Meadow Dynamics

The dynamic description of meadows until recently was based largely on inferences from spatial patterns and on the presence or absence of decreasers, increasers, and invaders (Sharsmith⁴ ). Recently Wood (1975) has described long-term changes in seven montane meadows based on the soil stratigraphy revealed in deep erosion gullies. DeBenedetti (1980) and DeBenedetti and Parsons (1979a) have been following meadow recovery after a natural wildfire in 1977. Benedict (1981) has initiated a study of the long-term development of subalpine meadows as revealed in the stratigraphy of soil cores collected in the Rock Creek drainage, Sequoia National Park, which will be directly comparable with Wood (1975).

Figure 2.
Siberian Outpost, Sequoia National Park. Example
of physiographic Type II subalpine meadow.

Classically meadows have been viewed as a seral stage in the hydrosere of a lake developing into a forest (Oosting 1956, and many others). Recent evidence suggests that this interpretation of meadow dynamics may be too restrictive (Benedict 1981). Two other possible hypotheses are: 1) meadow ecosystems, like any ecosystem, have changed through time as the climatic factors influencing meadows have changed; and 2) meadow ecosystems have changed the same amount or less than the surrounding forest vegetation over a given period of time. These two hypotheses are not necessarily alternatives to each other (Benedict 1981).

Evidence in support of the first hypothesis comes from three sources. Wood (1975) describes a generalized montane meadow stratigraphic sequence as: 1) a basal layer of alluvium deposited by pre-Holocene streams; 2) a paleosol dated at between 8,705 and 10,185 years B.P. developed under a mesic montane forest; 3) stratified sandy deposits dated at between 8,700 and 1,200–2,500 years B.P. and deposited under a fir, yellow pine, and lodgepole pine forest; and 4) stratified sedge peat, loams, and grus deposited since 2,500–3,000 years B.P. in a meadow environment. Based on this stratigraphic evidence, Wood (1975) suggests that meadow ecosystems can develop from, and develop to forest ecosystems, and that this is a result of climatic changes.

The second source of evidence for the first hypothesis comes from the widespread invasion of forest trees into meadows throughout the western United States (Dunwiddie 1977; Franklin etal . 1971; DeBenedetti and Parsons 1979b; Vale 1981 a,b). It has been suggested that this widespread tree invasion has resulted from excessive meadow grazing and climatic changes. It is only infrequently suggested that this is the result of successional processes. This implies that meadow ecosystems are in dynamic equilibrium with their total environment and that it is an oversimplification to view meadows only as stages in a hydrosere.

The third source of evidence that meadows are dynamically adjusted to their environment and climate comes from a man-induced experiment at Osgood Swamp near South Lake Tahoe, California (Benedict 1981). Osgood Swamp occupies a wet basin formed behind a morainal dam (Physiographic Type IA2, Adam 1967). When the morainal dam was artificially breached, the basin became drier simulating a dramatic change in climate. Subsequently, there was a massive invasion of Pinuscontorta subsp. murrayana into the meadow. This indicates that meadow vegetation changes not only as a result of successional processes but as a result of climatic and environmental changes. These changes can be either man-induced or natural.

Evidence for the second hypothesis that meadows are as stable as the surrounding forest has been discussed previously by Benedict (1981). Two sources of evidence support this hypothesis. Adam (1967) presents pollen diagrams from Osgood Swamp, and Soda Springs (near Tuolumne Meadows). These diagrams suggest that meadow vegetation is as stable or more stable than the surrounding forest vegetation. The second source of evidence comes from Wood (1975). From his stratigraphic work, seven of the meadows studied have been in existence since 1200–3000 years B.P., and two since 7700–9800 years B.P. This suggests that these meadows have been unstable over the past 10,000 radiocarbon years. In a similar manner, the forest vegetation at these same sites has also been unstable over the past 10,000 radiocarbon years (Wood 1975).