Old Growth on the Middle Fork Ranger District

Introduction

The persistence of old growth forest has been a matter of strong public interest over the last several decades.  There is a common perception these days that there is little old growth forest left in our public forests.  While old growth was preferentially harvested on the Middle Fork Ranger District from the mid 1920’s to the early 1980’s, there is still considerable acreage of this important, interesting, and beautiful forest type existing on the our District.  Before we discuss how much and where these forests exist, we should explain exactly what the term old growth means to us.  We’ll also discuss what kinds of processes allow for the persistence of old growth forests, as well as its destruction and renewal.

There are four basic forest types on the Middle Fork district, Douglas-fir dominated (with some western hemlock and western redcedar), mixed conifer (which contains Douglas-fir, grand fir, sugar pine, incense cedar, and ponderosa pine), Pacific silver fir, and mountain hemlock.  These are general categories; all tree species often occur as minor components in other types.  A group of Forest Service and university researchers developed a very specific definition for Douglas- fir dominated old growth in 1986 with a publication entitled “Interim Definitions for Old Growth Douglas-fir and Mixed Conifer Forests in the Pacific Northwest and California”.  This publication indicates that an old- growth forest has at last eight trees per acre larger than 32 inches in diameter, at least 12 trees per acre greater than 16 inches in diameter of species such as western hemlock and western redcedar which are tolerant of the shade produced by the larger trees, more than four dead standing trees per acre greater than 20 inches in diameter and 15 feet tall, and more than four dead trees per acre greater than 24 inches in diameter and 50 feet long lying on the ground.  On the Middle Fork District, stands with these characteristics may take as long as 250 + years to develop.

No such complete definitions exist for old growth characteristics of other forest types in our area.  We can generally consider the higher elevation Pacific silver fir and mountain hemlock stands to be old growth if they are more than 200 years old and/or have trees greater than 21 inches in diameter.  These typically higher elevation forests are often not as diverse as Douglas-fir old growth stands, structurally or in terms of species composition.  The species that occur in these higher elevation forests do not live as long nor get as large as Douglas-fir, and the dead trees decompose faster.  These forest types occur in areas with deep winter snow packs and not as many understory species are adapted to live in such harsh environments.   

Forest Succession

Douglas-fir is an especially long lived tree; it can grow to be as old as 1000 years.  If a Douglas-fir forest is not disturbed by fire, windthrow, or insects and diseases over its life, Douglas-fir trees will eventually be replaced by species such as western hemlock and western redcedar since the seedlings of these species are tolerant of the shade produced by mature trees. This forest of shade tolerant species is called a climax forest, since these species can theoretically regenerate themselves in perpetuity.   Douglas-fir seedlings and saplings cannot survive to maturity underneath an intact canopy of their parent trees; they need a rather large opening in the forest, typically as wide as the dominant trees are tall, to get enough sun to grow vigorously in height. 

While theoretically one could have an old growth stand composed entirely of western hemlock and redcedar, this is rare to the point that we have never had more than a few relatively small climax forest stands on the Middle Fork district.  The fire history on this southern portion of the Willamette National Forest is such that stands are nearly always killed naturally by a wildfire before all the Douglas-fir die of old-age or windthrow.  While there are occasional trees and small clumps of trees on the Middle Fork district that are 500 to 700 years old, the vast majority of the old growth forests in this area are around 300 to 400 years old.

After fires burn, a diverse set of plant species quickly re-vegetate the sites.  Many of those species are shrubs and herbs that require the high amounts of sunlight found away from the deep shade of tall trees.  At the same time young Douglas-fir trees begin to grow from seeds produced the year before the fire by the now dead large trees, or by adjacent survivors of the fire.  These young Douglas-fir and usually a few other minor tree species may eventually grow into an old growth forest, depending upon the density of tree seedlings and what disturbances they may be subject to over their potentially long life.

Development of Old Growth

In the scale of a human lifespan, forests, and in particular old growth forests, may seem to be eternal and unchanging, but this is not true on the time scale of the trees themselves.  The development of any forest is a constant process that usually has a beginning and end point, in the Northwest both closely associated with wildfire as discussed later, and slowly changing at all times.

Many may think that old growth forest contain trees of all ages, from venerable giants in excess of 1000 years old to seedlings several years old.  This sort of stand structure is not common in the fire-prone Douglas-fir forests of the Cascade Mountains.  In a typical 400 year old Douglas-fir forest there may be many small understory tree seedlings such as western hemlock which may be relatively young, and there may be some larger western hemlock and redcedar from one to several hundred years old which came into canopy openings created by past windthrow.  The occasional ground fires that may occur typically would remove this understory tree component.  The overstory Douglas-fir are typically all nearly the same age, or if the area reforested gradually due to a severe disturbance or lack of adequate seed sources, may vary in age by about 50 years.  Due to the relatively frequent occurrence of fire in the Douglas-fir ecosystem on the Middle Fork district, stands of trees seldom get to be more than 500 years of age.  As these forests age, individual trees die or are blown over, and large amounts of fuel accumulate on the forest floor.  At some point when wildfire comes to that particular forest stand, it burns so hot that the whole stand of trees is killed.

The development of old growth from a young stand created by fire or harvest requires some amount of low level disturbance, especially if the young trees are very densely established.  To become large, trees need room to grow, and a very dense stand of young trees can be quite spindly and subject to wind and snow damage.  A given area of forest may never develop old growth characteristics if the natural disturbance regime is frequent, or those characteristics may take a very long time to develop without any disturbance.  Small disturbance events in younger stands, such as individual or small group tree mortality from windthrow or insects and disease occurrence, may hasten the development of old growth by providing gaps in the forest which allow understory conifers, shrubs and herbs to proliferate. This opening in the forest canopy also allows the adjacent overstory trees to expand the width and depth of their crowns, and allows for development of a complex branching structure.  A very dense stand of young trees that is not disturbed may never provide for the development of large tree stems since many small trees complete with each other for water, nutrients and light, and there is not enough to provide for growth of large diameter stems before the next stand replacing fire.

Function of Old Growth

In addition to the aesthetic and spiritual values it holds for us humans, Douglas-fir old growth forests in particular provide habitat for a number of plant and animal species that either tend not occur in younger forest, or occur less abundantly elsewhere.  These habitats are provided by the protection from sunshine and wind that the deep and dense tree canopies provide, as well as the physical structures for nesting, hiding, and plant growth provided by standing and down dead trees, thick bark, complex crown structures and heavy branches, and undisturbed duff and litter layers.  Old growth trees also may provide a record of past climate and fire occurrence in their many annual growth rings.  Older forests also provide for stream protection and input of large down wood to the stream system that creates more complex aquatic habitat, shading of stream waters to keep them cool, and filters for sediments.

Old growth Douglas-fir forests also contain some of the highest amounts of stored biomass of any ecosystem on earth.  This stored biomass contains large amounts of carbon that would be released into the atmosphere as carbon dioxide if these forests burned in a wildfire.  As you can see from the following discussion of fire occurrence, these forests can only serve as relatively temporary carbon storage sites, as they will eventually burn.  

Fire

Old growth forests do not last forever and the most common event that ends their life is wildfire. On the average, over the last 400 years, about one quarter of the forests on the  Middle Fork Ranger District (some 180,000 acres) has been mostly or completely killed by wildfire each century. 

While many consider the western Cascade Mountains to be quite wet, the climate in the Pacific Northwest can be very dry during the late summer and early fall.  This climate regime and the flammable nature of coniferous tree needles and branches combine to produce an ecosystem that is subject to and adapted to wildfire.  Depending upon forest conditions, the time of year, and the local weather, these fires can be slow creeping ground fires that reduce the amount of flammable fuel on the forest floor, (while killing most of the understory vegetation), or they can be raging crown fires that may kill tens of thousands of acres of trees.  From examination of young stands that contain signs of past severe wildfire, it is apparent that however devastating wildfire may appear to be in the short-term, the forests destroyed by them almost always regenerate from seeds of the fire killed trees in one to ten years.

It has not been uncommon on the Middle Fork district to have a young forest get killed early in its life from another fire, as the dead trees from the first fires eventually fall to the ground and become large amounts of fuel which carries the second fire.  This ‘double burning”, defined here as two fires occurring on the same piece of land less than 30 years apart, can result in stands with little residual structural material, such as snags and down logs, from the previous stand as this material typically is consumed by the second fire.  We do not know the full extent of this type of fire regime on the Middle Fork district, but there are entire drainages exceeding three to four thousand acres in area which appear to have burned in this manner over the last 200 years.

Timber Harvest

Most of the early harvesting on the Willamette National Forest was concentrated in old growth forests.  This was because old growth typically has more net lumber per acre and that lumber is of higher quality since it is finer grained and has fewer knots.  And forest managers at that time thought that younger stands which had been regenerated by wildfires around the turn of the century needed to grow more before they were harvested.  Though considerable old growth was harvested between the 1920’s and early 1980’s, nearly one third of the Middle Fork District contains old growth forests today (see Table 2).  There are also many younger stands, typically from 100 to 200 years old (late-successional forest), which are in the process of developing old growth characteristics.

Old Growth Amounts and Locations

In response to the interest in old growth forests, the 1990 Willamette National Forest Land and Resource Management Plan designated a number of old growth groves (see Table 1).  These stands are representative examples of old growth forest types which are especially scenic and easily assessable to the public, either by being adjacent to main roads, or accessed by a short and easy hiking trail.  Fourteen old growth groves have been designated on the Middle Fork district, totaling about 2000 acres.  These stands were designated to provide for educational, aesthetic, and recreational experiences.

Table 1 ~ Designated Old Growth Groves on the Middle Fork District