Forest Health From Different Perspectives
T.E.Kolb, M.R. Wagner, and W.W. Covington
INTRODUCTION
The term " forest health" is
being increasingly used in the context of forestry and natural resource
management. For example, the term has been the subject of several recent
articles (e. g., Smith 1990, Btlrkman and Herte11992, Kessler 1992,Haack and
Byler 1993, Sampson and Adams 1994) and a recent Society of American Foresters
task force report, "Sustaining Long-Term Forest Health and
Productivity" (SAF 1993). Forest health is also increasingly used in
government mandates concerning forest management. For instance, the "Forest
Ecosystems and Atmospheric Research Act of 1988" mandated the USDA Forest
Service to develop surveys to monitor long-term trends in the health of forest
ecosystems (see Burkman and HerteI1992). Moreover, under new federal forest
management operating philosophies, such as ecosystem management, forest health
has emerged as a central objective for the desired future condition of forests
that replaces, to some extent, management for sustained commodity output (USDA
1993a, SAF 1993).
Despite its widespread use, the term "
forest health" is frequently used without a clear definition, making its
application to forest management difficult. In cases where the term has been
defined (e. g., McIntire 1988, Monnig and Byler 1992, USDA 1992, 1993a),
alternative definitions and viewpoints of forest health have not been thoroughly
discussed (however, see O'Laughlin et al. 1994). We feel that the overall
concept of forest health needs to be more thoroughly examined given its growing
use and importance as a management objective. Like it or not, foresters and
other natural resource professionals are currently, and will continue to be,
participants in ,public debates over land management where health analogies and
metaphors are used. The potential for miscommunication in such debates is great.
In fact, we believe that miscommunication about forest health is common in
discussions between parties which have very different expectations from the
forest. Therefore, it is essential that common definition
and conceptual understanding of forest health be agreed upon each time it is
introduced into the discussion. Moreover, the need for clarity is of
considerable importance given that a healthy forest is viewed as a desired
future condition and maintenance of forest health is viewed as a constraint that
may limit forest uses on public lands in the future.
In this paper, we discuss different
definitions of forest health, problems in scaling the concept of health from
individuals to ecosystems, and the relationship between forest health and pest
management, often using southwestern ponderosa pine, Pinus ponderosa var.
scopulorum, forests as an example. A central point of this paper is that
ambiguity should be minimized by defining the term when it is used, or at least
by discussing the concepts included in the term.
FOREST HEALTH DEFINITION
Aldo Leopold
Although forest health is a
relatively new term in forestry, notions of land health have existed for
millennia (Norton 1991, Callicott, 1992). Most contemporary views of forest
health stem from the writings of Aldo Leopold (Leopold 1949, Callicott and
Flader, 1992). In several of his essays, Leopold , decried widespread symptoms
of land " sickness," such as reductions in vegetation cover and
ensuing soil erosion, resulting from land abuse. He argued for the practice of
land health in which practitioners would seek to maintain the sustainability of
ecological conditions and processes by conserving the ecological integrity or
coevolved diversity of the land. Leopold supported the restoration of sample
native ecosystems present before industrialization of the American landscape.
These restored areas were to serve both as laboratories and as standards for
comparison in his practice of land health (Flader, 1974).
Utilitarian Perspective
More recent definitions of forest
health range between utilitarian (anthropocentric) and ecosystem (ecocentric)
perspectives. The utilitarian perspective emphasizes forest conditions which
directly satisfy human needs, while the ecosystem perspective emphasizes the
maintenance sustain- able ecosystems over the landscape, From a utilitarian
perspective, a desired state of forest health can be considered " a
condition where biotic and abiotic influences on forests (pests, pollution,
silvicultural treatments, harvesting) do not threaten management
objectives now or in the future" (McIntire, 1988, USDA 1993a). That is, a
forest is considered to be healthy if management objectives are satisfied, and
unhealthy if they are not.
"Consistency with objectives" is a theme common to both utilitarian
and ecosystem definitions of forest health. Failure to meet objectives, stated
by either human uses or ecological conditions, indicates an unhealthy forest.
The utilitarian perspective is perhaps more deeply rooted in the
" consistency with objectives" theme in that pests are traditionally
defined as organisms that interfere with intended uses of forests (Barbosa and
Wagner 1989). The " consistency with objectives theme" in forest
health definitions has been criticized in the context of ecosystem management
philosophy (Wagner 1994). On one hand, a healthy forest depends on meeting
management objectives, while on the other hand, a healthy forest is a management
objective according to recent ecosystem management philosophy. This results in
circular logic and creates a paradox where a desired state of forest health
depends on the occurrence of a healthy forest! Solutions to this paradox include
removal of the " consistency with objectives" theme . from forest
health definitions or removal of forest health as an objective of ecosystem
management.
The utilitarian definition implies that a
healthy forest can be described by many standards. A single forest condition
could be viewed as healthy from one perspective or use but unhealthy from
another. For example, a common component in southwestern ponderosa pine forests
is dwarf mistletoe, Arceuthobium vaginatum sbsp. cryptopodum. Dwarf
mistletoe is well-known to reduce the growth of ponderosa pine (Beatty 1982) and
increase mortality (Hawksworth and Geils 1990) and would be viewed as being
unhealthy from the perspective of wood fiber production. However, abundance and
species richness of birds is higher when dwarf mistletoe is present (Bennetts
1991) and the northern spotted owl nests in
witches' brooms caused by mistletoe in Douglas-fir, Pseudotsuga menziesii (Mirb.
) Franco, (Martin et al. 1992). Consequently from a perspective of bird species
habitat and diversity, the presence of dwarf mistletoe may constitute a healthy
condition. Thus, dependency on objectives can create obvious problems in
generating a definition of forest health, particularly when land management
objectives are not static.
The utilitarian perspective of forest
health is especially appropriate for those situations where management
objectives are unambiguous and consist of a small number of complementary human
uses. This situation is largely restricted to private industrial forest lands
which emphasize the production of wood fiber, and wilderness areas which
emphasize the preservation of natural processes (i.e., processes with minimal
human influence). Application of the utilitarian definition of forest health to
forest lands managed for multiple objectives, such as most of the National
Forest System, is a problem because management for multiple objectives
complicates the prioritization of objectives. Some authors have proposed a
return to a land management philosophy that allocates land to categories of
similar uses as a way to simplify the formulation of objectives and consequently
the evaluation of forest health (Seymour and Hunter 1992, Wagner 1994).
Ecosystem Perspective
Difficulties in application of the utilitarian perspective
of forest health to forest lands man- aged for multiple uses suggests the need
for an ecosystem perspective of forest health that emphasizes basic ecological
processes which characterize forest ecosystems whose presence on the landscape
can be sustained over time scales of at least many decades. Some examples of
forest health definitions from the ecosystem perspective are: " a forest in
good health is a fully functioning community of plants and animals and their
physical environment," and " a healthy forest is an ecosystem in
balance" (Monnig and Byler 1992). These examples provide a starting point
for thinking about forest health from an ecosystem perspective. Terms such as
"balance" and "fully functioning" are effective in steering
our thoughts towards ecosystem characteristics which appeal to many segments of
the public, especially those who believe that nature has an inherent
equilibrium, or balance. Unfortunately, most ecologists agree that ecosystems
tend to be chaotic in behavior, and not " in balance," especially when
viewed over long time periods.
Other ecosystem
definitions of forest health include the idea of resilience. For example "
a healthy forest is one that is resilient to changes.." Goseph et al.
1991), "the term forest health denotes the productivity of forest
ecosystems and their ability to bounce back after stress" (Radloff et al.
1991), or "forest health can be defined as the ability of a forest to
recover from natural and human-caused stressors" (USDA 1992). A related
idea is that a healthy forest is persistent on the landscape and recurs
following disturbance (Botkin 1994).
While we agree that
resilience to dramatic change at the landscape level may be a desired component
of a healthy forest, measuring the degree of resilience of a forest is
difficult. Although lack of resilience is evident a posteriori when a forest has
been significantly altered by stress or disturbance, the a priori presence of
resilience is difficult to quantify, especially in the absence of detailed
monitoring of physiological and ecological characteristics which promote
recovery following stress or disturbance. In other words, we really don't know
the degree of resilience of a forest until it has been exposed to and changed by
stress or " disturbance. Resilience is a useful ecological concept in the
context of ecosystem health. However, difficulty in quantifying resilience
suggests problems in its use in defining and measuring forest health.
A more useful definition
of forest health from an ecosystem perspective should include specific types and
rates of ecological processes and numbers and arrangement of structural elements
that lead to and maintain diverse, productive, forest ecosystems. This
perspective is based on a mechanistic view of forest ecosystems where important
ecological processes would be identified and objectively measured to assess the
health of the system. An example is given by Haskell et al. (1992) who offer
that a healthy ecosystem should be " free from distress syndrome." In
this context,
" distress syndrome" of an ecosystem is characterized by the following
group of symptoms (Rapport 1992): reduced primary productivity, loss of nutrient
capital, loss of biodiversity, increased fluctuations in key populations,
retrogression in biotic structure (a reversal of the normal successional
processes whereby opportunistic species replace species more specialized in
habitat and resource use in the absence of severe disturbance ), and widespread
incidence and severity of disease.
Unfortunately,
quantitative information on rates of essential ecosystem processes, such as net
primary productivity, nutrient cycling, or decomposition, and structural
characteristics, such as snags and landscape corridors, that create and maintain
diverse, productive, sustainable forest ecosystems is presently not available
for many regions. This type of information may be available for some forest
types in the future if efforts like the Environ- mental Monitoring and
Assessment Program, administered by the US. Environmental Protection Agency, are
adequately supported for at least the next several decades.
Of course, there are potential
problems with this highly quantitative approach to defining and measuring forest
health. One problem is the identification of threshold rates of important
ecological processes which lead to degraded resource conditions. In most cases,
knowledge of the "normal" range of temporal and spatial variation for
rates of important ecological processes is lacking. specification of
"normal" rates and trajectories of succession is a problem in some
regions. Techniques for understanding ranges of variability in ecosystem,
structure and processes in past times are being developed (Morgan et al. 1994).
However, the degree to which these techniques can be used to determine past
levels of all important ecological processes is uncertain. Some have suggested a
pre- European settlement baseline of range of variability for pine-dominated
forests which evolved under the influence of frequent, low-intensity fires (e.g.
Monnig and Byler 1992). Whether a baseline patterned after pre-European
settlement or other past forest conditions is appropriate for other forest types
is unclear.
Another potential problem with
the quantitative approach to defining and measuring forest health is the cost.
Despite the public's willingness to support environmental protection in surveys,
some of this apparent support may diminish when it is time to actually pay for
this level of research and monitoring. Given our current knowledge of ecosystem
ecology, long-term support for forest health research and monitoring will be
required in order to implement a highly quantitative approach to defining and
measuring forest health. Such an approach could yield scientifically defendable
data on the health of forest ecosystems if previously identified problems could
be surmounted.
In the absence of detailed
quantitative information on desired rates of ecosystem processes, present
definition of forest health from an ecosystem perspective should at least
include qualitative statements of the types of processes, structures,
and resources needed to support productive forests in
the sense of satisfying at least some of society's objectives. For example, we
consider a healthy forest ecosystem to have the following characteristics:
1) the physical environment, biotic
resources,
and trophic networks
to support productive forests during at least some seral stages;
2) resistance to dramatic change in
populations
of important
organisms within the ecosystem not accounted for by predicted successional
trends;
3) a functional equilibrium between supply
and demand of essential resources (water, nutrients, light, growing space) for
major portions
of the vegetation;
and
4) a diversity of seral stages, cover types,
and stand structures that provide habitat for many native species and all
essential ecosystem processes.
Specification within
these four criteria allow for definitions of forest health which span the gap
between landscapes which are natural, e. g. near pristine (i.e., pre-industrial
or presettlement characteristics) and landscapes which are artificial, e. g.
intensively managed for industrial uses.
We believe that a useful
ecosystem concept of forest health must consider patterns and rates of change in
forest composition and structure, or succession. This recognition of the
temporal variability of forest vegetation was noted by Leopold (1949) who
offered that "health is the capacity of the land for self-renewal."
Thus, a definition of forest health must consider the capacity for forest
replacement within the timespan of succession. Acceptable rates and patterns of
forest replacement following disturbance will vary widely among different
ecosystems and climatic regions, but
should reflect historical rates and patterns to the extent that
these rates and patterns sustain desirable ecosystems. For example, a long
succession to forest cover following disturbance is not necessarily an
indication of poor forest health if slow succession is a historical
characteristic of the ecosystem because of naturally harsh environmental
conditions.
Our definition also recognizes that
dramatic change in vegetation composition and structure following stress or
disturbance is inevitable over portions of a landscape. For example, small
openings in the canopy are common due to root disease, windthrow, and other
factors. Such openings are not necessarily unhealthy because they can increase
availability of resources to understory vegetation and tree regeneration and may
enhance values such as wildlife habitat and aesthetics. However, dramatic change
may be undesirable when it occurs at scales other than those experienced over
the recent evolutionary development of an ecosystem. For example, many
ecologists believe that fire suppression activities in the western United States
have led to the development of dense, homogeneous conifer stands over wide-
spread areas (e.g. Covington and Moore 1994). This is very different than the
mosaic of stand ages, structures, and species mixtures which were likely
maintained by fires prior to Euro-american settlement. Widespread, dense stands
are particularly prone to attack by bark beetles and other biological agents
which colonize heavily stressed trees.
The emphasis in our definition of forest
ecosystem health on the balanced availability of resources for portions of the
vegetation, instead of all the vegetation, recognizes succession as a process
which can occur, at least in part, because of changes in resource supply to
components of the vegetation. For example, the emergence of late- successional
species is partially a consequence of the decline of early successional species
resulting from their failure to acquire resources at levels sufficient to meet
their high nutritional and meta- bolic demands. In other words, there are
winners and losers when plants are competing for resources in a healthy forest.
Thus, we should not automatically assume that all instances of decline by a
single species, or groups of species with similar ecological characteristics
(i.e., early successional or pioneer types), reflect poor forest health.
Evaluation of forest health must be made within the context of successional
processes and ecosystem dynamics.
THE PROBLEM OF SCALE
Much of the current ambiguity about forest
health has arisen because of attempts to take a concept developed at the
individual organism level and elevate it to describe a landscape process. Most
dictionary definitions of "health" emphasize the condition or
functioning of a single organism. Extension of this concept to a complex system,
such as a forest, is based on the analogy between the functioning of an organism
and an ecosystem. Kessler (1992), for example, makes an analogy between the
health of a human and the health of a forest. This type of analogy is based on
the Clementsian concept of community ecology (Clements 1916) where the ecosystem
is viewed as a superorganism. Despite the apparent usefulness of the
superorganism analogy for describing the status of ecosystems, Oementsian
concepts have been discarded by most contemporary ecologists and thus"
cannot recommended for discussions of forest health.
There are other problems with the use of the term
"health" to describe the status of ecosystems (Ehrenfeld 1992). From a
scientific perspective, it is difficult to determine a normal state for
communities whose characteristics are often in flux because of disturbance. From
a practical perspective, attempts to define health in rigorous scientific terms
may diminish its present value as an intuitive, general concept. In fact,
Ehrenfeld (1992) concluded that health is not a valid ecological concept, but
does have value in communication between scientists and non-scientists regarding
the production of values by ecosystems. Although the limitations of the term
suggest that it should not be used in a rigorous ecological context, it is
likely
that "health" will continue to be used to describe and mandate
management objectives for forests.
Health has been applied to forest
ecosystems at several scales ranging from an individual tree to landscapes. The
concept becomes more ambiguous with increasing complexity of the system to which
it is applied. One definition of health, " absence of disease"
(Haskell et al. 1992), actually leads to a precise definition for an individual
tree because disease can be defined as a" deviation in the normal
functioning of a plant caused by some type of persistent agent" (Manion
1991). Forest pathology is a long-standing discipline in forestry that some
refer to as "the study of tree health" (Tattar 1978). In this context
the health of a tree can be evaluated by such indicators as crown condition,
growth rate, and external signs of disease-causing agents. A dead or dying tree
is not healthy.
The health of a stand is complex and
must consider many more dimensions than the health of a tree. The health of a
stand relates to the management objectives for that stand (utilitarian
perspective) and to the long-term functioning of the organisms and trophic
networks which constitute the stand (ecosystem perspective). Tree mortality in a
stand would not indicate an unhealthy condition as long as the rate of mortality
was not greater than the capacity for replacement. Stand objectives such as
wildlife habitat, soil and water protection, and preservation of biodiversity do
not require a healthy condition for all trees in the stand. A dead tree is not
healthy, but it may be part of a healthy stand! The health of a forest ecosystem
(i.e. large c watershed or landscape) is more complex than the health of a
stand. The health of a forest ecosystem depends both on society's objectives for
the forest (utilitarian perspective), and upon the interaction of biotic
(including humans) and abiotic processes that produce the range of habitats
required for continued existence of native species (ecosystem perspective).
A NEED FOR SIDEBOARDS
There is a clear
need to place bounds on the concept of forest health. Many forest pest
management specialists think of themselves today as forest health specialists.
For example, the USDA recently formed a "National Center of Forest Health
Management." The current emphasis of the center is on the development of
pest management strategies and technologies (USDA 1993b). However, based on our
definition of forest health, forest health specialists would require broad
training in physiology, ecology and ecosystem science. Traditional pest
management has primarily focused on the influences of insects and diseases on
commodity outputs. The role of insects and diseases in ecological processes is
frequently less emphasized in the traditional education of pest specialists,
although entomologists and pathologists are not without appreciation for the
ecological role of these organisms (Haack and Byler 1993, Clancy 1994,
Schowalter 1994). We suggest restricting the term "forest health" to
the definition of the role of biotic and abiotic agents in ecosystem level
processes. Pest management would then be a sub-discipline of forest health with
an emphasis on the influence of biotic and abiotic agents in the production of
commodity outputs. Entomologists and pathologists would continue and hopefully
in-
crease their examination of the role of insects and diseases in ecosystern level
processes.
EVALUATING FOREST HEALTH-SOUTHWESTERN PONDEROSA PINE FORESTS
Given our definition of a healthy forest ecosystem, when
is a forest considered to be unhealthy? The type of thinking needed to answer
this question can be illustrated by using ponderosa pine forests in the
southwestern United States as a case study. To address this question, we refer
to the four essential elements in our definition of forest eco- -system health:
1) physical and biotic resources to support forest cover; 2) resistance to
dramatic - change; 3) functional equilibrium been supply and demand of essential
resources, and 4) diversity of seral stages and stand structures. The physical
and biotic resources are presently in place to support ponderosa pine forests in
most areas of the Southwest that have historically supported them, except
perhaps some riparian sites. Using this criterion, our ponderosa pine forests
are probably healthy. However, for the other three criteria, it would be
difficult to argue that we have a healthy forest.
A significant threat of dramatic change in
forest composition and structure at the landscape level exists in much of the
southwestern ponderosa pine A , forest due to pine bark beetles, Dendroctonus
spp., Ips spp. These insects are well-known to reach outbreaks when
forest stand density exceeds the carrying capacity of the site (Sartwell and
Stevens 1971, Barbosa and Wagner 1989). Conditions are very favorable for pine
bark beetle in northern Arizona and "it is probably only a matter of time
before another large outbreak occurs" (Wilson and Tkacz 1994). Tree
mortality associated with widespread bark beetle outbreaks often increases the
risk of severe, stand-replacing wildfire over large areas.
Present high stand density and forest floor
accumulations in many southwestern ponderosa pine forests compared with
presettlement conditions (Covington and Moore 1992, 1994) has increased the
destructive potential of wildfires to the degree where there is a significant
risk of eliminating forest cover at the landscape level. These factors have also
created an imbalance between demand and supply of water, nutrients, and growing
space for major portions of the vegetation (Covington and Sackett 1986,
unpublished data on file with T. E. Kolb in the School of Forestry, Northern
Arizona University), especially herbaceous vegetation {Covington and Moore
1994). Nutrient cycling rates are likely low because of fire exclusion and the
lack of compensating factors such as microbial decomposition. This creates a
situation in which large nutrient reserves are found in forest floor .material
in a form unavailable to plants
The relatively homogeneous nature of the
southwestern ponderosa pine forest does not provide a balanced diversity of
seral stages, cover types, and stand structures. Underrepresented types include
native prairie vegetation, tree regeneration, and old growth {USDA 1993c).
Forests tend to be even-aged with a dense, uniform canopy and little recent
regeneration. These dense stand conditions were created by past grazing
practices, fire exclusion, and other environmental conditions favorable for pine
establishment in the early part of this century. Thus, many southwestern
ponderosa pine forests fail to meet three out of the four criteria needed to
satisfy our ecosystem definition of a healthy forest.
FOREST HEALTH SUMMARY
Although there are problems
with the use of health concepts to describe the complex array of factors that
influence ecosystems, the growing use of the term demands that natural resource
managers understand health issues. It is also important to recognize that one's
view of a healthy forest may vary considerably between utilitarian and ecosystem
perspectives, as well as over spatial scales.
One solution to the
present dichotomy which exists between utilitarian and ecosystem-centered
definitions of forest health is to combine elements of both viewpoints into a
single definition. For example, O'Laughlin et al. (1994) offer that "forest
health is a condition of forest ecosystems that sustains their complexity while
providing for Human needs."
Moreover, the
ecosystem perspective of forest health is not necessarily in conflict with the
utilitarian perspective if both are applied to large landscapes composed of a
mosaic of different stand ages, structures, and levels of management intensity
appropriate for satisfying the range of demands placed on the landscape by
society. Satisfaction of these demands will require maintenance over the
landscape of many native species and all of the ecosystem processes that
ultimately provide resources and habitat for their survival.
Current forest health problems were
caused by past lack of understanding of the importance of disturbance in forest
ecosystems and poor under- standing of public values by forest managers. Forest
health problems certainly exist in areas in the western United States where
conditions have been altered over the past several decades by concentrated
harvesting of early successional species or fire exclusion in fire-adapted
ecosystems (McIntire 1988, Covington and Moore 1992, Wickman 1992, O'Laughlin et
al. 1993, Covington and Moore 1994, Covington et al. 1994). However, we believe
that present concerns over forest health also reflect failures in defining
management objectives that are acceptable to society. In the absence of
well-defined and widely publicized objectives for forest management which
reflect the diversity of values held by society, forest health will continue to
be a concern even with dramatic breakthroughs in our scientific understanding of
forest ecosystem processes. On the other hand, public expectations must be
tempered with the under- standing that, in many cases, the range of values
potentially delivered by forests is limited by biological constraints to insure
sustainable forest ecosystems. Forest scientists arid managers are obligated to
clearly communicate these biological constraints to the public. In the current
political system of the United States, identification of priority objectives for
forest management within these bio- logical constraints is a public decision
which is often difficult and tedious and thus rarely achieved.
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