Schedule

"By using our knowledge of the factors that control the present distribution of a species, can we gain insight into the environmental changes that have influenced its distribution in the past?"

Objectives
1. Map the geographic distribution of western hemlock (Tsuga heterophylla) over the course of the late Quaternary along the coast of the Pacific Northwest from records of pollen distribution. (Note: the Quaternary era, which comprises the last 2.4 million years of earth history, is divided into the Pleistocene period, which extends from 2.4 million years BP to 10 000 years BP, and the Holocene [the "post-glacial"], which extends from 10 000 BP to the present).
2. Locate the glacial refuge for the species.
3. Estimate the date of post-glacial dispersal to the present range limit.
4. Reconstruct patterns of climatic change during the glacial and post-glacial periods from pollen abundance data.

Background
Many plant species rely on the wind to disperse their pollen. Pollen released by the strobili (male cones) of hemlocks is relatively heavy, and most falls to the ground within 100 - 500 m of the parent tree. The local pollen rain will therefore reflect the local vegetation, but because individual species produce variable amounts of pollen, it is usually not possible to determine the relative abundance of plant species with any precision. Pollen falling onto the soil surface decomposes fairly quickly, but is preserved in waterlogged soils (e.g. marshes and bogs) and in lake sediments. If the waterlogged soil or lake sediment is progressively buried, its changing pollen content is an archive which can be examined, and the changes in local vegetation reconstructed. We can then infer changes in the local environment (e.g. temperature, precipitation, fire frequency, etc.) from changes in the character of the local vegetation. This is the subject matter of the subdiscipline of biogeography/ecology called palynology.

The research efforts of palynologists and other geoscientists have produced the following (much simplified) picture of climatic change in the Pacific Northwest over the course of the late Quaternary:

1) Near-glacial conditions (with minor periods of glacial advance and retreat) persisted in the Pacific Northwest from about 40 000 years ago to about 24 000 years ago; the latter phase of this interval is known in the region as the "Olympia interglacial"
2) a major glacial advance (referred to locally as the "Fraser glaciation") ended this near glacial phase; the ice sheets reached their maximum extent, covering virtually all of B.C., and extending as far south as Olympia in Washington State, about 15 000 years ago.
3) ice retreat began shortly thereafter, and by 12 000 years ago the ice sheets had retreated into local valleys (Vancouver was probably ice-free by this date). By 9 000 years ago glaciers had retreated still further and were probably restricted to high mountain areas. Since then there has been little change in glacial extent.

Tasks and Questions
You are presented with a map showing the present distribution of western hemlock and the location of the major sites within its range where the pollen assemblage over the course of the late Quaternary has been determined from lake or bog cores. The pollen data are presented in terms of absolute or relative (%) abundance in a sample from a particular time period, and can be used to determine whether hemlock was rare or common, and whether its abundance was increasing or decreasing through time at a site.

1. Little Lake (site #1) in the Coast Range of Oregon provides us with a record of vegetation change for the last 42 000 years. From the pollen data, what kind of vegetation characterises this area at present (pollen zone LL-6)? List the four dominant tree species (or families) [2]

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2. What kind of vegetation occupied this area during the middle - late Holocene (pollen zone LL-5) [2]?

3. What kind of vegetation occupied this area during the early Holocene (pollen zone LL-4) [2]?

4. What kind of vegetation occupied this area during the Fraser glaciation (pollen zone LL-2) [2]?

5. What were the dominant plant species during the Olympia interglacial (pollen zone LL-1) [2]?

6. Carp Lake (site #3) lies to the east of the Cascades in south-central Washington. A core from there contains a record of vegetation change that stretches back about 125 000 years. The vegetation in the area at present consists of mixed stands of Douglas-fir, hemlock, oak and alder. Were there other times in the late Quaternary when similar forests occupied this area? [3]

7. Areas where species 'shelter' during period of glacial advance are called "glacial refuges". Infer the location of the refuge for western hemlock during the Fraser glaciation based on your interpretation of the Little Lake and Carp Lake cores. [3]

When the ice retreated after 15 000 years BP, the plants and animals in the glacial refuges recolonised the deglaciated terrain.

8. Use the information from the other sites to draw (on the map provided) the approximate northern limits of western hemlock distribution at 2 000, 4 000, 6 000, 8 000 and 10 000 years BP. [2 marks each = 10]

9. When (approximately) was the present distribution attained? [2]

10. Migration in the early postglacial period was particularly rapid. What does this suggest about the climate and the tree species? [3]

11. Is there any evidence that there were periods of retreat? [1]

12. On the basis of your answer to the previous question, has the climate of the Holocene been uniform in the Pacific Northwest? Is there evidence from the pollen diagrams that some periods since 10 000 years BP may have been warmer/cooler or wetter/drier than at present?[4]