Threat assessment for Limnanthes pumila ssp. pumila on Table Rocks ACEC

Upper and Lower Table Rocks, located northeast of Medford, Oregon, are collectively designated as an Area of Critical Environmental Concern (ACEC) by the Bureau of Land Management (BLM).  The Table Rocks are characterized primarily by vernal pool and mound habitats that support several rare species, including Limnanthes pumila ssp. pumila (née Limnanthes floccosa ssp. pumila), which is a state threatened and federal Species of Concern, and Callitriche marginata, a BLM Sensitive species.  The Oregon Biodiversity Information Center (ORBIC) has identified L. pumila ssp. pumila as a List 1 taxon, considered threatened with extinction or presumed extinct throughout its range (ORBIC 2016).  Limnanthes pumila ssp. pumila is a narrow endemic known only from the Table Rocks (U.S. Fish and Wildlife Service 2006).  Threats to the species and habitats at Table Rocks include invasive species, grazing, impacts associated with recreational use (e.g., trampling), and climate change.

Since 2006, the Institute for Applied Ecology (IAE) has monitored experimental plots to determine population trends and the effects of grazing, trampling, and invasive species on L. pumila ssp. pumila, and used transects to document plant community types, disturbances (including trails and animals activity), and distribution of habitat types (Gray et al. 2015).  In 2016, we monitored L. pumila ssp. pumila population plots on both Upper and Lower Table Rocks, and in high and low traffic areas to monitor for effects of recreation on Lower Table Rock.  In 2011-2013 we noticed a substantial increase in abundance and spread of annual invasive grasses, including Taeniatherum caput-medusae (medusahead) and Poa bulbosa (bulbous bluegrass) following a fire retardant drop that occurred in 2010 on Lower Table Rock.  In light of these changes, we added plant community monitoring transects in the affected area in 2013 and have monitored them since.  This area was burned in a prescribed fire in fall of 2015.  In this report, we focus discussion on population trends of L. pumila ssp. pumila on Upper and Lower Table Rocks, including the new community transects added in the fire retardant drop area.  In-depth discussion of past studies, including L. pumila ssp. pumila grass removal plots, trampling plots, monitoring of Callitriche marginata, and habitat quality surveys can be found in Gray et al. 2015.

Limnanthes pumila ssp. pumila

• The number of pumila ssp. pumila has fluctuated greatly between years with a steep decline from 2010-2013 in both number of plants and number of flowers per plant within monitoring plots on Lower Table Rock (2009-2012). In 2014 we observed slight increase in number of plants and number of flowers per plant within these plots to levels similar to in 2011.  In 2015 we observed the lowest number of L. pumila ssp. pumila over the course of this study.  2016 was a good year for the species with increases in number of plants per plot and number of flowers.
• Similar to in 2013 and 2015, in 2016 we observed a difference in density of pumila ssp. pumila in high and low traffic areas, where high traffic areas had fewer plants than low traffic areas. This indicates that recreation can influence this annual species, particularly in times where the population numbers are low.
• Similar to trends seen on Lower Table Rock, in plots established on Upper Table Rock in 2007, we observed an increase in number of plants and number of flowers per plant from 2015, which had the lowest numbers over the course of this study.
• We added six plots in high traffic areas of the southern end of Upper Table Rock in 2015, and six more plots in low traffic areas in 2016. This area has had increased number of visitors in recent years with many secondary trails cutting directly through habitat of pumila ssp. pumila and other native species.  We will monitor these plots over time to compare impacts of recreation traffic on the heavily used portion of Upper Table Rock.

Community monitoring of the fire retardant drop

• In 2016 we observed a decrease in cover of non-native grasses and litter both within and outside of the area impacted by the fire retardant drop. After low numbers in 2015, we observed an increase in pumila ssp. pumila in 2016.  Pool habitats declined from 2014 to 2015, but increased in 2016.
• Long-term monitoring plots were used to assess impacts of the prescribed fire that occurred in the area of the fire retardant drop in fall 2015. Mean number of plants tended to be greater in unburned plots whereas mean number of flowers tended to be greater in burned plots.  Litter cover varied greatly across plots but tended to be slightly higher in burned plots.  These results should be interpreted cautiously as plots were not set up to monitor fire effects.

Given the extreme annual variability observed, we recommend continued monitoring of these plots and transects to track population dynamics and the impacts of recreation traffic on this rare species and its habitat.