Lepidium latifolium (perennial pepperweed) is a weedy alien crucifer that has invaded wetlands throughout the western United States. We monitored L. latifolium invasion of an Elytrigia elongata (tall wheatgrass) community at the Honey Lake Wildlife Refuge in northeastern California. In 1993, a 40-m2 plot was delineated, at which time only two single plants of L. latifolium were present. Beginning in 1994, L. latifolium stem density was measured yearly until 2011. From 1994 through 2000, the density of L. latifolium increased to greater than 120 stems m−2. At its height of stem density and stature between 1998 and 2000, it appeared that E. elongata had been extirpated. From 2001 through 2006, stem density and plant stature of L. latifolium declined, but there were still areas of the plot where stem density exceeded 60 stems m−2. From 2007 through 2009, stem density decreased considerably and averaged less than 30 stems m−2 and a healthy recovery of E. elongata occurred. In the years 2010 and especially 2011, stem density increased, but individual plants were small in stature. Soil bicarbonate-extractable phosphorus data suggest that phosphorus availability may be crucial to the invasiveness of L. latifolium. Long-term biogeochemical cycling by L. latifolium may reduce soil phosphorus availability in deeper soil horizons and enrich availability in the soil surface, which alters the competitive relationship between L. latifolium and E. elongata.
Nomenclature: Perennial pepperweed. Lepidium latifolium L., tall wheatgrass, Elytrigia elongata Host (Nevski).
Management Implications: Over an 18-yr period, we monitored stem density of the exotic invasive crucifer Lepidium latifolium in an area at the Honey Lake Wildlife Refuge in northeastern California. Following a near record flood event in January 1997, in some areas density and stature of L. latifolium dramatically increased to form a near monoculture and appeared to have extirpated the perennial grass Elytrigia elongata planted in 1987 as nesting habitat. From its peak in 1998 to 2000, stem density and stature of L. latifolium has declined considerably and allowed E. elongata to once again assume dominance in many areas. The decline in stem density is not due to herbicide use or any management decisions. Instead, our evidence suggests that lowered precipitation and declining availability of soil phosphorous has tipped the competitive balance between L. latifolium and E. elongata. More research is needed to determine the range of soil phosphorus availability over soil depths that might constrain the invasiveness of L. latifolium. Our data, however, suggest that expensive control strategies for L. latifolium can be targeted to soils with high native levels of phosphorus availability; invasibility in areas with low phosphorus availability may naturally decline over time as phosphorus is bio-cycled by the plant.