YOSEMITE Vegetational Changes in Yosemite Valley NPS Occasional Paper No. 5

Plants of Cirsium vulgare (bull thistle) and Hypericum perforatum (Klamath weed) have been controlled chemically and by hand for the last 25 years; however, these were not new effects (Gibbens and Heady 1964). Civilian Conservation Corps workers in the 1930s removed weeds and young trees from the meadows, and a sizable area of the valley floor was cleared of underbrush in the 1890s. Man has maintained the meadows, probably by burning previous to 1850, and by cultivation, grazing, tree pulling, and camping between 1850 and 1916. The National Park Service has removed young trees from meadows by cutting, pulling, and, since 1970, by prescribed fire. Although Whitney (1868) recorded 300 ha (750 acres) of meadows in 1866, only 135 ha (340 acres) could be found on aerial photographs taken in 1960.

As noted earlier, the three most common trees were present on the meadow soils in dense stands of all ages. A complete tree canopy occupied a wet site in the Black Springs area and tree seedlings grew at the edges of poorly drained depressions. The present acreage of meadows would have been smaller or perhaps nonexistent without intentional and unintentional removal of young trees. To preserve the meadows in Yosemite Valley, they must be kept free of trees.

In addition to the influences of man, natural causes contributed to changes in meadow vegetation. Deposits from the annual spring floods, which occasionally were large, served to build the land surface and to fill abandoned river channels. Concurrently, erosion began to deepen the channel in the Merced River. In 1879 the channel was deepened by blasting the terminal moraine near Bridalveil Falls. Debris has been removed periodically from the channel to reduce flooding. Although the extent of these influences has not been substantiated, the meadows of today probably are drier than those of 100 years ago which results in more grass, more trees, and fewer of the Carex species than in Whitney's time.

Left alone, plant succession in Yosemite Valley would continue toward an undesirably thick canopy of forest trees (Fig. 19). Maintenance of vistas, from a grassy foreground to spectacular waterfalls and cliffs, requires forest openings with meadow vegetation (Fig. 20). However, manipulation to maintain meadows must be subtle to avoid impairment of natural processes and beauty. So much alteration of the meadows has occurred that they can no longer be restored to their primitive state. Therefore, the objective must be to maintain a natural appearance compatible with public enjoyment of Yosemite Valley as a unique mixture of forest and grassland with overpowering views of mountains, granite cliffs, and some of the highest waterfalls in the world.

Fig. 19 (Upper) The Black Springs area with Bridalveil Falls in the background, photographed by C. E. Watkins in 1866. (Lower) The same view, photographed by H. F. Heady in 1961, as indicated by the rock in the lower right-hand corner of both photos. Reduced meadow area has restricted views of striking cliffs and waterfalls.

Fig. 20. Vistas such as this one of Yosemite Falls must be maintained by removal of trees.