Abstract
(1) The pollen and plant macrofossils in sediments from three high-altitude lakes in the Sierra Nevada, California, document vegetation changes that have occurred over the last 12500 years. (2) Trees became established around the lakes by c. 10 000 years ago. Early Holocene forests were structurally different from those of today; they were more open than at present, with more montane chaparral shrubs. Several tree species characteristic of modern subalpine forests were rare and restricted to mesic habitats. Significantly drier conditions are inferred. (3) By c. 6000 B.P., effective precipitation had increased, as shown by an increase in subalpine conifers, principally Tsuga mertensiana (mountain hemlock) and Abies magnifica (red fir). (4) The upper altitudinal limits of many subalpine conifers began to fall c. 2500 B.P., coincident with the beginning of Neoglacial cooling. (5) The climatic interpretations are consistent with those previously described for the Pacific North-west region of North America, but not consistent with those for the American South-west.
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