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
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 2013). 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. In 2015, 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), particularly on Lower Table Rock.
In light of a recent dumping of fire retardant that occurred on a portion of Lower Table Rock in July 2010, we added plant community monitoring transects in the affected area in 2013 and have monitored them since. In this report, we focus discussion on population trends of L. pumila ssp. pumila and more recent analyses, 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, and monitoring of Callitriche marginata, and habitat quality surveys can be found in Appendices F-I.
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. Mean number of plants in plots declined from 44 to 6 from 2014 to 2015. In addition, number of flowers per plant also declined. This severe decline, coupled with the relatively low number seen in recent years is cause for concern.
- Similar to in 2013, in 2015 we observed markedly different cover 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 the lowest number of plants over the course of this study in 2015. Number of flowers per plant was also at one of the lowest levels over the course of this study.
Community monitoring of the fire retardant drop
- In 2015 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. Similarly, we observed a decline in the number of pumila ssp. pumila that occurred within plots. Pool habitats declined from 2014 to 2015, and in 2015, mound and pool habitats remain dominated by non-native grass cover. While it is promising that we have seen a decrease in non-native grass cover and litter cover both within and outside of the fire retardant drop in recent years, habitats continue to be impacted, particularly in mound and pool habitats.
2015 exhibited the lowest numbers of L. pumila ssp. pumila in plots on both Lower and Upper Table Rock. Given the recent decline observed in population size and the number of flowers per plant, we recommend continued monitoring of population dynamics on both Upper and Lower Table Rocks. Data suggest that the population on Lower Table Rock has experienced extreme annual variability, however the low numbers observed in recent years, particularly from 2010-2015, suggest that this species is struggling. In 2014 we observed a relatively high number of reproductive plants on Lower Table Rock, however this did not translate into a successful year in 2015. While pools were dominated by native species just a few years ago, in 2015 we found these habitats to be dominated by exotic grass cover, which can outcompete many native species endemic to these sensitive habitats. The impact of recreation, combined with the recent invasion of non-native grass species suggests that careful monitoring will be necessary to understand population trends and assess what management actions might be needed in the future. Likewise, high temperatures experienced in recent years combined with variable precipitation is likely greatly affecting population dynamics for rare species on both table rocks. Direct management targeted at combatting non-native species may be necessary for the perpetuation of rare species on the Table Rocks. Though both Upper and Lower Table Rock provide a valued recreation opportunity, limiting some access at sensitive times may decrease negative effects associated with trampling. Adding more signage, particularly on the southern end of Upper Table Rock would hopefully encourage hikers to stop trampling sensitive areas and the remaining L. pumila ssp. pumila habitat.