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Prescribed Fire Effects on Biological Control of Leafy Spurge



Herbicide drift from adjoining cropland top-killed varying percentages of leafy spurge on all Walsh block 1 ("Walsh-1") plots in June 1994. The fall-postburn plot on Storhoff-1 was lost to vandalism in the winter 1994-95. Data from these plots were excluded from all analyses.

Table 1. Mean initial vegetative conditions on 9 treatments blocks. Values are least-square means.
Block Leafy Spurge Grass Cover (%) Forb Cover (%) Bare Ground (%) Litter Depth (cm)
Density (stems m-2) Height (cm) Cover (%)
A11 173a 35a 59ab 29ab 2.7ab 4.9a 3.5ab
A2 269a 42a 79c 33a 2.5a 2.7a 4.7ab
K1 148a 39a 60ab 35b 2.7ab 3.3a 4.6ab
K2 222a 35a 68bc 44b 2.5a 9.9b 2.8a
S1 142a 41a 69bc 46bc 3.2bc 3.7a 5.1bc
S2 203a 37a 60b 34b 3.7c 5.5a 5.5bc
T1 148a 42a 65abc 41bc 2.5a 2.7a 6.9c
T2 129a 42a 52a 53c 2.5a 2.5a 6.2c
W2 211a 39a 69bc 14a 2.5a 3.7a 3.4ab
SE2 34 2 5 7 0.2 1.1 0.8
1 Block names: A=Arrowwood NWR, K=Kemmer WPA, S=Storhoff WPA, T=Thiesen WPA, W=Walsh WPA; numbers indicate block number.
2 Pooled standard error for variable. a-c Means within a column with different superscripts are different (p<0.05).

No pretreatment differences were detected among treatments for any of the vegetation variables measured in 1993 (p>0.11). However, differences (p<0.01) occurred among blocks for percent spurge cover, percent grass cover, percent forb cover, percent bare ground, and litter depth (Table 1). Leafy spurge stem density, height, and percent cover decreased, and grass percent cover increased, with increasing distance from center of plot (p<0.01, Table 2). No interactions were found between treatment and distance (p>0.11) except for spurge height (p=0.03).

Table 2. Mean initial vegetative conditions at distances of 1, 3, and 5 m from plot center. Values are least-square means.
Distance Leafy Spurge Grass Cover Forb Cover Bare Ground Litter Depth
Density Height Cover
(m) (stems m-2) (cm) (%) (%) (%) (%) (cm)
1 193b 40b 69c 34a 2.7a 3.4a 4.8a
2 190b 39ab 64b 36a 2.8a 5.6a 4.8a
3 163a 38a 60a 39b 2.7a 3.9a 4.7a
SE1 6.4 0.4 1.2 1.1 0.1 0.7 0.2
1 Pooled standard error for variable.
a-b Means within a column with different superscripts are different (p<0.05).

Experiment 1

In 1995 we collected ≥1 beetle in standard sweep samples on 8 of 9 fall-preburn and 7 of 9 spring-preburn plots, compared to only 10 of 26 unburned plots. The proportion of plots on which ≥1 beetle was captured (i.e. "colonized plots") did not differ between fall-preburn and spring-preburn plots (p=0.52), but was higher (p<0.01) on burned (fall-preburn and spring-preburn combined) than on unburned plots.

In 1995, mean population indexes (SE) were 1.9 (1.4), 9.9 (2.3), and 10.2 (2.3) for unburned, fall-preburn, and spring-preburn plots, respectively. Population indexes did not differ between the spring- and fall-preburn treatments (p=0.92), but were higher on burned than unburned plots (p<0.01). Comparison of treatment effects on population indexes using only colonized plots indicated the presence of larger populations on burned than unburned plots, with arithmetic means (SE) of 4.7 (3.3), 11.1 (3.0), and 13.1 (3.6) for unburned, fall-preburn, and spring-preburn plots, respectively.

The proportion of plots colonized varied among blocks (Table 3), suggesting that conditions were more suitable for beetles on some blocks than on others. Comparison of 1993 vegetation on colonized vs uncolonized plots using only the unburned plots revealed differences in litter depth (means of 3.6 cm colonized, 5.1 cm uncolonized, p=0.05) and percent bare ground (5.7% colonized, 3.0% uncolonized, p=0.02). No other differences in vegetation were found between colonized and uncolonized plots (p>0.10). Comparison of Tables 1 and 3 suggests a positive correspondence between overall colonization success and mean percent bare ground on the 9 blocks; bare ground averaged 5.5-9.9% on blocks Storhoff-2 and Kemmer-2, where beetles established on all plots, 3.7-4.9% on Arrowwood-1, Storhoff-1, and Walsh-2, where some unburned plots and all of the burned plots were colonized, and 2.5-3.3% on the remaining blocks, where none of the unburned, and only some of the burned, plots were colonized. No such relationship was apparent for litter depth.

Table 3. Colony establishment by block and treatment. Block designations as in Table 1.
Block Unburned Burned
  (no. colonizes/n. plots)
K2 3/3 2/2
S2 3/3 2/2
A1 2/3 2/2
S1 1/2 2/2
W2 1/3 2/2
T2 0/3 1/2
K1 0/3 1/2
A2 0/3 1/2
T1 0/3 1/2

Eight of 25 colonized plots were burned in fall 1995 or spring 1996 to evaluate effects of burning on established colonies (see experiment 2). Among the remaining 17 colonized plots that were not burned for experiment 2, standard sweep samples in 1996 indicated that populations had increased or remained constant on 5, and decreased or gone extinct on 6, of the preburn plots. Similar results were noted on the unburned plots, with 3 populations increasing or stable and 3 declining or extinct.

Auxiliary samples in 1996 revealed presence of small populations (1-7 captures/plot) on 4 of 8 plots where beetles were captured in standard sweep samples in 1995 but not in 1996. No beetles were collected in auxiliary samples in 1996 on any of the plots where no beetles were detected in standard sweep samples in 1995.

Joint treatment effects of fire and beetles on leafy spurge stem density from 1993 to 1994 and 1993 to 1995 are compared in Table 4, first using all plots, and then using only plots on which beetles were detected in both 1995 and 1996. Both data sets indicate that stem density increased more (p<0.01) on the burned than on the unburned plots in 1994, but that the net change from 1993 to 1995 did not differ between treatments (p>0.30). Distance from plot center had no effect on change in stem density, and no distance x treatment interaction was found in any of the comparisons (p>0.29).

Table 4. Change in mean number of leafy spurge stems m-2 (SE) by treatment and year. Values are least-square means.
Treatment All Plots Beetles Present 1995 & 1996
n 1993 to 94 1993 to 95 n 1993 to 94 1993 to 95
    (δ stems m-2) (δ stems m-2)   (δ stems m-2) (δ stems m-2)
Control 9 11.9 (24.1)a 18.9 (23.3)a 7 13.8 (24.6)a 24.8 (21.4)a
Unburned 26 21.6 (13.9)a 5.7 (13.8)a 7 31.7 (24.6)a -20.2 (21.4)a
Fall preburn 9 135.4 (24.1)b 37.0 (23.3)a 5 141.7 (29.1)b 31.3 (25.3)a
Spring preburn 9 110.1 (24.1)b 24.5 (23.3)a 5 124.8 (29.1)b 33.7 (25.3)a
a-b Means within a column with different superscripts are different (p<0.05).

Experiment 2

Beetles were collected in standard sweep samples in 1996 on all 12 plots used to evaluate effects of fire on established colonies. Populations increased on 10 plots and decreased on 2 plots (1 reference and 1 spring-burn). Mean increase in number (SE) of beetles captured from 1995 to 1996 was 51.5 (33.3), 70.8 (33.3), and 36.8 (33.3) for unburned, fall-burn, and spring-burn plots, respectively. Treatment differences were not significant (p=0.77).

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