Is vegetation in equilibrium with climate? How to interpret late-Quaternary pollen data

Abstract

Current methods for estimating past climatic patterns from pollen data require that the vegetation be in dynamic equilibrium with the climate. Because climate varies continuously on all time scales, judgement about equilibrium conditions must be made separately for each frequency band (i.e. time scale) of climatic change. For equilibrium conditions to exist between vegetation and climatic changes at a particular time scale, the climatic response time of the vegetation must be small compared to the time scale of climatic variation to which it is responding. The time required for vegetation to respond completely to climatic forcing at a time scale of 104 yr is still unknown, but records of the vegetational response to climatic events of 500to 1000-yr duration provide evidence for relatively short response times. Independent estimates for the possible patterns and timing of late-Quaternary climate changes suggest that much of the vegetational evidence previously interpreted as resulting from disequilibrium conditions can instead be interpreted as resulting from the individualistic response of plant taxa to the different regional patterns of temperature and precipitation change. The differences among taxa in their response to climate can lead a) to rates and direction of plantpopulation movements that differ among taxa and b) to fossil assemblages that differ from any modern assemblage. An example of late-Holocene vegetational change in southern Quebec illustrates how separate changes in summer and winter climates may explain the simultaneous expansion of spruce (Picea) populations southward and beech (Fagus) populations northward.

Ecology and the ratchet of events: Climate variability, niche dimensions, and species distributions

“…Ecologists often treat environmental variation as random fluctuations about a constant or gradually changing mean state-a convenient assumption for ecological modeling (21,22). Paleoecologists acknowledge rapid responses to abrupt climate shifts (e.g., the late-glacial Younger Dryas Event) (23,24), but have focused on gradual, biogeographic adjustments to orbitally driven climate changes (15,(25)(26)(27)(28).…”

“…Paleoecology, which exploits the great store of environmental and ecological history preserved in natural archives (14), has also benefited from the Grinnellian niche concept in explaining past ecological and biogeographic patterns (15)(16)(17)(18) and inferring past climates from fossil data (19). By extending ecological observations across a broad range of earth-system states, paleoecological records can reveal fundamental phenomena and processes that would likely go unrecognized in the observational and instrumental record [e.g., novel and disappearing climates (20)], and reveal hidden or overlooked assumptions in global-change applications of niche models and other tools.…”

Community ecology in a changing environment: Perspectives from the Quaternary

“…Environment assembly has received less attention in community ecology than the other two, although it underlies the recent explosion of empirical models predicting species distribution and abundance and community composition under various climate-change scenarios (74). Paleoecological support for environment assembly is very strong, resting on both theoretical (19,75) and empirical foundations (e.g., 20, 76-80). Paleoecologists generally look first to environmental change, particularly climate change, as a driver of the kinds of dynamics shown in Figs.…”

The late Quaternary vegetational history of northwest Greece

“…For example, there was an early expansion of Quercus followed by Ulmus, Tilia, Salix, Phillyrea, Eraxinus ornus, Eagus, Acer, Pistacia, and Eraxinus excelsior, a later expansion of Abies and Carpinus orientalis/ Ostrya carpinifolia and then a late appearance followed by expansion of Carpinus betulus. The delayed response to an increase in abundance of some taxa in this region was as a result of intrinsic and extrinsic parameters such as competition and climate (Webb 1986(Webb , 1988 rather than migration. All taxa except Carpinus betulus were present in the area from at least beginning of the postglacial and did not show the staggered appearance usually taken as being indicative of delayed migration.…”