Lecture 13: QUATERNARY VEGETATION HISTORY Evidence for Plant Movement: - Look at today's distribution however this is not always a good indication of past distributions. - One can infer that past distribution was greater for some species and not others, e.g. oak has a more southerly distribution Pine - Sagebrush - located towards arid/desert regions, as requires less precipitation Percent of total number of occurrences Use as palaotemperature indicators: Spruce Oak Big Sagebrush Pine Spruce Oak Big Sagebrush -40 -30-20 -10 0 10 20 0 5 10 15 20 25 30 January temperature ("?) July temperature ("?) Annual precipitation (mm) 10 1,000 10.000 - Local conditions can be reflected by plant macrofossils Figure 5 The present-day geographic and climatic distributions in western North America of four taxa that are well-represented in late Pleistocene pollen in the region (Little, 1971. 1976). The heights of the bars in the histograms indicate the percentage of the total number of occurrences of the taxon that falls within the climatic range indicated on the horizontal axis (Thompson et al., 1999a, b). - Aquatic plants >20 oC - during the Pleistocene interglacials indicates higher summer warming. - EXAMPLE: Maple leaves imprint - minimum winter temperature as cannot survive below O oC - this helps to constrain winter temperatures and suggests mild winters. Trapa natans (water chestnut): native to temperate Eurasia & Africa; >20ºC for fruiting - Last interglacial - suggested elevated summer warmth and milder winters. Najas minor (brittle water nymph): C & E Europe no further west than Belgium; >18°C - Temperatures are less well constrained for colder climates Important Points: - Younger Dryas termed so due to the Dryas shrub which was present during the cooler periods in Wales, Penines and Scotland - Pollen can be difficult to identify to species level, but can be more easily identified to the genus level. - Wormwood - can detect this pollen - this is present in periglacial conditions e.g. late cold stage conditions - Differential pollen production and dispersal - the amounts of pollen produced differ across genera - Confirs - Long-distance producers - produce very large quantities of pollen - Preservation - some pollen are preserved better than others - best preserved in neutral to more acidic conditions. · Less well constrained for cold climates Dryas octopetala (mountain avens): Arctic-alpine dwarf shrub Betula nana (dwarf birch): Far north of Europe and mountains >300m - Succession - succession of pioneering plants - interglacial to glacial succession - influx of deciduous species. Artemisia (wormwood), typical of periglacial steppes - Low levels of plant taxonomical precision in pollen - Regional vs. local signal - local vegetation live and die in situ, can result in macrofossils. - Contrasting limiting factors - e.g. altitude - tree snowline is suppressed in the area of snow - top snowline controlled by temperature, bottom snowline controlled by moisture. Deciduous forest Rates and Direction of Floral Migration: - Records from Britain indicate the direction of travel by species. - Different patterns are shown for