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Until now, Quaternary paleoecologists have regarded evolution as a slow process relative to climate change, predicting that the primary biotic response to changing climate is not adaptation, but instead (1) persistence in situ if changing climate remains within the species' tolerance limits, (2) range shifts (migration) to regions where climate is currently within the species' tolerance limits, or (3) extinction. We argue here that all three of these outcomes involve evolutionary processes. Genetic differentiation within species is ubiquitous, commonly via adaptation of populations to differing environmental conditions. Detectable adaptive divergence evolves on a time scale comparable to change in climate, within decades for herbaceous plant species, and within centuries or millennia for longer-lived trees, implying that biologically significant evolutionary response can accompany temporal change in climate. Models and empirical studies suggest that the speed with which a population adapts to a changing environment affects invasion rate of new habitat and thus migration rate, population growth rate and thus probability of extinction, and growth and mortality of individual plants and thus productivity of regional vegetation. Recent models and experiments investigate the stability of species tolerance limits, the influence of environmental gradients on marginal populations, and the interplay of demography, gene flow, mutation rate, and other genetic processes on the rate of adaptation to changed environments. New techniques enable ecologists to document adaptation to changing conditions directly by resurrecting ancient populations from propagules buried in decades-old sediment. Improved taxonomic resolution from morphological studies of macrofossils and DNA recovered from pollen grains and macroremains provides additional information on range shifts, changes in population sizes, and extinctions. Collaboration between paleoecologists and evolutionary biologists can refine interpretations of paleorecords, and improve predictions of biotic response to anticipated climate change.

Ecosystem responses to past climate change can provide insight into plausible scenarios of response to future change and can elucidate factors that may influence the overall predictability of such responses. I explore the utility of paleoecological studies for addressing questions about the predictability of ecosystem responses to climate change using Alaskan tree line ecosystems as a case study. Published studies were used to develop a regional analysis of patterns of recent tree line advance, and to estimate lags between recruitment onset and forest development beyond tree line. Tree line advance is ubiquitous, but asynchronous in time, occurring significantly earlier in the White Mountains in interior Alaska than in western Alaska or the Alaska Range. The mean lag between initiation of recruitment and forest development was estimated at approximately 200 years, similar to what modeling studies have found. Although continued advance of white spruce forests is the most likely scenario of future change, variability in the rate of forest response to warming may be likely due to limitation of spruce establishment in highly permafrost-affected sites, changes in seed dispersal and early establishment, and recent changes in the growth responses of individual trees to temperature. All of these factors may cause spruce populations to exhibit nonlinear responses to future warming, and uncritical extrapolation from recent trends is thus unwarranted.

Palaeoecological studies are yielding fresh insights into slow forest ecosystem processes that are rarely observed using standard ecological methods, yet have major impacts on ecosystem function. Regional pollen data describe the broad features of the regional spread of trees but yield few insights into the processes of stand invasion and the facilitating role of disturbance. Pollen and charcoal data from small forest hollows are used to complement regional data in the study of the spread of Picea abies and Fagus sylvatica into southern Scandinavia during the last 4000 years. P. abies spread as a migrating front and preferentially invaded successional Betula stands, which had become particularly widespread in the region during the last 1000 years as a result of human activity. The spread of P. abies also closely tracked the changing area of suitable regional climate. The spread of F. sylvatica was more directly linked to anthropogenic activities and disturbance by fire prior to stand establishment. F. sylvatica preferentially invaded rich deciduous stand types that had declined in abundance during the last 2000 years. A recent range reduction of F. sylvatica can also be ascribed to human activity. The stand-scale palaeoecological data show how site conditions and disturbance are more important rate-limiting factors for F. sylvatica than for P. abies and help explain why F. sylvatica spread shows a patchy dynamic rather than the smoother migrating front of P. abies.

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. Eariy-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.

Phenotypic differences among species, even closely related species, may translate into distinct effects on ecosystem dynamics. In lakes, the generalist grazer genus Daphnia often has marked effects on the abundance of primary producers, the rate of primary production, and rates of nutrient cycling. The effects are particularly distinct during the clear-water phase (CWP) when algal biomass is driven to extremely low values as Daphnia densities undergo an annual population increase. Here we show that the timing of the CWP in Onondaga Lake, New York, USA, has depended upon which Daphnia species were present in the water column. An analysis of the ephippia and diapausing eggs from the sediments reveals that long-term changes in the Zooplankton species assemblage tracks a history of chemical (especially salt waste) pollution. Prior to 1930 the assemblage was dominated by native D. pulicaria and D. ambigua. From 1930 to 1980, these species were replaced by D. exilis and D. curvirostris, two salinity-tolerant exotic species native to shallow salt pools of the southwestern United States and coastal brackish ponds of Europe, respectively. As industry was progressively shut down by government action over the period from the 1970s to the 1980s, the exotic species disappeared, and the two native taxa returned (plus D. galeata mendotae, which is also native to the region). As we have shown previously, the exotic species were identified either by hatching and rearing diapausing eggs (D. exilis) or by analysis of eggs using mtDNA (D. curvirostris). We interpret their role in seasonal algal dynamics in Onondaga Lake retrospectively using data collected in prior studies of the lake. The native Daphnia currently cause a typical spring CWP in late May and early June, whereas the exotic species caused an unusual late-summer (August-October) CWP during the period of maximum cyanobacterial density.