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Interface Forestry

forest stewardship in the wildland-urban interface

Notes and Research

Managing for Forest and Stand Health

This Explanatory notes includes three sections:

Definitions of forest health abound, some linked to ecological criteria, some linked to economic criteria. For the purposes of this discussion, I will separate stand health from forest health, though they are actually intertwined.

Stand Health

For the purposes of our work, Interface Forestry defines stand health as meeting three conditions:

  1. Production levels (volume growth) are near or equal to site potential for the given stand age. (Stands past CMAI, for instance, will grow less annual volume than they did at their peak, but will retain more sequestered carbon.)
  2. The stand is free of insect or disease endemic and resilient to epidemic.
  3. The stand is resilient to the effects of fire, meaning that a surface fire would not cause significant mortality in a stand, unless that stand is typical to a stand-replacing fire regime. A healthy stand typical of a frequent fire regime would not ordinarily provide the conditions for a surface fire to torch (rise into a tree crown) and carry into the canopy. Furthermore, the dominant species would not be sensitive to surface fires.

These criteria require some explanation and caveats, because discerning the difference between "healthy" and "unhealthy" forests can be complex and difficult (and is a subject of dispute in the professional and academic forestry discourse).

Part of this difficulty arises from the lack of healthy or "naturalistic" stands to serve as reference stands. Virtually all western forests have been logged, heavily grazed, or otherwise altered, and fire has been suppressed for over a century.

Another source of complexity and difficulty is that forests are changing in response to warmer, drier climates. One of the largest agents of change are airborne insects. For instance, in Montana, warmer temperatures and extensive drought-induced stress in trees have encouraged large epidemics of bark beetles (Dendroctenus spp.) and spruce budworm (Choristoneura occidentalis). In the afflicted areas present across the state, tree mortality is extensive, and even an otherwise healthy stand would suffer if exposed to an epidemic of this scale.

Further, insects and disease have always been part of forest ecology, on the stand and forest level. Consider the common root rot, Armillaria spp, also known as the edible honey mushroom, which is found all over the world. Ecologists identified one specimen in the Malheur National Forest (eastern Oregon, U.S.) that spanned 2,200 acres; they estimated it to be 2,400 years old. (See the Canadian Journal of Forest Research, April 2003.)

Considering the ubiquitous presence of Armillaria, which is commonly present in Montana, imagine a 20-acre Douglas-fir stand with an Armillaria presence. Perhaps a few trees in this stand have shown symptoms of stress or degradation from a root rot (including symptoms like resin streams on the bole or dying branches). And further, perhaps this small group of trees has increased and decreased over the years, but never more than a few trees. Do we deem this stand to be unhealthy? Perhaps these affected trees are on the edge of a grand fir stand growing within a riparian zone.

Finally, land use changes are disturbing and in some cases eliminating ecological functions over large areas, especially in the cases of wildlife-dependent functions. For instance, some migratory song birds, which are evolved to migrate from the tropics the northern lands along specific routes, are facing loss of hospitable habitat as a result of extensive development and logging. These birds once feasted upon insects, working with cold temperatures and frequent fires to maintain sub-epidemic insect populations.

So far this discussion has refereed to forest functions (processes), rather than to composition and structure. But function is dependent upon forest structure and composition, and so we make them part of our definition. In a healthy forest stand, structures and composition will resemble those of a naturalistic forest growing under a fire regime appropriate to its cover type.

The second condition of a healthy stand is that its stand structures, composition, and functions resemble those of a stand in a natural forest environment. For the sake of simplicity, a "natural forest environment" is one absent of logging, clearing, and fire suppression. (Evidence indicates that Native Americans routinely lit surface fires in northern forests for their own reasons. However, native people were nomadic and their settlement impacts were insignificant and ephemeral, in contrast with European settlers.) Because we presently have so few examples of such a stand, such a condition is idealized and must be constructed from what evidence we have. Thus, our representation of the naturalistic stand will have to be created using the resources at hand, including historical data (dating back before large-scale fire suppression), remnant data (observing remnants and fragments of old stands to plot large-tree diameter distribution and fire history), and knowledge of forest stand dynamics.

In an idealized healthy stand, stand structure will allow trees enough growing space (above and below ground) to meet their needs for nutrients, water, and sunlight for photosynthesis. Of course, photosynthesis provides the available energy for all plants (and secondarily, animals) to function as organisms. With trees, photosynthates meet several needs with varying priority; some needs will be met only after others are met. Generally, these needs include, in order of priority, these functions:

  1. Respiration.
  2. Leaf and fine root hair growth.
  3. Reproduction (flower and seed production).
  4. Diameter/xylem growth and resistance to diseases.

Thus, the tree must produce enough photosynthates to respire before it can allocate them to leaf production, with the last priority being resistance to insects. Clearly, stand structure, and in particular, stand density, affects function; if the stand is too dense, trees won't get enough light. Thus, they will produce fewer photosynthates, impairing their ability to resist insects and disease -- the last priority for photosynthetic allocation.

In any forest healthy or not, a tree's growing space and its associated access to sunlight, water, and nutrients is commonly obstructed only by other trees. Some tree species (and individuals) can thrive in shade better than others. These shade tolerant trees can thrive and reproduce in the shade and actually receive benefits from shade and other aspects of crowding, while shade intolerant trees cannot thrive and reproduce. Thus, on a given site, over time, the shade tolerant species will replace the intolerant species.

For example, in Montana, a common scenario is a mixed species stand composed of Douglas-fir and ponderosa pine, wherein the ponderosa pine (shade intolerant) is succumbing to the Douglas-fir (shade tolerant). Over more time, perhaps, a tree species even more shade tolerant will replace the original replacers. For example, subalpine fir, Abies lasciocarpa, is more shade tolerant than Douglas-fir, and we frequently encounter both species on the same stand. In time, such a process, known as succession will create a climax stand or forest, wherein the most shade-tolerant species (one or more) that can live on site (with available seed) will occupy the site. Of course, this undisrupted succession unfolds only in the absence of a significant disturbance.

However, disturbance is the rule rather than the exception in forest ecology. Disturbance vis-a-vis wildfire is especially prevalent in most forest types found in the northern Rockies -- particularly the ponderosa pine, Douglas-fir, western larch, and lodgepole pine forests. Further, until the advent of industrial logging and the associated suppression of wildfire, forest fire was the primary agent of disturbance. In general, on drier sites, fires burned more frequently but with less intensity than on wetter sites, and because fuels grow slower and before suppression, they did not build up (because they burned more often). Generally, a low-intensity fire will not reach into the canopy (tree crowns) without abundant ladder fuel, like small trees or low branches, which are generally unavailable in a fire-frequented landscape. Of course, exceptions occur due to extremes in fuel moisture and wind, forming a mosaic of dead and live trees.

The mixed Douglas-fir and ponderosa pine stand I discussed earlier would be a good example of such a dry site. It would also illustrate another principle, which is that some species are more tolerant of fire than others. Generally speaking, ponderosa pine is more tolerant of a low-intensity fire than Douglas-fir, which in turn is more fire-tolerant than subalpine fir. A ponderosa pine tree can generally withstand fifteen-minutes of surface fire for every inch of cork bark protecting the cambium and phloem. So, for example, a stand or clump cleared logging or high-intensity fire could go unburned for 20-30 years, and in that time, a fresh mixed cohort of trees -- say some Douglas-fir and ponderosa pine might regenerate. When a low-intensity fire did pass through the site, the Douglas-fir (with thinner bark and root hairs growing closer to the soil) and smaller ponderosa pine (with thinner bark and low lying branches) would likely succumb to root, stem, and leaf scorch while the larger ponderosa pine would be more likely to survive. Or, on another day, wind conditions might drive the fire right around a clump of Douglas-fir and take out the ponderosa pine. The result would be a mosaic of unburned, live burned, and dead burned trees. Over the years, as several fire cycles passed, favoring older trees with thicker bark and fewer low lying branches, the stand would mature to resemble a mosaic of older, larger trees interspersed with clumps of younger trees, dominated by old ponderosa pine, with pockets of Douglas-fir.

In contrast to a Douglas-fir/ponderosa pine stand, a subalpine fir or Engelman spruce (Picea engelmanii) stand is on a wetter site with species more shade tolerant and therefore much more overgrown. Such stand structures are highly favorable to a crown fire, so even before the fuel build-up resulting from fire suppression, when these stands burned they were usually destroyed, beginning anew the process of succession.

In some ways, we can rarely if ever identify a stand that meets the standard of our idealized stand. This lack of truly naturalistic stands is especially apparent if we consider function, which concerns processes, particularly the mechanics of the relationships between ecological entities (living and substrate). Usually, private (non-industrial) land is found near the urban-interface, where settlement and economic activities have severely disrupted wildlife ecology -- habitat, species, and population numbers, proportions, and cycles. This disruption is long-term and it is increasing. Since wildlife (small and "invisible" as well as large and charismatic) do affect plant ecology in myriad ways, this wildlife disruption means that over large areas, some naturalistic functions -- known and unknown -- are missing while others are intact. At best, we can achieve our forest health standard only part way in relation to forest functions.

Another important process and functional mechanism that has been lacking since the advent of fire suppression is, obviously, wildfire. Thus, we see many over-stocked stands composed of shade tolerant species that otherwise would be suppressed by fires. Remnants of shade-intolerant species that are still growing are usually display small crown ratios and signs of insect or disease damage. Even the shade tolerant species continue to grow and reproduce far beyond an optimal level for meeting all their photosynthetic priorities. Hence, millions of acres in the northern Rockies are dominated by overgrown Douglas-fir forests that would be dominated by old, widely space ponderosa pine under a natural fire regime without logging or development.

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Forest Health

In areas large enough for stand variability and large enough to buffer some of the effects of surrounding forestlands, forest health becomes an issue in addition to stand health. Forest health is similar to stand health. In a healthy landscape or forest, ecological structures, composition, and functions resemble the structures, composition, and processes of an idealized natural forest landscape. In addition, they are free of (or resilient to) pathogen and insect endemics, and they are able to withstand a surface fire with little crowning and mortality.

Because of the variability of the landscape, the characteristics of the relevant plant species, and the "stochastic" nature of wildfires and disturbances, the naturalistic landscape would consist of mosaics of stands at different stages and structures -- some younger, some older, some more dense, some less dense -- depending upon site-specific fire history, terrain, weather, and aspect and slope, among other variables. The overall effect would resemble a mosaic of even-age and uneven-aged cover types tending towards old-growth, but including areas of new growth or stages between new and old growth. Such diversified stand structures and the resulting ecotones (transition zones) would have the added benefit of encouraging wildlife species richness, given the lack of other impediments.

In the case of Douglas-fir and ponderosa pine forests, extremes in stand structure, such as highly overstocked stands, are likely to cover a small fraction of the landscape in a naturalistic setting and be concentrated in moister areas. However, those fractions that would support overstocking would also support the wildlife functions common to such a forest -- such as thermal and hiding cover for elk or deer.

In addition, as a management area expands, the likelihood of growing more species increases. In such a case, for example, southern aspects might be dominated by ponderosa pine and Douglas-fir while northern slopes might be mixed with a generous portion of western larch, with some subalpine fir in the draws.

In general, a healthy forest would support a diversity of plant and animal species, as well as stand structures and diversity of plant and animal functions (processes). As low-intensity surface fires would be frequent, old-growth structures (highly resistant to fire) would dominate sites with frequent fire intervals. Old growth structures would also likely dominate wetter sites, where fire regimes include long intervals between fire (often, over a century) giving stands time to mature before occasionally succumbing to stand-replacing fires.

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Managing for Stand and Forest Health

Managing forest and stand health involves an attempt to mimic a forest's natural balance of disturbance and growth, through forest activities and treatments, and consistent with an articulated vision of the future forest and a plan for achieving that condition over time. This articulated vision is called a target stand. A plan will usually call for periodic treatment of any given urban-interface stand to maintain safe fuel conditions and optimalize growth of remaining trees. Treatments range from thinning-from-below, to small-diameter logging, to broadcast burning. In some ecosystems, management for stand and forest health is compatible with the sustainable production of small logs.

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