Summary of Weber’s Milkvetch HFQLG Monitoring 2008‐2011 Kyle Merriam, Jim Belsher‐Howe, and Michelle Coppoletta 18 Jan 2012 Introduction Astragalus webberi (Webber’s milk vetch) is a USDA Forest Service (USFS) sensitive species known from only 13 occurrences. The species range is limited to about 25 miles of the Indian Creek and East Branch North Fork Feather River drainages of Plumas County, California. It is included in the California Native Plant Society’s inventory of rare and endangered plants on list 1B.2 for species that are rare, threatened, or endangered in California and elsewhere. Seven occurrences have been tracked since the late 1980’s. These occurrences had an estimated total of 2,178 individuals in 1989, but by 2008 it was estimated that these numbers had declined by 22 percent (USFS 2008). All 13 occurrences of A. webberi are found in disturbed areas such as the sides of roads, cut banks, and skid trails with exposed soil and high light conditions (Fig. 1). A. webberi often co‐occurs with other light requiring species such as oaks, shrubs and herbaceous understory species. Fig. 1. A. webberi plants growing in a road cut. Most occurrences are found in open, disturbed sites. Although the USFS has been conducting prescribed burning and thinning treatments to reduce fuel loads and restore forest health for several decades, rare plant populations are typically not included in project planning because of a lack of information on treatment effects and concerns over potentially extirpating rare populations. However, the decline of A. webberi on the Plumas National Forest has led USFS managers to consider taking a more active management approach. In 2008, the Plumas National Forest completed a hand thinning project within a portion of an A. webberi population, followed by a prescribed underburn conducted in 2010. The results of these first efforts to actively restore A. webberi populations are described here. Objectives The primary objectives of this study were to evaluate the effect of prescribed burning and thinning on A. webberi populations, and to quantify the environmental factors that promote Astragalus webberi size, density, flowering, and cover. Methods In June 2008 we established five circular plots within the proposed treatment unit, and four plots in an adjacent untreated area to serve as controls. Plots were re‐sampled annually in late June from 2008 through 2011. Data collected included: •
Number of A. webberi plants by size class based on stem number (<5 stems, 5‐10 stems, and >10 stems); and flowering status (flowering/non‐flowering); •
Stem number and flowering status of five permanently marked plants in both the control and treatment units; •
Ground cover including bare ground, rock, litter, coarse woody debris, moss, herbaceous plants, overstory canopy closure, and duff/litter depth; and •
Species cover and composition. Treatments Three different management activities occurred in the treatment unit between 2008 and 2011. After pre‐treatment data was collected in 2008, the area was thinned by hand crews in 2009 (Fig. 2). a b
Fig.2. Example of pre‐treatment conditions in the treatment unit in 2008 (a), and after hand thinning in 2009 (b). Surface fuels created during the hand thinning treatment were removed through pile burning in the fall of 2009, and hand lines were created by fire crews in preparation for the prescribed burning operation (Fig. 3). All existing A. webberi plants were avoided by these activities to minimize direct impacts, however small patches of ground disturbance were created throughout much of the treatment unit. Fig. 3.Hand line constructed in treatment unit in 2009 in preparation for the prescribed burn. In 2010, the treatment unit was prescribed burned (Fig. 4). The burn was conducted to avoid existing A. webberi plants. Fig. 4.Prescribed burning of treatment unit in 2010. The effect of the burn on A. webberi was evaluated in 2011. Post‐treatment conditions in the treatment unit are shown in Figure 5. Fig. 5. Post‐treatment conditions in treatment unit. Analysis Data were analyzed using SAS version 9.3. Differences in A. webberi density, cover, and stem number were evaluated by testing for a significant interaction effect between year and unit (treated vs. control) using two‐way ANOVA. Relationships between environmental variables and plant cover, density and stem number were explored using linear regression. Results Cover and density. A. webberi cover did not change in either the control or treatment units between 2008 and 2011. The total number of A. webberi plants increased in the treatment unit every year, but not in the control unit (Fig. 6). This was the result of an increase in the number of plants with less than five stems. 140
Plant Number
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Fig.6. A. webberi density significantly increased in the treatment unit between 2008 and 2011; plant numbers did not change in the control plots (p<0.001). The largest increase in A. webberi density occurred between 2009 and 2010 when hand lines were constructed to prepare the treatment unit for prescribed burning (Table 1). The second largest increase was recorded after the unit was hand thinned between 2008 and 2009. Plant numbers increased the least after implementation of the prescribed burn between 2010 and 2011. Table 1. A. webberi density in the treatment unit and treatment type implemented between 2008 and 2011. Year Plant Number
Treatment % Increase 2008 Pre‐Treatment 12 2009 Hand Thin 24 100 2010 Hand Lines 82 242 2011 Prescribed Burn 107 30 Stem and Flower Number. Although the total number of stems and the number of flowering stems of permanently tagged A. webberi plants varied between 2008 and 2011, the pattern of variation did not differ significantly between control and treatment units. This result suggests that variation in total stem number and flowering stem number was a response to climatic or other variables not associated with management activities in the treatment unit. Environmental Variables. Of all environmental variables measured, including the cover of bare ground, rock, litter, coarse woody debris, moss, herbaceous plants, as well as overstory canopy closure and duff depth, we found that only the amount of exposed bare ground was significantly associated with the density of A. webberi (Fig. 7). Plant Number (log)
R2=0.44, p<0.01
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Fig. 7. As the amount of exposed bare ground increased, so did the density of A. webberi plants. The average number of A. webberi stems per plant increased significantly in plots with less overstory canopy closure (Fig. 8). Stem Number
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Fig. 8. As the amount of overstory canopy closure increased, the number of stems on A. webberi plants decreased. The amount of exposed bare ground varied over the four year monitoring period in the treatment unit but not in the control. However, the changes in bare ground were relatively small, varying from between two and nine percent of total ground cover (Fig. 9). 12
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Fig.5. Bare ground cover changed significantly in the treatment unit between 2008 and 2011, but did not change in the control plots (p<0.001). Conclusions and Management Implications 
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Treatments conducted by the Plumas National Forest significantly increased the number of small A. webberi plants in the treatment unit. These plants likely germinated in response to the treatments. Bare ground cover was strongly related to the density of A. webberi. Treatments that exposed bare ground, particularly through the construction of hand lines, were most effective at increasing A. webberi density. Prescribed burning had a relatively small effect on the density of A. webberi. While A. webberi germination was stimulated by the exposure of bare ground, plant growth and size (as indicated by stem number), was enhanced by higher light conditions as a result of lower canopy closure. Future management should focus on protecting established individuals while applying landscape level treatments that: o Expose bare ground to stimulate germination of A. webberi (i.e. through the application of prescribed fire); and o Increase the amount of light that reaches the understory to promote growth of existing plants. Literature Cited USDA Forest Service. 2008. Conservation Assessment for Astragalus webberi. Prepared by Jim Belsher‐
Howe, Mt. Hough Ranger District, Plumas National Forest. 27 pp. Investigating the effect of prescribed fire on Webber’s milkvetch (Astragalus webberi) germination Michelle Coppoletta, Jim Belsher‐Howe, and Kyle Merriam 30 March 2012 Introduction Astragalus webberii is a geographically restricted legume species that is limited to 13 occurrences along a 25 mile stretch of the Indian Creek and East Branch North Fork Feather River drainages in Plumas County, California. Three occurrences are on lands managed exclusively by the Plumas NF, three are on the boundary of both private and Forest Service lands, and the remaining seven occurrences are on private lands. These occurrences support close to 2,000 individuals and cover approximately 10 acres. Webber's milk‐vetch grows in a variety of habitats that range from open, rocky areas to moderately dense stands of hardwoods and conifers. It appears to require open conditions where the dominant trees are widely spaced and enough sun reaches the ground to maintain adequate shrub and understory plant cover (USDA 2009). Field observations suggest that Webber’s milk‐vetch is tolerant of disturbance; most of the known occurrences are along highways, on stabilized cut‐banks, or on the edge of the forest. Recent monitoring results also suggest that thinning treatments may be beneficial to Webber’s milk‐vetch, as long as the treatments are applied at the appropriate scale and magnitude (Merriam et al. 2010). Overall, this species appears to be in decline; the number of individuals within seven of the occurrences has declined by 22 percent over the past 20 years (USDA 2009). Threats to this species include road maintenance and construction, trash dumping, vehicle parking, and timber harvest. Objectives The objective of this monitoring was to determine the effects of prescribed fire and hand thinning on Webber's milk‐vetch germination. To frame our analysis, we focused on the following questions: 1. Do prescribed fire and hand thinning treatments increase Webber's milk‐vetch germination one year after treatment? 2. Did pre‐treatment fuel manipulations influence fire behavior and severity within individual plots? 3. Did collection year influence germination or seedling vigor? Methods Fifteen seed plots were established in June 2010 to evaluate the effect of prescribed fire and hand thinning on Astragalus webberi seed germination. Twelve 2 x 2 meter seed plots, with 100 seeds per plot, were established within the treatment unit and three were established in an adjacent untreated area. The Figure 1. Planting seed in plots
treatment unit was hand‐thinned and materials piled and burned in 2009. Prescribed fire treatments were implemented in October of 2010. Prior to burning, woody fuels were arranged within nine of the seed plots to test the effect of fuel loading on germination. The untreated area was not thinned or burned. Prior to burning, all woody fuels were removed from the plots, weighed, and then re‐applied to simulate high (12.5 tons per acre), moderate (5 tons per acre), and low (no woody fuels) fuel loadings. Duff and litter depth was also measured for use as a covariate in the statistical analysis. Figure 2. Measuring woody fuels Figure 3. Plot treated during prescribed burn Flame length and rate of spread were used to calculate two measures of fire behavior (based on Rothermel and Deeming 1980): 
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Byram’s fireline intensity (Btu/ft/s) – estimates the rate of heat energy released per unit time per units length of the fire front Heat per unit area (Btu/ft2) – estimates the total amount of heat released per unit area as the flaming front of the fire passes. In general, for the same fireline intensity, faster rates of spread will direct less heat to the site, while slower moving fires will concentrate more heat to the site. Statistical analysis A one‐way analysis of variance (ANOVA) was used to compare germination rates among (a) the five different treatments: hand thinned with no underburn, hand thinned and underburned (with three different fuel loads), and no treatment; (b) the three levels of fire severity: unburned, lightly burned, and moderately burned; and (c) the treated and untreated plots. Germination rates were transformed (arcsine) prior to analysis to meet the assumption of normality. A multivariate analysis of covariance (MANCOVA) was used to examine the variation in fire behavior within the nine burned plots. Pre‐treatment fuel load was used as the predictor variable and the two measures of fire behavior, fireline intensity and heat per unit area, were included as correlated response variables. Duff and litter depth was also incorporated into the model as a covariate. Significance was determined using Wilks’ lambda. Post‐treatment severity was assessed using the National Park Service’s Fire Monitoring Handbook (2003). Plots were assigned a value of two if they were moderately burned and three if they were lightly burned; no plots were found to be scorched (4) or heavily burned (1). Because post‐treatment fire severity was divided into distinct categories, the effect of fuel load (high, medium, or low) was tested using Fisher’s exact test. Fisher’s exact test is used when you want to conduct a chi‐square test, but one or more of your cells has an expected frequency of five or less. A multivariate analysis of variance (MANOVA) was used to examine the effect of collection year (2009 vs. 2010) on two measures of seedling vigor (height and leaf number). Significance was determined using Wilks’ lambda. Results 1. Do prescribed fire and hand thinning treatments increase Astragalus webberii germination one year after treatment? Prescribed fire and hand thinning treatments do not appear to increase Astragalus webberii germination one year after treatment. Germination was not significantly correlated with pre‐treatment fuel load, fire behavior (i.e. fireline intensity or heat per unit area), or fire severity. In general, germination rates were low across all treatment types and ranged from 1.3 to 3.3 percent (Table 1, Figure 4). Table 1. Average number of seedlings and percent germination within treated and untreated plots. Treatment Type Hand thin, high fuels (12.5 tons/acre) Hand thin, moderate fuels (5 tons/acre) Hand thin, low fuels (no woody fuels) Hand thin, no burn No treatment ‐ Control Sample Size (n) 3
3
3
3
3
Mean number Average Percent of seedlings Germination 3 (± 1)
2.7% (± 0.6%)
3 (± 3)
3% (± 2.6%)
1 (± 2)
1.3% (± 1.5%)
2 (± 1)
3.3% (± 1.5%)
3 (± 2)
1.7% (± 0.6%)
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6.0
4.0
2.0
0.0
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HT, no burn
No treatment
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Figure 4 . The average percent germination of seeds within plots that were hand thinned (HT), hand thinned and burned with varying fuel loads, and within plots that received no treatment (controls). 2. Did artificial fuel manipulations influence fire behavior and severity within individual plots? Pre‐treatment fuel loadings did not have a significant effect on fire behavior, even with the variance associated with duff and litter removed. In order to more accurately examine this relationship, a larger study, with more replicates may be needed. Table 2. Measures of fire intensity and estimates of burn severity for plots treated with prescribed fire. All plots were hand thinned. Sample size for each plot was three. Treatment Type High fuel loads (12.5 tons/acre) Moderate fuel loads (5 tons/acre) Low fuel loads (no woody fuels) Fire behavior (averages) Fireline Intensity Heat per unit (Btu/ft2/s) area (Btu/ft2) Estimated burn severity Lightly burned Moderately burned (% plots) (% plots) 1.9 77.5 100 1.8 52.4 33 67 3.3 74.4 100 In contrast, fuel loadings did affect fire severity. All of the plots with high fuel loadings (12.5 tons per acre) resulted in moderately burned plots, while all of the plots with low fuel loadings (removal of all woody fuels) resulted in lightly burned plots. Plots with moderate fuel loadings (5 tons per acre) produced more mixed severity results. BurnSeverityIndex
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Figure 5. Estimated burn severity (based on Fire Monitoring Handbook) in response to pre‐treatment fuel loadings. Lower values represent higher severity; a value 2 of represents moderately burned and a value of 3 is lightly burned. 3. Did collection year influence germination? Seedling vigor (height or leaf number)? The year in which the seed were collected had no significant effect on germination or seedling vigor. Data are presented below in Table 3. Table 3. Summary of germination rate and seedling vigor for seed collected in 2009 and 2010. Seed Collection Year Percent Germination (averages) 2009 2010 2.4% (± 2.6%)
2.7% (± 2.3%)
Seedling vigor Height (mm)
Number of leaves 56 (±19)
2.6 (±0.6) 48 (±18)
2.8 (±0.7) Management implications Like many other legume species, the seeds of Astragalus webberii have a hard outer coat, which generally needs to be damaged or removed (i.e. scarified) in order to stimulate germination. The results of our monitoring suggest that habitat‐level treatments, such as prescribed fire and hand thinning alone, are not enough to achieve desired germination rates; therefore future reintroduction efforts should mechanically scarify the seeds (i.e. rub them between two layers of sandpaper) prior to planting in order to increase germination. Although not statistically significant in this study, our findings suggest that pre‐treatment manipulation of fuel loadings can be used to influence post‐treatment burn severity.