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by Sally Manning, Vegetation Scientist, May 17, 2006

Management of water resources according to the Water Agreement is supposed to be performed in a manner that avoids causing vegetation classified as type C to degrade into vegetation that could be classified as type B (or A). Stated more simply, a change in vegetation from grass-dominated meadow to shrub-dominated "scrub" is considered to be a significant adverse change if it results from inappropriate management of Owens Valley water resources. Recently, an analysis of vegetation data collected by the Inyo County Water Department (ICWD) revealed that conversion from meadow to scrub had occurred or was in the process of occurring in many parts of Owens Valley. The analysis also showed that actual conversion and higher rates of conversion were occurring in wellfields, especially wellfield areas where water tables and perennial plant cover had not recovered to baseline levels since the 1984-87 baseline period (see related article). An analysis of shrub conversion is documented in "Conversion of reinventoried parcels from meadow to scrub" by Sara J. Manning, January 9, 2006. Here, a synopsis is presented.

Determining whether a parcel should be categorized as "meadow" or "scrub" requires some definitions of terms. The LADWP comprehensive baseline inventory and mapping of vegetation, which took place 1984-87, distinguished meadow, scrub, and an intermediate plant community type identified as "shrub-meadow." Meadow and shrub-meadow were typically classified as type C (phreatophytic meadow), and parcels dominated by phreatophytic shrubs but with little or no grass cover were typically classified as type B. An investigation into the original baseline transect data showed that a threshold between type C meadow and type B scrub could be quantified. A clear dividing line between type C meadow and type B scrub occurred when the proportion of shrubs within a parcel, relative to the total cover of grasses plus shrubs, equaled or exceeded 0.8. If one were to view the vegetation from above, at a shrub proportion of 0.8, one would see that, of the area covered with living plants, 20% would be grass and 80% would be shrub. Shrubs actually "dominate" the parcel when a proportion of 0.5 is exceeded, but to cross the line from meadow through shrub-meadow into scrub, the shrub proportion (according to the baseline data) needs to be 0.8.

The 0.8 threshold was applied to the data from meadow parcels monitored in 2005. It was determined that, of the 50 type C meadow parcels, 12 had converted to scrub in 2005 (or before) because the proportion of shrubs equaled or exceeded 0.8. An additional 12 meadow parcels showed a statistically significant trend in increasing proportion of shrubs over time, but by 2005 had not yet crossed the threshold. Nearly all meadow parcels showed a higher proportion of shrubs in 2005 relative to baseline.

Water management practices since the mid 1980s can account for the conversion from meadow to scrub. Only one Control parcel (unaffected by groundwater pumping during 1987-92 drought) showed a conversion from type C to type B between the baseline period and 2005 (Table 1). This parcel, PLC106, has faulty baseline data: All available information strongly suggests the parcel existed as a type B scrub during the baseline period, not a type C meadow as assigned by LADWP in the parcel's final baseline classification. In contrast, more than 40% of the parcels still subject to the management constraints of the Drought Recovery Policy (DRP, see related article) converted from meadow to scrub, and they had an overall higher annual rate of increase in shrub proportion. A few wellfield parcels classified as generally recovered from the 1987-1992 drought (i.e., "DRPfree" = free from the constraints of the DRP) had converted from type C to type B. Overall, results for DRPfree parcels were somewhat intermediate between results for Controls vs results for DRP parcels (Table 1).

^* = Parcel PLC106 (see text)

Overall, the results of this analysis suggest that withdrawal of the water table without subsequent full water table recovery greatly accelerates the conversion of meadow to scrub. To avoid causing the accelerated rate of conversion within wellfield meadow parcels, management to allow full recovery of water tables to grass root zones is probably required.

The occurrence of wildfire was also examined to the extent allowed by this data set. Fire often kills shrubs. When a meadow parcel with a high water table burns, the existing perennial, native grasses resprout soon after the burn, but it typically takes many years for shrubs to re-establish to pre-burn levels. When sites without access to the water table burn, vegetation cover is lost, and non-native annual weeds or other non groundwater dependent species may subsequently invade the site. Therefore, periodic burning has the effect of limiting shrub encroachment in relatively healthy meadow sites, but it can have devastating consequences for sites where hydrologic conditions have been altered. Of the 68 parcels evaluated in this investigation, it was found that 12 had burned in the past two decades. Using this approximation, it could be estimated that a parcel has a chance of burning once about every 115 years. If the fire frequency rate is on the order of 115 years, it is unlikely that Control parcels, at the conversion rates calculated here, will achieve complete conversion, because the shrub cover is likely to be set back by fire long before the 0.8 threshold is reached. If vegetation in a wellfield parcel burns while it has access to the water table, it might show recovery to more meadow-like conditions. However, if the parcel has converted or is on the verge of converting to scrub, and access to the water table is minimal or negligible, fire will probably convert the parcel to bare ground and/or a sparse cover of non groundwater dependent species. In summary, fire may promote maintenance of type C meadow when the site's hydrology has not been compromised. In contrast, a meadow site without adequate water table conditions is at risk of being permanently destroyed should a fire occur.