Experimentación para asumir la novedad en las relaciones del taller

Terraced rice fi elds covering formerly forested hills of South China ’ s Yunnan province exem-plify the extent of anthropogenic land use changes and the intensity of harvests. A high-resolution image can be downloaded at http://upload.wikimedia.org/wikipedia/commons/1/

16/Terrace_fi eld_yunnan_china.jpg .

Adding Up the Claims 153

Three fundamental limitations make any large-scale, long-term accounting of human claims on the biosphere ’ s production challenging — and uncertain. First, these claims belong to three different categories that might simply be labeled extraction, manage-ment, and destruction, but a closer look shows that there are overlaps and blurred boundaries. Second, no single measure can adequately express the increasing extent and the overall magnitude of human claims on the biosphere. Third, although a combination of several revealing variables provides a better assessment of these claims, it does not bring a fully satisfactory appraisal because of the many uncer-tainties in quantifying the natural baselines and assessing the true extent of human interventions. Every one of these summary descriptions requires a closer examination.

Fishing and whaling, the killing of elephants for ivory, the capture of wild animals for the pet market, and the collecting of wild plants and mushrooms are common examples of extractive activities. Agriculture is the dominant activity in the manage-ment category ( “ constructive transformation ” could be another term): natural eco-systems are replaced by managed agroecoeco-systems that produce annual crops or are composed of perennial species, and although either the original vegetation has been completely replaced or its makeup has been changed beyond recognition, the site continues to produce phytomass and to support nonhuman heterotrophs. In con-trast, the destructive transformation of natural ecosystems alters or entirely elimi-nates a site ’ s primary production capacity as it destroys natural vegetation either to extract minerals or to expand settlements: industrial facilities and residential and commercial buildings also need associated space for requisite transportation net-works, the storage of raw materials, and waste disposal.

But complications and qualifi cations are obvious. Extractive activities target par-ticular species, and as long as they are well managed, they can continue with minimal adverse effects. But even in such cases there may be a great deal of associated damage, for the harvesting of a target species may kill or injure other species (even those belonging to a different class of animals, as exemplifi ed by dolphins caught in tuna purse seines). And once the target species gets overexploited or harvested to extinction, such actions will have changed not only the composition of affected communities and ecosystems but also their long-term dynamics and overall produc-tivity: an entire ecosystem can be affected by such degradation, sometime irrevers-ibly. Quantifying the human intervention only in terms of the specifi cally extracted biomass captures only part of the actual ecosystem effect. At the same time, even a complete removal of a key heterotroph may have no effect on the overall level of primary productivity or may actually enhance it: the removal of elephants, whose

heavy browsing of woody phytomass and toppling of trees help keep savannas open, is a key reason for the regrowth of shrubs and trees.

Permanent cropping replaces natural vegetation with domesticated species and creates new agroecosystems that are nearly always much less diverse in comparison with the original communities. The contrast is obviously greatest where the domi-nant cultivation takes the form of monocultures planted over vast areas. Crop rotations offer a more rational management approach, but even they will under-perform compared with their natural predecessors. Agriculture ’ s effects on the standing phytomass and on primary productivity fi t a clear pattern of overall loss in all instances where mature and rich natural ecosystems were replaced with a low-yielding annual crop in a climate whose short vegetation period makes only one harvest possible.

Converting a patch of a boreal forest to a wheat fi eld, a transformation common in medieval Europe or nineteenth-century Canada, replaced an ecosystem storing several hundred tons of aboveground phytomass per hectare with an agroeco-system whose peak preharvest aboveground phytomass was at best 3 t/ha. Bondeau et al. (2007) modeled this effect globally and found that global vegetation has been reduced by about 24% as a result of agriculture, and that the annual phytomass harvest was reduced by 6 – 9 Gt C during the 1990s. They also displayed their fi nd-ings in a map grading the phytomass storage and net primary productivity (NPP) difference between the existing farmland and previous natural vegetation.

For most of the world ’ s cultivated land the difference between carbon stocks in cultivated plants and in the vegetation of natural ecosystems that used to occupy their place is more than 2 kg/m ² (200 kg C/ha). Moreover, and somewhat counter-intuitively, the difference is greatest (more than 1 t C/ha) in some of the world ’ s most productive agricultural regions, including the Corn Belt in the United States and parts of Europe, Russia, and China, as even high wheat, corn, and rice yields remain far below the levels of carbon stored in mature forests or rich grasslands that were converted to fi elds. The only area where agriculture accumulates more carbon than natural vegetation would have done in the same places is where irriga-tion boosts productivity in arid regions, such as Egypt or Pakistan and northern India: there the crops will store more than the short-grass communities they replaced.

On a small scale, such gains are possible even in areas where the losses are domi-nant. Converting temperate grassland into a crop fi eld producing an annual harvest of corn with alfalfa as a winter crop can result in an overall phytomass and pro-ductivity gain. The NPP of a good European meadow may be 10 – 15 t/ha, while a modern corn crop will yield 8 – 10 t/ha of grain and an identical mass in stover, and

Adding Up the Claims 155

a single cut of nonirrigated alfalfa can add more than 5 t/ha, and as many as fi ve cuts are possible (Ludwick 2000). Bradford, Lauenroth, and Burke (2005) quantifi ed such a productivity gain for the Great Plains of the United States, the country ’ s premiere agricultural region, and concluded that compared with native vegetation, cultivation is increasing the region ’ s NPP by about 10%, or nearly 100 Mt/year. But while the primary productivity (and even a temporary phytomass storage) have been enhanced, the specifi c composition has been simplifi ed, and the biodiversity of the entire ecosystem (as well as some of its natural services: in the case of row crops such as corn, the key concern would be the compromised protection against soil erosion) has been reduced.

Matters are no easier when looking at the transformations of forests. Shifting agriculture may remove all natural vegetation within a cultivated patch, but it lets the original plant communities reassert themselves after a period of cropping, and if the regeneration period is relatively long and the temporary plantings are sur-rounded by still intact growth, the regenerated forest may regain most of its original biodiversity. A great deal of historical evidence shows that even some large-scale deforestation followed by decades or even centuries of permanent cropping is sub-stantially reversible.

One of the best-documented examples of this large-scale recovery is the case of Massachusetts ’ s forests. In 1700, some 85% of the state ’ s area was covered by forests, but 150 years later Henry David Thoreau noted in his Journals that “ our woods are now so reduced that the chopping this winter has been a cutting to the quick. ” By 1870, when the clearing reaching its greatest extent, only about 30% of the state was covered by trees, but by the end of the twentieth century the cover was back up to about 70% (Foster and Aber 2004). The history of Massachusetts forests also illustrates how the interplay of natural and human factors determines long-term outcomes. The 1938 hurricane, which damaged more than 70% of all wood volume in the Harvard Forest, was a perfect reminder of the fact that only a minority of trees in the U.S. Northeast can live out their maximum natural life span.

as the region is (infrequently but assuredly) subject to major hurricanes. And the massive death of hemlocks, originally one of three dominant species in the state ’ s natural forests, illustrates the impact of pests against whose attack there is no known defense.

Contrasts of single measures, even if they are such critical variables as the total standing phytomass or its annual productivity, cannot capture the true impact of human transformations of natural ecosystems — but neither can a list of species that have been endangered or eliminated by such changes, or a calculation of complex

indices designed to quantify the overall loss of biodiversity: after all, even simplifi ed anthropogenic ecosystems can be aesthetically pleasing, highly productive, and, when properly managed, also fairly resilient. The best way to judge the human impact is to look at a number of indicators. Chapter 10 concentrates on the evolution of postglacial global phytomass storage and on the land-cover changes brought about by food production, deforestation, industrialization, and urbaniza-tion. Chapter 11 takes apart the concept of NPP appropriation by humans, and chapter 12 concludes the book with some refl ections on the evolution, extent, impact, and future of the human presence in the biosphere.

The most obvious, and conceptually the simplest, indicator of the human impact on the biosphere ’ s productivity and phytomass storage is the total area of the natural ecosystems that have been transformed by human action. To put these changes into an evolutionary context, I will begin with a brief review of phytomass storage during the past 20 millennia, since the last glacial maximum (LGM), when North America north of 50 ° N and much of Europe beginning at only a slightly higher latitude were covered by massive continental glaciers. Primary productivity and carbon strongly rebounded during the next 15 millennia, reaching maxima by the mid-Holocene, some 5,000 – 6,000 years ago at the time of the fi rst complex civilizations. This was followed by millennia of locally severe and regionally substantial transformations whose global impact remained still relatively minor.

The most common kind of conversion was to create new fi elds by converting forests, grasslands, and wetlands (and in some regions also deserts) to new croplands and pastures. Next in overall importance have been the claims made on forest phytomass to remove timber and fi rewood and, starting in the nineteenth century, wood for making paper. These human transformations of ecosystems began to accelerate during the early modern era (after 1600) and reached an unprecedented pace and extent thanks to the post-1850 combination of rapid population increases and economic growth, marked by extensive urbanization, industrialization, and the construction of transportation networks.

Tracing these changes can be revealing even when most of the conclusions must be based on estimates and approximations, and a closer look is rewarded by a more nuanced understanding. At one extreme are those areas whose plant cover has been entirely lost by conversions to constructed impervious surfaces (such as pavement) in urban and industrial areas and transportation corridors: the primary production of these areas has been completely eliminated. At the other extreme are natural

10