STRUCTURALLY NOVEL BIOMES: A RESPONSE TO PAST WARMING IN BERINGIA

At northern high latitudes, biosphere responses to and interactions with climate warming are expected to be significant during the 21st century. Most predictions of climate–biosphere interactions rely on experiments and observations in contemporary landscapes, e.g., modern distributions of vegetation types and their structural features are used to delimit potential biosphere–atmosphere feedbacks. Paleorecords look beyond the present to examine vegetation configurations under climatic regimes that approximate future scenarios. To enhance the knowledge of arctic and subarctic ecosystems under varying climatic conditions, we analyzed pollen and macrofossil data from Beringia (northeast Siberia, Alaska, and northwest Canada; 130° E to 130° W) over the past 21 000 years, with a focus on structural and functional features of the vegetation. During the early Holocene (∼13 000– 10 000 cal yr BP), shrub tundra ecosystems responded to climate warming through a shift from shrub tundra to deciduous forest or woodland. Early-Holocene vegetation was structurally, and hence functionally, novel compared with today's dominant vegetation types. “Modern” boreal forest developed in the mid-Holocene (∼10 000–6000 cal yr BP), when evergreen conifers expanded in much of the region. The shift from tundra to deciduous forest could have happened rapidly and in situ as the result of individual (phenotypic) and/ or population-scale responses to climate warming. Because the structural and functional properties of deciduous forest differ from those of evergreen coniferous forest and tundra, deciduous boreal forest should be included in the range of future scenarios used to assess the probable feedbacks of vegetation to the climatic system that result from global warming at northern high latitudes.