~ CHAPTER 1 ~ FOREST DEGRADATION
(NOTE: Chapter 7, Sections (7-A) and (7-D) define terminology and units used below.)
CONTENTS: [A] Deforestation via Shifting Cultivation, [B] Tropical
Deforestation via Permanent Cultivation, [C]~Tropical Deforestation via
Grazing, [D] Tropical Deforestation via Urbanization, [E] Temperate
Deforestation, [F]~Sustainable-Yield on Tropical Forests, [G] Sustainable
Yield on Temperate Forests,
[H] Comparison of Wood Consumption to Sustainable Yield
Forest degradation has two main components - deforestation and violations of sustained-yield limits. Other components such as forest soil degradation and timber-quality degradation are neglected here. Deforestation must be interpreted in terms of the conversion of forestland to other uses -shifting agriculture (cropping or grazing, followed by extended periods of fallow), permanent agriculture (cropping or grazing with little or no fallow), or urban uses. Note then that cutting or burning trees may or may not reflect deforestation. The only sustained-yield limits considered here are those pertaining to wood production. Optimal harvest-cycles are 3-4 decades for wood pulp and firewood, and 9-12 decades for sawtimber. Shorter harvest cycles reduce sustainable wood-yield.
Forest degradation must be further sub-divided because tropical forest degradation processes are so different from those in temperate- and boreal forests. These differences largely reflect:
Thus forest degradation is divided here into 7 primary components:
Effects of these processes on human carrying capacity of the land are also considered.
Satellite imagery is making measurements of tropical deforestation a far more accurate science. Unfortunately only within the past decade has the science developed to its current refined state, so comparing old and new measurements is risky. The best estimate of tropical deforestation rate during the 1980s is 1,540,000 km2/ decade, or 8% of the current tropical forest inventory per decade. The regional breakdown is given in the review document. The breakdown of this rate, according to end-use, is:
[A] ~ Tropical Deforestation via Shifting Cultivation ~
The global inventory of tropical land under shifting cultivation (including fallow) is 3 million km2 (1980). This inventory is not considered to be forestland but to be part of the 10.3 million km2 of "forest-fallow and shrubland". (The remainder is probably abandoned cropland, abandoned grazing land and current grazing land.) About 90% of tropical forest soils can tolerate only brief periods of cropping or grazing (typically 3 years for cropping and 8 years for grazing), after which 20-30 years of fallow are required to restore soil fertility. Heavy, skilled applications of fertilizer can overcome this soil infertility problem, but high crop prices are required, and virtually all past efforts in this area have failed.
Estimates of the rate of converting new tropical forest to shifting cultivation are 500,000, 850,000 and 1,000,000 km2/ decade (one third of the land cleared each decade for shifting cultivation). So we take an average (800,000 km2/ decade) as the best estimate of the conversion rate. These figures date back to the early 1980s and are said to be doubling every 20 years. Growth of shifting cultivation reflects both the growth of the population of shifting cultivators and the long-term soil degradation caused by shifting cultivation cycles having gotten too short. Thus the relative rate of growth of area under shifting cultivation (0.8 million/3 million) should be significantly greater than the relative (%) rate of tropical population growth (about 25%/ decade).
The human carrying capacity of land under shifting cultivation is, at most, 10 people/km2 under subsistence levels, implying a carrying capacity of the 3 million km2 of land under shifting cultivation of 30 million people. Compare this to the 1980 population of shifting cultivators of 250 million. This is one of the two reasons why new tropical forestland must be constantly deforested (transferred to the non-forest "forest-fallow and shrubland" category). If all tropical forestland with poor soil (90% of 17.6 million km2 of open+ closed forest) were devoted to shifting cultivation, the carrying capacity of this land would be 158 million people - 63% of the current population of shifting cultivators.
[B] ~ Tropical Deforestation via Permanent Cultivation ~
The rate of conversion of tropical forestland to permanent cropland is estimated to be 300,000 km2/decade (an average of two values - 420,000 and 160,000 from the early 1980s). This presumably occurs on the 1,400,000 km2 (10%) of tropical forestland capable of supporting permanent cropping. Converting all of this tropical forestland to permanent cropland would increase the global cropland inventory by 10%.
[C] ~ Tropical Deforestation via Grazing ~
Data on the rate of conversion of tropical forest to grazing land cover a broad range. Seiler and Crutzen (1980) estimate a conversion rate of 600,000 km2/ decade. Houghton et al (1983) make a similar estimate. Some other estimates fall into the range of 130,000 to 150,000 km2/ decade, but these estimates are parts of analyses that estimate total tropical deforestation rates of well under the presently accepted value of 1,540,000 km2/ decade. There seems to be broad agreement that the great majority of tropical forest-to-grazing-land conversions occur in Central and South America. By 1980, over 72% of Brazil's forest clearing was due to ranching. Other South American data seem consistent. In Central America about 36% of tropical forest losses seem to be attributable to grazing. Tropical South America's share of total tropical deforestation is 610,730 km2/decade, while Central America and Mexico's share is 111,200 km2/ decade. These figures indicate a total rate of tropical deforestation via grazing of 480,000 km2/ decade plus whatever occurs elsewhere in the world. Thus the Seiler-Crutzen value of 600,000 km2/ decade seems most reasonable. These data date back to the early 1980s. Since then some government subsidies for converting tropical forests to grazing lands have been abolished, but human populations have increased significantly. So it is difficult to extrapolate this figure to the mid-1990s.
The carrying capacity of grazing land created from tropical forestland is estimated to be 4 cows/km2. If all 17.6 million km2 of tropical forestland were converted to grazing land, the grazing capacity of the resultant pastureland would be 70 million "cows". Compare this to the 1986-88 grazing-animal population of the world of 1930 million "cows" (counting a sheep or goat as 1/5 "cow" and a horse, buffalo or camel as 1.2 "cows") and a sustainable grazing capacity of the world's pasturelands of about 1000 million "cows".
The irony of all this is that grazing on land created from tropical forest is not economically viable, and must be heavily government-subsidized. (Nearly all (90%?) of grazing land created from tropical forest requires a 20-30-year fallow period following each 8 or so years of grazing.) The economic ($/ year) productivity of tropical forest is far greater than that of a like area of tropical rangeland.
[D] ~ Tropical Deforestation via Urbanization ~
The rate of conversion of tropical forestland to urban uses is estimated to be about 200,000 km2/ decade. This value is based on an analysis in a companion document to this review of deforestation ("Topsoil Loss - Causes, Effects and Implications"). The assumption is made that half the global rate of conversion of forest to urban uses occurs on tropical forests. This assumption may be suspect because temperate urbanization is primarily economic-growth-driven, while tropical urbanization is primarily population-growth-driven.
[E] ~ Temperate Deforestation ~
Temperate forest soils are mostly of good quality, so shifting agriculture is not practiced there. Rates of conversion of temperate forests to non-forest uses are so low that data are not available on how temperate deforestation is to be broken down by end-use (farming, grazing, urbanization). The table below provide perspective.
Global Inventory of Temperate Forest Land (Units: millions of
A small amount of temperate forest in southern South America is neglected. Biomass per unit-area in open forests is about 15% of that in closed forests on a stemwood basis and 27% on a total biomass basis. So open forests can be almost neglected.
Europe's deforestation rate is considered to be minus 2.5%/ decade. This reflects afforestation of surplus fields and drainage of peatlands (mainly in Nordic countries).
The projected decline in US commercial timberlands over the next five decades is 1.4%/ decade. It seems reasonable to assign a similar rate to the third of US forestlands that are non-commercial.
Total tree cover in the USSR is said to be increasing, implying a negative deforestation rate. This must reflect unexplained growth of remote forests, since accessible forests are shrinking.
Canadian deforestation data are non-existent, but the sparse population and large forest area suggest that the rate is probably close to zero.
Total temperate deforestation is thus extremely small -probably around zero +200,000 km2/ decade (0%+1%/ decade). Clearly, temperate deforestation is negligible relative to tropical deforestation.
[F] ~ Sustainable-Yield on Tropical Forests ~
Sustained yield should be considered in terms of both the spirit and substance of the issue. First the spirit. The three known estimates of the fraction of tropical forests under management plans that may secure their future productivity are less than 0.1%, 0.5% and 3% - all negligible in any meaningful sense. Those wishing to evade the issue will note the development and existence of "miracle" trees that produce wood fiber much faster than average. It has been found that "miracle" trees deplete tropical soil nutrients and water, causing yields to drop dramatically after one harvest. The weak link in tropical carrying capacity has always been soil, and all known evidence suggests that this will always be the case. Now for the substance of sustained yield.
One problem with evaluating sustainable yield of the world's forests is the data. Many "forests" are "non-stocked" (have no trees). Data is also inconsistent and/or grossly doctored by the national agencies reporting it. However data are increasingly being obtained by satellite imagery, so the problem should shrink over time. Hence it is ignored here. A second problem is that tropical wood-fiber harvests are doubling over time frames ranging from 15-30 years, so any statement about sustainability of a given yield must be given a time frame. A third problem is the inherent instability of Man-forest interactions. As pressures on forests increase, harvest-rotation times drop. This causes a cycle of declining productivity and declining harvest-rotation times. The cycle ends when every new sprout is nipped off upon emergence from the ground, soils erode away, and a wasteland replaces a forest. "Forest carrying capacity" is perhaps an oxymoron. Computing the productivity of wasteland may be more meaningful for the long term.
Current inventory of tropical forest stemwood biomass (FAO data)
(Areas (Col. 2) are in units of millions of km2.)
If these inventories were equally distributed among all age classes, assuming a rotation age of 75 year enables one to compute sustainable yields of stemwood given in the table below.
Global Sustainable Yields of Stemwood
Sustainable Yield/ year (Column 2 is in billions of m3/ year)
Because much inventory is old-growth, computed sustainable yields are significantly exaggerated here. Fuel-wood sustainable yields are under-estimated to the extent that branches and twigs may also be used as fuel. E.g. sustainable yield of fuel-wood in Northeast Thailand is estimated at 265 m3/ km2/ year. (When leaves and other soil litter are raked up and used as fuel, significant soil erosion occurs, as in South Korea.)
The Sustainable Yield table above also assumes 100% efficient timber harvests. Commercial logging yields vary from 3600-3900 m3/km2 in Southeast Asia to 1200-1300 m3/ km2 in Central Africa. Assuming a 75-year rotation gives corresponding sustainable yields of 48-52 m3/ km2/ year in southeast Asia. Even these yields would have to be reduced to be sustainable because much of the timber harvested is old growth. The reason for the disparity between these numbers and Col.4 in the table above is probably because only selected tree species are used as timber and the remainder are discarded, though these could be used for fuel wood.
[G] ~ Sustainable Yield on Temperate Forests ~
The sustainable yield of round wood (including fuel wood) on temperate forests depends upon what is assumed for the intensity of forest management. European forest management is high quality, stable (little old growth) and intense (requiring high wood prices and/or high government subsidies). The remainder of the temperate world uses non-intensive management, keeping wood prices low, but yields are about half of what could be obtained under European-style management. It seems best (though optimistic) to assume that, eventually, all temperate forests will be intensively managed in the European style. European wood productivity (total of all types of wood) is about 250 m3/ km2/ year. Using the fact that stemwood productivity on open forests is about 15% of that on closed forest, and taking the ratio of open- to closed forest area in Europe to be 0.161, one obtains a productivity of European closed forests of 280 m3/ km2/ year and an open-forest productivity of 42 m3/ km2/ year. Climatic similarities make it reasonable to take these figures as also characterizing the maximum sustainable yield in United States and Australia/ New Zealand forests once old growth is gone and management becomes intense.
Assuming these figures also characterize Canada and the former USSR seems implausible, given the harsher climate and harvest data. Canada produces 62 m3/ km2/ year under non-intensive management. Yet British Columbian forests, which contains 50% of Canada's commercially available timber resources, are being over-cut by about 30%; coastal rain forests will be gone in a few decades; New Brunswick softwood harvests exceed sustained yield by at least 15%, and experts believe that only a fraction of Canada's forests are being managed on a sustained-yield basis - and that, soon, severe timber shortages will force intensive management upon Canada's forests (following in Europe's footsteps). Thus, the notion that Canada could produce 250 m3/ km2/ year sustainably seems highly implausible. An optimistic sustainable productivity for Canada's forests would be 60 m3/ km2/ year on a non-intensive basis, and hence 120 m3/ km2/ year under intensive management. Assuming a ratio of open- to closed forest area of 0.652 thus gives a closed forest productivity of 180 and an open forest productivity of 27 m3/ km2/ year. Climatic similarity suggests sustainable yields of Soviet and Canadian forests to be about the same. (Canada's forests are 75% boreal, while Russia's are nearly all boreal.)
These figures can be compared to net primary production data as a rough check. Temperate forests fix 140-500 tonnes of carbon/ km2/ year, which translates to 310-1110 tonnes of dry organic matter/ year, or about 620-2220 m3/ km2/ year. The overwhelming bulk of this goes into leaves, twigs, roots, etc. so probably no more than 10% (60-220 m3/ km2/ year) could go into stem-wood. Thus the sustainable yield figures given above seem reasonably compatible with net primary production data.
The figures deduced above can now be used to compute a maximum sustainable yield of the world's temperate forest, and this is done in the table below.
Maximum Sustainable Yield of the World's Temperate Forests
Bear in mind that this table assumes intensive management in all temperate forests. Yields under non-intensive management would be about half of that, and the total sustainable yield under current management practice would be 1.78 billion m3/ year. Even non-intensive yields are optimistic. For example, eastern US forests suffer from "high-grading", harvesting of immature timber, and regeneration problems due to deer over-populations. Arid western US forests suffer regeneration failures due to clear-cutting in regions where regeneration is dependent on new growth growing in the shade of larger trees. European, and probably eastern US, forests suffer increasing mortality from air pollution.
The tables above indicate sustainable yields from closed, non-intensively managed temperate forests of 90-140 m3/km2/ year, versus 207 for tropical forests. This is roughly consistent with the fact that net primary productivities of tropical forests are about 50% larger than those of temperate forests. When societies resort to extreme-intensity forest management - harvesting even leaves, twigs and roots, then yields of dry organic matter can increase by up to a factor of 10 (much less if rotation-ages are reduced). However such management produces severe soil erosion and land-slides that damage lower-elevation croplands, grazing lands and urban areas, suggesting that carrying capacity of the land is probably decreased. It is certainly rendered non-sustainable.
[H] ~ Comparison of Wood Consumption to Sustainable Yield ~
The 1985-87 global rate of production of fuel wood and charcoal was 1.68 billion m3/ year, while industrial roundwood production was 1.57 billion m3/ year. However this fuel wood-charcoal production rate refers only to wood sold in the marketplace. A more recent estimate (91A1) of the global rate of burning of all fuel wood and charcoal is 2.64 billion m3/ year (1.45 Gt./ year). Wood consumption has been growing 26%/ decade (doubling every 3 decades). This permits an extrapolation of the 1985-87 roundwood production rates to a mid-1990s rate of 1.98 billion m3/ year. This gives a total mid-1990s global wood-consumption rate of 4.62 billion m3/ year. Temperate forests account for 40% of this (91M1).
The 1984 sustainable yield of tropical forests was computed above to be 2.69 billion m3/ year. Correcting for an 8%/decade decrease in tropical forest area between 1984 and the mid-1990s gives a sustainable yield of 2.47 billion m3/ year. The sustainable yield of temperate forests under current management-intensity levels is computed above to be 1.78 billion m3/ year. This gives a total global sustainable yield of wood of 4.25 billion m3/ year. If all temperate forests could be managed at the European management-intensity, the sustainable yield of temperate forests would increase to 3.17 billion m3/ year, and the total global sustainable yield would increase to 5.64 billion m3/ year. However it is far from clear that temperate nations outside of Europe have the social structure (and/or could afford the government subsidies) that intense forest management requires. No tropical nation shows even the remotest capacity for intense forest management.
A global wood-consumption rate of 4.6 billion m3/ year and a sustainable yield of 4.3 billion m3/ year indicate a rough balance between consumption and sustainable yield. Keep in mind however that:
Thus the next 2-3 decades could see profound changes in global wood markets and the character of the world's forests.
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