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Policy makers across the tropics propose that carbon finance could provide incentives for forest frontier communities to transition away from swidden agriculture (slash-and-burn or shifting cultivation) to other systems that potentially reduce emissions and/or increase carbon sequestration. However, there is little certainty regarding the carbon outcomes of many key land-use transitions at the center of current policy debates. Our meta-analysis of over 250 studies reporting above- and below-ground carbon estimates for different land-use types indicates great uncertainty in the net total ecosystem carbon changes that can be expected from many transitions, including the replacement of various types of swidden agriculture with oil palm, rubber, or some other types of agroforestry systems. These transitions are underway throughout Southeast Asia, and are at the heart of REDD+ debates. Exceptions of unambiguous carbon outcomes are the abandonment of any type of agriculture to allow forest regeneration (a certain positive carbon outcome) and expansion of agriculture into mature forest (a certain negative carbon outcome). With respect to swiddening, our meta-analysis supports a reassessment of policies that encourage land-cover conversion away from these [especially long-fallow] systems to other more cash-crop-oriented systems producing ambiguous carbon stock changes - including oil palm and rubber. In some instances, lengthening fallow periods of an existing swidden system may produce substantial carbon benefits, as would conversion from intensely cultivated lands to high-biomass plantations and some other types of agroforestry. More field studies are needed to provide better data of above- and below-ground carbon stocks before informed recommendations or policy decisions can be made regarding which land-use regimes optimize or increase carbon sequestration. As some transitions may negatively impact other ecosystem services, food security, and local livelihoods, the entire carbon and noncarbon benefit stream should also be taken into account before prescribing transitions with ambiguous carbon benefits.

The application of light detection and ranging (LiDAR), a laserbased remote-sensing technology that is capable of penetrating overlying vegetation and forest canopies, is generating a fundamental shift in Mesoamerican archaeology and has the potential to transform research in forested areas world-wide. Much as radiocarbon dating that half a century ago moved archaeology forward by grounding archaeological remains in time, LiDAR is proving to be a catalyst for an improved spatial understanding of the past. With LiDAR, ancient societies can be contextualized within a fully defined landscape. Interpretations about the scale and organization of densely forested sites no longer are constrained by sample size, as they were when mapping required laborious on-ground survey. The ability to articulate ancient landscapes fully permits a better understanding of the complexity of ancient Mesoamerican urbanism and also aids in modern conservation efforts. The importance of this geospatial innovation is demonstrated with newly acquired LiDAR data from the archaeological sites of Caracol, Cayo, Belize and Angamuco, Michoacán, Mexico. These data illustrate the potential of technology to act as a catalytic enabler of rapid transformational change in archaeological research and interpretation and also underscore the value of on-the-ground archaeological investigation in validating and contextualizing results.T ransformational changes are rare in archaeology and often are best recognized in hindsight (1). However, there are times when advances in technology are so far reaching that they serve as catalysts in transforming our understanding of both the past and the practice of archaeological research, thus triggering a scientific revolution as conceptualized by Thomas Kuhn (2). We believe that advances in the remote geospatial imaging of cultural landscapes, including ancient communities and their anthropogenic hinterlands, constitute such an archeological paradigm shift. Here we present newly acquired data from two recent archaeological projects in different parts of Mesoamerica showcasing the application of light detection and ranging (LiDAR) technology to ancient urban settlements. In both cases, the nearly 3D data are changing commonly held interpretations of societal development profoundly. The phenomenon described is not limited to archaeology; technical innovations have been noted as spawning advances in other areas of research, such as astrophysics (3). However, the magnitude of change enabled by the use of LiDAR technology is particularly apparent in the archaeology of Mesoamerica.Archaeological research revolves around temporal and spatial data; thus, advances in either parameter can be transformative. Histories of archaeology (1, 4) have noted the change in the discipline from an ancient antiquarian focus on objects to the current scientific enterprise. Great enablers of archaeological research have included various technological enhancements to spatial, temporal, and functional interpretations. Although many technical imp...

) are senior fell ows in envi ro n m en t a l s tu d i e s , Ea s t-West Cen ter, Ho n ol u l u , and Ngh i em Phuong Tuyen (email: tuyen @ u plands.ac.vn) are re se a rch ers at, and Le Trong Cuc (em a i l :c u c @ u plands.ac.vn) is dire ctor of , the Cen ter for Re sou rce and Envi ro n m ental Stu d i e s , Vi etnam Na tional Un ivers i ty, Ha n o i , 1 6 7 Bui Thi Xu a n , Ha n o i , Vi etn a m . Step h en Lei sz (e-mail: s j lv m @ f pt.vn) is GIS/monito ring and eva l u a tion advi so r, C A R E In tern a tional in