Competitiveness of the Exotic Silphium perfoliatum against the Native Urtica dioica: A Field Experiment
Table 5
Effects on the growth of S. perfoliatum and U. dioica and on reproductive parameters of S. perfoliatum.
Plant height
Parameter
S. perfoliatum
U. dioica
LM: < 0.001, adjusted R2 = 0.81, and n = 78
LM: < 0.001, adjusted R2 = 0.59, and n = 81
Df
Estimate
F value
Estimate
F value
Treatment
2
15.08
<0.001
3.37
0.040
Precipitation
1
0.01
167.13
<0.001
0.01
37.97
<0.001
Year of growth
1
0.20
7.97
0.006
0.02
0.900
Treatment × precipitation
2
1.53
0.223
12.09
<0.001
Treatment × year of growth
2
0.57
0.566
8.80
<0.001
Projection area
S. perfoliatum
U. dioica
LM, log-transformation: < 0.001 and adjusted R2 = 0.79 and n = 78
LM, log-transformation: < 0.001 and adjusted R2 = 0.81 and n = 81
Parameter
Df
Estimate
F value
Estimate
F value
Treatment
2
59.85
<0.001
92.11
<0.001
Precipitation
1
0.01
124.65
<0.001
0.01
90.62
<0.001
Year of growth
1
0.25
18.82
<0.001
0.67
10.25
0.002
Treatment × precipitation
2
5.33
0.007
2.19
0.119
Treatment × year of growth
2
1.57
0.214
9.59
<0.001
S. perfoliatum
Number of shoots per plant
Number of capitula per plant
GLM, Poisson-distribution: < 0.001 and n = 78
GLM, Poisson-distribution: < 0.001 and n = 78
Parameter
Df
Estimate
F value
Estimate
F value
Treatment
2
285.82
<0.001
258.14
<0.001
Precipitation
1
0.003
13.32
<0.001
0.017
191.08
<0.001
Growing season
1
0.469
30.07
<0.001
0.293
10.26
0.002
Treatment × precipitation
2
0.03
0.971
0.92
0.403
Treatment × year of growth
2
2.02
0.140
1.16
0.320
LM resp. GLM calculated with the resp. parameter as dependent variable. Independent variables were treatment (control without competition, surrounded by S. perfoliatum, and surrounded by U. dioica), precipitation (sum of April to August of the respective year), year of growth (2 to 4), and their interactions. Estimates are given for significant numeric parameters. They give the slope of the fitted regression line. Significant effects are highlighted in bold.
