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Vegetation changes on all temporal and spatial scales in response to natural climate variations. Forest communities are transient assemblages and tree adaptation cannot keep pace with changes in the selective environment.

Forest communities of the last ice age do not have counterparts on the modern landscape. Conversely, species that dominate present-day forests were rare during glacial periods. Because glacial periods are longer than interglacial periods, forest trees have been selected predominantly under conditions very different from today. Tree species dominating modern forests first became common 8,000-10,000 years ago, when they expanded northward from ice age refugia. Species spread at different rates and in different directions, reaching their current range limits only by 3000-5000 years ago. Thus, present-day forests are recent assemblages and should not be considered stable over evolutionary time scales. Although less extreme, the effects of environmental changes over the past several centuries and decades are evident at vegetation ecotones.

dendrochronology	Little Ice Age	Holocene
greenhouse warming	pack rat middens	palynology

LECTURE OUTLINE

Analysis of pollen and other plant "fossils"

Provides evidence of past conditions at different places on the Earth

Examples of pollen walls of insect-pollinated species

Useful for understanding

large-scale processes affecting the Earth's environment

basic nature of biotic responses to climate change

possible responses to future climate change

Example pollen diagrams form 2 lakes in the Olympic Mountains

Comparison of Earth's geography between present and peak of last glaciation (21K) Note: The environment during most of the past 2 million years has been similar to 21,000 years ago.

North American vegetation patterns in present and 8000 years ago. Arrows indicate areas of substantial change.

Evidence of very recent climate change.

Evidence of Little Ice Age and 20th century changes in Grindelwald glacier, Alps.

Upper: Treeform change in response to 20th century warming, WY.
Lower: Recession of Mt. Rainier glacier (LIA limits indicated in yellow) near Sunrise.

Trends in atmospheric concentrations of carbon dioxide (greenhouse gas), measured at Mono Loa Hawaii.

What are the consequences of processes occurring at different time and space scales.

Individual to landscape:

Landscape to globe: