Monitoring Lemhi penstemon (Penstemon lemhiensis) and spotted knapweed (Centaurea stoebe) in Big Hole National Battlefield: 2009-2014

Rodhouse, Thomas

Lemhi penstemon (Penstemon lemhiensis) is a rare regional endemic forb found only in a single county in Idaho and in four counties in Montana, including Beaverhead County where Big Hole National Battlefield (BIHO) is located. The species is thought to be at risk as a result of habitat loss and altered fire regimes, as well as from infestations of spotted knapweed (Centaurea stoebe). The species is thought to be particularly dependent on bare soil microsites for germination and to be easily outcompeted by both native (e.g., steppe bunchgrasses) and non-native (e.g., spotted knapweed) vegetation. Monitoring of Lemhi penstemon in the Battlefield began in 2007 with a census performed by student volunteers and staff that identified two subpopulations on the Howitzer Hill and Horse Pasture hillslopes. This information revealed that the Battlefield was home to a very large (the largest reported, NatureServe [2015]) population and was used to guide development of the sampling design which was implemented and modified during field work beginning in 2009 and conducted annually since. A third smaller subpopulation was discovered in 2011 near the Battlefield Visitor Center and has been incorporated into the monitoring program. Monitoring of the spotted knapweed population that is spatially coincident with the penstemon population has also been incorporated into the program since 2009. A protocol has been developed to guide monitoring that was submitted to the Pacific West Region for peer review in September 2014; reviewer comments were provided in September 2015 and the protocol is now being revised. Monitoring objectives are focused on evaluating status and trend of Lemhi penstemon and spotted knapweed population sizes within the context of long-term changes in vegetation cover and climate, and NPS management actions. Monitoring is explicitly integrated in to long-term goals of the NPS cultural landscape management program for the Battlefield. A replicated asymmetrical before-after-control-impact study design (Underwood 1994) has been superimposed on to the monitoring program to support causal inferences about fire effects on penstemon and knapweed populations. Information about weed control efforts are also being included in monitoring and analyses. The study design includes the use of three methodologies: 1) annual plant surveys beginning in 2009 across the Battlefield hillslopes using large spatially-balanced random samples (e.g., n=300) of temporary (i.e., a new sample is used each year) 50 m x 2 m (100 m2) belt transects (referred to as [spatially-] extensive plots) to estimate net changes in the population sizes of established (excluding seedlings) penstemon and knapweed plants on the two Battlefield hillslopes; 2) annual surveys since 2012 of 42 permanent 1 m x 2 m intensive study plots to estimate demographic rates in the hillslope and visitor center subpopulations; 3) an annual census since 2011 of established plants in the visitor center subpopulation. The two hillslope study areas have been divided in to 4 experimental blocks for prescribed fire treatments. The first fire treatment was applied within a single block in September 2014. A second block was planned to be burned in 2015 but weather and drought caused that to be deferred. However, the implementation of this replicated treatment design is highly contingent on annual knapweed survey results; if knapweed responds vigorously to fire and/or cannot be controlled, burn plans will be suspended. The first treatment was suspended for two years until it was clear that the increasing trend in knapweed observed during the first 3 years of study could be reversed, an indication of the willingness of Battlefield management to incorporate monitoring information into adaptive decision-making. This report summarizes the study design and pre-burn trends in penstemon and knapweed populations.

Monitoring Lemhi penstemon (Penstemon lemhiensis) and spotted knapweed (Centaurea stoebe) in Big Hole National Battlefield: 2009-2014

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