NOTES NOT UPDATED FOR SEVERAL YEARS -- DO NOT RELY ON STATISTICS PROVIDED HERE! I provide you here with the short version of the answer to this question (which is probably about all I'll have time to cover in class) and, below that, with a longer version. Take your pick! If you want to jump immediately to a focus on trends in the Pacific Northwestern US, you can click here.
We are presently experiencing a net loss (for all forest types except woodlands): of over 11 million acres per year (an area about equal to the size of Honduras). That was the average global rate between 1991-1995. The total forested area of the world, not including woodlands, is now about 23% smaller than it was in 1700. (Many of the summary data in this section are from "Vital Signs 1997" pages 96-97)
Most of the world's net deforestation in the 1990s was tropical forest loss, which averaged 12.6 million hectares a year (averaged over the first half of the 1990's). Despite public attention to the issue of tropical deforestation, damage has continued unabated since the 1980's when the average rate of loss was 12.8 mill hectares per year. During the first half of the 1990's, 4% of the world's tropical forests were lost. If losses continued at this rate, tropical forests would be gone within a few decades. ("Deforested" is defined by UN's FAO as depletion of crown cover of trees to <10%.)
Within the topics, Latin America and the Caribbean are losing the greatest amount in terms of absolute area, but Asia and Oceania have been losing greater proportions of their forests.
These figures mask important distinctions, in that the loss rate for "natural" forests (indigenous) has been greater than the loss of "forests" overall because the latter includes plantations, which increased by 1.7 mill ha over the first half of the 1990's.
Forest area is basically stable or increasing in N America, Europe, China, S. Korea and the former Soviet Union. Much of it is through planting of industrial forests (and some through conversion of abandoned agricultural lands back to forest).
In general, forests are being cut much faster than they are being replanted. Globally, the cut:replant ratio is about 10:1 (that is, 10 ha are cut for every 1 that is replanted to trees).
Deforestation is generally driven by the increasing human population. In many tropical regions, clearing for agriculture is the leading driver. This is variable though, as in some parts of the world, logging for lumber is as or more important as a driver.
CONSEQUENCES IN THE BROAD SENSE:
Deforestation adds a source of CO2 to the atmosphere and removes a sink for CO2.
Habitat destruction is the leading cause of species extinctions, which are now estimated to be between 20-75 species per day or 7300 - 27,000 per year. This extinction rate is similar to -- or maybe even faster than -- the rate that prevailed during the period of mass extinction that saw the loss of dinosaurs.
Global trends in forest cover: some facts and figures.
How much forest is there in the world, and how much of that is substantially unaltered from its pristine condition? (Note, please, that it can be tricky to compare statistics on deforestation as presented by different research groups. For example, logging itself is often not counted as deforestation, as logged over areas can regrow as forest. Thus, although forest biomass does change as result of logging, forest area doesn't necessarily, depending on the measure that one is using. There is also variability among estimates owing to factors such as definitions of "forested" land (e.g., are open woodlands included?) and types of data on forest cover being used (e.g., remotely sensed data or data obtained in other ways).)
Forests cover about 1/3 of the land surface of earth, including both closed forests and open savanna. Compared to the cover that existed before extensive human disturbance, this is a decrease (nor surprisingly). It is estimated that sub-Saharan African forest and woodland cover has decreased by about 44%; in Latin America, the decrease has been about 32%, and in SE Asia the decrease has been about 44% (Houghton May 94 Bioscience). ("State of the World, 1991," Chapter 5 has a nice table giving the present area of primary forest and the percentage % of original area that represents for various regions of world. )
Forests have been and are being used at a tremendous rate, for many purposes.
Conversion to agriculture (farming and grazing) in one form or another is the leading global cause of deforestation. This includes peasant agriculture ("shifting", but not shifting often enough to be sustainable) as well as larger corporate-style agricultural conversion. Agricultural conversion by peasants, driven by increasing populations and extreme poverty was estimated by some as being responsible for about 60% of global moist forest (tropical) deforestation in the late 1980's and early 1990's, and that percentage is probably increasing (Myers 1991). (Note, however, that some question these percentages, believing that corporate crop farming [which was promoted by some governments, such as Brazil's] has been responsible for more than this allows for.)
Another cause of deforestation is harvesting biomass for lumber, fuel, paper, and fodder for animals. The importance of this versus agricultural uses varies with region of the world. For example, in SE Asia, logging for timber export has, at least recently, been more important than clearing for agriculture as a cause of deforestation, while in S. America, logging for timber has been less important than clearing for agriculture. Something like a fifth of the world's population (over a billion people) are meeting their fuel wood needs only by cutting trees faster than they are growing (in Sudan wood is being burned 70% faster than replacement trees are growing and in Ethiopia, the rate is 150% faster [WorldWatch 9/93]).
What are trends in forest cover in various areas of the world?
In North America overall, forest area is roughly stable. (Note that this includes recent clearcuts that are regrowing as forest; that is, we aren't referring to pristine forest area.) In some areas, such as the NE US, reversion of farm to forest has happened, as agricultural land was abandoned for various reasons. In the US overall, about 15% of our original forests still exists in undisturbed condition, mostly in Alaska. ("Undisturbed" refers to human-induced disturbances, of course.) For the lower 48 states, only 2-5% of pristine forest area remains. In N. America, forest area is expected to shrink slightly over coming decades. We'll talk in some detail about the situation in the Pacific Northwest of the US later, but I'll note here that the current rate of harvest in British Columbia is phenomenal. BC has some of the most acreage of old temperate rain forest in the world, but only about 40% of the acreage remained uncut (as of 1993; WorldWatch) and the remainder was being cut rapidly. Logging in BC began in earnest post WW II, when the government offered "perpetually renewable" contracts for very little money in an attempt to stimulate the economy. Ninety five percent of BC's public forest land is legally available for timber extraction. BC (and Canada in general) do not have as well defined national forestry practices and other environmental laws (wuch as NEPA and the Endangered Species Act) as we do in the US, which makes it difficult to fight in any legal way for greater forest protection in BC. An irony, however: the US imports about 1/2 of BC's timber exports (or was during the early 1990's - I havent read a follow up), and there was a big increase in US demand for BC logs and lumber since the listing of the Northern Spotted Owl as threatened in the US. I guess that we'll just contribute to exploiting Canada instead of WA and OR's forests. In the early 1990's, BC's harvest rate was about 25% greater than the sustained yield rate according to the BC Ministry of Forests, and 3/4 of the acreage being cut supported trees that were over 150 years old - many considerably older than that.
Forested area in Europe appears to be increasing and is expected to continue to increase over the coming decades. Recall that the human population of Europe is stable (even shrinking). The biomass of European forests does appear to be increasing (92), and this may be where some of the "missing carbon" is going. Europe, however, has almost none of its original forest in undisturbed condition; the only significant remaining acreage is in Northern Sweden (excluding Russia and some other areas of the former Soviet Union).
By contrast, in most of Asia, Africa, and Latin America, the forest area is shrinking rapidly, particularly in the moist tropics. For example, in the early 1990's, Ivory Coast was being deforested at a rate of nearly 6% per year. Globally, it was estimated that we were losing about 15.4 million ha (38 million acres) of tropical forest yearly (1993), combining both moist and dry forest types. This is over 1 acre/second! About 10.6 million ha of this is closed forest (an area about the size of Ohio). Globally, this translates to a rate of about 0.8% of tropical biome area per year (1993 FAO report). The loss rate for moist tropical forest, globally, was estimated at about 0.6%/yr, while losses of moist deciduous and upland tropical forests were even faster (1 and 1.1%/yr respectively). If this rate of deforestation continued, we would lose most within the next few decades. Houghton, writing in the May 1994 issue of Bioscience, estimated that between 1850 and 1985, about 28% of forest area in Latin America was lost (mostly to crop and pasture land) and that, during that same time period, there was a 34-38% decrease in forest area in Southern and SE Asia and Africa. The biggest absolute rate of loss during the 1990's was in S. America, which was deforesting about 6.2 mill ha per year (which was 0.6% annual deforestation rte for moist tropical forests there). Brazil alone was losing about 1.5 mill ha/year. However, continental SE Asia was losing a larger proportion of its forests each yea; approximately 1.6% . This is particularly alarming, as SE Asia has some of world's most notable biodiversity "hotspots". The percentage loss rate for Central America was about 1.5% per year during the 1990's.
Before 1960, expansion of crop lands was greater outside the tropics than in (e.g., in North America, Europe, and the USSR). For China, most cropland expansion also occurred before 1960. However, since 1960, most expansion of crop area has taken place in tropical regions. Between 1850 and 1990, the area of global cropland more than doubled; as we've seen. About 1/2 of the world's croplands have been added in the last 90 years, while in the tropics the doubling took place in the last 50 years. (These patterns resemble those shown in the history of human population growth...)
The exceptions to forest trends in these regions are found in China and S Korea where forest areas have increased in recent times, owing in large part to vigorous re-planting programs. (Planting of tree plantations, essentially.)
To What Extent is Re-planting Offsetting Harvesting?
It is difficult to obtain good estimates of the extent to which tree planting is offsetting forest losses on a global basis. It appears, however, that the ratio (cut acreage to replanted acreage) is something on the order of 10: 1, globally. This includes both replanting of forested lands, and extension of plantations to new areas. Most of the replanting (or plantation extension) is taking place in China and the developed nations of the world. That is, we, globally, aren't replanting much of the land that is deforested each year.
We'll elaborate on this when we explore the case study of the Pacific Northwest in the US, but briefly, deforestation has implications for:
That is, clearing of forests causes many both short and long term changes in many aspects of the environment. Some effects are similar from one ecosystem to another but some vary with the ecosystem. I'll say just a bit here about some consequences of deforestation in moist tropical forests, and we'll talk later about consequences in the Pacific Northwest of the US.
When humid tropical lowlands are deforested, and not allowed to recover to trees for several years, soils that are nearly barren of nutrients are exposed; get baked, and turn essentially into brick. These tropical soils (laterites) are very nutrient poor -- most of the nutrients in such forests are nutrients tied up in living biomass, rather than being in the soil (from which they would leach rapidly, owing to the intense precipitation). When logging in these regions is followed by burning, overgrazing or overcultivation, deforestation is often merely the first step in the process of converting forest lands into barren wastelands - desertification (= bad degradation). That is, even though much of this land is fine for supporting forests, it isn't fine for supporting anything else over any significant length of time. In fact, about half of the Amazonian forest area that is cleared for pasture is abandoned within 10 years of its clearing, because of soil problems that render it unproductive. Only about half of the area of forest lost each year to agriculture in the moist tropics actually expands the area in productive agriculture. The other half is logged only to replace worn-out, abandoned lands (Houghton, May 1994 Bioscience ). It is not at all clear when (if) this land will recover -- the recovery process is slow and not well understood. Natural reestablishment of forests can be difficult when the cleared areas are large. For example, there are problems with seed dispersal. Many tropical species are dispersed by mammals and birds, and many of those won't venture into clearings or are lost when their habitat is destroyed. Further the environment is harsh in these clearings, and often unsuitable for the germination and establishment of most forest species. When fire is used to maintain pastures, it destroys many stump sprouts and seeds that would otherwise have accomplished the revegetation. (These tropical species are not like plants of the forests in our area of the world, which are adapted to being burned every now and then.) Another challenge to revegetation after deforestation in the humid tropics is that predation on seeds in clearings (by ants, particularly) can be intense. (It does appear, however, that unless the areas are reburned frequently [which destroys the soil seed bank] or are very large [posing challenges for the propagule flux], abandoned pasture will eventually re-grow [May 1994 BioScience].) (The situation described above is not the same as the traditional slash and burn, shifting agriculture long practiced in these regions. Under the traditional methods, the forest clearing was not as complete, nor did people try to use the land for agricultural purposes for many years in a row. This system was, apparently, much more sustainable - and had less impact on the forest ecosystems!)
At present in the world, we are faced simultaneously with increasing population, which needs more land and more forest products, and with decreasing forest area. These conflicting pressures will increase the pressure to intensify management of remaining forests to increase yields, and to increased areas of plantations. (This to address the need for forest products - the need for land for agriculture to feed the world's people is another question, and one that we've discussed earlier this term.) More intensive forest management will have pros and cons:
The tradeoff may be necessary to some extent and in some regions: we may have to do more intensive management of some areas if we are to preserve other areas as natural forests. However, as with so many things, compromise may also be possible. We'll discuss (shortly) some approaches to management of commodity forest lands which seek to minimize the ecological "cons" associated with this management --"ecosystem management." We'll see that it may not be necessary to intensively manage -- that is, we may be able to simultaneously manage lands for commodity production and for biodiversity and other values.
To read about the history of forest harvesting in the Pacific Northwest of the US, click forest harvesting. For reminders on how to move about within and among these pages, click "navigate." To return to the maaster table of contents for this BI 301 web site, click "contents."
Page maintained by Patricia S. Muir. Last updated November 18, 2002.