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).
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.