Ecological Archives E085-063-A4

Anurag A. Agrawal. 2004. Resistance and susceptibility of milkweed to herbivore attack: consequences of competition, root herbivory, and plant genetic variation. Ecology 85:2118–2133.

Appendix D. Effects of grass competition and time of day on photosynthetically active radiation reaching the base of milkweed plants on a clear day. I measured the amount of PAR reaching the base of milkweed plants with and without grass neighbors to assess the potential for light competition. Although this measure may overestimate the effect of competition because measures were taken at the base of plants, they nonetheless reflect the potential for light competition. At three time points throughout a clear day (8, 12, 16 hours), I measured PAR using a Licor 250 meter with quantum cosine-corrected sensor (LI-Corr, Lincoln, Nebraska, USA) (n = 20–34 per treatment-time combination). Light values for plants with and without neighbors were compared using separate variance t tests, which do not require equal variances among the two treatment groups (Wilkinson 1997). 

 
Time of day
 

8:00

12:00

16:00

 

Mean

SE

Mean

SE

Mean

SE

With grass

548

58

1006

84

361

33

Without grass

1115

20

1693

21

935

22

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t

9.33

 

7.93

 

14.35

 

df

36.8

 

37.2

 

33

 

P

<0.001

 

<0.001

 

<0.001

 

 

Literature cited

Wilkinson, L. 1997. Systat 7.0 - Statistics. SPSS Inc., Chicago, Illinois, USA.


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