Page 15 - Sierra Meadows Strategy

Legacy Impacts and Current On-going Threats to Meadows
In addition to conversion of meadows to other land uses
during the last century (e.g., inundation by reservoirs, drainage for agriculture or development, roads etc.) 60 widespread disturbances to meadows have occurred throughout the Strategy Area. Disturbances, whether from human activities or natural causes such as re, debris slide, or an extreme ood, can cause a cascade of events that can affect meadow function and the bene ts meadows provide61.
One of the most common characteristics of a degraded meadow is channel incision and/or gully creation. Incision can be caused by a number of different land use practices working alone
or in combination. The most common sources of incision are channelization, construction of roads or railroads, ditching, overgrazing, and logging. Heavy livestock grazing contributed to the degradation of meadows during the late 19th century62,63,55,15. Changes to meadows attributed to legacy overgrazing include gullying, desiccation, conifer encroachment, and changes in plant species composition, structure, and diversity64,55,8,63,65,15. Today conditions and grazing-use patterns are improving; however, in some cases impacts from grazing are still occurring66. Grazing management permits cattle removal or reduction in seasonal use or numbers. However, once damage has occurred in a meadow it can be exacerbated by natural climatic variation, affecting meadow hydrology67,68,69,34.
When stream channels in meadows become incised, or when a gully is created in a meadow with no pre-existing channel, the immediate effect is that water once stored in the rooting zone soil (primarily upper 1 yd.) drains down to the incised channel, lowering the water table and releasing subsurface groundwater from storage through the eroded channel or gully. This lowered water table has rami cations throughout the meadow, such
as more rapid runoff and decreased meadow water storage capacity70,71. During the growing season, a lower water table effectively changes the hydrologic regime experienced by the vegetation; when these conditions persist and no longer support the existing plant communities, other species tolerant of drier conditions will thrive and eventually could dominate the affected areas. In highly organic “peat” soils, a lowered water results in microbial oxidation of the organic matter, which could eventually lead to land-surface subsidence72.
Deeply incised channels and associated drawdown of groundwater can result in a destabilizing cascade of events: erosion channelizes ow and concentrates the erosive energy of oodwaters; down-cutting accelerates and, eventually the meadow surface, once a oodplain and recharge area during high ows, becomes a terrace; the terrace is cut off from the rewetting effect of seasonal oods; wet meadow vegetation is replaced by other drier vegetation types, with roots that
are incapable of stabilizing streambanks; bank erosion is exacerbated, and the channel widens. Likewise tributary channels and swales incise to match the new, lower elevation
of the main channel, and the result is a network of erosion
gullies that drain the meadow. Such positive feedback among hydrologic, uvial geomorphic and vegetative responses can exacerbate what may begin as a small perturbation, thereby hindering or preventing recovery without active restoration61. Thus, in many areas, meadows have been protected from grazing and other impacts for thirty years, but have still not recovered.
Long-lasting effects of soil compaction can also result in degraded meadow conditions, even where groundwater table elevations remain high73-78. Such effects include increased soil bulk density, reduced in ltration and water holding capacity and reduced root density. Soil compaction combined with selective grazing, can affect plant species composition by increasing the cover of grazing resilient species73,63,79.
A changing climate and altered re regime are also affecting meadow conditions in the Sierra Nevada. Fire suppression and an altered re regime have resulted in both conifer encroachment80,63 and hydrologic and sediment impacts associated with stand replacing res in meadow contributing areas (e.g., scouring peak ows and large sediment deposits in the downstream meadow;81,61). Climate change is affecting the spatial and temporal distribution of snow vs. water in the Strategy Area30. Some parts of the Strategy Area are expected to have more reduced snowpack than others, and many areas are expected to see increased frequency of extreme events, including drought, rain on snow, and large peak ows33,82. Forest res in contributing areas can combine with these
shifts in weather and hydrologic patterns to generate very
high peak ows and/or sediment deposits into the meadow channel and/or oodplain14. Healthy wet meadows, including fens, under saturated soil conditions, usually due to stable groundwater ows. These conditions are highly conducive to carbon accumulation over long time periods and the presence
of unusual ora and fauna. The bene ts from these meadows are particularly vulnerable to climate change. Fluctuations in snow and rain in uence water availability and thus the saturated conditions essential for existence of these meadows and the bene ts they provide83.
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