Now that we've looked briefly at trends in forest cover globally, and the implications of those trends, we'll focus on what has happened in the PNW as a case study. This furnishes a specific example of the kind of issues that come up in human influences on forests. Many of the concepts apply to north temperate forests in general. In this section, I'll provide an historical perspective on loss of old-growth forest in the region, including a discussion of forest fragmentation and consequences.
Subsequent sections provide information on the current situation with regard to harvesting of old growth from federal lands in the region, on what makes old-growth forests so special, and on the structural features of old-growth forest that seem most important in conferring its unique compositional and functional features.
(I hope to be able to provide information on conventional forest management and its influences on these structural, compositional and functional features, and a contrast between conventional forest management and emerging "ecosystem management," although I'm not sure that I'll have time to bring those notes up this (fall 2002) term.)
Historically forests occupied vast expanses of the presettlement landscape, despite episodes of wildfire and other natural disturbances. Disturbances, such as fire and wind storms, were important in creating a diverse mosaic of forests with different age classes and species compositions. At the turn of the century, most of western OR and WA's forests were old growth, but thee were forests of diverse ages, owing to natural disturbances.
When European settlers came, these forests were both a valuable resource (timber) and a hindrance to agricultural development, so their elimination began early and progressed to the point where a small fraction of the original forest remains, mostly on federal land. Most remains on federal land because intensive cutting on private lands preceded that on federal lands by several decades. In fact, non federal timber harvest has in recent history averaged 2/3 of the total harvest in the PNW (that is, 2/3 of the harvest has come from state and private lands).
It remains difficult to say how much old-growth forest is left in western WA and OR, in part because folks doing the assessing use different criteria to define "old growth." However, about 13% remaining was the consensus at President Clinton's "Forest Summit," held in Portland, OR in the early 1990's. That would be about 3.5 million acres of old-growth forest remaining, mostly on federal lands. If logging rates that prevailed during the 1980's had continued, it would all have been gone by 2010 or so (outside of preserves). Logging is not, however, continuing at those rates.
In fact, even more endangered in terms of total area, but less talked about is natural young forest (forests <20 years old). We have done such a good job with fire suppression that very little natural young forest exists today. This means that we have little in the way of models of how forests develop under natural conditions, and very little natural "feeder material" to produce new old growth.
Old-growth forests is all gone on private forestry company lands as near as we can tell.
One amazing thing is how fast the elimination of old-growth forests of the PNW happened. While logging began in some areas in the PNW in the 1830's, it wasn't really extensive until the turn of the century (that is, until the early 1900's). So, during less than a century, about 87% of the old-growth forest was logged.
To put this into perspective, we hear a lot about destruction of moist tropical forests, as in the Amazon. Perhaps 15- 30% of the ancient forest in Amazonia is gone (compared with about 87% here in the PNW). We also hear a lot about losses of wetlands; 50% of the US's original wetlands are gone. (Incidentally, tall grass prairie is the most endangered ecosystem in the US - over 98% of those lands have been converted to other uses, particularly to agriculture and urban/road expansion.)
The situation is even worse than these numbers indicate. We should consider not only the absolute amount of old-growth forest remaining, but also its spatial distribution.
The existing patch sizes of remaining old growth forests are skewed towards small patches. For example, on the Siuslaw National Forest (which is in the Coast Range of Oregon), in the mid-1980's, there were reportedly 319 old growth pieces remaining. How large were they?
In class, I showed an overhead of the change in distribution of old-growth forests on the Willamette National Forest of western OR over recent decades, in which you could readily see the increase in fragmentation over time; remaining acreage's of old-growth forest became smaller and smaller -- and increasingly isolated from each other.
Not only have pieces become smaller and increasingly isolated from one another, but the spatial distribution of remnants has also become very skewed. When logging first began, the valley bottoms were first to be cut, as they were accessible and near rivers (to float the logs out). Harvesting then slowly extended up slopes. The consequence is that much of the remaining old forest is at higher elevations, on steeper slopes, and relatively inaccessible and nonproductive. That is, little is left that typifies the lower elevation, highly productive systems; only about 20% of the remaining old growth is at lower elevations. So, most of the remaining old growth forests are small chunks at high elevations.
This means that there is less habitat diversity available in existing old growth than previously was present. One of the important values of unmanaged forests is that they are often reservoirs of species diversity (that is, support relatively large numbers of species), as compared to relatively species poor plantations. The greater the diversity of habitats encompassed by these undisturbed forests, the greater diversity of organisms they can support, both within them and across them.
Fragmentation is concerned with the size and isolation of remnant pieces of old-growth forest (or other kinds of ecosystems). Ideas about the effects of fragmentation on species diversity are derived from the theory of island biogeography, first put forth in the 1960's by R.H..MacArthur and E.O. Wilson in their now classic book, "The Theory of Island Biogeography," which was published in 1967. In the book, the developed a predictive model about the numbers of species that should be found on oceanic islands. They found that the numbers of species were well predicted as a function of an island's distance from the mainland (i.e., its isolation), size, and age. That is, species numbers = fn(size, distance, age). The equilibrium number of species is, according to the theory, determined as a balance between forces of immigration and extinction.
Larry Harris then applied these concepts to thinking about how to manage forest resources in the Pacific Northwest, in a book entitled "The Fragmented Forest" (listed on your supplementary reading list for this section of the course). He argues, basically that the smaller and more isolated a forest fragment is, the more extinction and less immigration there is likely to be of species that characterize the undisturbed condition.
When we are thinking about the size of remnant forest patches, it is important to remember that "edge effects" can be significant. For example, the microclimate at the edge of a forest patch in terms of incident radiation, wind, temperatures, and so forth, can be very different than the microclimate within the intact forest. In many cases, the edge microclimate may be unsuitable for the species that would normally be found in the intact forest, and suitable instead for other species, some of whom are not natives. In fact, populations of many "weedy" species (both plants and animals) thrive on edge habitat (including birds such as cow birds that parasitize nests of native birds
Further, while we think of an "edge" as being a line, ecologically it isn't a line but is a region. That is, some types of edge effects may extend >200 m into a remnant patch of forest. (Some estimates are more conservative; it depends on the response parameter being examined and the location of the patch.). So, returning to the patch size distribution for old-growth remnants on the Siuslaw National Forest, given above, a circular 40 acre remnant patch has a radius of 227 m and would be almost entirely affected by edge effects, if those extend 200 m! Patch shape becomes important here as well; long thin patches have more edge:interior area than would a circular patch of the same area, for example.
Beyond issues about edge effects, there are questions about whether patches (even when large enough to avoid typical edge effects) are large enough to support viably sized populations of species. What is a "viably sized population?" A population that is large enough that it is likely to persist indefinitely, given chance events that produce mortality and given the need for genetic diversity within populations. The smallest size that is likely to be able to persist is referred to as the minimum viable population size. Attempts have been, and are being made, to model this for various species. Calculations suggest that Yellowstone Park itself is not large enough to support a minimum viable population size of grizzly bears, for example. Obviously, the area needed to support a viable population will vary tremendously with the species being considered….
Connectivity between patches may also be very important for some species, hence the recent attention to leaving "corridors" between remnant patches. (It appears that some species use corridors and some don't.) From this perspective, roads become very important, as they increase fragmentation of habitat and may cut through what might otherwise be corridors. Some small mammals will not cross roads - they constitute virtually complete barriers to movements (like open water between islands would be for some completely terrestrial creature). Roads also, of course, allow increased access to humans and other weeds.
SUMMARY OF CONSEQUENCES OF FRAGMENTATION:
A DIGRESSION TO AMAZONIA - AN EXPERIMENTAL STUDY OF FRAGMENTATION AND ITS CONSEQUENCES:
A research project initiated in Amazonia in 1979 by the World Wildlife Fund and Brazil's National Institute for Research is intended to assess consequences of fragmentation for biodiversity in that region. It is known as the "biological dynamics of forest fragments project" (also as the "minimum critical size of ecosystems project"). It is examining how the size and spatial distribution of patches affects biodiversity. As part of an on-going conversion of forest to pasture land, with cooperation of ranchers, they established a replicated series of forest fragments of different sizes: 1, 10, 100, 200, and 1000 hectares in size. They surveyed the patches before the logging occurred, and then have monitored various responses in them after logging isolated them. (They conducted similar monitoring in equally sized pieces of forest within the intact forest for comparison.) One strength of this experiment is that the researchers had "before data" and then created the patches and are following them. Results will provide insight into the consequences of various degrees of fragmentation, with the hope that the understanding can help inform ecologically based forest management in the region.
Click current logging situation in the PNW to read about the current status of logging on federal lands in western WA, OR and northern CA; deforestation for an index of topics addressed related to forest harvesting globally and in the PNW; contents to return to the master Table of Contents for this BI 301 web site; or navigate for reminders on how to move about within and among these pages.
Page maintained by Patricia S. Muir. Laast updated November 18, 2002.