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Tropical mountains contain unique biological diversity, and are subject to many consequences of global climate change, exasperated by concurrent socioeconomic shifts. Glaciers are in a negative mass balance, exposing substrates to primary succession and altering downslope wetlands and streams. A review of recent trends and future predictions suggests a likely reduction in areas of open habitat for species of high mountains due to greater woody plant cover, accompanied by land use shifts by farmers and pastoralists along the environmental gradients of tropical mountains. Research is needed on the biodiversity and ecosystem consequences of successional change, including the direct effects of retreating glaciers and the indirect consequences of combined social and ecological drivers in lower elevations. Areas in the high mountains that are protected for nature conservation or managed collectively by local communities represent opportunities for integrated research and development approaches that may provide ecological spaces for future species range shifts.

The extent and the spatial patterns of landscape transformation we observe today are the result of the historic human settlement process, often dating back hundreds or thousands of years. Analyzing and reconstructing those historical patterns helps to advance the understanding of the dynamics and persistence of present-day ecosystems. This article explores this reconstruction by identifying and analyzing historic drivers of landscape change for seven periods between 1500 and 2000, and presents historical land use maps showing major trends and impacts on natural ecosystems. Historic land use maps were built using an ecosystem map of 1998 and a \"preclearing\" ecosystem map, by constraining the spatial change of transformed areas using data on accessibility to rivers and roads, elevation, slope, moisture availability, and settlement areas. We estimate the transformed area rose from approximately 15 Mha in 1500 to 42 Mha in 2000, and land use changed from cropping in 1500 to predominantly grazing in 2000. Demographic impacts of colonization and the introduction of cattle were major drivers of change, but rates and trends of land cover change varied between regions and from period to period. The most impacted ecosystems have been the Andean and tropical dry forests, with the most recent trends toward clearing of humid lowland forests, especially in the Amazon and Pacific. Some landscapes have been subject to strong human influence continuously for more than 500 years, whereas others have been transformed for less than thirty years. We discuss the relevance of a historical approach for guiding conservation goals, ecological restoration efforts, and research hypotheses.

The classic story of historical land-cover change in the United States suggests that agricultural clearing in the 1800s was followed by agricultural abandonment at the turn of the twentieth century and subsequent forest regrowth-often referred to as a forest transition. Most descriptions present statistical data from historical censuses and surveys to make this case. Here we show that the historical data on cropland and forest area change for the United States need to be interpreted with care. Some earlier studies have exaggerated the extent of cropland abandonment and forest regrowth by failing to account for changes in definitions of croplands over time and changes in political boundaries in the case of forests. We reexamined the historical data to find that cropland and forest area for the United States as a whole have not undergone large-scale abandonment and regrowth but rather stabilized around the mid-twentieth century. Moreover, we find that, consistent with local and regional case studies, croplands were indeed abandoned in the eastern portions of the continent accompanied by forest regrowth, but there was compensating cropland expansion and forest clearing in the west. Our study suggests the need to exercise caution when using historical data to understand land-cover change and for developing theories such as forest transition. [Supplemental material is available for this article. Go to the publisher's online edition of Annals of the Association of American Geographers for the following free supplemental resource: (1) a table of cropland harvested area for the states of the United States from 1879 to 2002.]

Coupled human and natural systems (CHANS) are characterized by many complex features, including feedback loops, nonlinearity and thresholds, surprises, legacy effects and time lags, and resilience. Agent-based models (ABMs) are powerful for handling such complexity in CHANS models, facilitating in-depth understanding of CHANS dynamics. ABMs have been employed mostly on a site-specific basis, however. Little of this work provides a common infrastructure with which CHANS researchers (especially nonmodeling experts) can comprehend, compare, and envision CHANS processes and dynamics. We advance the science of CHANS by developing a CHANS-oriented protocol based on the overview, design concepts, and details (ODD) framework to help CHANS modelers and other researchers build, document, and compare CHANS-oriented ABMs. Using this approach, we show how complex demographic decisions, environmental processes, and human-environment interaction in CHANS can be represented and simulated in a relatively straightforward, standard way with ABMs by focusing on a comparison of two world-renowned CHANS: the Wolong Nature Reserve in China and the Chitwan National Park in Nepal. The four key lessons we learn from this cross-site comparison in relation to CHANS models include how to represent agents and the landscape, the need for standardized modules for CHANS ABMs, the impacts of scheduling on model outcomes, and precautions in interpreting \"surprises\" in CHANS model outcomes. We conclude with a CHANS protocol in the hope of advancing the science of CHANS.

In arid regions of the world, the conversion of native vegetation to agriculture requires the construction of an irrigation infrastructure that can include networks of ditches, reservoirs, flood control modifications, and supplemental groundwater pumping. The infrastructure required for agricultural development has cumulative and indirect effects, which alter native plant communities, in parallel with the direct effects of land use conversion to irrigated crops. Our study quantified historical land cover change over a 150-year period for the Walker River Basin of Nevada and California by comparing direct and indirect impacts of irrigated agriculture at the scale of a 10,217 km 2 watershed. We used General Land Office survey notes to reconstruct land cover at the time of settlement (1860-1910) and compared the settlement-era distribution of land cover to the current distribution. Direct conversion of natural vegetation to agricultural land uses accounted for 59 percent of total land cover change. Changes among nonagricultural vegetation included shifts from more mesic types to more xeric types and shifts from herbaceous wet meadow vegetation to woody phreatophytes, suggesting a progressive xerification. The area of meadow and wetland has experienced the most dramatic decline, with a loss of 95 percent of its former area. Our results also show Fremont cottonwood, a key riparian tree species in this region, is an order of magnitude more widely distributed within the watershed today than at the time of settlement. In contrast, areas that had riparian gallery forest at the time of settlement have seen a decline in the size and number of forest patches.

The Three Gorges Project (TGP) in the mountainous middle reach of the Yangtze River entailed large dam construction and farmland inundation by reservoir. Concurrently, national policy mandated defarming and reforesting extensive erosion-prone slope farmlands in the region. This study evaluated social-ecological impacts of two parallel land-cover change projects on farmland supply, carrying capacity, resettlement, and alternative employment. The integrated research adopted field assessment, interviews, questionnaire surveys, and remote sensing and geographic information system techniques. Analysis of high-resolution QuickBird images and thematic maps generated digital maps on land use, farmland quality, and standardized farmland. The two projects, development versus conservation, accentuated and disseminated impacts on rural communities. Reservoir inundation eliminated the best farmlands in riverside villages and aggravated population pressure. Widespread poor farmlands on steep slopes required extensive defarming and reforestation. The double blow reduced carrying capacity and brought economic hardship in impoverished areas. Villages beset by farmland deficits encountered difficulty decanting the surplus population because of resistance to intervillage and nonlocal resettlement. Migrant labor, as transient rural-urban migration operating outside the official regime, offered alternative employment and supplementary income to alleviate overpopulation and poverty. This safety valve was neither equally accessible to households nor assisted by the government. The simultaneous execution of resettlement and reforestation called for a systematic rural-urban migration program, engagement of local people in policy formulation, and provision of incentives in the form of long-term market benefits and livelihood assurances. The findings offered hints and precautions on the joint implementation of major land-cover change projects in developing regions.