Does a trend in declining stem density of Lepidium latifolium indicate a phosphorus limitation? A case study

Authors: Blank, Robert - Bob; Morgan, Tye

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2014
Publication Date: 11/5/2014
Citation: Blank, R.R., Morgan, T.A. 2014. Does a trend in declining stem density of Lepidium latifolium indicate a phosphorus limitation? A case study. Invasive Plant Science and Management. 7:526-531.

Interpretive Summary: Perennial pepperweed is an exotic crucifer that had invaded wetlands throughout the intermountain west. We monitored the invasion of a wildlife refuge in northeastern CA for over 18 years. Surprisingly, perennial pepperweed density has declined and pre-existing vegetation has been invigorated. Soil tests suggest that, as deep soil reserves of phosphorus is depleted due to biocycling, the competitiveness of perennial pepperweed declines.

Technical Abstract: Lepidium latifolium L. (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 CA. 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 has been 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 suggests its availability may be crucial to the invasiveness of L. latifolium. We hypothesize that 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. elongate.