The quantity and quality of land use directly and indirectly relates to many “grand challenges” in sustainability science (Vitousek, 1997; Rindfuss et al., 2004; Global Land Project, 2005; Steffen et al., 2007; Turner et al., 2007). Land use is a major driver for habitat encroachment and biodiversity loss (Sala et al., 2000), for the alterations of global biogeochemical cycles (Gruber and Galloway, 2008; Postel et al., 1996; Vitousek et al., 1997) and for soil degradation (Lal, 2004). Changes in land use and subsequent changes in land cover play a central role in the global carbon cycle and significantly contribute to anthropogenic climate change (Brovkin et al., 2004; Canadell et al., 2007; McGuire et al., 2001; Watson et al., 2000). On the other hand, land use provides the nutritional basis for humans and thus of any socioeconomic system, and is intrinsically linked to food security (Ayres, 2007; Foley et al., 2005; Millennium Ecosystem Assessment, 2005). Research on global land use has a long tradition, reaching back to the work of G.P. Marsh (1865) and A. Von Humboldt (1849). It gained momentum in sustainability research in the mid-1970s, when the impact of land use on the global surface albedo was recognized (Lambin et al., 2006). Since then, many aspects of land use have been assessed, quantified and mapped across spatio-temporal scales. Two aspects of land use changes can be distinguished: (a) Changes in land cover, i.e. alterations of biophysical characteristics of the Earth's surface, e.g. by expansion or contraction of a certain land use type; a prominent example would be the expansion of agricultural fields into pristine forests. (b) Changes in land use intensity, denoting changes in the levels of socioeconomic inputs (e.g., labour, resources, water, energy or capital) and/or altered output (value or quantity) per unit area and time. Changes in intensity need not result in changes in land cover, but cause ecological changes within the same land cover type. Increasing land use intensity stands in an inverse relation to land expansion for increasing production. Consequently, a major effect of intensification may be to “spare” land, e.g. for wilderness conservation, by concentrating production on other areas (Tilman, 2001). Indeed, this effect is often assumed to be essential for many sustainability aspects, as it allows to reduce area demand and avoid considerable carbon emissions from deforestation (Burney et al., 2010) or habitat encroachment (Green et al., 2005). In the future, safeguarding the land-sparing effect of intensification could become decisive, given the rising nutritional and energy demands of a growing world population, and the concomitant need to protect the shrinking untouched habitats of the Earth, rich in biodiversity and carbon. Moreover, many policies that aim at harnessing land use for the goals of climate change mitigation, such as strategies aimed at expanding bioenergy production, or at reducing greenhouse gas emissions from deforestation and forest degradation (REDD), will probably not be effective without the land sparing effect of intensification. On the other hand, many technologies required for intensification are associated with detrimental ecological impacts, such as the accumulation of toxins in food, ecosystem and soil degradation, groundwater and air pollution, or biodiversity loss (IAASTD, 2009; Matson et al., 1997; Millennium Ecosystem Assessment, 2005; Tilman, 2001). Such processes negatively affect the ability of ecosystems to sustain vital ecosystem services, thereby running the risk of jeopardizing human well-being in the long run (Foley et al., 2005). Thus, it will become imperative to find ways of sustainable intensification (Tilman et al., 2002) that allow reaping its land-sparing benefits while at the same time avoiding the detrimental social and ecological effects. However, the interrelation between intensification and expansion of land use is far from trivial. Empirical analyses of Rudel et al. (2009) on the interrelation between past trajectories in cropland expansion and intensification resulted in inconclusive findings. At the national scale, land use intensification was paired with a decline or stasis in cropland area between 1970 and 2005 only in countries that “externalized” agricultural production (e.g. grain imports) or preserved land with explicit land conservation programs (Rudel et al., 2009). These counterintuitive findings may be explained not only by large data gaps and uncertainties (Grainger, 2009), but also by feedback loops of higher order, such as a rebound effect of consumption to increased production, that overcompensated the land-sparing effect (Lambin and Meyfroidt, 2011). This altogether casts doubts on the straightforward interpretations or scenario-based extrapolations of the beneficial effects of land intensification strategies. These feedback loops of land transitions are active across a wide range of spatial and temporal scales (Global Land Project, 2005; Lambin and Geist, 2005; Bennett and Balvanera, 2007; Erb et al., 2009b; Lambin and Meyfroidt, 2011). To take such feedbacks into account is indispensable, but it poses a formidable challenge to land change science (Turner et al., 2007), as it requires innovative methods and new perspectives that allow for the construction of sound causal chains between the various factors, mechanisms, determinants and constraints that underpin land-use intensification processes. In this commentary, I discuss the potential contribution of an extension of the socioeconomic metabolism concept (Ayers and Simonis, 1994; Ayres, 1989; Fischer-Kowalski and Huttler, 1998) by accounts that create an integrated picture of socio-ecological flows (Erb et al., 2008; Haberl et al., 2004; Krausmann et al., 2004) to global land system science. Such an approach could help to develop an analytical framework for conceptualizing and reporting on the complex, systemic interactions related to land use intensification, including feedbacks between production and consumption. It thus might give guidance for data collection and analysis, and so enhance the understanding of the interplay between land expansion and intensification.

中文翻译：

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社会生态代谢方法如何有助于促进我们对土地利用强度变化的理解

土地利用的数量和质量直接和间接地与可持续性科学中的许多“重大挑战”相关（Vitousek，1997；Rindfuss 等，2004；全球土地项目，2005；Steffen 等，2007；Turner 等， 2007）。土地利用是栖息地侵占和生物多样性丧失的主要驱动因素（Sala 等，2000），以及全球生物地球化学循环的改变（Gruber 和 Galloway，2008；Postel 等，1996；Vitousek 等，1997）和土壤退化（Lal，2004 年）。土地利用的变化和随后的土地覆被变化在全球碳循环中发挥着核心作用，并对人为气候变化做出了重大贡献（Brovkin 等人，2004 年；Canadell 等人，2007 年；McGuire 等人，2001 年；Watson 等人）等人，2000）。另一方面，土地利用为人类以及任何社会经济系统提供了营养基础，并且与粮食安全有着内在联系（Ayres，2007 年；Foley 等人，2005 年；千年生态系统评估，2005 年）。全球土地利用研究有着悠久的传统，可以追溯到 GP Marsh (1865) 和 A. Von Humboldt (1849) 的工作。在 1970 年代中期，当土地利用对全球地表反照率的影响得到承认时，它在可持续性研究中获得了动力（Lambin 等人，2006 年）。从那时起，土地利用的许多方面都在时空尺度上进行了评估、量化和映射。可以区分土地利用变化的两个方面： (a) 土地覆盖的变化，即地球表面生物物理特性的改变，例如某种土地利用类型的扩张或收缩；一个突出的例子是将农田扩展到原始森林。(b) 土地利用强度的变化，表示每单位面积和时间的社会经济投入水平（例如劳动力、资源、水、能源或资本）和/或改变的产出（价值或数量）水平的变化。强度的变化不一定会导致土地覆被的变化，但会导致同一土地覆被类型内的生态变化。增加土地利用强度与增加生产的土地扩张成反比。因此，集约化的主要影响可能是通过将生产集中在其他地区来“节省”土地，例如用于荒野保护（Tilman，2001）。事实上，这种影响通常被认为对许多可持续性方面至关重要，因为它可以减少面积需求并避免因森林砍伐（Burney 等人，2010 年）或栖息地侵占（Green 等人，2005 年）而产生的大量碳排放。将来，鉴于不断增长的世界人口对营养和能源的需求不断增加，以及随之而来的需要保护地球上不断缩小的、未受破坏的、富含生物多样性和碳的栖息地，保护集约化的土地节约效应可能具有决定性意义。此外，许多旨在利用土地利用来缓解气候变化目标的政策，例如旨在扩大生物能源生产或减少因森林砍伐和森林退化 (REDD) 造成的温室气体排放的战略，如果没有土地，可能就不会有效。集约化的保留效应。另一方面，集约化所需的许多技术与有害的生态影响有关，例如食物中毒素的积累、生态系统和土壤退化、地下水和空气污染、或生物多样性丧失（IAASTD，2009；Matson 等，1997；千年生态系统评估，2005；Tilman，2001）。这些过程对生态系统维持重要生态系统服务的能力产生负面影响，因此从长远来看有危害人类福祉的风险（Foley 等，2005）。因此，必须找到可持续集约化的方法（Tilman et al., 2002），既能获得节约土地的好处，又能避免有害的社会和生态影响。然而，土地利用的集约化和扩张之间的相互关系绝非微不足道。Rudel 等人的实证分析。（2009）关于农田扩张和集约化的过去轨迹之间的相互关系导致了不确定的发现。在全国范围内，1970 年至 2005 年间，只有在“外部化”农业生产（例如粮食进口）或通过明确的土地保护计划保护土地的国家，土地利用集约化与耕地面积的下降或停滞相结合（Rudel 等，2009）。这些违反直觉的发现不仅可以用大的数据差距和不确定性来解释（Grainger，2009），还可以用更高阶的反馈循环来解释，例如消费对增加产量的反弹效应，过度补偿了土地节约效应（Lambin 和梅弗罗特，2011）。这完全使人们对土地集约化战略的有益效果的直接解释或基于情景的推断产生了怀疑。这些土地转换的反馈回路在广泛的空间和时间尺度上都很活跃（全球土地项目，2005；兰宾和盖斯特，2005；贝内特和巴尔瓦内拉，2007；Erb 等人，2009b；兰宾和梅弗罗特，2011）。考虑到这些反馈是必不可少的，但它对土地变化科学提出了巨大的挑战（Turner 等，2007），因为它需要创新的方法和新的视角，以便在各种因素之间建立健全的因果链，支持土地利用集约化进程的机制、决定因素和制约因素。在这篇评论中，我讨论了社会经济新陈代谢概念扩展的潜在贡献（Ayers 和 Simonis，1994；Ayres，1989；Fischer-Kowalski 和 Huttler，1998）通过创建社会生态流动的综合图景（Erb等人，2008 年；Haberl 等人，2004 年；克劳斯曼等人，2004 年）到全球土地系统科学。这种方法有助于开发一个分析框架，用于概念化和报告与土地利用集约化相关的复杂、系统的相互作用，包括生产和消费之间的反馈。因此，它可能为数据收集和分析提供指导，从而增强对土地扩张与集约化之间相互作用的理解。