordinator; Alberta
Environmental Centre, Vegreville, AB.
DANI WALKER, Technician, Wildlife Biology; Alberta Environmental Centre,
Vegreville, AB.
GREG RADSTAAK, Wildlife Database Coordinator; Alberta Environmental
Centre, Vegreville, AB.
ZACK FLORENCE, Biometrician; Alberta Environmental Centre, Vegreville,
AB.
HAI VAN NGUYEN, Biometrician; Alberta Environmental Centre, Vegreville,
AB.
ROBERT M.R. BARCLAY, Associate Professor, Ecologist; Department of
Biological Sciences, University of Calgary, Calgary, AB.
DAVE HALLIWELL, Research Scientist, Meteorologist; Canadian Forest
Service, Edmonton, AB.
PETER KERSHAW, Associate Professor, Biogeographer; Department of Earth
and Atmospheric Sciences, University of Alberta, Edmonton, AB.
MARK DALE, Professor, Plant Community Ecologist; Department of Biological
Sciences, University of Alberta, Edmonton, AB.
Preface
Executive summary
Research team
Acknowledgements
1. Introduction
2. Relationships between microclimate and stand age and structure in aspen
mixedwood forests in Alberta.
3. Changes in forest structure and floral composition in a chronosequence of
aspen mixedwood stands in Alberta
4. Changes in snags and down woody material characteristics in a chronosequence
of aspen mixedwood forests in Alberta
5. Changes in understory composition for a chronosequence of aspen mixedwood
stands in Alberta
6. Relationships between nonvascular species and stand age and stand structure
in aspen mixedwood forests in Alberta.
7. Bird species richness and abundance in relation to stand age and structure in
aspen mixedwood forests in Alberta.
8. Relationships between mammal biodiversity and stand age and structure in
aspen mixedwood forests in Alberta.
9. Abundance of ungulates in relation to stand age and structure in aspen
mixedwood forests in Alberta.
10. Relationships between bats and stand age and structure in aspen mixedwood
forests in Alberta.
11. Relationships between northern flying squirrels and stand age and structure
in aspen mixedwood forests in Alberta
12. Changes in vertebrate communities in relation to variation in forest
community attributes: a comparison of bird and mammal communities
13. General summary
14. Recommendations
15. References
i. Glossary
ii. List of abbreviations
iii. Species list
iv. Stand maps
Intensive forest harvest (e.g.,
clearcut logging of 60-70 year rotations) in boreal mixedwood forests, by
simplifying the structure of young stands and reducing the frequency of older
stands, will not maintain abundances of flora and fauna at levels found in
unmanaged forests.
In recognition that conservation of ecological processes and biodiversity
requires a landscape approach embodying both commercial forests and reserve
networks (Thomas et al. 1990; Franklin 1993), this chapter describes four
components of aspen mixedwood forest management (structuring forests, variable
harvest rotation intervals, maintaining mixedwood forests, and mixedwood forest
reserves) that require implementation. These practices, although important to
correcting current problems with forest management, must be complemented by a
societal philosophy that recognizes the value of stand structure and landscape
forest patterns in maintaining biota prior to decisions involving allocations of
trees to fiber production. Such a new approach differs markedly from previous
annual allowable cut (AAC) calculations in Alberta that were based on
merchantable landbase, wood growth and yield projections, and optimum
"fiber" rotation age.
A new forest management paradigm receiving considerable attention in North America argues that managed forests should approximate stand and landscape patterns historically created by natural disturbance regimes (Hansen et al. 1991; Hunter 1993; Swanson et al. 1993). The basic assumption of this management model is that plants and animals are adapted to natural disturbances and are more likely to maintain viable populations if human land-uses create forest structures similar to those left by fire, insect outbreaks, floods, and windthrow.
Stand-replacing fires are considered the dominant disturbance agent of the boreal forest (Rowe and Scotter 1973; Van Wagner 1978; Johnson 1992) and are largely responsible for maintaining variance in stand age, size, shape, and structure (Zackrisson 1977; Eberhart and Woodard 1987; Engelmark et al. 1993). As such, commercial forests with structural variability similar to those of fire-dominated landscapes may offer a preferred risk management strategy for protecting both biota and ecological processes in Alberta's boreal mixedwood forests.
The considerable variance in plant community structures that occurred within and between seral stages, and the variety of forest community attributes to which abundances of wildlife and plants species were correlated, indicate that reliance on a "fine filter" species-by-species approach to conserving biodiversity is unlikely to succeed. Such a species-based approach will lead to conflicting objectives involving species with disparate habitat requirements. Opportunities to resolve conflicts and manage all species are further constrained because the majority of biota in mixedwood boreal forests are invertebrates, and many of these taxa have yet to be identified or their life histories described.
If the most salient features of aspen mixedwood forests are a diverse biota associated with a diversity of forest structures, forest management approaches that propagate appropriate frequencies of structures in space and time may hold the greatest promise for maintaining the integrity of this ecosystem.
The following recommendations are
logical extensions of findings from this study and are consistent with the
principles of forest ecosystem management.
1. Maintain Structure in Forests
Abundances of many species of plants, birds, and mammals were positively related
to residual structures (green trees, snags, down woody material [DWM]) that
originated from the previous stand cohort or were created by the
stand-initiating fire.
Therefore, to maintain plant and wildlife communities in young forests,
logging practices should more closely approximate the amount and variability of
structures created or retained by fire.
Current wood utilization standards in Alberta, however, encourage clearcut
logging that removes all merchantable tree biomass from cutblocks (Alberta
Timber Harvest Planning and Operating Ground Rules 1994).
A presumed advantage of retaining structures on cutblocks is the reduction in
years required before post-harvest stands acquire characteristics associated
with old stands. For example, retaining green trees at harvest will lead to the
production of large trees, and subsequently to large snags and canopy gaps
earlier than would occur in a even-age stand of trees following unstructured
clearcut logging.
Clearly, fire and clearcut logging differ in the amount of wood removed from the
stand and in the structure of the new stands they initiate. Logging companies,
however, are reticent to retain amounts of structure similar to those found in
young stands following fire, as such a proposal would require a significant
reduction in the annual allowable cut (AAC) of many companies (Maser 1994). A
more moderate approach involves the modification of the cutblock utilization
standards that direct logging practices. Rather than requiring the forest
industry to remove all merchantable wood from cutblocks, regulations should
encourage retention of structure in the form of live trees, snags, and DWM.
Although we do not know the precise amounts of structure required on mixedwood
cutblocks to maintain wildlife populations or ecological processes, our general
understanding indicates that significant increases in retention of live trees,
large snags, and DWM are warranted.
As a first approximation to maintaining the ecological function of young aspen
mixedwood forests, managers should consider retaining snags and DWM in amounts
similar to those of pre-fire origin measured on our young stands. These residual
levels can be viewed as conservative estimates, as our young stands had 20-30
years since the fire event for residual green trees to become snags, for snags
to fall and become DWM, and for DWM to rot. In young stands, snags greater than
10 cm DBH had mean densities of 19/ha (13 S.E.M), while coarse DWM volumes were
~50 m3/ha (12 S.E.M.). As abundances of both snags and coarse DWM were highly
variable, it is important to maintain stand-to-stand variation in these
structures.
Determinations of which stands are to be highly structured and which stands are
to be lightly structured will depend in large part on the amount of residual
trees, snags, and coarse DWM that exist at each cutblock at time of harvest.
Where possible, the distribution of DWM on cutblocks should approximate the
patterns found in young forests following fires. As such, localized
concentrations of DWM, such as tall slash piles near landings of whole-tree
harvesting systems, should be discouraged. Stump-side delimbing, where feasible,
provides a mechanism for maintaining DWM on the cutblock. Where whole-tree
harvesting systems are used, slash should be redistributed throughout the
cutblock using skidders. In cutblocks where soil compression is a serious
concern, slash could be re-distributed during the winter season.
Recently, forest companies have intentionally retained live trees and snags in
clumps within cutblocks to approximate variation created by fire (Plates 35, 36,
37). The practice of clumping residual trees, although advisable, should not be
implemented exclusively until the prevalence of clumped and dispersed residual
materials have been measured, and their effects on forest flora and fauna
assessed. If a goal of forest ecosystem management is to approximate the
variability in green trees, snags, and DWM left by fire, then a variety of
forest harvest strategies may have merit, including unstructured clearcuts,
structured clearcuts, patch cutting, shelterwood harvest, and selection logging.
Constraints and Research Needs
Although our project provides a general insight into the importance of DWM,
further research on which forest structures to retain at harvest, and in what
abundances and locations, needs to be conducted to determine a fair compromise
between the long-term ecological and economic sustainability of public forests.
Although patterns of retention for green trees and snags following fire may
offer a general template for harvest planning, further research is required to
indicate the most appropriate frequencies and locations of each attribute on the
cutblock. Proper forecasting of these structures during succession in managed
forests will require research on decay rates of snags and DWM, success of green
tree retention, and fall down rates of snags in post-logged forests. Stand level
research is also needed on optimal frequencies of snags required to maintain
populations of cavity-dwelling birds and mammals, optimal levels of DWM required
to maintain decomposition processes, nonvascular populations, and soil
fertility, and optimal levels and size of live trees retained to produce a
spatially heterogeneous canopy and canopy gaps.
Retention of live trees and snags does not constitute a safety hazard for
mechanical harvesters in aspen forests where regeneration is vegetative (Steneker
1976), but standing snags are considered dangerous when manual felling is
involved or when planting and tending operations occur subsequent to harvest
(Occupational Health and Safety Act 1983). Further research is required to
evaluate what types and placement of residual materials are required to maintain
ecosystem health while securing worker safety.
However, ignoring the ecological importance of forest structures for reasons
of human safety is not an acceptable rationale if such practices compromise both
forest integrity and the ability of forests to provide wood products over the
next several rotations.
Managers should understand that anthropic reconstruction of fire-related
structures or "old-growthness" in managed forests will be of limited
value if the ecosystem processes which maintain the integrity of forests are
impaired through intensive forestry.
Other potential problems with reconstruction of stand structure include: 1) many
forest community attributes covary and it is unclear which are most important to
retain, 2) some forest components may be time dependent and can not be rushed,
and 3) stand level treatments of structure may not provide adequate habitat
unless landscape level treatments are also implemented.
2. Adopt Variable Rotation Ages for Forest Harvest
Forest companies in Alberta commonly manage the merchantable forest landscape
with a narrow range of harvest rotation ages to facilitate a constant forest age
class frequency that provides an even timber supply (Alberta Timber Harvest
Planning and Operating Ground Rules 1994). The 60-70 year rotation that is used
by the forest industry for Alberta's aspen mixedwoods approximates maximum
annual increment (MAI) of aspen and was chosen to maximize wood production (Kabzems
et al. 1986; Peterson and Peterson 1992). Although stands of this age have
favorable live tree characteristics for pulp production, they are structurally
simple (closed canopy, few large trees, snags, and DWM). In general, a
commercial mixedwood forest landscape would have a truncated age class and have
fewer of the structures that are associated with old stands. Although data
confirming this pattern are unavailable for comparisons of unmanaged and
commercial boreal mixedwood forests, many species of plants and wildlife in
Pacific Northwest forests have been adversely affected by intensive forestry
(Ruggiero et al. 1991; Thomas and others 1993). These studies raise concerns
whether recurrent logging of 60-70 year rotations can maintain viable
populations of plant and wildlife species whose preferred habitat is late
successional aspen mixedwood forests.
One corrective approach to this issue is for the forest industry to
de-emphasize their reliance on a narrow rotation interval and a balanced age
class structure of the forest, and to select a range of rotation ages that more
closely approximates the variability produced by natural fire return intervals.
Research on fire frequency in boreal mixedwood forests in Alberta suggests that
fires were generally frequent with a short fire return interval and created a
landscape where frequency of stands declined with increasing stand age (Murphy
1985). Based on this natural pattern, a smaller portion of the landscape could
be harvested at long intervals while the majority of stands would be harvested
at shorter return intervals.
The practice of rushing mixedwood forests through early successional stages,
using softwood planting and tending techniques, may have adverse ecological
effects. For example, our study identified shrubs and aspen saplings as equal to
or more important than canopy and deadwood structures to which vertebrate
species abundance was positively related. The low frequency of young (0-20
years) stands in the boreal mixedwood forests of northeast Alberta, possibly the
result of fire suppression, may have caused shrubs and sapling stands to be
limiting to some bird and mammal populations. However, high rates of logging on
mixedwood forests using natural regeneration of aspen should significantly
increase abundance of both shrubs and saplings during the next few decades.
Constraints and Research Needs
Far more research is required to quantify the spatial and temporal variance in
fire return intervals and stand age before society and industry can evaluate the
optimum landscape age structure that meets both ecological and economic goals.
The use of a fire disturbance model as a basis for harvest rotation intervals is
predicated on the assumption that logging will largely replace wildfire on the
mixedwood forest landscape. Such a transition appears to have occurred in many
U.S. forests (Agee 1994; Johnson et al. 1994), but fires in boreal mixedwood
forests may be less controllable (Attiwill 1994).
3. Maintain the Mixedwood Forest by Adopting a Mixedwood Management Model
Many species of plants and wildlife are associated with stands containing
both aspen and spruce, yet forest regulations may lead to the "unmixing"
of hardwood/softwood stands.
Although monocultures occur naturally in the boreal mixedwood forest, most stand
canopies contain both hardwood and softwood tree species (Alberta Forest Service
1985; Peterson and Peterson 1992). Current forest management regulations in
Alberta require FMA holders operating on a mixedwood landbase to regenerate
softwood volumes equal to those harvested (Alberta Timber Harvest Planning and
Operating Ground Rules 1994).
The most common practice for spruce regeneration involves site preparation
(usually scarification), manual planting of seedlings, and subsequent stand
tending to reduce competition of seedlings with surrounding plants (Smith 1986).
Because much of the spruce harvested from previously unharvested mixedwood
forests are solitary or from small clumps <5 ha, it is a common practice for
managers to aggregate spruce regeneration at larger spatial scales (20-40 ha;
Alberta Timber Harvest Planning and Operating Ground Rules 1994). This softwood
regeneration strategy, coupled with short-rotation aspen logging where site
preparation for conifers does not occur, is likely to alter successional
trajectories and lead to spruce stands and aspen stands that are separated
spatially. On mixedwood cutblocks where conifer regeneration is not encouraged
by silviculture, spruce densities may decline because few cone-bearing spruce
trees may be present and/or appropriate germination substrate (large DWM) may be
absent (Kabzems and Lousier 1992).
Thus, current silviculture practices encourage monodominant stand canopies,
and may unmix the tree species that define the mixedwood forests (McDougall
1988).
"Unmixing the mixedwood forests" may jeopardize wildlife species and
ecological processes reliant on a mixed-species tree canopy. For example, the
relative abundances of approximately 25% of bird and mammal species in this
study were positively related to densities of white spruce. Conifer trees, by
providing forage, cover, or nesting resources, may allow some species to use
mixedwood forests that otherwise would not.
Alternative silvicultural strategies that encourage natural regeneration
strategies for spruce in aspen mixedwood stands, are required.
One option involves leaving some spruce trees to serve as current or future seed
sources. Conifers retained during harvest may experience less wind-related
mortality if they are positioned at the upwind border of the cutblock (Navratil
et al. 1994) or if they are protected from wind by other conifers or aspen trees
(Wasel, personal communication). In addition, because large DWM provides
germination substrate for white spruce (Rowe 1955), large logs, snags, and live
trees should be retained on cutblocks to ensure adequate amounts of germination
substrate throughout forest succession. Finally, because white spruce seedlings
often grow in exposed soils (Neinstaedt and Zasada 1990), forest companies might
consider planting spruce seedlings on mixedwood cutblock access roads.
Constraints and Research Needs
Management of mixedwood forests for wood production from both aspen and
spruce presents several challenges, including land tenure allocations, harvest
involving multiple entries, and regeneration and growth of understory conifers.
Effective management of mixedwood forests will require government and industry
to re-evaluate growth and yield estimates, silvicultural practices, and
regeneration standards. Although untested in the field, strategies that rely on
natural regeneration of mixedwood canopies may be economically viable because of
reduced scarification, planting, and tending costs, and because of greater total
yield from both aspen and spruce. Conversely, mixedwood harvest systems may not
regenerate conifers quickly and may result in lower AAC for softwood operators.
The forest industry is unlikely to widely adopt natural regeneration of white
spruce as a silvicultural option until researchers have quantified rates of
natural regeneration and growth, evaluated DWM as a germination substrate,
documented spatial relationships of seed trees, and better understand spruce
mortality factors of spruce such as aspen, snowshoe hares, and marsh reed grass
(Calamagrostis spp).
4. Establish Aspen Mixedwood Forest Reserves as Ecological Benchmarks
Although forest companies in Alberta are exploring "forest ecosystem
management" (Franklin 1989; Hunter 1990) as a strategy for balancing
commodity and ecological concerns, the long-term success of this approach
remains uncertain (Stanley 1995).
As such, the establishment of unmanaged forest reserves of appropriate age,
size, and composition within the forest landscape remains a prudent risk
management strategy for conserving biodiversity and ecological function
(Franklin 1993).
Because clearcut logging in Alberta's boreal mixedwood forests has a short
history (most commercial forests are less than half way through their first
rotation), we have no evidence that current forest practices are sustainable
over long periods of time (i.e., multiple harvest rotations). If problems are
encountered with maintaining viable plant and wildlife populations, forest
regeneration, or ecological processes in forests managed for wood production,
the scientific community will require a reserve system of natural forests to
make comparisons and conduct experiments.
Because much of the aspen mixedwood forests has been allocated for fiber
production, this plant community must become adequately represented in the
boreal forest reserve system. The deletion of unmerchantable forest types,
riparian buffer strips, and steep riverine slopes from the merchantable landbase
(Alberta Timber Harvest Planning and Operating Ground Rules 1994) may provide
significant environmental benefits, but at present there have been no
evaluations of how adequate these other forest types are for conserving species
and ecological processes characteristic of aspen mixedwood forests.
The diversity of ages, sizes, shapes, and juxtaposition found in boreal
mixedwood forest landscapes (Peterson and Peterson 1992) emphasizes the need for
a reserve system that maintains reasonable frequencies of natural stands
spatially and temporally positioned in the landscape (Baker 1992).
Such reserves, particularly old growth forests, may also offer cultural and
aesthetic values to society (Burton et al. 1992; Maser 1994), maintain core
populations of threatened species, and produce dispersing individuals or
propagules that may be important to the maintenance of plants or wildlife
populations in the surrounding managed landscape (Franklin and Forman 1987; Noss
1991).
Finally, reserves represent ecological benchmarks against which society can
evaluate the integrity of forests managed for commercial land-uses such as
forestry.
Constraints and Research Needs
Central to the issue of sustainable forestry is whether high utilization levels
(e.g., conventional clearcut logging) and short rotation ages (60-70 years) can
be sustained over multiple harvests. Answering this question will require
lengthy research projects involving growth and yield monitoring from permanent
sample plots (PSP) in both harvested and reserve systems. Such monitoring
programs will provide government and industry with an ongoing mechanism to
assess the viability of harvesting systems. Forest managers need to be able to
detect small changes in site productivity, as minor losses to plant or wildlife
productivity with each rotation may have profound consequences to forest systems
when considered over meaningful ecological time (100s-1000s of years).
To be effective as ecological benchmarks, reserves need to be large enough for
natural disturbances to occur in appropriate size, frequency, and intensity
(Pickett and Thompson 1978; The Nature Conservancy 1982; Noss 1987; Baker 1992).
Allowing natural levels of wildfire may not be possible within small reserves
surrounded by land-use practices that view natural disturbances unfavorably. A
protected area network based on large areas where conflicts with competing
land-uses are minimal may be more likely to support natural disturbances of
appropriate type and frequency. Large, "semi-natural" boreal mixedwood
forests in western North America include Wood Buffalo National Park and Primrose
Air Weapons Range in Alberta and Prince Alberta National Park in Saskatchewan.
Even within these areas, however, natural fire regimes may not be present as
fire suppression and other land-uses occur. Further discussion is required on
whether reserves should be representatively established within the boreal
mixedwood biome, embedded within the commercial forest, or both. Because the
degree of ecological similarity between harvested and reserve forests may
diminish with increasing distance, appropriate attention to geographical
positioning of reserves in the landscape matrix is required (Franklin 1993).
It would be unwise to rely solely on reserve networks to conserve biological
diversity, however, as reserve systems of sufficient scale to embody the range
of natural variability of landform, disturbance events, biota, and ecological
processes may not be attainable (Baker 1989; Wilcove 1989). Reserve networks
should therefore be viewed as complementary to harvest strategies involving
structured cutblocks, multiple rotation ages, and mixedwood management.
The degree of similarity between natural lands and human land-use practices,
however, should influence the area of reserves required (Harris 1984; Franklin
1993). By embedding reserve networks within a matrix of land-uses that adopt
ecological principles, it may be possible to create a landscape useful for both
commodity production and conservation of biodiversity (Harris 1984; Hansen et
al. 1991; Mladenoff et al. 1993). In landscapes manipulated intensively (e.g.,
conventional clearcut logging), a large amount of the area should be established
as reserves to serve as ecological benchmarks and to compensate for problems
caused by intensive forestry.
In contrast, forestry manipulations that are less intensive (e.g., structured
clearcuts, selection logging) and that better approximate natural disturbances
in intensity, size, and frequency, may offer less ecological risk and therefore
require less area allocated to reserves.
Obstacles and Opportunities
Albertans are beginning to realize that consideration of ecological issues after
fiber allocations have been made and after forest industry infrastructures are
complete, severely compromises our ability to find solutions that offer both
economic and ecological sustainability. A new forest management model, one that
addresses both ecological and forestry concerns at appropriate spatial and
temporal scale, is required. However, the remarkable complexity of aspen
mixedwood forests, and our rudimentary understanding of this ecosystem, have
been used by different interest groups to either justify the abolishment of
commercial forestry or to suggest that it is premature to change current forest
harvest strategies. A more reasonable approach to directing forest management
practices is offered; a model based on maintaining variability in stand age,
stand size, and stand structure following patterns created by such natural
disturbances as fire.
The implementation of this model will require extensive scientific study of
boreal mixedwood forests and an extension program that informs Albertans about
issues relating to both the ecological and economic consequences of forest
land-use practices. The implementation of this model will be difficult, however,
as historic approaches to timber allocations have not adequately considered
non-fiber attributes. Forest structure that has been historically allocated for
timber or pulp may now be required as components of wildlife habitat or
ecological processes. For these reasons, further allocations of the few
remaining uncommitted public forests in Alberta is inadvisable and not
consistent with current national and international directions in forest
stewardship. In the context of forest ecosystem management principles, these
unallocated forests are required as unlogged reserves, and as areas where
existing forest harvest operations can expand to provide the latitude needed to
implement alternative silviculture practices, structured cutblocks, a mixedwood
management model, and variable rotation ages.
Studies such as the one presented in this report have repeatedly shown strong
relationships between stand age, stand structure, and biota. The patterns that
define these relationships can be used by resource managers to mitigate adverse
effects of forestry through modification of operational practices (e.g.,
structured clearcuts, multiple rotation ages, mixedwood management models).
Although this approach to understanding forest ecosystems, and how they respond
to landuses, has merit, it must be complemented by research that focuses on
fundamental ecological processes. The range of ecological processes affected by
intensive forestry is far from fully known (Maser 1994), but includes nutrient
cycling, soil fertility, plant germination, and competitive interactions of
organisms. In particular, more research should be directed to invertebrates and
fungi (Hawksworth 1991; Olson 1992), because these organisms may be critical to
tree germination, growth, and mortality, and to nutrient cycling (Trappe and
Luoma 1992; Franklin 1993).
Although commercial forestry is presently foremost in the public's mind as a
threat to forest ecosystems, the effects of the oil and gas sector and
agriculture should not be underestimated. Both of these landuses excise
considerable areas, either permanently or semi-permanently, from Alberta's
boreal forest each year, and cause extensive fragmentation to the remaining
forest landscape.