HANPP An Introduction

Plants absorb solar radiation and, through photosynthesis, transform it into chemically stored energy. This process is called primary production. A part of the fixed energy is used for the plant's metabolism; the remainder either results in an accumulation of biomass stocks or nourishes humans, animals, fungi, or microorganisms; that is, it becomes part of heterotrophic food chains (Odum 1971). Net primary production (NPP) is the net amount of primary production after the costs of plant respiration (i.e., the energy needed for the plant's metabolism) are included; it equals the amount of biomass produced. HANPP is the fraction of NPP appropriated by humans and has been used to assess human domination of the earth's ecosystems (Vitousek et al. 1986, 1997; Whittaker and Likens 1973).

Definition of HANPP

Vitousek et al. (1986) calculated HANPP using three different definitions. The most narrow definition regarded only biomass used by society (e.g., food, timber) as appropriated, the intermediate definition additionally included the NPP of human-dominated ecosystems (e.g., cropland), and the third definition also considered an assessment of the NPP foregone because of human-induced changes in ecosystem productivity (e.g., ecosystem degradation).

Vitousek's first and second definition could lead to problematic results, however. As demonstrated for Austria, changes in agricultural technology increased aboveground productivity on agricultural land by a factor of 2.6 from 1830 to 1995 (Krausmann 2001). Consider, for example, 1 hectare of this cropland: According to Vitousek's intermediate definition (also used by Rojstaczer et al. 2001), one would find an increase in HANPP of about 2.6 because of the increase in harvest, although the NPP remaining in the ecosystem stayed near zero, because the increase in the agroecosystem's productivity was compensated for by a similar increase in harvest.

On the other hand, regarding all NPP of human-dominated ecosystems as appropriated is also problematic: In forest plantations and grasslands a large fraction of the NPP remains in the ecosystem and supports food chains not directly controlled by humans. This argument has already been used to question the HANPP concept altogether (Davidson 2000).

Wright (1990), who was interested primarily in the possible impact of HANPP on biodiversity, proposed to define HANPP as the difference in NPP available in (hypothetical) undisturbed ecosystems and the amount of NPP actually available to support heterotrophic food chains. This definition seems to overcome some of the problems associated with Vitousek's approach. However, Wright excluded activities such as logging and biomass burning in forests on the ground that harvest in forests, though removing energy, does not result in a long-term reduction of productivity of the land for wild species if forests are allowed to regrow. Although this argument may be correct as long as nutrient-rich parts (e.g., leaves) remain in the forest, it does not justify the exclusion of wood harvests from the definition of HANPP because harvest and biomass burning are very important for forest ecology (Harmon et al. 1986).

We have therefore defined HANPP (Haberl 1997) by measuring changes in the availability of NPP for ecological processes induced by alterations of the productivity of vegetation that result from land use and extraction of NPP from ecosystems through biomass harvest, including wood harvest in forests. HANPP is thus the difference between NPP0, the NPP of potential vegetation (Tuxen 1956), and NPPt, the part of the NPP of actual vegetation (NPPact) remaining in ecosystems after harvest (NPP^)1

HANPP can be expressed as material (kilograms dry matter), substance (kilograms carbon), or energy flow (joules) or as a percentage of NPP0 (%HANPP = HANPP/NPP0 X 100).

This definition of HANPP is appropriate for interregional comparisons and time series analysis. By monitoring HANPP and its various components, such as NPPact, NPPt, and NPPh, we can evaluate the impacts of different land use practices on ecosystem energetics and their socioeconomic performance: land use may increase or reduce productivity, it may leave more or less energy in the ecosystem, it may yield rich or poor harvests, and so on. Thus, we are also able to observe a possible decoupling of biomass harvest and HANPP (Krausmann 2001). This definition of HANPP does not exag gerate human impact by including all NPP of human-dominated ecosystems as appropriated. HANPP includes only the amount of biomass actually harvested, on top of the NPP prevented by human land use. It is possible to assess HANPP in great spatial detail by combining statistical data with land cover data derived from remote sensing (Haberl et al. 2001).

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