[R] mixed-effects model with two fixed effects: interaction

[Ilona Leyer]

Dear all,
In a greenhouse experiment we tested performance of 4 different species (B,H,P,R) under 3 different water levels in 10 replications. As response variable e.g. the number of emerging sprouts were measured on three dates. A simple Anova considering every measurement date separately shows a higly significant effect of species and moisture (and partly the interaction of both). The mixed-effects model with species and moisture shows a highly significant effect of species and moisture as well. However, when I included the interaction the t-values of the species dropped strongly and the SE increase and the results for the species are not significant anymore. For me this does not seem plausible. Has anybody an idea, how this can be interpreted and if I have done a mistake in calculating the data? 

Thanks in advance for any help!
Ilona


model1<-lme(sprouts~species+moisture,random=~time|ID)
model2<-lme(sprouts~species*moisture,random=~time|ID)


Fixed effects: sprouts ~ species + moisture 
                         Value Std.Error  DF   t-value p-value
(Intercept)           7.971267  1.330500 240  5.991180  0.0000
speciesH             -6.459344  1.536329 114 -4.204400  0.0001
speciesP            -10.063604  1.536329 114 -6.550421  0.0000
speciesR             -5.051894  1.536329 114 -3.288288  0.0013
moisturemoist         2.228835  1.330500 114  1.675185  0.0966
moisturewaterlogged  17.111149  1.330500 114 12.860688  0.0000


Fixed effects: sprouts ~ species * moisture 
                                  Value Std.Error  DF   t-value p-value
(Intercept)                    4.831965  1.750970 240  2.759594  0.0062
speciesH                      -4.464197  2.476245 108 -1.802809  0.0742
speciesP                      -3.986787  2.476245 108 -1.610013  0.1103
speciesR                      -0.809376  2.476245 108 -0.326856  0.7444
moisturemoist                  3.505506  2.476245 108  1.415654  0.1598
moisturewaterlogged           24.766934  2.476245 108 10.001811  0.0000
speciesH:moisturemoist        -0.457291  3.501939 108 -0.130582  0.8963
speciesP:moisturemoist        -2.458125  3.501939 108 -0.701932  0.4842
speciesR:moisturemoist        -2.555356  3.501939 108 -0.729697  0.4672
speciesH:moisturewaterlogged  -5.597498  3.501939 108 -1.598400  0.1129
speciesP:moisturewaterlogged -15.538272  3.501939 108 -4.437048  0.0000
speciesR:moisturewaterlogged -10.206874  3.501939 108 -2.914635  0.0043