Lens-pod milk-vetch (Astragalus lentiformis), Mabie DFPZ Fuels Treatment Study, Beckwourth
Ranger District, Plumas National Forest.
Lynée Crawford, Botanist,
Kyle Merriam, Province Ecologist
Michelle Coppoletta, Botanist
and Colin Dillingham, Ecologist
Nov 30, 2010
Lens-pod milk-vetch
Purpose
This monitoring was conducted to determine the effect of prescribed fire on the rare lens-pod milkvetch (Astragalus lentiformis), which is a member of the legume family. To determine if prior treatments
would influence the effect of prescribed burning on lens-pod milk-vetch, plots were established in areas
that previously had been treated by mechanical thinning, hand thinning, and in untreated control areas.
However, burning activities were either cancelled or did not impact any plots, so the effect of burning
treatments on lens-pod milk-vetch could not be evaluated. Instead, trends in lens-pod milk-vetch
abundance over time, relationships between lens-pod milk-vetch and environmental variables, and
unplanned impacts from off-highway vehicles (OHVs) are investigated in this report.
Methods
In 2006 eight permanent, circular monitoring plots were randomly established within mechanical thin
unit 62, and hand thin units 67, 70 and 142. Unit 70 contained a control area used to establish a control
plot. Plots were 25 feet in diameter and the centers of the plots were marked with rebar. The following
data were collected:
1. Number of adults
2. Number of seedlings
3. Litter and duff depth
4. Canopy cover
Mechanical and hand thinning occurred in 2004 and 2005 prior to plot establishment in 2006. Data was
collected in the summers of 2006, 2009 and 2010. OHV impacts occurred sometime between 2006 plot
establishment and 2009 data collection.
Results
Repeated measures ANOVA found a significant interaction effect between year and treatment type
(mechanical thin, hand thin, mechanical thin plus OHV-impacts, and control F=7.58, p=0.014) on
numbers of lens-pod milk-vetch individuals. Least squares post-hoc analysis determined that changes in
lens-pod milk-vetch numbers within the OHV-impact plots between years were responsible for the
significant interaction effect. There was no significant interaction between year and treatment type for
all other treatments (mechanical thin alone, hand thin, and control). See Figure 1 below for a summary
of the results.
Figure 1: Numbers of lens-pod milk-vetch individuals by treatment type over time. Hand thin and Mechanical thin
treatments occurred prior to 2006. Off-highway vehicle impacts occurred between 2006 and 2009 plant counts.
140
120
100
Control
80
Hand Thin
60
Mech Thin
40
Mech Thin_OHV
20
0
2006
2009
2010
*Figure by Kyle
Merriam, Province Ecologist
Linear regression analysis found that canopy cover significantly influenced lens-pod milk-vetch
abundance (Figure 2).
Figure 2. Duff depth was significantly related to lens-pod milk vetch abundance. Linear regression found that this
2
variable explained almost 32% of the variation in plant numbers (R =0.32, p=0.03).
Figure 3. Off-highway vehicle (OHV)
impacts to Lens-pod milk-vetch, Plot 3:
2006 before OHV impacts (top left) and
2009 after OHV impacts (top right).
Photo bottom left is close up of OHV
trail system.
Conclusions
OHV user created trails and subsequent use have significant negative impacts on the number of
individuals of lens-pod milk-vetch (Figure 1). There was some evidence of rebound in population
numbers in 2010, however, lens-pod milk-vetch abundance was still lower than prior to the OHVimpacts in 2006. Populations of lens-pod milk-vetch remained stable in mechanical thin and hand thin
treatments over time (years 1 through 6 post-treatment) and there was not a significant change in
numbers.
Lens-pod milk-vetch was more abundant in plots with lower duff depths, but was absent in areas
impacted by off highway vehicles with no duff. Based on these findings, treatments that reduce duff
depths such as low intensity fire, would likely be beneficial
In order to determine conclusive impacts to lens-pod milk-vetch from mechanical thin, hand thin, and
prescribed fire (either underburn or pile burn) treatments, additional monitoring plots should be
established within future treatment areas prior to treatment implementation.