摘要：As climate changes, how will terrestrial vegetation respond? Because the fates of many biogeochemical, hydrological and economic cycles depend on vegetation, this question is fundamental to climate change science but extremely challenging to address. This is particularly true in the Arctic, where temperature change has been most acute globally (IPCC 2007) and where potential feedbacks to carbon, energy and hydrological cycles have important implications for the rest of the Earth system (Chapin et al 2000). It is well known that vegetation is tightly coupled to precipitation and temperature (Whittaker 1975), but predicting the response of vegetation to changes in climate involves much more than invoking the limitations of climate envelopes (Thuiller et al 2008). Models must also consider efficacy of dispersal, soil constraints, ecological interactions, possible CO2 fertilization impacts and the changing impact of other, more proximal anthropogenic effects such as pollution, disturbance, etc (Coops and Waring 2011, Lenihan et al 2008, Scheller and Mladenoff 2005). Given this complexity, a key test will be whether models can match empirical observations of changes that have already occurred. The challenge is finding empirical observations of change that are appropriate to test hypothesized impacts of climate change. As climate gradually changes across broad bioclimatic gradients, vegetation condition may change gradually as well. To capture these gradual trends, observations need at least three characteristics: (1) they must quantify a vegetation attribute that is expected to change, (2) they must measure that attribute in exactly the same way over long periods of time, and (3) they must sample diverse communities at geographic scales commensurate with the scale of expected climatic shifts. Observation networks meeting all three criteria are rare anywhere on the globe, but particularly so in remote areas. For this reason, satellite images have long been used as a means of tracking retrospective changes in Arctic and boreal vegetation. These images are attractive because they are consistent over time, are good at mapping vegetation, are available for areas difficult to reach on the ground, and are of broad geographic extent. In a now-classic study, Myneni et al (1998) used historical reanalysis of AVHRR image data to document changes in vegetation phenology at continental scales in the northern hemisphere, finding patterns of change consistent with impacts of increased growing season in boreal and near-polar regions. The year 2000 launch of the MODIS sensors has allowed even more robust assessment of vegetation change in the Arctic (de Beurs and Henebry 2010) and at global scales (Zhao and Running 2010). Despite opening a window into vegetation change in the Arctic, these studies provide a relatively coarsely filtered view of change. To track trends occurring before the year 2000, researchers are constrained to the large pixel sizes of the AVHRR instrument (nominally 1 km, but typically 4–8 km for derived datasets). Even the finer grain of MODIS (250 m to 1 km resolution) obscures many important natural and anthropogenically derived spatial patterns. The effects of climate change may exacerbate contrasts in competitive status of different vegetative groups (Klady et al 2011, Pieper et al 2011, Seastedt et al 2004). Resolving mechanisms of response requires empirical observation at the scale of individual vegetative communities. Thus, the new work of Fraser et al (2011) represents a critical milestone in climate change related monitoring of Arctic vegetation. Their work is important in three ways. First, the authors provide the first spatially explicit, continuous record of long-term trends in Arctic vegetation condition at a pixel resolution of 30 m. Based on Landsat Thematic Mapper (TM) data reaching back to the mid 1980s, the work required the overcoming of several key methodological challenges to build a dataset from which trend data could be extracted. While other studies have used TM data to map change across two to three points in time to evaluate change in boreal or arctic vegetation under climate change (Masek 2001, Ranson et al 2004), the time-series analysis of Fraser et al (2011) allows the detection of subtle trends not typically discernible with two-date comparisons. Second, the findings of Fraser et al confirm that vegetation in the Arctic appears to be responding to warming, particularly winter warming. Their maps represent a basic empirical dataset whose spatial patterns should be replicated by model-based approaches. Third, the comparatively fine resolution of this study relative to previous satellite studies allows Fraser et al to more precisely locate where greening occurs. They note increases in shrub cover, confirming findings from studies that used other approaches to reach similar conclusions (Stow et al 2004, Tape et al 2006). They also find evidence that both shrub and herbaceous growth occurs more in landscape positions already favorable to productive vegetation, such as lower hillslopes and valleys, and less so on uplands. Such findings will certainly factor into discussions of the mechanism by which warming could facilitate vegetative growth in Arctic communities (Forbes et al 2010, Sturm et al 2005). The work of Fraser et al (2011) also adds to a growing body of work leveraging free Landsat data from the US Geological Survey's (USGS) archive. As the longest-running continuous satellite image dataset for land processes, Landsat data provide unparalleled witness to the enormous changes occurring on Earth since 1972 (Wulder et al 2008). By opening the US holdings of the Landsat archive to all humans on the planet (Woodcock et al 2008), the USGS in 2008 catalyzed a blossoming of approaches to capture and characterize that change (Goodwin et al 2010, Hais et al 2009, Hansen et al 2010, Huang et al 2010, Kennedy et al 2010, Potapov et al 2011, Vogelmann et al 2009). While the USGS archive has been dominated by imagery from the United States, recent efforts by the USGS to repatriate data stored in international archives are adding new historical images to the archive every day. With persistence and the goodwill of collaborating countries, this effort may someday allow analyses similar to that of Fraser et al across broader expanses of the Earth, providing further insights into the mechanisms and manifestations of climate change. References Chapin F S et al 2000 Arctic and boreal ecosystems of western North America as components of the climate system Glob. 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